Spatial distribution and regulating factors of the carbonate system in the Indonesian Seas
Wentao Hao¹, Shiping Lei², Faisal Hamzah³, Jinyan Pang¹, Yan Li¹, Xianghui Guo^1
1: State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen 361102
²: Ningde Ocean Innovation Center, Ningde, 352100, China
³: 2Ministry of Marine Affairs and Fisheries, Institute for Marine Research and Observation, Bali, Indonesia
The Indonesian seas, as the only low-latitude conduit connecting the tropical Pacific and Indian Oceans, serve as a critical hub for global ocean circulation and oceanic carbon cycling. The Indonesian Throughflow (ITF) flowing through this region acts not only as the primary pathway for heat and water mass transport, but also as the principal carrier of dissolved inorganic carbon (DIC) exchange between the two oceans. Its vigorous vertical mixing and cross-basin transport profoundly influence the carbonate system balance and carbon budget at both regional and global scales. Based on dissolved oxygen (DO), DIC, and nutrient measurements sampled along the two major ITF passages during November–December 2023, this study investigates the spatial distribution patterns of DIC and the controlling factors. In the western passage, DIC concentrations generally exhibit a decreasing trend along the main flow path, yet DIC levels in the southern Makassar Strait are 12.3 μmol/kg higher than those in the upstream Sulawesi Sea. Analysis reveals that the intrusion of Sulu Sea waters affects the biogeochemical characteristics of the western Sulawesi Sea, while freshwater input from the Mahakam River generates a local salinity and DIC minimum in the central Makassar Strait. In the southern Makassar Strait, the eastward transport of Java Sea waters leads to a modest increase in DIC concentrations. In the eastern passage, the Halmahera Sea and the Maluku Sea serve as the two primary entrance for Pacific waters into the Indonesian seas. Following the intrusion of South Pacific waters into the Halmahera Sea, topographic constraints restrict the water exchange with the Seram Sea and the Lifamatola Strait, causing a portion of high-DIC water to be retained within the Halmahera Sea. North Pacific waters, after splitting in the Sulawesi Sea, branch southward into the Maluku Sea, subsequently flowing through the Lifamatola Strait, the Seram Sea, the Banda Sea, and ultimately exiting into the Indian Ocean via the Ombai Strait. Along this pathway, DIC concentrations exhibit a marked increasing trend, with values observed in the Ombai Strait approximately 40 μmol/kg higher than those at the Maluku Sea stations. Furthermore, a quantitative estimate of DIC variation and the net contribution of biogeochemical processes along the flow path of the western passage was conducted.

Keywords: Indonesian Seas, Indonesian Throughflow, water mass, dissolved inorganic carbon, biogeochemical process