Using recent hydrographic data from the Indian Ocean, we have derived an empirical relation that predicts PO₄^3- concentrations (µg - atoms/l) from the concentration of dissolved O₂ and temperature (°C), where PO₄^3- = 4.82 - 2.26 log T -0.30 dissolved O₂. The ^dgr¹³C and ^dgr¹⁸O of certain species of planktonic foraminifera have been demonstrated to be functions of apparent oxygen utilization (AOU = 205.0 - 100^dgr¹³C) and temperature (calculated from the ^dgr¹⁸O paleotemperature equation). Utilizing the down-core ^dgr¹⁸O and ^dgr¹³C of planktonic foraminifera, substituted into the equation above, permits determinations of paleophosp ate concentrations for fossil oceans. In rewritten form: PO₄^3- = (43.5(16.0) - 7.43 log T) - (43.5 O₂ saturated seawater (ml/l) - 205.0 - 100 ^dgr¹³C)/145.0. A comparison of the PO₄^3- values calculated from recent forams collected from 14 sites in the Indian Ocean with recent hydrographic values showed no significant differences (M = 0.91, B = 0.28, R = 0.89).

Having measured the isotopic concentrations of ^dgr¹³C and ^dgr¹⁸O obtained from 5 species of planktonic foraminifera from two cores in the Arabian Sea, we attempted down-core paleophosphate determinations. Foraminifera were collected from three time horizons in each core, the 0, 9, and 18 thousand years before present (KYBP) isochrons as determined from ^dgr¹⁸O ice-volume curves, using a linear age model. Our results indicate that the average surface and deep water (approximate O₂ minimum depth) paleophosphate concentrations for the three isochrons are, respectively: 0.575 and 2.07 at 18 KYBP, 0.205 and 2.30 at 9 KYBP, and 0.33 and 1.92 for the Holocene. These determinations may indicate greater productivity or upwelling at 9 KYBP relative o the Holocene at 18 KYBP.

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