Variability and stability of anthropogenic CO₂ in Antarctic Bottom Water observed in the Indian sector of the Southern Ocean, 1978–2018

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.