View source for Stable oxygen isotopes in foraminifera

The fractionation of [[D18O | oxygen isotopes]] between [https://en.wikipedia.org/wiki/Water water] and [https://en.wikipedia.org/wiki/Calcium_carbonate calcium carbonate] is temperature sensitive, which prompted [https://en.wikipedia.org/wiki/Harold_Urey H.C. Urey] in 1947 <ref>Urey, H. C., Epstein, S., & McKinney, C. R. (1948). Method for measurements of paleotemperatures. Bulletin of the Geological Society of America (abstract), 59, 1359-1360. </ref> to propose that this phenomenon could be used as a [https://en.wikipedia.org/wiki/Paleothermometer paleothermometer]. 

The oxygen isotopic composition of a carbonate samples is measured by [https://en.wikipedia.org/wiki/Mass_spectrometry mass spectrometry] using CO<sub>2</sub> gas released when carbonate samples are treated with 100% [https://en.wikipedia.org/wiki/Phosphoric_acid phosphoric acid] in an evacuated vessel at a controlled temperature, typically 90°C <ref> McCrea, J. M. (1950). On the isotopic chemistry of carbonates and a paleotemperature scale. Journal of Chemical Physics, 18, 849-857.</ref>. The stable oxygen isotope rations are usually expressed using the [[Isotope | &delta;-notation]] referenced to the Vienna Pee Dee Belemnite (VPDB). 

The temperature dependence of the fractionation had been determined by several groups of investigators and has been expressed in different forms. The classic temperature equation derived by Epstein et al. (1953) ,<ref>Epstein, S., Buchsbaum, R., Lowenstam, H. A., & Urey, H. C. (1953). Revised carbonate-water isotopic temperature scale. Geological Society of America Bulletin, 64, 1315-1326. 
</ref> takes the form:

<math> T(^\circ C)=16.9-4.2(\delta^{18}O_{c}-\delta^{18}O_{sw(VSMOW)})+0.13(\delta^{18}O_{c}-\delta^{18}O_{sw(VSMOW)})^2</math>

where &delta;<sup>18</sup>O<sub>sw</sub> represents the stable oxygen isotopic composition of seawater in which the carbonate [https://en.wikipedia.org/wiki/Precipitation_(chemistry) precipitated]. 

However, paleothermometry based on the temperature dependence of the fractionation of oxygen isotopes between marine carbonates and water must satisfy a set of assumptions concerning: (1) the existence of isotopic equilibrium between oxygen in the water and in biogenic calcite of the presence of "vital effects", (2) the preservation of the isotope composition of oxygen in the solid carbonate shell as subsequent [https://en.wikipedia.org/wiki/Dissolution_(chemistry) dissolution] and/or replacement by calcium carbonate that equilibrated at a different temperature with water having a different isotopic composition may occur, (3) the presence of crystals of [https://en.wikipedia.org/wiki/Diagenesis diagenetic] calcite in foraminiferal shells, whose oxygen isotope composition is difference from that of the shell, and (4) the constancy of the isotope composition of water in the oceans <ref name="sharp2007"> Sharp, Z. (2007). Principles of stable isotope geochemistry. Upper Saddle River, New Jersey: Pearson Prentice Hall. </ref>. 

Violating these assumptions can limit the use &delta;<sup>18</sup>O of [https://en.wikipedia.org/wiki/Biogenic_substance biogenic] calcite as a true paleothermometer. Planktonic foraminifera tend to secrete their shells in isotopic equilibrium with seawater. However, divergence from equilibrium, termed "vital effects", has been observed and is related to [https://en.wikipedia.org/wiki/Environment_(biophysical) environmental] factors such as sunlight intensity, temperature stress, and nutrient supply <ref name= "sharp2007" />. This had led to species-specific calibration equation <ref name="bemis1998"> Bemis, B. E., Spero, H. J., Bijma, J., & Lea, D. W. (1998). Reevaluation of the oxygen isotopic composition of planktonic foraminifera: Experimental results and revised paleotemperature equations. Paleoceanography, 13(2), 150-160. </ref>. 

Oxygen isotope rations of biogenic calcite can be changed by two diagenetic processes: (1) the addition of new carbonate by cementation, and (2) the dissolution of unstable carbonate and the re-precipitation of a stable mineral. Great care is taken to avoid the effects of diagenesis, but there is no foolproof method to prove that no diagenesis has taken place. 

The stable oxygen isotopic composition of surface waters is influenced by [https://en.wikipedia.org/wiki/Evaporation evaporation], [https://en.wikipedia.org/wiki/Precipitation precipitation], [https://en.wikipedia.org/wiki/Mixing_(physics) mixing] and [https://en.wikipedia.org/wiki/River river] runoff. These processes also control [https://en.wikipedia.org/wiki/Salinity sea surface salinity], and therefore, there is a strong linear relationship between local salinity and &delta;<sup>18</sup>O<sub>sw</sub> <ref> LeGrande, A. N., & Schmidt, G. A. (2006). Global gridded data set of the oxygen isotopic composition in seawater. Geophysical Research Letters, 33, L12604. doi:10.1029/2006GL026011 </ref>. On longer time scales, the &delta;<sup>18</sup>O<sub>sw</sub> of the entire ocean is also controlled by ice volume changes that accompany the glacial-interglacial cycles. During glacials, the &delta;<sup>18</sup>O<sub>sw</sub> increases when isotopically light water is transferred from the ocean to the continental ice sheets. Therefore, an estimation of paleo-salinity and ice volume changes must be made in order to use &delta;<sup>18</sup>O<sub>calcite</sub> as a paleothermometer. Conversely, if the temperature at which the biogenic calcite precipitated can be estimated through other means, then the &delta;<sup>18</sup>O<sub>calcite</sub> can be used to infer past changes in &delta;<sup>18</sup>O<sub>sw</sub>.

==References==
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