Continental margin sediments have been identified as the dominant sink in the marine budget of cadmium (Cd). The isotopic composition of this important output flux is, however, unknown. Here we present, with measurements on the Argentine continental margin, the first observational constraints on the isotopic composition of Cd in modern marine oxic and sub-oxic sediments. We identify two main removal mechanisms of Cd; in organic material, and by sulfide formation. Surface margin sediments (0–0.5 cm), with dissolved O₂ below detection from ∼0.5 cm, are isotopically lighter than overlying oxygenated waters. A mass balance for these surface sediments indicates that Cd is present dominantly as organically-bound particulate Cd. In sub-surface sediments, Cd concentrations increase in the zone of nitrate reduction, and attain similar isotopic compositions as the water that overlies the sediment (i.e. ∼0.35‰ in deep waters). These observations are consistent with a downward diffusive flux of seawater Cd and redox-driven quantitative removal of that Cd during sulfide precipitation. In combination, these two routes of Cd removal lead to burial of isotopically light organic Cd in margin sub-oxic sediments that enables the global isotopic Cd budget to be balanced.

大陆边缘沉积物已被确定为镉（Cd）海洋预算中的主要汇。但是，这个重要的输出通量的同位素组成是未知的。在这里，我们通过对阿根廷大陆边缘的测量，提出了对现代海洋含氧和亚含氧沉积物中Cd同位素组成的第一个观测约束。我们确定了两种主要的Cd去除机理：在有机物质中，并通过硫化物形成。表面边缘沉积物（0-0.5厘米），溶解有O ₂低于0.5 cm处的同位素比上方的含氧水轻。这些表面沉积物的质量平衡表明，Cd主要以有机结合的颗粒Cd的形式存在。在地下沉积物中，Cd的浓度在硝酸盐还原区增加，并获得与覆盖在沉积物中的水相似的同位素组成（即深水约为0.35‰）。这些观察结果与海水Cd的向下扩散通量和硫化物沉淀过程中氧化还原驱动的Cd定量去除相一致。结合起来，这两种去除Cd的途径导致在边缘亚含氧沉积物中掩埋同位素轻的有机Cd，从而使全球同位素Cd预算得以平衡。