Comparison of 210Po, 234Th and Sediment-Trap Based Export Fluxes in the Northern Gulf of Mexico

The northern Gulf of Mexico (NGOM) is one of the well-studied areas of global ocean, especially after the Deepwater Horizon oil spill in 2010 and yet direct estimates of upper ocean POC fluxes from this region is practically nonexistent. In oligotrophic waters of the open Gulf of Mexico, particulate carbon is the main source of particles and POC flux is the key mechanism for the removal of metals and other particle reactive contaminants like polycyclic aromatic hydrocarbons from the upper ocean. Disequilibria between natural radioisotope pairs ²³⁸U-²³⁴Th and ²¹⁰Pb-²¹⁰Po as well as sediment traps have been widely used to measure particle export fluxes from the upper ocean on time scale of few days to months. The present work measured the vertical profiles of total and particle size-fractionated ²¹⁰Pb, ²¹⁰Po and ²³⁴Th activities, together with particulate carbon concentrations in the Gulf of Mexico during April 2012 and 2013. In spite of the difference in time scale both ²¹⁰Po and ²³⁴Th based estimates are in reasonably agreement with sinking POC fluxes, caught in sediment traps, and each tracer provides unique information about the magnitude and efficiency of the ocean's biological pump. POC flux estimates ranged between 22-41 mgCm^-2day^-1 at 150m to 9-29 mgCm^-2day^-1 at 350m. POC export efficiencies ranged between 0.04- 0.10 at 150m which is similar to export efficiencies of 0.11-0.15 calculated from satellite based export production models.