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

Abstract

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 (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 ( 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, 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 CO 2 , 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.

Decadal‐Scale Increases of Anthropogenic CO2 in Antarctic Bottom Water in the Indian and Western Pacific Sectors of the Southern Ocean

“…Although we found large positive values throughout the water column east of 120°E, they were not matched in the ΔCFC‐12 results. If upwelling of old deep water increased AOU (Pardo et al, ) in an earlier cruise, increases of anthropogenic CO 2 between two cruises are calculated to be high. In addition, enhanced biological production can affect the disequilibrium of CO 2 between atmosphere and sea surface (Arrigo et al, ) such that the upper water layers can draw more anthropogenic CO 2 from the atmosphere.…”

GLODAPv2.2019 – an update of GLODAPv2

“…Broadly speaking, the pH data in GLODAP have been obtained using a variety of methods (e.g., potentio-metric measurements, and spectrophotometric measurements with purified or impure dyes). The pH values produced by these different approaches have documented pH-dependent offsets from one another (Carter et al, 2013;Liu et al, 2011;Patsavas et al, 2015;Yao et al, 2007) that challenge the viability of the uniform adjustments applied (Carter et al, 2018). While we have continued to apply such uniform offsets for this update, we have chosen the higher initial minimum adjustment limit of 0.01, which is twice that used for GLO-DAPv2 (0.005), to minimize the possibility of false corrections.…”

“…For GLO-DAPv2, we have registered more than 120 applications. Examples include model evaluation (Beadling et al, 2018;Goris et al, 2018;Tjiputra et al, 2018;Ward et al, 2018), model initialization (Orr et al, 2017), water mass analyses (Jeansson et al, 2017;Peters et al, 2018;Rae and Broecker, 2018), ocean acidification (Fassbender et al, 2017;García-Ibáñez et al, 2016;Perez et al, 2018) calibration of Argo biogeochemical sensor measurements (Bushinsky et al, 2017;Johnson et al, 2017), calibration of multiple linear regression (MLR) and neural-network-based methods for biogeochemical data estimation (Bittig et al, 2018;Carter et al, 2018;Fry et al, 2016;Sauzède et al, 2017), contextualization of paleo-oceanographic data (Glock et al, 2018;Sessford et al, 2018), and calculation of inventory, transport, and variability of ocean carbon (DeVries et al, 2017;Fröb et al, , 2018Gruber et al, 2019;Panassa et al, 2018;Pardo et al, 2017;Quay et al, 2017). A full list of GLODAPv2 citations is provided at https://www.glodap.info/index.php/ glodap-impact/ (last access: 17 September 2019).…”

Coccolithophore populations and their contribution to carbonate export during an annual cycle in the Australian sector of the Antarctic zone

“…Therefore, the location of sediment traps at the 61 • S site allows for the assessment of dissolution changes, if any, of coccolithophore assemblages between the two critical dissolution depth horizons: the CSH and CCD. Notably, both the progressive uptake of anthropogenic CO 2 and increased upwelling of naturally CO 2 -rich deep waters over the past 20 years is leading to the shallowing of these features (Pardo et al, 2017).…”