Search

The main purpose of the present study, conducted in the Piedemonte Llanero of Colombia, was to identify the role of covariates that influence soil properties under different land uses by analyzing the dynamics of selected soil properties in natural and agricultural areas. Soil samples were collected from 90 different sites throughout the region, 45 from areas with agricultural activities and 45 natural areas or areas with low anthropic intervention. The studied soils were strongly acidic (pH < 5.5), had low organic matter contents (1.73% and 2.34% on average in agricultural soils and natural areas, respectively) and, in general, represented poor conditions for agricultural production. The use of multivariate analysis showed that altitude was an important influence on soil properties. Moreover, it was found that land use influenced soil properties, which may limit productive capacity. The results presented here help to clarify the relationships between soils, climate, and landforms in an equatorial climate. [ABSTRACT FROM AUTHOR]

The methodical principles of the analysis of biogenic silica accumulation in the pelagic zones of the Pacific and Indian oceans are considered. Two quantitative approaches of lithological analysis are compared: method of absolute mass by A.D. Arkhangel'skii and volumetric method by A.B. Ronov. It is established that the concept of 'two oceans' ('ice' and 'non-ice') may be applied to both the oceans. Differences in the evolution of the silica accumulation rate in the Pleistocene between high- and low-latitude oceans are revealed. [ABSTRACT FROM AUTHOR]

L'auteur présente les résultats des mesures des intensités de pluie et des affaiblissements dus à la pluie effectuées au Congo à la fréquence de 7 GHz. Il propose une relation empirique permettant de calculer la distribution des intensités de pluie en zone équatoriale et tropicale qui est bien vérifiée par les données expérimentales, tant pour les faibles que pour les fortes intensités de pluie. Il utilise certaines des méthodes de prévision dont on dispose actuellement pour établir les statistiques des affaiblissements dus à la pluie, notamment celles de S. H. Lin, F. Fedi, R. Crane. Les affaiblissements calculés avec chacune de ces méthodes sont comparés aux valeurs expérimentales. Measurements of rainfall rates and of attenuations due to rain at 7 GHz have been carried out in Congo. An empirical relationship for the prediction of rainfall rates in equatorial and tropical climates has been derived, and reveals good agreement with experimental data. Some rain attenuation prediction models (S. H. Lin, F. Fedi, R. Crane) are checked with our experimental data. [ABSTRACT FROM AUTHOR]

An atypical lateritic weathering profile overlying micaceous quartzites in central Cameroon was studied to understand its morphology and the behavior of major and trace elements (including rare earth elements; REE). Mineralogical analyses were performed by X-ray powder diffraction (XRPD). Major and trace element geochemistry were determined using X-ray fluorescence (XRF) and inductively coupled plasma-mass spectrometry (ICP-MS), respectively. The studied micaceous quartzites consist of 1 to 2 cm thick quartzitic platelets separated by millimeter to multi-millimeter beds of ferromagnesian minerals (biotite, hornblende, garnet and, epidote), muscovite, graphite, and magnetite. According to chemical index of alteration, mafic index of alteration, index of lateritization, the A – Fe – Mg – Ca – Na – K diagrams, and the Weathering index of granite, the intense hydrolysis of ferromagnesian beds resulted in the genesis of soil unit consisting of quartz, muscovite, illite, kaolinite, anatase, and hematite which occur within rotten quartzites or coarse saprolites (CS). Thus, the weathering profile presents the following macromorphological organization: micaceous quartzite/CS-soil/CS-soil/CS-soil/CS-soil/ CS-soil/nodular materials/loose stony clayey soil. The pH-Eh diagrams indicate that formation of these soils within the saprolitic zone is the result of differential weathering that occurred under acidic-reductant conditions, which enhanced the rapid weathering of ferromagnesian mineral beds. The main pedogenetic processes along the weathering profile are translocation of REE, the transformation of primary minerals into neoformed-clays and Fe-oxide-hydroxides formation (associated mainly with light REE losses), and the addition of organic matter in surface horizons (associated with an accumulation of transition metals, high field strength elements, and heavy REE). [ABSTRACT FROM AUTHOR]