Attainment of isotopic equilibrium between ocean water and the benthonic foraminifera genus Uvigerina: isotopic changes in the ocean during the last glacial

It is shown that even when the oxygen isotopic composition of foraminiferal tests is examined using non-standard analytical methods, the measurements may be expressed in terms of the PDB standard. In order to investigate departure from isotopic equilibrium, comparison must be made not with an empirical relationship determined for molluscs in a limited temperature range, but with a relationship based on inorganic calcite precipitation in the temperature range of interest. By such a comparison it is found that Uvigerina deposits its test at or near isotopic equilibrium in the temperature range 0.8 °G to 7 °C. Values for the isotopic composition of tests of this genus which lived during the last glacial can only be explained in terms of a change in the mean oxygen isotopic composition of the oceans which probably exceeded 1 %elox about 20 000 years, and exceeded 0.5%« for about 70 000 years.

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