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

• Published in: Biogeosciences
• Main Authors: P. C. Pardo, B. Tilbrook, C. Langlais, T. W. Trull, S. R. Rintoul
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2017
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; Antarctic; Southern Ocean; Antarc*
• Online Access: https://doi.org/10.5194/bg-14-5217-2017; https://doaj.org/article/93621450ede5475a8da878b914cc0c2c

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 ( γ n ). 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 CO 2 (∼ 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 CO 2 . 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 ...