Pulsed, cross-shelf export of terrigenous dissolved organic carbon to the Gulf of Mexico

The export of terrigenous dissolved organic carbon (tDOC) and other river-borne material across the continental shelf boundary has important ramifications for biological productivity and the cycling of continentally derived bioelements in the ocean. Recent studies revealed the 275-295 nm spectral slope coefficient of chromophoric dissolved organic matter (CDOM), S275-295, is a reliable tracer for terrigenous dissolved organic carbon (tDOC) in river-influenced ocean margins. Here an empirical algorithm for the accurate retrieval of S275-295 from ocean color was developed and validated using in situ optical properties collected seasonally in the northern Gulf of Mexico. This study also demonstrated S275-295 is a robust proxy for tDOC concentration in this environment, thereby providing a means to derive surface tDOC concentrations on synoptic scales and in quasi-real time using remote sensing. The resulting tDOC-algorithm was implemented using Aqua-MODIS in a retrospective analysis of surface tDOC concentrations over the northern Gulf of Mexico between July 2002 and June 2013. Large pulses of tDOC were observed in continental-slope surface waters off the Mississippi River delta, indicating cross-shelf export of tDOC was sporadic and exhibited considerable interannual variability. Favorable winds following an anomalously high discharge from the Mississippi-Atchafalaya river system always coincided with a major export event, and in general, cross-shelf export was enhanced during years of anomalously high discharge. The tDOC-algorithm will find applicability in the assessment of future climate-and human-induced changes in tDOC export, in biogeochemical models of the continental shelf, and in the validation of high-resolution coastal models of buoyancy-driven shelf circulation.