Louis Scott

Grassland development under glacial and interglacial conditions in southern Africa: review of pollen, phytolith and isotope evidence

Abstract Pollen evidence suggests that grasslands were well established in southern Africa by the Late Tertiary. Evidence for grassland composition in the region during the Quaternary includes published accounts of isotopes, grass phytoliths and pollen of both grasses and woody plants from a wide range of different environments. Isotope data were derived from speleothems (stalagmites), fossil bones, and fossil tooth enamel and plant material in fossil hyrax dung. The different data types suggest that, with perhaps the exception of the dry southern Kalahari region, temperate grassland consisted of a relatively increased C3- to C4-grass ratios during the Last Glacial Maximum (LGM). Cold winter temperature extremes in the southern high latitude and altitude regions and a persistent winter rainfall pattern over the Cape region during the LGM probably limited the distribution of C4 grasses and canceled out any advantages gained from lowered CO2 concentrations in the atmosphere. In contrast, in the tropics where marked seasonal temperature fluctuations were lacking, C4-grass growth was favored.

A record of rapid Holocene climate change preserved in hyrax middens from southwestern Africa

The discovery of sensitive paleoenvironmental proxies contained within fossilized rock hyrax middens from the margin of the central Namib Desert, Africa, is providing unprecedented insight into the region’s environmental history. High-resolution stable carbon and nitrogen isotope records spanning 0–11,700 cal (calibrated) yr B.P. indicate phases of relatively humid conditions from 8700–7500, 6900–6700, 5600–4900, and 4200–3500 cal yr B.P., with a period of marked aridity occurring from 3500 until ca. 300 cal yr B.P. Transitions between these phases appear to have occurred very rapidly, often within <200 years. Of particular importance are: (1) the observed relationship between regional aridifi cation and the decline in Northern Hemisphere insolation across the Holocene, and (2) the signifi cance of suborbital scale variations in climate that covary strongly with fl uctuations in solar forcing. Together, these elements call for a fundamental reexamination of the role of orbital forcing on tropical African systems, and a reconsideration of what factors drive climate change in the region. The quality and resolution of these data far surpass any other evidence available from the region, and the continued development of this unique archive promises to revolutionize paleoenvironmental studies in southern Africa.

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.

Late glacial interhemispheric climate dynamics revealed in South African hyrax middens

Our ability to identify the timing and extent of past major climate fluctuations is central to understanding changes in the global climate system. Of the events that have occurred in recent geological time, the Younger Dryas (YD, 13–11.5 ka), an abrupt return to near-glacial conditions during the last glacial–interglacial transition (ca. 18–11.5 ka), is one of the most widely reported. While this event is apparent throughout the Northern Hemisphere ([Peteet, 1995][1]), evidence for its occurrence in the Southern Hemisphere remains equivocal due to a lack of well-dated terrestrial records. Here we report high-resolution stable carbon and nitrogen isotope records obtained from a rock hyrax midden, revealing the first unequivocal terrestrial manifestation of the YD from the southern African subtropics. These results provide key evidence for the relative influence of the YD, and suggest that a subtropical-temperate transition zone existed along the oceanic Subtropical Front (∼41°S) across the Southern Hemisphere, with the Northern Hemisphere exerting a strong influence on all but the higher latitudes of the Southern Hemisphere after the Heinrich Stadial 1 (15 ka). [1]: #ref-22

Lower timberline in central Colorado during the past 15,000 yr

A 15,000 yr paleoenvironmental record at the elevation of lower timberline in central Colorado suggests two major paleoclimatic changes during that interval: (1) at 10,000 yr B.P., from cool-moist conditions to warm-moist conditions and (2) at 4,000 yr B.P., to warm-dry conditions. This sequence can be interpreted as a shift from precipitation predominantly originating from Pacific winter storms (before 10,000 yr B.P.) to a predominantly monsoonal-type regime (lasting until 4,000 yr B.P.) to the present pattern. Contemporaneity of higher upper timberline and lower lower timberline during the 10,000 to 4,000 yr B.P. interval is not contradictory but a result of changes in those circulation patterns.

Pollen analysis of Iron Age cow dung in southern Africa

Thick accumulations of consolidated cow dung occur in ancientkraals (byres or corrals) in the bushveld and highveld areas of Zimbabwe, Botswana, and South Africa dating from the last 2000 years. They originated from long-term cattle herding by Iron Age people. The “vitrified” or baked dung deposits are thought to be a product of the burning of cow dung as fuel, either for domestic purposes or for iron smelting. In order to establish the palaeoecological potential of this material, 36 samples of cow dung from archaeological sites within the present-day savanna and grassland biomes were analyzed for pollen and other microfossils. Of the samples, 29 contained pollen together with other microfossils that support a faecal origin of the material such as sordariaceous ascospores,Thecaphora, Gelasinospora, andChaetomium, and eggs of the intestinal parasiteTrichuris. Similar microfossils were also found in recent fresh cow dung from the same study areas. The presence of pollen grains and spores in most of the Iron Age samples lead to the assumption that they survived the burning because fire temperatures were not high enough to destroy them. Pollen in these cow dung pieces is apparently sealed and can be preserved under open-air conditions at sites under which pollen in other deposits like soils, will decay away. Good pollen preservation and palynomorph diversity were found with mainly Poaceae, and secondly Chenopodiaceae and Cyperaceae as the most important pollen types, while trees and shrubs indicating savanna are rare. In the case of the samples that came from the subtropical savanna biome the latter result is unexpected and suggests that the cattle were kept in more open vegetation than the woody environments of today. Recent cow dung samples reflect the composition of present-day vegetation by showing considerably higher proportions of tree pollen than the fossil assemblages.

