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

Abstract

We report water column dissolved and particulate 210Pb and 210Po profiles along with ancillary data from the northern Gulf of Mexico continental shelf collected during the summers of 2011 and 2012. The dissolved 210Po/210Pb ratio in bottom water was >1 at 10 out of 12 stations whereas the 210Po/210Pb activity ratio in surface water was approximately 0.2–1.3. This lower dissolved 210Po/210Pb surface ratio indicates that 210Po is generally more efficiently scavenged than 210Pb in surface layers. The particulate 210Po/210Pb ratio was 1.6–5.1 in surface water and 2.5–10.4 in bottom water indicating that 210Po tends to be more enriched in deep water particulate material as compared to surface material. The 210Po and POC are significantly correlated (r2=0.93) with the POC/210Po ratio varying between 205 and 2094μmolCdpm−1. These general patterns suggest that 210Po is scavenged from the surface waters and regenerated or added to bottom waters relative to 210Pb. 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 O2 concentration and 210Po excess. 210Po 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 210Po remobilization that produces the observed water column 210Po distributions.