The Internal Cycling of Zinc in the Ocean

The internal cycling of zinc (Zn) in the ocean has been a longstanding mystery. Particularly, puzzling is the strong correlation between Zn and silicate (SiO44-), but not phosphate (PO43-), even though Zn is involved with cell functions that regulate PO43- uptake and are unrelated to SiO44- uptake. To help solve this mystery, we use an artificial neural network to produce global maps of dissolved Zn, and then use a diagnostic model to infer rates of uptake and regeneration for Zn, SiO44-, and PO43-. We find that plankton in the Southern Ocean account for 62 (32)% of global Zn uptake. The plankton Zn:PO43- uptake ratio increases by more than tenfold from the low latitudes to the Southern Ocean, a much larger range than expected from culture studies, suggesting controls from factors such as iron availability. Reconstruction of particulate Zn (PZn), phosphorus (PP), and biogenic silica (PSi) fluxes reveals that PZn remineralizes like PP, and not like PSi. However, a small flux of PZn into the deep ocean is not matched by an equivalent flux of PP, which is likely due to the combined effects of desorption of scavenged Zn and the input of hydrothermal Zn in the deep ocean. This small difference in the remineralization of PZn and PP, combined with the patterns of surface uptake, eliminates the correlation between Zn and PO43- in the deep ocean and causes a tight correlation between Zn and SiO44-. This coincidental correlation cannot be expected to hold for past and future states of the ocean. Plain Language Summary Marine phytoplankton account for half of global net primary production. These plankton require zinc (Zn) to grow, but the cycle of Zn in the ocean is not well understood. We combined observations and a model to determine that plankton use Zn at highly variable rates, with lowest rates in the tropical oceans and highest rates in the Southern Ocean. This bioassimilated Zn reenters water column as swiftly as phosphorous. However, Zn is enriched relative to phosphorous in the deep ocean due to additional inputs of Zn from seafloor hydrothermal vents, and from desorption of Zn that has been scavenged onto sinking particles. These processes cause the hitherto mysterious correlation between Zn and silicon, an element not involved in the Zn cycle.