Chromium isotopes in marine hydrothermal sediments

Abstract

Hydrothermal chromium (Cr) cycling contributes to marine Cr inventories and their Cr isotopic composition, yet Cr isotope effects associated with this cycling remain poorly documented. Here we determine the distribution, isotopic composition, and diagenetic mobility of Cr in hydrothermal sediments from the distal flank of the South East Pacific Rise (SEPR, DSDP-site 598). We find that Cr is primarily associated with the metalliferous iron (oxyhydr) oxide and detrital components of the sediment (0.4–3.6 mg kg−1), whereas Cr concentrations are much lower in the dominant carbonate phase (<0.03mg kg−1). The Cr:Fe ratio of the metalliferous component, however, decreases with increasing depth below the sediment water interface, with an apparent loss of >80% Cr from the sediment relative to Fe. We propose this loss is tied to oxidation of authigenic Cr(III) to Cr(VI) followed by diagenetic remobilization and efflux from the sediment pile. The bulk δ53Cr composition of the SEPR sediments is isotopically light (−0.24 to −0.57 ± 0.05‰) and the authigenic δ53Cr is as light as −1.2 ± 0.2‰. We argue that this light Cr isotopic composition results from the partial reduction of oxic seawater-bearing Cr(VI) by hydrothermal vent fluids enriched in Fe(II)aq. Diagenetic oxidation of the reactive Cr pool by Mn-oxides and loss of Cr(VI) from the sediment may further deplete the sediment in 53Cr during diagenesis. The δ53Cr composition of the detrital Cr fraction of the sediment (average δ53Cr composition = −0.05 ± 0.04‰) falls within the igneous silicate earth (ISE) range, revealing that detrital Cr delivered to this region of the Pacific ocean is unfractionated, and has carried a relatively constant δ53Cr composition over the last 5.7 million years. Together our results show that light δ53Cr compositions in hydrothermal sediments are imparted through a combination of processes previously overlooked in the marine Cr biogeochemical cycle, and that the δ53Cr composition of such sediments may provide a rich source of information on paleo-marine redox conditions.