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

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 4 summer repeats of the WOCE/JGOFS/CLIVAR/GO-SHIP 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 (CANT) uptake on the variability of dissolved inorganic carbon (DIC) for 8 water mass layers defined by neutral surfaces (ϒn). CANT 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 remineralization of organic matter or intensification of upwelling. The range of estimates for the increases of CANT 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 remineralization. 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 CANT trends. For SAMW, the CANT increase tracked rising atmospheric CO2. As a consequence of the general DIC increase, decreases in total pH (pHT) and aragonite saturation (ΩAr) were found in most water masses, with the upper ocean and the SAMW layer presenting the largest trends for pHT 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, ΩAr = 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, probably linked to a positive trend in the Southern Annular Mode. 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 CANT storage rates (~ 0.41 ± 0.20 mol m−2 yr−1) twice that of the upper layers.

Pardo P. C., Tilbrook B., Langlais C., Trull T. W. & Rintoul S. R., 2017. Carbon uptake and biogeochemical change in the Southern Ocean, south of Tasmania. Biogeosciences Discussions 1-43. Article.

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