Nickel and its isotopes in organic-rich sediments: implications for oceanic budgets and a potential record of ancient seawater

Abstract Nickel (Ni) is a biologically active element that displays a nutrient-like depth distribution in the modern oceans. Recent studies of Ni isotopes have highlighted the fact that, in common with many other transition metals, the Ni stable isotope composition, expressed as δ 60 Ni, of the dissolved phase is heavier than the inputs, at +1.3 to +1.7‰. The sedimentary outputs that control the high δ 60 Ni of the ocean, coupled with records for past seawater, could potentially yield new information on the past Earth system, but these are currently not well understood. Here we present the first Ni abundance and isotope data for a key output, that associated with Ni uptake into organic matter, at productive upwelling regions and elsewhere. We investigate the distribution of Ni and its isotopes in two fractions separated from the bulk sediment, an HF-digestible fraction, extracted with HF–HCl, and an organic-sulphide-rich fraction. The organic-sulphide fractions exhibit a range in δ 60 Ni, from +0.86 to +1.83. Systematic relationships between Ni concentrations, total organic carbon and Ni isotopes suggest that the organic-sulphide fraction originates in the photic zone, and is delivered to the sediment as a closed system, despite the possibility of transfer of Ni to sulphide within it. Authigenic Ni in the bulk sediment is dominated by the HF-digestible fraction which, in Ni-enriched sediments where the detrital correction is small, is very close to the modern deep ocean, at δ 60 Ni = +1.2‰. These data suggest that organic-rich sediments beneath upwelling zones, while they are an important output flux of Ni from the oceans, do not solve the isotope balance problem because their δ 60 Ni is almost identical to modern seawater. On the other hand, the approach adopted here involving the analysis of the two fractions, both traces the fractionation imparted by biological uptake as well as recording the δ 60 Ni of contemporary seawater, suggesting potential for understanding the past oceans.