Revealing the pace of river landscape evolution during the
Quaternary: recent developments in numerical dating methods
Gilles Rixhon a, *
, Rebecca M. Briant b
, Stephane Cordier c
, Mathieu Duval d
, Anna Jones e
,
Denis Scholz f
a
University of Cologne, Institute of Geography, Zülpicher Str. 45, 50674 Cologne, Germany
b
Birkbeck University of London, Department of Geography, Environment and Development Studies, London, UK
c
University Paris Est Creteil Val de Marne, Department of Geography, Paris, France
d
Centro Nacional de Investigacion sobre la Evolucion Humana (CENIEH), Burgos, Spain
e
Edge Hill University, Department of Geography, Ormskirk, UK
f
University of Mainz, Institut of Geosciences, Mainz, Germany
a r t i c l e i n f o
Article history:
Received 29 February 2016
Received in revised form
18 July 2016
Accepted 8 August 2016
Available online xxx
Keywords:
Numerical dating method
Fluvial archives
Quaternary
14
C dating
Luminescence dating
ESR dating
230
Th/U dating
Terrestrial cosmogenic nuclides dating
a b s t r a c t
During the last twenty years, several technical developments have considerably intensified the use of
numerical dating methods for the Quaternary. The study of fluvial archives has greatly benefited from
these enhancements, opening new dating horizons for a range of archives at distinct time scales and
thereby providing new insights into previously unanswered questions. In this contribution, we separately
present the state of the art of five numerical dating methods that are frequently used in the fluvial
context: radiocarbon, Luminescence, Electron Spin Resonance (ESR), 230
Th/U and terrestrial cosmogenic
nuclides (TCN) dating. We focus on the major recent developments for each technique that are most
relevant for new dating applications in diverse fluvial environments and on explaining these for nonspecialists.
Therefore, essential information and precautions about sampling strategies in the field
and/or laboratory procedures are provided. For each method, new and important implications for
chronological reconstructions of Quaternary fluvial landscapes are discussed and, where necessary,
exemplified by key case studies. A clear statement of the current technical limitations of these methods is
included and forthcoming developments, which might possibly open new horizons for dating fluvial
archives in the near future, are summarised.
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction
Unravelling processes and rates of long-term landscape evolution,
focusing on the evolution of river drainage systems, has been a
core topic in the earth surface sciences since Davis’s (1899) pioneering
work more than a century ago. Since then, river terrace
sequences and/or related landforms have thus been extensively
used as geomorphic markers across the world. However, assigning
chronologies to these sequences and related river sediments or
landforms has constantly been challenging. Until the late 20th
century, this goal was often achieved using diverse methods that
provide relative age information on Quaternary fluvial deposits.
Such methods included: correlation with the alpine glacial
chronology (e.g. Brunnacker et al., 1982), soil chronosequences (e.g.
Engel et al., 1996), palaeomagnetism (e.g. Jacobson et al., 1988),
clast seismic velocity (e.g. Crook, 1986), weathering rind analysis
(e.g. Colman and Pierce,1981), obsidian hydration (e.g. Adams et al.,
1992), amino-acid racemization of terrestrial molluscs (e.g. Bates,
1994) or correlation to Marine Isotope Stages (MIS) via mammalian
(e.g. Schreve, 2001) and molluscan (e.g. Preece, 1999) biostratigraphy.
Combining these methods often yielded insightful relative
chronologies for Quaternary terrace flights (e.g. Knuepfer, 1988;
Schreve et al., 2007).
Whilst methodological improvements to some of these techniques
have since been achieved (e.g. Penkman et al., 2007 for
amino-acid racemization), in most instances, relative dating
methods have been progressively supplemented by dating methods
delivering absolute numerical ages over the last two or three decades.
With the exception of radiocarbon dating, which has been
applied since Libby's seminal paper (Libby et al., 1949), the
* Corresponding author.
E-mail address: grixhon@uni-koeln.de (G. Rixhon).
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Please cite this article in press as: Rixhon, G., et al., Revealing the pace of river landscape evolution during the Quaternary: recent developments
in numerical dating methods, Quaternary Science Reviews (2016), http://dx.doi.org/10.1016/j.quascirev.2016.08.016
development of most of these geochronometers occurred in relation
to major theoretical and/or technical improvements in the late
20th century. For instance, although cosmic rays were discovered in
1912 by the Nobel laureate Victor Hess, only the development of
accelerator mass spectrometers (AMS) in the 1980s enabled measurements
of cosmogenic nuclide concentrations (e.g. Klein et al.,
1982) and thus their use as a geochronometer (e.g. Nishiizumi
et al., 1986). Likewise, Electron Spin Resonance (ESR) spectroscopy,
already outlined in the mid 1930s (Gorter, 1936), was first
successfully applied as a dating tool only 40 years later (Ikeya,
1975).
In the framework of this FLAG (Fluvial Archives Goup) special
issue, we present and discuss the recent major dating advances
offered by modern numerical methods in diverse fluvial environments.
Five methods are discussed: radiocarbon, Luminescence,
Electron Spin Resonance, 230
Th/U and terrestrial cosmogenic
nuclide (TCN) dating. They were specifically selected amongst the
array of Quaternary dating methods because (i) they are commonly
used in the fluvial context, (ii) they have all experienced major
theoretical and/or technical developments during recent decades,
(iii) they require different dateable material and thereby may also
yield information about a wide range of fluvial processes and environments,
(iv) they have different time ranges of application, but
altogether, span the last million years (Fig. 1). Detailing all theoretical
principles of the individual techniques is beyond the scope
of this contribution. Instead, the focus is on relevant major technical
developments and how these enabled new dating applications
for different kinds of fluvial archives in distinct settings. The
pathways of dateable material within fluvial systems are detailed in
Fig. 2. Fundamental information and precautions about sampling
strategies in the field and/or laboratory procedures are also provided.
Whilst these are well known by geochronologists, they have
not often been published and need therefore to be clarified to nonspecialists
who intent to collect samples for dating. For each
method, new and important implications for chronological reconstructions
of Quaternary fluvial landscapes are also discussed
and, if necessary, exemplified. Case studies published in outputs
related to former FLAG activities and using one (or more) of these
dating method(s) are listed in Table 1. Current technical limitations
and probable forthcoming developments are also addressed.
2. Radiocarbon dating of fluvial deposits
Radiocarbon dating has been a common method applied to
fluvial deposits in those settings where organic material is readily
preserved within sequences, i.e. partially or fully waterlogged parts
of the floodplain system, including channels and overbank deposits
(Fig. 2). As a technique it has contributed significantly to
understanding key questions, both about palaeoenvironmental
information contained within fluvial deposits (e.g. Kasse et al.,
1995) and about periods of river activity (e.g. Macklin et al.,
2005). The accuracy with which age estimates can be gained
from ever smaller samples has improved significantly over the
60e70 years since the first development of the technique. This is
partly due to the increasingly routine use of accelerator mass
spectrometry (AMS) measurements of smaller samples (~1 mg in
some cases, Ruff et al., 2010, but more robustly 5e6 mg, Brock et al.,
2010). Another important development has been the significant
international cooperation involved in calibrating radiocarbon
measurements against independent annually-resolved records to
account for natural variability in the concentration of atmospheric
14
C, culminating most recently in the IntCal13 dataset (Reimer et al.,
2013).
The 14
C dating method can be applied to any material that
contains carbon. This includes: cellulose-containing materials
(wood, seeds, plant remains), charcoal and charred material, carbonates
(including corals, foraminifera, shells), collagen-containing
materials (bone, tooth, antler, ivory), hair, and bulk sediment. Many
of these are found within fluvial deposits in more temperate environments,
where preservation conditions are favourable, but not
all are in situ (Fig. 2). Therefore, when considering the radiocarbon
dating of fluvial deposits, we need to consider the issue of provenance
and reworking. In addition, calibration, reservoir effects and
appropriate pretreatments are also relevant to fluvial archives in
lakes, but reviewed elsewhere (Brauer et al., 2014).
All present-day carbon-bearing material contains three naturally
occurring carbon isotopes. Of these, 14
C is radioactive, with a
half life of 5730 ± 40 years (Godwin, 1962). The source of this 14
C is
cosmic ray activity in the atmosphere. This enters the global carbon
cycle when it is oxidised to CO2, and concentrations are very low
compared to 12
C and 13
C. Conventional radiocarbon ages are
calculated from measured concentrations of 14
C, using either beta
counting methods or, meanwhile more commonly, AMS. To allow
consistency with earlier analyses, these are reported using the
original Libby half life of 5568 years (e.g. Stuiver and Polach, 1977;
Reimer et al., 2004). They are also corrected for fractionation processes
that occur during measurement, as described by Brauer et al.
(2014). Because of the multiple stages at which differences can
occur within the calculation of a radiocarbon age, they should be
reported in detail according to the conventions described by
Millard (2014).
2.1. Provenance and reworking of radiocarbon samples in the fluvial
environment
A feature of fluvial systems is the wide range of depositional
Fig. 1. Dateable ranges of the five numerical dating methods detailed in this contribution. Black rectangles refer to time spans within which the methods usually provide reliable
results; dashed rectangles represent challenging time periods. Luminescence methods are divided into two rows: the first row represents the routinely applied techniques (OSL:
optically stimulated; IRSL: infrared stimulated, including pIRIR) and the second row the techniques currently under development (TT: thermally transferred; RF: radiofluorescence).
ESR dating on quartz and U-series/ESR dating of tooth enamel as well as surface exposure dating and burial dating with terrestrial cosmogenic nuclides (TCN) are also divided
because of the different dating principles.
G. Rixhon et al. / Quaternary Science Reviews xxx (2016) 1e232
Please cite this article in press as: Rixhon, G., et al., Revealing the pace of river landscape evolution during the Quaternary: recent developments
in numerical dating methods, Quaternary Science Reviews (2016), http://dx.doi.org/10.1016/j.quascirev.2016.08.016
environments which may be found within a single catchment,
including, for instance, river channel, floodplain and floodbasin
deposits. These differ in frequency of depositional events, deposited
grain sizes and likely presence of in situ organics (Fig. 2). The
nature and rate of fluvial activity within a reach determine the
spatial distribution of depositional environments and their preservation
within the alluvial record (Lewin and Macklin, 2003). The
depositional context of a radiocarbon-dated sample determines its
suitability for answering questions about the timing of events
within a fluvial system. Where possible, a distinction should
Fig. 2. Sketch representing the dateable deposits/landforms and the pathways of dateable material for 14
C, OSL/IRSL, ESR, 230
Th/U and TCN dating in both braided and meandering
fluvial systems. Transport pathways and temporary storages of both inorganic (gravel, sand, silt) and organic (bone, charcoal, opercula, seed, shell and tooth) materials on hillslopes
and in the fluvial system are also represented.
Table 1
Case studies published in outcomes relative to former FLAG activities using one (or more) numerical dating method(s) detailed in the text.
FLAG special issue/dating method Radiocarbon Luminescence ESR 230
Th/U TCN
Quaternaire 15, issue 1e2 (2004) Briant et al. Chausse et al.
Flez and Lahousse Jain et al. Despriee et al. Veldkamp et al.
Salvador et al. Mol et al. Voinchet et al.
Kuzucuoglu et al.
Mathieu et al.
Quaternary International 189,
issue 1 (2008)
Fontana et al.
Kalicki et al.
van der Schriek et al.
Geomorphology 98, issue 3e4
(2008)
de Moor et al.
Proceedings of the Geologists'
Association, 121, issue 2 (2010)
Coltorti et al. Lauer et al.
Kasse et al. Martins et al.
Vandenberghe et al.
Vis et al.
Geomorphology, 165e166 (2012) Cordier et al. Anton et al.
Cunha et al.
Ramos et al.
Geomorphologie, Relief, Processus,
Environnement, issue 4 (2012)
Le Jeune et al.
Piovan et al.
Boreas 43, issue 2 (2014) Wolf et al. Cordier et al. Zhu et al. Rixhon et al.
Zhu et al. Wang et al.
Quaternaire 26, issue 1 (2015) Garnier et al. Harmand et al.
