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

• Published in: Geochimica et cosmochimica acta 2020-09, Vol.285, p.207-224
• Main Authors: Nasemann, Philipp, Janssen, David J., Rickli, Jörg, Grasse, Patricia, Frank, Martin, Jaccard, Samuel L.
• Format: Article
• Language: English
• Subjects: Chromium isotopes; Oxygen minimum zones
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2020.06.027

The marine chromium (Cr) cycle is still insufficiently understood, in particular the mechanisms modulating the spatial distribution 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 concentration [Cr] and its isotopic composition (δ53Cr), whereby isotopically light Cr(III) is removed in surface waters and oxygen minimum zones (OMZs), and subsequently released to deeper waters from remineralized particles or sediments. Seawater [Cr] and δ53Cr 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 µmol/kg. We found [Cr] ranging from 1.5 to 5.5 nmol/kg, associated with δ53Cr variations between +1.59 and +0.72‰, but no systematic relationship to dissolved oxygen concentrations. 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]