Trivalent chromium isotopes in the eastern tropical North Pacific oxygen-deficient zone
Trivalent chromium isotopes in the eastern tropical North Pacific oxygen-deficient zone
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DOI：
10.1073/pnas.1918605118
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发表时间：
2021-02
期刊：
影响因子：
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通讯作者：
Tianyi Huang;S. B. Moos;E. Boyle
中科院分区：
文献类型：
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作者：
Tianyi Huang;S. B. Moos;E. Boyle
铬同位素已被用于研究古代地球大气中的氧含量，并似乎有希望作为古氧化还原的代理。然而，很少有研究关注它在现代海洋中的循环。在这里，我们分析了缺氧区的样本，并研究了铬的形态和同位素如何对极低氧环境做出反应，这有助于解开铬的氧化还原循环和相关的同位素分馏。热带北太平洋东部缺氧区三价铬的同位素组成比总溶解铬和残留六价铬轻。这一发现支持优先减少同位素轻铬发现在理论和实验研究中，是根本的铬同位素作为古氧化还原代理的使用。由于三价[Cr（III）]和六价铬[Cr（VI）]之间的分馏，铬（Cr）同位素比率的变化正被地质学家用来推断过去环境中的氧条件。然而，在现代海洋中，几乎没有关于Cr的信息来证实这些推论。两种铬形态之间的转化是海洋铬循环的重要过程。在这里，我们提出的配置文件的六价和三价铬的浓度和稳定同位素比从东热带北太平洋（ETNP）缺氧区（ODZ），支持理论和实验研究预测，较轻的铬优先减少在低氧环境中，残留的溶解铬变得更重，由于去除颗粒反应性铬（III）下沉颗粒。Cr（III）最大值主要发生在ODZ的上部，这意味着微生物活性（依赖于有机物的沉降通量）可能是这种转化的主要机制，而不是取决于氧化还原电位的物种之间的简单无机化学转化。
Significance Chromium isotopes have been used to study ancient earth atmospheric oxygen levels and seem promising as a paleoredox proxy. However, few studies have focused on its cycling in the modern ocean. Here we analyzed samples from oxygen-deficient zones and investigated how chromium speciation and isotopes respond to extremely low oxygen environments, which helps to unravel the chromium redox cycling and the associated isotopic fractionation. The isotopic composition of trivalent chromium in the eastern tropical North Pacific oxygen-deficient zones is lighter than total dissolved chromium and residual hexavalent chromium. This finding supports the preferential reduction of isotopically light Cr found in theoretical and experimental studies and is fundamental to the usage of Cr isotopes as a paleoredox proxy. Changes in chromium (Cr) isotope ratios due to fractionation between trivalent [Cr(III)] and hexavalent [Cr(VI)] are being utilized by geologists to infer oxygen conditions in past environments. However, there is little information available on Cr in the modern ocean to ground-truth these inferences. Transformations between the two chromium species are important processes in oceanic Cr cycling. Here we present profiles of hexavalent and trivalent Cr concentrations and stable isotope ratios from the eastern tropical North Pacific (ETNP) oxygen-deficient zone (ODZ) which support theoretical and experimental studies that predict that lighter Cr is preferentially reduced in low-oxygen environments and that residual dissolved Cr becomes heavier due to removal of particle-reactive Cr(III) on sinking particles. The Cr(III) maximum dominantly occurs in the upper portion of the ODZ, implying that microbial activity (dependent on the sinking flux of organic matter) may be the dominant mechanism for this transformation, rather than a simple inorganic chemical conversion between the species depending on the redox potential.