9416088 HONEYMAN Conventional wisdom has held that thorium reacts with all non- aqueous phase ligands (i.e., macroparticles and colloids) to the same degree with thorium distribution among those ligands being proportional to ligand concentration. There is, however, emerging evidence that thorium reacts differently with the various compo- nents of the "particle" pool. This evidence suggests that thorium may be highly fractionated within the pool of marine colloidal material (COM) and, thus, has a complicated path through the res- ervior of COM. For example, recent work points to a puzzling aspect of marine colloid behavior: colloid turnover times estimated from U-238/Th-234 disequilibrium are relatively short for all molecular weight fractions: several weeks or less depending on the environment; in contrast, 14C data yields old apparent ages for low molecular weight colloids and modern ages for the higher MW fractions (Santschi et al., 1994). Understanding the chemical relationship between thorium and components of COM should provide access to thorium as a tracer for components within the carbon cycle.
