Biogeochemical controls on dissolved zinc cycling in the Equatorial Pacific Ocean

Zinc (Zn)-containing metalloenzymes facilitate the uptake and fixation of dissolved inorganic carbon and phosphorus (P) in marine phytoplankton, coupling the Zn biogeochemistry with biological cycling in the ocean. Furthermore, dissolved Zn (dZn) distribution in the global ocean is strongly linked with surface biogeochemistry, water mass formation and circulation in the Southern Ocean. However, additional controls of basin-scale biogeochemical processes on Zn cycling outside the Southern Ocean still remain uncertain. We present dZn distribution measured along the GEOTRACES cruise section, GP11, in the Equatorial Pacific Ocean (EPO) and discuss the relative roles of regional biological cycling and large-scale physical circulation in controlling the observed distribution. Dissolved Zn and P in the surface and thermocline waters (< 400m) exhibit an overall positive linear relation, albeit with an apparent kink at ~1μM of P: strong correlation above the kink and uniform, low dZn concentration below the kink. The estimated dZn to P ratio (0.74 ± 0.07) from the linear relation in the thermocline waters is comparable to the observed Zn/P uptake ratio (0.68) for the equatorial Pacific picoplankton, which dominate the phytoplankton biomass in the region, and larger than that reported over similar potential density range in the source region of the thermocline waters (~0.18–0.32). This indicates the important control of organic matter regeneration on observed dZn variations in the upper water column of EPO. Anomalously high dZn concentrations are observed close to the South American margin in the oxygen-deficient sub-surface waters, suggesting Zn sourced from remineralization of Zn-rich biogenic particles and/or authigenic Zn sulfide phases in resuspended reducing margin sediments. However, this signal decreases offshore due to advective mixing with low dZn ambient waters. In the deeper waters (> 500m), we used an extended optimum multiparameter water mass model and end-member composition of water masses to demonstrate that water mass mixing predominantly governs the dZn distribution in the EPO, while the impact of organic matter regeneration and reversible particle scavenging is limited. Overall, our study highlights the influence of local phytoplankton trace metal uptake stoichiometry in the upper waters (< 400m) and water circulation on the dZn distribution in deeper waters (> 500m) of the EPO, and offers new insights into the inter-basin variability in Zn biogeochemical cycling in the Pacific Ocean.