Physics of Aquatic Systems II

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1 Contents of Session 5 Physics of Aquatic Systems II 5. Stable Isotopes - Applications Some examples of applications Stable isotopes as markers of water origin Stable isotopes in process studies Stable isotopes as paleoclimate proxies Literat.: Mook Vol. 3, ch. 2,4; Vol. 4, ch. 5 Werner Aeschbach-Hertig Institute of Environmental Physics University of Heidelberg Stable isotopes as "colour": Trace water masses marked with certain isotopic signature Quantify mixing between different water masses Hydrograph separation Mixing between rivers Cross-sections through Rhine: Slow lateral mixing with tributaries Why do tributaries have higher δ 18 O than the Rhine? Altitude effect! (lower catchment) 3 4 Mixing of river water with groundwater ( 1 f ) δ = fδ + δ m r g Groundwater Hydrograph separation Separation of peak in discharge into fresh rain water and groundwater River water from Siegenthaler, 1997, In: Lectures in Isotope Hydrology 5 from Fitts,

2 Hydrograph separation using stable istopes Hydrograph separation using stable istopes Result: Prestorm groundwater contributes strongly to storm discharge "Push effect" Qtotal = Qgroundwater + Qrain δt δ r Qg = Qt Qt δt = Qg δg + Qr δ r δ g δ r 7 from Siegenthaler, 1997, In: Lectures in Isotope Hydrology 8 Components of runoff 2. Stable isotopes as process tracers: Identify processes that affect the isotopic signature Trace isotope fractionating processes Trace transport and exchange processes from Kendall and McDonnell, Origin of saline groundwaters in arid regions What is the origin of the high salinity of samples 56, 57, 59? Lake groundwater interaction and dating Delay and damping of seasonal variations in groundwater from Gonfiantini et al., In: Isotope techniques in groundwater hydrology 1974, IAEA Evaporation! Evaporation lines have low (< 8) slope in δd-δ 18 O plots 11 from Stichler & Moser, In: Isotope Hydrology 1979, IAEA 12

Reconstruct paleoprecipitation (amount effect) Archives: A) Stored precipitation Ice (polar

cores Polar ice sheets and alpine glaciers are archives of precipitation Stable isotopes (δ

flow models Records of > 400 kyr obtained from > 3000 m cores in Antarctica warm cold Raw

, Nature 1999, 399:429-436 15 Dating by layer counting (upper part) and by ice flow model

3 Water- Rock Interaction 3. Stable isotopes as paleoclimate proxies: Reconstruct paleotemperature (temperature effect) Reconstruct paleoprecipitation (amount effect) Archives: A) Stored precipitation Ice (polar ice sheets, glaciers) Groundwater B) Carbonates (Oxygen in CO 3 ) Carbonate shells of foraminifera etc. in sediments Stalagmites, corals Example 1: Ice Cores Stable isotopes in polar ice cores Polar ice sheets and alpine glaciers are archives of precipitation Stable isotopes (δ 18 O, δ 2 H) in ice are proxies for temperature Dating is possible by layer counting and flow models Records of > 400 kyr obtained from > 3000 m cores in Antarctica warm cold Raw data from Vostok (Antarctica), Petit et al., Nature 1999, 399: Dating by layer counting (upper part) and by ice flow model (lower part) (from Broecker, 1995) 16 Calibration of δ 18 O Thermometer in Ice How Cold was the Ice-Age in Greenland? Stable Isotopes δ 18 O ~ 8 Slope: 0.67 / C T ~ 12 C Borehole Temperatures T ~ 23 C Mismatch by factor of 2! T had been underestimated 17 Dahl-Jensen et al., 1998, Science 282:

Slopes of δ-t relationship Spatial (latitudinal) slope (temperature effect): Annual means, same time, different places Temporal (seasonal) slope

different times, same place Example 2: Groundwater Aquifers are archives of precipitation Dating by 14 C on DIC Records of > 30 kyr obtained from many

