Stable isotope ratios measured on carbonate tests from the Kara Sea

River discharge of Ob and Yenisei to the Kara Sea is highly variable on seasonal and interannual time scales. River water dominates the shallow bottom water near the river mouths, making it warmer and less saline but seasonally and interannually more changeable than bottom water on the deeper shelf. This hydrographic pattern shows up in measurements and modelling, and in stable isotope records (delta18O, delta13C) along the growth axis of bivalve shells and in multiple analyses of single benthic foraminiferal shells. Average isotope ratios increase, but sample-internal variability decreases with water depth and distance from river mouths. However, isotope records of bivalves and foraminifera of a sediment core from a former submarine channel of Yenisei River reveal a different pattern. The retreat of the river mouth from this site due to early Holocene sea level rise led to increasing average isotope values up core, but not to the expected decrease of the in-sample isotope variability. Southward advection of cold saline water along the palaeo-river channel probably obscured the hydrographic variability during the early Holocene. Later, when sediment filled the channel, the hydrographic variability at the core location remained low, because the shallowing proceeded synchronously with the retreat of the river mouth.

鄂毕河（Ob）与叶尼塞河（Yenisei）向喀拉海（Kara Sea）的径流量在季节和年际尺度上存在显著变化。河口附近浅水区的底层水体以河流水为主，相较于深水陆架区的底层水体，其水温更高、盐度更低，但季节与年际变化幅度更大。这一水文学格局可在实测数据、数值模拟结果，以及沿双壳类（bivalve）壳体生长轴获取的稳定同位素（stable isotope）记录（δ¹⁸O、δ¹³C），还有单颗底栖有孔虫（benthic foraminifera）壳体的多组分析中得到体现。平均同位素比值随水深增加以及距河口距离增大而升高，但样品内部的同位素变异性则随之降低。然而，取自叶尼塞河古水下河道沉积岩芯的双壳类与有孔虫同位素记录，却呈现出截然不同的格局。受早全新世（early Holocene）海平面上升影响，河口自此站位后退，导致岩芯向上（沉积年代逐渐变新的方向）的平均同位素比值持续升高，但并未出现预期的样品内部同位素变异性降低现象。早全新世期间，沿古河道向南平流的冷盐水团大概率掩盖了当时的水文学变异性。后续当沉积物充填该河道后，岩芯站位处的水文学变异性始终维持在较低水平，这是因为水深变浅过程与河口后退同步进行。