Ocean Acidification in the Southern Indian Ocean between 1994, 2007 and 2018.

Abstract:

The oceans are the largest exchangeable reservoir of carbon on Earth and the Southern Ocean is one of the most important areas in the world for the air-sea exchange of CO₂ due to the formation of intermediate and deep water masses there. Anthropogenic input of CO₂ to the atmosphere is causing increased Ocean acidification (OA) as seawater takes up CO₂ to remain in equilibrium with the atmosphere. The Southern Ocean is thought to account for around 40% of the global uptake of this anthropogenic carbon, making it particularly susceptible to OA. OA occurs as dissolved CO₂ reacts with water, and the resulting DIC (dissolved inorganic carbon) forms carbonic acid (H₂CO₃) and dissociates into bicarbonate (HCO₃^-) and carbonate (CO₃^2-) ions. Increasing DIC causes the pH of seawater to drop and the saturation state (Ω) to decrease. To analyze the change in OA in the Southeastern Indian and Southern Ocean, DIC and pH were measured on water samples from the CROCCA-2S (Coring to Reconstruct Ocean Circulation and Carbon Dioxide Across 2 Seas) cruise in November-December 2018. We use these new data to calculate seawater Ω. These results are compared to those obtained from the same region during the CLIVAR (Climate and Ocean: Variability, Predictability and Change) voyage in 2007-2009 and WOCE (World Ocean Circulation Experiment) in 1994-95. Comparison between our data and this previous data shows that the DIC concentration has increased through time. Our results will be used to assess OA over the last three decades.