Stable oxygen isotopes in foraminifera

The fractionation of oxygen isotopes between water and calcium carbonate is temperature sensitive, which prompted H.C. Urey in 1947 to propose that this phenomenon could be used as a paleothermometer.

The oxygen isotopic composition of a carbonate samples is measured by mass spectrometry using CO₂ gas released when carbonate samples are treated with 100% phosphoric acid in an evacuated vessel at a controlled temperature, typically 90°C . The stable oxygen isotope rations are usually expressed using the δ-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) , takes the form:

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

where δ¹⁸O_sw represents the stable oxygen isotopic composition of seawater in which the carbonate 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 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 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 .

Violating these assumptions can limit the use δ¹⁸O of 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 environmental factors such as sunlight intensity, temperature stress, and nutrient supply . This had led to species-specific calibration equation .

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 evaporation, precipitation, mixing and river runoff. These processes also control sea surface salinity, and therefore, there is a strong linear relationship between local salinity and δ¹⁸O_sw . On longer time scales, the δ¹⁸O_sw of the entire ocean is also controlled by ice volume changes that accompany the glacial-interglacial cycles. During glacials, the δ¹⁸O_sw 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 δ¹⁸O_calcite as a paleothermometer. Conversely, if the temperature at which the biogenic calcite precipitated can be estimated through other means, then the δ¹⁸O_calcite can be used to infer past changes in δ¹⁸O_sw.