A study of the carbon dioxide system in the northern Indian Ocean during premonsoon

Abstract

Extensive data sets on the carbon dioxide system in the northern Indian Ocean have been obtained during the premonsoon season. A marked increase (~50 ¼M) in the total carbon dioxide (TCO₂) content is observed in deep (≥1 km) waters of the northern Arabian Sea between approximately 10 and 20°N; contributions from soft tissue decomposition and skeletal solution to the observed increase are about the same. In the Bay of Bengal, the corresponding increase in TCO₂ observed from 11 to 19°N is relatively small (17.5 ¼M). Significantly, the contribution of the organic decomposition in this region is only 1/7 of the increase in TCO₂. This, together with the earlier reports on large sinking fluxes of particulate organic carbon and pronounced near-bottom anomalies, suggests rapid sinking of organic matter with little decomposition in the water column due to the immense fluvial inputs of lithogenic matter into the Bay of Bengal. The greater extent of organic decomposition in subsurface waters of the Arabian Sea is reflected by generally lower pH and higher pCO₂ levels. This also causes the deep waters in the Arabian Sea to be more corrosive to CaCO₃ than those in the Bay of Bengal. However, the TCO₂ concentrations in the deep Bay of Bengal are slightly higher as compared to those in the Arabian Sea, presumably due to the greater CaCO₃ dissolution in deep waters en route the Bay of Bengal. The atmospheric fluxes of CO₂ are found to be larger in the Arabian Sea (2.9 mmol/m₂/d) than in the Bay of Bengal (0.33 mmol/m₂/d) during the premonsoon season. The overall annual evasion rate of CO2 from the Arabian Sea and Bay of Bengal has been estimated as ≥ 50 Tg C. These results confirm that the northern Indian Ocean serves as a significant source of atmospheric CO₂.