Temporal Variability of Particulate Organic Carbon Fluxes in the Northern Gulf of Mexico

Abstract

There is limited data of Particulate Organic Carbon (POC) export from the Northern Gulf of Mexico and this work presents the first estimation of the efficiency of the “biological pump” in this region. In oligotrophic oceans like the Gulf of Mexico, POC is the main source of particles and POC flux is the key mechanism for the removal of particle reactive contaminants and pollutants from upper ocean water column. Particle-reactive, the naturally occurring radionuclides are useful tracers of the sinking flux of organic matter from the surface to the deep ocean. The disequilibrium between natural radioisotope pair 238U-234Th as well as sediment traps have been widely used to measure particle export fluxes from surface ocean water column. Another radionuclide pair, 210Pb-210Po, can also be used for the same purpose but has not been as widely adopted till now. The present work measured the vertical profiles of total and particle size-fractionated 210Po and 234Th activities, together with Particulate Organic Carbon (POC) concentrations during an opportunistic cruise in the month of April in 2012 and 2013 in and around the Deep Water Horizon spill site (28°44’N, 82°22’W) in the Gulf of Mexico. Both 210Po and234Th reasonably predict 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 0.54-4.25% at 150m to 0.43-3.08% at 350m and showed an inverse linear relation with the surface productivity, contrary to the idea that higher primary productivity leads to higher export. However more research is needed to better understand the biological pump in this context, which not only impacts the atmospheric CO2 sequestration, but also is the main source of nutrient to benthic organisms.