Assessing the Fate of Terrigenous Dissolved Organic Carbon in River-influenced Ocean Margins

Abstract:

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. 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, we demonstrate how a lignin-based optical proxy for tDOC and a shelf-wide mass balance approach can be used to quantitatively assess the fate of tDOC discharged on a river-influenced ocean margin. Such a mass balance was carried out in the northern Gulf of Mexico and revealed that ~40% of the tDOC discharged by Mississippi-Atchafalaya River System between March 2009 and March 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 ~94% of the tDOC mineralization on an annual basis, and suggested that photochemical transformations of tDOC enhanced biomineralization by ~50% in the mixed layer. Direct photomineralization accounted for a relatively small fraction (~6%) of the tDOC mineralization on an annual basis. This quantitative assessment provides direct evidence confirming 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. In this presentation, we also demonstrate how the remote sensing of ocean color can be used to trace tDOC and its cross-shelf export and further advance our understanding of the fate of tDOC in ocean margins.