Investigation of reductive chromium removal and associated stable isotope fractionation within the Oxygen Minimum Zone offshore Peru
Mineralogy, Petrology and Geochemistry
Final Report Abstract
Dissolved seawater Cr concentrations and stable isotope compositions were investigated along depth profiles across the Peru margin Oxygen Minimum Zone. Altogether 52 new data points were added to the global inventory of seawater Cr data, completing a previously unstudied area of the global ocean. Samples covered a wide spectrum of dissolved oxygen concentrations and biogeochemically distinct settings. Although there is no single relationship between dissolved oxygen concentrations and Cr dynamics across the shelf, slope and open ocean settings, our data imply that large-scale Cr reduction and removal in OMZs requires suboxic conditions and occurs primarily in shelf settings. The fundamental responsible mechanisms remain poorly constrained at this stage, but observations are sufficient to set the direction for future research. Spatial confinement of Cr reduction and removal to the shelf and coinciding anomalies in nitrate and nitrite concentrations may hint towards a link to denitrification, but specific reducing agents in the water column may also be involved as electron donor. The global impact of Cr removal in suboxic/anoxic shelf waters is difficult to assess. So far, this mechanism was only observed at few locations above the Peruvian shelf between 3° and 17°S, and only in suboxic waters (this study). The suboxic/anoxic shelf area offshore Peru covers roughly 68000 km2, which is negligible compared to the global ocean surface area. On the contrary, Cr removal above the Peruvian shelf appears to be efficient and some samples imply a Cr deficit of ~50% compared to the initial water composition, lost within a fraction of the entire shelf length. Whether the strength of this removal mechanism varies over time is not known, but it is conceivable that the dynamic conditions in the Peruvian upwelling area also facilitate fluctuations in Cr removal. And given that suboxic shelf areas cover a much wider surface globally, suboxic Cr removal could be a significant process within the global marine Cr cycle.
Publications
- (2020). Chromium reduction and associated stable isotope fractionation restricted to anoxic shelf waters in the Peruvian Oxygen Minimum Zone. Geochimica et Cosmochimica Acta, 285, pp. 207-224
Nasemann, P., Janssen, D.J., Rickli, J.D., Grasse, P., Frank, M., Jaccard, S.L.
(See online at https://doi.org/10.1016/j.gca.2020.06.027)
