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Research article 11 Sep 2018

Research article | 11 Sep 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Distribution and cycling of terrigenous dissolved organic carbon in peatland-draining rivers and coastal waters of Sarawak, Borneo

Patrick Martin1, Nagur Cherukuru2, Ashleen S. Y. Tan1, Nivedita Sanwlani1, Aazani Mujahid3, and Moritz Müller4 Patrick Martin et al.
  • 1Asian School of the Environment, Nanyang Technological University, Singapore 5 639798, Singapore
  • 2CSIRO Oceans and Atmosphere Flagship, Canberra ACT 2601, Australia
  • 3Department of Aquatic Science, Faculty of Resource Science & Technology, University Malaysia Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia
  • 4Swinburne University of Technology, Faculty of Engineering, Computing and Science, 93350 Kuching, Sarawak, Malaysia

Abstract. South-East Asia is home to one of the world's largest stores of tropical peatland, and accounts for roughly 10% of the global land-to-sea dissolved organic carbon (DOC) flux. We present the first-ever seasonally-resolved measurements of DOC concentration and chromophoric dissolved organic matter (CDOM) spectra for six peatland-draining rivers and coastal waters in Sarawak, north-western Borneo. The rivers differed substantially in DOC concentration, ranging from 120–250µmolL−1 (Rajang river) to 3,100–4,400µmolL−1 (Maludam river). All rivers carried high CDOM concentrations, with a350 in the four blackwater rivers between 70–210m−1, and 4–12m−1 in the other two rivers. DOC and CDOM showed conservative mixing with seawater except in the largest river (the Rajang), where DOC concentrations in the estuary were elevated, most likely due to inputs from the extensive peatlands within the Rajang delta. Seasonal variation was moderate and inconsistent between rivers. However, during the rainier north-east monsoon, all marine stations in the western part of our study area had higher DOC concentrations and lower CDOM spectral slopes, indicating a greater proportion of terrigenous DOM in coastal waters. Photo-degradation experiments revealed that riverine DOC and CDOM in Sarawak are photo-labile: up to 25% of riverine DOC was lost within five days of exposure to natural sunlight, and the spectral slopes of photo-bleached CDOM resembled those of our marine samples. We conclude that coastal waters of Sarawak receive large inputs of terrigenous DOC that is only minimally altered during estuarine transport, and that any biogeochemical processing must therefore occur mostly at sea. It is likely that photo-degradation plays an important role in the degradation of terrigenous DOC in these waters.

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Short summary
The carbon cycle is a key control over the Earth's climate. Every year, rivers deliver a lot of organic carbon to coastal seas, but we do not know what happens to this carbon, particularly in the tropics. We show that rivers in Borneo deliver carbon from peat swamps to the sea with at most minimal biological or chemical alteration in estuaries, but that sunlight can rapidly oxidise this carbon to CO2. This means that South-East Asian seas are likely hotspots of terrestrial carbon decomposition.
The carbon cycle is a key control over the Earth's climate. Every year, rivers deliver a lot of...