Water Mass Conversion in the Glacial Subarctic Pacific (54°N, 148°W): Physical Constraints and the Benthic‐Planktonic Stable Isotope Record

Abstract

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 δ18O records are 1.1–1.3‰, less than half the amplitude observed at core sites at similar latitudes in the North Atlantic; these data imply that a strong, negative δw 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) δ18O and δ13C gradients of >1‰, which suggest that a thermohaline link between Pacific deep waters and the Subarctic Pacific mixed layer did not exist during the late glacial. Epibenthic δ13C 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.