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.