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）现今可补偿向北流动的北大西洋流挪威分支与伊尔明厄分支，该环流驱动北欧热泵系统。末次盛冰期（Last Glacial Maximum, LGM）时，冰盖不仅收缩了丹麦海峡的通口宽度，还因冰盖引发的地壳均衡与海面升降效应，使该海峡深度较现今（约630米）降低约130米。上述因素叠加水团南北密度梯度减弱，被认为会限制甚至逆转冰期丹麦海峡溢流的强度。为更好地限定这些边界条件，我们基于丹麦海峡南北两侧站点的4条新的和4条已发表的近底栖（epibenthic）与浮游（planktic）稳定同位素记录，首次重建了冰期丹麦海峡溢流的演化序列。近底栖δ¹⁸O（delta18O）与δ¹³C（delta13C）最大值的时空分布显示，冰期中层水深处存在南北向密度梯度，势密度σ₀（sigma0）从~28.7降至28.4/28.1，表明北欧海域曾发生致密且强通风水团的对流混合。但大西洋中脊顶部极高的近底栖δ¹³C值则记录了冰岛以南另一个冰期北大西洋中层水（Glacial North Atlantic Intermediate Water）的对流单元，不过该单元的密度显著更低（σ₀~28.1）。水团的南北密度梯度或许意味着，冰期丹麦海峡溢流与现今一样向南流动，并补给了冰期北大西洋深层水（Glacial North Atlantic Deep Water）——该深层水在伊尔明厄海盆中俯冲于冰期北大西洋中层水之下。