| Summary: | Rates of carbon and peat accumulation were studied in a series of peat landforms within discontinuously frozen peatlands near Fort Simpson, Northwest Territories. An extended distribution of the White River volcanic ash layer was used as a chronostratigraphic horizon to ensure a consistent time span of peat deposition among peat cores and to allow a large core sample size without the expense of radiocarbon dating. Apparent recent carbon accumulation rates measured over the past 1200 years were not significantly different among rich fen, peat plateau, and collapse fen (means 13--14 g C M-2 yr-1). Poor fen and bog mean accumulation rates were 20--22 g C M-2 yr -1 and were not significantly different from each other. Microtopography and water table position appear to be important controls on both carbon and vertical peat accumulation rates. A regional survey incorporating measurements from other parts of the southwestern Northwest Territories and the southeastern Yukon shows rates similar to those near Fort Simpson. The aggradation of permafrost results in 50 and 65% decreases in carbon and vertical peat accumulation rates, respectively. Carbon and peat accumulation continue to decrease significantly with both increasing permafrost maturity and the number of ground fires. The internal degradation of permafrost results in nearly a doubling in carbon accumulation rates, yet permafrost degradation at the margins of a peat plateau results in carbon accumulation rates similar to the peat plateau. Clymo's (1984) carbon accumulation model was applied to cores from each landform in addition to a core spanning the entire developmental history of the peatland. Results indicate that true carbon accumulation and sequestration efficiency rates in ombrotrophic peatlands are lower in the upper Mackenzie Valley than for other boreal regions, primarily owing to high decomposition rates. The cessation of carbon accumulation is being approached. The model also serves to highlight the dangers of using apparent and true carbon accumulation rates interchangably. Apparent and true carbon accumulation rates are significantly lower than published rates from other parts of northern Canada, Finland, and the Former Soviet Union. Low and variable summer precipitation in the region may be a significant factor through increased aerobic decomposition and/or decreased plant production caused by moisture stress.
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