238U [sbnd]234U and 232Th [sbnd]230Th in the Baltic Sea and in river water

Abstract

The concentration ( C) of dissolved 238U, 234U, 232Th and 230Th 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‰. 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,δ 234U * = 1000‰ was calculated for the fresh water source in the northern Baltic. A study of the Kalixälven River over an annual cycle yields highδ 234U 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 μm) for the 232Th 230Th load at high discharge. 232Th/ 238U in river water is about 40 times less than in detrital material. In the brackish water, C 232 Th drops 2 orders of magnitude in the low salinity region ( < 5‰), reaching a value close to that of sea water at a salinity of 7.5‰. Almost all of the riverine 232Th must be deposited in the low-salinity regions of the estuary. The 230Th/ 232Th in river waters is about twice the equilibrium value for 232Th/ 238U (3.8). In the brackish waters, 230Th/ 232Th is greater by a factor of 10–100 than both river water and SW. The big increase in 230Th/ 232Th 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 232Th in river water is carried by detrital particles, whereas the other phase (solution, colloidal) has a much higher 232Th/ 232Th. Entering the estuary, the detrital particles sediment out rapidly, whereas the non-detrital phase is removed more slowly, causing a marked increase in 230Th/ 232Th in the brackish water. In SW, 230Th/ 232Th is closer to river input and detrital material than in brackish water. We conclude that in the deep sea, 232Th is almost exclusively dominated by windblown dust and can be used to monitor dust flux. The 230Th excess in Baltic rivers is produced in U-rich, 232Th-poor peatlands and trapped in authigenic particles and transported with the particles. Time scales for producing the 230Th excess are ∼ 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.