RECENT PERMAFROST MELT IN PEATLANDS OF NORTHWESTERN CANADA
ROBINSON, STEPHEN D. St. Lawrence University.
Park, Joe . GDT Canada.
The presence or absence of permafrost is an extremely important control on rates of carbon storage in peatlands (Camill, 1999; Robinson and Moore, 2000). Most studies that suggest that a climate warming will result in a net carbon source to the atmosphere through increased decomposition are from sites with continuous permafrost. The response of carbon pools in the discontinuous permafrost zone may be more complicated as peatland-permafrost relationships are influenced more by vegetation patterns and hydrology, and result in peat landforms with significantly different rates of carbon storage (Robinson and Moore, 1999).
The Mackenzie Valley, Northwest Territories, is one of the major peatland regions in Canada, yet it remains relatively unstudied by researchers. Peatlands cover approximately 15% of the Mackenzie Valley north of 60oN, but with large areas approaching 100% peat cover. Permafrost conditions range from sporadic discontinuous in the southern part of the valley, extensive between approximately 62 and 64oN, and continuous north of 64 or 65oN. The region is also characterized by low precipitation and is affected by a significant number of wildfires.
Around and south of Fort Simpson, peatlands are a dominant component of the landscape. Permafrost temperatures in peatlands of this area are only slightly below 0oC (Robinson and Moore, 2000). This region of discontinuous permafrost adds an important and dynamic complication to the question of carbon storage, and much of this permafrost is expected to melt with only a slight warming (Wright et al., 2000). A significant warming trend is noted in climate records for both Fort Simpson and Norman Wells, and the Mackenzie Valley region has undergone the most warming of any region in Canada over the past century (data from Environment Canada).
The delineation of frozen and unfrozen peatland areas is relatively simple using aerial photographs and some forms of satellite imagery. In general, frozen peatlands are forested and appear dark on photographs, while unfrozen regions are of lighter tones. Utilizing imagery collected over a long time span, combined with GIS analytical techniques, allows the calculation of recent rates of thaw. We used a time-series of aerial photographs and high-resolution satellite images spanning at least 50 years to quantify thaw at four locations (total 6 sites) ranging in latitude from 60 to 64oN.
Significant thaw of permafrost has occurred at all sites over the past 50 years, ranging from a 33.9 to 79.2% increase in unfrozen peatland area. On average, the amount of thawed peatland increases by 1% per year in this region. There is a trend of lesser increases in thawed area with increasing latitude. Mean lateral thaw (the lateral movement of the frozen - unfrozen boundary) at different sites ranges from 7.0 to 18.6 m over the time period of study. This translates into a range of 0.127 to 0.351 m per year over the 50-55 years of study. Maximum thaw rates reach over 1.0 m per year at several sites. There are also locations at the Liard site (61o 26íN) where very large peat plateaus, up to 200 x 225 m have completely thawed in the 53 years between images. Overall, there are significant ranges in retreat rates, with some margins retreating as much as 45 m, and some showing no retreat or even minor permafrost aggradation, however the general trend is towards significant, widespread permafrost thaw in peatlands of the southern Mackenzie Valley.
Many of the thawed areas at the Liard River sites are becoming more and more interconnected and contiguous over time, resulting in improved drainage and a decrease in the dominance of permafrost. A result of the dominance of unfrozen peatlands and improved drainage is the establishment of drier, shrub and tree-dominated non-permafrost peatlands that have higher carbon accumulation rates than permafrost-dominated sites. In contrast, sites other than the Liard River area are still permafrost-dominated, although the trend is certainly towards ongoing thaw, the coalescence of thaw features, the development of an integrated drainage basin, and a landscape becoming more dominated by unfrozen peatlands. These differences may be due to changing hydrological conditions associated with the loss of permafrost on a large scale. Recent evidence from Camill (1999), combined with this assessment based on a time series of images, suggests that thaw features may follow a consistent succession pattern changing to drier, raised communities in as little as 50-80 years. It is possible that high methane fluxes common following permafrost thaw may not be sustainable over the long-term, as the raised unfrozen communities have much lower fluxes (Liblik et al., 1997).
Camill, P. 1999. Peat accumulation and successsion in the boreal permafrost peatlands of Manitoba, Canada. Ecoscience 6, p. 592-602.
Liblik, L.K., Moore, T.R., Bubier, J.L., and Robinson, S.D. 1997. Methane emissions from wetlands in the zone of discontinuous permafrost: Fort Simpson, Northwest Territories, Canada. Global Biogeochemical Cycles, 11, p. 485-494.
Robinson, S.D. and Moore, T.R. 1999. Carbon and peat accumulation over the past 1200 years in a landscape with discontinuous permafrost, northwestern Canada. Global Biogeochemical Cycles, 13, p. 591-602.
Robinson, S.D. and Moore, T.R. 2000. The influence of permafrost and fire upon carbon accumulation in high boreal peatlands, Northwest Territories, Canada. Arctic, Antarctic and Alpine Research, 32, p. 155-166.
Wright, J.F., Smith, M.W., and Taylor, A.E. 2000. Potential changes in permafrost distribution in the Fort Simpson and Norman Wells regions. Pages 197-207 in L.D. Dyke and G.R. Brooks, eds. The Physical Environment of the Mackenzie Valley, Northwest Territories: a Baseline for the Assessment of Environmental Change. Geological Survey of Canada Bulletin 547, p. 197-207.
Previous | Abstract Index | Next
Copyright © 2001 INSTAAR, Univ. of Colorado