Temporal changes in surface partial pressure of carbon dioxide and carbonate saturation state in the eastern equatorial Indian Ocean during the 1962–2012 period

Abstract. Information on changes in the oceanic carbon dioxide (CO₂) concentration and air–sea CO₂ flux as well as on ocean acidification in the Indian Ocean is very limited. In this study, temporal changes of the inorganic carbon system in the eastern equatorial Indian Ocean (EIO, 5° N–5° S, 90–95° E) are examined using partial pressure of carbon dioxide (pCO₂) data collected in May 2012, historical pCO₂ data since 1962, and total alkalinity (TA) data calculated from salinity. Results show that sea surface pCO₂ in the equatorial belt (2° N–2° S, 90–95° E) increased from ∼307 μatm in April 1963 to ∼373 μatm in May 1999, ∼381 μatm in April 2007, and ∼385 μatm in May 2012. The mean rate of pCO₂ increase in this area (∼1.56 μatm yr⁻¹) was close to that in the atmosphere (∼1.46 μatm yr⁻¹). Despite the steady pCO₂ increase in this region, no significant change in air–sea CO₂ fluxes was detected during this period. Ocean acidification as indicated by pH and saturation states for carbonate minerals has indeed taken place in this region. Surface water pH (total hydrogen scale) and saturation state for aragonite (Ω_arag), calculated from pCO₂ and TA, decreased significantly at rates of −0.0016 ± 0.0001 and −0.0095 ± 0.0005 yr⁻¹, respectively. The respective contributions of temperature, salinity, TA, and dissolved inorganic carbon (DIC) to the increase in surface pCO₂ and the decreases in pH and Ω_arag are quantified. We find that the increase in DIC dominated these changes, while contributions from temperature, salinity, and TA were insignificant. The increase in DIC was most likely associated with the increasing atmospheric CO₂ concentration, and the transport of accumulated anthropogenic CO₂ from a CO₂ sink region via basin-scale ocean circulations. These two processes may combine to drive oceanic DIC to follow atmospheric CO₂ increase.