Evolution of the Oceanic 13C 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.

Automated summary

The study revealed a decrease in delta C-13(DIC) in the surface ocean of the Southeastern Indian Ocean over the last decade, primarily attributed to the uptake of anthropogenic CO2, with a significant impact observed in the Subantarctic Mode Water (SAMW). The research estimated that the annual storage of anthropogenic carbon in the SAMW of the southeastern Indian Ocean increased significantly between 2008 and 2018 compared to previous years.