ALBERT

Description: Benthic (Uvigerina spp., Cibicidoides spp., Gyroidinoides spp.) and planktonic (N. pachyderma sinistral, G. bulloides) stable isotope records from three core sites in the central Gulf of Alaska are used to infer mixed-layer and deepwater properties of the late glacial Subarctic Pacific. Glacial-interglacial amplitudes of the planktonic delta18O records are 1.1-1.3 per mil, less than half the amplitude observed at core sites at similar latitudes in the North Atlantic; these data imply that a strong, negative deltaw anomaly existed in the glacial Subarctic mixed layer during the summer, which points to a much stronger low-salinity anomaly than exists today. If true, the upper water column in the North Pacific would have been statically more stable than today, thus suppressing convection even more efficiently. This scenario is further supported by vertical (i.e., planktic versus benthic) delta18O and delta13C gradients of 〉1 per mil, which suggest that a thermohaline link between Pacific deep waters and the Subarctic Pacific mixed layer did not exist during the late glacial. Epibenthic delta13C in the Subarctic Pacific is more negative than at tropical-subtropical Pacific sites but similar to that recorded at Southern Ocean sites, suggesting ventilation of the deep central Pacific from mid-latitude sources, e.g., from the Sea of Japan and Sea of Okhotsk. Still, convection to intermediate depths could have occurred in the Subarctic during the winter months when heat loss to the atmosphere, sea ice formation, and wind-driven upwelling of saline deep waters would have been most intense. This would be beyond the grasp of our planktonic records which only document mixed-layer temperature-salinity fields extant during the warmer seasons. Also we do not have benthic isotope records from true intermediate water depths of the Subarctic Pacific.

Description: Stable isotope records of coexisting benthic foraminifers Uvigerina spp. and Cibicidoides spp. and planktonic G. ruber (white variety) from Site 724 are used to study the late Pleistocene evolution of surface and intermediate water hydrography (593 m water depth) at the Oman Margin. Glacial-interglacial d18O amplitudes recorded by the benthic foraminifers are reduced when compared to the estimated mean ocean changes of d18Oseawater . Epibenthic d13C remains at its modern level or is increased during glacial times. This implies that Red Sea outflow waters which are enriched in d18Oseawater and d13C (Sum CO2) have been replaced during glacial periods by intermediate waters still positive in d13C (Sum CO2) but more negative in d18Oseawater. Glacial-interglacial amplitudes of the planktonic d18O record exceed those of the mean ocean d18Oseawater variation and imply decreased surface water temperatures (SST) during glacial times. Throughout most of the records these cooling events correlate with enhanced rates of carbon accumulation. However, both negative (colder) SST and positive Corg accumulation rate anomalies do not correlate with potential physical upwelling maxima as inferred from the orbital monsoon index. This is in conflict with the established hypothesis that upwelling in the estern Arabia Sea should be strongest during maxima of the southwest monsoon.