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

Abstract. Biogeochemical change in the water masses of the Southern Ocean, south of Tasmania, was assessed for the 16-year period between 1995 and 2011 using data from four summer repeats of the WOCE–JGOFS–CLIVAR–GO-SHIP (Key et al., 2015; Olsen et al., 2016) SR03 hydrographic section (at ∼ 140° E). Changes in temperature, salinity, oxygen, and nutrients were used to disentangle the effect of solubility, biology, circulation and anthropogenic carbon (C_ANT) uptake on the variability of dissolved inorganic carbon (DIC) for eight water mass layers defined by neutral surfaces (γⁿ). C_ANT was estimated using an improved back-calculation method. Warming (∼ 0.0352 ± 0.0170 °C yr⁻¹) of Subtropical Central Water (STCW) and Antarctic Surface Water (AASW) layers decreased their gas solubility, and accordingly DIC concentrations increased less rapidly than expected from equilibration with rising atmospheric CO₂ (∼ 0.86 ± 0.16 µmol kg⁻¹ yr⁻¹ versus ∼ 1 ± 0.12 µmol kg⁻¹ yr⁻¹). An increase in apparent oxygen utilisation (AOU) occurred in these layers due to either remineralisation of organic matter or intensification of upwelling. The range of estimates for the increases in C_ANT were 0.71 ± 0.08 to 0.93 ± 0.08 µmol kg⁻¹ yr⁻¹ for STCW and 0.35 ± 0.14 to 0.65 ±  0.21 µmol kg⁻¹ yr⁻¹ for AASW, with the lower values in each water mass obtained by assigning all the AOU change to remineralisation. DIC increases in the Sub-Antarctic Mode Water (SAMW, 1.10 ± 0.14 µmol kg⁻¹ yr⁻¹) and Antarctic Intermediate Water (AAIW, 0.40 ± 0.15 µmol kg⁻¹ yr⁻¹) layers were similar to the calculated C_ANT trends. For SAMW, the C_ANT increase tracked rising atmospheric CO₂. As a consequence of the general DIC increase, decreases in total pH (pH_T) and aragonite saturation (Ω_Ar) were found in most water masses, with the upper ocean and the SAMW layer presenting the largest trends for pH_T decrease (∼ −0.0031 ± 0.0004 yr⁻¹). DIC increases in deep and bottom layers (∼ 0.24 ± 0.04 µmol kg⁻¹ yr⁻¹) resulted from the advection of old deep waters to resupply increased upwelling, as corroborated by increasing silicate (∼ 0.21 ± 0.07 µmol kg⁻¹ yr⁻¹), which also reached the upper layers near the Antarctic Divergence (∼ 0.36 ± 0.06 µmol kg⁻¹ yr⁻¹) and was accompanied by an increase in salinity. The observed changes in DIC over the 16-year span caused a shoaling (∼ 340 m) of the aragonite saturation depth (ASD, Ω_Ar =  1) within Upper Circumpolar Deep Water that followed the upwelling path of this layer. From all our results, we conclude a scenario of increased transport of deep waters into the section and enhanced upwelling at high latitudes for the period between 1995 and 2011 linked to strong westerly winds. Although enhanced upwelling lowered the capacity of the AASW layer to uptake atmospheric CO₂, it did not limit that of the newly forming SAMW and AAIW, which exhibited C_ANT storage rates (∼ 0.41 ± 0.20 mol m⁻² yr⁻¹) twice that of the upper layers.