Isotopically Light Cd in Sediments Underlying Oxygen Deficient Zones
Isotopically Light Cd in Sediments Underlying Oxygen Deficient Zones
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DOI：
10.3389/feart.2021.623720
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发表时间：
2021-03
期刊：
影响因子：
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通讯作者：
Le-Tian Chen;S. Little;K. Kreissig;S. Severmann;J. McManus
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作者：
Le-Tian Chen;S. Little;K. Kreissig;S. Severmann;J. McManus
镉是海洋中的一种微量金属，部分原因是它的浓度与磷酸盐的浓度相似。然而，在缺氧区存在着偏离全球平均溶解Cd/PO4关系的情况，在那里Cd相对于磷酸盐被耗尽。这种脱钩被认为是硫化镉(CDS)在下沉有机质内还原微环境中沉淀的结果。我们研究了东北太平洋大陆边缘几个上升流地点沉积的富有机沉积物中的Cd浓度和Cd同位素组成。与地壳物质相比，这些沉积物都富集了Cd。我们计算出Cd的净累积速率在2.6至12.0×107mol/年之间，高于先前的估计，但处于最近公布的水柱Cd降水引起的海洋沉降量估计的低端。富有机沉积物中镉的同位素组成(δ=+0.02±0.14‰，n=26；2SD)轻于深海(+0.3±0.1‰)。然而，大陆边缘环境中成岩作用期间的同位素分馏作用似乎很小。因此，富有机沉积物的轻Cd同位素组成可能反映了Cd的同位素光源。浮游植物对光Cd的非定量生物吸收是向沉积物提供光Cd的一种可能方式，这意味着Cd同位素可以作为过去海洋生产力的示踪剂。然而，据预测，水柱中的镉降水会优先从水柱中隔离轻的Cd同位素，这可能会混淆Cd作为示踪剂的作用。我们还观察到与固相Fe浓度升高相关的轻Cd同位素组成，表明Fe氧化物相对Cd的清除可能对沉积物的轻Cd同位素组成有贡献。沉积物中Cd同位素轻质的多种可能来源，以及Cd在水柱中复杂的颗粒循环的证据，使Cd同位素作为古生产力替代物的直接应用受到了质疑。
Cadmium is a trace metal of interest in the ocean partly because its concentration mimics that of phosphate. However, deviations from the global mean dissolved Cd/PO4 relationship are present in oxygen deficient zones, where Cd is depleted relative to phosphate. This decoupling has been suggested to result from cadmium sulphide (CdS) precipitation in reducing microenvironments within sinking organic matter. We present Cd concentrations and Cd isotope compositions in organic-rich sediments deposited at several upwelling sites along the northeast Pacific continental margin. These sediments all have enriched Cd concentrations relative to crustal material. We calculate a net accumulation rate of Cd in margin settings of between 2.6 to 12.0 × 107 mol/yr, higher than previous estimates, but at the low end of a recently published estimate for the magnitude of the marine sink due to water column CdS precipitation. Cadmium in organic-rich sediments is isotopically light (δ 114/110CdNIST-3108 = +0.02 ± 0.14‰, n = 26; 2 SD) compared to deep seawater (+0.3 ± 0.1‰). However, isotope fractionation during diagenesis in continental margin settings appears to be small. Therefore, the light Cd isotope composition of organic-rich sediments is likely to reflect an isotopically light source of Cd. Non-quantitative biological uptake of light Cd by phytoplankton is one possible means of supplying light Cd to the sediment, which would imply that Cd isotopes could be used as a tracer of past ocean productivity. However, water column CdS precipitation is also predicted to preferentially sequester light Cd isotopes from the water column, which could obfuscate Cd as a tracer. We also observe notably light Cd isotope compositions associated with elevated solid phase Fe concentrations, suggesting that scavenging of Cd by Fe oxide phases may contribute to the light Cd isotope composition of sediments. These multiple possible sources of isotopically light Cd to sediments, along with evidence for complex particle cycling of Cd in the water column, bring into question the straightforward application of Cd isotopes as a paleoproductivity proxy.