Stable carbon and oxygen isotope ratios of foraminifera from Holocene sediments of the North Atlantic

The Denmark Strait Overflow (DSO) today compensates for the northward flowing Norwegian and Irminger branches of the North Atlantic Current that drive the Nordic heat pump. During the Last Glacial Maximum (LGM), ice sheets constricted the Denmark Strait aperture in addition to ice eustatic/isostatic effects which reduced its depth (today ~630 m) by ~130 m. These factors, combined with a reduced north-south density gradient of the water-masses, are expected to have restricted or even reversed the LGM DSO intensity. To better constrain these boundary conditions, we present a first reconstruction of the glacial DSO, using four new and four published epibenthic and planktic stable-isotope records from sites to the north and south of the Denmark Strait. The spatial and temporal distribution of epibenthic delta18O and delta13C maxima reveals a north-south density gradient at intermediate water depths from sigma0 ~28.7 to 28.4/28.1 and suggests that dense and highly ventilated water was convected in the Nordic Seas during the LGM. However, extremely high epibenthic delta13C values on top of the Mid-Atlantic Ridge document a further convection cell of Glacial North Atlantic Intermediate Water to the south of Iceland, which, however, was marked by much lower density (sigma0 ~28.1). The north-south gradient of water density possibly implied that the glacial DSO was directed to the south like today and fed Glacial North Atlantic Deep Water that has underthrusted the Glacial North Atlantic Intermediate Water in the Irminger Basin.

丹麦海峡溢流（Denmark Strait Overflow, DSO）如今可平衡驱动北欧热泵的北大西洋洋流北向分支——挪威洋流与伊尔明厄洋流——所输送的流量。末次盛冰期（Last Glacial Maximum, LGM）时，除冰川海面升降与均衡效应使海峡现今约630米的水深降低约130米外，冰盖还收缩了丹麦海峡的通道孔径。上述因素叠加水团南北向密度梯度的减弱，被认为会限制甚至逆转末次盛冰期丹麦海峡溢流的强度。为更好地限定此类边界条件，本研究借助丹麦海峡南北两侧站位的4条新获得与4条已发表的底栖与浮游有孔虫稳定同位素记录，首次重建了冰期丹麦海峡溢流序列。底栖有孔虫δ¹⁸O与δ¹³C最大值的时空分布显示，中等水深范围内存在σ₀（势密度）约28.7至28.4/28.1的南北向密度梯度，表明末次盛冰期北欧海域存在高密度、强通风水体的对流过程。不过，大西洋中脊顶部的极高底栖δ¹³C值表明，冰岛以南还存在一个冰期北大西洋中层水的对流单元，但其密度极低（σ₀≈28.1）。水体密度的南北向梯度或许表明，冰期丹麦海峡溢流与现今一样向南流动，并为伊尔明厄海盆中俯冲到冰期北大西洋中层水之下的冰期北大西洋深层水提供了物质补给。