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 (δ 60 Ni NIST 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 x 10 8 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.

Bioactive Trace Metals and Their Isotopes as Paleoproductivity Proxies: An Assessment Using GEOTRACES‐Era Data

“…Dissolved Ni is added to the ocean from rivers, hydrothermal vents, and dust (Figure 25). All three sources possess δ 60 Ni between +0.1‰ and +0.8‰, which is lighter than deep ocean seawater (see recent summaries by Gueguen & Rouxel, 2021; Little et al., 2020). In general, these Ni sources are not of a sufficient magnitude to generate deviations in deep ocean dissolved δ 60 Ni, and are therefore unlikely to compromise use of δ 60 Ni as a paleoproductivity tracer (similar to δ 114 Cd; Section 6).…”

“…The main output flux of Ni from seawater is believed to be burial with Mn oxides, which have a wide range of reported Ni isotope compositions. While hydrogenetic ferromanganese crusts are generally isotopically heavy, from about +0.9 to +2.5‰ (Gall et al., 2013; Gueguen et al., 2021, 2016), Mn‐rich pelagic clays possess δ 60 Ni between −0.8 and +1.0‰ (Little et al., 2020; Gueguen & Rouxel, 2021). The Ni isotope difference between sources and sinks implies the existence of a missing “heavy” Ni source.…”

Nickel isotopes link Siberian Traps aerosol particles to the end-Permian mass extinction

“…Therefore, at present, the budget of Ni isotopes at steady-state between inputs and outputs has an apparent imbalance 35 – 37 , 43 , 45 . However, δ 60 Ni values ranging from −0.2‰ to −0.8‰ were reported from the sediments that were deposited under oxygenated bottom water conditions at water depths of >3 km in the eastern Pacific 46 . These light δ 60 Ni values are interpreted to have resulted from diagenetic remobilisation, or mineralogical transformation of birnessite to todorokite, or possible scavenging of light Ni by Fe oxides or Fe-rich authigenic clays 46 .…”

Nickel Isotopes Link Siberian Traps Aerosol Particles to the End-Permian Mass Extinction

“…between inputs and outputs has an apparent imbalance [35][36][37]43,45 . However, δ 60 Ni values ranging from −0.2‰ to −0.8‰ were reported from the sediments that were deposited under oxygenated bottom water conditions at water depths of >3 km in the eastern Pacific 46 . These light δ 60 Ni values are interpreted to have resulted from diagenetic remobilisation, or mineralogical transformation of birnessite to todorokite, or possible scavenging of light Ni by Fe oxides or Fe-rich authigenic clays 46 .…”