科研之友
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摘要
%26apos;Heuweltjies%26apos; occur throughout Namaqualand and the western and southern Cape coasts: broad, low-relief termitaria of the harvester termite Microhodotermes viator which show different vegetation patterns to the surrounding soils, distinguishable even in satellite images. There is a direct relationship between soil hardpan occurrence and the heuweltjies. Characteristically in the semi-arid areas the hardpans grade outwards from a central sepiolitic petrocalcic horizon laterally through %26apos;(petro) sepiolitic/palygorskitic%26apos; (variable in the degree of cementation) to the petroduric horizon on the edges, in a landscape in which these hardpans are otherwise absent. The aim of the study was to investigate the genesis of these hardpans associated with the heuweltjie mound, with a particular focus on the build-up of calcite and sepiolite. The micromorphological study examined a transect through a representative sepiolite-bearing heuweltjie on the coast near the Olifants River mouth, and revealed evidence of termite activity in the central (petro)calcic part of the mound. This concentration of calcite and Mg-rich clay in the centre can be explained by termite foraging, as the regionally characteristic Ca- and Mg-rich foliage is moved into the centre of the mound, facilitating the precipitation of calcite and sepiolite as bacterial decomposition and subsequent leaching modify the soil solution in the mound. The increase in coarse/fine ratio from the centre outward showed that accumulation of calcite and sepiolite displaced the original sand matrix by a greater degree than the silica accumulation, consistent with the topographically raised surface in the centre. Limpid yellow, low birefringent nodules, some with pseudo-negative uniaxial interference figures, showed a fibrous nature under ESEM, and their low Ca, and molar Mg/Si ratios of 0.64 to 0.68 (ESEM-EDX) were consistent with sepiolite. The presence of the sepiolite, with its Mg-rich composition and hydrophilic character, together with organic acids generated from the vegetation collected by the termites, is considered to be an explanation for the formation of %26apos;ooids%26apos;, radial calcite crystals associated with a core of sepiolite. Colourless, non-calcareous, pseudo-uniaxial negative spherulites were present in fresh termite excrement from an active heuweltjie near Stellenbosch. These were either produced in the termite gut itself or had been ingested along with herbivore dung by the termites. The presence of faecal spherulites in termite excrement is significant since it shows that faecal spherulites can be distributed over a wider area than that directly associated with mammalian herbivores.
- 出版日期2013-1
