Pw. Swarzenski et al., Uranium biogeochemistry across the redox transition zone of a permanently stratified fjord: Framvaren, Norway, MAR CHEM, 67(3-4), 1999, pp. 181-198
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.