The Equatorial Pacific Ocean (EPO) is one of the major open-ocean upwelling regions, supporting about one-fifth of the total marine net primary production and playing a key role in the global carbon cycle. Phytoplankton growth in vast areas of the central and eastern EPO is limited by the low supply of bio-essential micronutrients, particularly iron (Fe), to the euphotic zone. The processes controlling micronutrient distributions in the EPO, however, remain poorly constrained due to sparse observations. We present sectional distributions of dissolved (d) and particulate (p) micronutrients including Fe, manganese (Mn), nickel (Ni), copper (Cu), zinc (Zn) and cadmium (Cd), along a zonal GEOTRACES transect, GP11, in the EPO. The study aimed to identify the biogeochemical controls on micronutrient distributions and examine potential links between EPO trace metal cycling and trans-equatorial and global biogeochemical processes.
In the upper water column (depths <300 m), the strong zonal Equatorial Undercurrent (EUC) was observed to influence micronutrient distributions through (1) the transport of Fe- and Mn-rich water and particles from the western Pacific margin toward the eastern EPO and (2) the west to east shoaling of the EUC and thermocline, regulating vertical mixing of dNi, dCu, dZn and dCd in the euphotic zone. In the mesopelagic and deep waters (depths >300 m), large-scale ocean circulation and water mass mixing, organic matter (OM) remineralization, and particle exchange processes exert distinct but varying controls on distributions of different micronutrients. Constraints from particulate data and water mass mixing modelling results indicate benthic inputs of dNi and dCu to deep waters (depths >2500 m) from OM-rich sediments in the open eastern and central EPO. In contrast, sinking and deposition of lithogenic mineral-rich particles in the western EPO result in enrichments of dFe and pFe throughout the water column. Overall, the study reveals the coupled roles of water mass mixing, particle dynamics and benthic inputs in regulating micronutrient distributions across the EPO and provides new mechanistic insights into trace metal cycling and its role in regulating marine productivity in a globally significant upwelling system.

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