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

[摘要] Information on changes in the oceanic carbon dioxide (CO₂) concentrationand air–sea CO₂ flux as well as on ocean acidification in the IndianOcean is very limited. In this study, temporal changes of the inorganiccarbon system in the eastern equatorial Indian Ocean (EIO, 5° N–5° S, 90–95° E) are examined using partialpressure of carbon dioxide (pCO₂) data collected in May 2012,historical pCO₂ data since 1962, and total alkalinity (TA) datacalculated from salinity. Results show that sea surface pCO₂ in theequatorial belt (2° N–2° S, 90–95° E)increased from ∼307 μatm in April 1963 to∼373 μatm in May 1999, ∼381 μatm inApril 2007, and ∼385 μatm in May 2012. The mean rate ofpCO₂ 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 changein air–sea CO₂ fluxes was detected during this period. Oceanacidification as indicated by pH and saturation states for carbonate mineralshas indeed taken place in this region. Surface water pH (total hydrogenscale) 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, anddissolved inorganic carbon (DIC) to the increase in surface pCO₂ andthe decreases in pH and Ω_arag are quantified. We find thatthe increase in DIC dominated these changes, while contributions fromtemperature, salinity, and TA were insignificant. The increase in DIC wasmost likely associated with the increasing atmospheric CO₂concentration, and the transport of accumulated anthropogenic CO₂ from aCO₂ sink region via basin-scale ocean circulations. These two processesmay combine to drive oceanic DIC to follow atmospheric CO₂ increase.

[学科分类] 地球化学与岩石