Biogeochemical cycling of chromium and chromium isotopes in the sub-tropical North Atlantic Ocean

• Published in: Frontiers in Marine Science
• Main Authors: Wang, Wenhao, Goring-Harford, Heather, Kunde, Korinna, Woodward, E. Malcolm S., Lohan, Maeve C., Connelly, Douglas P., James, Rachael H.
• Other Authors: Natural Environment Research Council, China Scholarship Council
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Frontiers Media SA 2023
• Subjects: North Atlantic
• Online Access: http://dx.doi.org/10.3389/fmars.2023.1165304; https://www.frontiersin.org/articles/10.3389/fmars.2023.1165304/full

Chromium (Cr) is a redox-sensitive element and because Cr isotopes are fractionated by redox and/or biological processes, the Cr isotopic composition of ancient marine sediments may be used to infer changes in past seawater oxygenation or biological productivity. While there appears to be a ‘global correlation’ between the dissolved Cr concentration and Cr isotopic composition of seawater, there is ongoing debate about the relative importance of external sources and internal cycling on shaping the distribution of dissolved Cr that needs to be resolved to validate the efficacy of using Cr isotopes as a paleo proxy. Here, we present full water column depth profiles of total dissolved Cr (Cr(VI)+Cr(III)) and dissolved Cr isotopes (δ 53 Cr), together with ancillary data, for three stations along a transect (GEOTRACES GApr08) across the sub-tropical North Atlantic. Concentrations of dissolved Cr ranged between 1.84 and 2.63 nmol kg -1 , and δ 53 Cr values varied from 1.06 to 1.42‰. Although atmospheric dust, hydrothermal vents and seabed sediments have the potential to modify the distribution of Cr in the oceans, based on our observations, there is no clear evidence for substantial input of Cr from these sources in our study region although benthic inputs of Cr may be locally important in the vicinity of hydrothermal vents. Subsurface waters (below the surface mixed layer to 700 m water depth) were very slightly depleted in Cr (by up to ~0.4 nmol kg -1 ), and very slightly enriched in heavy Cr isotopes (by up to ~0.14‰), relative to deeper waters and the lowest Cr concentrations and highest δ 53 Cr values coincided with lowest concentrations of colloidal (0.02 to 0.2 μm size fraction) Fe. We found no direct evidence for biological uptake of dissolved Cr in the oligotrophic euphotic zone or removal of Cr in modestly oxygen depleted waters (O 2 concentrations ~130 μmol kg -1 ). Rather, we suggest removal of Cr (probably in the form of Cr(III)) in subsurface waters is associated with the formation of colloid aggregates ...