Comparative evaluation of sediment trap and 234Th-derived POC fluxes from the upper oligotrophic waters of the Gulf of Mexico and the subtropical northwestern Pacific Ocean

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 > 150 μm) 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 (120 m) ranged from 71 to 94 mg C m − 2 day − 1 , while 234Th-derived POC fluxes using POC/ 234Th ratios in the 10–50 µm and 50–150 µm varied from 71 to 150 mg C m − 2 day − 1 . In 2006, the sediment trap POC flux at 120 m ranged from 24 to 67 mg C m − 2 day − 1 , while the 234Th-derived POC fluxes in the 10–50 µm fraction were comparable or somewhat higher, ranging from 71 to 119 mg C m − 2 day − 1 . The POC fluxes in 2006, calculated by using POC/ 234Th ratios in the 1–10 µm and the 50–150 µm fractions were much higher, ranging from 847 to 1369 mg C m − 2 day − 1 . Correlations with biological and chemical parameters support a likely mechanism of sinking aggregates of haptophytes (0.2–20 µm) 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 (10–15 µm) suspended particles rather than larger (50–150 µm) 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 10–50 μm fraction contained the largest proportion of POC (22–41%), followed by the 50–150 μm (22–37%), the > 150 μm (15–27%), and the 1–10 μm (17–23%) fraction. The partitioning of 234Th in trap-collected particles was slightly different from that of POC, with the 1–10 μm fraction representing the largest proportion (27–48%) of 234Th flux. Together, the < 50 μm particles contributed, on average, 52 ± 6% of POC, which suggests that the POC/ 234Th ratios traditionally derived from large (> 50 µm) 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 (> 50 µm) pump-collected particles.