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

• Published in: Ocean Science
• Main Authors: Mahieu, Léo, Monaco, Claire, Metzl, Nicolas, Fin, Jonathan, Mignon, Claude
• Format: Text
• Language: English
• Published: 2020
• Subjects: Antarctic; Cape Darnley; Darnley; Indian; Southern Ocean; Weddell; Weddell Sea; Antarc*
• Online Access: https://doi.org/10.5194/os-16-1559-2020; https://os.copernicus.org/articles/16/1559/2020/

Antarctic Bottom Water (AABW) is known as a long-term sink for anthropogenic CO 2 ( C ant ), 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 C ant 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 C ant concentrations increased significantly in the AABW, from an average concentration of 7 µ mol kg −1 calculated for the period 1978–1987 to an average concentration of 13 µ mol 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 C ant concentrations are about twice as large. Our analysis shows that total carbon ( C T ) and C ant increasing rates in the AABW are about the same over the period 1978–2018, and we conclude that the long-term change in C T is mainly due to the uptake of C ant 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 ( C T total alkalinity, A T dissolved oxygen, O 2 ) properties. A surprising result is the apparent stability of C ant concentrations in recent years despite the increase in C T and the gradual acceleration of atmospheric CO 2 . The interannual variability at play in AABW needs to be carefully considered in the extrapolated estimation of C ant sequestration based on sparse observations over several years.