Scavenging and fractionation of thorium vs. protactinium in the ocean, as determined from particle–water partitioning experiments with sediment trap material from the Gulf of Mexico and Sargasso Sea

Particle–water partition coefficients (Kd) for thorium (Th) and protactinium (Pa) were calculated for sediment trap material collected from the Gulf of Mexico (GOM) in the upper 140 m and archived sediment trap material collected in the mesopelagic and bathypelagic Sargasso Sea (500, 1500 and 3200 m sediment trap samples from the Oceanic Flux Program (OFP) site off Bermuda). Results showed that Kd values for Th were generally greater than for Pa when values were smaller, but converging to a 1:1 line at higher values. Furthermore, results showed that, though the contents of polysaccharides and calcium carbonate are significantly correlated for all samples, log Kd of Th (and Pa) values are correlated with the polysaccharide content while no correlation is apparent with CaCO3, Mn and Fe. Since polysaccharides are not generally regarded as strongly chelating agents for actinides, we hypothesize that other co-occurring organic phases originating from the matrix of carbohydrate-rich extracellular polymeric substances (EPS), could be responsible for binding the actinides. These data provide, for the first time, direct evidence for the role of carbohydrate-rich EPS being responsible for the differential scavenging of these two radioisotopes, i.e., their fractionation.