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yvind Paasche Jrund Raukleiv Strmse Svein Olaf Dahl Henriette Linge 《Geomorphology》2006,82(3-4):430-452
The arctic islands of the Lofoten-Vesterålen archipelago in northern Norway have a wide distribution of weathered land surfaces commonly located above 250 m with several apparent similarities. In order to investigate the characteristics of (deep) weathering in this region, northern Langøya and Hadseløya were chosen for in-depth analyses. Eight weathering profiles were excavated from various surfaces, and the stratigraphies were logged in detail. Material was collected throughout the weathering horizons, and all samples were subsequently analysed for clay mineralogy (< 63 μm fraction) and grain size distribution. The sampling strategy was complemented by samples from additional saprolites and other landforms such as moraines and rock glaciers. The XRD results indicate that the presence of secondary minerals, such as gibbsite (Al(OH)3) and kaolinite (Al2Si2O5(OH)4), are very common throughout the profiles. Gibbsite is an extreme end product of silicate weathering and usually associated with a warmer and more humid climate, as found in Scandinavia during the Tertiary. The grain size analyses (< 63 μm) show that the finer silt fractions (< 8 μm) tend to be high in the profiles (20–40%), with significant amounts of clay (5–15%) demonstrating that the regolith itself is susceptible to frost sorting mechanisms.10Be exposure dates from in situ quartz knobs on tors and boulders of local origin suggest > 40,000 years of subaerial conditions. Considering the steady surface erosion, this figure should be viewed as an absolute minimum age estimate. Mapping of the superficial sediments and geomorphological features of the study areas has revealed several common morphological features, which indicate dominance of glacial and periglacial processes in the areas lying below the lower boundary of blockfields (c. 250 m). The weathering mantles are not a periglacial end product, but rather a relict tertiary landform that were modulated by permafrost processes as well as biological processes at later stages. The regolith cover constrain the vertical extension of warm-based Quaternary ice sheets challenging the notion of a parabolic ice mass consuming every mountain top of Lofoten and Vesterålen. 相似文献
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Chemical weathering gradients are defined by the changes in the measured elemental concentrations in solids and pore waters with depth in soils and regoliths. An increase in the mineral weathering rate increases the change in these concentrations with depth while increases in the weathering velocity decrease the change. The solid-state weathering velocity is the rate at which the weathering front propagates through the regolith and the solute weathering velocity is equivalent to the rate of pore water infiltration. These relationships provide a unifying approach to calculating both solid and solute weathering rates from the respective ratios of the weathering velocities and gradients. Contemporary weathering rates based on solute residence times can be directly compared to long-term past weathering based on changes in regolith composition. Both rates incorporate identical parameters describing mineral abundance, stoichiometry, and surface area.
Weathering gradients were used to calculate biotite weathering rates in saprolitic regoliths in the Piedmont of Northern Georgia, USA and in Luquillo Mountains of Puerto Rico. Solid-state weathering gradients for Mg and K at Panola produced reaction rates of 3 to 6×10−17 mol m−2 s−1 for biotite. Faster weathering rates of 1.8 to 3.6×10−16 mol m−2 s−1 are calculated based on Mg and K pore water gradients in the Rio Icacos regolith. The relative rates are in agreement with a warmer and wetter tropical climate in Puerto Rico. Both natural rates are three to six orders of magnitude slower than reported experimental rates of biotite weathering. 相似文献
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Chemical weathering studies in relation to geomorphological research in southeastern Canada 总被引：1，自引：0，他引：1
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. 相似文献
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