Chapter 3 U-Series Geochemistry in Weathering Profiles, River Waters and Lakes

Abstract

This chapter summarizes the recent applications of U-series nuclides to study weathering and river transport. The chapter focuses on the determination of time-scales of continental weathering and erosion, based on U-series analysis in soils, rivers, as well as in lakes. Analysis of U-series nuclides in soils and alterites can provide information on the behavior of radionuclides during weathering in the natural environment and also on the nature and time-scale of weathering processes. The chapter also indicates that determination of the time-scales of weathering processes from U-series data in soils requires a full understanding of the processes that control U–Th–Ra fractionation in regoliths. In parallel with the development of U-series disequilibrium studies in soils and weathering profiles U-series disequilibria in rivers have been used to establish weathering mass balances and/or to determine weathering time-scales at the scale of a watershed. Studies of U- and Th-series nuclides in lake waters provide information on their geochemical behavior and mixing characteristics of lakes. Some of these studies complement the investigations in river waters. Lake sediments, on the other hand, hold records of recent environmental changes. Measurements of U–Th nuclides in these sediments and associated mineral precipitates can help chronologically decipher the records in them. It has been shown that U-series disequilibria in lake water and sediment could also provide a wealth of information about the physico-chemical processes operating in lakes, such as the source and fate of elements in sediment and water, the residence time of particles or radionuclides in lakes, mixing or turnover time-scales of lakes. The chapter briefly presents some of the results obtained in these areas of research. These findings are certainly one of the major advances in U- and Th-series nuclide applications in the earth sciences and have laid the foundation for further developments in this field.