Abstract
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 (Ωarag), 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 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.
Highlights
Over the past decade, the global ocean took up atmospheric carbon dioxide (CO2) at a rate of about 2.5 Pg C yr−1 (1 Pg = 1015 g), roughly a quarter of all the anthropogenic CO2 released from fossil fuel burning, cement production, and land-use change (Le Quéré et al, 2014)
We find that sea surface pressure of CO2 (pCO2) increased at a mean rate of 1.56 ± 0.08 μatm yr−1 from 1963 to 2012 (Fig. 4a)
We estimated the surface pCO2 trend along the Equator from 89.5 to 94.5◦ E, and found that since the International Indian Ocean Expedition (IIOE, 1960–1965), surface pCO2 increased from 307 ± 4 μatm in April 1963 to 392 ± 6 μatm in May 2012 (Fig. 4b), with a mean rate of ∼ 1.64 μatm yr−1
Summary
The global ocean took up atmospheric carbon dioxide (CO2) at a rate of about 2.5 Pg C yr−1 (1 Pg = 1015 g), roughly a quarter of all the anthropogenic CO2 released from fossil fuel burning, cement production, and land-use change (Le Quéré et al, 2014). The oceans’ uptake of atmospheric CO2 plays an important role in slowing down the increase of atmospheric CO2 (Sabine et al, 2004; Takahashi et al, 2009) and the global climate change. It is important to accurately document changes of the oceanic CO2 sink in order to accurately project future atmospheric CO2 levels and global climate change (Takahashi and Sutherland, 2013). Xue et al.: Temporal changes in surface pCO2 and carbonate saturation state
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