Thallium isotope composition of the upper continental crust and rivers -: An investigation of the continental sources of dissolved marine thallium

The thallium (T1) concentrations and isotope compositions of various river and estuarine waters, suspended riverine particulates and loess have been determined. These data are used to evaluate whether weathering reactions are associated with significant T1 isotope fractionation and to estimate the average T1 isotope composition of the upper continental crust as well as the mean T1 concentration and isotope composition of river water. Such parameters provide key constraints on the dissolved T1 fluxes to the oceans from rivers and mineral aerosols. The T1 isotope data for loess and suspended riverine detritus are relatively uniform with a mean of 82115 T1 = -2.0 +/- 0.3 (epsilon(205) T1 represents the deviation of the (205)T1/(203)T1 isotope ratio of a sample from NIST SRM 997 T1 in parts per 10(4)). For waters from four major and eight smaller rivers, the majority were found to have T1 concentrations between 1 and 7 ng/kg. Most have T1 isotope compositions very similar (within +/- 1.5 epsilon(205) T1) to that deduced for the upper continental crust, which indicates that no significant T1 isotope fractionation occurs during weathering. Based on these results, it is estimated that rivers have a mean natural T1 concentration and isotope composition of 6 +/- 4 ng/kg and epsilon(205)T1 = -2.5 +/- 1.0, respectively. In the Amazon estuary, both additions and losses of T1 were observed, and these correlate with variations in Fe and Mn contents. The changes in T1 concentrations have much lower amplitudes, however, and are not associated with significant T1 isotope effects. In the Kalix estuary, the T1 concentrations and isotope compositions can be explained by two-component mixing between river water and a high-salinity end member that is enriched in T1 relative to seawater. These results indicate that T1 can display variable behavior in estuarine systems but large additions and losses of T1 were not observed in the present study. Copyright (c) 2005 Elsevier Ltd