The fate of terrigenous dissolved organic carbon in a river-influenced ocean margin

The mineralization of terrigenous dissolved organic carbon (tDOC) discharged by rivers can impact nutrient and trace metal cycling, biological productivity, net ecosystem metabolism, and air-sea CO2 exchange in ocean margins. However, the extreme heterogeneity of river-influenced ocean margins represents a major challenge for quantitative assessments of tDOC transformations and thereby obscures the role of tDOC in biogeochemical cycles. Here a lignin-based optical proxy for tDOC and a shelf-wide mass balance approach were used to quantitatively assess the fate of tDOC discharged from the Mississippi-Atchafalaya River System (M-ARS) to the Louisiana shelf. The mass balance revealed that similar to 40% of the tDOC discharged by the M-ARS during March 2009-2010 was mineralized to CO2 on the Louisiana shelf, with two thirds of the mineralization taking place in the mixed layer. A strong seasonality in tDOC mineralization was observed, with mineralization rates severalfold higher during summer than during winter. Independent assessments of specific mineralization processes indicated biomineralization accounted for similar to 94% of the tDOC mineralization on an annual basis and suggest that photochemical transformations of tDOC enhanced biomineralization by similar to 50% in the mixed layer. Direct photomineralization accounted for a relatively small fraction (similar to 6%) of the tDOC mineralization on an annual basis. This quantitative assessment directly confirms that ocean margins are major sinks of the tDOC discharged by rivers and indicates that tDOC mineralization rates in the shelf mixed layer are sufficiently large to influence whether the Louisiana shelf is a net sink or source of atmospheric CO2. Key Points About 1 Tg of terrigenous DOC is mineralized on the Louisiana shelf annually Mineralization of terrigenous DOC is 2.5 times higher in summer than winter Biomineralization is the dominant mineralization process for terrigenous DOC