Middle-to late-Holocene moisture changes in the desert of northwest Namibia derived from fossil hyrax dung pollen

New pollen results and radiocarbon dating from fossil hyrax middens derived from the edge of the northern Namib Desert address the shortage of continental palaeobotanical evidence in arid Namibia by providing evidence for the environmental conditions during the mid to late Holocene in the region. The results obtained reflect long-term stability in the area throughout most of the sequence. Higher than modern moisture availability is suggested between c. 6 and 1 ka BP by the increased abundance of Poaceae, Cyperaceae or Chenopodiaceae, which respond rapidly by flowering after modest quantities of rainfall. Around 1 ka and recently arid conditions seem to have prevailed, with a decrease in Poaceae and an increase in Acanthaceae. Other palaeoecological evidence from the local and regional surroundings that focused on fluvial deposits, marine palynology and mineralogy confirms this mid-Holocene increase in more effective rainfall.

Siliceous microfossils as late-Quaternary paleo-environmental indicators at Braamhoek wetland, South Africa

A peat-sequence covering the last 16 ka (16 000 cal. yr BP) from Braamhoek wetland, eastern South Africa, was analysed in terms of phytolith and diatom composition. The fossil peat was rich in phytoliths, while diatoms were less prominent, probably as a result of degradation during wetland plant growth associated with silica uptake. With this study we present the first continuous phytolith and diatom record from South Africa covering the Late Pleistocene and Holocene period. The phytolith assemblages indicate a clear dominance of C3-grasses within the wetland throughout the sequence. The fossil diatom record infer changes in past moisture conditions. Unlike the modern wetland, which is dominated by benthic and aerophilic diatoms, the Late Pleistocene—early Holocene wetland favoured growth of planktonic species. Abundance of planktonic diatoms suggests three main phases when water depth was deeper than today; at c.13.6 ka, 11.3 ka and 10.4—10.0 ka. These indications of past fluctuations in humidity mostly provide confirmation of previously published indications of pollen, charcoal fragments and isotopes in the same core, but the siliceous microfossil data also help to refine the paleo-environmental interpretation of the sequence.

Between Agulhas and Benguela: responses of Southern African climates of the Late Pleistocene to Current Fluxes, Orbital Precession and the Extent of the Circum-Antarctic Vortex

Terrestrial records for the Middle Pleistocene of Southern Africa are sparse and mostly fragmentary because the landscape was largely devoid of suitable depositories. Fortunately, one continuous lacustrine sequence, that of the Tswaing impact crater near Pretoria, South Africa (Fig. 1), spans the period from ∼200 kyr to the present. Recent analyses of marine cores off the west coast of Southern Africa have, in addition, vouchsafed important proxy evidence of palaeo-wind regimes, periods of aridity and terrestrial flora. A generalised, but largely consistent, picture of palaeoenvironmental fluctuations during Isotope Stage 6 can therefore be drawn.

Phytoliths and Pollen, the Microscopic Plant Remains in Pliocene Volcanic Sediments Around Laetoli, Tanzania

We analyzed sediment samples collected from several localities at different stratigraphic levels at Laetoli (i.e., Lower Laetolil Beds [LLB], Upper Laetolil Beds [ULB] and the overlying Upper Ndolanya Beds [UNB]) to establish a record of vegetation succession spanning intermittent periods between 4.3 and 2.66 Ma during the Pliocene. No reliable pollen spectra were found, but phytoliths, especially those of grasses (Poaceae), were investigated. A considerable time interval of deposition for the sequence, combined with a relatively low sample yield, allowed us to present only a low-resolution sequence of environmental changes, but one with marked grass cover variation. Grass was a ubiquitous, but never a dominant vegetation component in the LLB, ULB and the UNB sequences, with a general succession from mainly C3 grass types in the LLB and older ULB levels to more C4 grass types in the younger ULB and UNB. The record lends support to fossil herbivore analyses and δ13C isotope studies, which suggest more heterogeneous habitats and a combination of C3/C4 grassland conditions in the ULB and UNB sequences (Andrews 1989; Kingston and Harrison 2007; Kovarovic and Andrews 2007). Productive samples suggest wet, C3 conditions in the LLB and potentially dry, C3 conditions in the lower part of the ULB. A shift from drier to more mesic C4 grass conditions is recorded in the upper part of the ULB. Arid C4 grassland environments occurred during UNB deposition.