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The northern Gulf of Mexico (NGOM) is one of the well-studied areas of global ocean, yet direct estimates of upper ocean particulate organic carbon (POC) fluxes from this region are limited. The present work reports vertical fluxes of POC from the oligotrophic region of NGOM utilizing short-lived radionuclide pairs 234 Th/ 238 U and 210 Po/ 210 Pb. In spite of the difference in time scale both 210 Po and 234 Th based estimates are in reasonable agreement with sinking POC fluxes, caught in sediment traps. POC flux estimates ranged between 22–41 mg C m − 2 day − 1 at 150 m and 6–40 mg C m − 2 day − 1 at 250 m. The average export efficiency at base of euphotic zone (E z ) was found to be 0.07 ± 0.03 while the export ratio (T 100 ) at 100 m below euphotic zone was found to be 0.66 ± 0.18 indicating that most of the attenuation of NPP in this region is set in the surface layer (low E z ratio) and there is relatively little flux attenuation in the subsurface (high T 100 ) which is typical for oligotrophic settings. Satellite based export efficiencies predicted by the Laws and Dunne models are on average found to be two times higher than the in situ observations while estimates from the Henson model were found to be similar or lower. This is consistent with the observation on a global scale where we find export estimates from in situ data to be consistently lower than those predicted by the Laws export model for the temperature range of 20–25 °C. The discrepancy between modelled estimates and in situ measurements of POC fluxes highlights the fact that global empirical models of satellite based POC fluxes that only consider temperature are overly simple and may need further refinement for ocean biome specific scaling to accurately predict export fluxes. [ABSTRACT FROM AUTHOR]

Response actions to the Deepwater Horizon oil spill included the injection of ∼771,000 gallons (2,900,000 L) of chemical dispersant into the flow of oil near the seafloor. Prior to this incident, no deepwater applications of dispersant had been conducted, and thus no data exist on the environmental fate of dispersants in deepwater. We used ultrahigh resolution mass spectrometry and liquid chromatography with tandem mass spectrometry (LC/MS/MS) to identify and quantify one key ingredient of the dispersant, the anionic surfactant DOSS (dioctyl sodium sulfosuccinate), in the Gulf of Mexico deepwater during active flow and again after flow had ceased. Here we show that DOSS was sequestered in deepwater hydrocarbon plumes at 1000-1200 m water depth and did not intermingle with surface dispersant applications. Further, its concentration distribution was consistent with conservative transport and dilution at depth and it persisted up to 300 km from the well, 64 days after deepwater dispersant applications ceased. We conclude that DOSS was selectively associated with the oil and gas phases in the deepwater plume, yet underwent negligible, or slow, rates of biodegradation in the affected waters. These results provide important constraints on accurate modeling of the deepwater plume and critical geochemical contexts for future toxicological studies.