210Po and 210Pb cycling in a hydrothermal vent zone in the coastal Aegean Sea

To quantify the potential enhancement of naturally-occurring Po-210 and Pb-210 that may result from the high sulfur-reducing and sulfur-oxidizing regimes associated with hydrothermal vents, sinking particles from both inside and outside vent areas and benthic molluscs grazing on or living near bacterial mats in the vent zone were collected off Milos Island (Aegean Sea) and analyzed for their Po-210 and Pb-210 content. There was no significant difference in the range of Po-210 specific activities measured in particulate material collected by sediment traps in a control area and in the vent area; the resultant Po-210 levels were of the same order of magnitude as literature values reported for other Mediterranean coastal areas. Pb-210 levels in sinking particles from the control site tended to be higher than those measured in the vent zone, as demonstrated by the lower Po-210/Pb-211 ratios observed in particles from the control site. Nevertheless, these Pb-210 levels were also comparable with similar Pb-210 data reported for the northwestern Mediterranean Sea. The Po-210 and Pb-210 vertical particulate fluxes were, on average, higher in the vent zone as a consequence of the higher particle flux. This observation indicates that vents can indirectly control the flux of these natural radionuclides by affecting the types and amount of particles produced in hydrothermal areas. The Po-210 levels measured in a gastropod and a bivalve living on or near the microbial mat in the vent zone were higher than values reported for non-vent gastropods and bivalves from the NW Mediterranean Sea, an observation which suggests that an enhanced food chain transfer of Po-210 may occur in the vicinity of vents off Milos Island. Nevertheless, the lack of a general enhancement of Po-210 and Pb-210 in the marine particulate samples collected indicates that any input of these radionuclides through venting activity may have a minimal effect in the surrounding environment. (C) 2001 Elsevier Science B.V. Ail rights reserved.