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

• Published in: Biogeosciences
• Main Authors: L. Xue, W. Yu, H. Wang, L.-Q. Jiang, L. Feng, L. Gao, K. Li, Z. Li, Q. Wei, C. Ning
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2014
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; Indian; Ocean acidification
• Online Access: https://doi.org/10.5194/bg-11-6293-2014; https://doaj.org/article/76333de5893347e8a020252fd6a3384e

Information on changes in the oceanic carbon dioxide (CO 2 ) concentration and air–sea CO 2 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 ( p CO 2 ) data collected in May 2012, historical p CO 2 data since 1962, and total alkalinity (TA) data calculated from salinity. Results show that sea surface p CO 2 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 p CO 2 increase in this area (∼1.56 μatm yr −1 ) was close to that in the atmosphere (∼1.46 μatm yr −1 ). Despite the steady p CO 2 increase in this region, no significant change in air–sea CO 2 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 p CO 2 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 p CO 2 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 CO 2 concentration, and the transport of accumulated anthropogenic CO 2 from a CO 2 sink region via basin-scale ocean circulations. These two processes may combine to drive oceanic DIC to follow atmospheric CO 2 increase.