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

Copper and its Isotopes in Organic-Rich Sediments: From the Modern Peru Margin to Archean Shales

“…The approach adopted here, whereby we separate and analyse the two operationally defined fractions (HFD and OPF), provides more information on the distribution and behavior of Cu and other elements in organic-rich sediments than can be achieved with a traditional bulk digestion. As described and demonstrated in Ciscato et al [40], the extraction of organic matter in the OPF is nearly complete and there is virtually no method-induced carbon isotope fractionation. The presence of small amounts of Al in the OPF fraction is almost certainly due to residual fluoride salts [40], a common feature of HF digestion of silicates [44] that should not affect the distribution of Cu among the different fractions.…”

“…As described and demonstrated in Ciscato et al [40], the extraction of organic matter in the OPF is nearly complete and there is virtually no method-induced carbon isotope fractionation. The presence of small amounts of Al in the OPF fraction is almost certainly due to residual fluoride salts [40], a common feature of HF digestion of silicates [44] that should not affect the distribution of Cu among the different fractions. The OPF fraction is also known to contain very small sulphide grains that are intimately associated with the organic matter in the OPF, but scanning electron microscope (SEM) observations demonstrate that these survive the HF treatment [40].…”

“…The presence of small amounts of Al in the OPF fraction is almost certainly due to residual fluoride salts [40], a common feature of HF digestion of silicates [44] that should not affect the distribution of Cu among the different fractions. The OPF fraction is also known to contain very small sulphide grains that are intimately associated with the organic matter in the OPF, but scanning electron microscope (SEM) observations demonstrate that these survive the HF treatment [40]. The digestion tests conducted here (see details in Section 3 and in Supplementary Information) suggest that mobilization of Cu from such grains, or from the organic matter of the OPF, by the HF digestion is minimal.…”

“…The TOC contents and carbon isotope composition of the Peru Margin sediments have already been presented and discussed in Ciscato et al [40]. However, they are tabulated again in Table 1 for easier reference.…”

“…The extracted OPFs of Peru Margin samples were analyzed for their TOC and δ 13 C as detailed in Ciscato et al [40] and the isotope ratios are reported in the conventional notation with respect to the V-PDB (Vienna Pee Dee Belemnite) standard:…”