Gulf Of Mexico
Map of the sampling locations in the Gulf of Mexico continental shelf and shelf slope. The “S” stations sampling on a transect south of the mouth of the Atchafalaya River were conducted in August 2011 (circle). The sampling on “T” transect running from the mouth of the Mississippi River to Southwest were conducted in June 2013 (Triangle).
An important pathway for CO2 sinking in the ocean is via the “biological pump”, driven by the production of particulate organic carbon (POC) in the surface ocean and its subsequent export to the deeper ocean via vertical settling. Here we examine the vertical fluxes of POC in the continental slope of the northern Gulf of Mexico by utilizing the short-lived radionuclide pair 210Po – 210Pb. The total activities of 210Po and 210Pb ranged from 2.5 ± 0.35 to 16.52 ± 1.98 dpm 100L-1 and from 3.47 ± 0.45 to 24.49 ± 3.42 dpm 100L-1 respectively with the exception of a higher activities of 210Po (87.36 ± 11.38 dpm 100L-1) and 210Pb (32.25 ± 4.35 dpm 100L-1) reported from the near bottom water in the hypersaline anoxic Orca basin. In the upper 150 m of the water column, the inventories of 210Pb were consistently greater than those of 210Po while the total overall water column inventories of 210Po and 210Pb were found to be near equilibrium. The 210Po fluxes varied between 9.8 ± 6.34 dpm m-2 d-1 and 30.7 ± 12.06 dpm m-2 d-1 with corresponding POC fluxes between 8.6 mg C m-2 d-1 and 37.65 mg C m-2 d-1. The fluxes showed a general decreasing trend further offshore, closely following a similar trend in net primary productivity across the slope. The export efficiency ratios “e-ratios” of the biological pump over this slope region was found to vary between 0.027 and 0.11 averaging at 0.05 ± 0.01, which is low. The 210Po based and the model-based e-ratio estimates were different however they fall within the range of previous model-based estimates.