Changes in ocean acidification evaluated in the southern Indian ocean from observations over the last 30 years

Chair: Kumiko Azetsu-Scott

Renaud Gomez (1)*, Claire Lo Monaco (1), Nicolas Metzl (1)

1 Sorbonne Universités (UPMC, Univ Paris 06)-CNRS-IRD-MNHN, LOCEAN Laboratory, 4 place Jussieu, Paris, 75005, France

Background
The Southern Ocean is an important sink for anthropogenic CO2 and a region of Subantarctic Mode Water (SAMW) formation. This results in a large accumulation of anthropogenic carbon (Cant) in the ocean between 30°S and 50°S, which leads to a decrease in seawater pH. This acidification process could impact marine organism, the biological carbon pump and thus global climate.

Methods
We used total alkalinity (TA) and dissolved inorganic carbon (DIC) measurements collected during 10 oceanographic cruises in the Southern Indian Ocean from 1985 to 2012 to estimate the decadal trends in pH and the saturation state for aragonite in the Antarctic and Subantarctic surface waters (ASW and SASW respectively) and also in the SAMW. We also identify the different drivers behind the observed trends.

Findings
Over this period, the aragonite saturation state decrease is much higher in the SASW (0.20±0.07/decade) than in the SAMW and the ASW (0.09±0.02/decade) due to a maximum increase of DIC. Concerning the pH the observed decrease is statistically similar in the three studied waters (from 0.018±0.005/decade to 0.025±0.003/decade). In the SASW and the SAMW the pH decrease is mainly attributed to a DIC increase, whereas in the ASW interannual changes of carbon parameters play a significant role.

Conclusions
Over the last 30 years significant pH and aragonite decrease were observed in the three studied waters. The role of natural variability or climate-induced changes in the carbonate chemistry need to be better understood for future estimations of ocean acidification parameters