210Po-210Pb distribution and carbon export in the northern Gulf of Mexico continental slope

Abstract

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 100 L-1 and from 3.47 ± 0.45 to 24.49 ± 3.42 dpm 100 L-1 respectively with the exception of higher activities of 210Po (87.36 ± 11.38 dpm 100 L-1) and 210Pb (32.25 ± 4.35 dpm 100 L-1) 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 but within the range of previous estimates.