J.C. Vogel

Late Quaternary paleotemperatures derived from a speleothem from Cango Caves, Cape Province, South Africa

Abstract An oxygen isotope temperature record over a large part of the past 30,000 yr has been obtained for the southern Cape Province of South Africa by combining data on the isotopic composition of a stalagmite from a deep cave with that of a confined groundwater aquifer in the same region. Results show that temperatures during the last glacial maximum were on average about 6°C lower than those today, with peaks up to 7°C lower. A detailed analysis of the past 5000 yr suggests multiple fluctuations, with generally lower temperatures (1–2°C) around 4500 and 3000 yr B.P. The carbon isotopic composition of the stalagmite indicates significant vegetation changes between the late Pleistocene and today, and also during the second half of the Holocene.

The Pretoria Saltpan: a 200,000 year Southern African lacustrine sequence

Abstract The Pretoria Saltpan is a circular crater 1130 m in diameter and is situated some 40 km N of Pretoria (lat. 25°34′30″/long. 28°04′59″E). A recent tube sampling and core drilling programme has revealed an infilling consisting of some 90 m of fine lacustrine sediments (chiefly organic muds, underlain below 30 m depth by micrites) which rest upon a further 61 m of coarse clastic debris. Granite bedrock was encountered at − 151 m. Broad sedimentary zones correspond with major phases in the evolution of the crater lake. Superimposed cyclical patterns of accumulation reflect environmental changes on millenial to seasonal timescales. 14C age determinations on algal debris from the upper 20 m of the core indicate a mean rate of sedimentation of about 1 m/2000 yr, suggesting that the lacustrine sequence may span almost 200,000 yr. Over this period major environmental changes are apparent from sedimentological, chemical, mineralogical and isotopic analyses of the core and studies of the pollen spectra and diatom assemblages present within it. This long continental sequence is therefore providing a high-resolution palaeoenvironmental record for southern mid-latitudes over much the same period as is covered by the Vostok ice-core in Antarctica.

Episodic late quaternary palaeogully erosion in northern KwaZulu-Natal, South Africa

Abstract Hillslope deposits in northern KwaZulu-Natal, South Africa are incised by gullies (dongas) which expose sequences of buried palaeosols developed within a succession of discontinuity-bounded colluvial sedimentary units. Infilled and buried palaeogully topography attests to at least four geomorphic cycles, comprising gully cut-and-fill and palaeosol formation, on hillslopes in the region during the past 135 ka. Past attempts at dating colluvial stratigraphic units were limited to radiocarbon dating of associated palaeosol organic matter and authigenic soil carbonates. This study presents a preliminary set of Infra Red Stimulated Luminescence (IRSL) age determinations pertaining to deposition of the succession of colluvial units infilling and burying palaeogullies at several sites of stratigraphic, geochronological and palaeoenvironmental importance in the region. Interpretation of the IRSL and 14 C dates in the context of cyclical hillslope processes suggests that past episodes of palaeogully erosion and infill were not synchronous and individual palaeosol landsurfaces were diachronous during the late Pleistocene.

Late Quaternary hillslope evolution recorded in eastern South African colluvial badlands

Abstract Infrared stimulated luminescence (IRSL) ages have been obtained from a sequence of sandy colluvial deposits in northern KwaZulu–Natal. Comparison of the IRSL ages on sand-sized, potassium-rich feldspars, with radiocarbon dates on A-horizon organic matter from buried palaeosols within the colluvial succession shows good agreement between the dating techniques. When compared with palaeoenvironmental records, the data suggest that the colluvium accumulated during arid stages of the Late Quaternary, whilst pedogenesis repeatedly occurred at intervals of hillslope stability which may reflect periods of greater humidity. Temperature does not seem to have been a forcing factor in the landscape development. Extensive colluviation took place during the arid Last Glacial Maximum, but the occurrence of several palaeosols indicates that it was a time of fluctuating climatic conditions.

