Natural U-Th series radionuclide studies of estuarine processes

Abstract

The natural uranium and thorium decay series provides a series of elements, the chemistry and decay times of which reveal key estuarine processes. In turn, estuarine processes impose disequilibria on the series which can define the rates of many estuarine processes. Radionuclide studies have been conducted in the Delaware Estuary and its pervasive salt marshes over the past 10 years. In the Delaware Estuary, uranium shows conservative behavior, except in the upper reaches during the summer season. Both in the estuary and in the surrounding salt marsh sediments, important uranium uptake processes occur concurrent with a minimum in alkalinity and maximum in phosphate. This suggests that iron/phosphate based scavenging processes out-compete the carbonate-complexes which normally stabilize uranium in estuarine and sea waters. Such uptake of uranium in th world's estuaries could constitute a significant global sink for marine uranium. Thorium and lead in the Delaware Estuary show dominant particle association. During periods of high productivity, the super(210)Pb granddaughter, super(210)Po, seasonally shows dissolved activity excesses, particularly in tidal waters of salt marshes. The polonium excesses persist out onto shelf areas as the lead parent is removed by particulate sedimentation processes. The polonium excesses are believed to be associated with coastal processes of organic association and complexation.