234Th scavenging and its relationship to acid polysaccharide abundance in the Gulf of Mexico

Size-fractionated particulate Th-234 and acid polysaccharides (APS) were collected from stations along a transect in the Gulf of Mexico, in order to examine the role of APS content in controlling the extent and rates of Th-234 scavenging in the ocean and to explore, for the first time, the relationship between Th scavenging and biochemical composition of particulate matter. Oceanographically consistent vertical profiles of dissolved and particulate Th-234 concentrations were observed, with a considerable Th-234 deficit relative to U-238 in the upper water column and in bentbic nepheloid layers, but reaching secular equilibria between Th-234 and U-238 in intermediate waters. Within the total particulate Tb-234 pool (>0.5 mum), the 10-53 mum fraction had the largest share of Th-234 (37-57%), followed by the >53 mum (13-36%), the 1 - 10 mum (10-21%), and the 0.5-1 mum (8-17%) fractions, resulting in a decrease of POC/(234) Th ratios with increasing particle size. Residence times of Th-234 in size-fractionated particles, calculated with a serial multi-box model, were, as expected, consistently shorter than those for total particulate Th-234, with the shortest residence times (< 0.5 day at coastal stations and < 1 - 5 days at deep stations) observed in the smaller particulate fractions (0.5- 10 mum), and the large particles >53 mum. These results suggest that submicron and micronsized particles are the most important intermediary in the Th scavenging and that Th-234 on smaller particles (< 10 mum) can coagulate into the 10-53 mum particles very rapidly, within a time scale of < 1 day. A positive correlation between Th-234/POC and OC-Pormalized total APS content was observed, suggesting that exopolymeric fibrillar APS, the surface active substances in seawater, are the most effective organic compounds for Th(IV) scavenging. Most importantly, residence times of particles in the size ranges of 1-10 and the >53 mum were also significantly and inversely correlated with uronic acid (URA, a fraction of total APS) concentrations, indicating that the APS content controls not only rates and amounts of Th-234 sorption, but also rates of coagulation of particles. Thus, the biochemical composition of marine particles needs to be considered in improved Th(IV) scavenging models. (C) 2002 Elsevier Science B.V All rights reserved.