Towards balancing the oceanic Ni budget

Abstract

Nickel isotopes are a novel and promising tracer of the chemistry of past ocean environments, but realisation of this tracer's potential requires a comprehensive understanding of the controls on Ni burial in the marine sedimentary archive. An outstanding puzzle in the marine budget of Ni, first recognised in the 1970s, is a major imbalance in the known inputs and outputs to and from the ocean: the sedimentary outputs of Ni are much larger than the inputs (rivers, dust). Much more recently, it has also been recognised that the outputs are also considerably isotopically heavier than the inputs. In this study, we find light Ni isotope compositions (δ60NiNIST SRM986 = −0.2 to −0.8‰) for Mn-rich sediments from the eastern Pacific compared to Fe-Mn crusts (at about +1.6‰). These data suggest that diagenetic remobilisation of isotopically heavy Ni leads to a significant benthic Ni flux (estimated at 0.6−2.3×108 mol/yr), similar in magnitude to the riverine flux, to the ocean. Diagenetic remobilisation of Ni may occur either via cycles of Mn-oxide dissolution and precipitation, with associated Ni sorption and release, or during mineralogical transformation of birnessite to todorokite. A minor role for retention of isotopically light Ni by Fe oxides or Fe-rich authigenic clays is also proposed. Overall, a benthic flux of isotopically heavy Ni (at about +3‰) can balance the marine Ni budget, pinpointing diagenesis as a key missing piece of the Ni puzzle.