Colloidal organic carbon and 234Th in the Gulf of Maine

The role of colloids in the biogeochemical cycling of organic carbon and 234Th was examined in the Gulf of Maine during the summer of 1996 and 1997. The colloidal fraction was separated using a 1 kiloDalton (kDa) cross-flow ultrafiltration (CFF) membrane. The mass balance of CFF was carefully checked in order to evaluate the performance of the CFF system, since this is one of the primary concerns when using CFF methods. For bulk organic carbon (OC), the membrane showed excellent recoveries (94–107%) as long as preconditioning techniques were employed. Although our mass balances for 234Th were higher than previous efforts (>55% for most samples), significant losses still occurred. Furthermore, a large percentage of the “lost” 234Th was irrecoverable even after the CFF membranes were cleaned with weak acid and base rinses. The colloidal organic C (COC) represented 16–25% of the organic carbon pool, and was correlated with the Chl a fluorescence maximum. The profile of colloidal 234Th, 7–36% of the total 234Th, followed a similar pattern. The difference between the mass balances for 234Th and OC suggests that 234Th may follow only a fraction of COC within the water column. This fraction appears to be the more reactive component of the COC pool. Given the higher and more consistent 234Th recoveries found using our CFF techniques, we were able to constrain the turnover rate of COC to between 3 and 30 days. Turnover rates of dissolved and particulate OC varied from 14 to 41 and 19 to 68 days, respectively.