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

Abstract

The Subantarctic Zone (SAZ), which covers the northern half of the Southern Ocean between the Subtropical and Subantarctic Fronts, is important for air-sea CO 2 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 CO 2 partial pressure (pCO 2 ) and hydrographic properties covering a full annual cycle in the SAZ. The amplitude of the seasonal cycle in pCO 2 (∼60 μatm), from near-atmospheric equilibrium in late winter to ∼330 μatm in midsummer, results from opposing physical and biological drivers. Decomposing these contributions demonstrates that the biological control on pCO 2 (up to 100 μatm), 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.

Carbon uptake and biogeochemical change in the Southern Ocean, south of Tasmania

“…Li and Peng, 2002;Najjar, 2009). Metzl et al (1999) and Shadwick et al (2015) observed that the uptake of CO 2 over the sub-Antarctic zone (SAZ, between the SAF and the STF) in summer is mostly controlled by biological processes. If a change in remineralisation rates has occurred, i.e.…”

Utilizing the Drake Passage Time-series to understand variability and change in subpolar Southern Ocean pCO<sub>2</sub>

“…Figure 3 indicates how inconsistent Southern Ocean pCO 2 data density is in space and time. Even given the sampling efforts of the DPT, repeated occupations of SR03 south of Australia [Shadwick et al 2015], along…”

Annual nitrate drawdown observed by SOCCOM profiling floats and the relationship to annual net community production

“…Measurements of the annual production of oxygen [ Riser and Johnson , ; Bushinsky and Emerson , ] or depletion of nitrate [ Plant et al ., ] using sensors mounted on profiling floats can provide direct estimates of ANCP after the Redfield ratio is used to convert the measured quantity to carbon uptake. While these sensors can be deployed on a variety of other platforms, such as moorings [ Shadwick et al ., ] or gliders [ Nicholson et al ., ], the high operational costs of these platforms [ Rudnick et al ., ] generally preclude large arrays from being deployed [ Rudnick , ]. The Argo program [ Riser et al ., ] has demonstrated that very large arrays of profiling floats can be operated at the global scale.…”