Seasonal decoupling of particulate organic carbon export and net primary production in relation to sea-ice at the shelf break of the eastern Bering Sea: Implications for off-shelf carbon export

Particulate organic carbon (POC) export fluxes and net primary production (NPP) rates are used to assess seasonal patterns in the export ratio (e-ratio=POC export/NPP) in relation to proximity of the sea-ice edge near the shelf break of the eastern Bering Sea during 2008-2010. POC fluxes were relatively low in April (4.61.6, trap, and 5.74.3 mmol C m(-2) d(-1), Th-234-derived) and increased in May-early June (19.913.3 and 17.08.8 mmol C m(-2) d(-1)). POC export reached a maximum in mid-June-mid-July (30.0 +/- 12.6 and 48.1 +/- 17.4 in 2009; 33.1 +/- 27.6 and 57.0 +/- 68.4 mmol C m(-2) d(-1) in 2010) and decreased by late July (13.1 +/- 4.7 and 14.1 +/- 8.0 mmol C m(-2) d(-1)). NPP rates were relatively high and export fluxes low near the ice-edge in spring leading to e-ratios <0.25. In early summer, POC export exceeded NPP at individual stations and resulted in e-ratios >1, which is attributed to a temporal decoupling, or offset, of spring NPP and export during summer. While these observations reveal a seasonal progression in POC export and the e-ratio, there is no direct relationship to sea-ice proximity. Furthermore, based on a water column-sediment Th-234 budget, the off-shelf export of POC during spring-summer is estimated to be 24 +/- 35 mmol C m(-2) d(-1), which represents an off-shelf e-ratio of 0.07 and 0.21 for contemporaneous seasonally averaged daily NPP and 0.17 and 0.52 for historical monthly averaged daily NPP. An implication is that off-shelf POC transport may represent a seasonal net sink for CO2 in this and other polar shelf regions.