Uranium, thorium and lead nuclides in a Delaware salt marsh sediment

Abstract

Natural isotopes of uranium (234, 238), thorium (232, 230) and lead (210) have been assayed in sediments from a lower Delaware salt marsh. Uranium is concentrated in the top sections of the core and decreases with depth (3 to 1 d min⁻¹ g⁻¹). A significant proportion of this uranium has been authigenically extracted from seawater as demonstrated by an excess ratio of ²³⁴U:²³⁸U (1.03-1.14). Thorium (232) is rather constant (2.4-4.7 parts 10⁻⁶ for all but one sample), while the concentration of ²³⁰Th is deficient with respect to its uranium parent. In contrast to the deep sea, the fixation of authigenic reduced uranium dominates over the scavenging of ²³⁰Th in these tidal sediments. Excess ²¹⁰Pb shows an exponential decay over the length of the core, and when corrected for a correspondingly significant exponential decrease in organic carbon and salt yields a sedimentation rate of 0.47 cm year⁻¹. Although this rate approximates the local apparent rate of sea-level rise, a lower limit for the rate of salt marsh accumulation, the actual rate is higher suggesting net filling and acccretion of salt marshes. The total integrated flux of ²¹⁰Pb to this and other salt marsh surfaces in the eastern U.S. is calculated to more often exceed the reported local atmospheric flux. It appears that salt marshes may scavenge ²¹⁰Pb, as other trace metals, from flooding waters.