A new explanation for extreme 234U238U disequilibria in a dolomitic aquifer

High 234U238U activity ratios are found in the shallow groundwater of the phreatic Transvaal Dolomite Aquifer. The aquifer is uranium poor, while the waters are oxygen rich and young. Tritium and 14C are used to show that the disequilibrium increases the longer the water resides in contact with the dolomite host. The 228Ra and 222Rn isotopes were used as supporting evidence that ion exchange between the 238U series nuclides in the water and the carbonate wall rock is necessary in assisting in the fractionation process. The Wolkberg cave speleothems preserve a record of the uranium isotopic fractionations that evolved as water flowed through the aquifer. Extremely variable and elevated 234U238U ratios (of 2–12) are characteristic. Individual caverns may exhibit very large 234U238U variability in their drip water and in derivative carbonate precipitates (speleothems). Deeper chambers, where water has spent a longer time in contact with the aquifer, tend to exhibit a greater degree of uranium isotopic fractionation in their speleothems. The recorded fractionation process has been an ongoing one from at least the middle-late Pleistocene to the present. The speleothem study supports the contention that ion exchange processes that produce sufficient enrichment of uranium on carbonate can provide ideal conditions for generating uranium isotopic fractionation in the water of carbonate aquifers (in the absence of redox changes and within a uranium-poor environment).

Evaluation of luminescence-dating procedures applied to late-Holocene colluvium near St Paul's Mission, Natal, South Africa

Colluvial deposits provide an interesting challenge for luminescence-dating techniques because of the short and varied light-exposure histories of the grains prior to deposition. Results of thermoluminescence (TL) and infrared stimulated luminescence (IRSL) measurements are presented for two stacked colluvial deposits from Natal. A radiocarbon date for bulk organic matter in the A horizon of the soil formed within the surface of the lower unit provides a means of assessing the suitability of various luminescence procedures for such material. The overlying colluvium contains grains bleached to varying extents, as shown by the range of EDs for the coarse-grain feldspar separate obtained using the single-aliquot IRSL method. For this sample all age estimates (either by IRSL or TL) are too large compared with the radiocarbon age, indicating that even the IRSL signal was not zeroed in all grains before they were buried. The underlying colluvium contains grains bleached more uniformly, resulting in a smaller range in the values of single-aliquot EDs; the agreement of the IRSL age with the radiocarbon age indicates that the IRSL signals of all grains were zeroed prior to deposition. The TL results for this sample suggest that the signals were not totally zeroed, but had experienced considerably more light at deposition than the grains from the overlying unit.

Twentieth century changes in montane vegetation in the eastern Free State, South Africa, derived from palynology of hyrax dung middens

The dating and pollen analysis of a hyrax dung deposit in a mountain rock shelter (Rooiberg Shelter II) are compared with that in a previous study from the same mountain range at the rural town Clarens, in South Africa. Calibration of radiocarbon measurements from the dung deposit provides different possibilities for the age of the sequence. Unlikely dates can be eliminated on the basis of pollen stratigraphy, comparisons with a previously studied accumu- lation from the last 30 yr, artificially increased radiocarbon levels in the upper samples as result of nuclear arms testing after 1954, the presence of historically introduced exotic elements, and the assumption of a relatively constant rate of dung accumulation. According to these consider- ations we suggest that the dung started accumulating at the beginning of the twentieth century. The pollen contents show marked changes in composition, indicating mainly open grass veg- etation with fynbos in the first half of this century and woody vegetation in the second half. A first marked increase of the woody component is estimated to have occurred around 1950, but it only became permanent in the 1960s. The fluctuating pollen sequence can best be interpreted in terms of the combined effects of rainfall changes, fire and stock grazing, the latter of which increased together with town expansion in the area during the course of this century. Considering historical events recorded in the area and the region in general, the results suggest that pollen in hyrax dung is a good recorder of vegetation change. Copyright