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.

Cr isotope geochemistry is being explored in the context of a variety of geological problems as well as the environmental remediation of pollutant Cr(VI). There is a strong Cr isotope fractionation during reduction of oxidized Cr(VI) to reduced Cr(III). We present chromium concentration and Cr isotope data for samples from highly reducing environments ([O-2] < 2 mu mol/kg) in the Eastern Tropical North Pacific (ETNP) Oxygen Deficient Zone (ODZ) off of Mexico and the deep Santa Barbara Basin off of California. Total dissolvable Cr in the upper ETNP ODZ is slightly depleted (by up to 0.8 nmol/kg) and delta Cr-53 is up to 0.1-0.2 parts per thousand heavier compared to oxic waters of the same density seen at the SAFe station (30 degrees N, 140 degrees W), presumably both a result of reduction of Cr(VI) and removal of light Cr(III) by sinking particles. The Cr depletion and Cr isotope fractionation peak at the same depth as the highest delta N-1(5) of NO3- and decrease within the equally oxygen-deficient waters below, implying that microbial reduction dependent on the sinking organic matter flux may be the mechanism of Cr reduction. These data are consistent with a fractionation mechanism with a net isotope fractionation factor of epsilon approximate to -0.44 parts per thousand. In the deepest anoxic waters of the Santa Barbara Basin in July 2014, dissolved ( <0.2 mu m) Cr is depleted by up to 1.8 nmol/kg and delta Cr-53 is up to 0.5 parts per thousand heavier compared to SAFe station waters of the same density. This is consistent with a net isotope fractionation factor of epsilon approximate to -0.65 parts per thousand. At the Santa Barbara Basin site, it is possible that abiotic Fe(II) reduction (from Fe(II) diffusing out of reducing continental shelf sediments) also contributes to Cr reduction in addition to the microbial reduction mechanism.

Although the cycling of chromium (Cr) in the ocean may have direct implications on the Cr isotope system's application as a paleo-redox proxy, little seawater Cr isotope data has been published. This may be partly due to the analytical challenges associated with the preconcentration and the purification of the nano-molar trace metal Cr from the seawater matrix. Here, we present a reliable method for the determination of accurate and precise Cr-53/Cr-52 isotope ratios of total chromium (= Cr(III) + Cr(VI)) in seawater, which could be modified to be redox species-specific. Seawater acidification induces the slow conversion of Cr(VI) to Cr(III). Total Cr of the sample and an equilibrated Cr-50-Cr-54 double spike is preconcentrated as Cr(III) using Mg(OH)(2) coprecipitation. Cr captured by the Mg(OH)(2) pellet is oxidized to Cr(VI) using (NH4)(2)S2O8 under carefully controlled pH, temperature and time settings, in order to avoid H2O2 generation and to control sulfur speciation. Taking advantage of the differing charges of Cr(III) and Cr(VI), three AG 1-X8 columns separate Cr from the salt matrix and isobaric interferences V, Ti, and Fe (column 1), sulfur polyatomic interferences (column 2), and Fe traces (column 3). Analysis is conducted on a MC-ICP-MS IsoProbe featuring a hexapole collision cell in low resolution mode. When pure solutions of SRM 979 are processed via this low blank method (similar to 0.017 nmol of Cr), delta Cr-53 w.r.t. SRM 979= + 0.02 +/- 0.06 parts per thousand (+/- 2 SD) is obtained (n = 10). Using this technique, the first full water column profile of Cr isotope ratios at a station in the Pacific Ocean (SAFe station, 30 degrees N 140 degrees W) was generated and demonstrates high oceanographic consistency. This article is part of a special issue entitled: "Cycles of trace elements and isotopes in the ocean - GEOTRACES and beyond" - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. Gonzalez.

We review the current distribution of lead and lead isotopes in the ocean with regard to the evolving pattern of human emissions during the past decades and centuries.