David J. Nash

World atlas of desertification.

This text aims to summarize the state of scientific knowledge on the drylands of the globe. It explores the current stage of understanding of desertification as well as its extent and possible solutions. The book argues that desertification is one of the worlds most pressing environmental problems and that it is a global issue which is accelerating. This edition has been revised and expanded to include updated computer images of desertification, as well as fuller descriptions and explanations of the issues concerned. Using detailed data of the physical and chemical status of soil degradation provided by the Global Assessment of Soil Degradation, the book is fully referenced and covers topics including: desertification and global warming; monitoring on the ground and by remote sensing; vegetation and degradation; local action; cultural factors; financial issues; land reclaimations; the political economy and desertification; and desertification and refugees.

Geochemical sediments and landscapes

1. Introduction: Geochemical Sediments in Landscapes (David J. Nash and Sue J. McLaren). 2. Calcrete (V. Paul Wright). 3. Laterite and ferricrete (Mike Widdowson). 4. Silcrete (David J. Nash and J. Stewart Ullyott). 5. Aeolianite (Sue J. McLaren). 6. Tufa and travertine (Heather A. Viles and Allan Pentecost). 7. Speleothems (Ian J. Fairchild, Anna Tooth, Andrea Borsato and Silvia Frisia). 8. Rock varnish (Ronald I. Dorn). 9. Lacustrine and palustrine geochemical sediments (Eric P. Verrecchia). 10. Terrestrial evaporites (Allan R. Chivas). 11. Beachrock and intertidal precipitates (Eberhard Gischler). 12. Nitrate deposits and surface efflorescences (Andrew S. Goudie and Elaine Heslop). 13. Analytical techniques for investigating terrestrial geochemical sediments (John McAlister & Bernie J. Smith). 14. Geochemical sediments and landscapes: general summary (Sue J. McLaren and David J. Nash).

Late Pleistocene wetting and drying in the NW Kalahari: an integrated study from the Tsodilo Hills, Botswana

Abstract The sediments and landforms at the Tsodilo Hills, in the northwestern Kalahari desert, provide an opportunity to directly investigate the late Quaternary wetting and drying of the region from evidence at a single site. Lacustrine carbonates, including incorporated molluscs and diatoms, a lake shoreline feature and stabalised linear dunes were investigated for their constituent palaeoenvironmental signals. Chronometric control is provided by calibrated 14 C , AMS and OSL dating. The evidence suggests that linear dune construction has not occurred since the Last Glacial Maximum, with particular development from 36 to 28 ka . Lake stands indicating wetter regional conditions than present occurred at 40– 32 ka , with more seasonal conditions from 36 ka , and at 27– 12 ka with a possible drying out at 22– 19 ka . Data are consistent with other independent studies from the region, and with recent evidence obtained from Atlantic cores off the coast of Namibia. It is concluded that careful consideration of multi-proxy data from a single location can assist in resolving discrepancies that arise from independent studies of lake, cave and dune records in the Kalahari.

Kalahari valley calcretes: their nature, origins, and environmental significance

Abstract Calcretes that form in non-pedogenic settings have been widely reported in the geomorphological and geological literature, yet they are still poorly understood in comparison with pedogenic varieties. This may be because there are assorted types of non-pedogenic calcretes (often loosely referred to as groundwater calcretes, but encompassing groundwater, phreatic, open valley or confined channel calcrete types) forming within vadose and phreatic environments in different geomorphological settings. Relatively few studies have described the detailed petrological characteristics of such calcretes, leading Wright and Tucker (Calcretes; International Association of Sedimentologists Reprint Series, (1991) Vol. 2, Blackwell Scientific Publications, Oxford, p. 10) to suggest that “much more work is needed to define the ‘groundwater calcrete facies’ and to devise criteria for its recognition” so that the various non-pedogenic calcretes can be identified within the Quaternary as well as further back in the geological record. Most descriptions of groundwater calcretes are highly generalistic and do not take into account the variability that may occur as a result of calcretes forming in different positions within a landscape and affected by separate controls. Thus, the range of groundwater calcretes that exists now needs to be studied individually to identify patterns within the distinct types. As a step towards expanding our knowledge of the variety of non-pedogenic calcretes, this paper describes the petrology, micromorphology, and mode of development of one of these types of calcrete, namely that which forms within valley locations (but not in definite fluvial channels). Late Quaternary to Holocene valley calcretes are described from various dry valleys ( mekgacha ) across the Kalahari region of central Botswana, with the majority of samples collected from trunk and tributary valleys of the Okwa. Samples have been analysed in thin section and under scanning electron microscope in order to determine the carbonate matrix type and calcrete micromorphology. In general, most samples consist of grains of quartzose Kalahari sand cemented by fine crystalline, often glaebular, grain-coating and pore-filling micrite. Cement types are fairly consistent and not as micromorphologically variable as has been noted for other Kalahari calcretes. Biological inputs are prevalent in many samples and include networks of calcified rootlets. These characteristics reflect the fact that the calcretes formed in a relatively near-surface environment with relatively high rates of evaporation.

