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Abstract: Diatom assemblages in suspended particulate matter (SPM), surface sediments, and piston core samples in the southern Bering Sea and the western subarctic Pacific were examined. The distribution of diatom assemblages in the southern Bering Sea shows not only differences in species composition between the SPM and surface sediment samples, but also preferential dissolution of different species in the upper water column or surface sediments. Furthermore, we present the downcore profiles of selected diatom taxa for four piston cores to indicate changes in paleoceanographic conditions, e.g., seasonal productivity, sea-ice and surface-water circulation, of the glacial and deglaciation periods during the late Quaternary. In particular, we propose five models for the annual cycling of the water masses in the western subarctic Pacific and the southern Bering Sea during the late Quaternary. Significant regional differences in diatom accumulation rates and taxonomic composition between the southern Bering Sea and the western subarctic Pacific reflect spatial shifts in water masses caused by the variable extent of sea-ice cover and cold low-salinity surface waters in these regions. [Copyright &y& Elsevier]

Abstract: High-resolution analyses of geochemical parameters (biogenic opal, calcium carbonate, organic carbon, and nitrogen) and microfossil assemblages (diatoms and radiolarians) on Core MD01-2412 clarified detailed paleoceanographic changes such as sea-ice cover and biological production in the southwestern Okhotsk Sea during the last 115kyr. An age model of Core MD01-2412 was established based on δ18O stratigraphy, accelerator mass spectrometer (AMS) 14C, and tephrochronology. Sea-ice history reconstructed by siliceous microplankton records indicated that the present sea-ice condition was formed during the last 8kyr. Only during Marine Isotope Stage (MIS) 2 was the duration of sea-ice cover in this region much longer than that of today (4–5 months a year). Two diatom species, Thalassionema nitzschioides and Fragilariopsis doliolus, revealed that the Soya Warm Current Water (SWCW) flowed into the Okhotsk Sea near the site of Core MD01-2412 during the last 12–14kyr and during MIS 5a, and was associated with sea-level rise. Biological productivity rapidly increased during MIS 1, associated with sea-ice retreat. Two major increases of organic carbon (OC) contents (wt%) and Corg/N ratios were observed, and the timings of these events were 15.8–16.7ka (Event 1) and 13.1–13.6ka (Event 2). Corresponding to these events, the abundance of Cycladophora davisiana, an intermediate water dwelling radiolarian species, increased. This high C. davisiana abundance can be correlated to the input of terrestrial organic matter from the submerged shelf to the intermediate water. Apart from the radiolarians, the production of diatoms in the surface waters was suppressed by the development of well-stratified surface water along with sea-ice melting during the early Holocene. Diatom production increased gradually during the last 10kyr with enhanced vertical mixing. [Copyright &y& Elsevier]

Abstract: Changes in biological productivity of the Bering Sea have been evaluated for the late Quaternary based on biogenic opal, calcium carbonate, and microfossils in two piston cores. Biological productivity increased during Marine Isotope Stage (MIS) 1 after the last glaciation. During the last deglaciation, two pronounced peaks of calcium carbonate content were observed at ca. 2400m water depth, which can be explained by the “CaCO3 compensation hypothesis” and coccolithophore blooms. Simultaneous with the CaCO3 peaks, oxygen-poor deep water was apparently distributed in the Aleutian Basin based on benthic foraminiferal assemblages. The low-O2 events seem to be related to deep-water circulation and/or elevated productivity in the northwest Pacific. After the CaCO3 peak events, biogenic opal contents and diatoms increased gradually, associated with enhanced vertical mixing with an inflow of the Alaskan Stream through the eastern Aleutian Arc passes. [Copyright &y& Elsevier]