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 Î1/4atm in April 1963 to ∼373 μ1/4atm in May 1999, ∼381 μ1/4atm in April 2007, and ∼385 μ1/4atm in May 2012. The mean rate of pCO2 increase in this area (∼1.56 μ1/4atm yrg-1) was close to that in the atmosphere (∼1.46 μ1/4atm yrg-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 (Ωarag), calculated from pCO2 and TA, decreased significantly at rates of g-0.0016 ± 0.0001 and g-0.0095 ± 0.0005 yrg-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 Ωarag 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.