Evaporation induced 18
O and 13
C enrichment in lake systems: A global
perspective on hydrologic balance effects
Travis W. Horton a, *
, William F. Defliese b
, Aradhna K. Tripati b, c
, Christopher Oze a
a
Department of Geological Sciences, Private Bag 4800, University of Canterbury, Christchurch 8140, New Zealand
b
Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, 595 Charles Young Drive, Box 951567, Los Angeles, CA 90095-
1567, USA
c
Department of Atmospheric and Oceanic Sciences, Institute of the Environment and Sustainability, University of California, Los Angeles, 595 Charles Young
Drive, Box 951567, Los Angeles, CA 90095-1567, USA
a r t i c l e i n f o
Article history:
Received 3 January 2015
Received in revised form
23 June 2015
Accepted 29 June 2015
Available online 23 July 2015
Keywords:
Evaporation
Hydrologic balance
Lakes
Lacustrine
Stable isotopes
Oxygen isotopes
Carbon isotopes
Clumped isotopes
Hydrology
Paleohydrology
Isotope hydrology
Hydroclimate
Climate change
Paleoclimate
Paleoelevation
Paleothermometry
Carbonate
Western U.S.
Sierra Nevada
Southern Alps New Zealand
a b s t r a c t
Growing pressure on sustainable water resource allocation in the context of global development and
rapid environmental change demands rigorous knowledge of how regional water cycles change through
time. One of the most attractive and widely utilized approaches for gaining this knowledge is the analysis
of lake carbonate stable isotopic compositions. However, endogenic carbonate archives are sensitive to a
variety of natural processes and conditions leaving isotopic datasets largely underdetermined. As a
consequence, isotopic researchers are often required to assume values for multiple parameters, including
temperature of carbonate formation or lake water d18
O, in order to interpret changes in hydrologic
conditions. Here, we review and analyze a global compilation of 57 lacustrine dual carbon and oxygen
stable isotope records with a topical focus on the effects of shifting hydrologic balance on endogenic
carbonate isotopic compositions.
Through integration of multiple large datasets we show that lake carbonate d18
O values and the lake
waters from which they are derived are often shifted by >þ10‰ relative to source waters discharging
into the lake. The global pattern of d18
O and d13
C covariation observed in >70% of the records studied and
in several evaporation experiments demonstrates that isotopic fractionations associated with lake water
evaporation cause the heavy carbon and oxygen isotope enrichments observed in most lakes and lake
carbonate records. Modeled endogenic calcite compositions in isotopic equilibrium with lake source
waters further demonstrate that evaporation effects can be extreme even in lake records where d18
O and
d13
C covariation is absent. Aridisol pedogenic carbonates show similar isotopic responses to evaporation,
and the relevance of evaporative modification to paleoclimatic and paleotopographic research using
endogenic carbonate proxies are discussed.
Recent advances in stable isotope research techniques present unprecedented opportunities to overcome
the underdetermined nature of stable isotopic data through integration of multiple isotopic
proxies, including dual element 13
C-excess values and clumped isotope temperature estimates. We
demonstrate the utility of applying these multi-proxy approaches to the interpretation of paleohydroclimatic
conditions in ancient lake systems. Understanding past, present, and future hydroclimatic
systems is a global imperative. Significant progress should be expected as these modern research
techniques become more widely applied and integrated with traditional stable isotopic proxies.
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
Ongoing climate change induced shifts in water balance will be
exacerbated by the increased demand for food, energy resources,
and fresh water in many parts of the world in coming decades
(IPCC, 2014). Our ability to anticipate how regional hydrologic
systems will respond to modern era climate change is largely
informed by our understanding of how similar systems have
responded in the past. In the context of these globally significant
challenges, paleohydroclimate research remains one of the most
relevant sub-disciplines of modern geoscience.
Endogenic lacustrine minerals are particularly attractive
* Corresponding author.
E-mail address: travis.horton@canterbury.ac.nz (T.W. Horton).
Contents lists available at ScienceDirect
Quaternary Science Reviews
journal homepage: www.elsevier.com/locate/quascirev
http://dx.doi.org/10.1016/j.quascirev.2015.06.030
0277-3791/© 2015 Elsevier Ltd. All rights reserved.
Quaternary Science Reviews 131 (2016) 365e379
recorders of paleohydroclimate due to the fact that changes in
mineral chemistry are directly linked to changes in water balance in
many settings. Stable oxygen isotope proxies are the most widely
applied proxies in this regard due to their large kinetic fractionations
during evaporation (Leng and Marshall, 2004): it has long
been known that as a liquid evaporates, the residual fluid becomes
enriched in the less abundant heavy isotope(s) (Urey et al., 1932).
However, lakes are complex systems and it can be challenging to
isolate the effects of evaporation and changing water balance from
other effects based on oxygen (or hydrogen) isotopes alone. Thus,
there is an urgent need to develop and apply multi-proxy approaches
to interpreting terrestrial paleohydroclimate, particularly
in areas where water resource scarcity is a looming problem.
Using western U.S. water isotope and global Quaternary lake
carbonate datasets compiled from the literature, we show that
evaporation induced isotopic fractionations in lake systems are
often >þ10‰ from unmodified meteoric waters for d18
O. Such large
shifts in isotopic composition challenge the basic assumptions
regarding temperature of mineral formation or source water
composition applied in many stable isotopic studies underscoring
the underdetermined nature of single element isotopic records.
Most of the lake carbonate records included in the global dataset
we present show significant positive covariation between d18
O and
d13
C highlighting the need to take into account all possible controls
on stable isotopic proxies, including evaporation effects, for both
the Quaternary climate change and paleotopographic research.
In an effort to improve our understanding of what happens to
water when it evaporates, we performed a simple experiment: we
allowed natural water samples to evaporate and we analyzed their
evolving stable carbon and oxygen isotopic compositions. Over the
course of our six-day long evaporation experiment, isotopic compositions
changed by >þ10‰ in both d18
O and d13
C-dissolved
inorganic carbon (DIC), consistent with the thermodynamics of
kinetic fractionation, empirical evidence from endogenic carbonate
archives, isotopic monitoring of modern hydrologic systems, and
similar evaporation experiments performed under a variety of
environmental conditions. The observed positive linear covariation
between d18
O and d13
C follows the same narrow range in slope (ca.
0.7 to 1.2) as is present in a global compilation of Quaternary
lacustrine carbonate isotopic archives (n ¼ 57) and laminated
lacustrine carbonates reported here providing a quantitative basis
for recognizing evaporative effects in endogenic carbonate isotopic
records.
2. Materials and analytical methods
2.1. Global lake carbonate stable isotope compilation
The global database of published Quaternary lake carbonate
records we compiled includes more than 11,200 dual C and O stable
isotopic analyses on endogenic lake carbonate samples spanning a
>100 latitude range and >4800 m altitude range (Table 1). Modern
aridity index values (i.e. precipitation:evaporation) were extracted
for each location (Table 1) from a global aridity index raster
(Trabucco and Zomer, 2009) using ArcGIS. Many of the stable isotopic
records were downloaded from the open access NCDC on-line
database while other records were sourced from the primary
literature. Please refer to the primary references listed in Table 1 for
detailed information on all of these previously published dual
element stable isotopic records.
