The isotopic composition of authigenic chromium in anoxic marine sediments: A case study from the Cariaco Basin

Chromium (Cr) isotopes are an emerging proxy for tracking redox processes at the Earthʹs surface. However, there has been limited exploration of the Cr isotope record of modern and recent marine sediments. The basic inorganic chemistry of Cr suggests that anoxic marine basins should factor prominently in the global Cr cycle and that sediments deposited within anoxic basins may offer a valuable Cr isotope archive throughout Earthʹs history. Here, we present δ 53 Cr data from sediments of the Cariaco Basin, Venezuela—a ‘type’ environment for large, perennially anoxic basins with a relatively strong hydrological connection to the global oceans. We document a marked positive shift in bulk δ 53 Cr values following the termination of the Last Glacial Maximum, followed by relative stasis. Based on a suite of independent redox proxies, this transition marks a switch from oxic to persistently anoxic and sulfidic (euxinic) depositional conditions within the basin. We find good agreement between two independent approaches toward estimating the δ 53 Cr composition of authigenic Cr in euxinic Cariaco Basin sediments and that these estimates are very similar to the δ 53 Cr composition of modern open Atlantic Ocean seawater. These data, together with considerations of reaction kinetics and mass balance within the Cariaco Basin, are consistent with the hypothesis that anoxic marine settings can serve as a chemical archive of first-order trends in seawater δ 53 Cr composition. Additionally, the Cariaco Basin data suggest that there has been secular stability in the average δ 53 Cr value of Atlantic seawater over the last ∼15 kyr.