G. Rixhon et al. / Quaternary Science Reviews xxx (2016) 1e23 3
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therefore be made between radiocarbon dates from within thick
sedimentary units and those collected at or close to boundaries
between units. The former provide a single age for processes, such
as vertical sediment accretion or lateral channel migration, operating
over an extended time period (Lewin et al., 2005), while the
latter constrain the timing of events in the river system that produced
sedimentological changes (Macklin and Lewin, 2003). In Late
Pleistocene to Holocene settings, where detailed sedimentological
information is more commonly preserved, radiocarbon dates on
fluvial units have been classified as river activity ages, in minerogenic
sediment, or river stability ages, on peat or palaeosols
(Zielhofer and Faust, 2008). Sedimentary units indicative of river
activity and river stability may be produced simultaneously in
different depositional environments within a single reach
(Zielhofer and Faust, 2008). Radiocarbon dates close to sediment
unit boundaries provide maximum or minimum ages for events
which produced features such as reversals in fining upwards
sediment sequences or renewed fluvial sedimentation above a peat
or palaeosol (Macklin et al., 2005). Radiocarbon-dated samples
from a unit directly below such a sedimentological change, giving a
maximum age (‘change after’ dates), are regarded as the most
reliable indicator of the age of the event which produced the
change in sedimentation rate or grain size (Macklin et al., 2010). In
older deposits, where fewer units are amenable to radiocarbon
dating, such precise analysis of how the dates relate to fluvial activity
is less feasible. Nonetheless, the sedimentological setting
should be assessed in a similar way so that the age estimate obtained
can be most effectively interpreted. In addition, care should
be taken to interpret the different transport pathways of the type of
material to be dated.
Further to their diversity, river catchment systems are highly
dynamic and material can be transported varying distances from
the original source. It can also be kept in storage on hillslopes or
within floodplains and released into the channel tens to thousands
of years afterwards (Fig. 2). Therefore, when radiocarbon dating
material from fluvial deposits, the possibility of reworking must
always be borne in mind. This can be especially problematic in
relation to carbon-bearing material that does not easily break down
in transport (Fig. 2), for example wood, bone and some shells
(either because they are calcitic, such as shell opercula, or because
they are light and travel in suspension rather than bedload).
Therefore, it is essential to date only the identifiable fraction of the
deposit (Table 2). In addition to being identifiable, it is necessary to
exercise common sense over how likely the material isolated is to
have been contemporaneous with deposition (e.g. not choosing
material suggesting a temperate climate if preserved within cold
stage deposits).
2.2. Calibration
Due to natural variability in cosmic ray production and exchange
between different carbon stores (i.e. ocean, terrestrial ecosystems
and atmosphere) the concentration of 14
C in the atmosphere varies
over time. For this reason, to convert radiocarbon ages to calendar
ages, a detailed calibration curve has been constructed from independently
dated (often annually resolved) records including treerings,
varves, corals and speleothems. The tree-ring curve extends
to 13,900 cal years BP (Reimer et al., 2013) and is the most robust
part of the curve. The extension beyond this to 50,000 cal years BP
is based on multiple datasets which diverge from each other in
places, creating larger errors. The radiocarbon community meets
regularly to review this curve and the most recent data set is
IntCal13 (Reimer et al., 2013) e or SHCal13 for the Southern
Hemisphere (Hogg et al., 2013), which is now included in all calibration
software (e.g., CALIB, OxCal, BCal). The output of calibration
is an interval of possible calendar ages that correspond to the 14
C
age calculated from the measured 14
C concentration. Often multiple
intervals correspond to the measured concentration.
2.3. Freshwater reservoir effects and radiocarbon dating of fluvial
archives
When dating plant or shell material, the preferred habitat of the
species used is crucial. If the material to be dated is from an aquatic
species, the chemistry of the water body must be taken into account.
The presence of ‘old carbon’ which is ‘dead’ with respect to
radiocarbon can lead the 14
C of the water to have an apparent age.
This apparent age is then transferred to the material being dated.
This issue has been known for many years, with very early studies
showing that Potamageton, an aquatic plant which is believed to
photosynthesise within the water column, yielded an apparent age
in modern hardwater lakes of ~2000 years (Deevey et al., 1954). For
this reason, the first choice of material to date would instead be a
plant which photosynthesises directly with the atmosphere such as
Scirpus or Carex (Deevey et al., 1954).
Determining the freshwater reservoir effect in lakes (where
some fluvial archives are found) is based on the assumption that
the effect has remained constant and can be corrected for. In relation
to rivers, this is more problematic because most studies (e.g.
Deevey et al., 1954) have been carried out in lakes. A recent study of
water, plants and animals from rivers in northern Germany
(Philippsen, 2013) showed significant temporal variability in the
scale of reservoir age in both the dissolved inorganic carbon (DIC)
from river water itself (a range of 1527e3044 years) and the
reservoir age in aquatic plants (a range of 350e2690 years). Of
particular interest is the finding that the radiocarbon values from
river water are directly related to the balance between groundwater
and precipitation inputs to the system. When precipitation
was higher before samples were taken, associated radiocarbon
reservoir ages of river water were younger. This means that the
freshwater reservoir effect in fluvial deposits is likely to be present,
but to varying degrees. The main way to avoid this issue is to date
only terrestrial species of plants or molluscs (Table 2), which requires
the investigator to develop some skills in fossil identification.
Brauer et al. (2014, p.49) recommend for lake or speleothem sequences
that where samples known to be affected by a freshwater
reservoir effect have been measured, these “must be corrected prior
to calibration by subtraction of the age offset estimated using the
measured 14
C concentration of known age samples”. In the fluvial
setting, given the demonstrable variability in freshwater reservoir
effect in relation to discharge, and the known large fluctuations in
discharge regime over the time period of the radiocarbon technique,
this is unlikely to be possible. The radiocarbon dating of
aquatic species should therefore be avoided unless a 2000 year
uncertainty is sufficient to answer the research question being
posed.
2.4. Laboratory pretreatments
Because the concentrations of 14
C are so low in materials used
for radiocarbon dating, the possibility of contamination with
modern carbon is always present. Contamination during sample
preparation can be avoided as detailed in Table 2. Removing
contamination that has accumulated in situ requires laboratory
pretreatment (Table 2) and becomes more crucial for samples near
the limit of the radiocarbon technique between 30,000 and 50,000
14
C years BP, because the amount of 14
C present within the sample
is so low that any contamination has a much larger effect (Fig. 3).
This is particularly important in many discontinuous fluvial sequences
where problems with dating cannot be detected in the
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in numerical dating methods, Quaternary Science Reviews (2016), http://dx.doi.org/10.1016/j.quascirev.2016.08.016
context of a vertical sequence.
Significant progress has been made in recent years in providing
more reliable radiocarbon ages on bone and charcoal from this
older time period (e.g. Higham et al., 2006; Bird et al., 1999) and
these pretreatments should be used for these materials. However,
fluvial sequences sometimes lack these dating materials, which are
best preserved in dry, alkaline conditions (the opposite of the wet,
acidic conditions often present in fluvial systems for the preservation
of environmental material). There are as yet no ‘stand-out’
preferred pretreatments for the shell or seed material more
commonly preserved in fluvial deposits, and advice should be
sought from the radiocarbon laboratory with which you are
working if the samples are likely to be near the limit of the
technique.
3. Luminescence dating of fluvial deposits
The Optically Stimulated Luminescence (OSL) dating method is
currently one of the most commonly applied to fluvial sediments
because it directly dates the sand/silt grains of which such sediments
are often composed. These grains enter the river system
from hillslopes and then travel in suspension through the fluvial
system, passing into and out of storage before final deposition
(Fig. 2). This method was developed during the 1980s as an alternative
to Thermoluminescence (TL). It became widely used ~15
years ago, in particular due to the development of the Single Aliquot
Regenerative (SAR) protocol (Murray and Wintle, 2000, 2003)
which replaced previous additive approaches. The SAR protocol
enables multiple age estimates to be measured from a single
sample, generating more accurate final ages (Duller, 2008).
Continuous improvements to precision and accuracy have occurred
during past decades, giving the OSL method a key role in the dating
of Quaternary fluvial archives. Many publications in special issues
of the FLAG include an OSL-based geochronological approach
(Table 1). Questions that have been answered by using this
approach encompass, for example, the timing of phases of fluvial
activity in relation to climate (e.g. Briant et al., 2004) or the dating
of terrace bodies associated with archaeology (e.g. Cunha et al.,
2012; this issue/2016).
Several relevant reviews related to the technical details of optical
dating of fluvial deposits have been published (e.g. Wallinga,
2002; Rittenour, 2008), so the principles and basic procedures are
here only briefly mentioned. Instead, we focus particularly on what
researchers working on fluvial archives need to know to successfully
apply this method to their samples. This includes sampling
strategies, new protocols and statistical treatment of data required
Table 2
Sampling and laboratory techniques to improve accuracy in radiocarbon dating fluvial deposits. AMS ¼ accelerator mass spectrometry.
Issue Sampling/laboratory solution
1) Dating suitable material
Age difference between 14
C dated sediment deposit and the
deposit/event for which an age is required
Select samples for dating according to sedimentary context and, where possible, from close to boundaries
between sedimentary units (c.f. ‘change after’ dates (Macklin et al., 2010).
Danger of reworking of fossil material either whole or as
organic detritus (e.g. Rogerson et al., 1992)
Date only the identifiable fraction of the deposit e e.g. thoroughly cleaned specific plant macrofossils,
shells or bones (e.g. Turney et al., 2000). This is only possible because of AMS techniques which allow
dating of small samples.Danger of field contamination by modern organic detritus
2) Freshwater reservoir effect
Danger of carbon uptake from carbonate rich water rich in ‘old’
carbon
Date only macrofossils from terrestrial species which do not photosynthesise under water (e.g. Carex,
Scirpus).
3) Pretreatments to remove contaminants
Danger of secondary carbonate from post-depositional
groundwater infiltration
Dilute HCl pre-treatment (first step of the mild ABA below).
Danger of humic acid infiltration from higher in the profile e
especially if overlain by peat
For younger samples (<~25 14
C yr BP): mild acid-base-acid (ABA) washes as standard pretreatment.
For older samples, other pretreatments are recommended to remove more contamination:
 Ultrafiltration on bone (e.g. Higham et al., 2006)
 ABOx-SC (acid, base, wet oxidation - stepped combustion) on charcoal (Bird et al., 1999)
 No clear favoured protocols as yet for seeds or shells
4) General considerations
Sampling and laboratory preparation  Ensure laboratory space and all equipment being used for preparation has never previously come into
contact with radioactive elements (e.g. from biological researchers using 14
C as a tracer element e
Zermano et al., 2004)
 Avoid organic packaging such as paper
 Process and store sample in deionised water only
 Clean working conditions, avoiding contact of samples or equipment with paper where possible
 Powderless laboratory gloves
 Visual checks for contamination
 Dry samples soon after identification to prevent fungal growth during storage
 Submit as large a sample size as possible e preferably >1.4 mg carbon content, i.e. >5 mg dry weight
(Brock et al., 2010)
Fig. 3. The impact of modern contamination (0.25e2% by weight) on measured 14
C
ages (thin lines) compared to the 1:1 or uncontaminated line (thickest line). After
Pigati et al. (2007).
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to derive reliable age estimates.
The luminescence method is based on the estimation of the
impact of radiation on the crystalline structure of minerals such as
quartz (optically stimulated, OSL) and feldspar (infrared stimulated,
IRSL) while they are shielded from light (e.g. Duller, 2004). The
radiation (a, b, g) comes from radionuclides which are present in
the mineral and its natural environment, mainly U, Th (and their
decay products) and K, with a small proportion from cosmic particles.
This radiation leads to the trapping of electrons in crystalline
lattice defects. The total amount of trapped electrons within a
crystal is proportional to the total energy (dose) absorbed by the
crystal, which naturally increases with time. As soon as the mineral
is exposed to sunlight, especially during its transport, trapped
electrons are released from the traps. This generates the emission of
light (the luminescence signal) which can be measured following
light stimulation (Huntley et al., 1985). The age of the sediment is
then estimated by dividing the DE (equivalent dose) by the dose
rate (the rate at which the sediment is exposed to natural
radiation).
