Comparative evaluation of sediment trap and Th-234-derived POC fluxes from the upper oligotrophic waters of the Gulf of Mexico and the subtropical northwestern Pacific Ocean
Academic Article
Overview
Research
Identity
Additional Document Info
Other
View All
Overview
abstract
To better understand the inter-relationships between particulate organic carbon (POC) fluxes and phytoplankton and bacteria biomass and production, we compared POC fluxes determined in sediment traps and approaches based on size-fractionated (1-10, 10-50, 50-150 and >150m) suspended particulate 234Th and POC concentrations in oligotrophic sections of the Gulf of Mexico during August 2005 and May 2006 and in the oligotrophic northwestern Pacific Ocean during 2009. In 2005, the sediment trap POC flux near the bottom of the euphotic zone (120m) ranged from 71 to 94 mg Cm-2day-1, while 234Th-derived POC fluxes using POC/234Th ratios in the 10-50m and 50-150m varied from 71 to 150mgCm-2day-1. In 2006, the sediment trap POC flux at 120m ranged from 24 to 67mgCm-2day-1, while the 234Th-derived POC fluxes in the 10-50m fraction were comparable or somewhat higher, ranging from 71 to 119mgCm-2day-1. The POC fluxes in 2006, calculated by using POC/234Th ratios in the 1-10m and the 50-150m fractions were much higher, ranging from 847 to 1369mgCm-2day-1. Correlations with biological and chemical parameters support a likely mechanism of sinking aggregates of haptophytes (0.2-20m) of higher density held together by Th-complexing and uronic acid containing exopolymeric substances. The observations that 234Th (and POC) is mainly associated with medium-sized (10-15m) suspended particles rather than larger (50-150m) ones may be caused by the use of a one-filter method and standard filtration and processing procedures that were applied here for collecting suspended particles. This then raises the question of what constitutes representative material from the ocean that settles on the characteristic time scale of 234Th. As a comparison, size-fractionated trap-collected particles in the oligotrophic northwestern Pacific Ocean showed that the 10-50m fraction contained the largest proportion of POC (22-41%), followed by the 50-150m (22-37%), the >150m (15-27%), and the 1-10m (17-23%) fraction. The partitioning of 234Th in trap-collected particles was slightly different from that of POC, with the 1-10m fraction representing the largest proportion (27-48%) of 234Th flux. Together, the <50m particles contributed, on average, 526% of POC, which suggests that the POC/234Th ratios traditionally derived from large (>50m) pump-collected particles may not accurately reflect the majority of sinking particles. Therefore, estimated POC fluxes may be significantly biased using a conventional 234Th based approach, i.e., using POC/234Th ratios from a single filter obtained from large (>50m) pump-collected particles. 2010 Elsevier B.V.
