Isotope ratios, variations

The conclusions of Hurt s study of year-by-year oxygen isotope ratios in 72 years of S. gigantea are thus supportive of the conclusions of the CIAP study [49] that solar variations influence the abundances of many kinds of chemical species in the stratosphere, and therefore influence the.amount of solar energy they absorb and re-radiate to earth, and therefore influence the surface temperature of the earth and especially the surface temperatures of the oceans. It is the surface temperature of the oceans which produces the phenomena we have discussed the isotope ratio variations in rain and hence in tree rings, the isotope ratio variations in the Greenland ice cap, in the organic carbon and uranium concentrations in sea cores, and furthermore variations of the sea surface temperature produces variations in the carbon-14 to carbon-12 ratio fractionation at the sea air interface and hence in the carbon-14 content of atmospheric carbon dioxide and hence in the carbon-14 content of tree rings. 

Transport balance or box models have been used by many workers in the past in efforts to understand the trace element and isotope characteristics of the Earth s major silicate reservoirs, i.e. continental crust, and upper and lower mantle (e.g. Jacobsen Wasserburg 1979 Zartman Haines 1988). Although simple mass balance calculations can be applied to present-day trace element concentrations and Pb, Nd and Hf isotope compositions of major reservoirs, e.g. continental crust and depleted mantle, to test the hypothesis that these reservoirs are complementary, transport balance models are needed to test ideas on their evolution in time. The reason is that the isotope ratio variations are the result of time-integrated trace element variations in the reservoir, modified by fluxes between them. Below, recent transport balance models in which the evolution of the continental crust is examined using Th-U-Pb (Kramers T olstikhin 1997) and... 

Mearns, E. W. McBride, J. J. 1999. Hydrocarbon filling history and reservoir continuity of oil fields evaluated using Sr/ Sr isotope ratio variations in formation water, with examples from the North Sea. Petroleum Geoscience, 5, 17-27. 

Also motivated by the desire to constrain further the history of oxidation of the atmosphere and oceans, Frei et al. [37] determined the isotopic compositions of Cr in banded iron formation (BIF) samples, representing the Archean (>2.5 Ga ago) and Proterozoic (2.5 Ga to 542 Ma ago) Eons. They anticipated that isotopic variations in those rocks would correlate with changes in global redox conditions over time because of previous work reporting isotope ratio variations correlated with the Cr oxidation state in natural samples [38, 39], experiments [40, 41], and theoretical calculations [42]. 

If the interpretation of Cr isotope ratio variation by Frei et al. [37] is valid, Cr isotope ratios provide an extremely valuable complement to other geochemical proxies for paleoredox conditions, since they reflect atmospheric, rather than marine, oxygenation and can be measured in a rock type that is not suitable for applying other paleoredox proxies, such as Mo or S isotope ratios. Some critical assumptions in Frei et al. s study [37] must be verified, namely (i) that oxidative weathering of Cr(III) from rocks indeed results in an isotopically heavy pool of dissolved Cr and (ii) that reduction and coprecipitation of Cr with Fe oxyhydroxides either does not fractionate Cr isotopes or results in complete removal of Cr from the water column in continental shelf environments where BIF are deposited. Although Cr isotope ratio behavior is not yet fully understood at a fundamental level, several studies have been published that provide important insights into this system. 

Processes that result in isotope ratio variations can be divided into three general categories (1) isotope exchange reactions such that isotopes of an element are redistributed among different molecules or different phases of the same molecule at thermodynamic equilibrium (2) unidirectional processes where the reaction rates of chemical or physical reactions of isotopic species differ and (3) radiogenic decay. 

Carbon has two stable isotopes and C) of -98.89% and -1.11% abundance, respectively. Carbon compounds are exchanged between the oceans, atmosphere, biosphere and lithosphere. Significant isotope ratio variations exist within these groups due to both kinetic and equilibrium isotope effects. Representative carbon isotope abundance variations are shown in Figure 9. 

Galy, A., Pomies, C., Day, J. A., Pokrovsky, O. S., and Schott, J. (2003) High precision measurement of germanium isotope ratio variations by multiple collector-inductively coupled plasma mass spectrometry. J. Anal. At. Spectrom., 18(2), 115-9. 

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