Distribution and Cycling of Nickel and Nickel Isotopes in the Pacific Ocean

Nickel stable isotopes (delta 60Ni) provide insight to Ni biogeochemistry in the modern and past oceans. Here, we present the first Pacific Ocean high-resolution dissolved Ni concentration and delta 60Ni data, from the US GEOTRACES GP15 cruise. As in other ocean basins, increases in delta 60Ni toward the surface ocean are observed across the entire transect, reflecting preferential biological uptake of light Ni isotopes, however the observed magnitude of fractionation is larger in the tropical Pacific than the North Pacific Subtropical Gyre. Such surface ocean fractionation by phytoplankton should accumulate isotopically lighter Ni in the deep Pacific, yet we find that North Pacific deep ocean delta 60Ni is similar to previously reported values from the deep Atlantic. Finally, we find that seawater dissolved delta 60Ni in regions with hydrothermal input can be either higher or lower than background deep ocean delta 60Ni, depending on vent geochemistry and proximity. A habitable Earth relies on the growth of microscopic plants, called phytoplankton, in the ocean. Phytoplankton help regulate Earth's climate and their growth requires various nutrients in seawater, including a suite of metal elements such as iron, zinc, and nickel (Ni). These metals are called trace metals in the ocean because their concentrations are extremely low in seawater, yet they are essential micronutrients. Therefore, it is important to understand how much and how these elements are supplied to and removed out of the ocean, and how they are used by phytoplankton. To study this question, we analyzed Ni isotope ratios of hundreds of seawater samples collected from the Pacific Ocean-which is the ratio of naturally occurring Ni atoms with different numbers of neutrons and therefore slightly different masses. We find that surface seawater is more enriched in heavier Ni isotopes than the deep seawater, in particular around the tropical Pacific. The deep ocean has similar Ni isotope ratios across different ocean basins, from the Atlantic to the Pacific. Seawater Ni isotope ratios are impacted by rivers near Alaska, submarine volcanos near Hawai'i, and a submarine volcanic chain called the East Pacific Rise. Enrichment of heavier Ni isotopes is observed in the surface ocean across the GP15 transect, particularly in the equatorial Pacific North Pacific deep ocean Ni isotope composition is nearly homogeneous and similar to previously reported values from other ocean basins Hydrothermal activity can both decrease and increase seawater Ni isotope composition, depending on vent geochemistry and proximity