Seasonal and interannual changes in particulate organic carbon export and deposition in the Chukchi Sea

Particulate organic carbon (POC) export fluxes were estimated in the shelf-slope region of the Chukchi Sea using measurements of (Th-238U)-Th-234 disequilibria and the POC/Th-234 ratio in large (> 53-mu m) particles. These export fluxes were used in conjunction with rates of primary productivity and benthic carbon respiration to construct a POC budget for this shelf-slope region. Samples were collected along a series of shelf-basin transects in the spring ( May - June) and summer ( July - August) of 2004. These stations were previously occupied during the ice covered ( spring) and open water ( summer) seasons of 2002, allowing for an interannual comparison of export flux. In contrast to 2002, when open water POC fluxes were significantly higher than in the ice-covered period, POC export fluxes in 2004 were similar during the spring ( average = 19.7 +/- 24.8 mmol C m(-2) d(-1)) and summer ( average = 20.0 +/- 14.5 mmol C m(-2) d(-1)). The high POC fluxes measured during the spring are attributed to a plankton bloom, as evidenced by exceptionally high rates of primary productivity (average = 124.4 +/- 88.1 mmol C m(-2) d(-1)). The shelf-slope budget of particulate organic carbon indicates that 10 - 20% of primary productivity was exported below 50 m but was not consumed during benthic carbon respiration or burial and oxidation in underlying sediments. Furthermore, a water column - sediment budget of Th-234 indicates that particulate material is retained in shelf sediments on a seasonal basis.