Tracing Bering Sea Circulation With Benthic Foraminiferal Stable Isotopes During the Pleistocene

Abstract:

As the largest ocean basin, the Pacific helps to set the global climate state, since its circulation affects mean ocean properties, air-sea partitioning of carbon dioxide, and the distribution of global oceanic poleward heat transport. There is evidence that during the Last Glacial Maximum (LGM) the subarctic Pacific contained a better-ventilated, relatively fresh intermediate water mass above ~2000 m that may have formed locally. The source and spatial extent of this water mass is not known, nor do we know how formation of this water mass varied during Pleistocene glaciations with different orbital and ice sheet boundary conditions. Here we present a 0.5 My multi-species benthic stable isotope record from Site U1345 (1008 m) on the northern Bering slope and a 1.0 My record from U1339 (1868 m) from the Umnak Plateau in the southeastern basin. We find that the relatively well-ventilated low-δ¹⁸O intermediate water reaches 1000 m in the Bering Sea during MIS2 and 6, but that the hydrographic divide between this water mass and poorly-ventilated deep water was shallower than 1000 m for the previous three glaciations. We also compare Bering Sea piston core and IODP Expedition 323 Uvigerina data from the Holocene and LGM with the modern hydrography, and to previously published profiles from the Okhotsk Sea and Emperor Seamounts. We find that the carbon and oxygen stable isotope signatures of well-ventilated water in the Bering and Okhotsk Seas are distinct, suggesting that there may have been intermediate water formation in both basins during the LGM. However, interpretation of the δ¹³C data is complicated by offsets between Uvigerina species and bottom water δ¹³C of DIC, so this hypothesis will require confirmation by other proxies.