Heavy nickel isotope compositions in rivers and the oceans

Abstract

Nickel is a biologically-active trace metal whose dissolved concentration depth profiles in the ocean show nutrient-like behaviour. If the pronounced removal of nickel from the dissolved phase in the surface ocean, and its return in the deep, is associated with an isotopic fractionation nickel isotopes may be able to yield constraints on the precise biogeochemical processes involved. Here we present the first nickel isotope data for seawater along with data for the dissolved phase of rivers, one of the principal sources of nickel to the oceans.The dissolved phase of rivers exhibits substantial variability in both Ni concentration and δ60Ni: from 2.2 to 35nmolkg−1 and +0.29 to +1.34‰, respectively. The most striking result from the nickel isotope analyses of rivers is that they are substantially heavier (by up to 1‰ for δ60Ni) than the range for silicate rocks on the continents, a finding that is analogous to that for other transition metal isotope systems. If the data presented here are close to representative of the global riverine flux, they suggest an annual input of Ni to the oceans of 3.6×108moles, and a discharge- and concentration-weighted δ60Ni average of +0.80‰. The relationship between Ni isotopes and concentrations shows similarities with those for other transition metal isotope systems, where the main control has been suggested to be isotopic partitioning between the dissolved phase and particulates, either in the weathering environment or during transport.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.