Dynamics of the Carbonate System in the Western Indonesian Seas During the Southeast Monsoon

Abstract

We present a unique water column data set of dissolved inorganic carbon (DIC) and total alkalinity (TAlk) from a cruise to the western Indonesian Seas during the southeast monsoon, covering the Karimata Strait, western Java Sea, and Sunda Strait. Salinity-normalized TAlk (NTAlk) in the surface water ranged 2,297–2,348 μmol kg−1, very close to typical values observed in the tropical ocean. In the Karimata Strait, the Kapuas River plume was observed, featuring low salinity, DIC, and TAlk. In the western Java Sea, where waters were well mixed, we observed relatively homogeneous distributions of salinity, DIC, and TAlk. In the Sunda Strait, waters intruding from the Java Sea occupied the upper layer, and below was the Indian Ocean water with lower values of salinity, DIC, and TAlk. In its deep portion, depth profiles of normalized DIC and NTAlk were very similar to those observed in the Indian Ocean. Physical processes and air-sea gas exchange exerted predominant controls on the carbonate system in the Karimata Strait and western Java Sea. While both processes play large roles in the Sunda Strait, a net DIC removal of 31 ± 23 μmol kg−1 in the surface mixed layer were revealed. The drawdown of DIC is consistent with an overall supersaturation of dissolved oxygen (102–107%), suggesting significant organic carbon production. In the subsurface-intermediate waters of the Sunda Strait mainly influenced by the advection of Indian Ocean water, a net DIC consumption of 54 ± 45 μmol kg−1 was distinct, likely stimulated by the nutrients supplied from the Indian Ocean.