Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139.
Massachusetts Institute of Technology/Woods Hole Oceanographic Institution Joint Program in Oceanography, Cambridge, MA 02139.
Proc Natl Acad Sci U S A. 2021 Feb 23;118(8). doi: 10.1073/pnas.1918605118.
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.
由于三价 [Cr(III)] 和六价 [Cr(VI)] 之间的分馏导致铬同位素比值发生变化，地质学家正在利用这一变化来推断过去环境中的氧条件。然而，现代海洋中关于铬的信息很少，无法为这些推断提供依据。两种铬形态之间的转化是海洋铬循环中的重要过程。在这里，我们展示了来自东热带北太平洋（ETNP）缺氧区（ODZ）的六价和三价铬浓度和稳定同位素比值的剖面，这些剖面支持理论和实验研究，这些研究预测在低氧环境中较轻的铬优先被还原，并且由于沉降颗粒上颗粒反应性 Cr(III) 的去除，残留的溶解铬变得更重。Cr(III)最大值主要出现在 ODZ 的上部，这意味着微生物活性（取决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。
