Selenium geochemistry and isotopic composition of sediments from the Cariaco Basin and the Bermuda Rise: a comparison between a restricted basin and the open ocean over the last 500 ka.

Broad changes in atmospheric oxygen have been identified for the last 4 Ga whereas changes in ocean oxygenation are not as well constrained. Mo isotope ratios have already been suggested as a potential global ocean palaeoredox proxy. However, a shorter oceanic residence time, theoretically greater isotope shifts, and no reliance on free H2S to alter its isotope composition, suggest Se isotope ratios could be a better global ocean palaeoredox proxy. The Cariaco Basin, on the Venezuela margin, has been the subject of numerous palaeoceanographic studies. Alternating laminated and non-laminated sediments indicate cyclical changes in water column oxygenation. It is therefore an ideal natural laboratory in which to study these effects on Se isotopes. A comparison with the Bermuda Rise, in the western North Atlantic, over the same time period, allows an evaluation of the spatial variations of sedimentary Se isotope signatures. Over the last interglacial-glacial transition, laminated (interglacial) sediments from the Cariaco Basin are characterised by high Se (~10 ppm) and high δ82/76Se (~0.03‰). In contrast, sediments from non-laminated (glacial) sediments have low Se (~0.5 ppm) and negative δ82/76Se (~-0.97‰). These end members result from two modes of Se cycling. Laminated sediments are dominated by organically-bound Se and low Se/Corg. Bioturbated sediments have a higher proportion of authigenic Se and higher Se/Corg. Over older interglacial-glacial cycles, the relationship between the two end-members and interglacial-glacial states becomes less clear. Similar end-members to those in the Cariaco Basin exist in the Bermuda Rise except the low δ82/76Se end-member has lower Se concentrations. As in the Cariaco Basin, variations are controlled by local climate rather than overall interglacial-glacial state. A comparison of the δ82/76Se values for the two sites show similarities which suggest that Se isotope ratios might not be just recording local changes.