Chromium reduction and associated stable isotope fractionation restricted to anoxic shelf waters in the Peruvian Oxygen Minimum Zone
Chromium reduction and associated stable isotope fractionation restricted to anoxic shelf waters in the Peruvian Oxygen Minimum Zone
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DOI：
10.1016/j.gca.2020.06.027
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发表时间：
2020-09
影响因子：
5
通讯作者：
Philipp Nasemann;D. J. Janssen;J. Rickli;P. Grasse;M. Frank;S. Jaccard
中科院分区：
文献类型：
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作者：
Philipp Nasemann;D. J. Janssen;J. Rickli;P. Grasse;M. Frank;S. Jaccard
海洋铬（Cr）循环的认识仍然不够，特别是在海水中溶解的稳定的Cr同位素的空间分布的调节机制。其主要氧化态Cr（VI）和Cr（III）之间的氧化还原转化被认为是溶解的Cr浓度[Cr]与其同位素组成（δ 53 Cr）之间观察到的紧密的反对数关系的原因，由此同位素轻的Cr（III）在表面沃茨和氧最小区（OMZ）中被去除，在秘鲁边缘OMZ的一系列深度剖面中研究了海水[Cr]和δ 53 Cr，涵盖范围从2到242 µmol/kg的宽范围溶解氧浓度。[Cr]在1.5 ~ 5.5 nmol/kg之间，δ 53 Cr在+1.59 ‰ ~+0.72‰之间变化，但与溶解氧浓度无系统关系。然而，明显不同的海水剖面观察以上的缺氧/缺氧货架相比，那些位于进一步离岸，与大量的铬去除限制在缺氧或缺氧环境的货架上。这表明，单独的低氧条件（[O2] < 5 µmol/kg）不足以解释Cr的大量去除。鉴于环境条件下，铬可以减少空间仍然受到限制，因此，在海洋铬循环中的作用可能是小的。此外，一些观察结果证实了以下假设，即Cr还原不一定伴随着所形成的Cr（III）立即吸附到颗粒上，导致其从溶解相中除去。相反，通过颗粒部分去除Cr（III），留下残留的溶解Cr（III）池，可能在低氧沃茨中更普遍。
The marine chromium (Cr) cycle is still insufficiently understood, in particular the mechanisms modulating the spatial distribution of dissolved stable Cr isotopes in seawater. Redox transformations between its main oxidation states, Cr(VI) and Cr(III), have been held accountable for the observed tight inverse logarithmic relationship between the dissolved Cr concentration [Cr] and its isotopic composition (δ53Cr), whereby isotopically light Cr(III) is removed in surface waters and oxygen minimum zones (OMZs), and subsequently released to deeper waters from remineralized particles or sediments.Seawater [Cr] and δ53Cr were investigated in a series of depth profiles across the Peruvian margin OMZ, covering a wide spectrum of dissolved oxygen concentrations ranging from 2 to 242 µmol/kg. We found [Cr] ranging from 1.5 to 5.5 nmol/kg, associated with δ53Cr variations between +1.59 and +0.72‰, but no systematic relationship to dissolved oxygen concentrations. However, distinctly different seawater profiles were observed above the suboxic/anoxic shelf compared to those located further offshore, with substantial Cr removal restricted to suboxic or anoxic environments on the shelf. This suggests that suboxic conditions ([O2] < 5 µmol/kg) alone are not sufficient to account for substantial Cr removal. Given that environmental conditions under which Cr can be reduced remain restricted spatially, the role of this sink in the marine Cr cycle may therefore be small. Additionally, some observations corroborate the assumption that Cr reduction is not necessarily accompanied by immediate adsorption of the formed Cr(III) onto particles, leading to its removal from the dissolved phase. Instead, partial removal of Cr(III) via particles, leaving a residual dissolved Cr(III) pool, may be more widespread in suboxic waters.