2.2. Western U.S. modern water stable isotope compilation
We compiled modern meteoric, river, and lake water d2
H and
d18
O values from a variety of published sources (Friedman, 2000;
Coplen and Kendall, 2000; Friedman et al., 2002; Henderson and
Shuman, 2009). This compilation includes 799 individual meteoric
water analyses, 3875 river water analyses, and 247 lake water
analyses predominantly from the western U.S. (Fig. 1). Aridity index
values were extracted for each location as described above. For
more information regarding these data, please refer to the primary
sources.
2.3. Evaporation experiments
In an effort to document the stable isotopic response to evaporation,
we determined d2
H, d18
O, and d13
C-DIC values for three
250 ml natural water samples allowed to evaporate in 500 ml opentop
beakers on a laboratory bench in the climate-controlled (21 C)
stable isotope analytical facility at the University of Canterbury
(Christchurch, New Zealand). All three samples were collected in
Southland, New Zealand (one upland river sample, one groundwater
sample, one coastal lowland stream sample) as part of a
larger regional isotope hydrology investigation. Threaded 250 ml
sample collection bottles were over-filled in the field and stored at
4 C prior to analysis. Hydrogen and oxygen stable isotopic compositions
were determined using a Picarro, Inc. Liquid Water
Isotope Analyzer and two-point (i.e. stretch-and-shift) normalized
to the SMOW-SLAP scale based on replicate analysis of International
Atomic Energy Agency (IAEA) certified reference waters
SMOW2 and SLAP. GISP, IAEA-TEL1, IAEA-TEL2, IAEA-TEL3, IAEATEL4
check standards were also analyzed at regular intervals across
the analytical sequence for quality control and quality assurance
purposes. d2
H values are precise to <1.0‰ and d18
O values are
precise to <0.1‰.
d13
C-DIC values were determined using the Sp€otl (2005)
method. In brief, 1 ml sub-samples were injected into ultra-high
purity (>99.999%) helium flushed 10 ml borosilicate exetainer
vials pre-loaded with 103% phosphoric acid. Dissolved inorganic
carbon (DIC) derived CO2, liberated by the acidewater reaction, was
analyzed using a ThermoFinnigan GasBench II coupled to a ThermoFinnigan
DeltaVþ
isotope ratio mass spectrometer operating
under a continuous flow of ultra-high purity helium. d13
C-DIC
values were two-point normalized to the VPDB scale based on
replicate analysis of IAEA certified reference materials NBS18 and
NBS19. All d13
C-DIC values are precise to <0.10‰.
Each water sample was analyzed and massed at least once daily
over the duration of the six-day evaporation experiment. Approximately
65% of the initial water volume evaporated over the course
of the experiment for all three samples.
2.4. Stable isotopic analysis of lake carbonates
Quaternary (Mono Lake tufa) and middle Miocene laminated
lacustrine carbonates (Barstow Fm. tufa, California, U.S.A.; Bannockburn
Fm. oncholite, Otago, New Zealand) were analyzed for
d18
O and d13
C in the stable isotope analytical facility at the University
of Canterbury. Hand samples were cut into flat slabs and
milled at 0.5 mm resolution using a diamond-coated Dremel tool
parallel to the growth axis. d18
O and d13
C of acidified (103% phosphoric
acid) sample powders were determined using a ThermoFinnigan
GasBench II coupled to a ThermoFinnigan DeltaVþ
isotope
ratio mass spectrometer operating under continuous ultra-high
purity He flow conditions. d18
O and d13
C values were two-point
normalized to the VPDB scale based on replicate analysis of IAEA
certified reference materials NBS18 and NBS19. A MERCK carbonate
internal lab standard was also analyzed at regular intervals
throughout each analytical sequence. All laminated lacustrine carbonate
d18
O and d13
C values are precise to <0.10‰.
D47 values for the middle Miocene Barstow Fm. Tufa, middle
T.W. Horton et al. / Quaternary Science Reviews 131 (2016) 365e379366
Miocene Bannockburn Fm. oncholite and Quaternary Mono Lake
tufa samples were determined using a ThermoFinnigan MAT 253
dual inlet gas source mass spectrometer (“Chewbacca”) in the Tripati
Lab at the University of California, Los Angeles, which has been
modified to simultaneously measure masses 44e49 and is equipped
with a custom built extraction device (Passey et al., 2010). For
each analysis, 5 mg of sample powder was digested with 105 wt.
percent H3PO4 using a common acid bath apparatus held at 90 C.
The resulting CO2 was cryogenically isolated and further purified by
a gas chromatograph. Purified CO2 was transferred to the mass
spectrometer, and simultaneously analyzed for d18
O, d13
C, and D47.
D47 values were corrected for acid fractionation using an empirically
derived acid fractionation offset of 0.092‰ (Henkes et al.,
2013; Defliese et al., 2015), and are reported on the absolute
reference frame (Dennis et al., 2011). D47 values were converted to
temperatures (see Supplementary Information) using a variety of
calibrations, (Ghosh et al., 2006; Eagle et al., 2013; Zaarur et al.,
2013; Tang et al., 2014; Defliese et al., 2015), which have been
adjusted to a common acid fractionation reference frame (Defliese
et al., 2015). Water d18
O values were calculated by using the
measured carbonate d18
O value in conjunction with the measured
D47-based temperature according to the carbonate-water oxygen
isotope equilibrium fraction equation of Kim and O'Neil (1997).
The water and lake carbonate data we report is a combination of
primary data original to this study and previously published datasets.
The reader is referred to the original sources for all previously
published data in both the text and figure captions.
3. Results
3.1. Western North America water isotopes
Meteoric waters from western North America closely approximate
the global meteoric water line (GMWL) and span an ~35‰
range in d18
O (Fig. 2a; Friedman, 2000; Coplen and Kendall, 2000;
Freidman et al., 2002; Henderson and Shuman, 2009). Yet, some
of these meteoric water samples, specifically those at the more
positive end of the dataset, plot significantly to the right of the
GMWL reflecting the effects of evaporation during rainout and
resulting in a more negative regional meteoric water line y-intercept
value (2.38) than is generally assigned to the GMWL (~10).
River waters from western North America show a similar
pattern, albeit with a dampened range in d18
O (~30‰ versus ~35‰).
Such signal dampening is easily explained by source water mixing
along hydrological flow paths. The more negative y-intercept value
of western North America river water (1.59) in comparison to
meteoric water from the same region (2.38) is almost certainly the
result of additional surface water evaporation (Fig. 2b) either in
Table 1
Compiled Quaternary lake carbonate stable isotopic record information including primary published sources.
Lake Number of
records
Latitude
(
)
Longitude
(
)
Lake elevation
(m.a.s.l.)
