Palynology of late quaternary deposits from the central plateau, South Africa

[摘要] English: Pollen analyses of organic sediments from Clarens, in the eastern Free State,Florisbad, in the central Free State and Blydefontein, in the Karoo were carried out toinvestigate the mechanism of vegetation and climatic changes in the South Africaninterior during the Holocene.The methods of sampling, chemical processing and the construction of pollendiagrams and their zonation are described. In order to assist with the interpretation of pastenvironmental changes, the relationship between physiography, the present climate andvegetation, is addressed. The nature of the different sites, their deposits and theradiocarbon dates obtained, are discussed. Furthemore, each sequence is described andinterpreted in terms of past vegetation and climate changes.According to these reconstructions, the earliest phase recorded for the study areais for Florisbad, around 6500 to 6000 yr BP. This period was relatively dry and warmwith high evaporation rates, as indicated by the presence of the local halophyticvegetation. The regional vegetation consisted of some grasses and Asteraceae shrubs andherbs. The period between 6000 and 5500 yr BP is not documented in any of thesequences, but it seems the drier phase continued throughout this time. Between 5400 and5000 yr BP, conditions were sub-humid with dry episodes in Blydefontein, whileFlorisbad experienced dry conditions with reduced summer rains.The next phase between 4800 and 4000 yr BP is characterized by a completechange in the environmental conditions that include increased precipitation and a coolerclimate in the interior. This is shown by the sequences from Blydefontein and Florisbad.The vegetation had a good grass cover with indications of cooling characteristic of theNeoglacial' phase which occurred worldwide some time after 5000 yr BP. Seemingly,harsh conditions prevailed in the interior after ca. 4000 yr BP, as poor pollen preservationin sequences from Blydefontein and Florisbad indicate. The Blydefontein sediments showa very high percentage of trilete spores around 3950 yr BP, a possible sign of selectivepreservation, just below the barren part of the sequence. Although the Clarens sequence(Elim) dates havesome discrepancies, it also indicates a high percentage of trilete spores,especially for the period between 3000 and 4000 yr BP. The vegetation at Elim duringthis phase was open as a result of cool, dry conditions. The high percentage of triletespores in Blydefontein and in Elim indicates poor pollen preservation that might becaused by the dessication of the sediments. All of the evidence for the period 3000-4000yr BP points to dry conditions.The Florisbad sequence indicates the return of sub-humid conditions from ca.3140 yr BP, while the Blydefontein sediments of this stage are still sterile. A decline ingrasses accompanied by a rise in Chenopodiaceae/ Amaranthaceae pollen is seen inFlorisbad ca. 2000 yr BP, possibly indicating a drier phase with increased evaporation.This situation was reversed around 1710 yr BP. Grassy vegetation appears atBlydefontein at ca. 1980 yr BP, suggesting a change to sub-humid conditions.The environmental changes since around 6500 yr BP to 1710 yr BP from Elim,Florisbad and Blydefontein agree with most of the available indications from the interiorand with simulation models. Conditions in the interior only became sub-humid laterduring the Holocene altithermal, ca. 6500 yr BP, although the temperature was alreadyhigh at this time. The period between 4000-2000 yr BP requires further research as mostof the available sediments either have a hiatus or show an over-representation of triletespores. This phase has been interpreted as indicative of harsh conditions for thepreservation of pollen.

[发布机构] University of the Free State