Abstract
Isotopic analyses have been performed on narrow size ranges of seven species of planktic foraminifera picked from 43 core-top locations in the Indian Ocean. Comparison of observed δ 18O for each species with calculated profiles of equilibrium δ 18O at each location yield three distinct depth habitat preferences. For surface-dwelling species, Globigerinoides ruber and Globigerina bulloides, observed oxygen isotopic compositions are in close agreement with the seasonal range of calculated δ 18O for calcite precipitated in surface waters. Observed δ 18O values for Orbulina universa, Neogloboquadrina dutertrei, and Pulleniatina obliquiloculata are consistently enriched relative to calculated δ 18O for surface conditions. In the northern Indian Ocean, test precipitation for O. universa, N. dutertrei, and P. obliquiloculata occurs over a narrow range of temperature (22°C–24°C), with no systematic relationship with sea-water σ t . In contrast, Globorotalia inflata and G. truncatulinoides observed δ 18O apparently reflect temperature and salinity variation along the 26.8 σ t isopycnal.
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