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.