Terrestrial and marine POC fluxes derived from 234Th distributions and δ13C measurements on the Mackenzie Shelf

Water column deficits of Th-234 relative to U-238 in the Mackenzie Shelf, Cape Bathurst Polynya, and Amundsen Gulf were used to estimate sinking fluxes of particulate organic carbon (POC) in these areas. The Th-234 fluxes were converted to marine and terrestrial POC fluxes using the POC/Th ratio on filterable particles > 70 mu m and delta C-13 measurements to determine the fraction of marine and terrestrial POC. In June/July 2004, the greatest Th-234 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 to 59 dpm m(-2) in the Cape Bathurst Polynya. The delta C-13 values of POC in the > 70-mu m particles filtered in situ pumps ranged from -25.1 parts per thousand to -28 parts per thousand. Using a two-end-member mixing model with marine POC = -21.4 parts per thousand and terrestrial POC = -28 parts per thousand 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 highest values in the Cape Bathurst Polynya. Fluxes of marine POC in the polynya average similar to 5 mmol C m(-2) d(-1) at 50 m in June 2004 and increase to similar to 12 mmol C m(-2) d(-1) in July. Comparable fluxes are observed at 100 m in June, but values decrease to similar 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 (similar to 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.