Some observations on the biogeochemical cycling of zinc in the Australian sector of the Southern Ocean: a dedication to Keith Hunter

In this study we investigated the distribution of dissolved and particulate zinc (dZn and pZn respectively) and its isotopes in the Subantarctic Zone as part of a Geotraces Process voyage. dZn and pZn depth profiles contrasted each other, with dZn showing depletion within the euphotic zone while pZn profiles showed enrichment. Fitting a power law equation to the pZn profiles produced an attenuation factor of 0.82, which contrasted values for particulate phosphorus, cadmium and copper. The results indicate that zinc has a longer regeneration length scale than phosphorus and cadmium, but shorter than copper. The differential regeneration of pZn relative to that of particulate phosphorus likely explains why dZn appears to have a deeper regeneration profile than that of phosphate. The dZn isotope (δ66Zndissolved) profiles collected across the Subantarctic Zone showed differing profile structures. For one station collected within an isolated cold-core eddy (CCE), δ66Zndissolved showed surface enrichment relative to deep waters. The corresponding pZn isotope profiles within the CCE did not show enrichment; rather, they were subtly depleted in surface waters and then converged to similar values at depth. Zinc isotope fractionation can be explained through a combination of fractionation processes associated with uptake by phytoplankton, zinc complexation by natural organic ligands and zinc regeneration from particulate matter

Seasonality of biological and physical controls on surface ocean CO2 from hourly observations at the Southern Ocean Time Series site south of Australia

The Subantarctic Zone (SAZ), which covers the northern half of the Southern Ocean between the Subtropical and Subantarctic Fronts, is important for air-sea CO2 exchange, ventilation of the lower thermocline, and nutrient supply for global ocean productivity. Here we present the first high-resolution autonomous observations of mixed layer CO2 partial pressure (pCO(2)) and hydrographic properties covering a full annual cycle in the SAZ. The amplitude of the seasonal cycle in pCO(2) (similar to 60 mu atm), from near-atmospheric equilibrium in late winter to similar to 330 mu atm in midsummer, results from opposing physical and biological drivers. Decomposing these contributions demonstrates that the biological control on pCO(2) (up to 100 mu atm), is 4 times larger than the thermal component and driven by annual net community production of 2.45 +/- 1.47 mol C m(-2) yr(-1). After the summer biological pCO(2) depletion, the return to near-atmospheric equilibrium proceeds slowly, driven in part by autumn entrainment into a deepening mixed layer and achieving full equilibration in late winter and early spring as respiration and advection complete the annual cycle. The shutdown of winter convection and associated mixed layer shoaling proceeds intermittently, appearing to frustrate the initiation of production. Horizontal processes, identified from salinity anomalies, are associated with biological pCO(2) signatures but with differing impacts in winter (when they reflect far-field variations in dissolved inorganic carbon and/or biomass) and summer (when they suggest promotion of local production by the relief of silicic acid or iron limitation). These results provide clarity on SAZ seasonal carbon cycling and demonstrate that the magnitude of the seasonal pCO(2) cycle is twice as large as that in the subarctic high-nutrient, low-chlorophyll waters, which can inform the selection of optimal global models in this region

Distinct iron cycling in a Southern Ocean eddy

Mesoscale eddies are ubiquitous in the iron-limited Southern Ocean, controlling ocean-atmosphere exchange processes, however their influence on phytoplankton productivity remains unknown. Here we probed the biogeochemical cycling of iron (Fe) in a cold-core eddy. In-eddy surface dissolved Fe (dFe) concentrations and phytoplankton productivity were exceedingly low relative to external waters. In-eddy phytoplankton Fe-to-carbon uptake ratios were elevated 2–6 fold, indicating upregulated intracellular Fe acquisition resulting in a dFe residence time of ~1 day. Heavy dFe isotope values were measured for in-eddy surface waters highlighting extensive trafficking of dFe by cells. Below the euphotic zone, dFe isotope values were lighter and coincident with peaks in recycled nutrients and cell abundance, indicating enhanced microbially-mediated Fe recycling. Our measurements show that the isolated nature of Southern Ocean eddies can produce distinctly different Fe biogeochemistry compared to surrounding waters with cells upregulating iron uptake and using recycling processes to sustain themselves