The relationship between oxygenation, productivity and circulation in the Bering Sea during late Pleistocene glacial-interglacial cycles

Abstract:

Changes in high-latitude biological and physical oceanographic processes have the potential to affect global climate. The efficiency of the biological pump, ocean circulation, and vertical mixing are all drivers of changes in ocean-atmosphere carbon dioxide flux and the CO2 storage of the ocean. It is hypothesized that lower sea level and expanded sea ice during glacial periods caused the expansion of nutrient-poor North Pacific Intermediate Water (NPIW). This in turn led to increased nutrient utilization in surface waters and greater CO2 storage in the glacial North Pacific. However, there are few long proxy records of the expansion of glacial NPIW. In this study, we present an opal wt. % record (productivity), a nitrogen isotope record (nutrient utilization), and a high resolution bioturbation index (seafloor oxygen concentration) from IODP site U1345. This site is in the middle of the present-day oxygen minimum zone (1008 m water depth) on the northern Bering Sea continental slope. These data span approximately 162 m of sediment representing the last 550 ky. Like other records from the region, the U1345 nitrogen isotope record is relatively high during glacial periods, consistent with the hypothesis that nutrient utilization is more complete during glacials. Homogenous, well-bioturbated sediment, reflecting relatively high seafloor oxygen concentration is associated with low to medium opal weight percent, reflecting low rates of diatom production. Laminated sediment (low oxygenation) is associated with medium to high opal weight percent (high productivity). It is clear that export production does not uniquely determine seafloor oxygenation. In addition, there is no evidence of consistently higher oxygenation during glacials, which we would expect with expanded glacial NPIW. This suggests the NPIW didn’t extend to the depth of U1345, and/or a mechanism besides productivity and circulation drove preservation of laminations at this site