It should be noted that the sensitivity of mineral grains to optical
stimulation is highly variable, making some depositional settings
inherently more successful than others. “Quartz grains that have
undergone repeated cycles of bleaching and deposition tend to
become sensitized … and so for some samples a large fraction of
quartz grains will yield a measurable OSL signal … [In contrast],
samples from any environment can show poor sensitivity and
highly-skewed sensitivity distributions … [where] … 95% of the
combined OSL signal comes from less than 5% of the grains”
(Cunningham and Wallinga, 2012, p. 17). In addition, the
commonly-used SAR protocol may not be applicable for all samples.
Standard tests for the appropriateness of the SAR protocol include
the use of a dose response test (i.e. can the laboratory protocol
successfully remeasure a known dose?) and the recycling ratio (i.e.
does the test dose successfully correct for sensitivity changes during
measurement?). However, recent experimental work suggests
that these may be insufficient tests (Guerin et al., 2015). It is
possible that this is due to an initial sensitivity change that is not
corrected for by the use of the response to a test dose.
3.1. Causes of age underestimation
The complexities involved in generating a luminescence signal
mean that in some cases it is not possible to provide a reliable age
determination, either over- or underestimating the age of the
sediment. Underestimation may occur when the mineral is saturated.
This means that all traps have been filled with electrons, thus
preventing additional trapping. The measured signal will hence
only reflect a part of the burial duration, and the obtained age must
be considered as a minimum age. Saturation explains why the OSL
method cannot be applied to old sediments. The age limit varies
between minerals, but quartz often saturates at doses of
~200e300 Gy (Table 3, Wintle and Murray, 2006). This makes it
difficult to date sediments beyond 150 ka (except when the dose
rate is quite low).
Feldspars in contrast saturate at higher doses and may in theory
be used to date Middle Pleistocene sediments (Table 3). However,
feldspars are affected by anomalous fading. This is the spontaneous
eviction of electrons from deep traps without light stimulation
(Wintle, 1973) which can then lead to age underestimation. Several
procedures have been developed to detect and correct for anomalous
fading by estimating fading rates (Huntley and Lamothe,
2001). Another approach is the post-IR-IRSL procedure (Thomsen
et al., 2008). This procedure is based on the measurement of an
elevated temperature (>200 C) post-IR IRSL signal immediately
after the IRSL measurement (typically performed at 50 C). The
post-IR IRSL signal is characterised by a higher stability and thus
yields lower fading rates (Buylaert et al., 2009). However, the postIR
IRSL signal is harder to bleach than the IRSL signal.
3.2. Fluvial transport and incomplete bleaching: a main source of
age overestimation
The physical principles behind OSL suggest that the method is
well suited to the study of fluvial sediments, since it allows direct
dating of the last transport-and-sedimentation process (Table 3).
However, in addition to the mineral-related issues described above,
a key issue related to the dating of fluvial sediments is the potential
occurrence of incomplete bleaching. This phenomenon occurs
when grains have not been exposed to sufficient daylight during
transport. In this case, a part of the measured OSL signal is formed
by electrons that remained trapped despite the fluvial transport
(inherited component; Murray and Olley, 2002). This leads to an
overestimation of the age, which is significant in the case of young
sediments (less than 2 ka; Jain et al., 2004), but may also affect
older sediments. For this reason, the detection and avoidance of
incomplete bleaching is fundamental to obtain reliable burial ages
to infer the timing of deposition.
In the case of fluvial sediments, these should be selected for
sampling to maximise bleaching in the depositional setting. The
degree of bleaching of the individual grains depends on two main
parameters (Stokes et al., 2001): transport length and the transport
conditions. Sufficient transport is necessary to ensure complete
bleaching of the signal. Studies focusing on transport length
showed that the inherited signal was significantly reduced after a
transport of several tens or hundreds of km (Murray and Olley,
2002). The second parameter refers to the way the grains are
transported and includes, amongst others, the water turbidity and
the channel depth. Grains that have been transported in a deep
water column (leading to strong attenuation of the solar spectrum)
and/or in turbid water may therefore be incompletely bleached
(e.g. Ditlefsen, 1992). Settings in which samples are more or less
likely to be completely bleached are represented in Fig. 2. However,
the expertise of the researcher must be employed at the site to truly
maximise the likelihood of sampling completely bleached material,
since the presence of turbid water or a deep water column will
usually leave a sedimentary signature.
Table 3
A brief overview of the luminescence dating method applied to quartz and feldspar grains extracted from sediment.
Quartz (SAR) Quartz (TT-OSL) Feldspar (IRSL) Feldspar (pIR-IRSL)
Upper dating range Present-day
Lower dating range 200-300 Gy e i.e. ~150 ka (depending on dose rate) ~950 ka Unclear due to anomalous fading e ~300 ka? ~950 ka
Main strength of the application Can date fluvial sediments directly.
Can date beyond the C-14 dating time range.
TT-OSL and pIR-IRSL can extend back to c. 1 Ma.
Standard precision Standard errors are usually ~10% (5e15%).
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3.3. The importance of the sampling strategy
Following from this, the sampling strategy should aim to collect
the potentially best bleached grains, keeping in mind that the OSL
method is mainly applied to sand- (100e250 mm) or silt- (4e11 mm)
sized grains. This makes it necessary to perform fine sedimentological
investigations to interpret the depositional locations (i.e.
channel/palaeochannels, point bar, crevasse splay, floodplain deposits).
Most sediments analysed to date have been collected in
channels or point bars (Figs. 2 and 4), as these are more clearly
associated with significant transport of the grains. OSL dating of
floodplain deposits is less common, but possible especially in the
case of sandy facies (Keen-Zebert et al., 2013). Considering the
sedimentation process is also very important, as the exposure to
sunlight will be different in a flood dominated river (typical of
Mediterranean or semi-arid areas) or a less ephemeral river. In the
latter, presence of a more regular water flow will allow grains to be
more completely bleached, while in the former case mass transport
associated with floods may prevent complete zeroing (Bartz et al.,
2015).
In common with all depositional locations, the sampling should
ideally be performed in thick (>30 cm both above and below the
sample) homogeneous layers, to ensure that the dose rate estimation
is as simple as possible (Fig. 4a). This is particularly important if
the field scientist does not have access to a field gamma spectrometer
which can capture the dose rate from this full radius of
gamma radiation (Fig. 4b). In the common case of a thinner bed
surrounded by inhomogenous sediments, detailed attention should
be paid to the ‘micro-stratigraphy’ and small samples for laboratory
dose rate measurement taken from all sediment types within a
30 cm radius of the sample. These can have significantly different
dose rates (clays are often higher, gravels lower) and this can be
adjusted for using the methods published by Aitken (1985) if such
samples are taken. It is worth being aware, however, that the
greater complexity of dose rates and lower likelihood of complete
bleaching may make the results from such samples hard to
interpret.
The choice of the mineral to be studied as a dosimeter is also
crucial, if a choice is possible. Both theoretical work and comparative
analyses by Wallinga et al. (2001) showed for Upper Pleistocene
to Holocene sediments that quartz was a preferred dosimeter.
The quartz grains are more rapidly bleached than feldspars (a few
seconds vs a few tens of seconds, Huntley et al., 1985), and not
affected by anomalous fading. For older deposits, trade offs must be
made, and feldspars may be selected to allow dating of older deposits,
with anomalous fading effects taken into account and corrected
for as well as possible.
This sedimentological approach is fundamental in selecting the
grains with the best properties for dating. However, it may in some
cases not be sufficient to avoid heterogeneous bleaching. There are
measurement protocols that seek to avoid partial bleaching by
measuring or reporting only the well-bleached component within a
sample (e.g. ‘early background subtraction’, Cunningham and
Wallinga, 2010, or combined IR and OSL stimulation, Jain et al.,
2005). However, none of these methods have become mainstream
approaches as yet. It is also worth noting that field investigations
and sampling may benefit from the use of recently
developed portable readers (Sanderson and Murphy, 2010). These
make it possible to broadly estimate luminescence intensities and,
when combined with in situ gamma spectrometry, the depositional
age. Whilst the precision is too low for this to replace laboratory
measurements, it may be a useful tool in the case of complex
Fig. 4. a) A good sampling location for Luminescence dating at Stanswood Bay, Hampshire, England (Briant et al., 2006). The thickest sand bed at the base is Mesozoic in age, but
samples were taken from thick sand beds above the gravel channel (as shown and above the photo out of view); b) Less optimal sampling location for Luminescence dating at
Barton on Sea, Hampshire, England (Briant et al., 2006). Field gamma spectrometry was undertaken to mitigate the complex dose rate effect of the thinner sand lenses.
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depositional patterns, to detect potentially problematic samples
and guide sampling strategies (Stone et al., 2015). It has yet to be
tested on fluvial sediments, or at the lower luminescence intensities
typical of temperate-zone samples.
3.4. Detection of incomplete bleaching during OSL measurements
and statistical treatments to address this issue
Incomplete bleaching can be detected while performing luminescence
measurements in the laboratory. Large-scale assessments
can be made firstly by measuring both quartz and feldspars for a
given sample. As the dosimeters have different bleaching rates,
obtaining comparable ages provides evidence for complete zeroing
of the sediments prior to burial (Colarossi et al., 2015). The testing
of modern analogues (recent sediments transported under conditions
similar to those under study) may also be useful (e.g. Geach
et al., 2015), provided such sediments are available.
It is possible to statistically separate different parts of the
luminescence signal to isolate the ‘fast’ component, which is most
easily bleached (e.g. Singareya and Bailey, 2004). The most common
way of detecting incomplete bleaching in the laboratory, however,
is through investigation of the distribution of multiple age estimates
from a sample. The SAR protocol is based on the measurement
of multiple equivalent doses (from aliquots or single grains)
for a given sample. The number of aliquots used varies, but
Rodnight (2008) proposed 50 aliquots as a minimum based on
analysis of a poorly bleached fluvial sample. In some case higher
values are required or lower may be sufficient (Galbraith and
Roberts, 2012). It is important that these measurements are performed
on small aliquots or single grains to avoid averaging of the
signal across the aliquot.
The initial assumption is that a fully bleached sample will yield
consistent DE values (excluding analytical uncertainty). Therefore,
the presence of scattering in the DE distribution is taken as an
indication that some aliquots have been incompletely bleached.
Whilst this is commonly represented as a histogram or probability
density function, recently many workers have started to use radial
plots which allow the inclusion of information on the precision of
each DE (e.g. Galbraith, 2010; Fig. 5). Use of appropriate statistical
methods for plotting and choosing an average DE has been made
simpler for the non-specialist by the recent development of the R
package for Luminescence dating (Kreutzer et al., 2012). The
overdispersion parameter, defined as the remaining dispersion after
having considered the uncertainty sources associated with the
measurement, is seen as an indicator of the likely presence of
partial bleaching (Colarossi et al., 2015). However, it is difficult to
propose a single threshold value for this since other parameters
also influence overdispersion (Thomsen et al., 2012).
Following investigation of the shape of the distribution, the DE
value used for the final age determination is derived from several
‘age models’ (Lauer et al., 2010), all available in the R package for
Luminescence (Fuchs et al., 2015). The most commonly used are the
Common Age and Central Age Models (combining the calculation of
overdispersion with that of the weighted mean), which are
appropriate when the overdispersion is zero or low, respectively
(no significant evidence for partial bleaching). The Minimum Age
Model (Galbraith and Laslett, 1993) is used for samples with higher
overdispersion values to identify the most well bleached aliquots
and bases the age estimate on these. Finally the Finite Mixture
Model (Galbraith and Green, 1990) can be applied to single grains
only (Galbraith and Roberts, 2012) and allows the detection of
discrete populations. In all these cases, however, the choice of the
age model to be used is often subjective, since there is no set
threshold value of overdispersion to use for choosing between
different age models. Bayesian methods have been used for a
number of years by the radiocarbon community and are useful in
robustly identifying outliers and thereby increasing precision. Such
approaches have recently been tested on OSL samples (e.g.
