Controls of 234Th removal from the oligotrophic ocean by polyuronic acids and modification by microbial activity

To gain new insights into the variability of particulate organic carbon (POC) fluxes and to better understand the factors controlling the POC/Th-234 ratios in suspended and sinking particulate matter, we investigated the relarionships between POC/Th-234 ratios and biochemical composition (uronic acids, URA; total carbohydrates, TCHO; acid polysaccharides, APS; and POC) of suspended and sinking matter from the Gulf of Mexico in 2005 and 2006. Our data show that URA/POC in sediment traps (STs). APS/POC in the suspended particles, and turnover times of particulate Th-234 in the water column and those of bacteria in STs inside eddies usually increased with depth, whereas particulate POC/Th-234 (10-50 mu m) and the sediment-trap parameters (POC flux, POC/Th-234 ratio, bacterial biomass, and bacterial production) decreased with depth. However, this trend was not the case for most biological parameters (e.g., phytoplanktan and bacterial biomass) or for the other parameters at the edges of eddies or at coastal-upwelling sites. In general, the following relationships were observed: 1) Th-234/POC ratios in STs were correlated with APS flux, and these ratios in the 10-50 pm suspended particles also correlated with URA/POC ratios; 2) neither URA fluxes nor URA/POC ratios were significantly related to bacterial biomass; 3) the sum of two uronic acids (G2, glucuronic, and galacturonic acid, which composed most of the URA pool) was positively related to bacterial biomass; and 4) the POC/Th-234 ratios in intermediate-sized particles (10-50 mu m) were close to those in sinking particles but much lower than those in >50 mu m particles. The results indicate that acid polysaccharides, though a minor fraction (similar to 1%) of the organic carbon, act more likely as proxy compound classes that might contain the more refractory Th-234-binding biopolymer, rather than acting as the original Th-234 scavenger compound. Moreover, these acid polysaccharides, which might first be produced by phytoplankton and then modified by bacteria, also influence the on-and-off piggy-back processes of organic matter and Th-234, thus causing additional variability of the POC/Th-234 in particles of different sizes. (C) 2010 Elsevier B.V. All rights reserved.