Nickel (Ni) is a crucial micronutrient for phytoplankton and plays a key role in ocean biogeochemistry. Nickel isotope ratios (δ⁶⁰Ni) in seawater offer a powerful tool to explore Ni biogeochemistry in the modern ocean and serve as a potential proxy for reconstructing past marine redox conditions, climate change, and biogeochemical processes. However, current methods for purifying Ni from seawater for isotope analysis are time-consuming, limiting the application of Ni isotope ratios as a biogeochemical tracer. Here, we first present a new rapid purification procedure for Ni isotope analysis using an automated chromatography system, which facilitates Ni isotope analysis. This new method enable us to investigate several key aspects of Ni biogeochemistry in the ocean, including: (1) the oceanic Ni mass balance, by quantifying the fluxes and δ⁶⁰Ni values of Ni sources and sinks, (2) the internal cycling of Ni in the ocean, such as biological uptake, regeneration, and scavenging, by analyzing seawater samples from a Pacific GEOTRACES transect, and (3) Ni isotope fractionation by marine phytoplankton, based on culturing experiments.

In order to better understand the mass balance of Ni and Ni isotopes in the modern ocean, we measured trace metal concentrations and δ⁶⁰Ni values in sediment porewaters from two sites off the Southern California. We found that benthic fluxes of Ni from continental margin sediments with active Mn cycling represent a new source of isotopically heavy dissolved Ni to the ocean. This provides valuable insight into the global oceanic Ni mass balance. We also analyzed over 600 seawater Ni isotope samples from the US GEOTRACES Pacific Meridional Transect (GP15), providing the first high-resolution seawater δ⁶⁰Ni dataset in the Pacific Ocean. We observe increasing δ⁶⁰Ni values in the surface ocean across the entire GP15 transect, a homogeneous deep ocean δ⁶⁰Ni across ocean basins, and distinct hydrothermal impact on seawater δ⁶⁰Ni. Our culturing experiments further demonstrate that phytoplankton preferentially assimilate isotopically light Ni, but different species exhibit varying Ni isotope fractionation and Ni quotas, thus contributing uniquely to oceanic Ni biogeochemical cycling.

nickel, GEOTRACES, isotopes, phytoplankton, mass balance