Cunningham and Wallinga, 2012; Guerin et al., 2015). Cunningham
and Wallinga (2012) applied a combination of bootstrap likelihoods
and Bayesian methods to young (<1 ka), partially bleached samples
from a vertical floodplain sequence in the Netherlands. The bootstrap
likelihoods were used to provide a probability density function
for each sample that was statistically appropriate for Bayesian
analysis. This approach was useful in this setting, but can only be
applied where there is sufficient sample density for the stratigraphical
relationships to be known and the age distributions to
overlap.
The need for such complex statistical treatment of the results
may be considered a drawback of the luminescence dating method,
since the obtained age is dependent on the model used. However,
when explained fully and justified in relation to luminescence
characteristics, this approach leads to greater confidence in the
robustness of the results. The selection of the “best” model then
Fig. 5. Comparison of two common methods of plotting data (82 aliquots of aeolian quartz). The radial plot on the right is able to show both precision and equivalent dose on the
same plot. This is not possible with the histogram on the left, nor with commonly used probability density plots. Figures 1 and 2 of Galbraith (2010).
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derives from a rigorous analysis of all the available data, including
not only the measurement values, but also the field and sedimentological
evidence (which can be useful for example to assess the
bleaching potential of the sediments). Furthermore, recent developments
in the use of Bayesian statistics hold out a hope that a
single approach to determining equivalent dose may soon be
possible where stratigraphical relationships are clear.
3.5. A key issue for the future: extending OSL dating to the Middle
Pleistocene
Whilst fluvial sediments of Middle Pleistocene age have been
dated, especially using IRSL on feldspars, extending the age range to
older sediments remains a major issue (Table 3). This is also of
significant importance for the FLAG community as it will allow a
longer-term reconstruction of valley evolution during the Pleistocene.
Several protocols have been developed to date older sediments,
including the pIR-IRSL method discussed above.
For quartz, a new approach is the measurement of the Thermally
Transferred OSL signals (TT-OSL; Wang et al., 2006). These signals
are observed after stimulation and heating of the quartz grains and
result from a complex charge transfer associated with the heating.
As they saturate at much higher doses than the OSL signal, they
might be used for dating older sediments (Table 3). Arnold et al.
(2015) compared single-grain TT-OSL and pIR-IRSL at the Atapuerca
hominin site where independent age control is available.
When they used measurement temperatures of 225 C, they found
good agreement for both methods from ~240 to 930 ka, though pIRIRSL
measurements at 290 C gave overestimates. Arnold et al.
(2015) argue therefore that multiple methods should be used in
extended range dating, since each is more reliable in different
settings. This view seems also relevant for the new developments
in Luminescence dating, such as the Infra-Red Radio-Fluorescence
(IR-RF) or the Violet Simulated Luminescence, for which further
investigations are required prior to validate their suitability for
dating ancient fluvial archives (Schmidt et al., 2015). It is worth
noting that these approaches do not address uncertainties in estimating
dose rates, which remain significant also at older ages.
4. Electron Spin Resonance (ESR) dating in fluvial
environments
ESR is a radiation exposure (or palaeodosimetric) dating method
based on the evaluation of the natural radiation dose absorbed by
materials over geological times. The first application of ESR as a
geochronologic tool was published by Ikeya (1975) on stalagmites
from Japanese caves. Since then, the method has been used on a
wide range of materials including phosphates, carbonates, and
silicates (see review in Ikeya, 1993). The most popular applications
in fluvial settings are undoubtedly on fossil teeth and optically
bleached quartz grains extracted from sediment, either for targeted
dating of a given site/section (e.g. Falgueres et al., 2006; Santonja
et al., 2014) or for the establishment of a comprehensive chronological
framework for terrace staircases (e.g. Voinchet et al., 2004;
Antoine et al., 2007; Cordier et al., 2012). As with Luminescence
dating, ESR dating is based on the quantification of charge trapped
in the crystalline lattice of a material under the effect of natural
radioactivity. These trapped charges give rise to an ESR signal
whose intensity is proportional to the radiation dose absorbed by
the sample over time. The ESR age equation is similar to that used in
luminescence dating and the standard analytical procedure consists
in determining the two main parameters: the equivalent dose
(DE) and the dose rate. DE is obtained using ESR spectroscopy, by
artificially aging the samples at increasing doses in order to
describe the behaviour of the studied signal. The dose rate is usually
assessed by a combination of in situ and laboratory measurement
using a wide range of different analytical techniques and corrected
for the density of the material, its geometry and water content (see
Grün, 1989; Duval, 2016).
4.1. ESR dating of fossil teeth: on the importance of modelling
uranium incorporation into dental tissues
ESR dating of fossil teeth has been first proposed in the mid-
1980s as an alternative to fossil bones (see an overview by Duval,
2015 and references therein). The main difficulty of this application
lies in the complexity of the system that has to be considered
for the dose rate evaluation. A tooth is made from different dental
tissues (dentine, enamel and, sometimes, cement). They have
different characteristics in terms of composition and thickness but
all contribute to the irradiation of the enamel layer. Additionally,
dental tissues are known to behave as open systems for U-series
elements. In other words, teeth frequently experience delayed Uuptake
or U-leaching processes. As a consequence, it is crucial to
model the kinetics of the incorporation of U into each dental tissue
in order to obtain an accurate estimation of the dose rate. The most
common, and reliable, method is using the U-series data collected
for each dental tissue in combination with the ESR dose evaluation
(i.e., the so-called combined U-series/ESR dating approach; see
Grün et al., 1988 and Grün, 2009). Further detail is found in a recent
review by Duval (2015) and Table 4.
4.2. ESR dating of sedimentary quartz grains: the choice of signal to
measure
Similar to OSL, ESR dating of sedimentary quartz is based on the
study of light-sensitive signals whose intensity is reset (bleached)
under sunlight exposure during sediment transportation. Once the
sediment is buried, and thus sheltered from sunlight, paramagnetic
centres are created and the ESR signal intensity increases as a result
of the interaction of natural radioactivity with the quartz sample.
Quartz has several paramagnetic centres associated with crystal
defects (for a detailed review, see Ikeya,1993; Preusser et al., 2009),
but the most widely used since the first dating application by
Yokoyama et al. (1985) are undoubtedly the Titanium (Ti) and the
Aluminum (Al) centres. Because Al is the major trace element found
in quartz (Preusser et al., 2009), the ESR signal associated with the
Al centre can be observed in any sample. It also usually presents
high intensities (Fig. 6a) and signal-to-noise ratio values, ensuring
high precision measurements (Duval, 2012). However, the Al signal
shows relatively slow bleaching kinetics (the signal requires several
hundred hours of UV laboratory irradiation to reach a minimum
value, see Fig. 6b), and it cannot be fully reset under sunlight
exposure as there is a residual ESR intensity that cannot be
bleached (Fig. 6b; Toyoda et al., 2000). This residual level should be
assessed (usually via bleaching experiments using sunlight simulators)
in order to avoid dose overestimations. In contrast, the Ti
centres (Ti-Li and Ti-H mostly in quartz samples) show much faster
bleaching kinetics and no residual (i.e. unbleachable) ESR intensity.
However, measurements are significantly longer and less precise
than those of the Al centre given the very low ESR intensities that
are usually measured (Fig. 6a, Duval and Guilarte, 2015). Further
detail about ESR dating of optically bleached quartz grains may be
found in the recent reviews by Toyoda (2015) and Tissoux (2015),
while basic information is also given in Tables 4 and 5.
4.3. Fluvial environment and ESR dating: main specificities
Depending on the material dated, there may be different impacts
from the fluvial environment on the ESR dating results. Unlike
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in quartz, the ESR signal measured in tooth enamel is not light
sensitive and thus cannot be reset during transportation. However,
transport and depositional conditions can indirectly impact the ESR
results, in particular regarding the preservation state of the sample,
as they may fragment and weaken dental tissues, thus favouring
post-depositional processes and in particular U-uptake or leaching.
Additionally, a review by Grün (2009) showed that the U-uptake
kinetics into dental tissue is significantly different depending on
the sedimentary environment: teeth found in cave sites most
frequently document earlier U-uptake compared with those found
in open air sites, which also show more frequent occurrences of Uleaching.
This is most likely due to differences in the sedimentological
context. Cave sites, as closed environments, usually offer
more stable geochemical conditions over time. In contrast, open air
Table 4
A brief overview of the ESR dating method applied to fossil teeth and optically bleached quartz grains extracted from sediment. “Modified from Duval (2016).” Further details
regarding the dating time range of each application may be found in Duval (2016) and references therein.
Fossil tooth enamel Optically bleached quartz grains extracted from sediment
Dated event Burial of the fossil tooth (usually assumed to happen
shortly after the death of the animal)
Last exposure of the sediment to sunlight
Main specificity of the application Dental tissues are open-systems for U: U-uptake needs
to be modelled (combined U-series/ESR dating approach).
Light-sensitive ESR signals (same basic principles as OSL dating).
Presence of a residual (non-bleachable) ESR intensity for the Al centre.
Upper dating range Present-day ~10 ka
Lower dating range Early Pleistocene Miocene (Al-centre)
Optimum dating range 40e800 ka 200 kae2 Ma
Main strength of the application Direct dating of hominin and animal fossil remains
beyond the C-14 and U-series dating time range.
May date beyond the OSL dating time range.
Standard precision Standard errors are usually ~10% (5e15%). Standard errors are usually ~10% (5e15%).
Fig. 6. a) Examples of ESR spectra of Al and Ti centres measured in quartz. Modified from Duval and Guilarte (2015); b) Decay of the ESR intensity of the different centres Al, Ti-Li
and Ti-H under UV exposure. This laboratory bleaching experiment was performed with a SOL2 sunlight simulator (Dr H€onle) on a quartz sample from the Moree-Villeprovert
locality, France.
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sites are frequently found as the result of erosion processes that
may induce modifications of the hydrological environment and
cause recent mobilisation of radioelements impacting the original
isotopic signature of the teeth.
Fluvial transport has a direct impact on the ESR signals
measured in quartz as it is known to induce resetting by either
exposure to natural sunlight (Toyoda et al., 2000) or mechanical
effects (Liu and Grün, 2011). Similarly to OSL, the degree of
bleaching of the ESR signals depends on the length and conditions
of transport (see Section 3.2.). In a recent study, Voinchet et al.
(2015) studied the impact of a series of parameters such as the
grain size, transport mode and water turbidity to evaluate the most
suitable conditions for optimum bleaching. Based on their results,
higher bleaching levels are apparently achieved for 100e200 mm
grains in comparison with other fractions and for fluvial transport
under clear water conditions (see overview in Fig. 2).
4.4. Sampling precautions
When dating sedimentary quartz, sampling precautions are
very similar to those for Luminescence dating (see 3.3), i.e. the
choice of a suitable sedimentary setting and suitably thick beds for
simplicity of dose-rate estimation (Fig. 4). Although ESR signals
bleach much slower than OSL ones, it is nevertheless important to
minimise the exposure of the raw sediment to the sunlight during
sampling, as in luminescence dating. According to the results
shown by Voinchet et al. (2015), sediment showing a nonnegligible
fraction of medium sands (mostly 100e200 mm) and
transported and deposited in a clear-water fluvial environment
should be targeted for sampling, as they potentially offer the most
suitable bleaching conditions. In addition, in situ measurements of
natural radioactivity should be undertaken (especially if the immediate
surrounding sedimentary environment is not homogeneous,
see Fig. 4b), in order to obtain an accurate estimation of the
gamma dose rate. Additional small bags of sediment are also usually
collected at the ESR sampling spot for future laboratory analysis,
e.g. for water content evaluation and analysis of radioelement
concentration. When dating teeth, samples have usually already
been collected during the archaeo-palaeontological excavation and
are thus chosen from collections. It is important to make sure that
exact original (geographical and stratigraphical) location of the
selected tooth is well-known and the corresponding layer/outcrop/
site is still accessible to enable complementary fieldwork sampling
and dose rate measurements. Ideally, it is recommended to ask the
archaeologists and/or palaeontologists to collect the sediment
attached to the tooth during the excavation. This is essential for a
correct evaluation of the beta, and sometimes gamma, dose rate
component(s). The apparent preservation state of the tooth matters
as well, as previous studies have shown a strong correlation between
macroscopic cracks in dental tissues and preferential
migration of U-series elements (Duval et al., 2011).
