U-238-U-234 AND TH-232-TH-230 IN THE BALTIC SEA AND IN RIVER WATER

The concentration (C) of dissolved U-238, U-234, Th-232 and Th-230 in fresh and brackish waters from the Baltic Sea were determined using TIMS. The brackish waters range in salinity from that of sea water (SW) to 2.5 parts per thousand. C-238U in oxygen-saturated, surface waters is well correlated with salinity and shows quasi-conservative behavior, as does Sr. Samples from the redox water interface show depletion in C-238U, demonstrating that dissolved U is being removed by FeMn oxyhydroxides. From a simple mixing relationship for the brackish water, delta(234)U* = 1000 parts per thousand was calculated for the fresh water source in the northern Baltic. A study of the Kalixalven River over an annual cycle yields high delta(234)U during spring and summer discharge and lower values during fall and winter, showing that different sources contribute to the U load in the river during different seasons. C-232Th and C-230Th in river water are governed by the discharge, reflecting the importance of the increased abundance of small particles (< 0.45 mu m) for the Th-232-Th-230 load at high discharge. Th-232/U-238 in river water is about 40 times less than in detrital material. In the brackish water, Cu-232Th drops 2 orders of magnitude in the low salinity region (< 5 parts per thousand), reaching a value dose to that of sea water at a salinity of 7.5 parts per thousand. Almost all of the riverine Th-232 must be deposited in the low-salinity regions of the estuary. The Th-230/Th-232 in river waters is about twice the equilibrium value for Th-232/U-238 (3.8). In the brackish waters, Th-230/(232) is greater by a factor of 10-100 than both river water and SW. The big increase in Th-230/Th-232 in the Baltic Sea waters over the riverine input indicates that the Th isotopes enter the estuary as a mixture of two carrier phases. We infer that about 96% of Th-232 in river water is carried by detrital particles, whereas the other phase (solution, colloidal) has a much higher Th-230/Th-232. Entering the estuary, the detrital particles sediment out rapidly, whereas the non-detrital phase is removed more slowly, causing a marked increase in Th-230/Th-232 in the brackish water. In SW, Th-230/Th-232 is closer to river input and detrital material than in brackish water. We conclude that in the deep sea, Th-232 is almost exclusively dominated by windblown dust and can be used to monitor dust flux. The Th-230 excess in Baltic rivers is produced in U-rich, Th-232-poor peatlands and trapped in authigenic particles and transported with the particles. Time scales for producing the Th-230 excess are similar to 2000-8000 yr. This is younger than, but comparable to, the time of the latest deglaciation, which ended some 9000 yr ago when the mires were forming. These results have implications for the possible mobility of actinides stored in repositories.