Calcium carbonate, biogenic opal, and organic carbon content and d15N ratios of sediments from the Bering Sea

Millennial-scale paleoceanographic changes in the Bering Sea during the last 71 kyrs were reconstructed using geochemical and isotope proxies (biogenic opal, CaCO3, and total organic carbon (TOC), nitrogen and carbon isotopes of sedimentary organic matters) and microfossil (radiolaria and foraminifera) data from two cores (PC23A and PC24A) which were collected from the northern continental slope area at intermediate water depths. Biogenic opal and TOC contents were generally high with high sedimentation rates during the last deglaciation. Laminated sediment depositions during the Early-Holocene (EH) and Bølling-Allerød (BA) were closely related with the increased primary productivity recorded by high biogenic opal and TOC contents and high d15N values. Enhanced surface-water productivity was attributed to increased nutrient supply from strengthened Bering Slope Current (BSC) and from increased amount of glacial melt-water, resulting in high C/N ratios and low d13C values, and high proportion of Rhizoplegma boreale during the last deglaciation. In contrast, low surface-water productivity during the last glacial period was due to depleted nutrient supply caused by strong stratification and to restricted phytoplankton bloom by extensive sea ice distribution under cold climates. Extensive formation of sea ice produces more oxygen-rich intermediate-water, leading to oxic bottom-water conditions due to active ventilation, which favored good preservation of oxic benthic foraminifera species. Remarkable CaCO3 peaks coeval with high biogenic opal and TOC contents in both cores during MIS 3 to MIS 4 are most likely correlated with Dansgaard-Oeschger (D-O) events. High d15N and d13Corg values during D-O interstadials support increased surface-water productivity resulting from nutrients supplied mainly by intensified BSC. During the EH, BA and D-O interstadials, dominant benthic foraminifera species indicate dysoxic bottom-water conditions as a result of increased surface-water productivity and weak ventilation of intermediate-water with mitigated sea ice development caused by strengthening of the Alaskan Stream. It is of note that the bottom-water conditions and formation of intermediate-water in the Bering Sea during the last glacial period are related to the variation of dissolved oxygen concentration of the bottom-water in the northeastern Pacific and to strong ventilation of intermediate-water in the northwestern Pacific. Thus, the millennial-scale paleoceanographic events in the Bering Sea during the D-O interstadials are closely associated with the intermediate-water ventilation, ultimately leading to weakening of North Pacific Intermediate Water.