Continental runoff is a major source of freshwater, nutrients and terrigenous material to the Arctic Ocean. As such, it influences water column stratification, light attenuation, surface heating, gas exchange, biological productivity and carbon sequestration. Increasing river discharge and thawing permafrost suggest that the impacts of continental runoff on these processes are changing. Here, a new optical proxy was developed and implemented with remote sensing to determine the first pan-Arctic distribution of terrigenous dissolved organic matter (tDOM) and continental runoff in the surface Arctic Ocean. Retrospective analyses revealed connections between the routing of North American runoff and the recent freshening of the Canada Basin, and indicated a correspondence between climate-driven changes in river discharge and tDOM inventories in the Kara Sea. By facilitating the real-time, synoptic monitoring of tDOM and freshwater runoff in surface polar waters, this novel approach will help understand the manifestations of climate change in this remote region.

[1] A novel method to accurately retrieve DOC concentrations (±4%) from CDOM absorption coefficients, a g (l), at l = 275 and 295 nm is presented. By using these two wavelengths, the method exploits useful information about the ratio of a g (l) to [DOC] contained in the 275-295 nm spectral slope coefficient, S 275−295 . This approach was developed using data (n = 222) collected on a seasonal basis in surface waters of the Northern Gulf of Mexico. The approach is demonstrated to accurately and consistently estimate [DOC] for all seasons and for a broad [DOC] range of 63-611 mM. Application to coastal waters of the Beaufort Sea (n = 33) demonstrated that similar performance can be expected in other river-influenced ocean margins after parameterization using local data. Applicability to other marine environments remains to be tested but such assessment can be pursued immediately where appropriate data have already been collected. Citation: Fichot, C. G., and R. Benner (2011), A novel method to estimate DOC concentrations from CDOM absorption coefficients in coastal waters, Geophys. Res. Lett., 38, L03610,

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 ~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 ~94% of the tDOC mineralization on an annual basis and suggest 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 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.

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