Trivalent chromium isotopes in the eastern tropical North Pacific oxygen-deficient zone

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 particl-ereactive 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.

Automated summary

Research findings in the eastern tropical North Pacific (ETNP) oxygen-deficient zone (ODZ) support theoretical and experimental studies predicting that lighter chromium is preferentially reduced in low-oxygen environments and residual dissolved chromium becomes heavier due to removal of particl-reactive Cr(III) on sinking particles. Microbial activity, dependent on sinking organic matter flux, may be the dominant mechanism for chromium transformation, rather than a simple inorganic chemical conversion.