SCAVENGING OF THORIUM ISOTOPES BY COLLOIDS IN SEAWATER OF THE GULF OF MEXICO

A suite of surface-water samples from the Gulf of Mexico was analyzed to ascertain the extent of association of Th isotopes (Th-232, Th-234) with colloids and the role of colloidal material in scavenging Th from the water column. These are the first measurements of naturally occurring colloidal Th. The fraction of Th-232, Th-234 associated with colloids (i.e., 10,000 Dalton < colloids < 0.4 mum) ranged from 10 to 78% of the Th passing 0.4 mum Nucleopore cartridge filters. Colloid mass concentrations were much larger than the corresponding 0.4 mum filter-retained particle concentrations. The conditional partitioning constants for Th-234 with colloids, K(c), and filter-retained particles, K(f), are comparable in magnitude. Thorium scavenging rate constants decreased in value with increasing distance from the coast (0.164 to 0.007 d-1), and this is attributed to the decreasing particulate-matter concentration from coastal to deeper Gulf waters. In addition, there exists a highly significant, positive correlation between values of the Th scavenging rate constant and the fraction of 0.4 mum filter-passing Th associated with colloids. An average residence time of 6 days, with a range of 4-26 days, was calculated for the characteristic time scale of colloid transfer through the particle size spectrum, including sedimentation. The large fraction of Th-234 which was found to be associated with colloids suggests that Th isotopes can be used as in-situ "coagulometers," tracing the aggregation of colloidal material with, or into, large aggregates of filter-retained sizes.