4.5. Current challenges in ESR dating
4.5.1. ESR dating of fossil tooth enamel: improving resolution and
removing unstable components of the ESR signal
In comparison with quartz, ESR dose reconstruction of fossil
tooth enamel is more straightforward. The composition of the ESR
signal and its dose response has been extensively studied in recent
decades. The modern development of ESR analyses of enamel
fragments now enables the differentiation of the relative contribution
of non-oriented CO2
À
radicals (NOCORs) vs. the oriented ones
(CORs) (e.g. Grün et al., 2008). Additionally, Joannes-Boyau and
Grün (2011) showed that laboratory gamma irradiation produces
additional unstable NOCORs in comparison with natural
irradiation, which may lead to dose underestimation (~30%) if this
contribution is not removed. The authors acknowledge, however,
that this value should not be considered as universal and extrapolated
to any samples, as it may depend on many parameters (e.g.,
age, type, species). In contrast, more recent investigations indicate
that this preferential creation of an unstable component may not be
systematic, being rather sample dependant (Duval and Grün, unpublished
data). Consequently, from these results it seems that
each sample should be independently assessed. However, as an
additional complication, it should be mentioned here that most of
the dating studies are performed on enamel powder. One of the
major current challenges would thus be to develop an analytical
procedure that enables an easy identification of these unstable
NOCORs using enamel powder. In that regard, using the microwave
saturation characteristics of the different groups of CO2
À
(Scherbina
and Brik, 2000) may be an avenue worth exploring in the future.
High resolution LA-ICP-MS U-series analyses has recently
demonstrated the spatial heterogeneity of the distribution of Useries
elements in dental tissues (Duval et al., 2011). This analytical
tool has rapidly become essential for studying U-mobility and may
be particularly useful to identify domains in the teeth that are
suitable for ESR dating. However, the use of this technique raises
new issues. There is a difference in resolution when comparing ESR
and the ICP-MS methods. Currently, in situ laser ablation ICP-MS Useries
analysis can be performed with a resolution of a few tens of
mm. In contrast, the spatial variation of the ESR signal intensity in
tooth enamel has rarely been studied, and ESR bulk analyses are
usually performed on several hundreds of mg of enamel powder.
This difference in resolution may become a non-negligible source of
uncertainty in ESR dating, especially for old samples for which the
dose rate associated with dental tissues is the major factor in the
total dose rate calculation. Future challenges will thus consist of
developing new approaches to reduce the amount of sample
required for ESR analyses and obtain spatially resolved data. This
may now be seriously envisaged through the use of high sensitivity
X-band resonators and with the development of a specific analytical
procedure for quantitative measurements in Q-band spectroscopy
based on only a few mg of enamel (Guilarte et al., 2016).
Additionally, although it is for the moment extremely complicated
to integrate spatially-resolved ESR and U-series data for age calculations,
the recent development of DosiVox (software for
dosimetry simulations) opens new possibilities for modelling dose
rates from complex geometries and heterogeneous spatial distributions
of radioelements (Martin et al., 2015).
4.5.2. Avoiding and minimizing the effect of scatter and incomplete
bleaching in ESR dating of sedimentary quartz
One of the main difficulties in ESR dating of quartz is to achieve
repeatable measurements ensuring reproducible DE results. This
reproducibility is lower than that obtained with tooth enamel, not
only because measurements close to liquid N2 temperature require
a very stable experimental setup, but also because of the heterogeneity
of the quartz samples and the strong angular dependence
of the signal within the cavity. Extensive work has been performed
recently to optimise the conditions of measurements for both the Al
and Ti centres (e.g. Duval and Guilarte Moreno, 2012; Duval and
Guilarte, 2015; Duval, 2012).
In parallel to this work, another major challenge is in reducing
the uncertainty on the final DE value. As noted by Toyoda (2015),
some approaches developed in OSL dating are definitely worth
exploring in ESR dating. Perhaps the most obvious is the use of the
regenerative dose protocol instead of the additive dose protocol
that is routinely used in ESR. This protocol would not only provide
more precise DE results but also significantly shorten the analytical
time (i.e. fewer aliquots to be measured and lower irradiation dose
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values). However, several previous attempts employing optical
bleaching resetting have shown somewhat contrasting results
regarding the presence of sensitivity changes (Tissoux et al., 2007;
Beerten and Stesmans, 2006). Other approaches are less obviously
fruitful. For example, although single grain dating using Q-band
ESR spectroscopy has been tested to identify partial bleaching
among a grain population, it is currently too complicated to be
applied routinely (Beerten and Stesmans, 2006).
The main challenge, however, is in minimising the uncertainty
regarding possible incomplete bleaching of the signal during
sediment transportation. Most dating studies use the Al centre
even though laboratory bleaching experiments indicate that
several hundreds of hours of exposure to UV are required for the
ESR signal to decay to a plateau (e.g. Toyoda et al., 2000; see also
Fig. 6b). These values would correspond to several tens of days of
sunlight, which understandably leads many authors to question the
possibility of the Al centre actually reaching its residual ESR intensity
during transportation. However, Voinchet et al. (2007)
demonstrated that the signal was fully reset (to its residual level)
after only 1 km of transportation in the Creuse river, France.
Additionally, at the Vallparadís site (Spain), Al-ESR ages were found
to be in good agreement with the combined US-ESR ages on fossil
teeth and data from magneto- and bio-stratigraphy (Duval et al.,
2015a). These two examples demonstrate that any definitive
conclusion derived from laboratory bleaching experiments should
be considered with caution. It is possible that other processes, not
yet understood, are involved in the bleaching of the signal in natural
conditions.
As a consequence of the uncertainty that may arise from the
bleaching of the Al centre, ESR age results based on this centre
should be considered as maximum possible ages: the true age of
the deposits being either similar or younger. To constrain this uncertainty,
a few strategies are available. The use of modern
analogue samples collected from nearby river banks may provide
some useful information regarding resetting of the signal. This
approach is, however, based on the assumption that transportation
and bleaching conditions are similar to those in the past, which is
not always plausible. Another approach is to use independent age
control to verify the age results (see the example of Vallparadís,
Duval et al., 2015a). However, the best option is undoubtedly the
Multiple Centres (MC) approach proposed by Toyoda et al. (2000).
The authors proposed the systematic measurement of both the TiLi
and Al centres in quartz samples in order to check whether they
would provide consistent results (Table 5). If the Al centre yields an
age estimate older than that of the Ti centre, this is interpreted as
incomplete bleaching of the Al signal. In this case, the Ti-Li age
should be considered a closer estimate to the burial age of the
deposits. Although ESR measurements following the MC approach
are highly time consuming, it has provided promising results (see
Rink et al., 2007; Duval et al., 2015b). The use of this MC may soon
become a standard requirement in ESR dating of optically bleached
quartz grains.
Lastly, another Ti centre, Ti-H, presents great potential worth
investigating for dating purposes. It is known to bleach much faster
and to be more radiosensitive than the Ti-Li (see Fig. 6b; Duval and
Guilarte, 2015), which would make it a good candidate for dating
deposits younger than 200 ka. It is, however, unclear for the
moment whether it provides reliable dose estimations. Indeed, the
weakness of the signal intensity makes it very complicated to
measure in all samples, resulting in low measurement precision
(Table 5; Duval and Guilarte, 2015).
5. 230
Th/U-dating of fluvial deposits
The 230
Th/U-dating method is based on the radioactive decay in
the natural decay chain of 238
U and was developed in the 1960s
(Broecker, 1963; Kaufman and Broecker, 1965). Since then, the
precision and accuracy of the method has progressively increased,
primarily due to major technical advances. Whereas alpha spectrometry
was widely used until the 1990s (Goldstein and Stirling,
2003), the use of thermal ionisation mass spectrometry (TIMS)
(Edwards et al.,1987) represented a major advance at the end of the
1980s. This reduced the time required for an analysis from a week
to several hours, decreased sample size from 10e100 g to 0.1e1 g
and, most importantly, improved precision from percent to permil
levels and extending the dating range from 350 to 600 ka
(Goldstein and Stirling, 2003). In the last two decades, the application
of multi-collector inductively coupled plasma mass spectrometry
(MC-ICPMS) has led to further substantial improvements
(Goldstein and Stirling, 2003; Scholz and Hoffmann, 2008). The
considerably higher ionisation and transfer efficiency for U and Th
isotopes of the MC-ICPMS technique leads to higher count rates, in
turn resulting in more precise and accurate 230
Th/U-ages.
Furthermore, measurement times (~10e20 min) and sample sizes
are again substantially lower than for TIMS. In addition to the
technical advances, the half-lives of both 230
Th and 234
U have been
re-determined (Cheng et al., 2000, 2013), also leading to more
precise 230
Th/U-ages. During the last decade, procedures for laserablation
(LA) MC-ICPMS 230
Th/U-dating of carbonates have been
developed (e.g. Eggins et al., 2005; Mertz-Kraus et al., 2010). This
technique has very large potential since it offers in situ dating at
extremely high spatial resolution (in the range of 10e100 mm),
requires no sample preparation and is extremely fast and, thus,
enables very high sample throughput. In return, the analytical
precision is much lower than for conventional 230
Th/U-ages (a few
percent compared to epsilon levels).
In undisturbed natural materials with an age of several million
years, the activity of the parent (i.e. 238
U) and the daughter isotopes
(i.e. 234
U and 230
Th, respectively) is in secular equilibrium. This state
of equilibrium, however, can be disturbed by several natural processes,
which is the basic principle of all U-series disequilibrium
dating methods. In aqueous environments, the major reason for
disequilibrium between U and Th is the different geochemical
behaviour of the two elements. Whereas U is soluble, Th is
Table 5
Summary of the main features usually observed for the three paramagnetic centres Al, Ti-Li and Ti-H. Relative characterisation is provided: (þþþ) ¼ high, (þþ) ¼ medium,
(þ) ¼ low. Further details and additional references may be found in the text.
Al centre Ti-Li centre Ti-H centre
Signal-to-noise ratio (S/N) þþþ þþ þ
Precision of the measurements þþþ þþ þ
Dose response curve No apparent saturation at
high doses (>60 kGy)
Non-monotonic behaviour
(maximum intensity ~6e10 kGy)
Non-monotonic behaviour (maximum
intensity ~3e8 kGy)
Bleaching kinetics (speed) þ þþ þþþ
Residual ESR intensity (unbleachable
component)
Yes No No
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insoluble in natural waters and, thus, mainly transported adsorbed
onto particles. As a consequence, groundwater, rivers, lakes and
seawater contain significant amounts of dissolved U, but essentially
no Th. During formation of secondary carbonates, U is thus incorporated,
whereas Th is not. Consequently, secular equilibrium is
disturbed, and the initial activity of 230
Th is zero. If the decay system
remains closed after deposition (i.e. no U and Th isotopes are
lost or added subsequently), the activity ratios of (234
U/238
U) and
(230
Th/238
U) return to the state of secular equilibrium (e.g. Bourdon
et al., 2003, activity ratios are indicated in parentheses in the
following). The temporal evolution of the activity ratios (in
particular the increase of 230
Th due to the decay of 234
U and 238
U)
allows dating of the time of carbonate formation (i.e. the timing of
the establishment of disequilibrium) and, thus, the age of the carbonate
phase. This is, however, only possible if two basic requirements
are fulfilled: (i) no presence of initial 230
Th and (ii) the
system remained closed after deposition. If one of these assumptions
is violated, the resulting 230
Th/U-age may be substantially
inaccurate.
