Chromium reduction and associated stable isotope fractionation restricted to anoxic shelf waters in the Peruvian Oxygen Minimum Zone

Nasemann, Philipp; Janssen, David J.; Rickli, Jörg; Grasse, Patricia; Frank, Martin; Jaccard, Samuel L. (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. Elsevier Science 10.1016/j.gca.2020.06.027

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The marine chromium (Cr) cycle is still insufficiently understood, in particular the mechanisms modulating the spatial dis- tribution of dissolved stable Cr isotopes in seawater. Redox transformations between its main oxidation states, Cr(VI) and Cr (III), have been held accountable for the observed tight inverse logarithmic relationship between the dissolved Cr concentra- tion [Cr] and its isotopic composition (d53Cr), whereby isotopically light Cr(III) is removed in surface waters and oxygen min- imum zones (OMZs), and subsequently released to deeper waters from remineralized particles or sediments.
Seawater [Cr] and d53Cr were investigated in a series of depth profiles across the Peruvian margin OMZ, covering a wide spectrum of dissolved oxygen concentrations ranging from 2 to 242 mmol/kg. We found [Cr] ranging from 1.5 to 5.5 nmol/kg, associated with d53Cr variations between +1.59 and +0.72‰, but no systematic relationship to dissolved oxygen concentra- tions. However, distinctly different seawater profiles were observed above the suboxic/anoxic shelf compared to those located further offshore, with substantial Cr removal restricted to suboxic or anoxic environments on the shelf. This suggests that suboxic conditions ([O2] < 5 mmol/kg) alone are not sufficient to account for substantial Cr removal. Given that environmen- tal conditions under which Cr can be reduced remain restricted spatially, the role of this sink in the marine Cr cycle may there- fore be small. Additionally, some observations corroborate the assumption that Cr reduction is not necessarily accompanied by immediate adsorption of the formed Cr(III) onto particles, leading to its removal from the dissolved phase. Instead, partial removal of Cr(III) via particles, leaving a residual dissolved Cr(III) pool, may be more widespread in suboxic waters.

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