布朗大學(xué)同位素地球化學(xué)Applications-in-paleoclimate-studies-1

1、Application of oxygen and hydrogen isotopes in paleoclimate studies:An introduction,Topics in this section,Basis for paleoclimate reconstructions from isotope ratios Common inorganic proxies for paleoclimate studies based on O and H isotope ratios 18O of hard fossils in marine sediment, coral (annua

2、l bands) 18O andD of ice cores,Objectives,Understand the basic principles of paleoclimate reconstruction using stable isotopes A general understanding of different (inorganic) approaches for paleoclimate reconstructions,Isotopic fractionation factor =f(T) Equilibrium isotope fractionation is strongl

3、y dependent on the temperature of the reaction, because it is a thermodynamic reaction Empirical relationship (overleaf of the text) 1000lnx-ydx-dy=a106/T2 + b103/T + c,Basis I: temperature dependency of equilibrium isotope fractionation,Two phases, one reservoir,When two phases, x and y attained eq

4、uilibrium with a common oxygen reservoir (e.g., H2O) at a specific temperature, the differences in their 18O values is a function of temperature, you also have 1000lnx-y=A*106/T2+Bx-y So, we can determine temperature of equilibrium from isotopic measurement of the two phases preserved in the sedimen

5、ts,The isotope exchange reactions must have reached isotopic equilibrium The isotopic compositions were not altered after the establishment of equilibrium =f(T), is known from the experimental data,Three basic pre-requirements,“The estimation of the paleotemperatures of the ancient oceans by measure

6、ment of the oxygen isotope distribution between calcium carbonate and water, suggested by H.C.Urey (1947), is one of the most striking and profound achievement of modern nuclear geochemistry” Harmon Craig, 1965,Paleothermometry in the oceans,Basis II: Changes in the isotopic ratios of the source res

7、ervoir when climate changes,Temperature dependence of isotopic ratios of O and H isotopes in precipitation Effect of precipitation amount (Amount effect) Effect of aridity/evaporation on precipitation isotopic ratios Sometimes complications arise when climate changes cause both variations in fractio

8、nation factor , and the source isotopic ratios,Isotopic cycle,Isotopic exchange reaction between CaCO3 and H2O during the formation of shell carbonates, with =Rc/Rw=(18O/16O)calcite/(18O/16O)water At 25, =1.0286 Therefore, calcite is enriched in 18O by 28.6 relative to sea water at 25oC 103(-1), and

9、 the is a function of temperature (note a = 1.0306 at 15oC),Foraminifera shells,Foraminifera tests,Benthic foram Planulinoides biconcavus from Bass Strait. Source: Unknown.,Benthic foram Elphidium crispum from Bass Strait. Source: Unknown.,Benthic foram Cibicides cygnorum from Bass Strait, common in

10、 Victoria.,Planktic foram Turborotalia inflata from Bass Strait, moderately common in Victorian waters.,Foram d18Oc and T,An experimental equation determined by Epstein (1953), then modified by Craig (1965), T (oC)=16.9 - 4.2(c-w) + 0.13(c-w)2 c is 18O of calcium carbonate shells formed by the small

11、 marine animals called foraminifera.,Compare,The fractionation factor between d18O of calcite and temperature (ONeil et al., 1969) is 1000ln18Oc-w = 2.78 (106/T2) -2.89 T is in oK This equation is often used for high temperature reactions when calcite precipitates from water,Foram d18O changes in oc

12、ean sediment core,Questions?,Are observed d18O changes temperature changes or changes in d18Oseawater ? The answer is: both, same direction In the last glacial maximum, on average the ocean water d18O increased by 1.26 due to transfer of light water to ice caps For each 1oC decrease in temperature,

13、the shell is enriched relative to water by an additional 0.23 (hence, a 5oC T decrease will create 1.15 d18O enrichment in shells, similar to the source water effect,Global hydrological cycle,-30 (Greenland) to -55 (Vostok, Antarctica),0 ,Climate and isotope changes,The values of w18O Emiliani and S

14、hackleton (1974) estimated w=1.01.4 during glacial-interglacial cycles of Pleistocene epoch, which could be used to estimate temperature changes Measure 18O of both planktonic and benthic foraminifera, assuming the temperature of deep oceans remained constant. 18Obenthic record mainly variation of 1

15、8Ow of sea water. Temperature changes can be estimated from pelagic and W Alkenone paleothermometer, established by G. Eglinton et al. at University of Bristol, UK, using UK37 index.,How to differentiate temperature effect from sea water effect?,Isotopic disequilibrium Some forams produce shells tha

16、t are not in equilibrium with water (“vital effect”). 18OC is a function of T as well as other biological factors: Choose species with little vital effects Diagenesis: Mineralogical and chemical changes occur after the death of the organisms, altering d18Oc Effect of mineral composition Calcite Arag

17、onite Shells can be a combination of both minerals,Additional problems,Vital effect,Calcite and aragonite,Why are we living in “ice age”?,Other mineral thermometers in ocean sediments,Biogenic silica (Opal): diatoms, radiolarians, sponges Problems: Opal is poorly crystallized, contains organic matte

18、r and water (and OH-) Needs vigorous procedures to remove water, OH- and organic matter, in order to obtain d18O of the opal Use of chert (chemically precipitated SiO2) has been successful, however (Knauth and Epstein, 1978) more useful for longer geological time scale (continuous deposition hard to

19、 find),Other mineral thermometers in ocean sediments,Fractionation of biogenic silica d18O and water: toC = 169 - 4.1(dsi dw + 0.5) Fractionation of biogenic phosphate d18O and water toC = 111.4 -4.3(dp dw + 0.5) The slopes for carbonate (4.2), opal (4.1) and phosphate (4.3) are so similar, that pai

20、ring these minerals cannot allow precise calculation of temperature (after eliminating the dw),Carbonate, opal and phosphate,Corals as high resolution archive,Clipperton Atoll, coral d18O-El-Nino,Linsley et al, Paleoceanography 15, 322-335 (2000),Strength/Limitations,Very useful for El Nino-Southern

21、 Oscillations studies Short lived, a few hundred years Limited geographic occurrences,Ice core from polar ice caps (Antarctica, Arctic/Greenland, High mountains like Himalayas and even tropics) Hydrogen and oxygen isotopic ratios of ice water are mainly a function of temperature Ice cores can be use

22、d to measure trace gas concentrations, chemical impurities of terrestrial and marine origin, other trace compounds or isotopes, cosmogenic isotopes, extraterrestrial material, and aerosols of volcanic and anthropogenic origin In sites with high accumulation rates, isotopically enriched summer layers

23、 and depleted winter layers can be recognized,Paleoclimate records from ice core,Temperature-isotope relationship,Ice core studies,Antarctic ice sheet sites (domes),Antarctic ice drilling sites,Greenland sites,Ice core studies,Ice core studies,Ice core studies,Ice core d18O variations,Ice core information,Vostok Records,(Petit et al., 1999, Nature 399, 429-436),CO2 and CH4 in Vostok,(Petit et al., 1999, Nature 399, 429-436),Dome C, Antarctica