Variability and stability of anthropogenic CO<sub>2</sub> in Antarctic Bottom Water observed in the Indian sector of the Southern Ocean, 1978–2018

Abstract

Abstract. Antarctic Bottom Water (AABW) is known as a long-term sink for anthropogenic CO2 (Cant), but the sink is hardly quantified because of the scarcity of observations, specifically at an interannual scale. We present in this paper an original dataset combining 40 years of carbonate system observations in the Indian sector of the Southern Ocean (Enderby Basin) to evaluate and interpret the interannual variability of Cant in the AABW. This investigation is based on regular observations collected at the same location (63∘ E–56.5∘ S) in the framework of the French observatory OISO from 1998 to 2018 extended by GEOSECS and INDIGO observations (1978, 1985 and 1987). At this location the main sources of AABW sampled is the low-salinity Cape Darnley Bottom Water (CDBW) and the Weddell Sea Deep Water (WSDW). Our calculations reveal that Cant concentrations increased significantly in the AABW, from an average concentration of 7 µmol kg−1 calculated for the period 1978–1987 to an average concentration of 13 µmol kg−1 for the period 2010–2018. This is comparable to previous estimates in other Southern Ocean (SO) basins, with the exception of bottom water close to formation sites where Cant concentrations are about twice as large. Our analysis shows that total carbon (CT) and Cant increasing rates in the AABW are about the same over the period 1978–2018, and we conclude that the long-term change in CT is mainly due to the uptake of Cant in the different formation regions. This is, however, modulated by significant interannual to multi-annual variability associated with variations in hydrographic (potential temperature, Θ; salinity, S) and biogeochemical (CT; total alkalinity, AT; dissolved oxygen, O2) properties. A surprising result is the apparent stability of Cant concentrations in recent years despite the increase in CT and the gradual acceleration of atmospheric CO2. The interannual variability at play in AABW needs to be carefully considered in the extrapolated estimation of Cant sequestration based on sparse observations over several years.

Physical and biological controls on anthropogenic CO2 sink of the Ross Sea

“…An important component of this uptake occurs in Antarctica's continental shelves and coastal polynyas (Arrigo et al, 2008a), where high primary production and formation of Dense Shelf Water (DSW) have the potential to efficiently sequester the C ant into the deep ocean over centennial time scales (Sigman et al, 2010). However, notable discrepancies exist among different Antarctic shelf areas (Arrigo et al, 2008b;Lee et al, 2017), and regional estimates of net CO 2 uptake (Mahieu et al, 2020) come with large uncertainties (McKinley et al, 2017;Gruber et al, 2019). This limited knowledge is mostly due to sparse observations (Lenton et al, 2013) and incomplete evaluations of the different processes that govern the ocean carbon dynamics, such as melt water influence, sea surface temperature, ocean circulation, wind regimes, and biological processes (Takahashi et al, 2012).…”

Assessment of Annual Physico-Chemical Variability via High-Temporal Resolution Monitoring in an Antarctic Shallow Coastal Site (Terra Nova Bay, Ross Sea)

“…Even the local character of our study, TNB, is where the DSW, the precursor for the AABW, orginate and bring information on water masses from local coastal systems [20]. The increase in anthropogenic CO 2 storage in the AABW recorded in the past 40 years was accompained by a long-term change in total carbon concentration due to anthropogenic CO 2 uptake of the different formation regions [33]. All these changes were modulated by significant interannual to multi-annual variability associated with variations in physicochemical properties (i.e., temperature, practical salinity, DO, TA, pCO 2 ) orginating at local other than regional scales involving the ecosystems.…”

“…As a result of the heterotrophic activities, this surface water becomes supersaturated with CO 2 and undersaturated in terms of DO [30]. The carbonate system in the Ross Sea and the variability associated with different water masses were investigated in a summer campaign in 2008 [31], showing that the highest pH T values in TNB (about 8.03 at 200 m of depth) are measured where waters are oversaturated (more than 1) in terms of both calcite and aragonite [32,33].…”

East–west contrasting changes in Southern Indian Ocean Antarctic Bottom Water salinity over three decades

“…Antarctic Bottom Water (AABW), generally defined as seawater with a potential temperature (θ) less than 0 °C or neutral density (γ n ) greater than 28.27 kg m −3 , forms near the Antarctic continental margins 1 – 5 . AABW ventilates the abyssal ocean, sequesters heat and carbon, and regulates the global overturning circulation and atmospheric carbon dioxide 4 – 7 . There are four identified regions of AABW formation; AABW has been reported to have slightly different properties in each of them 4 , 5 , 8 – 14 (Fig.…”

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Figure 1 Schematic of the primary sources and paths of Antarctic Bottom Water (AABW) in Southern Indian Ocean (SIO), with ocean depth highlighted. Schematic pathways of AABW (blue dashed arrows) and Lower Circumpolar Deep Water (LCDW; thick red arrows) based on previous works 1 , 3 , 5 , 7 – 9 , 13 – 20 , 28 , 30 – 32 are superimposed with ocean depth (shaded). Source water formation regions (Weddell Sea Deep Water, WSDW; Cape Darnley Bottom Water, CDBW; Adélie Land Bottom Water, ALBW; and Ross Sea Bottom Water, RSBW) are marked using solid arrows and labels (red for WSDW and RSBW, cyan for CDBW and ALBW).

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