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 (delta C-13(DOC)). However, several limitations can hamper the use of delta C-13(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 delta C-13(DOC). The stable isotope ratio of non-exchangeable hydrogen in the DOC (delta H-2(n)) has emerged as a new tool that can potentially overcome these limitations: (1) delta H-2(n) has a large separation between aquatic and terrigenous endmembers (>50 parts per thousand) and (2) it is not subject to reservoir effects caused by tDOC mineralization. Here, we evaluate the potential of delta H-2(n) obtained from solid phase-extracted dissolved organic matter (SPE-DOM), by comparing it to delta C-13(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 delta H-2(n) of SPE-DOM and delta C-13(DOC) of bulk DOC was well correlated (r(2) = 0.42), and there was no significant difference in f(terr) between the two isotope systems. In fact, delta H-2(n) displayed slightly stronger correlations with salinity and CDOM optical properties compared to delta C-13(DOC). Our results indicate that delta H-2(n) of SPE-DOM is effective for quantifying tDOC across coastal gradients, potentially offering greater sensitivity than delta C-13(DOC), and is a viable alternative in settings where delta C-13(DOC) is inadequate.