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Ocean Science An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/os-2020-37
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/os-2020-37
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  04 May 2020

04 May 2020

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A revised version of this preprint was accepted for the journal OS and is expected to appear here in due course.

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

Léo Mahieu1, Claire Lo Monaco2, Nicolas Metzl2, Jonathan Fin2, and Claude Mignon2 Léo Mahieu et al.
  • 1Ocean Sciences, School of Environmental Sciences, 4 Brownlow Street, Liverpool L69 3GP, UK
  • 2LOCEAN-IPSL, CNRS, Sorbonne Université, Paris, France

Abstract. Antarctic bottom waters (AABWs) are known as a long term sink for anthropogenic CO2 (Cant) 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 Cant 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 Cant 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 Cant concentrations are about twice as large. Our analysis shows that the CT and Cant increasing rates in 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 anthropogenic CO2 in the different formation regions. This is however modulated by significant interannual to pluriannual variability associated with variations in hydrological (ϴ, S) and biogeochemical (CT, AT, 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 Cant 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 Cant sequestration based on sparse observations over several years.

Léo Mahieu et al.

Léo Mahieu et al.

Léo Mahieu et al.

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Short summary
We investigated the evolution of anthropogenic CO2 (Cant) in the Antarctic Bottom Water in the Southern Indian Ocean since 1978 based on observations from 16 reocupations. We found that the Cant and the dissolved inorganic carbon increased at about the same rate over the 40-years period. However, the data also show large interannual variations and a surprising stability of Cant in the last decade likely reflecting the variability of bottom waters formations and circulation in the Southern Ocean.
We investigated the evolution of anthropogenic CO2 (Cant) in the Antarctic Bottom Water in the...
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