Transport of U- and Th-series nuclides in a Baltic Shield watershed and the Baltic Sea

The transport of Th, Ra, and Pb-210 from a continental source region and th rough an estuarine environment was investigated. Unlike previous studies, h ere both short- and long-lived nuclide data were obtained for river inputs, river water, and estuarine waters. The mire-rich Kalix River drainage basi n was chosen as a typical northern shield area because this river may repre sent typical waters flowing into the Arctic and northern seas. Groundwaters from bedrock and glacial tills have comparable Th isotope conc entrations and do not exhibit significant Th isotopic shifts relative to ho st rocks. The extensive peat deposits of the basin receive groundwater disc harges and concentrate Th and U (but not Ba and Ra), which cause high Th-23 0/Th-232 ratios in mire waters. However, mire outflows do not have a signif icant impact on Th and Ra isotopic compositions of the river. Overall weathering characteristics for the basin are obtained from the rive r data. The Th-230/Th-232, Ra-228/Ra-226, and Ra-226/Ba river ratios are co mparable to those of source rocks, consistent with similar release rates of these nuclides from U-, Th-, and Ba-bearing minerals. River ratios of (Th- 230/U-238)(AR) and (Ra-226/U-238)(AR) are <1, so that Th and possibly Ra ar e accumulating in the weathering regions, and the weathering profile is sti ll evolving. Low (Ra-228/Th-232)(AR) and (Ra-226/Th-230)(AR) ratios indicat e that Th is preferentially retained over Ra. River (Th-234/U-238)(AR) rati os are greater than (Th-230/U-238)(AR) ratios and suggest that in systems w here river inputs are well characterized, these ratios can be used to calcu late Th transit times through the watershed. Filtration data indicate that although a dominant fraction of the Th is transported in the river on parti cles, the rest is almost entirely carried by colloids. The Kalix River discharges into the Baltic Sea. Model calculations for the transport of Th and Ra isotopes in the Baltic Sea show that the high ratios of (Th-234/U-238)(AR) found here reflect long Th residence times relative to particle scavenging of similar to 50 d. The water column Th-232 budget i s dominated by eolian inputs. The Ra-226 concentrations may be higher than those of water inflows, with less than or equal to 60% derived from underly ing sediments. A greater fraction of Ra-228 is derived from sediments to ba lance the decay of Ra-228 within the water column during the 35-yr residenc e time of water in the Baltic. The Baltic (Ra-228/Ra-226)(AR) ratios, which are relatively constant over a range of salinities, are fortuitously simil ar to those of the river inflows. Copyright (C) 2001 Elsevier Science Ltd.