Ground truthing the Cd/Ca-carbon isotope relationship in foraminifera of the Greenland-Iceland-Norwegian Seas

In order to examine whether the paleoceanographic nutrient proxies, d13C and cadmium/calcium in foraminiferal calcite, are well coupled to nutrients in the region of North Atlantic Deep Water formation, we present da ta from two transects of the Greenland-Iceland-Norwegian Seas. Along Transect A (74.3°N, 18.3°E to 75.0°N, 12.5°W, 15 stations), we measured phosphate and Cd concentrations of modern surface sea water. Along Transect B (64.5°N, 0.7°W to 70.4°N, 18.2°W, 14 stations) we measured Cd/Ca ratios and d13C of the planktonic foraminifera Neogloboquadrina pachyderma sinistral in core top sediments. Our results indicate that Cd and phosphate both vary with surface water mass and are well correlated along Transect A. Our planktonic foraminiferal d13C data indicate similar nutrient variation with water mass along Transect B. Our Cd/Ca data hint at the same type of nutrient variability, but interpretations are hampered by low values close to the detection limit of this technique and therefore relatively large error bars. We also measured Cd and phosphate concentrations in water depth profiles at three sites along Transect A and the d13C of the benthic foraminifera Cibicidoides wuellerstorfi along Transect B. Modern sea water depth profiles along Transect A have nutrient depletions at the surface and then constant values at depths greater than 100 meters. The d13C of planktonic and benthic foraminifera from Transect B plotted versus depth also reflect surface nutrient depletion and deep nutrient enrichment as seen at Transect A, with a small difference between intermediate and deep waters. Overall we see no evidence for decoupling of Cd/Ca ratio and d13C in foraminiferal calcite from water column nutrient concentrations along these transects in a region of North Atlantic Deep Water formation.

为探究古海洋营养盐代用指标（有孔虫方解石中的δ¹³C和镉钙比）在北大西洋深水形成区域是否与营养盐存在良好耦合关系，本研究呈现了格陵兰-冰岛-挪威海两个断面的数据。沿断面A（北纬74.3°、东经18.3°至北纬75.0°、西经12.5°，共15个站位），我们测定了现代表层海水的磷酸盐和镉浓度。沿断面B（北纬64.5°、西经0.7°至北纬70.4°、西经18.2°，共14个站位），我们测定了岩心顶部沉积物中浮游有孔虫Neogloboquadrina pachyderma sinistral的镉钙比和δ¹³C。结果表明，断面A中镉与磷酸盐浓度均随表层水团变化，且存在良好相关性。断面B中浮游有孔虫的δ¹³C数据显示，营养盐随水团变化的趋势与之相似。镉钙比数据也暗示了类似的营养盐变化特征，但由于数值偏低接近该技术的检测限，导致误差较大，从而限制了对结果的解读。此外，我们还测定了断面A三个站位的水深剖面中镉和磷酸盐浓度，以及断面B底栖有孔虫Cibicidoides wuellerstorfi的δ¹³C。断面A的现代海水深度剖面显示，表层营养盐浓度较低，而深度大于100米处的浓度保持稳定。断面B中浮游与底栖有孔虫的δ¹³C随深度变化的曲线同样反映出表层营养盐匮乏、深层富集的特征（与断面A一致），但中层与深层水体间存在微小差异。总体而言，在北大西洋深水形成区域的这两个断面中，未发现有孔虫方解石的镉钙比和δ¹³C与水体营养盐浓度存在解耦的证据。