6. Conclusions
Our incubation data provide the first direct evidence of Cr release from biogenic particles and demonstrate that the release of Cr is mechanistically independent from the regeneration of major elements in organic matter (e.g. C, N, P), and may instead be related to oxidative release facilitated by Mn oxides. Our pore water data indicate large benthic Cr fluxes as an important local process, and possibly an important component of the ocean’s Cr budget. This benthic flux likely reflects the release of Cr scavenged onto particles in the water column, rather than ‘new Cr’ from the dissolution of lithogenic material. δ53Cr data from both bottom waters and incubations identify that release from biogenic particles follows the global δ53Cr–[Cr] array, demonstrating that Cr release, either in deep waters or surface sediments, can explain the high [Cr] end-member of the global δ53Cr–[Cr] array. Furthermore, our data confirm that biogenic export from the surface ocean and release at depth is important in shaping distributions of both [Cr] and δ53Cr throughout the global ocean. Our compilation provides clear evidence of [Cr] accumulation in deep waters, supporting a deep regeneration cycle for Cr that is decoupled from organic matter respiration and includes a potential benthic source. This regeneration is likely particularly strong below tropical Pacific OMZs where the local [Cr] maxium may result from Cr scavenged in OMZ waters being released in underlying oxic waters and sediments. Intermediate waters show stable or decreasing [Cr], reflecting minimal release from particles and a potential net water column Cr sink via scavenging. Because both [Cr] and δ53Cr are impacted by biological and physical processes, and pore water data indicate active cycling in near-surface seidments, the role of these processes with respect to Cr paleoredox proxy viability should be incorporated into future δ53Cr O2-reconstruction applications.