Uranium biogeochemistry across the redox transition zone of a permanently stratified fjord: Framvaren, Norway

During August 1995, the vertical concentration profile of dissolved and par ticulate uranium exhibited strong non-conservative characteristics in the u pper 30 m of Framvaren Fjord. There was a pronounced peak in both particula te (> 0.2 mu m; 1.09 nM) and dissolved (< 0.2 mu m: 17.06 nM) uranium in th e finely stratified waters at the O-2/H2S interface which is positioned wel l within the euphotic zone at about 20-21 m. Such concentration maxima at t he redox boundary are also observed for dissolved organic carbon (DOC), Sr and Pa. Dissolved U levels seen in the water column from 18 m down to 30 m exceeded the high salinity (salinity = 35)U concentrations (13.63 +/- 0.84 nM; Chen, J.H., Edwards, R.L., Wasserburg, G.L., 1986, U-238, U-234 and Th- 232 in seawater. Earth Planet Sci. Lett. 80, 241-251.) observed uniformly i n the open ocean. A prolific population of S microbes (e.g., Chromatium, Ch lorobium sp.) flourishes at the O-2/H2S interface. The source of elevated U at the redox boundary must be due to microbial uptake and subsequent relea se processes rather than dilution from oceanic uranium. Uranium oxidation s tate determinations in waters from 1, 22 and 30 m depth reveal that reduced U(IV) is not present in significant abundance, and that the chemical and/o r biological reduction of hexavalent uranium is largely inhibited. Our resu lts suggest that U and other trace constituents such as DOG, Sr, Ba, Fe(II) , Mn(II) are greatly modified by direct and indirect microbial transformati on reactions which are most concentrated across the redox transition zone i n Framvaren Fjord. (C) 1999 Elsevier Science B.V. All rights reserved.