Parameter: Ice volume (and temperature) Proxy: δ 18 O in carbonate (CaCO 3 ) shells of foraminifera Archive: Forams in ocean sediments Dating: 14 C,

4 Slopes of δ-t relationship Spatial (latitudinal) slope (temperature effect): Annual means, same time, different places Temporal (seasonal) slope (seasonal effect): Monthly means, different times, same place Needed for palaeotemperature reconstruction: Longterm temporal slope Annual means, different times, same place Example 2: Groundwater Aquifers are archives of precipitation Dating by 14 C on DIC Records of > 30 kyr obtained from many aquifers Stable isotopes in palaeogroundwaters Stable isotopes in palaeogroundwaters Example 3: The Oceanic δ 18 O Record Climate Parameter: Ice volume (and temperature) Proxy: δ 18 O in carbonate (CaCO 3 ) shells of foraminifera Archive: Forams in ocean sediments Dating: 14 C, indicators, orbital tuning The Oceanic δ 18 O Record and Marine Isotope Stages warm less ice cold more ice 23 from Broecker, 1995: The Glacial World According to Wally 24

The oceanic δ 18 O record and marine isotope stages Ice sheets of the last glacial maximum (LGM) and today 25 26 Stable Isotopes in Ice, Ocean and Foraminifera Present day: Ocean: 97

less water Ice: 5 % of water on Earth, δ 18 O -40 Ocean: ~ 94 % of water, δ 18 O enriched to ~ 0.03 40 = 1.

$isotope fractionation 27 Stable Isotopes in Ocean Sediments δ 18 O record from carbonate shells of benthic foraminifera Temperature effect: T-dependence of water/carbonate$ stable isotopes in precipitation Temperature effect: No strict, physical relationship Continental and altitude effects due to progressive rainout Amount effect can be dominant in

stable isotopes in precipitation Temperature effect: No strict, physical relationship Continental and altitude effects due to progressive rainout Amount effect can be dominant in

5 The oceanic δ 18 O record and marine isotope stages Ice sheets of the last glacial maximum (LGM) and today Stable Isotopes in Ice, Ocean and Foraminifera Present day: Ocean: 97 % of water on Earth, δ 18 O = 0 (standard) Mean depth of the ocean: 3800 m Ice: 2 % of water on Earth, δ 18 O -35 Last glacial maximum (LGM): Sea level ~ 120 m lower: 120/ % less water Ice: 5 % of water on Earth, δ 18 O -40 Ocean: ~ 94 % of water, δ 18 O enriched to ~ = 1.2 Benthic (bottom dwelling) foraminifera CaCO 3 shells in isotopic equilibrium with deep sea water mainly record deep water δ 18 O ice volume Complication: T-dependence of eq. isotope fractionation 27 Stable Isotopes in Ocean Sediments δ 18 O record from carbonate shells of benthic foraminifera Temperature effect: T-dependence of water/carbonate fractionation Ice volume effect: Ice caps depleted, glacial ocean enriched in heavy isotopes from Broecker, 1995: The Glacial World According to Wally 28 Summary Factors influencing stable isotopes in precipitation Temperature effect: No strict, physical relationship Continental and altitude effects due to progressive rainout Amount effect can be dominant in tropics Applications of stable isotopes Markers: Decompose mixtures, e.g. hydrograph separation Process studies: Evaporation, water exchange & flow, WRI, Paleoclimate reconstruction: Ice cores, Groundwater: Isotope/temperature relationship? Ocean sediments: Mainly records of ice volume 29

XII. Heidelberger Graduiertenkurse Physik. Climate Research. Werner Aeschbach-Hertig Pier-Luigi Vidale

XII. Heidelberger Graduiertenkurse Physik Climate Research Werner Aeschbach-Hertig Pier-Luigi Vidale Part 1: Paleoclimate Session 1 (Tuesday): Introduction and motivation Basics of the Earth's climate