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. Plain Language SummaryResearch on the carbonate system in the tropical regime of the Indonesian Seas is minimal, which hampers our understanding to its essential role in interbasin exchanges of material and energy via the Indonesian throughflow. Here we present a unique data set to examine the dynamics of the carbonate system in the western Indonesian Seas. Based on DIC and TAlk relationships, we show that water mass mixing in the western Indonesian Seas during the southeast monsoon is dominated by zonal wind-mixed waters from the plume of the Kapuas River, the Java Sea and South China Sea mixed water, and the subsurface Indian Ocean water. These processes resulted in homogenous distributions of physical properties and carbonate system parameters. However, biologically mediated DIC consumption occurred in the surface mixed layer of the Sunda Strait, which led to an increase in dissolved oxygen saturation, the saturation state of aragonite (Ω arag ), and pH. Overall, our region was a source of atmospheric CO 2 as previously reported, although the controlling processes may vary with respect to time at both seasonal and interannual timescales.

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