Mireille Bouchard

Chemical weathering studies in relation to geomorphological research in southeastern Canada

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 Montreal (Quebec). 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.

Comparison of streamflow between pre and post timber harvesting in Catamaran Brook (Canada)

Abstract The forest industry plays a major role in the economy of eastern Canada. The recreational fishery also represents an important source of revenue for this area. Therefore, there is concern over the potential economic effects and ecological impacts from logging operations on aquatic habitats. The present study deals with the comparison of streamflow between pre and post timber harvesting at Catamaran Brook (New Brunswick, Canada) to identify any potential changes to the hydrological regime. Studies were carried out on two sub-basins of Catamaran Brook, namely the Middle Reach (mid-basin) and the Upper Tributary 1. The harvested area at the Middle Reach represented 2.3% of this sub-basin while 23.4% of Upper Tributary 1 was harvested. It was noted that during both the calibration and timber harvesting phases, meteorological conditions (e.g. precipitation, runoff) contributed to relatively high natural variability. When studying changes on an annual and seasonal basis for the basin cut at 2.3% (i.e. Middle Reach) and using a control basin for comparison, no changes were detected to the annual water yield, seasonal runoff and streamflow timing between the calibration and timber harvesting phases. On a summer rainfall event basis, no changes were detected at the Middle Reach and the Upper Tributary 1 when studying relations between precipitation and stormflow (obtained through hydrograph separation). Alternatively, changes were detected in relations between peak flows and precipitation ( p When comparative studies were carried out on peak flow and stormflow between sub-basins (using the Middle Reach as control for treatments vs. the most affected site, i.e. Upper Tributary 1), significant changes were detected in peak flow ( p

Characteristics and significance of two pre-late-Wisconsinan weathering profiles (Adirondacks, USA and Miramichi Highlands, Canada)

Abstract The properties of two weathering profiles developed from granitic rocks and covered by deposits of Wisconsinan age were studied in order to characterize saprolites and non saprolite residuum of the glaciated area of northeastern North America. The Warrensburg profile (Adirondacks, New York) is more than 9 m thick and formed on a hornblende granitic gneiss. Total chemical analysis of the sandy saprolite shows slight losses compared to bedrock. Kaolinite is the main weathering product in the

Palaeolandforms and morphotectonic evolution around the Baie des Chaleurs (eastern Canada)

Abstract A morphological study of the Baie des Chaleurs area, between northern New Brunswick and the Gaspe Peninsula (eastern Canada), leads to the identification of several types of palaeolandforms in the landscapes of the northeastern Appalachians. One of them, the exhumed sub-Carboniferous palaeosurface, was recognized along the shores of the Baie des Chaleurs, around the western part of the Carboniferous basin — the Maritimes Basin — which underlies the Gulf of Saint Lawrence. By analyzing the conditions of its exhumation and its relationships with escarpments limiting the higher planation surfaces of inner Gaspesie and western New Brunswick, it was possible to identify recent deformations. These are en bloc tilting and uplift, flexuring, and faulting; they partly reflect the reactivation of Carboniferous or older structures. The study of palaeolandforms has already proved to be an appropriate method for reconstructing the morphological evolution of basement areas. In this paper, it is applied to the study of the evolution of an emerged part of the eastern Canada rifted margin after the opening of the North Atlantic Ocean.

Influences stationnelles sur l'alteration chimique des sols derives de till (Sherbrooke, Que., Canada)

Abstract In a small watershed of the canadian Appalachians, five trenches were studied in two catenas facing each other, one forested, the other under recent prairie vegetation (100 years of age), to investigate the character of chemical weathering in these sites. On the top of the forested catena there was a humo-ferric fragic podzol, and further down, and orthic gleysol. The other catena had a humo-ferric fragic podzol, then, downwards, a grey gleyified luvisol and a luvic fragic gleysol. Percolating waters near these trenches were sampled regularly, at various depth. The distribution of the ions on the meadow side indicated on one hand that H 4 SiO 4 , Na + and Mg 2+ in percolating waters are related to the total volume of soil through wich waters percolate, that the dissolution of aluminium is accelerated in acid sites (upslope) and that the solubility of iron reaches a maximum in the zone influenced by the water table. On the other hand, calcium, which is easily liberated in the high sections, is in part fixed when the solutions get concentrated, iron and aluminium are soon traped, silicium is less mobile than the basic cations. The constituants of primary minerals organize as follows: concentration of Al 2 O 3 and Fe 2 O 3 upslope, when the contrary is for the basic oxydes and SiO 2 ; this trend stands out more clearly in the top 30 cm. Thus on top of the forested catena, chemical weathering is intense, the aluminium and iron oxydes are highly evacuated in this podzol. At the same altitude, in the prairie podzol, which probaly had 100 years ago a less acidifying vegetation (hardwoods) than in the first case, feldspars and micas are degraded and we find mostly transformed minerals (montmorillonite — Al). Minerals are very much less degraded downslope because of their confined position and the high ionic content of the percolating waters.