Browsing by Author "Chadwick, Peter J"

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    The effects of leaf harvesting on the morphology, reproduction and sap production of the Cape Aloe (Aloe Ferox)
    (2004) Greengrass, Catherine; Chadwick, Peter J; Hoffmann, Timm
    Harvesting of Aloeferox was estimated (in 1996) to bring in an annual income ofR4 million per year to rural communities alone - a vital source of income for full-time tappers who rely solely on Aloe ferox as their only form of financial survival. They are joined on occasion by casual-tappers who rely on tapping to supplement their incomes in times of hardship (Newton and Vaughan 1996). Landowner's also benefit by allowing harvesting on their land in exchange for a percentage of the harvest (Newton and Vaughan 1996). Trade in bitters sap or gel far outweighs that of other plant parts so care is taken by harvesters to keep the plants alive for future tapping. Leaves from only the lower third of the leaf rosette ( 10 - 15 leaves) are harvested, and 2 - 4 cm at the base of the leaves are left to prevent damage to vascular tissues (Newton and Vaughan 1996, Mac Farlane 2004). Populations are only harvested every 18 - 36 months allowing plants to recover from the loss of leaf material. Plants suffering from insect-infestation or disease are not harvested to avoid weakening the plant further and possibly causing death (Newton and Vaughan 1996). Effort is also made by landowners to relocate plants growing in areas they plan to transform and to monitor harvesting activities on their properties (Newton and Vaughan 1996). The vested interest in keeping the plants alive has inadvertently assured the preservation of populations at past harvesting levels although no formal conservation or resource management policy presently exists for A. ferox (Newton and Vaughan 1996). Effects of harvesting, which are not limited to the direct effect of leaf removal on the plant but include indirect effects - such as reduced reproductive output, reduced sap production and even trampling of recruits - have nonetheless raised concern for the long-term survival of populations (Newton and Vaughan 1996). This study addressed some of the indirect effects of harvesting on Aloe ferox on both population and individual plants levels. On the population level plant density, mortality, susceptibility to disease and sizeclass structure was compared between unharvested and harvested populations. On the individual plant level vegetative traits, flower production and sap production were compared between unharvested and harvested plants. One of few other studies on ecology of A. ferox by Hoffman (1988) assessed some of these characteristics for a population in Port Elizabeth, South Africa.
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    A study of the berg Aukas-type Pb-Zn-V deposits in the Otavi mountain land, Namibia
    (1993) Chadwick, Peter J; Frimmel, Hartwig E
    This study is focussed on the petrographic and isotopic aspects of the crystallization processes taking place in carbonate-hosted Berg Aukas-type Zn-Pb-V deposits of the Otavi Mountain Land in Northern Namibia. Numerous studies have shown that ore formation in Mississippi Valley-type deposits bears a close relationship with carbonate diagenesis and basin evolution. The application of isotope (Sr , O, C) techniques as regards to timing and mechanisms of ore-forming processes, as a possible tool in exploration, is especially emphasized in this study. Detailed petrographic and isotope-geochemical investigations were carried out in the Late Precambrian deposits of Berg Aukas, situated just north of the town of Grootfontein, in addition to the Odin Prospect situated to the east of Kombat Mine. Both ore deposits display many similarities, but also show differences concerning the facies-bound character of the host rock, which is related to the paleogeographic position and the tectonic framework. In spite of these differences, similar complex diagenetic textures and structures suggest a comparable diagenetic evolution. Isotope-geochemical analyses have been performed on samples of consecutive crystallization generations, or dolomite types. This was checked petrographically using cathodoluminescence, and sample homogeneity was monitored by means of X-ray diffraction. The results reveal significant isotopic trends, which are interpreted to reflect the diagenetic evolution of the respective basin. A consistent strontium isotope trend is noted, and in both cases studied, the later crystallization generations are enriched in radiogenic strontium, as compared to the earlier ones, which tend to approximate the strontium isotope composition of contemporaneous seawater. The increase in ⁸⁷Sr/⁸⁶Sr ratios in subsequent diagenetic generations is explained with the pore fluids evolving towards more radiogenic compositions with advancing diagenetic stages. The influence of basinal brines enriched in radiogenic strontium as a consequence of the reaction with Rb-bearing phases, especially detrital (clay) minerals hosted by underlying basement rocks, is considered to be most likely. Stable isotope investigations carried out on identical samples also show certain trends, though these are less pronounced compared to the strontium studies. In general, the later crystallization generations are depleted in the heavy isotope of carbon and oxygen, which is explained in terms of changing physico-chemical conditions of the system, in combination with changing fluid compositions during the course of diagenetic evolution. By combining petrographic and isotopic observations, the genetic hypothesis for the ore deposits at Berg Aukas and Odin can be significantly constrained. In both cases, a model is proposed based on basinal brine migration during advanced diagenetic stages. These brines were enriched in radiogenic strontium due to the interaction with basement rocks. Thus, the combination of petrographic and isotope-geochemical methods has proved to constitute a powerful tool in tracing the diagenetic evolution, and in an exploration sense may be used to identify imprints of fluid circulation, which mobilized and concentrated metals during various stages of basin evolution.