THE OXYGEN-ISOTOPE COMPOSITION OF TROPICAL OCEAN SURFACE-WATER DURING THE LAST DEGLACIATION

Evidence of abrupt climate change in each of the tropical oceans can be found in high-deposition rate sediments from the continental margins. New AMS C-14-dated sediments from the Arabian Sea reveal a two-step depletion of surface-ocean O-18/O-16 during the last deglaciation, with a return to more enriched values approximately 10.5 ka BP. The two-step depletion can be correlated among isotope records derived from foraminifer calcite in the Arabian Sea, the Gulf of Mexico, the Sulu Sea and Gulf of California, confirming the existence of the two-step depletion and deltaO-18-enriched event at 10.5 ka BP in at least three tropical sites outside the North Atlantic. To determine whether the isotopic enrichment at 10.5 ka BP was caused by local cooling of the surface ocean, we estimated sea surface temperatures over this interval using quantitative paleo-ecological techniques. There is no evidence for cooling in the Arabian Sea, Sulu Sea or Gulf of Mexico, consistent with the hypothesis that the enrichment at 10.5 ka BP is related to a change in the isotopic composition of the tropical surface ocean and not change in the temperature of calcification. We conclude that the cooling at higher latitudes that occurred at 10.5 ka BP did not extend southward to the tropical ocean. More cores from high-sedimentation rate sites in the tropics are needed to identify the cause and extent of the deltaO-18 signal, however these preliminary results are consistent with two meltwater pulses during the deglaciation that distributed isotopically depleted water into each of the tropical oceans, with a slow-down in meltwater flux at 10.5 ka BP. The most likely transport route for the isotopically depleted water from the melting northern hemisphere ice sheets to these isolated tropical ocean basins is via the relatively fast circulation of the atmosphere, although other pathways are possible.