Frederiksen Jesper Allan, Klaebe Robert Matthew, Farkaš Juraj, Swart Peter Koenraad, Frei Robert
Department of Geosciences and Natural Resource Management, University of Copenhagen, Denmark.
Department of Earth Sciences, The University of Adelaide, SA 5005, Australia.
Sci Total Environ. 2022 Feb 1;806(Pt 2):150565. doi: 10.1016/j.scitotenv.2021.150565. Epub 2021 Sep 25.
The distribution of cadmium (Cd) within the oceans strongly suggests that it is used as a nutrient by marine phytoplankton. Biologically induced removal of Cd from modern surface waters is accompanied by an isotopic fractionation leaving surface-waters enriched in isotopically heavy Cd. This first study focusses on tying the Cd isotopic record preserved in modern shallow platform carbonates of the Great Bahama Bank (GBB) to conditions in the upper water column, and provides a base for future studies aiming at reconstructing past bioproductivity levels in ancient ocean/basin surface waters. In addition, we compare δCd values with previously published chromium (Cr) isotope values and link signals of bioproductivity with redox conditions in the surface waters. The GBB core samples yield [Cd] (21-188 μg/kg), which increases with depth alongside changes in carbonate mineralogy related to sediment supply and diagenesis. The δCd values of these carbonates are mainly positively fractionated with an average of 0.11‰ ± 0.17 (2σ; n = 17) relative to the NIST 3108 reference standard. Unlike previously observed for Cr isotopes, there is no control of δCd values by relative abundances of the carbonate polymorphs aragonite and calcite in the studied profile. Likewise, δCd values are not correlated to major and trace element (e.g. Ca, Mg, Mn and Sr) contents. We postulate that the burial and diagenetic processes of carbonate cannot modify the Cd isotope signals. Using the experimental fractionation factor for Cd into calcite (-0.45‰), calculated seawater δCd of +0.56 ± 0.17‰ is in agreement with values for modern North Atlantic Surface Seawater. This study's results suggest that δCd values in carbonates are a reliable tool for reconstruction of bioproductivity levels in past surface seawaters, and open new possibilities in combination with Cr isotopes to link these with past ocean redox.
海洋中镉（Cd）的分布强烈表明它被海洋浮游植物用作一种营养物质。生物诱导从现代表层水体中去除镉伴随着同位素分馏，使得表层水体中富含同位素较重的镉。这项首次研究聚焦于将保存在大巴哈马浅滩（GBB）现代浅海台地碳酸盐中的镉同位素记录与上层水柱的条件联系起来，并为未来旨在重建古代海洋/盆地表层水体过去生物生产力水平的研究提供了基础。此外，我们将δCd值与先前发表的铬（Cr）同位素值进行比较，并将生物生产力信号与表层水体中的氧化还原条件联系起来。GBB岩芯样本的[Cd]含量（21 - 188μg/kg）随深度增加，同时伴随着与沉积物供应和成岩作用相关的碳酸盐矿物学变化。相对于NIST 3108参考标准，这些碳酸盐的δCd值主要呈正分馏，平均值为0.11‰±0.17（2σ；n = 17）。与先前观察到的铬同位素不同，在所研究的剖面中，文石和方解石这两种碳酸盐多晶型的相对丰度对δCd值没有控制作用。同样，δCd值与主量和微量元素（如Ca、Mg、Mn和Sr）含量也没有相关性。我们推测碳酸盐的埋藏和成岩过程不会改变镉同位素信号。利用镉进入方解石的实验分馏系数（-0.45‰），计算得出的海水δCd值为+0.56±0.17‰，与现代北大西洋表层海水的值一致。这项研究的结果表明，碳酸盐中的δCd值是重建过去表层海水生物生产力水平的可靠工具，并与铬同位素相结合为将这些与过去海洋氧化还原联系起来开辟了新的可能性。
