Novel insights into the origin of dissolved organic carbon in the Sunda Shelf Sea using stable isotope ratios of hydrogen (d2H) and carbon (d13C).

Rivers deliver ~0.25 Pg C year^-1 of terrigenous dissolved organic carbon (tDOC) from land to shelf seas. As tDOC moves along the river, coastal ocean and deep ocean continuum, it undergoes complex biogeochemical processing that results in both chemical alteration and remineralisation. Remineralisation of tDOC to CO₂ can contribute significantly to coastal ocean acidification and CO₂ emissions to the atmosphere. Our understanding of tDOC processing in coastal seas is still limited, in part because it is challenging to distinguish between marine and terrigenous DOC. The stable carbon isotope ratios (d¹³C) of the dissolved inorganic and organic carbon pools are commonly used to quantify tDOC, because terrestrial vegetation is typically more isotopically depleted (-32 to -25 ‰) compared to marine organic carbon (-24 to -20 ‰). However, this relatively small difference between the marine and terrigenous end-members can introduce large uncertainties in d¹³C-based estimates, particularly if tDOC originates from both C3 and C4 vegetation. End-member isotope ratio values with larger separation could potentially help to better quantify tDOC. Recent studies in freshwater ecosystems have shown that the stable isotope ratios (d²H) of the carbon bound non-exchangeable hydrogen fraction of dissolved organic matter (DOM) typically differs by more than 50 ‰ between terrestrially derived and aquatically derived dissolved organic matter. However, d²H has not yet been used as a tracer for tDOC in marine environments.

Here, we present results from a one year-long monthly time series of δ¹³C and δ²H at a coastal location in Southeast Asia’s Sunda Shelf Sea, where the southwest monsoon delivers a seasonal input of tDOC from tropical peatlands on Sumatra. We found that δ²H of solid-phase extracted DOM, as measured after dual water steam equilibration, ranged between -130 to ­­-150 ‰ during the southwest monsoon, but between -160 to -167 ‰ during other months. Fresh tDOC from peatland-draining rivers had values close to -100‰, and decreased somewhat upon partial photodegradation, while DOM produced in plankton enrichment cultures had values around -174‰. Values of d¹³C of DOC ranged from -25.5 to -23.0 ‰ during the southwest monsoon, and between -23.0 to -21.0 ‰ at other times. We will present preliminary mass balance calculations to estimate tDOC concentrations based on δ¹³C and δ²H. Our results suggest that δ²H can be a sensitive tracer of tDOC in the marine environment.