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 (CANT) uptake on the
variability of dissolved inorganic carbon (DIC) for eight water mass layers
defined by neutral surfaces (γn). 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
remineralisation of organic matter or intensification of upwelling. The range
of estimates for the increases in 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 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 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
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 CANT
storage rates (∼ 0.41 ± 0.20 mol m−2 yr−1) twice
that of the upper layers
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