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>Geobotanical and biogeochemical investigations in the Otavi Mountainland, South West Africa with particular reference to the influences of the calcareous nature of soils and bedrock on the uptake of toxic metals by plants
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Geobotanical and biogeochemical investigations in the Otavi Mountainland, South West Africa with particular reference to the influences of the calcareous nature of soils and bedrock on the uptake of toxic metals by plants
Investigations of the vegetation of the Otavi Mountainland in northern South West Africa were undertaken during the period 1970 - 1972. These had two primary objectives; firstly an evaluation of the influence of individual environmental factors on the distribution of vegetation units and secondly, an assessment of the roles of geobotany and biogeochemistry in exploration for copper, lead and zinc ores in the particular environment characteristic of this part of Africa. Regional reconnaissance studies were followed by detailed field investigations involving the collection of soil and plant samples for subsequent laboratory analysis for trace elements. The results of these investigations form the subject of this thesis. The study area is characterised by a sequence of predominantly calcareous rocks, mainly dolomites and limestones, of the Nosib Formation and Damara System, both of Proterozoic age. The more resistant rocks build striking mountain ranges which rise abruptly from broad flat floored valleys which are covered with younger depositional material. The ranges carry forms of Open Low Savanna Woodland, a low tree Savanna in which the trees are relatively closely spaced, while the valleys carry a Low Tree and Shrub Savanna in which the trees are widely to closely spaced in a fairly dense grass cover. The geobotanical investigations disclosed close distributional relationship between vegetation associations and lithological units. They revealed the considerable influence of calcrete and calcareous bedrock on the composition of the vegetation associations. Change of species distribution imparting differences of pattern within vegetation associations and the occurrence of possible indicator plants were discovered in the vicinities of known mineralised areas. The biogeochemical investigations revealed a number of areas with weak anomalies in the copper, zinc and lead contents of the leaves and twigs of the trees and shrubs which were sampled. Some of these anomalies occurred over areas with outcropping mineralised bedrock where they supported and confirmed the geochemical anomalies disclosed by soil sampling. Others having occurred in areas with deep soil and/or thick calcrete cover where geochemical values were either low or weekly anomalous. Overall the results suggested that biogeochemistry provides a complement to geochemistry, that it is capable of detecting mineralised bedrock beneath cover of soil and calcrete.
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