The behaviour of Ni and its isotopes during early diagenesis of Mn oxide-rich abyssal sediments

The use of nickel (Ni) and its isotope system as biogeochemical tracers of past ocean environments requires a sound understanding of the oceanic mass balance and the sinks that control output fluxes. Recent efforts have focused in particular on quantifying and understanding the size and Ni isotope composition of the most complicated and largest sink of Ni from the dissolved oceanic pool, the output to oxic Mn-rich sediments. The precise processes controlling Ni and its isotopes during early diagenesis in oxic sediments are not yet fully understood, but could impact estimates of the net oxic sink used in oceanic mass balance calculations. To address this issue, we present Ni concentration and isotope data from fully oxic Mn-rich sediments and corresponding porewater samples from 3 stations in the Equatorial North Pacific, south of Hawai’i. As in other abyssal Mn-rich sediments, Ni is well correlated with Mn in the solid phase, suggesting that Mn oxides are the dominant vector by which Ni is supplied to these sediments. Authigenic Ni isotope compositions in the solid phase are isotopically lighter than global deep seawater Ni (∼1.33 ‰) and range from 0.52 to 1.06 ‰, within the range of other abyssal Mn-rich sediments (0.26 to 1.08 ‰). At all three stations, the porewater results indicate the presence of a reactive Ni pool that is mobilised into the aqueous phase near the top of the cores. Nickel concentrations in the porewaters are significantly higher at the core top (30–63 nM) than in seawater (∼8 nM), decrease to seawater concentrations downcore, and correlate with porewater DOC concentrations. Additionally, porewater Ni isotopes are heaviest (∼1.8 ‰) near the sediment–water interface, where DOC levels are highest, and decrease monotonically to values that are close to, but slightly lower than, the deep ocean Ni isotope composition (∼1.3 ‰) further down in the cores. The relationship between Ni concentration, isotopes and DOC in the porewater suggests that Ni is remobilised into porewater from Mn oxides due to lowering of pH as a result of the remineralisation of organic matter. The heavy isotope composition of porewater Ni is likely controlled by isotope fractionation between residual solid Mn oxide Ni (light) and organically-complexed heavy aqueous Ni. This isotopically heavy pore fluid Ni leaves the sediment via a diffusive benthic flux. Despite this reactive zone very close to the sediment–water interface, these diagenetic processes barely impact sediment geochemistry, suggesting that the buried sink of Ni to Mn oxide-rich sediments is accessible via analysis of solid sediment in the upper few 10 s of cm at such sites.