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. Key Points The Zn:PO43− uptake ratio varies by approximately tenfold across latitude and is modulated by Fe availability Zn remineralizes similar to PO43− in the upper ocean, but its accumulation in deep waters exceeds that of PO43− The strong Zn‐SiO44− correlation is caused by a combination of surface uptake, desorption from particles, and hydrothermal inpu

Inter-laboratory study for the certification of trace elements in seawater certified reference materials NASS-7 and CASS-6

Certification of trace metals in seawater certified reference materials (CRMs) NASS-7 and CASS-6 is described. At the National Research Council Canada (NRC), column separation was performed to remove the seawater matrix prior to the determination of Cd, Cr, Cu, Fe, Pb, Mn, Mo, Ni, U, V, and Zn, whereas As was directly measured in 10-fold diluted seawater samples, and B was directly measured in 200-fold diluted seawater samples. High-resolution inductively coupled plasma mass spectrometry (HR-ICPMS) was used for elemental analyses, with double isotope dilution for the accurate determination of B, Cd, Cr, Cu, Fe, Pb, Mo, Ni, U, and Zn in seawater NASS-7 and CASS-6, and standard addition calibration for As, Co, Mn, and V. In addition, all analytes were measured using standard addition calibration with triple quadrupole (QQQ)-ICPMS to provide a second set of data at NRC. Expert laboratories worldwide were invited to contribute data to the certification of trace metals in NASS-7 and CASS-6. Various analytical methods were employed by participants including column separation, co-precipitation, and simple dilution coupled to ICPMS detection or flow injection analysis coupled to chemiluminescence detection, with use of double isotope dilution calibration, matrix matching external calibration, and standard addition calibration. Results presented in this study show that majority of laboratories have demonstrated their measurement capabilities for the accurate determination of trace metals in seawater. As a result of this comparison, certified/reference values and associated uncertainties were assigned for 14 elements in seawater CRMs NASS-7 and CASS-6, suitable for the validation of methods used for seawater analysis