Processes controlling nickel and its isotopes in anoxic sediments of a seasonally hypoxic bay

Abstract

Nickel (Ni) is a biologically active metal whose reactivity and isotope fractionation in the marine realm are strongly influenced by biological and redox-related processes, giving the stable isotope system potential for studying past ocean environments. Reducing, organic-rich, sediments constitute an important sink of Ni from the modern ocean. Importantly, at open ocean upwelling margins, these kinds of sediment record the isotope composition of the modern deep ocean. Thus, records of their Ni isotope composition in the past have the potential to record the past deep ocean isotope composition and the oceanic isotope mass balance. However, the detailed processes controlling the upwelling sink are not fully understood. Here, we address this issue through data for sediments, porewaters and the water column of Kiel Bight in the Western Baltic Sea. This setting preserves sediments that have similar characteristics to those of open ocean upwelling margins, allowing us to study specific controlling processes in a well constrained setting.

In common with sediments from open-ocean upwelling settings, Ni is well-correlated with carbon in solid sediment, suggesting delivery of Ni via rain of organic carbon from the water column. Overall, porewaters at all sites studied show increasing Ni concentrations from around 10 nM near the sediment–water interface to as high as 50 nM at 25 cm depth. This increase is correlated with increases in ammonia concentrations, suggesting release of Ni from anaerobic respiration of organic matter. However, porewater Ni/NH₄ ratios are always lower than Ni:N of water column suspended particulate matter, suggesting an additional process that removes Ni from the porewater. Porewater sulphide also increases with depth, from as low as zero at the sediment–water interface to levels as high as 3 mM at 25 cm. Overall, porewater Ni isotopes become heavier with depth, from bottom water δ⁶⁰Ni around +0.5 to +1‰, to values as high as +2.3‰ at depth. All these observations strongly suggest that Ni is removed from porewater into a solid sulphide. Mass balance indicates that over 90% of the Ni delivered in organic material to the sediment–water interface is transferred from organic matter into solid sulphide. Upward diffusive fluxes lead to the loss of a small amount back to the water column via a benthic flux. Given the large proportion of Ni retained within the sediment, the loss of such Ni does not strongly impact the isotope composition of the buried pool. These data are crucial in clarifying the processes controlling the size and isotope composition of organic-rich sediments on upwelling margins.