234Th-derived particulate organic carbon fluxes in the northern Barents Sea with comparison to drifting sediment trap fluxes

Abstract

Large-volume sampling of 234Th was conducted to estimate particulate organic carbon (POC) export in conjunction with drifting sediment trap deployments in the northern Barents Sea in July 2003 and May 2005. 234Th-derived POC fluxes averaged 42.3 ± 39.7 mmol C m − 2 d − 1 in 2003 and 47.1 ± 30.6 mmol C m − 2 d − 1 in 2005. Sediment trap POC fluxes averaged 13.1 ± 8.2 mmol C m − 2 d − 1 in 2003 and 17.3 ± 11.4 mmol C m − 2 d − 1 in 2005, but better reflected the transient bloom conditions that were observed at each station within a season. Although 234Th fluxes agreed within a factor 2 at most stations and depths sampled, sediment trap POC fluxes were lower than large-volume POC flux estimates at almost every station. This may represent an under-collection of POC by the drifting sediment traps or, conversely, an over-collection of POC by the large-volume sampling of 234Th. It is hypothesized that the offset between the two methods is partly due to the presence of the prymnesiophyte Phaeocystis pouchetii, which potentially causes a large variation in > 53-μm POC/ 234Th ratios. Due to the large proportion of dissolved carbon or mucilage released by P. pouchetii, and because it is thought that P. pouchetii does not contribute significantly to the vertical export of biogenic matter in the Barents Sea, the application of large-volume sampling of 234Th may yield relatively high, and possibly inaccurate POC/ 234Th ratios. Hence, POC fluxes derived from 234Th sampling may be inappropriate and drifting sediment traps might be a more reliable method to measure the vertical export of biogenic matter in regions that have recurrent P. pouchetii blooms, such as the Barents Sea.