^(238)U-^(234)U and ^(232)Th-^(230)Th in the Baltic Sea and in river water

Abstract

The concentration (C) of dissolved ^(238)U,^(234)U,^(232)Th and230Th 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^(234)U^* = 1000‰ was calculated for the fresh water source in the northern Baltic. A study of the Kalixälven River over an annual cycle yields high δ^(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 μm) for the ^(232)Th-^(230)Th load at high discharge. ^(232)Th/^(238)U in river water is about 40 times less than in detrital material. In the brackish water, C_(232Th) 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 ^(232)Th must be deposited in the low-salinity regions of the estuary. The ^(230)Th/^(232)Th in river waters is about twice the equilibrium value for ^(232)Th/^(238)U (3.8). In the brackish waters, ^(230)Th/^(232)Th is greater by a factor of 10–100 than both river water and SW. The big increase in ^(230)Th/^(232)Th 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 ^(232)Th in river water is carried by detrital particles, whereas the other phase (solution, colloidal) has a much higher ^(232)Th/^(232)Th. Entering the estuary, the detrital particles sediment out rapidly, whereas the non-detrital phase is removed more slowly, causing a marked increase in ^(230)Th/^(232)Th in the brackish water. In SW, ^(230)Th/^(232)Th is closer to river input and detrital material than in brackish water. We conclude that in the deep sea, ^(232)Th is almost exclusively dominated by windblown dust and can be used to monitor dust flux. The ^(230)Th excess in Baltic rivers is produced in U-rich, ^(232)Th-poor peatlands and trapped in authigenic particles and transported with the particles. Time scales for producing the ^(230)Th 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.

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