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

• 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-2020-37; https://os.copernicus.org/preprints/os-2020-37/

Antarctic bottom waters (AABWs) are known as a long term sink for anthropogenic CO 2 (C ant ) but is hardly quantified because of the scarcity of the observations, specifically at an interannual scale. We present in this manuscript 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 frame 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 fresh and younger Cape Darnley bottom water (CDBW) and the Weddell Sea deep water (WSDW). Our calculations reveal that C ant concentrations increased significantly in AABW, from about + 7 µmol kg -1 in 1978–1987 to + 13 µmol kg -1 in 2010–2018. This is comparable to previous estimates in other SO basins, with the exception of bottom waters close to their formation sites where C ant concentrations are about twice as large. Our analysis shows that the C T and C ant increasing rates in 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 anthropogenic CO 2 in the different formation regions. This is however modulated by significant interannual to pluriannual variability associated with variations in hydrological (ϴ, S) and biogeochemical (C T , A T , 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 C ant sequestration by AABWs is more variable than expected and depends on a complex combination of physical, chemical and biological processes at the formation sites and during the transit of the different AABWs. The interannual variability at play in AABW needs to be carefully considered on the extrapolated estimation of C ant sequestration based on sparse observations over several years.