HR: 1330h
AN: GC12A-0157    [PDF]
TI: Riparian Permafrost Dynamics Associated With Climatic Changes at Tree Line During the Late Holocene
AU: * Vallee, S
EM: Sheila.Vallee@bio.ulaval.ca
AF: Departement de biologie and Centre d'etudes nordiques, Universite Laval, Cite universitaire, Quebec, Qc G1K 7P4 Canada
AU: Payette, S
EM: Serge.Payette@bio.ulaval.ca
AF: Departement de biologie and Centre d'etudes nordiques, Universite Laval, Cite universitaire, Quebec, Qc G1K 7P4 Canada
AB: Thawing of permafrost has been a widespread phenomenon in the Northern Hemisphere since the beginning of the 20th century. Because future global warming is expected to be more pronounced in high latitude regions, it is of major interest to evaluate the influence of forcing factors on permafrost dynamics. In subarctic Quebec, most palsas are found in peatlands. The formation and degradation of these periglacial landforms are influenced by several regional and local factors such as air temperature, depth of snow cover and peat insulation. However, mineral palsas are also observed on river margins. Due to their peculiar position near the river bed, the dynamics of these mineral palsas are influenced by another factor: water level fluctuations associated with variations in winter (snow) precipitation. Studying such terrestrial ecosystems with a high hydrologic component can help distinguish the relative influence of temperature and precipitation on permafrost dynamics. In this study, we have examined mineral palsa dynamics along the Boniface river (subarctic Quebec) in relation to changes in precipitation and temperature. A detailed mapping of palsas and thermokarst ponds allowed us to compare permafrost landforms in 1957 (from aerial photographs) and in 2001 (using data from field surveys). Dates of formation and decay of 15 palsas were assessed from radiocarbon dating of paleosols and tree-ring dates of establishment and mortality of black spruce and planeleaf willow. Two major periods of palsa formation were associated with periods of climatic cooling: around 1300 BP, and around the 15th and 18th centuries (Little Ice Age). Between 1957 and 2001 the area occupied by palsas decreased by 29% whereas thermokarst ponds increased by 76%. No new palsa developed during this period. For the 15 palsas studied, degradation began at the end of the 19th century and climaxed during the 20th century. Palsa dynamics is closely related to distance from the river. The youngest palsas were located in the river bed and were most affected by thawing processes (an average of 48% of area lost). The older palsas were at 1.2 to 14.7 m from the river margin and their decay was less pronounced (an average of 19% of area lost). The spatio-temporal distribution of palsas suggests that changes in water level is one of the most important factor influencing riparian palsa dynamics. As the river retreats, sediments are exposed to frost penetration, leading to the formation of mineral frost mounds along to the spatio-temporal gradient. When the water level increases, the degradation is more important for palsas directly in conctact with the water compared to those located on river margins.
DE: 1600 GLOBAL CHANGE (New category)
DE: 1620 Climate dynamics (3309)
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Global Climate Change [GC]
MN: 2003 Fall Meeting