Modern aridity index
(P:E)
Age (yrs) Source(s)
Ahung 1 31.62 92.06 4575 0.570 <9000 Morrill et al., 2006
Bangong 1 33.70 79.00 4241 0.130 <11,000 Fontes et al., 1996
Bear 1 42.00 À111.33 1805 0.324 modern Dean et al., 2007
Big Soda 1 39.52 À118.88 1216 0.096 <100 yrs Rosen et al., 2004
Bonneville 2 41.00 À114.00 1570 0.162 <45,000 Nelson et al., 2005; Benson et al., 2011
Cahuilla 1 33.40 À116.05 À24 0.042 <20,000 Li et al., 2008
Castor 1 48.53 À119.55 596 0.328 <6000 Nelson et al., 2011
Chiquita 1 À30.90 À62.85 67 0.565 <250 Piovano et al., 2004
Crawford 3 43.47 À79.95 ~150 1.022 <13,000 Yu and Eicher, 1998
Deep 1 47.68 À95.37 411 0.730 <12,000 Hu et al., 1999
Elk 2 45.87 À95.80 366 0.673 <12,000 Smith et al., 1997
Farewell 1 62.55 À153.63 320 0.819 <2059 Hu et al., 2001
Fayetteville
Green
1 43.03 À75.97 70 1.148 <3186 Kirby et al., 2002
Foy 1 48.17 À114.36 1004 0.587 <2500 Stevens et al., 2006
Jellybean 1 60.35 À134.80 730 0.491 <7556 Anderson et al., 2005
Junin 1 À11.02 À76.12 4082 0.935 <18,000 Seltzer et al., 2000
Keche 1 68.02 À146.92 740 0.496 <3450 Chipman et al., 2012
Kepler 2 61.55 À149.21 26 0.731 <792 Gonyo et al., 2012
Lisan 2 31.50 35.00 À200 0.280 <60,000 Katz et al., 1977; Kronfeld et al., 1988
Medicine 1 44.82 À97.35 519 0.599 <10,600 Valero-Garces et al., 1995
Mono 6 38.00 À119.00 1945 0.296 <300 Li and Ku, 1997; Li et al., 1997; Benson et al., 2003;
Natron/Magadi 1 À2.42 36.00 538 0.231 >200,000 Hillaire-Marcel and Casanova, 1987 (P3)
Owens 4 36.43 À117.95 1084 0.170 <155,000 Menking et al., 1997; Benson et al., 2002
Pumacocha 1 À11.89 À75.05 4635 1.147 <2300 Bird et al., 2011
Pyramid/
Lahontan
6 40.00 À119.50 1250 0.126 <30,000 Benson et al., 1996; Benson et al., 2002; Benson et al.,
2013
Qinghai 1 37.06 100.30 3192 0.479 <18,000 Xu et al., 2006
Ruidera/Alcaraz 1 38.93 À2.90 950 0.392 <5000 Andrews et al., 2000
San Luis 1 37.68 À105.72 2300 0.198 <17,000 Yuan et al., 2013
Seven Mile 1 62.18 À136.38 520 0.454 <1100 Anderson et al., 2011
Siling 1 31.75 89.00 4500 0.399 <15,000 Morinaga et al., 1993
Steisslingen 1 47.80 8.92 446 1.205 <15,000 Mayer and Schwark, 1999
Taravilla 1 40.65 À1.97 1100 0.525 <11,000 Valero-Garces et al., 2008
Turkana 1 3.58 36.12 360 0.097 <105 Abell et al., 1982
Twiss 1 43.45 À79.95 ~150 1.009 <13,000 Yu and Eicher, 1998
Wallywash 1 17.97 À77.81 7 0.989 <120,000 Holmes et al., 2007
Xiangshui 1 25.42 107.88 310 1.086 <4280 Liu et al., 2011
Zabuye 1 31.35 84.07 4421 0.347 <30,000 Wang et al., 2002
Zoige 1 33.95 102.35 3400 0.774 <140,000 Wu, 1997
Total 57
T.W. Horton et al. / Quaternary Science Reviews 131 (2016) 365e379 367
through-flowing lakes or the rivers themselves.
Although originating from meteoric and riverine sources,
modern lake waters in western North America clearly depart from
the GMWL (Fig. 2c). Lakes from less arid environments generally
plot closer to the GMWL, although some humid and sub-humid
environment lakes are shifted to the right of the GMWL along a
regional evaporation line of slope ~5.2 (dark blue and blue triangles
in Fig. 2c). In contrast, most lakes in semi-arid to arid environments
plot well to the right of the GMWL unequivocally due to the effects
of lake water evaporation.
The more pronounced effect of evaporation on lake water isotopic
compositions is best demonstrated through quantification of
the positive d18
O shift away from the GMWL recognized in most
western North America lakes (Fig. 2d). We determined the
evaporation induced d18
O shift (i.e. change in d18
O value) for individual
meteoric, river and lake water samples by subtracting the
observed d18
O value from the corresponding d18
O value at the
intersection of the GMWL (d2
H ¼ 8 Â d18
O þ 10) and an intermediate
and representative slope 4.5 evaporation line. The median
d18
O shift for the 248 lake water samples we compiled is þ4.6‰,
with a minimum of À1.3‰ at Bonneville Salt Flat (January, 1993)
and a maximum of þ23.5‰, also at Bonneville Salt Flat and in the
same calendar year (July, 1993).
These results demonstrate the extreme isotopic sensitivity of
lake systems to evaporation induced kinetic fractionation effects,
particularly in arid and semi-arid environments. A statistical
comparison of meteoric, river and lake water d18
O shift values indicates
that, on average, meteoric waters are not significantly
Fig. 1. Aridity index (P:E) map of western North America (equidistant conic projection) showing modern meteoric (white snowflakes), river (blue circles) and lake (light blue
triangles) water sample collection sites (Friedman, 2000; Coplen and Kendall, 2000; Friedman et al., 2002; Henderson and Shuman, 2009). 10 of the 38 globally distributed
Quaternary paleolake systems, comprising 24 of the 57 dual element stable isotopic records compiled as part of this review, are also shown (red stars).
T.W. Horton et al. / Quaternary Science Reviews 131 (2016) 365e379368
different than river waters (p ¼ 0.34; two-tailed t-test), yet lake
waters are highly significantly different than both meteoric and
river waters (p ¼ 10À36
and p ¼ 10À41
, respectively; two-tailed t-
test).
The fact that lake water isotopic compositions do not reflect
meteoric water isotopic compositions is further demonstrated by a
comparison of observed lake water d18
O values and modeled
meteoric water d18
O values (Online Isotopes in Precipitation
Calculator; Bowen, 2014) at each lake's location (Fig. 3). Surprisingly,
relatively few lakes exhibit more negative d18
O values than
local meteoric water despite the cordilleran physiography of
western North America: lakes should be recharged by higher altitude
meteoric water sources that are 18
O depleted relative to local
meteoric recharge. Yet, the empirical pattern present in the data we
compiled is quite the opposite (Fig. 3): the by far majority of lakes
are 18
O enriched relative to local meteoric water. Interestingly, even
lakes in humid environments can be enriched in 18
O by up to ~12‰
in comparison to local meteoric water, and lakes in semi-arid to
arid environments can exhibit 18
O enrichments on the order
of þ15‰ (Fig. 3).
The results presented above provide important context to our
discussion of d18
O and d13
C covariation in the following sections.
