The Global Biogeochemical Cycle of Chromium at the Earth's Surface

Abstract

The biogeochemistry of Cr and its cycling in Earth's surface environments is reviewed. A synthesis and critical evaluation of the major processes controlling Cr mobility and isotope composition (δ⁵³Cr) is presented, taking a source to sink view beginning with Cr mobilization from Earth's crust. Transport and cycling in inland waters and input to the oceans are discussed. Anthropogenic mobilization of Cr results in contributions to the atmosphere and inland waters that are of similar orders of magnitude as natural processes. The principal sources of Cr to the oceans are rivers and diffusive fluxes from marine sediments. Internal cycling of Cr in the ocean is largely controlled by reductive removal onto particles, particularly in O₂-depleted waters, and redistribution through ocean circulation. Chromium removal from the oceans occurs primarily in organic carbon-rich, O₂-poor shelf sediments. Despite theoretically poor mobility of Cr(III), reductive removal in anoxic waters is non-quantitative. As a result, isotope fractionation drives δ⁵³Cr offsets in removed Cr as well as residual dissolved Cr(III), which accumulates in anoxic water, compared to the corresponding source. The implications for δ⁵³Cr-based reconstructions are discussed, along with an outlook for future proxy applications based on the processes controlling Cr incorporation into sediments. The roles of different source and sink processes are quantified in an updated mass balance for the global ocean. Finally, priority topics for future research are suggested, which at present are the primary uncertainties of the modern Cr biogeochemical cycle and aspects of the Cr isotope mass balance.