230
Th/U-dating can, in principle, be applied to all materials
whose formation is accompanied by a constrained disequilibrium
between U and Th. The materials most widely dated by the 230
Th/Umethod
are fossil reef corals and speleothems (Scholz and
Hoffmann, 2008; Edwards et al., 2003), which can, in general, be
accurately and precisely dated up to an age of 600 ka. However,
with increasing sensitivity of both LA and MC-ICPMS systems,
increasing precision may be achieved enabling high-precision in
situ 230
Th/U-dating (i.e., without prior sample preparation) at very
high spatial resolution. This may be particularly useful for impure
carbonates found in fluvial deposits in order to analyse the most
pristine fractions of a dirty sample. Examples of successful dating of
inclusions in fluvial deposits by the 230
Th/U-method (Fig. 2) include
pedogenic carbonates and calcretes deposited in alluvial fans and
river terraces (e.g. Candy et al., 2004; Kelly et al., 2000; Ludwig and
Paces, 2002; Sharp et al., 2003) as well as tufa and travertine
(Schulte et al., 2008; Candy and Schreve, 2007). All these deposits
have in common that they form subsequently to the deposition of
fluvial sediments, such as fans and terraces (Fig. 2). Thus, they can
only provide a minimum age for the fluvial deposits with which
they are associated (Blisniuk et al., 2012). Carbonates that have
been mobilised subsequent to deposition (e.g. flood events or
washed in from slopes, Fig. 2) are not expected to provide reliable
230
Th/U-ages because they are (i) most likely affected by postdepositional
diagenesis and (ii) difficult to relate to a depositional
context (Fig. 2). Extensive reviews of the 230
Th/U-dating methodology
can be found in the classic books by Ivanovich and Harmon
(1992) and Bourdon et al. (2003).
5.1. 230
Th/U-dating of secondary carbonates in fluvial archives:
main issues
In general, carbonates deposited in fluvial and lacustrine environments
are difficult to date by the 230
Th/U-method. In many
cases, samples of fluvial and lacustrine deposits contain very large
amounts of detrital Th, which represents a violation of one of the
basic requirements of the dating method. Since Th is mainly
transported adsorbed onto particles, it is generally associated with
relatively fast flowing water, which has the potential to transport
these particles. In particular, carbonates associated with alluvial
fans thus often contain substantial amounts of detrital Th (Fig. 2).
However, pedogenic carbonates may also contain high amounts of
detrital Th, which is mobilised from the overlying horizons (Fig. 2).
These materials are thus often referred to as impure carbonates or
dirty calcites (e.g., Kaufman, 1993). Initial Th is often associated
with a silicate or clay fraction. Whereas the preparation of pure
carbonate samples is relatively straightforward (e.g. Yang et al.,
2015), the preparation of impure carbonates may be more elaborate
due to the presence of an insoluble residue. Various approaches
to deal with insoluble residues have been proposed (see
5.2.2). Initial (also often referred to as detrital) 230
Th is generally
accompanied by 232
Th, which is the most abundant naturally
occurring isotope of Th. 232
Th does not occur in the decay chain of
238
U and is, in contrast to 230
Th, not produced by the decay of 234
U
and 238
U. Elevated content of 232
Th is clear evidence for the presence
of initial 230
Th, and its concentration even provides a measure
for the degree of contamination. For (230
Th/232
Th) activity ratios
<20, a correction for detrital contamination is definitely required
(Schwarcz, 1989). Other studies have suggested even higher
thresholds for (230
Th/232
Th) necessitating a correction for detrital
contamination (Richards and Dorale, 2003). Potential correction
techniques that have been shown to be successful for fluvial deposits
are discussed in Sections 5.2.1 and 5.2.2. In addition, postdepositional
open-system behaviour is not uncommon for secondary
carbonates deposited in fluvial environments, which is even
more complicated to detect and account for (see 5.2.3.). For marine
samples, such as corals, open system behaviour can be detected by
comparing the initial (234
U/238
U) activity ratio of the sample with
the (234
U/238
U) activity ratio of modern seawater (e.g. Edwards
et al., 2003). In terrestrial environments, this is not possible due
to the highly variable (234
U/238
U) activity ratio in river, lake and
groundwater. Thus, successful 230
Th/U-dating of fluvial carbonates
has been restricted to a relatively small number of case studies,
which are characterised by the high U content (238
U > 1 mg/g) of the
dated material.
5.2. Approaches developed to date secondary carbonates by 230
Th/U
Two general correction methods to account for detrital Th have
been developed: a priori estimation of the (230
Th/232
Th) activity
ratio of the detrital phase and isochron techniques. In rare cases,
secondary carbonates associated with fluvial deposits, such as tufa
and travertine, may be very clean, and a correction for initial 230
Th
may not be necessary. For instance, Schulte et al. (2008) established
a chronology for the fluvial terrace sequence from the River Aguas
basin, Iberian Peninsula, by 230
Th/U-dating of travertine. The
(230
Th/232
Th) activity of some of their samples is larger than 20, and
a correction for initial 230
Th is not required. Candy and Schreve
(2007) obtained 230
Th/U-ages on fluvial and colluvial tufa deposits
from southern England with sufficient precision to correlate
discrete periods of temperate climate with individual warm substages
during MIS 7. Although the U content of their samples is
relatively low (ca. 0.1 mg/g), the (230
Th/232
Th) activity of the majority
of samples is>20.
5.2.1. A priori estimation of the (230
Th/232
Th) activity ratio of the
detrital phase
The average 232
Th/238
U weight ratio of the upper continental
crust is ~3.8 (Wedepohl, 1995). Assuming secular equilibrium between
230
Th, 234
U and 238
U for the detrital component, the
(230
Th/232
Th) activity ratio of the initial Th is ~0.9 (Hellstrom, 2006).
Based on the measured content of 232
Th, the amount of initial
(detrital) 230
Th can thus be estimated and subtracted from the
measured concentration of 230
Th. This approach is often referred to
as a priori estimation of the detrital phase and may provide
reasonable ages. However, the initial (230
Th/232
Th) activity ratio is
highly variable and associated with large uncertainties. Usually, an
uncertainty of 50% is assumed (Hellstrom, 2006). Propagation of
this substantial uncertainty to the corrected 230
Th/U-age may lead
to highly elevated age uncertainties and even ages with zero significance
(Kaufman, 1993; Wenz et al., 2016). Despite these large
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uncertainties, a priori estimation of the (230
Th/232
Th) activity ratio
of the detrital phase has been successfully applied to date fluvial
deposits by the 230
Th/U-method. For instance, Adamson et al.
(2014) obtained a large number of ages for fluvial deposits in
Montenegro by 230
Th/U-dating of carbonate benches and calcite
rinds. This study is particularly remarkable because the U content of
the studied samples was relatively low (<1 mg/g). However, many
samples also have very low 232
Th, resulting in (230
Th/232
Th) activity
ratios >20. Ludwig and Paces (2002) determined 230
Th/U-ages on
pedogenic silica-carbonate clast rinds and matrix laminae from
alluvium in Crater Flat, Nevada, employing the TSD-technique,
whereas Sharp et al. (2003) dated pedogenic carbonate clastrinds
from gravels of glacio-fluvial terraces in the Wind River Basin,
Wyoming. The success of both studies is mainly based on the
high U content of the samples. Blisniuk and Sharp (2003) determined
the age of two well-preserved fluvial terrace surfaces in
central Tibet by 230
Th/U-dating of pedogenic carbonate rinds on
clasts in the terrace deposits.
5.2.2. Isochron methods
The second approach to account for initial or detrital 230
Th is the
isochron methodology. For isochron 230
Th/U-dating of impure
carbonates, various procedures for sample preparation have been
proposed (e.g. total sample dissolution (TSD), leachate-leachate (L/
L), leachate-residue (L/R), Bischoff and Fitzpatrick, 1991; Kaufman,
1993; Ku and Liang, 1984; Luo and Ku, 1991; Schwarcz and
Latham, 1989). In addition, several statistical methods for the
evaluation of the isochron data have been developed (Ludwig,
2003). In general, the isochron method is more flexible than the a
priori approach and provides more reliable ages with smaller uncertainties
(Wenz et al., 2016). However, the application of the
isochron methodology is based on two assumptions: all subsamples
(i) must have the same age and (ii) should contain
different amounts of the same detrital component (i.e. with the
same (234
U/238
U) and (230
Th/238
U) ratios). Unfortunately, the latter
assumption in particular is not fulfilled for many impure carbonate
samples (Ludwig, 2003; Wenz et al., 2016), again leading to large
age uncertainties and corrected ages with low significance.
Isochron techniques have also been successfully applied for 230
Th/
U-dating of fluvial deposits. For instance, a stratigraphically
consistent chronology based on isochron 230
Th/U-ages determined
on pedogenic calcretes has been reported for alluvial terrace sequences
from the Sorbas Basin, south-eastern Spain (Candy et al.,
2004, 2005; Kelly et al., 2000). However, Candy et al. (2005) have
shown that dating of mature calcretes is much more difficult than
dating of immature calcretes, as has been revealed by the isochron
statistics. Nevertheless, it may also be possible to determine a
reliable age for mature calcretes if a large number of sub-samples
from a single horizon are dated. Other studies aiming to date
fluvial deposits were not successful in accounting for initial 230
Th
by isochron techniques. For instance, Kock et al. (2009) attempted
230
Th/U-dating of pedogenic carbonate crusts from fluvial gravels of
the River Rhine, and compared them with internally coherent OSL
ages. Most of their U-series data scattered widely on isochron diagrams
suggesting multiple components of initial 230
Th that are not
related to detrital 232
Th. A significant fraction of the initial 230
Th
may originate from bacterial activity and Th transport on organic
colloids. This suggests that samples in which bacteria could have
contributed to carbonate precipitation should be avoided.
5.2.3. Accounting for open-system behaviour
One option for detecting open-system behaviour of 230
Th/Uages
of fluvial deposits is through comparison with independent
ages (e.g. Blisniuk et al., 2012; see 7). Another option is consideration
of the stratigraphical context of the deposited samples, i.e.
whether the determined (corrected) 230
Th/U-ages are in stratigraphical
order within a sedimentary sequence. This approach is
currently used to identify ages representing outliers, probably
because the applied correction techniques were not successful or
due to post-depositional open-system behaviour. This approach
has been proved to be successful for the aragonitic lacustrine sediments
from Lake Lisan, the Last Glacial precursor of the Dead Sea,
which have been extensively studied by 230
Th/U-dating (e.g.,
Torfstein et al., 2013). These sediments contain high amounts of U
(>3 mg/g), and different approaches have been used to obtain corrected
ages, including isochrons (Schramm et al., 2000), a priori
estimates of the detrital (230
Th/232
Th) activity ratio (Schramm et al.,
2000) and an iterative approach independently evaluating the
composition of the detrital component for every set of coeval
samples (Torfstein et al., 2013). Furthermore, several authors
recently have suggested algorithms for speleothems including
stratigraphical constraints in order to estimate the (230
Th/232
Th)
activity ratio of the detrital component (Hellstrom, 2006; RoyBarman
and Pons-Branchu, 2016). These algorithms may also be
very useful for fluvial samples deposited in a clear stratigraphical
context.
6. Terrestrial cosmogenic nuclides (TCN) dating of fluvial
deposits and landforms
Terrestrial or in situ-produced cosmogenic nuclides, by contrast
to those produced in the atmosphere such as 14
C (see 2), are the
products of interactions between secondary particles created during
the cosmic ray shower and the atoms’ nuclei in minerals. The
development of the AMS technology in the early 1980s (e.g. Klein
et al., 1982), which allowed measurements of isotopic ratios as
low as 10À15
at that time (presently 10À16
has been reached), represented
a decisive milestone, enabling the use of TCN as a dating
tool, as already proposed by Davis and Schaeffer (1955). In parallel,
a tremendous amount of work has taken place and aimed at understanding
the physical properties and processes involved in the
production of the most commonly used nuclides in the Earth sciences,
i.e. 3
He, 10
Be, 21
Ne, 26
Al and 36
Cl (e.g. Nishiizumi et al., 1986).
A particular emphasis was on the determination and refinement of
their respective production rates according to the different production
pathways, mostly involving fast neutron- (spallation) and
muon-induced reactions (Gosse and Phillips, 2001). This is well
exemplified by the strongly debated determination of both the
production rate of 10
Be in quartz and the half-life of this radionuclide
(Gosse and Phillips, 2001; Dunai, 2010). Moreover, the use of
these nuclides as geochronometers required integrating the variability
of production rates in space and time, hence the build-up of
scaling factors (e.g. Dunai, 2000). Extensive reviews about terrestrial
cosmogenic nuclides, including basic principles and concepts
as well as dating approaches and general applications in the Earth
sciences, can be found in Gosse and Phillips (2001) and Dunai
(2010).
