From swamp to sea: Quantifying terrestrial dissolved organic carbon in a tropical shelf sea using hydrogen isotope ratios

Dissolved organic carbon (DOC) is a key component of coastal biogeochemical cycles, but its composition and reactivity depend on the relative contribution of autochthonous aquatic versus allochthonous terrigenous DOC (tDOC). In complex coastal waters, tDOC is commonly quantified using the bulk DOC stable carbon isotope ratio (δ¹³C_DOC). However, several limitations can hamper the use of δ¹³C_DOC in marine ecosystems, such as (1) the narrow and often overlapping separation of the autochthonous and allochthonous endmembers, and (2) mineralization of tDOC to dissolved inorganic carbon creates a reservoir effect such that autochthonous DOC can acquire a terrigenous-like δ¹³C_DOC. The stable isotope ratio of non-exchangeable hydrogen in the DOC (δ²H_n) has emerged as a new tool that can potentially overcome these limitations: (1) δ²H_n has a large separation between aquatic and terrigenous endmembers (>50‰) and (2) it is not subject to reservoir effects caused by tDOC mineralization. Here, we evaluate the potential of δ²H_n obtained from solid phase-extracted dissolved organic matter (SPE-DOM), by comparing it to δ¹³C_DOC and chromophoric DOM (CDOM) optical properties. We collected samples at a site in Southeast Asia’s Sunda Shelf that experiences substantial seasonal variation in tDOC input, driven primarily by the monsoon-induced physical advection of peatland-derived tDOC. Over a 1-year monthly time series, the terrigenous fraction of DOC (f_terr) determined using δ²H_n of SPE-DOM and δ¹³C_DOC of bulk DOC was well correlated (r² = 0.42), and there was no significant difference in f_terr between the two isotope systems. In fact, δ²H_n displayed slightly stronger correlations with salinity and CDOM optical properties compared to δ¹³C_DOC. Our results indicate that δ²H_n of SPE-DOM is effective for quantifying tDOC across coastal gradients, potentially offering greater sensitivity than δ¹³C_DOC, and is a viable alternative in settings where δ¹³C_DOC is inadequate.