Heavy nickel isotope compositions in rivers and the oceans

In stark contrast to the rivers, the dataset for seawater is very homogeneous, with 2SD of the entire dataset being only twice the analytical reproducibility. The second main feature is that seawater is distinctly heavier in Ni isotopes than rivers. The average δ60Ni is 1.44 ± 0.15‰ (2SD), and only 2 of the 29 seawater analyses have a Ni isotopic composition that is lighter than the heaviest river. The lack of an isotopic shift associated with the drawdown of nickel concentrations in the surface ocean suggests that the cycling of nickel between the surface and deep ocean is not associated with a pronounced isotopic fractionation. The isotopic data also present a mass balance problem. The main output of nickel from the oceans (sorption to Fe-Mn oxides) appears to be similar in isotopic composition to the dissolved phase, yet the riverine input is lighter than the dissolved pool. This observation either requires other inputs that are isotopically heavy, or an output that is isotopically light. Further large inputs, over and above the dissolved riverine source, appear to be required by what we know of the elemental budget, but the isotopic mass balance suggests that such an input needs to be isotopically much heavier than both the riverine dissolved load and the lithogenic isotopic composition of nickel.