Award: OCE-1736896

Nickel and copper are important nutrients for algae which live in the oceans, where they are the base of the food chain for important fisheries, and where they help to draw down carbon dioxide. Just as humans require these important nutrients to be healthy, the microscopic photosynthetic algae which live in the oceans require small amounts of nickel and copper in order to survive. Humans obtain nickel, copper, and other nutrients from the food we eat, while algae in the oceans must obtain these nutrients from seawater. Therefore, it is important to understand how these metals get into seawater, and how they are distributed thorughout the global oceans. The primary goal of this project was to measure nickel (Ni) and copper (Cu) in the North Pacific Ocean, using samples collected during the US GEOTRACES GP15 oceanographic cruise. Hundreds of seawater samples were collected during this cruise, at locations from the coast of Alaska, down past Hawaii, over the equator, and into the South Pacific near Tahiti, at depths ranging from the surface ocean down to near the ocean floor thousands of meters below. These samples were then returned to our laboratories, where we extracted Ni and Cu to measure both their concentrations in seawater, and their stable isotope ratios.The concentrations of Ni and Cu both decrease towards the surface ocean. This reflects the fact that phytoplankton in the surface ocean are taking these metals out of seawater to use as nutrients, then subsequently dying, and sinking into the deep ocean, taking their nickel and copper with them. By measuring exactly how much Ni and Cu concentrations decrease in the upper ocean, we can better understand how much is removed by phytoplankton. Metal stable isotope 'fingerprints' provide unique information about nickel and copper. Specifically, we find that there are more heavy nickel isotopes in the surface ocean than at depth, reflecting the fact that phytoplankton are preferentially removing the lighter isotopes of nickel. Additionally, we find that nickel isotopes are especially heavy in the South Pacific ocean near the equator, which suggests an especially high demand for Ni by phytoplankton living here, perhaps for the process of nitrogen fixation. These data contribute to the global database of Ni and Cu concentrations and stable isotope ratios produced from the GEOTRACES program. By combining our data with other data from across the world, we will better understand how these important algal nutrients affect life in the oceans. Last Modified: 01/10/2023 Submitted by: Seth G John