Investigation of reductive chromium removal and associated stable isotope fractionation within the Oxygen Minimum Zone offshore Peru

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Chromium (Cr) is a transition metal that occurs only at trace levels in seawater (~80 to 550 ng/kg). The distribution of stable Cr isotopes is thereby highly susceptible to redox changes, which has promoted development of stable Cr isotopes as a proxy for the evaluation of redox conditions and oxygenation in the water column. First investigations have revealed an unexpected heterogeneity of seawater Cr isotopes hypothesized to derive from surface water removal of reduced Cr species and associated isotope fractionation in Oxygen Minimum Zones (OMZs), possibly utilizing phytoplankton as removal vector. In order to test this hypothesis and better disclose the extend of surface and deep water Cr isotope variability, a two-year project investigating Cr isotopes in seawater and sediment samples collected along the Peru Margin is proposed for a DFG research fellowship. The Peru Margin hosts the most prevalent OMZ found in the modern-day oceans and covers major gradients in coastal upwelling and benthic flux intensities, producing variations in nutrient supply that affect primary production. It thus provides a unique setting to conduct a detailed study assessing the influence of changing biogeochemical and redox conditions on Cr isotope behaviour in the marine environment and across the seawater-sediment boundary. Gaining an enhanced understanding of the factors controlling Cr isotope fractionation is a prerequisite for establishing Cr isotopes as a novel isotope proxy, and is greatly assisted by the wealth of information already available for the selected samples and the entire Peruvian upwelling region.The required analytical methods facilitating Cr isotope analysis of seawater and sediment samples including chemical pre-concentration and purification techniques as well as advanced methods in isotope ratio mass spectrometry, are routinely applied at the Institute of Geological Sciences, University of Bern, allowing for prompt first results. The output of the proposed research project will provide critically needed insights into Cr isotope effects internal to the oceans with far-reaching implications for the interpretation of sedimentary Cr isotope records and our understanding of the marine carbon cycle and nutrient biogeochemistry.