Depending on the aim of the study and/or the fluvial or lacustrine
environment where it takes place, numerical ages based on
concentration measurements of cosmogenic nuclides can be undertaken
either via surface exposure dating or burial dating (Fig. 2).
Both dating approaches are presented in this section. Note that the
material that has to be dated undergoes pre-exposure to cosmic
rays during (i) bedrock exhumation, (ii) temporary storage on
hillslopes and (iii) transport and/or temporary storage in the fluvial
system (Fig. 2). This accumulation of cosmogenic nuclides inventories
prior to the depositional event is known as inheritance
(Anderson et al., 1996). Whereas surface exposure dating of depositional
landforms is highly sensitive to this process (see 6.1), this
inherited component allows the dating of a burial event (see 6.2). In
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fluvial settings, surface exposure dating first provided numerical
ages for alluvial fans (Siame et al., 1997; Van der Woerd et al., 1998)
and river terraces, both bedrock strath terraces (Burbank et al.,
1996; Leland et al., 1998) and alluvium-mantled terraces
(Anderson et al., 1996; Repka et al., 1997). In lacustrine environments,
surface exposure dating of palaeo-shorelines provides information
about former lake-level highstands (Rades et al., 2013).
Burial dating can be applied to in cave-deposited alluvium (Granger
et al., 1997) or deeply buried fluvial or lacustrine sediments (Kong
et al., 2009).
6.1. Surface exposure dating
The calculation of exposure ages requires both high-precision
AMS measurements of nuclide concentrations and the determination
of the site-specific nuclide production rate. The latter must
integrate the use of specific scaling factors and the potential
topographic or self shielding of cosmic rays at the sampling location
(Dunai, 2010). As fluvial sediments or related landforms very often
contain quartz-bearing material, surface exposure ages are usually
determined via concentration measurements of 10
Be (Fig. 7a, Dunai,
2010), sometimes used alongside 26
Al (e.g. Repka et al., 1997;
Rixhon et al., 2011). However, alternative nuclide species are produced
in other minerals, such as 3
He in olivine and pyroxene or 36
Cl
in calcite (see Gosse and Phillips, 2001; Dunai, 2010), thereby
allowing other lithologies to be dated (e.g. Baynes et al., 2015). The
dateable range in surface exposure dating of fluvial environments
varies strongly according to the setting and the employed nuclide(s)
(Fig. 1). The lower age range very much depends on the
detection limit of the AMS, hence the production rates, but late
Holocene exposure ages of bedrock strath surfaces were obtained
(Leland et al., 1998, see 6.1.2.). On the other hand, surface exposure
dating with 10
Be, because of its long half-life (i.e. ~1.36 Ma), permits
pre-Quaternary applications under specific conditions without
saturation being reached (Dunai, 2010).
In many instances, surface exposure ages of fluvial depositional
surfaces, especially alluvial fans, were formerly based on concentration
data obtained from individual clasts or boulders lying on
these (Fig. 7a, b; e.g. Siame et al., 1997; Van der Woerd et al., 1998).
However, Schmidt et al. (2011) emphasized the need of caution
when inferring exposure ages from such TCN concentration data;
diverse geomorphological processes acting on a surface might
indeed represent a considerable source of uncertainty. These
encompass inheritance (Fig. 2), post-depositional weathering,
erosion or covering by sediments and even by snow (e.g. Anderson
et al., 1996; Rixhon et al., 2011). Whereas inheritance might lead to
an overestimate of the true exposure age, all other processes tend
to reduce the cosmogenic inventory near the dated surfaces and
thereby result in age underestimations. An unequal distribution
and/or intensity of these stochastic processes across the surface
might result in a significant spread in apparent exposure ages
(Owen et al., 2014). For this reason thorough field observations and
descriptions are an absolute prerequisite for surface exposure
sampling (see field template in Dunai, 2010).
6.1.1. Depth profile dating of depositional surfaces (alluvial fans,
alluvium-mantled terraces)
The depth profile sampling technique may overcome some of
the uncertainties related to these geomorphological processes
(Anderson et al., 1996). It allows simultaneous computation of
exposure time (i.e. the abandonment time of the landform), the
post-depositional denudation rate of the landform and inheritance
(Braucher et al., 2009; Hidy et al., 2010). This approach consists of
sampling the fluvial sediments at regular depth intervals (Fig. 7c),
taking advantage of the spallation-dominated production at or near
the surface and the muon-dominated production at greater depth
(Braucher et al., 2009). Given the physical properties of these particles,
an exponential decrease of TCN concentrations along the
depth profile is expected and can be modelled by Monte Carlo
simulations (Fig. 7d, see the user-friendly simulator of Hidy et al.,
2010). However, because this method is very sensitive to any
post-depositional reworking processes (e.g. cryo or bioturbation
…), one should avoid sites where such processes have occurred.
The depth profile technique is particularly useful for dating alluvial
fans and fill terraces (Fig. 7d, e.g. Repka et al., 1997; Le Dortz
et al., 2011; Rixhon et al., 2011). In contrast to the pioneering
studies on alluvial fans (e.g. Siame et al., 1997), almost all recent
works systematically combined surface concentration data with
depth profile data to better constrain the inheritance and the postdepositional
evolution of the landform (e.g. Le Dortz et al., 2011;
Schmidt et al., 2011; Owen et al., 2014). Where the petrographic
composition of fan - or terrace - sediments is favourable, it is
advisable to perform an internal control by comparing concentrations
of different nuclides. For instance, quartz-bearing and calcitebearing
materials enable 10
Be and 36
Cl concentration measurements,
respectively (Le Dortz et al., 2011). As lateral or vertical
offsets disrupting fan surfaces represent an excellent geomorphological
marker for crustal deformation, surface exposure ages allow
quantifying average slip rates along main fault lines for the Middle/
Late Pleistocene and/or the Holocene (e.g. Siame et al., 1997; Le
Dortz et al., 2011). Also, surface exposure dating of fan surfaces
may likewise provide valuable information about climatic forcing
on fan formation (e.g. Owen et al., 2014). Depth profile concentration
data of terrace sediments are commonly used to quantify
incision rates by sampling vertically-spaced levels within terrace
sequences (e.g. Repka et al., 1997). Alternatively, diachronic abandonment
times of geometrically-correlated terraces along a hydrological
network allow inference of long-term propagation rates
of a specific incision wave from the main trunk into its (sub-)tributaries
(Rixhon et al., 2011).
6.1.2. Surface exposure dating of strath terraces
An alternative application of the surface exposure method
consists of dating bedrock surfaces of strath terraces (Fig. 2). This
term is used here to describe laterally-carved benches in steep
valley flanks, especially in actively uplifting orogens (e.g. Himalayas),
and are often characterised by smooth polished surfaces or
sculpted erosional features (Fig. 7e, Burbank et al., 1996; Leland
et al., 1998). Inheritance usually does not represent a major issue
for strath terraces since they are erosional landforms. Provided that
the bedrock surface is still pristine, one can assume insignificant
weathering or erosion after strath abandonment. If the strath was
not covered by temporary alluvium or landslide deposits subsequent
to terrace abandonment (see Leland et al., 1998), the calculation
of the exposure time is straightforward (Fig. 7f, g). To check
the representativeness of bedrock samples and to take concentration
variability into account, we recommend the nested sampling
strategy of Reusser et al. (2006). The thin alluvial cover can also be
sampled if it is present (e.g. Reusser et al., 2006). Surface exposure
dating of strath terraces in diverse gorge settings highlighted, for
instance, (i) differential rock uplift related to major thrust activity in
active orogens (Burbank et al., 1996; Leland et al., 1998), (ii)
regional, climatically-driven incision of rivers along a passive
margin (Fig. 7f, g, Reusser et al., 2006) or (iii) the impact of extreme
flood events for canyon formation related to significant knickpoint
retreat (Baynes et al., 2015).
6.2. Burial dating
In contrast to surface exposure dating, which relies on
G. Rixhon et al. / Quaternary Science Reviews xxx (2016) 1e23 15
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continuous accumulation of TCN, burial dating is based on the
differential decay of at least two nuclides, where at least one of
them is a radionuclide e for full details, see Granger and Muzikar
(2001) and Granger (2014). The nuclide pair 26
Al/10
Be is
frequently employed because they are both produced in quartz and
their production ratio is fundamentally independent from latitude
and altitude and varies only slightly with depth (Dunai, 2010;
Granger, 2014). In the case of the pair 26
Al and 10
Be, burial dating
is based on a two step exposure/shielding episode of any quartzbearing
material. First, the latter accumulates nuclide inventories
during exhumation of bedrock and transport/storage on hillslopes
and in the drainage network (Fig. 2), i.e. the inherited component.
Fig. 7. Surface exposure dating: application of distinct sampling strategies to different fluvial archives or landforms (left), exemplified by dating results (right). a) Sandstone boulder
lying at the surface of an alluvial fan, sampled for 10
Be concentration measurement (Escondida creek, Andean Precordillera, Argentina). After Schmidt et al. (2011); b) Mean surface
exposure ages (bold numbers), calculated from 10
Be and 26
Al concentration measurements in individual clasts samples, for three fan terraces displaced by Holocene strike-slip
faulting activity (NE Tibet). The young age cluster on T1 is attributed to the occurrence of a recent flash flood (light arrow) whereas the four samples from T1, T2 and T3 with
much older apparent ages (dark arrows) are supposed to have been reworked from older deposits but may also have a higher inherited content. After Van der Woerd et al. (1998); c)
Sketch of TCN concentrations along a depth profile (bold black curve) in an alluvial sequence deposited in a single event, highlighting a concentration decrease with depth. Red
curves represent supposed frequency distribution of nuclide concentrations of individual clasts, illustrating the need to amalgamate tens of clasts. Modified after Ivy-Ochs and
Kober (2008); d) measured 10
Be concentrations with 1s error bars along a ~4.5 m-deep profile in terrace sediments (Ourthe river, Ardenne massif, Belgium) and modelled
curves based on 10 or 9 samples (bold and dashed curves, respectively). Modified after Rixhon et al. (2011); e) two distinct levels of fluvially-carved strath terraces; both bedrock
surfaces were sampled for 10
Be concentration measurements (Susquehanna river, Appalachian mountains, USA). After Reusser et al. (2006); f) Sketch summarizing the Late
Pleistocene incision in the Susquehanna river based on 10
Be concentrations of distinct strath terraces (with a minimum age for the upper, strongly eroded surface); g) Normalized
cumulative probability curves based on the sample numbers of fig. f for the three lower terrace levels. After Reusser et al. (2006). (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)
G. Rixhon et al. / Quaternary Science Reviews xxx (2016) 1e2316
Please cite this article in press as: Rixhon, G., et al., Revealing the pace of river landscape evolution during the Quaternary: recent developments
in numerical dating methods, Quaternary Science Reviews (2016), http://dx.doi.org/10.1016/j.quascirev.2016.08.016
Whilst the amount of both nuclides in any given clast or grain is
impossible to predict due to stochastic individual exposure history,
10
Be and 26
Al concentrations are related as they are produced in the
same material over the same time period, resulting in a26
Al/10
Be
surface concentration ratio of ~6.75:1 (Dunai, 2010; Granger, 2014).
Second, the quartz-bearing material is rapidly buried (see 6.2.1.),
implying a cessation of production (Fig. 2). Exploiting the differential
radioactive decays of both nuclides, the preburial ratio decreases
with increasing burial duration according to the
corresponding half-lives of each nuclide (Dunai, 2010). The time
range of application of burial dating extends into the Pliocene (~up
to 5 Ma; Fig. 1) but the current analytical precision of 26
Al measurements
in AMS implies uncertainties of (at least) ~60e100 ka
(Dunai, 2010; Granger, 2014).