The unequivocal and potentially extreme effects of evaporation on
Fig. 2. Compiled western North America meteoric (a), river (b) and lake (c) water d2
H and d18
O compositions reported in Friedman (2000), Coplen and Kendall (2000), Friedman et
al. (2002), and Henderson and Shuman (2009). Regional stable isotopic water line equations represent the least-squares linear regression fits through all data points plotted in each
panel. Black star (aec) represents standard mean ocean water (SMOW) and thick black line (aec) represents the global meteoric water line (GMWL). Lake water symbols (c) are
color coded according to the modern aridity index at each sampling site. Panel (d) shows frequency distributions and whisker-box plots of the d18
O shift away from the global
meteoric water line along an evaporation line of slope 4.5 for each sample type. Vertical lines in the whisker-box plots (d) represent, minimum, lower quartile, median, upper
quartile, and maximum d18
O shift values observed.
Fig. 3. Comparison of modeled meteoric water d18
O values (Bowen, 2014) and
observed lake water d18
O values reported in Friedman (2000), Coplen and Kendall
(2000), Friedman et al. (2002), and Henderson and Shuman (2009). Meteoric water
values were calculated for the same month in which each lake water sample was
collected. Symbols are color coded according to the modern aridity index at each lake
water sampling site. Solid line represents a 1:1 (i.e. no difference) relationship. Labeled
dashed lines represent þ10‰ and þ20‰ 18
O lake water enrichments relative to local
temporally matched meteoric water.
T.W. Horton et al. / Quaternary Science Reviews 131 (2016) 365e379 369
surface water isotopic compositions present major challenges to
single element (i.e. O or H) proxies of temperature, elevation, water
source apportionment, atmospheric circulation conditions and any
combination of these environmental conditions. However, these
same results confirm that stable isotopic lacustrine archives are
potentially powerful recorders of hydrologic balance.
3.2. Evaporation experiment
The stable isotope community has long known of the potentially
extreme effects of evaporation on the isotopic composition of liquids.
After all, Urey himself applied this knowledge when he
demonstrated the existence of deuterium through evaporative
enrichment of liquid hydrogen (Urey et al., 1932). Yet, the liquid
itself is not the only phase affected by the chemical consequences of
evaporation.
As water evaporates its chemistry changes: concentrating dissolved
phases, increasing alkalinity and forcing changes in equilibrium
conditions. One of the most obvious consequences of
evaporation is the formation of sedimentary evaporites including
some lacustrine carbonates (e.g. trona and tufa). The isotopic effects
of evaporation on endogenic carbonates have been a research focus
of the terrestrial paleoclimate community for many years (e.g.
Stuiver (1970)), and one of the most commonly reported observations
is O and C stable isotopic covariation.
Coupled 18
O and 13
C enrichment in endogenic carbonates has
been documented and discussed at some length in the limnological
(Stuiver, 1970; Talbot, 1990; Li and Ku, 1997; Huang et al., 2014),
pedological (Cerling and Quade, 1993; Quade et al., 2007; Ding
et al., 2014), and speleological communities (Hendy, 1971;
Mickler et al., 2006; Dreybrodt and Deininger, 2014). Oxygen and
carbon stable isotopic covariation has also been documented in
evaporating Dead Sea brines (Stiller et al., 1985), degassing epithermal
systems (Zheng, 1990) and laboratory experiments (Ufnar
et al., 2008; Abongwa and Atekwana, 2013). Although several hydrological
models incorporate evaporative effects in their isotopic
determinations (Appelo, 2002; Benson and Paillet, 2002; Cappa
et al., 2003; Jones et al., 2005; Jones and Imbers, 2010), only one
of these models explicitly addresses the coupled effects of evaporation
on oxygen and carbon isotope fractionation (Deininger et al.,
2012). However, the model of Deininger et al. (2012) specifically
applies to thin-film evaporation associated with stalagmite formation
in caves rather than surface water evaporation in lakes.
Although evaporative effects on lake water and lake carbonate
isotopic compositions have been highly studied, an integrated understanding
of how evaporation influences the coupled carbon and
oxygen isotope compositions of surface waters remains elusive.
In fact, the general lack of dual O and C stable isotopic data for
evaporating solutions in the primary literature is somewhat surprising
given the widespread empirical demonstration of isotopic
covariation in many carbonate lakes and soil environments.
Although we do not intend to fill this gap in the literature here, the
evaporation experiment data we report reinforce what many have
inferred from empirical results and isotopic theory: evaporation
causes 18
O enrichment in water and 13
C enrichment in DIC.
All three water samples that we allowed to evaporate showed
18
O and 2
H enrichment according to well-known kinetic fractionation
processes (Cappa et al., 2003). Over the six-day evaporation
period, all three samples lost ~65% of their mass to evaporation at
near constant evaporation rates (Fig. 4a). The stable O and H isotopic
response to evaporation increased residual water d18
O values
by ~11e13‰ and d2
H values by ~45e55‰ over the course of the
experiment. All three samples followed similar linear evaporation
trends away from the GMWL (Fig. 4b).
As each of the samples evaporated, clear progressive 13
C
Fig. 4. Evaporation experiment results for three South Island, New Zealand, water
samples, including one upland river (circles), one groundwater sample (inverted triangles)
and one coastal lowland stream (upright triangles). Symbols in all panels are
color coded by %-water evaporated. Panel (a) shows the near constant evaporation rate
of all three samples, panel (b) shows the d2
H and d18
O isotopic response to evaporation,
and panel (c) shows the d18
O and d13
C-DIC response. Thick black line and black
star in (b) represent the GMWL and SMOW, respectively. Thin black lines in all panels
represent least-squares linear regression fits. Each sample was allowed to evaporate
for ~6 days in a 500 ml beaker at 21 C.
T.W. Horton et al. / Quaternary Science Reviews 131 (2016) 365e379370
enrichments in DIC, on the order of 1e2‰ per day, occurred over
the six-day experiment. From these data and the corresponding
water d18
O values, we calculated the equilibrium fractionation (Kim
and O'Neil, 1997) O and C stable isotopic compositions of calcite
that would have formed from these evaporating waters at 21 C
(Fig. 4c). Although not as strongly linear as the O and H isotopic
response, statistically significant (p < 0.05; t-test) positive covariation
was observed in all three samples. The relatively constant
linear covariation slope (m ¼ 0.7 to m ¼ 1.2) is notably similar to
what has been observed in the lacustrine carbonate rock record
(Horton and Oze, 2012) suggesting systematic isotopic fractionations
occur as excess CO2 degasses from the water body, 13
C
enriched bicarbonate ions (and carbonate ions above pH > 8.3)
further dominate the DIC pool as alkalinity increases in response to
evaporation, and the water molecules themselves become 18
O
enriched as they evaporate (Zheng,1990; Valero-Garces et al.,1999;
Cappa et al., 2003; Dreybrodt and Deininger, 2014).
A number of different processes can cause 13
C enrichment of the
DIC pool, including carbon isotope fractionation caused by an
increased rate of aquatic photosynthesis relative to respiration and
decomposition, equilibrium exchange with atmospheric carbon
dioxide, and inputs of carbon derived from magmatic or marine
limestone sources. Of these processes, only equilibrium exchange
with atmospheric carbon dioxide is a possible factor influencing
our experimental results. Thus, our findings suggest that evaporation
induced increases in alkalinity (i.e. increased bicarbonate
concentration) drives DIC 13
C enrichment through isotopic equilibrium
exchange with CO2 (ca. 8‰ at 25 C in the HCOÀ
3 ÀCO2
system; Mook et al., 1974). This interpretation and our results are
consistent with mechanistic models of the isotopic response of DIC
during evaporation (Abongwa and Atekwana, 2013; Dreybrodt and
Deininger, 2014).
