Evolution of the Oceanic C-13 Suess Effect in the Southeastern Indian Ocean Between 1994 and 2018

The decrease in delta C-13 of dissolved inorganic carbon (delta C-13(DIC)) owing to uptake of anthropogenic CO2 (the oceanic C-13 Suess effect) in the Southeastern Indian Ocean over the last decade was calculated using an extended multiparameter linear regression technique. Samples collected on the CROCCA-2S (Coring to Reconstruct Ocean Circulation and Carbon Dioxide Across 2 Seas) cruise in November-December 2018 were compared to samples from the CLIVAR (Climate and Ocean: Variability, Predictability, and Change) and OISO (Ocean Indien Service d'Observation) programs from 2007 to 2009. Surface ocean delta C-13(DIC) decreased by an average of -0.53 +/- 0.04 parts per thousand across this period, at an average rate of -0.053 +/- 0.004 parts per thousand per year. This rate of delta C-13(DIC) change is an increase from -0.021 +/- 0.024 parts per thousand per year between 1994 and 2008. We find that the interior water mass most impacted by the oceanic C-13 Suess effect between 2008 and 2018 was Subantarctic Mode Water (SAMW), within which delta C-13(DIC) decreased by -0.044 +/- 0.002 parts per thousand per year. Using previously published relationships between the oceanic C-13 Suess effect and anthropogenic carbon, we estimate the annual storage of anthropogenic carbon within SAMW in the southeastern Indian Ocean has increased from similar to 2.0 +/- 0.2 mu mol/kg per year between 1994 and 2008 to 5.5 +/- 0.6 mu mol/kg per year between 2008 and 2018.