Temporal variability in dissolved inorganic carbon, δ13CDIC, and anthropogenic CO2 in the North Indian Ocean from 1995 to 2016: assessing the influence of anthropogenic CO2

The dynamics of dissolved inorganic carbon (DIC), stable carbon isotopes of DIC (δ13CDIC), and anthropogenic CO2 (CO2ant) in the upper 500 m of the water column were examined in two upwelling-favourable regions: the Sri Lankan Dome (SLD) and the central Bay of Bengal (BOB) in the Northern Indian Ocean over the period 1995 to 2016. This study aimed to investigate the spatiotemporal variability of these carbon parameters and assess the influence of CO2ant in these oceanic environments. Data from the GLODAPv2.2022 database, including cruise-based biogeochemical bottle measurements, were utilized to examine temporal trends in DIC and δ13CDIC. The TrOCA (Tracer combining Oxygen, Carbon, and Alkalinity) approach was employed to calculate CO2ant. Although DIC concentrations showed minimal variability across the water column in both the SLD and central BOB, significant fluctuations in CO2ant and δ13CDIC were observed in the upper 50 m in both regions between 1995 and 2016. Specifically, δ13CDIC values in the upper 50 m decreased by 0.45 ‰ (at a rate of 0.021 ‰ yr-1) in the SLD and by 0.41 ‰ (at a rate of 0.02 ‰ yr-1) in the central BOB over the study period. This decline is likely attributable to the combined effects of upwelling of remineralized DIC and increased CO2ant invasion in the upper 50 m of these oceanic regions, occurring at rates of 0.93 µmol kg-1 yr-1 in the SLD and 1.97 µmol kg-1 yr-1 in the central BOB. Additionally, a weaker correlation between δ13CDIC and CO2ant was observed in the central BOB, whereas a stronger correlation in the SLDsuggests that the invasion of isotopically lighter CO2ant contributed significantly to the observed depletion of δ¹³CDIC in both regions from 1995 to 2016. These findings underscore the significant role of anthropogenic CO2 in influencing carbon dynamics in the upper ocean of these upwelling-prone regions.