| Abstract: | Although chemical weathering provides fundamental information relevant to geomorphology, the subject has been overlooked during the 20th century in Canada. This paper provides an overview of the current state of Canadian research on chemical weathering in southeastern Canada and takes into account three spatial and temporal contexts: (1) the formation of bedrock morphology by chemical weathering, (2) occurrences, characteristics and age of saprolites and (3) contemporary chemical denudation rates. Long-term geomorphological evolution of southeastern Canadian landscapes shows that chemical weathering has played an important role. An example is taken from the Laurentide region of the Canadian Shield north of Montréal (Québec). The present topography resulted from the stripping of the former weathering mantle and from the probable subsequent modification of the weathering front, first by the action of hillslope processes and rivers and then by glaciers, before and during Plio-Pleistocene times. The present landscape reflects the timing of the formation of erosion surfaces, and of the stripping of the Paleozoic cover rocks and exposure of the Shield. Since the late seventies, several isolated occurrences of saprolite-soil profiles have been discovered in eastern Canada and prompted a renewal of the study of these materials about the Cenozoic evolution of these regions. One of the problems in this field of research is the dating of saprolites and their inclusion in a chronostratigraphic framework, along with the other Cenozoic surficial deposits and landforms. Because of the multiple factors involved in the development of secondary minerals in saprolites during the course of weathering, it is preferable to distinguish the dating of saprolites from the study of their mineralogical and geochemical evolution. Fortunately, several new techniques are becoming available for the absolute dating of surficial deposits and saprolites, including the use of cosmogenic radionuclides. Saprolites provide a strong potential field of research for our understanding of the geological evolution of eastern Canada during the Cenozoic. Contemporary weathering and erosion rates are fields of research that have gained increasing interest recently, since modeling landscape geochemical response can be applied to various environmental stress situations, such as acidification by rain and forest harvesting. Rock-type may be the main factor explaining the large differences between watersheds. In fact, variability of cation removal in the temperate zone is probably most closely related to flow-paths of water. Investigations, at different scales, from entire watersheds to slopes to individual pedons, highlight the problem. In the Catamaran Brook watershed (New Brunswick), water chemistry is explained by a mix of groundwater and soil solution from the horizons at the base of the floodplain soils. Geochemical mass-balances based on net outputs give little information on the weathering reactions of primary minerals, the weathering products or on the nature of the weathering processes that provide the dissolved load of streams. Mineralogic and petrographic analysis of selected soil pedons are necessary to determine weathering reactions and their role as sources or sinks for bases, silica, aluminum and iron in the various compartments through which water percolates before it reaches the stream. |
