- © The Mineralogical Society Of America
Rivers carry the products of continental weathering, and continuously supply the oceans with a broad range of chemical constituents. This erosional signature is, however, uniquely moderated by biogeochemical processing within estuaries. Estuaries are commonly described as complex filters at land-sea margins, where significant transformations can occur due to strong physico-chemical gradients. These changes differ for different classes of elements, and can vary widely depending on the geographic location. U- and Th-series nuclides include a range of elements with vastly different characteristics and behaviors within such environments, and the isotopic systematics provide methods for investigating the transport of these nuclides and other analog species across estuaries and into the coastal ocean.
There are numerous types and definitions of estuaries (see Dyer 1973). In this paper, it is simply considered to be a region where freshwater and seawater mix, to emphasize the chemical focus of the issues involved here. This region can either be contained within a river channel or extend onto the shelf, with effects often extending well into an adjoining ocean basin (Fairbridge 1980). These are complex ecological, biogeochemical, and hydrodynamic systems, with strong gradients in the concentrations and composition of micro- and macro particulate matter as well as dissolved organic and inorganic species. Particle-reactive radionuclides and trace elements being transported across estuaries are subject to partitioning between the dissolved, colloidal and particulate phases. The variable chemical nature of the U- and Th-series radionuclides is evident in their distributions in an estuarine environment. Such radionuclides can effectively serve as tracers to identify the sources, fate and transport of particles and colloids as well as other pollutants that behave similarly to these nuclides. These nuclides also have a wide range of half-lives that can be used to examine processes over various time scales. For example, Th and Ra daughter isotopes have …