Published in Journal of Geophysical Research: Oceans 04 January 2020

The Indonesian Seas are the only low-latitudeinterbasin pathway; known as the Indonesian throughflow (ITF),they are an essential component of the earth’s climate system. As part of thermohaline circulation, the ITF has received muchattention, largely due to its vitally important role in the transport of mass,heat, and freshwater. However, the generalbiogeochemistry of the Indonesian Seas and their role in the global carboncycle remain largely unknown, despite emerging research efforts across multiplefields. This knowledge gap is in striking contrast to the importance of theIndonesian Seas to global marine ecosystems. Understanding the dynamics of thecarbonate system in this region is a prerequisite to meet the challenges ofthis vulnerable ecosystem and similar coastal tropical environments under ongoingocean warming and acidification.

We present aunique dataset to examine the dynamics of thecarbonate systemin the western Indonesian Seas during the southeast monsoon, covering theKarimata Strait, western Java Sea, and Sunda Strait. Salinity-normalized total alkalinity (NTAlk) in thesurface water is very close to typical values observed in the tropical ocean. In the western Java Sea, where waters werewell mixed, we observed relativelyhomogeneous distributions of salinity, dissolved inorganic carbon(DIC), and total alkalinity (TAlk). In the Sunda Strait, waters intruding from the Java Seaoccupied 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 NDIC and NTAlk were very similar tothose observed in the IndianOcean.

Based on DIC and TAlk relationships, weshow that water mass mixing in the westernIndonesian Seas during the southeastmonsoon is dominated by zonal wind-mixedwaters from the plume of the Kapuas River, the JavaSea and South China Sea mixed water and the subsurfaceIndian Ocean water. Physical processes and air-sea gas exchange exertedpredominant controls on the carbonate system in the Karimata Strait and western JavaSea,while biologically-mediatedDIC consumption occurred in the surface mixed layer of the Sunda Strait, which led to an increasein dissolved oxygen saturation, the saturation state of aragonite (Ω_arag), and pH. Overall ourregion was a source of atmospheric CO₂ as previously reported,although the controlling processes may vary with respect to time at both seasonal and interannual timescales.

Figure: Biologically-mediated DIC variations represented by ΔDIC (a) and ΔpH and ΔΩ_arag (b) in thesurface mixed layer (upper 40 m) of the Sunda Strait. (c) Dissolved oxygensaturation (DO%) in the upper 40 m of the western Indonesian Seas during thesoutheast monsoon of June 2015. Greydashed lines in panels (a) and (b) denote no net change in ΔDIC, ΔpH, and ΔΩ_aragdue to equal amounts of addition and removal, and in panel (c) denotes DO% =100% with equilibrium reached between the sea surface and the atmosphere.

Hamzah,F., Agustiadi, T., Susanto, R. D., Wei, Z., Guo, L., Cao, Z., & Dai, M. (2020). Dynamics of the Carbonate System in the Western Indonesian Seas During theSoutheast Monsoon. Journal of Geophysical Research: Oceans, 125, e2018JC014912. https://doi.org/10.1029/2018JC014912