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

Abstract

Water column deficits of 234Th relative to 238U 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 234Th 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 to 59 dpm m−2 in the Cape Bathurst Polynya. The δ13C 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 highest values in the Cape Bathurst Polynya. 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.