The distribution of Ni and Ni isotopes in the North Pacific

As one of the biologically active elements, nickel (Ni) is essential to at least known seven enzymes, including those which influence the global carbon, nitrogen, and oxygen cycles. In the modern ocean, Ni exhibits a nutrient-type depth profile, with lower Ni concentrations in the surface seawater and higher Ni concentrations in the deep seawater. Thus, the lowest Ni concentrations occur in the oligotrophic ocean (e.g., the North and South Pacific subtropical gyres) where the Ni concentration of surface seawater can be as low as 2nM. The oligotrophic ocean also exhibits obvious Ni isotope fractionations, suggesting that biological Ni uptake drives isotope fractionation.

Here we present the results of the distribution of Ni concentrations and nickel stable isotopes (δ⁶⁰Ni) in the North Pacific, incorporating data from the US GEOTRACES Pacific Meridional Transect (GP15) and Gradients cruises. Surface seawater in the North and South Pacific subtropical gyres have lower Ni concentrations (as low as ~2.1nM), consistent with previous studies. Ni exhibits a typical nutrient-type profile and the depth of the maximum Ni concentration is around 2500m. Another striking feature of the concentration-depth profile is that it is very consistent as all data points seem to fall on the same trend curve, which might indicate that the Ni cycling throughout the GP15 transect is controlled by a few dominant processes (e.g., biological uptake and remineralization). In the North Pacific subtropical gyre, the δ⁶⁰Ni values increase from deeper seawater towards surface seawater. The δ⁶⁰Ni value of surface seawater is around 1.7‰ and of deeper seawater is around 1.4‰, indicating a biological preference for the uptake of lighter nickel isotopes.

A few exceptionally high Ni concentrations are observed in the vicinity of the Loihi hydrothermal vents. These anomalously high Ni data are located near 55°N and 18.9°N (Loihi station), with extremely high Fe and Mn concentrations indicative of hydrothermal activity. The Ni isotopic compositions of these samples may reveal the isotopic composition of the hydrothermal end-member and partition the Ni in seawater into initial seawater Ni and hydrothermal Ni.