| Summary: | Water column deficits of 234Th relative to 238 U in the Mackenzie shelf, Cape Bathurst Polynya and Amundsen Gulf were used to estimate sinking fluxes of POC in these areas. 234 Th fluxes were converted to marine and terrestrial POC fluxes using the POC/Th ratio on filterable particles >70 μm and δ13C measurements to determine the fraction of marine and terrestrial POC. In June/July 2004, the greatest 234Th deficits (0-100 m: 56-95 dpm m -2) were observed in the Mackenzie outer shelf. Deficits in the upper 100 m ranged from 3–59 dpm m-2 in the Cape Bathurst Polynya. δ 13C values of POC in the >70 μm particles filtered in situ pumps ranged from -25.1‰ to -28‰. Using a two-end member mixing model with marine POC = -21.4‰ and terrestrial POC = -28‰ shows that terrestrial POC is most evident at the Mackenzie shelf stations but is present throughout the region. The fraction of marine POC ranged from 0 to 59% in the area in June/July 2004, with the highest values in the Amundsen Gulf. Fluxes of marine POC in the polynya average ∼5 mmol C m-2 d-1 at 50 m in June 2004 and increase to ∼12 mmol C m -2 d-1 in July. Comparable fluxes are observed at 100 m in June, but values decrease to ∼6 mmol C m-2 d -1 at 100 m in July. These fluxes are greater than estimates of organic carbon remineralization and burial in sediments of the polynya (∼3 mmol m-2 d-1), suggesting that POC may be exported out of the area, effectively remineralized by microbial activity in the twilight zone or incorporated into biomass.
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