Lead-210 and Polonium-210 disequilibria in the northern Gulf of Mexico hypoxic zone

We report water column dissolved and particulate Pb-210 and Po-210 profiles along with ancillary data from the northern Gulf of Mexico continental shelf collected during the summers of 2011 and 2012. The dissolved Po-210/Pb-210 ratio in bottom water was >1 at 10 out of 12 stations whereas the Po-210/Pb-210 activiy ratio in surface water was approximately 0.2-13. This lower dissolved Po-210/Pb-210 surface ratio indicates that Po-210 is generally more efficiently scavenged than Pb-210 in surface layers. The particulate Po-210/Pb-210 ratio was 1.6-5.1 in surface water and 2.5-10.4 in bottom water indicating that Po-210 tends to be more enriched in deep water particulate material as compared to surface material. The Po-210 and POC are significantly correlated (r(2) = 0.93) with the POC/Po-210 ratio varying between 205 and 2094 mu mol C dpm(-1). These general patterns suggest that Po-210 is scavenged from the surface waters and regenerated or added to bottom waters relative to Pb-210. The addition of Po to bottom water (either in the dissolved or particulate phase) likely requires a sedimentary source of Po, relative to Pb, to the overlying water column. Dissolved oxygen concentrations and water column stratification vary throughout the region, and we find no correlation between dissolved O-2 concentration and Po-210 excess. Po-210 enrichment does, however, appear to be coupled to the release of the redox sensitive trace metals Fe and Mn and remineralization of silica in bottom waters to some extent. We suggest that the cycling of these redox sensitive metals, coupled with the degradation of organic matter is the likely driving mechanism for Po-210 remobilization that produces the observed water column Po-210 distributions. (C) 2014 Elsevier B.V. All rights reserved.