Geochemistry on the Earth’s surface for analytical geochemists 2c. Geochemistry of the hydrosphere Tento učební materiál vznikl v rámci projektu Rozvoj doktorského studia chemie č. CZ.02.2.69/0.0/0.0/16_018/0002593 Geochemistry of the hydrosphere A. Hydrogeochemistry – Properties of water – Acid-base reactions – pH, alkalinity, acidity – Redox reaction – Complexation , colloids B. Hydrosphere – Water cycle, reservoirs, flows – Ocean – Development of the hydrosphere in the geological past HYDROSPHERE Distribution of mass and heat Hydrosphere • Water on the Earth's surface (and near it) – liquid, gaseous and frozen • Reservoirs: • ocean 97.5% • freshwater 2.5% – 1.85% (74% of freshwater) permanent polar ice cap – 0.64% (98.5% of the rest) groundwater • atmosphere, surface water (streams, lakes) 0.01% Reservoirs Ground water • less than 1% of the total amount of water • 40× more than in freshwater lakes • more than 98% of unfrozen water in the hydrological cycle • Most in altitude up to 750 m • a volume equivalent to a layer of 55 m of water on the continents Ocean • 71% of the surface, average depth 3.8 km • 97.5% of water on Earth • 1370 × 106 km3 of water OCEAN Composition of ocean water • Very homogeneous • Main components: Na+, Ca2+, Mg2+, K+, Cl−, SO4 2−, HCO3 − – 99% of dissolved substances in most waters (ocean and river) • Concentrations vary by up to 10%, but ion ratios by only 1% • Total ocean water salinity between 33 and 38‰ • Minori and trace elements more variable – Especially nutrients Components of ocean water A. Conservative (Br−, Cl−, Na+, Mg2+) – The concentration relative to other conservative components is constant – Constant concentration with depth – Long residence times (> 106 years) B. Non-conservative (recycled…) – Variable concentration – Recycled by activities of organisms • Organic C, H4SiO4, Ca2+, NO3 − and PO4 3− – Adsorption on surfaces of solid particles • Sn , Mn , Pb • Division sometimes based on use by living organisms ( biolimiting , etc.) Gradient • Main gradient controlled by biological processes • Biota consume elements and produce organics near the surface • Decomposition of organic matter descending to depth • With the exception of oxygen, abundance of most substances increases with depth • Surface water warm • Deep water cold • Transition zone = thermocline – Influenced by latitude and season – Determines the use of substances by biota Adapted from Misra (2012) Salinity • Highest at the surface – evaporation • 30‰ (estuary) to 40‰ (shallow waters in arid regions) Adapted from Misra (2012 ByPlumbago-Ownwork,CCBY-SA3.0, https://commons.wikimedia.org/w/index.ph p?curid=23016243 Gases in water • Concentration of gases A. Given by solubility B. Controlled by metabolic processes of organisms • More than the equilibrium oxygen concentration at the water surface – Produced by photosynthesis • Oxygen minimum given by decomposition of org. matter sinking from the surface zone • O2 increase in depth via the oxygenated water input by ocean currents • There is not enough organic matter to consume all the oxygen • Carbon is consumed by photosynthesis and the production of CaCO3 shells – These dissolve again in the depth due to high concentration of CO2 • Ocean pH buffered in the 7.8–8.4 range Adapted from Misra (2012) Ocean mass balance • Previously, rivers were considered a source of dissolved salts in the ocean – Sediments show stable composition of oceans since Cambrium – At the present input, the composition would be reached in several tens of millions of years Adapted from Misra (2012 Ocean mass balance • Steady state • Why are there differences is ocean water and sea water? • Other processes changing the composition of ocean water: 1. Biological processes – Effect on recycled components (Organic C, CO2, O2, H4SiO4, Ca2+, NO3 − and PO4 3−) – Bodies, boxes, decomposition of organics 2. Interaction of underwater volcanism and seawater 3. Reaction of water with particles from the continent (silicates, especially clay minerals) – Transformation of minerals, sorption 4. Transfer of particles to the atmosphere (e.g. droplets from surf) 5. Precipitation of evaporites – Na+, Ca2+, Cl− and SO4 2− 6. Element-specific processes (e.g. nitrification/denitrification) Adapted from Misra (2012) The formation of the oceans • Based on the isotopic composition (18O) • Ocean formed about 4.3±0.1 Ga • Requirement for origin of life • Essential for the carbon cycle – protection of the planet from overheating (Venus) The origin of water • Unlikely to form by synthesis from H and O or oxidation of hydrocarbons • Most likely sources: 1. Comets 2. Watery asteroids 3. Phyllosilicates 4. "wet" planetesimals • Each has its pros and cons Comets • For – High temperature near the Sun – problém of condensation of volatile components – Existence of magma ocean – possible loss of water from accretion • Against – Isotopic composition of water in known comets (but we do not know the composition of Kuiper belt objects) – If they are representative, they represent max. 30% of water on Earth Watery asteroids • Water a significant component of some chondrites (up to 9%) • Asteroid bombardment used to be more intense • Isotopes again show that they are not the main source of water Phyllosilicates • Formation of hydrated phyllosilicates in the asteroid belt (thermodynamically stable) and then transport to Earth • Same problems as watery asteroids Wet planetesimals • Water is sorbed onto the surface of cosmic dust • Cosmic dust forms planetesimals • Accretion • Water comes directly from the creation of the Earth • Problem – there shouldn't be any "dry" asteroids – Anhydrous chondrites would have to be the decay product of metamorphosed and dehydrated bodies Chemical history • Since the formation of the oxygen atmosphere, the subsurface part of the ocean has been oxygenated – Conditions in deeper parts unclear • Throughout the Phanerozoic, the surface is well oxygenated – there are large fluctuations in depth – Sedimentation of black shales in anoxic conditions – Links to extinctions? Chemical history • Composition evolution can be estimated from the sediments • Fluid inclusions in evaporites • Nothing about Hadean • In the Archean, the composition is not dissimilar to the present one – Na+, K+, Ca2+, Mg2+ and Cl − – Carbonate sediments, but no evaporites • Enough HCO3 − , but little SO4 2− • In the Proterozoic, the content of sulfates slowly increases, the content of CO2 decreases (concurrently with the atmosphere) – An increase in the pH of ocean water • At the end of the Precambrian, conservative elements composition very similar to today – In the Cambrian, an increase in Ca2+, significant changes in recycling elements Tento učební materiál vznikl v rámci projektu Rozvoj doktorského studia chemie č. CZ.02.2.69/0.0/0.0/16_018/0002593 Resources • Images without a specified source are public domain, with a free license or copyright or used with the permission of doc. Zeman. • Images from the following publications are also used: – Appelo, C. A. J., & Postma, D. (2005). Geochemistry, groundwater and pollution : (2nd ed.). Leiden: AA Balkema publishers. – Clark, I. (2015). Groundwater Geochemistry and Isotopes. CRC Press. 442 p. ISBN 978-1-4665-9174-5 (eBook - PDF) – Misra , K. (2012). Introduction to geochemistry: principles and applications. Wiley-Blackwell. 438 p. ISBN 978-1-4443-5095-1. – Oki and Kanae 2006: available from http://www.u- tokyo.ac.jp/en/about/publications/tansei/14/science_1.html – Ryan, P. (2014). Environmental and low temperature geochemistry. John Wiley and Sons. 402 p. ISBN 978-1-4051- 8612-4 (pbk.)