6.2.1. Complete and fast burial: dating of in cave-deposited
alluvium
Fast and complete burial of sediments requires two basic assumptions
(Granger and Muzikar, 2001). First, the time span over
which incomplete shielding occurs is much shorter than the subsequent
burial duration. Second, shielded sediments are buried
sufficiently deeply, i.e. in practice !30 m, implying an insignificant
production through muons at depth. Given that these prerequisites
are frequently met for in-cave deposited alluvium (Fig. 2), the
sampling of these sediments is one of the most straightforward
applications of burial dating (Dunai, 2010). River sediments washed
into abandoned phreatic tubes in limestone valley walls characterize
the last time the passage was at the local water table (Fig. 8a,
Anthony and Granger, 2007). Alluvium-filled multi-level cave systems
thus mimic alluvium-mantled terrace staircases and, as such,
also record the regional incision history of river systems (Fig. 8b,
Anthony and Granger, 2007). The selection of suitable sampling
sites should ensure that abandoned and alluvium-filled phreatic
tubes were not contaminated by any reworked material from an
older (or younger) depositional episode (Dunai, 2010). The solution
of the complete and fast burial dating equations is graphically
expressed on the so-called erosion-burial diagram, where the
26
Al/10
Be ratio is plotted against 10
Be concentrations (Fig. 8c). This
approach has provided valuable new insights into long-term incision
rates in diverse tectonically-active (Fig. 8b, e.g. Stock et al.,
2004) and moderately-uplifted (e.g. Granger et al., 1997; Anthony
and Granger, 2007) settings, or in river catchments marked by
enhanced glacial deepening (H€auselmann et al., 2007).
6.2.2. Overcoming incomplete shielding: isochron burial dating of
fill terraces
The !30 m overburden thickness as a prerequisite for a complete
shielding is unfortunately not often met in cases of river
terraces, even in thick fill terraces (Fig. 8d). In these instances,
incomplete shielding of the fluvial sediment to the cosmic rays may
imply significant postburial production through deeplypenetrating
muons (Granger and Muzikar, 2001). As postburial
production is very difficult to constrain, it may become a considerable
issue to produce reliable burial ages. This problem was
overcome by the isochron burial dating method (Balco and Rovey,
2008), which involves the sampling of several pebbles from the
same stratigraphical layer at the base of the river terrace (Erlanger
et al., 2012; Darling et al., 2012). It relies on the fact that these clasts
are likely to have originated from different source areas in the
catchment (Erlanger et al., 2012). As the latter is subject to variable
production and/or surface erosion rates, and clasts have variable
transport and/or storage time within the fluvial system, they have
Fig. 8. Burial dating: application of two sampling strategies to different fluvial archives (left), exemplified by dating results (right). a) Horizontal, abandoned phreatic tube, partly
filled with river sediments (Cumberland river catchment, Appalachian mountains, USA). Note the regular elliptic cross-section of the former phreatic passage. After Anthony and
Granger (2007), photo: D. Granger; b) Topographic cross-section across the South Fork river canyon (Sierra Nevada, USA) displaying the multi-level cave system in which burial
dating was performed. Note the significant decrease of incision rates toward present. After Stock et al. (2004); c) erosion-burial diagram, the bold line represents the 26
Al/10
Be ratio
in steadily eroding rocks whereas the dashed curves refer to equal burial duration (dotted lines refer to pre-burial erosion rates). All samples plot beneath the bold line and have
therefore experienced burial (New river, Appalachian mountains, USA). After Granger et al. (1997); d) ~10 m-thick, gravel terrace body overlain by a tephra layer, sampled at the base
for isochron burial dating because of insufficient shielding to cosmic rays (Gunnison river, Colorado plateau, USA). After Darling et al. (2012), photo: L. Crossey; e) Graphical
representation of burial dating isochron for a gravel terrace, the burial age is calculated from the slope of the regression line (Sundays river, South Africa). After Erlanger et al. (2012).
G. Rixhon et al. / Quaternary Science Reviews xxx (2016) 1e23 17
Please cite this article in press as: Rixhon, G., et al., Revealing the pace of river landscape evolution during the Quaternary: recent developments
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distinct pre-burial histories resulting in different 10
Be and 26
Al
inherited concentrations. Sampling the same stratigraphical layer
implies an identical postburial production for each of them; this
parameter can thus be treated as a constant among samples
(Erlanger et al., 2012). On the graphical representation of isochron
burial dating (26
Al concentration plotted against 10
Be concentration),
the burial age is calculated from the slope of the regression
line (Fig. 8e). Used on a well-preserved terrace flight in South Africa
(Sundays River), this approach yielded valuable terrace ages for
inferring Late Cenozoic incision rates (Erlanger et al., 2012).
6.3. Future potential of TCN dating
In addition to the commonly employed TCN (3
He, 10
Be, 21
Ne, 26
Al
and 36
Cl), the use of further radionuclides may extend the application
time span of TCN. On the one hand, the long-lived 53
Mn
nuclide, given its half-life of 3.7 ± 0.4 Ma, has the potential to unravel
exposure histories older than 10 Ma in iron-bearing materials,
although it requires AMS technologies with higher energies than
those presently attained in order to reduce the analytical uncertainty
(Schaefer et al., 2006). On the other hand, in situ-produced
14
C, with its short half-life, is able to reveal short-term sediment
storage time within large floodplains (Hippe et al., 2012). These
values can be compared with long-term estimates of sediment
production when they are used in combination with 10
Be and 26
Al
(Hippe et al., 2012). Also, coupling 21
Ne concentrations with the
nuclide pair 10
Be and 26
Al, all measured in the same quartz-bearing
material, improves both the accuracy and the time range of
26
Al/10
Be burial dating (Balco and Shuster, 2009).
7. Application of multiple numerical dating methods to
single fluvial sequences
In this contribution, we have focused on the major recent developments
of five numerical dating techniques and showed how
these have enabled new dating applications in diverse fluvial settings
at different time scales. However, two main recommendations
must be borne in mind. First, there is no ideal numerical dating
method that can provide accurate age results on any kind of sample
and in any context. The use of a dating method, even the most
established one such as 14
C, is limited by a range of intrinsic constraints
and based on some implicit assumptions. Because the latter
are rarely openly stated, expectations regarding numerical dating
methods from non-geochronologists are sometimes unreasonable.
Setting more realistic expectations from non-specialists is a key aim
of this paper. Second, each method presented here, when applied to
fluvial archives or landforms, may encounter specific methodological
issues. This, in turn, may bias the “true” age of the event that
has to be dated: for instance, age overestimation of a fluvial
depositional event may be caused by the reworking of organic
material (14
C), incomplete bleaching of the quartz dosimeter (OSL
and ESR), inaccurate estimation of the initial (230
Th/232
Th) activity
ratio (230
Th/U) or inherited nuclide concentrations (TCN).
To overcome some of these limitations, we therefore strongly
recommend applying three different approaches. Each of these is
exemplified by case studies, including a discussion how the combination
of these dating methods may strengthen the chronological
framework of fluvial archives. First, provided that the petrographic
composition of the fluvial sediments is favourable, some of these
dating methods may allow an internal cross-check. For instance,
surface exposure ages are strengthened when 10
Be concentrations
are measured alongside 36
Cl concentrations from quartz-bearing
and calcite-bearing material, respectively (e.g. Le Dortz et al.,
2011). The same holds for luminescence dating: Colarossi et al.
(2015) comparatively analysed OSL (quartz) and post-IR IRSL
(feldspar) signals from identical samples collected in Quaternary
river sediments (South Africa) to test whether the second dosimeter
can reliably date partially bleached sediments. Notwithstanding
the statement that the post-IR IRSL225 signal was the most
adequate because of the fastest bleaching kinetics, age convergence
and divergence were both observed for younger (<20 ka) and older
(>50 ka) samples, respectively (Colarossi et al., 2015). Further
research is however required to understand the cause(s) of this
discrepancy for older fluvial material.
Second, as stated by Brauer et al. (2014), it is of major importance
to produce independent chronologies obtained from different
dating methods, provided that the nature and the characteristics of
the fluvial deposits allows it (e.g. Table 1). A common combination
involves radiocarbon and OSL dating to yield robust chronologies
for Late Pleistocene/Holocene fluvial sequences (e.g. de Moor et al.,
2008). Moreover, age discrepancies between these two dating
methods may give further insights into methodological issues. For
instance, based on directly comparable paired OSL and 14
C ages of
Late Pleistocene terrace deposits from Eastern England, Briant and
Bateman (2009) showed that ages inferred from both methods are
either consistent (<29e35 ka) or divergent (>29e35 ka) (Fig. 9a).
The systematic age underestimation of 14
C dating beyond this limit
is attributed to secondary contamination of older organic material
by low levels of modern carbon (Fig. 3); it was thus suggested that
conventionally pre-treated 14
C ages !29e35 ka should be treated
with great caution (Briant and Bateman, 2009). Likewise, some of
the case studies mentioned in this contribution take advantage of
rarely used combinations between OSL, ESR, 230
Th/U and TCN
dating (Chausse et al., 2004; Kock et al., 2009; Le Dortz et al., 2011;
Blisniuk et al., 2012). For instance, Blisniuk et al. (2012) applied a
combination of 10
Be exposure dating with 230
Th/U-dating to
constrain the deposition of mid-Holocene to late Pleistocene alluvial
fans (California). Three sampling strategies were implemented
for the first method: top surface of individual large boulders
(Fig. 7a), amalgamate of surface clasts and depth profile (Fig. 7c).
The second method involved the sampling of post-depositional
pedogenic carbonate from sub-surface clast-coatings. 230
Th/U
ages (minimum ages) are convergent or slightly younger than TCN
ages (maximum ages if not corrected for inheritance and assuming
zero denudation), thereby proving the usefulness of this combined
approach in obtaining reliable depositional ages of fan deposits
(Fig. 9b). Furthermore, the computing of 10
Be depth profile ages of
Late Pleistocene alluvium was made easier by the valuable minimum
age information inferred from 230
Th/U dating (Fig. 9b).
Third, as well as the parallel use of two or more independent
methods from the same fluvial sequences, a few exploratory studies
have attempted to merge the dating principles of distinct methods.
For instance, Guralnik et al. (2011) developed an innovative
approach using a mathematical framework for consistently incorporating
10
Be concentration data along a depth profile with OSL
ages from a single alluvial section (Fig. 9c, d). This model is based on
three parameters and solves an integrated, co-dependent and selfconsistent
set of equations and assumes fluvial aggradation at a
constant rate, with uniform cosmogenic inheritance, followed by
terrace abandonment and subsequent preservation and exposure
of its surface (Fig. 9c, d). This scenario of terrace evolution may be
validated or rejected by comparing model depth concentration data
and model OSL ages to real observations (Guralnik et al., 2011).
As a conclusion, establishing reliable chronologies for Quaternary
fluvial sequences has strongly benefited from such applications
of multiple dating methods. We finally recommend
combining age results of numerical methods with chronological
information obtained from relative dating methods. This is particularly
well exemplified by the study of Antoine et al. (2007), synthesizing
age results in the Somme valley (northern France), where
G. Rixhon et al. / Quaternary Science Reviews xxx (2016) 1e2318
Please cite this article in press as: Rixhon, G., et al., Revealing the pace of river landscape evolution during the Quaternary: recent developments
in numerical dating methods, Quaternary Science Reviews (2016), http://dx.doi.org/10.1016/j.quascirev.2016.08.016
diverse numerical (14
C, TL, IRSL, ESR, 230
Th/U) and relative (palaeomagnetism,
mammalian biostratigraphy and amino-acid racemization)
methods were implemented. In addition, control for ESR
dating was internally provided by cross-checking results of optically
bleached quartz grains with U-series/ESR dating of tooth
enamel. This multi-dating approach enabled Antoine et al. (2007)
to build a coherent and robust chronostratigraphical interpretation
of the terrace sequence of the Somme valley for the last 1 Ma.
Acknowledgements
M. Duval's research was funded by a Marie Curie International
Outgoing Fellowship of the EU’s Seventh Framework Programme
(FP7/2007-2013), awarded under REA Grant Agreement No. PIOFGA-2013-626474.
D. Scholz acknowledges funding of the DFG
(SCHO 1274/9-1). Two anonymous reviewers are acknowledged for
their thoughtful suggestions on an earlier version of this
manuscript.
David Bridgland is also acknowledged for his editorial work and
for having improved the English of the manuscript.
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