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

• DOC crucial in coastal biogeochemical cycles, but its behavior varies based on sources. • Bulk DOC stable isotope ratio (δ 13C DOC) faces limitations in complex coastal waters. • Issues like narrow separation and reservoir effects in δ 13C DOC addressed by deuterium (δ 2H n). • The result shows δ 2H n and δ 13C DOC agree well in quantifying the terrigenous DOC fraction. • The δ 2H n proves comparable effectivity in quantifying tDOC, offering an alternative in settings where δ 13C DOC is inadequate. 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 (δ 13C DOC). However, several limitations can hamper the use of δ 13C 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 δ 13C DOC. The stable isotope ratio of non-exchangeable hydrogen in the DOC (δ 2H n) has emerged as a new tool that can potentially overcome these limitations: (1) δ 2H 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 δ 2H n obtained from solid phase-extracted dissolved organic matter (SPE-DOM), by comparing it to δ 13C 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 δ 2H n of SPE-DOM and δ 13C 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, δ 2H n displayed slightly stronger correlations with salinity and CDOM optical properties compared to δ 13C DOC. Our results indicate that δ 2H n of SPE-DOM is effective for quantifying tDOC across coastal gradients, potentially offering greater sensitivity than δ 13C DOC , and is a viable alternative in settings where δ 13C DOC is inadequate. [ABSTRACT FROM AUTHOR]