Contribution
Summary
Bremer, Hanna
Zeitschrift für Geomorphologie, Supplementary Issues Volume 54 Supplementary Issue 1 (2010), p. 303 - 311
publié: May 1, 2010
DOI: 10.1127/zfg_suppl/54/2010/303
ArtNo. ESP023105401012, Prix: 29.00 €
Abstract
Nearly 50 years of research and discussion of tropical geomorphology and geoecology is the background of the following results and proposals. The database (CRD) on which the study is based consists of 30 parameters for 938 regolith samples, 93 sand samples and 189 rock and crust samples. The principle difference that needs to be kept in mind when applying rules from the extratropics is that surface renewal in many extratropical regions during glacial times contrasts with widespread inheritance of old relief elements in the tropics. The intensity of weathering of regoliths not only involves past processes but also the potential for current geoecological processes. Material transport is a significant part of geoecology. Subterranean transport makes itself apparent in the intensity of weathering and regolith characteristics. Displacements on the surface are correlated to geomorphological processes recognized by observation of landforms and regolith features (autochthon-allochthon, recent – ancient). The diagnostic quality of primary and secondary minerals and their associations for weathering intensity are generally accepted fundamentals of tropical weathering. Types of weathering fronts at various scales and small features on rock surfaces reveal the broad range of transformation processes under different ecological conditions. Characteristics of tropical regoliths which differ from those in the extratropics are the relatively uniform grain size ranges with an average of 50% sand and little clay, high pore (macropore) volume (median 20%) and the stable structure provided by pseudosand and crystallisation. These features explain many individual results. Material flow in regoliths is more rapid and distinct than in the extratropics and it has a strong lateral component. Not only is clay removed subterraneanly but also pipes form as well as surface depressions. This means export of solid substances as well and either slow sinking or collaps of the cavities. Stratified or irregular deposits may form. Discontinuities can be explained by interflow and, in conjunction with bioturbation, stonelines form. The fundamentals are discussed in detail to allow temporal and spatial classification of geoecological processes. However, a universally accepted geoecological time scale is not yet available for the tropics. In the savanna soil building processes are less effective due to a semi-humid climate and inherited soils became truncated during long periods of time. The effect of climate changes is difficult to establish with any degree of certainty in the humid tropics. “Aged” soils form by removal of clay and/or iron from the topsoil and by residual enrichment of sand and/or pisoliths near the surface. In contrast to the extratropics, the surface is not renewed during glacial times, palaeosols and aged soils are much more widely distributed and a spatially more varied soil pattern forms. Zonal (savanna-rainforest), regional (petrovariance and tectonovariance) und local categories (catena, divergent weathering) have different impacts on geoecological processes. Quantification of erosion and weathering is limited not only due to the limited number of absolute dates but also for methodical reasons, especially in regard to the difficulty of ascertainment of subterranean removal.
Mots-clefs
extratropics • regolith • crystallisation • subterranean • removal of clay • savanna • rainforest • glacial times