3.3. Global pattern of O and C isotopic covariation
Tens if not hundreds of research investigations into lake carbonate
isotope variability have been conducted in the twenty-five
years since Talbot (1990) published his review of the paleohydrological
interpretation of lake carbonate stable isotopic records. In an
effort to further explore and update the paleohydrological interpretation
of lacustrine isotopic archives, we compiled 57 lake carbonate
dual element (i.e. O and C) stable isotopic records (Table 1)
representing 38 different globally distributed lake systems published
in 46 different primary sources, of which 42 were published
between 1990 and 2015. These 57 records were selected because
they span a broad range of geographic, physiographic, climatic and
hydrologic conditions including high and low latitude environments,
closed and open basin hydrologies, low and high altitudes,
and humid to arid hydroclimates. The most salient pattern that
emerges from the analysis of this compilation is O and C stable
isotopic covariation.
Of the 57 lake records analyzed, 41 show a statistically significant
(p < 0.05, t-test) positive covariation between d18
O and d13
C, 3
show a statistically significant (p < 0.05, t-test) negative covariation
(Medicine Lake, Valero-Garces et al., 1995; Wallywash Pond,
Holmes et al., 2007; Seven Mile Lake; Anderson et al., 2011), and 13
show no significant isotopic covariation (Fig. 5). The average linear
regression slope of the 41 records with a significant positive
covariation trend is 0.62, across slope values ranging between 1.67
and 0.1. The average range in d18
O values spanned by these 41 records
is 6.9‰, with a maximum range of 13.5‰ and minimum
range of 1.2‰. Nearly three-quarters (i.e. 72%) of the lake records
we compiled show statistically significant positive covariation
trends spanning isotopic compositional ranges that are difficult to
explain in the absence of extreme isotopic modification by
evaporative enrichment of lake waters in both 18
O and 13
C.
4. Discussion
4.1. From source to sink: O and C isotopes in Quaternary lake
systems
Oxygen and carbon stable isotopic covariation in endogenic
lacustrine carbonates is not uncommon (Talbot, 1990). Yet, relatively
few studies have analyzed the oxygen and carbon isotopic
composition of both modern lake water samples and lake source
(i.e. inflow) water samples, making it difficult to quantify the extent
to which lacustrine processes, including evaporation, modify
source water O and C chemistries. In an effort to quantify these
source water modification effects on the stable isotopic composition
of lake carbonates, we modeled individual lake carbonate
isotopic compositions using two different approaches and
compared these results to observed isotopic records.
In our first analysis we only consider oxygen isotope compositions.
As part of this analysis, we modeled the oxygen isotope
composition of calcite that would have formed from mean summertime
precipitation (Bowen, 2014) at mean summer month air
temperatures (Hijmans et al., 2005) according to temperature
dependent oxygen isotope equilibrium fractionation (Kim and
O'Neil, 1997):
1000lna(Calcite-H2O) ¼ 18.03(103
TÀ1
) À 32.42 (1)
where T is temperature in kelvins and a is the temperature
dependent calcite-water fractionation factor. We then compared
these model d18
O values to the average d18
O observed in all 57 of
the global lake carbonate records we compiled (Fig. 6). In this
model, the calculated d18
O values assume that lake calcite formed
from unmodified local meteoric water. Although this analysis represents
an overly simplified model of oxygen isotope equilibrium
fractionation in lake carbonates, it is similar to the approach used in
27 of the 45 (i.e. 60%) of the published sources (see Table 1) we used
in our data compilation.
Similar to what was observed in our comparison of modern
meteoric water and lake water d18
O (Fig. 3), the observed average
lake carbonate d18
O values are more positive than the modeled
summer month meteoric water derived calcite d18
O values (Fig. 6).
Lakes in humid environments generally plot closer to the 1:1 line,
suggesting lakes in these environments are less impacted by
evaporative modification, yet 46 of the 57 lake records analyzed (i.e.
81%) plot to the right of the 1:1 line consistent with evaporative
modification of lake water d18
O. Forty-two percent of the lake
carbonate d18
O records are >5‰ shifted towards more positive d18
O
than would be expected for summer month carbonate precipitates
derived from unmodified local meteoric water. Although many
lakes with vastly different modern aridity index values show
similar offsets between modeled and observed d18
O, lakes from
currently arid and semi-arid environments have a much larger
average d18
O offset (5.4‰) than sub-humid and humid environment
lakes (2.0‰).
This model analysis of 57 global Quaternary lake carbonate d18
O
records demonstrates the challenges of working with underdetermined
stable isotopic systems. By assuming temperatures of
carbonate formation or source water oxygen isotope composition,
or any other environmental parameters (e.g. atmospheric circulation;
seasonally variable recharge sources; etc.) that may similarly
influence these two unknowns in carbonate mineral-water oxygen
isotope equilibrium fractionation equations, researchers effectively
pre-determine the primary control on carbonate d18
O values. For
example, had we assumed winter month (or pre-interglacial)
T.W. Horton et al. / Quaternary Science Reviews 131 (2016) 365e379 371
temperatures of formation in our analysis rather than modern
summer month temperatures we would have generated more
positive modeled carbonate d18
O values. Conversely, had we used
mean annual or weighted mean annual precipitation stable isotopic
compositions rather than summer month precipitation values, we
would have generated more negative modeled carbonate d18
O
values. In either instance the likely interpretations rely heavily on
the assumptions that underlie the analysis. Such overly simplistic
approaches reinforce the challenges associated with traditional
single element stable isotopic investigations of complex hydrologic
and climatic systems.
In our second analysis, we examined a subset of Quaternary
lacustrine carbonate isotopic records where complementary modern
lake and source water oxygen and carbon isotope data were
also available in the literature. From these data, we were able to
separately calculate the O and C stable isotopic compositions of
calcite forming in isotopic equilibrium with both source water and
lake water. Because Quaternary lake carbonate dual element isotopic
records were also available for these lake systems, we were
able to graphically present each lake's complete stable isotopic
profile, including modeled source water derived calcite, modeled
lake water derived calcite, and observed lake carbonate compositions,
on individual bivariate plots (Fig. 7). There are several key
insights to be gained from these isotopic profiles.
First, modeled source water derived calcite compositions (green
stars, Fig. 7) are several permil more negative than modeled lake
water derived calcite compositions (red stars, Fig. 7) in both d18
O
and d13
C. In the context of the evaporation experiment results we
present above, the ~8‰e~15‰ differences between source water
calcite and lake water calcite isotopic compositions is best
explained by high levels of evaporation (i.e. the equivalent of the
isotopic response to >50% losses to evaporation). Determining the
extent to which source water O and C isotopic compositions are
modified by evaporation can now be determined with relative ease.
