Biogeochemical controls on dissolved micronutrient (Ni, Cu, Zn, Cd) distributions in the Equatorial Pacific Ocean

The Equatorial Pacific Ocean (EPO) is one of the most productive regions in the ocean with upwelling of nutrient-rich subsurface waters and strong outgassing of CO 2 , and thereby playing a prominent role in the global carbon cycle. We report distributions of dissolved (d) and particulate (p) trace metals (TM) including nickel (dNi), copper (dCu), zinc (dZn) and cadmium (dCd), along the GEOTRACES transect, GP11, in the EPO during a developing El Niño phase, associated with anomalous surface winds and currents. Using estimates of surface dTM fluxes and water mass mixing in deeper waters, we assessed biogeochemical controls on dTM distributions in the water column. Surface waters (depths < 100 m) showed dTMs minima and significant correlations between pTM and particulate phosphorus, indicating phytoplankton uptake of TMs and low atmospheric input. A large proportion (64–99%) of dNi, dZn and dCd in the surface mixed layer (24–51 m) was supplied by vertical upwelling, which was strongly influenced by the west-east shoaling of the thermocline and transport of sub-surface waters by the Equatorial Undercurrent. Moreover, spatial variability in vertical fluxes controlled biological Cd uptake in the EPO. In mesopelagic and deep waters (depths > 200 m), water mass mixing and organic matter (OM) remineralization largely governed the dCd distribution. The dNi, dCu and dZn distributions and their relationships with macronutrients were determined by particle scavenging of remineralized Ni, Cu and Zn and/or their retention (slower remineralization) to particles. Strong enrichments in dNi and dCu were observed in deep waters (depths > 2500 m) of the eastern and central EPO, which could not be explained by water mass mixing and OM remineralization. Particulate data indicated a limited influence of reversible scavenging and deep-water remineralization, with the supply from sediments likely supporting the dNi and dCu increases in deep waters. Our results underscore the need to better quantify benthic trace metal fluxes across Pacific abyssal sediments, as sedimentary inputs may substantially influence deep ocean micronutrient reservoirs and their eventual redistribution to the upper ocean.