Abstract

Activity profiles of U-234, U-238 and Pb-210, as well as the U-234/U-238 ratio, and Pb-210(xs) radiochemical parameters and macrobenthic abundance in sediments were measured and calculated at different times during a year (1991), along a transect between the head and the mouth of Concepcion Bay, Chile. This bay is a semi-enclosed shallow embayment with a strong seasonality in its hydrographic and biological characteristics due to wind driven upwelling. Upwelling of oxygen-poor and nutrient-rich water occurs and organic detritus reaches the seabed, creating a sub-oxic and anoxic environment during, at least, 57% of the year. U-238 and U-234 activities in the sediments show a gradual increase from 1.57 to 6.07 dpm g(-1) from the surface to 15 cm depth in the sediments. Under suboxic-anoxic conditions, this pattern is consistent with a reduction of U(VI) to U(IV) and its subsequent removal from pore-water. A U-234/U-238 ratio of 1.10+/-0.08 indicates an authigenic origen. The U-238 accumulation rate and inventory show a significant increase from Station 1 (head of the bay) to Station 3 (mouth of the bay) from 348 to 794 dpm cm(-2) kyr(-1) and of 14 to 32 dpm cm(-2), respectively. The organic-rich and highly reducing sediments also show an increase in sulphate-reduction rates, sulfur speciation and bioturbation activities of macrobenthos. These patterns suggest that the extent of U enrichment in these sediments is regulated by bacterial sulphate reduction rate and bioturbation and bioirrigation activities. The estimated U-238 accumulation rates are higher than those reported for other anoxic environments, representing an important U sink in marine sediments. The geochemical behaviour of U seems to be very sensitive to the redox condition and this characteristic could become an important paleoceanographic tool to identify changes in oceanographic conditions during the last millennium.