Late quaternary sea-ice and sedimentary redox conditions in the eastern Bering Sea – Implications for ventilation of the mid-depth North Pacific and an Atlantic-Pacific seesaw mechanism

On glacial-interglacial and millennial timescales, sea ice is an important player in the circulation and primary productivity of high latitude oceans, affecting regional and global biogeochemical cycling. In the modern North Pacific, brine rejection during sea-ice freezing in the Sea of Okhotsk drives the formation of North Pacific Intermediate Water (NPIW) that ventilates the North Pacific Ocean at 300 m to 1000 m water depth. Glacial intervals of the late Quaternary, however, experienced a deepening of glacial NPIW to at least 2000 m, with the strongest ventilation observed during cold stadial conditions of the last deglaciation. However, the origin of the shifts in NPIW ventilation is poorly understood. Numerical simulations suggest an atmospheric teleconnection between the North Atlantic and the North Pacific, in response to a slowdown or shutdown of the Atlantic meridional overturning circulation. This leads to a build-up of salinity in the North Pacific surface ocean, triggering deep ventilation. Alternatively, increased sea-ice formation in the North Pacific and its marginal seas may have caused strengthened overturning in response to enhanced brine rejection. Here we use a multi-proxy approach to explore sea-ice dynamics, sedimentary redox chemistry, and benthic ecology at Integrated Ocean Drilling Program Site U1343 in the eastern Bering Sea across the last 40 ka. Our results suggest that brine rejection from enhanced sea-ice formation during early Heinrich Stadial 1 locally weakened the halocline, aiding in the initiation of deep overturning. Additionally, deglacial sea-ice retreat likely contributed to increased primary productivity and expansion of mid-depth hypoxia at Site U1343 during interstadials, confirming a vital role of sea ice in the deglacial North Pacific carbon cycle

Holocene Sedimentation History in the Southern Novaya Zemlya Trench

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Late Quaternary environmental changes on the southeastern slope of the Sea of Okhotsk inferred from benthic foraminifera

Benthic foraminifera were studied in 117 sediment samples from a 1112-cm-long core obtained from the Kamchatka continental slope (52°02.514′ N, 153°05.949′ E) at a sea depth of 684 m. The section covers the last 180 ky, from marine isotopic stage (MIS) 6 to the present time. The substantial quantitative and taxonomic changes in the assemblages of benthic foraminifera reflect the climatic and paleoceanographic variations. The insignificant contents of foraminiferal tests in the sediments that accumulated during glaciations (MIS 6, MIS 5(d-a)-MIS 2) suggest a minimal organic flux to the sea bottom. During deglaciation and in the Holocene (MIS 1) and, particularly, in the interglacial optimum (MIS 5e), the organic flux to the bottom significantly increased. Sestonophagous species prevailed in the foraminiferal assemblages of glacial periods, when the production of the young Sea of Okhotsk Intermediate Water (SOIW) increased. The assemblages of warm periods (MIS 1 and 5e) are mainly composed of detritophagous species. Now, conditions favorable for these species exist in the bottom areas influenced by the old Pacific waters. During the warm interglacial optimum (MIS 5e), when the SOIW production decreased, its thickness became reduced and the boundary with the Pacific water mass substantially rose (probably by 200-400 m). During MIS 1, the decrease in the SOIW production and the rise of its lower boundary were less significant

Late Quaternary changes of the oxygen conditions in the bottom and intermediate waters on the western Kamchatka continental slope, the Sea of Okhotsk

Microfossil data on the foraminifers and radiolarians in the sediment core KOMEX LV28-44-3, the Kamchatka slope in the eastern Sea of Okhotsk, exhibit the changes in the water oxygen conditions during the last 146 ky. The paleoenvironmental proxies are the radiolarian species Cycladophora davisiana as indicator of the upper intermediate water ventilation, and the benthic/planktonic foraminifers as indicators of the bioproductivity and bottom water oxygenation. In case of sediment core LV28-44-3, the bottom water represents the lower intermediate one, so that conclusions on paleoenvironments are applicable for the most range of the local intermediate water. The well-oxygenated intermediate and near-bottom waters existed in the area of study during the penultimate glaciation of MIS 6, Early Weichselian initiation of the last glaciation within MIS 5b–d, and last glacial stages of MIS 3–2. The intervals of the short low-O2 bottom events with suboxic conditions (dissolved O2 in water 0.3–1.2 ml/l) occur during the last interglacial MIS 5e (Eemian stage) 125 to 113 ka, and during the last deglaciation 17.5 to 6.5 ka. Eemian low-O2 bottom events are associated with the high bioproductivity of the subsurface water but poor ventilation on the upper intermediate depths. The low-O2 bottom events during the last deglaciation occurred at the high bioproductivity of the subsurface water and active ventilation on the upper intermediate depths