Triple-oxygen isotope analysis of water, an excellent indicator of
evaporation effects (Passey et al., 2014), is becoming more common
as researchers begin utilizing current generation off-axis integrated
cavity output spectrometers (Berman et al., 2013) and high sensitivity
isotope ratio mass spectrometers (Barkan and Luz, 2005), and
d13
C-DIC analyses are similarly more accessible to the geochemical
research community (e.g. Bass et al. (2014)). We encourage researchers
interested in both modern and ancient lake systems to
Fig. 5. Global aridity index map (a) showing location (red stars) of the fifty-seven Quaternary lake carbonate dual element d18
O and d13
C records (>11,200 total data points), from
thirty-eight discrete lake systems, compiled as part of this review. Panel (b) presents the least-squares linear regression fits to these isotopic records. Black regression lines
correspond with records exhibiting a statistically significant correlation between d18
O and d13
C. Red regression lines correspond with records exhibiting no significant correlation.
Dashed line represents a 1:1 (i.e. m ¼ 1) relationship. All data were normalized to the mean isotopic composition of each record using an additive shift.
T.W. Horton et al. / Quaternary Science Reviews 131 (2016) 365e379372
pursue these multi-proxy approaches to advancing our understanding
of source water modification during lacustrine residence.
A crucial aspect of such endeavors will be to study both the source
water, lake water and carbonate precipitation as an integrated and
temporally dynamic system, rather than discrete hydrologic components
suitable for study in isolation.
Second, measured Quaternary carbonate d18
O and d13
C values
generally plot along a slope ~1 positive linear covariation trend that
also includes the modeled source and lake calcite compositions (i.e.
the line connecting green stars to red stars shown in Fig. 7). The
general agreement between measured paleolake carbonate compositions
and modeled modern lake calcite compositions demonstrates
that this approach provides a reasonably accurate first-order
approximation of endogenic lake carbonate d18
O and d13
C values.
The lack of overlap between modeled source water derived calcite
compositions and observed paleolake compositions reinforces the
fact that meteoric derived source waters are modified by inelake
processes. Just as we have shown above for modern lakes, paleolake
carbonates do not reflect meteoric derived source water isotopic
compositions. Based on the results of our evaporation experiments,
we argue that the relatively systematic slope ~1 positive linear
covariation trends, linking both modeled calcite and measured
paleolake carbonate isotopic compositions, is further evidence that
evaporation during lake residence is the primary driver of these
individual lake isotope profiles.
Third, lake carbonate d18
O and d13
C values show severe isotopic
modification due to lake water evaporation regardless of whether
or not they exhibit isotopic covariation (Fig. 7). Strong positive
covariation between lake carbonate d18
O and d13
C values has been
associated with closed-basin hydrology or lakes with very long
residence times, implying that evaporative effects are the primary
driver of these covariant trends (e.g. Talbot (1990)).
Our analysis reinforces these interpretations, yet it also demonstrates
that even lake carbonate records lacking significant
positive covariation trends can also be severely impacted by the
effects of evaporation. Several Quaternary lake carbonate records,
notably those from arid environment closed-basins including Mono
Lake cores MLB-003D (Benson et al., 2003), MLC-001T (Benson
et al., 2003), ML91-FC3 (Li et al., 1997) and Pyramid Lake core
PLC-97.3 (Benson et al., 2002), do not show significant covariation
trends. Yet, according to our analysis, the d18
O and d13
C values
measured in these cores are ~10‰e~15‰ enriched in both 18
O and
13
C along a slope ~1 trend relative to modeled modern source water
derived calcite compositions (Fig. 7). These extreme isotopic enrichments
relative to less evolved inflow water isotopic compositions
is demonstrated by the positions of the green stars (i.e. inflow
water derived carbonate compositions) shown in Fig. 7 relative to
the positions of the red stars (i.e. lake water derived carbonate
compositions). The Mono Lake data presented in Fig. 7e provide an
excellent example.
Several of the Mono Lake carbonate records we compiled
(Fig. 7e) do not exhibit statistically significant positive covariation
between d18
O and d13
C. Slopes of these statistically not significant
linear regressions fit to these records range between À0.02
and þ 0.77. Applying the logic presented in Talbot's seminal review
of carbonate lake paleohydrology (Talbot, 1990), it would be
tempting to interpret that Mono Lake was not hydrologically closed
during these intervals due to a shift in hydrologic balance towards
relatively wetter conditions. However, the lack of isotopic covariation
and associated variability in regression line slopes are the
consequence of the fact that these metrics are determined by
comparing the lake carbonate values to themselves. By comparing
these same data to the isotopic compositions expected from inflow
waters (Fig. 7e) and other records from Mono Lake that do exhibit
significant covariation (Fig. 7a), our analysis reveals that these intervals
instead represent some of the most isotopically enriched
periods captured by any of the Mono Lake archives.
The most plausible mechanism of such severe heavy isotope
enrichments is evaporation. Given the overlap between the d18
O
and d13
C values observed in these Mono (Fig. 7e) and Pyramid Lake
(Fig. 7f) records and other records from the same lakes (Fig. 7a, b),
we suggest that a stable, yet highly arid, climatic condition was
present during deposition of these lake carbonates. The possibility
that extreme isotopic modification by evaporation fails to impart
significant positive O and C covariation trends on lake carbonate
archives is problematic in that it increases the probability that
isotopic evaporation effects may pass unrecognized by the
researcher.
4.2. Relevance to other research topics
The data and interpretations we present above are not restricted
to the lacustrine Quaternary paleoclimate community. The underlying
hydrochemical processes and conditions at the core of our
review and analysis are key topics in modern research on regional
hydrologic cycles and water resource management, as well as the
broader limnology, biogeochemistry and aquatic ecology communities.
In the context of stable isotopic proxies, however, the data
and analysis we present is particularly relevant to both the pedogenic
carbonate and paleotopographic research communities.
Pedogenic carbonate formation is most commonly associated
with arid to semi-arid environments (Jenny, 1994). Thus, it is
reasonable to assume that if evaporation is the dominant process
controlling stable O and C isotopic compositions in endogenic
lacustrine carbonates, then similar effects should be expected in
pedogenic carbonates. To explore this possibility, we plotted the
seminal western U.S. aridisol pedogenic carbonate d18
O and d13
C
values reported by Cerling and Quade (1993) and classified each
value by its site-specific modern aridity index (Fig. 8a). The stable
isotopic data reported by Cerling and Quade (1993) correspond
with average soil carbonate isotopic compositions observed below
30 cm depth in modern soils.
As would be expected from isotopic enrichment due to evaporation,
modern soil carbonates from the western U.S. describe a
statistically significant positive covariation trend (p ¼ 4.7À10
, t-test)
Fig. 6. Modeled isotopic equilibrium summer month lake calcite d18
O values (see text
for details) plotted against the average lake carbonate d18
O value in each of the fiftyseven
Quaternary records we compiled. Symbols are color coded according to modern
aridity index values with whiskers showing ±1s. Solid black line represents a 1:1
(i.e. no difference) relationship. Labeled dashed lines correspond
with À5‰, þ5‰, þ10‰ and þ15‰ 18
O depletion/enrichments in observed lake carbonate
isotopic composition relative to modeled summer lake calcite compositions.
T.W. Horton et al. / Quaternary Science Reviews 131 (2016) 365e379 373
with a slope very close to 1 (m ¼ 1.03; Fig. 8a). The observation that
pedogenic carbonates from hyper-arid and arid environments
exhibit the most positive d18
O and d13
C values, while semi-arid to
humid environment soils show the most negative values, further
suggests that evaporation is the dominant control on both the oxygen
and carbon isotope composition of these soil carbonate samples
at >30 cm depth (Fig. 8a). More recent research on the stable
isotopic composition of arid environment soil carbonates support
this interpretation (Quade et al., 2007). Future research based on the
stable isotopic composition of pedogenic soil carbonates, whether
for paleoclimatic or paleotopographic purposes, will benefit significantly
from multi-proxy research approaches that provide quantitative
constraints on the effects of soil water evaporation (e.g.
