The effects of disequilibria in the uranium and thorium decay chains on burial dose rates in fluvial sediments

The potential of radioactive disequilibria as tracers and chronometers of weathering processes has been recognised since the 1960’s (e.g., Rosholt et al. 1966; Hansen and Stout 1968). This interest results from the dual property of the nuclides of the U and Th radioactive series (1) to be fractionated during water-rock interactions and (2) to have radioactive periods of the same order of magnitude as the time constants of many weathering processes and chemical transfers to ground and river waters. Therefore, the study of radioactive disequilibria in surface environments should help to bring information about the nature, the intensity but also the time-scale of the water-rock interactions produced during weathering and related chemical transfers. These different properties have justified many of the studies on U-Th series in weathering profiles and river waters.

Rosholt (1982), Scott (1982) and Osmond and Ivanovich (1992) gave a synthesis of the studies of U-series in weathering and surface hydrology up to the 1980’s and the 1990’s respectively. Here, we present the main directions taken in these domains over the last decade. They were partly stimulated by the analytical developments made in the measurement of the medium half-life nuclides of 238U series (i.e., 234U-230Th-226Ra) in the mid-1980’s, namely, the use of thermal ionisation mass spectrometry (TIMS) (e.g., Chen et al. 1986; Edwards et al. 1987; Cohen et al. 1991; Chabaux 1993;Chabaux et al. 1994) and more recently the use of MC ICP-MS (Turner et al. 2001; Robinson et al. 2002) for U and Th isotope analysis. Details of these developments are given in this volume (Goldstein and Stirling 2003). Compared to the radioactive counting methods previously used, the new techniques permit (1) a reduction by one order of magnitude or more of the size of the sample required for …

U-Th-Ra Fractionation During Weathering and River Transport

Chemical mobilizations in laterites: evidence from trace elements and 238U-234U-230Th disequilibria

Publisher Summary 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.

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

Publisher Summary Zinc deficiency has been observed most extensively in the United States and Australia, but it occurs under field conditions in at least 20 different countries. Zinc deficiencies of plants are dramatic because of combinations of chlorosis, rosetting, dieback, and depressed or abnormal vegetative growth. It occurs on a variety of soils. Because zinc deficiency of plants is highly dramatic, it seems that it would have been identified long ago, but the rapid onset of the disease and the marked deterioration of affected plants were taken as evidence that the cause was not nutritional. Affected soils are usually of pH 6.0 or higher. Often they are sandy, although deficiencies have been found on fine-textured soils and on mucks and peats. Treatments with lime and phosphate fertilizers have been reported to induce zinc deficiencies in crops under some conditions. Old corral and barnyard sites often show zinc deficiencies.

Perry R. Stout

Papers

Isotopic distributions of uranium and thorium in soils.

The micronutrient cations iron, manganese, zinc, and copper: their uptake by plants from the adsorbed state