Export fluxes of particulate organic carbon in the Chukchi Sea: A comparative study using 234Th/ 238U disequilibria and drifting sediment traps

Abstract

Large-volume sampling of 234Th and drifting sediment trap deployments were conducted as part of the 2004 Western Arctic Shelf–Basin Interactions (SBI) spring (May 15–June 23) and summer (July 17–August 26) process cruises in the Chukchi Sea. Measurements of 234Th and particulate organic carbon (POC) export fluxes were obtained at five stations during the spring cruise and four stations during the summer cruise along Barrow Canyon (BC) and along a parallel shelf-to-basin transect from East Hanna Shoal (EHS) to the Canada Basin. 234Th and POC fluxes obtained with in situ pumps and drifting sediment traps agreed to within a factor of 2 for 70% of the measurements. POC export fluxes measured with in situ pumps at 50 m along BC were similar in spring and summer (average = 14.0 ± 8.0 mmol C m − 2 day − 1 and 16.5 ± 6.5 mmol C m − 2 day − 1 , respectively), but increased from spring to summer at the EHS transect (average = 1.9 ± 1.1 mmol C m − 2 day − 1 and 19.5 ± 3.3 mmol C m − 2 day − 1 , respectively). POC fluxes measured with sediment traps at 50 m along BC were also similar in both seasons (31.3 ± 9.3 mmol C m − 2 day − 1 and 29.1 ± 14.2 mmol C m − 2 day − 1 , respectively), but were approximately twice as high as POC fluxes measured with in situ pumps. Sediment trap POC fluxes measured along the EHS transect also increased from spring to summer (3.0 ± 1.9 mmol C m − 2 day − 1 and 13.0 ± 6.4 mmol C m − 2 day − 1 , respectively), and these fluxes were similar to the POC fluxes obtained with in situ pumps. Discrepancies in POC export fluxes measured using in situ pumps and sediment traps may be reasonably explained by differences in the estimated POC/ 234Th ratios that arise from differences between the techniques, such as time-scale of measurement and size and composition of the collected particles. Despite this variability, in situ pump and sediment trap-derived POC fluxes were only significantly different at a highly productive station in BC during the spring.