Investigation of planktic foraminifera from the Norwegian-Greenland Sea

The vertical density gradients in the Nordic Seas are crucial for the preconditioning of the surface water to thermohaline sinking in winter. These gradients can be reconstructed from paired oxygen isotope data in tests of different species of planktonic foraminifera, the isotopic signatures of which represent different calcification depths in the water column. Comparison of d18O values from foraminiferal tests in plankton hauls, sediment traps, and nearby core top samples with the calculated d18Ocalcite profile of the water column revealed species-specific d18O vital effects and the role of bioturbational admixture of subfossil specimens into the surface sediment. On the basis of core top samples obtained along a west-east transect across various hydrographic regions of the Nordic Seas, d18O values of Turborotalita quinqueloba document apparent calcification depths within the pycnocline at 25-75 m water depth. The isotopic signatures of Neogloboquadrina pachyderma (s) reflect water masses near and well below the pycnocline between 70 and 250 m off Norway, where the Atlantic inflow leads to thermal stratification. Here, temperatures in the calcification depth of N. pachyderma (s) differ from sea surface temperature by approximately -2.5°C. In contrast, N. pachyderma (s) calcifies very close to the sea surface (20-50 m) in the Arctic domain of the western Nordic Seas. However, further west N. pachyderma (s) prefers somewhat deeper, more saline water at 70-130 m well below the halocline that confines the low saline East Greenland Current. This implies that the d18O values of N. pachyderma (s) do not fully reflect the freshwater proportion in surface water and that any reconstruction of past meltwater plumes based on d18O is too conservative, because it overestimates sea surface salinity. Minimum d18O differences (<0.2per mil) between N. pachyderma (s) and T. quinqueloba may serve as proxy for sea regions with dominant haline and absent thermal stratification, whereas thermal stratification leads to d18O differences of >0.4 to >1.5per mil.挪威海（Nordic Seas）中的垂直密度梯度，是冬季表层水形成温盐下沉的关键预条件因子。此类密度梯度可通过不同种类浮游有孔虫（planktonic foraminifera）壳体的配对氧同位素数据重建，其同位素特征代表了水柱中不同的钙化深度。通过对比浮游拖网、沉积物捕获器（sediment traps）及邻近岩芯表层样品（core top samples）中获取的有孔虫壳体δ¹⁸O（d18O）值，与计算得到的水柱方解石δ¹⁸O（d18Ocalcite）剖面，研究揭示了种特异性的δ¹⁸O生命效应，以及亚化石标本（subfossil specimens）经生物扰动混合进入表层沉积物的作用。基于沿挪威海不同水文区域的东西向断面采集的岩芯表层样品，五叶透压球虫（Turborotalita quinqueloba）的δ¹⁸O值记录了其在25~75 m水深的密度跃层（pycnocline）内的表观钙化深度。厚壁新方球虫（Neogloboquadrina pachyderma (s)）的同位素信号，反映了挪威近海70~250 m处密度跃层附近及下方的水团（water masses），该区域受大西洋流入流影响形成热层化（thermal stratification）。此处，厚壁新方球虫钙化深度处的水温与海表温度相差约-2.5℃。与之相反，在挪威海西部的北极海域，厚壁新方球虫的钙化位置极接近海表（20~50 m）。然而，再往西方向，厚壁新方球虫偏好栖息于盐度跃层（halocline）下方70~130 m处的高盐水体——该盐度跃层限制了低盐的东格陵兰洋流（East Greenland Current）。这表明厚壁新方球虫的δ¹⁸O值无法完全反映表层水中的淡水占比，基于δ¹⁸O开展的古融水羽流（meltwater plumes）重建结果过于保守，因其高估了海表盐度。厚壁新方球虫与五叶透压球虫之间的最小δ¹⁸O差值（<0.2‰），可作为盐度层化占主导、热层化缺失海域的代用指标（proxy）；而热层化会导致二者的δ¹⁸O差值达到>0.4‰至>1.5‰。