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

Abstract

Size-fractionated particulate 234 Th 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 234 Th 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 234 Th concentrations were observed, with a considerable 234 Th deficit relative to 238 U in the upper water column and in benthic nepheloid layers , but reaching secular equilibria between 234 Th and 238 U in intermediate waters. Within the total particulate 234 Th pool (>0.5 μm), the 10–53 μm fraction had the largest share of 234 Th (37–57%), followed by the >53 μm (13–36%), the 1–10 μm (10–21%), and the 0.5–1 μm (8–17%) fractions, resulting in a decrease of POC/ 234 Th ratios with increasing particle size. Residence times of 234 Th in size-fractionated particles, calculated with a serial multi-box model, were, as expected, consistently shorter than those for total particulate 234 Th, 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 μm), and the large particles >53 μm. These results suggest that submicron and micron-sized particles are the most important intermediary in the Th scavenging and that 234 Th on smaller particles (<10 μm) can coagulate into the 10–53 μm particles very rapidly, within a time scale of <1 day. A positive correlation between 234 Th/POC and OC-normalized 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 μm 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 234 Th 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.