Horton and Oze, 2012; Ji et al., 2014; Passey et al., 2014).
Fig. 7. d13
C versus d18
O bivariate plots for Quaternary lake carbonate records where modern source (i.e. inflow) water and lake water d18
O and d13
C-DIC values were available from
the literature. Green stars represent the isotopic equilibrium composition of calcite forming in equilibrium with source water at each site's mean annual air temperature. Red stars
represent the isotopic equilibrium composition of calcite forming in equilibrium with lake water at each site's mean annual air temperature. Quaternary lake carbonate data symbols
as indicated in panel legends. Solid black line represents a 1:1 relationship; dashed lines represent ±5‰ differences between lake carbonate d18
O and d13
C. Mono Lake tufa data are
original to this study. See Table 1 for all other primary data sources. Modern hypsometric altitudes (zhyp) are shown in (aef).
T.W. Horton et al. / Quaternary Science Reviews 131 (2016) 365e379374
The stable isotope paleotopography community will almost
certainly benefit from similar approaches for similar reasons. Just as
the Quaternary paleoclimate community must address the underdetermined
nature of stable isotopic proxies available from
terrestrial archives, the stable isotope paleotopography community
must also take care to accurately interpret authigenic mineral isotopic
records sensitive to changing water balance in evolving
orographic rain shadows. Failing to account for the isotopic effects
of evaporation and uplift induced shifts in regional hydroclimatic
conditions, could lead to erroneous paleoelevation interpretations
on the order of kilometers (Horton and Oze, 2012).
As recognized in a smaller lake carbonate compilation (Horton
and Oze, 2012), both the Quaternary lake carbonate and modern
soil carbonate records we investigated here indicate there is an
apparent quantitative relationship between hypsometric altitude
and positive linear covariant trend y-intercept value (i.e.13
C-excess;
Fig. 7; Fig. 8). The dual element 13
C-excess paleoelevation approach
takes advantage of the systematic nature of the empirically and
experimentally observed positive covariation slope m z 1 and
combines this relationship with the well-known altitude effect on
meteoric water d18
O values to estimate hypsometric altitudes
within ±500 m errors (Horton and Oze, 2012). Application of this
proxy to modern soil carbonates from the Mojave Desert-Great
Basin region (Fig. 8a), as well as middle Miocene laminated lacustrine
carbonates from the modern Sierra Nevada (Fig. 8b; Barstow
Fm. tufa; Mud Hills, California) and Southern Alps (Fig. 8b; Bannockburn
Fm. oncholite; Otago, New Zealand) orographic rain
shadows, yields hypsometric elevations on the order of 1.5 km for
all three records. These 13
C-excess elevations agree well with
modern elevations across the Great Basin (~1700 m mean elevation
in the north; ~900 m mean elevation in the south; Saltus and
Thompson, 1995) and published paleotopographic interpretations
for the Sierra Nevada (Mulch et al., 2006; Cassel et al., 2009).
However, a ~1.5 km high ‘proto’ Southern Alps at ~15 Ma is an
entirely new interpretation that provides important detail otherwise
lacking in the only other paleoelevation study on the mountain
range (Chamberlain et al., 1999).
The 13
C-excess approach to determining paleoelevations represents
one way future researchers can overcome the stable isotopic
evaporation problem. Yet, the most robust interpretations of
paleoelevation are likely to result from combining the 13
C-excess
approach with complementary proxies of both evaporation (e.g.
triple oxygen stable isotope analysis; Passey et al., 2014) and temperature
(e.g. clumped isotope analysis; Petryshyn et al., 2015).
4.3. Multi-proxy methods
The research we present demonstrates that the underdetermined
nature of stable isotopic systems is a key challenge in
paleoenvironmental research. The sensitivity of terrestrial systems
to a number of environmental variables is both a strength and
weakness of stable isotope proxy methods. However, recent advances
in stable isotope analytical techniques create unprecedented
opportunities to determine multiple independent proxy
datasets for the same suite of samples. Using some of the same lake
carbonate samples discussed above, we below demonstrate the
utility of applying coupled d18
O and D47 temperature estimates to
assessments of paleohydroclimatic conditions.
We determined d18
O and D47 values for a previously toppled
modern era Mono Lake tufa tower collected near the South Tufa
visitor's area (U.S. Department of Agriculture, Special Use Permit:
LVD050047T issued to Horton). d18
O values determined on individual
carbonate laminations present in this tufa are ~13‰ more
positive than would be expected for Mono Lake inflow water
derived carbonate (Fig. 7a). Paleotemperature estimates for two D47
analyses of this same sample (Table 2) are 15.8 C (±4.3 C) and
15.4 C (±3.3 C). Inputting the d18
O values and D47 paleotemperature
estimates to the Kim and O'Neil (1997) oxygen isotope
equilibrium fractionation equation yields lake water d18
O values
of À2.3‰ (±0.9‰). and À1.4‰ (±0.7‰) VSMOW (Table 2). These
values are 13‰e14‰ more positive than published Mono Lake
inflow water d18
O values and 1‰e2‰ more negative than present
day Mono Lake water (Li et al., 1997). By combining traditional
carbonate d18
O data with more recently developed D47 clumped
isotope analytical methods we were able to isolate and solve for a
single variable (d18
Owater) in the calcite-water equilibrium fractionation
equation. Our multi-proxy approach demonstrates that
Mono Lake water was highly modified by evaporation at the time of
tufa formation, consistent with the interpretations published by
others (Li and Ku, 1997; Li et al., 1997; Benson et al., 2003) and the
basin's arid hydroclimatic setting in the rain shadow of the Sierra
Nevada.
Fig. 8. d13
C versus d18
O bivariate plots for Cerling and Quade's (1993) modern aridisol soil carbonates from the western U.S. Great Basin/Mojave Desert region (a), and middle
Miocene (~15Ma) laminated lacustrine carbonates from the Mojave Desert's Mud Hills area Barstow Formation and Otago New Zealand's Lake Manuherikia sequence Bannockburn
Formation at Vinegar Hill (b). Pedogenic soil carbonate symbols in (a) are color-coded based on modern aridity index. Solid black line represents a 1:1 relationship; dashed lines
represent ±5‰ differences between lake carbonate d18
O and d13
C. Modern regional hypsometric altitude (zhyp) is shown in (a). Middle Miocene lacustrine carbonate data in (b) are
original to this study.
