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

Abstract

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.

Plio-Pleistocene deep-sea ventilation in the eastern Pacific and potential linkages with Northern Hemisphere glaciation

“…S7E) (34). The benthic foraminiferal δ 13 C record is interpreted to reflect changes in lower circumpolar deep water (LCDW) intensity (34) because different water masses have distinct δ 13 C values (35). A comparison between our MFN metal records and the sediment grain size record from Ocean Drilling Program (ODP) site 1123, located northeast of the Chatham Islands, records of the Pacific deep western boundary current (36), also highlights similar variations (fig.…”

“…S7, C to F, and table S1). Specifically, we observe synchronous changes in MFN elemental content data (Ni, Mn, and Cu) and the benthic foraminiferal δ 13 C record of core MV0502-4JC from the Southern Ocean (fig. S7E) (34).…”

“…1) covers >70% of the total oceanbottom region and 30 to 40% of the total global water mass (11). It has high dissolved oxygen concentration (12) and isolates CO 2 in the deep ocean (13,14). The long-term evolution of AABW remains, however, poorly known.…”

Summer trends and drivers of sea surface fCO<sub>2</sub> and pH changes observed in the southern Indian Ocean over the last two decades (1998–2019)

“…For the water-column data obtained between 1998 and 2011 secondary quality controls were performed in the frame of the CARINA (Carbon Dioxide in the Atlantic Ocean) and GLODAP-v2 (Global Ocean Data Analysis Project version 2) projects (Lo Monaco et al, 2010;Key et al, 2015;Olsen et al, 2016). For OISO cruises conducted since 2012 (not yet included in GLODAP-v2) we proceeded to a data quality control following the same protocol as for GLODAP-v2 (comparison with deep waters), and we found no systematic bias for the properties measured during these cruises as described in detail by Mahieu et al (2020). All the data (and metadata) will be delivered to OCADS (Ocean Carbon and Acidification Data System) NCEI (National Centers for Environmental Information) database and to GLODAP-v2-2023.…”

“…In addition, surface samples were collected every 4 to 8 h for salinity, nutrients and chl a. Analytical methods for these cruises followed the protocols used since 1998 described previously (e.g. Metzl et al, 2006;Mahieu et al, 2020;Lo Monaco et al, 2021).…”