Large deficiency of polonium in the oligotrophic ocean's interior

The naturally occurring radionuclide (210)Po is typically deficient relative to its parent (210)Pb in the surface ocean due to preferential removal by biota, while in near equilibrium or excess below the surface mixed layer due to rapid regeneration from sinking organic matter. However, a strikingly large deficit of (210)Po is observed in the oligotrophic Sargasso Sea's interior. This argues against the general concept that the removal of reactive elements depends on the population of settling particles. A (210)Po mass balance model suggests that rather than downward transport, polonium (proxy for S, Se, and Te) is taken up efficiently by bacteria (i.e., cyanobacteria) and transferred to higher trophic levels (i.e., nekton) in this environment. In contrast, in productive areas of the ocean, sulfur group elements seem to reside in the subsurface ocean for much longer periods as taken up by abundant free-living bacteria (non-sinking fine particles). This study sheds new light on global biogeochemical cycling of sulfur group elements in association with microbial roles, and suggests that (210)Po may be useful as a tracer of nitrogen fixation in the ocean. (C) 2001 Elsevier Science B.V. All rights reserved.