Thorium isotopes as indicators of scavenging rates in the Venice Lagoon

Abstract

The naturally occurring thorium isotopes ²²⁸Th and ²³⁴Th, produced in sea water from decay of ²²⁸Ra and ²³⁸U, respectively, were used to estimate the rate of scavenging onto particle surfaces and the rate of removal of particles from the water column of the Venice Lagoon. Large water samples (1000-2000 L) were collected at three sites in the shallow (<1 m) waters of the lagoon. Samples were pumped through two 0.5-μm wound filter cartridges to remove particles and then through two MnO₂-impregnated cartridges to extract dissolved Th. Activities of particulate ²³⁴Th ranged from 510 to 1335 μBq L^-1 and dissolved ²³⁴Th was <350 μBq L^-1. Relative to calculated ²³⁸U activities in the lagoon, the ²³⁴Th data yielded mean residence times as short as 2 h for the scavenging of dissolved ²³⁴Th onto particles and 12 h for the removal of particulate ²³⁴Th. Resuspension rates of 0.6 to 8 mg cm^-2 day^-1 were estimated from the data on dissolved and particulate ²³⁴Th, these values being comparable to those determined by sediment traps (1.8-9.5 mg cm^-2 day^-1) at the same sites. These results suggest that Th and other similarly reactive trace metals are removed rapidly from the waters of the Venice Lagoon to the sediments.