T.W. Horton et al. / Quaternary Science Reviews 131 (2016) 365e379 375
Application of this multi-proxy method is similarly instructive
for middle Miocene laminated lacustrine carbonates exposed in the
central Mojave Desert (Barstow Formation tufa) and central Otago
in Aotearoa/New Zealand's South Island (Bannockburn Formation
oncholite). d18
O values determined for carbonate powders drilled at
0.5 mm spacing across a 100 mm wide section of the Barstrow
Formation tufa do not covary with d13
C (r2
¼ 0.03; n ¼ 201; Fig. 8b)
suggesting evaporative effects may not have been a primary control
on the isotopic composition on the water filling this 15 million yearold
lake basin. However, D47 paleotemperature estimates for the
Barstow tufa are relatively warm (48.3 C; Table 2), yielding lake
water d18
O values of À0.4‰ (±0.3‰) VSMOW (Table 2) under
equilibrium calcite-water fractionation conditions. In this case,
combining d18
O and D47 paleotemperature estimates suggests that
the Barstow Formation tufa may have been fed by hot springs
generated by local magmatic systems present in the central Mojave
during the middle Miocene (Glazner et al., 2002).
In a third example, multi-proxy d18
O and D47 values determined
for an oncholite preserved in central Otago's ~17 million year-old
Bannockburn Formation are consistent with paleontologic, palynologic
and sedimentologic interpretations of paleolake Manuherikia's
natural environment, climate and physiography. The
occurrence of mekosuchine crocodile bones and subtropical casuarinas
and palm pollen in Manuherikia sediments suggest a relatively
warm subtropical climate not unlike that of modern northern
New Zealand (Schwarzhans et al., 2012). The D47 paleotemperature
estimates we determined (Table 2) reinforce this interpretation
returning values that range between 17.7 C (±3.9 C) and 24.6 C
(±2.6 C), similar to the range in modern-era monthly average air
temperatures in Northland, New Zealand, and several degrees
warmer than the modern-era mean annual air temperature of
central Otago.
Bannockburn Formation oncholite sedimentary structures and
textures suggest these laminated lacustrine carbonates formed in
the shallow littoral zone of an ephemeral sub-basin in the paleolake
Manuherikia system (Lindqvist, 1994). If true, we would expect
Manuherikia oncholites to exhibit isotopic compositions consistent
with evaporative modification of lake water. The significant
(p < 0.05; t-test) positive covariation between d18
O and d13
C in the
Bannockburn Formation sample (Fig. 8b) and the equilibrium
fractionation lake water d18
O values (À0.1‰ ±0.8‰ to þ2.6‰
±0.6‰ VSMOW; Table 2) determined using the above D47 paleotemperature
estimates and associated oncholite d18
O values provide
strong empirical evidence that these samples indeed formed
in the presence of evaporated lake water. Integrating these findings
with the 13
C-excess paleoelevation proxy results which suggest the
presence of a ~1.5 km high catchment hinterland in the Manuherikia
system (see Section 4.3, above) provides further paleoenvironmental
detail that cannot be achieved through application of
traditional single element stable isotopic proxy methods.
5. Conclusions
A review of globally distributed Quaternary lake carbonate
stable isotopic records and modern water isotopic compositions
from western North America reinforces what the scientific community
has known for decades: evaporation enriches surface waters
and associated authigenic minerals in both 18
O and 13
C. This
empirical pattern of O and C heavy isotope enrichment is corroborated
by simple evaporation experiment results, and is consistent
with the thermodynamics of kinetic fractionation of evaporating
water, degassing of dissolved CO2, and 13
C enrichment of DIC as
alkalinity increases at Earth surface temperatures. Of utmost
importance is the recognition that the isotopic effects of evaporation
can be extreme, often in excess of 10‰ for both d18
O and d13
C.
Traditional single-element isotopic records fail to record the full
extent of these evaporative effects due to their sensitivity to myriad
environmental conditions and the difficulty in determining unmodified
source water compositions from terrestrial archives.
However, the systematic nature of evaporation induced positive
covariation in d18
O and d13
C for both lacustrine and pedogenic
carbonates presents opportunities to apply dual element and
multi-proxy methods to this isotopic evaporation problem even in
systems where covariation is not immediately apparent. Integration
of traditional O and C isotopic compositions with clumped
isotope and triple-oxygen proxies has the potential to produce
detailed empirical data-based interpretations of hydroclimatic and
paleoenvironmental conditions in terrestrial settings on a variety of
spatial and temporal scales. Our research demonstrates that the
integration of multiple stable isotopic proxy methods not only
improves paleoclimate research outcomes but also paleotopographic
interpretations of geological archives (e.g. Chamberlain and
Poage, 2000; Horton et al., 2004; Horton and Chamberlain, 2006;
Sjostrom et al., 2006). As they become more widely applied, we
anticipate that similar multi-proxy research approaches will help
improve our understanding of modern hydrological responses to
rapidly changing environmental conditions.
Acknowledgments
We thank the U.S. Department of Agriculture e Forest Service
for granting TWH a special use permit (authorization ID:
LVD050047T) allowing for the collection of previously broken (i.e.
toppled) laminated carbonate deposits/tufas at Mono Lake, Inyo
National Forest, California, U.S.A. We thank Dave Eiriksson for his
University of Puget Sound senior thesis research and fieldwork on
the California laminated lacustrine carbonates, and Clinton Rissmann
and Michael Killick of Environment Southland (Invercargill,
New Zealand) for providing the water samples used in our evaporation
experiments. We would also like to thank Matt Jones for his
invitation to submit this manuscript and his contribution to the
Table 2
Stable isotope data (d13
C, d18
O, D47), D47-temperature estimates and reconstructed d18
Owater.
Sample (Age) Number of
analyses
d13
C (‰)a
d18
O (‰)a
D47 (‰)b
D47-temperature
(
C)c
d18
Owater
(‰)d
Mono Lake tufa interior (modern) 3 7.77 ± 0.06 À2.78 ± 0.04 0.734 ± 0.013 15.8 ± 4.3 À2.3 ± 0.9
Mono Lake tufa outer laminations (modern) 4 8.84 ± 0.01 À1.76 ± 0.02 0.735 ± 0.010 15.4 ± 3.3 À1.4 ± 0.7
Barstow Fm. tufa (Middle Miocene) 4 À0.50 ± 0.05 À7.16 ± 0.02 0.648 ± 0.004 48.3 ± 1.8 À0.4 ± 0.3
Bannockburn Fm. oncholite interior laminations (Middle Miocene) 4 3.50 ± 0.04 0.30 ± 0.02 0.708 ± 0.008 24.6 ± 2.8 2.6 ± 0.6
Bannockburn Fm. oncholite outer laminations (Middle Miocene) 4 9.95 ± 0.05 0.89 ± 0.04 0.721 ± 0.005 19.7 ± 1.8 2.2 ± 0.4
Bannockburn Fm. oncholite bulk homogenate (Middle Miocene) 4 3.64 ± 0.05 À0.94 ± 0.06 0.728 ± 0.012 17.7 ± 3.9 À0.1 ± 0.8
a
Relative to V-PDB.
b
On absolute reference frame.
c
Determined using the calibration of Defliese et al. (2015).
d
Relative to V-SMOW.
T.W. Horton et al. / Quaternary Science Reviews 131 (2016) 365e379376
scientific community as editor of this Water Isotope Systematics
special issue of Quaternary Science Reviews. We acknowledge
support from DOE grant DE-FG0213ER16402 and NSF grants EAR-
1352212, EAR-1325054, and EAR-0949191. The original manuscript
was improved by thoughtful and constructive reviews provided by
two anonymous referees.
Appendix A. Supplementary information
Supplementary information related to this article can be found
at http://dx.doi.org/10.1016/j.quascirev.2015.06.030.
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