Hydrochemistry measured on water bottle samples during POLARSTERN cruise ANT-VIII/2@en

On a section between 72°S and 42°S and a transect between 60°E and 10°E through the Weddell Sea and the southernmost eastern Atlantic Ocean, the water column was sampled on 72 stations, and the stable carbon isotopic composition of total dissolved inorganic carbon (delta13C(SumCO2)) as well as the stable oxygen isotopic composition of seawater (delta18O) was determined. These data were compared with potential temperature, salinity, dissolved oxygen and phosphate data from the same stations. The observed delta13C(SumCO2)/PO4[3-] relationship in the deep Weddell Sea strongly differs from the global Redfield-driven deep water relationship. We attribute this to enhanced thermodynamic fractionation at sites of bottom water formation that decouples the nutrient signal from the delta13C(SumCO2) signal not only in surface and intermediate water masses but also in deep and bottom water. Different, water-mass specific thermodynamic imprints due to different modes of bottom water formation are assumed to cause the observed deviation from the global delta13C(SumCO2)/PO4[3-] relationship in the deep Weddell Sea. The influence of increased photosynthetic fractionation, i.e., a more negative than low-latitude isotopic organic carbon composition, is shown to be minor. As a result, Recent Weddell Sea deep and bottom water delta13C(SumCO2) is by 0.4-0.5 per mil higher than expected if solely biologic fractionation would occur. A discussion of simple hypotheses of Weddell Sea deep and bottom water formation during glacial times reveals that regardless of what scenario is considered, the thermodynamic imprint on Southern Ocean deep water would increase. This makes it difficult to explain low glacial delta13C values observed in benthic foraminifera from the subpolar Southern Ocean as being calcified in Antarctic source bottom water and thus is in support of hypotheses looking for additional sites of deep water formation.本数据集针对南纬72°至42°区间、东经60°至10°横贯威德尔海（Weddell Sea）与东大西洋最南部海域的水体柱样，共布设72个采样站位，测定了总溶解无机碳的稳定碳同位素组成（δ¹³C(SumCO₂)）以及海水的稳定氧同位素组成（δ¹⁸O）。将上述同位素数据与同站位的位势温度、盐度、溶解氧及磷酸盐浓度数据进行对比分析。威德尔海深部观测到的δ¹³C(SumCO₂)/PO₄³⁻关系，与全球雷德菲尔德（Redfield）驱动的深层水关系存在显著差异。我们将该差异归因于底层水形成区域增强的热力学分馏作用：该过程不仅在表层与中层水团中，同时也在深层与底层水团中，将营养盐信号与δ¹³C(SumCO₂)信号解耦。不同底层水形成模式所带来的、具有水团特异性的热力学印记，被认为是造成威德尔海深部δ¹³C(SumCO₂)/PO₄³⁻关系偏离全球模式的原因。增强的光合分馏作用（即有机碳同位素组成较低纬度海域更偏负）的影响被证明较为微弱。因此，若仅发生生物分馏作用，现代威德尔海深层与底层水的δ¹³C(SumCO₂)值将比预期高出0.4‰~0.5‰。针对冰期时期威德尔海深层与底层水形成的若干简单假说进行讨论后发现，无论采用何种情景，南大洋深层水所受的热力学印记均会增强。这使得难以将亚极地南大洋底栖有孔虫（benthic foraminifera）中观测到的冰期低δ¹³C值，解释为在南极源底层水中钙化形成的结果，因此该结论支持了寻找额外深层水形成位点的相关假说。