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 ( 232Th, 234Th) 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 232Th, 234Th associated with colloids (i.e., 10,000 Dalton < colloids < 0.4 μm) ranged from 10 to 78% of the Th passing 0.4 μm Nucleopore cartridge filters. Colloid mass concentrations were much larger than the corresponding 0.4 μm filter-retained particle concentrations. The conditional partitioning constants for 234Th 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 μm 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 234Th 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.