A new purification method for Ni and Cu stable isotopes in seawater provides evidence for widespread Ni isotope fractionation by phytoplankton in the North Pacific

Nickel and copper are cofactors in key phytoplankton enzymes and the stable isotope composition of Ni and Cu (δ60Ni and δ65Cu) in seawater have the potential to identify major processes that influence their biogeochemistry. However, accurate analysis of δ60Ni and δ65Cu is challenging because of the difficulties in separating these metals from interfering elements in the seawater matrix. Here we report a fast and simple method for purification of Ni and Cu from seawater samples that is able to completely remove interfering elements Mn, Ti, Cr, and Fe. This method was verified by analyzing four reference materials that contain significant levels of interfering elements (powdered plankton, natural soils, and two marine sediments). Using this technique, we generated a dataset of 49 seawater δ60Ni and δ65Cu measurements from the upper water column of the North Pacific Ocean, which show preferential uptake of light Ni isotopes by phytoplankton (αbio-sw = 0.9997 ± 1) but no net fractionation of Cu isotopes. This new method simplifies treatment of seawater samples for Ni and Cu isotope analysis, enabling high-throughput investigations of δ60Ni and δ65Cu throughout the global ocean. © 2020