Silica and carbonate relationships in silcrete-calcrete intergrade duricrusts from the Kalahari of Botswana and Namibia

Abstract Silcrete-calcrete intergrade duricrusts (surface or near-surface chemically precipitated crusts with a cement comprising a mixture of silica and CaCO 3 ) have been widely identified in the geological, geomorphological and pedological literature, but have not, to date, been systematically described or classified. This paper presents a review of previous definitions of the end members of the silcrete-calcrete spectrum and subsequently identifies the major silica-carbonate relationships within intergrade types are identified on the the Kalahari of Botswana and Namibia. Three main intergrade types are identified on the basis of silica-carbonate associations: duricrusts where secondary silica occurs within a calcareous matrix; varieties where secondary carbonate occurs within a siliceous matrix; and materials where silica and carbonate matrix cements appear to have been precipitated contemporaneously or in close succession. Within each of these three groups, sub-types are identified dependent upon whether secondary materials have replaced or been emplaced within a pre-existing duricrust. Finally, a practical procedure for the simple definition of silcrete-calcrete intergrade duricrusts is suggested based upon a combination of bulk chemical and thin-section analyses.

Multiple calcrete profiles in the Tabernas Basin, southeast Spain: their origins and geomorphic implications

The paper describes a sequence of Pliocene(? ) to Quaternary age calcretes developed within alluvial fan and fluvial gravels in the Tabernas Basin, Almeria Province, southeast Spain. Calcrete profiles are described from sites adjacent to major tributaries of the Rambla de Tabernas. Six distinct calcrete units are identified within the basin. These have variable distributions but have developed in an identifiable evolutionary sequence. Two pairs of calcrete units are widely present across the basin preserving two former land surfaces. Each of the former land surfaces has been planated and subsequently buried by alluvial fan or fluvial gravels. A massive calcrete unit is present at the base of each gravel sequence, immediately in contact with the underlying bedrock, with a less well developed calcrete unit situated at the top of the gravel sequence. The lowest two calcrete units within the basin are more spatially restricted and are confined to the floors and flanks of incised drainage lines. The geochemistry, macro- and micromorphological properties and geomorphological positions of the calcrete units are outlined and, on the basis of this information, their mode of origin identified. Two main modes of calcrete genesis appear to be present: massive calcretes situated in direct contact with bedrock are suggested to have formed by groundwater processes, whilst calcretes situated at the top of gravel sequences are likely to have developed by pedogenic processes. Calcrete genesis is subsequently considered in the context of the reconstruction of the early phases of landscape development, and is suggested to have been controlled by phases of uplift and stability within the Tabernas Basin.

Temperature variability over Africa during the last 2000 years

A growing number of proxy, historical and instrumental data sets are now available from continental Africa through which past variations in temperature can be assessed. This paper, co-authored by members of the PAGES Africa2k Working Group, synthesises published material to produce a record of temperature variability for Africa as a whole spanning the last 2000 years. The paper focuses on temperature variability during the ‘Medieval Climate Anomaly’ (MCA), ‘Little Ice Age’ (LIA) and late 19th–early 21st centuries. Warmer conditions during the MCA are evident in records from Lake Tanganyika in central Africa, the Ethiopian Highlands in northeastern Africa, and Cango Cave, the Kuiseb River and Wonderkrater in southern Africa. Other records covering the MCA give ambiguous signals. Warming appears to have been greater during the early MCA (c. ad 1000) in parts of southern Africa and during the later MCA (from ad 1100) in Namibia, Ethiopia and at Lake Tanganyika. LIA cooling is evident in Ethiopian and southern African pollen records and in organic biomarker data from Lake Malawi in southeastern tropical Africa, while at Lake Tanganyika the temperature depression appears to have been less consistent. A warming trend in mean annual temperatures is clearly evident from historical and instrumental data covering the late 19th to early 21st centuries. General warming has occurred over Africa since the 1880s punctuated only by a period of cooling in the mid 20th century. The rate of temperature increase appears to have accelerated towards the end of the 20th century. The few long high-resolution proxy records that extend into the late 20th century indicate that average annual temperatures were 1–2°C higher in the last few decades than during the MCA.

Siliceous duricrusts as palaeoclimatic indicators: evidence from the Kalahari desert of Botswana

Abstract Previous attempts to identify the environmental conditions associated with the development of siliceous duricrusts have used differences in bulk chemical compositions and cement mineralogy amongst other criteria to distinguish between samples. High (up to 3.00%) levels of TiO2, together with petrographic characteristics, have been used as an indicator of past wetter climatic conditions, often failing to fully emphasise the potential importance of local controls upon micromorphology and chemistry. In southern Africa, silcretes from the Cape Coastal zone associated with deep-weathering profiles have been attributed to formation under wetter conditions whereas Kalahari silcretes are suggested to have formed under more arid, alkaline conditions. Petrographic and geochemical analyses of silcretes from five locations within the Kalahari Desert of Botswana (one site associated with deeply-weathered bedrock) indicate the great micromorphological complexity of Kalahari silcretes. It is suggested that variations in the host material composition of Kalahari silcretes are an important control of their chemistry. Silcretes have developed from unconsolidated Kalahari Sand with variations in the mineral content of the host sediment appearing to cause variation in silcrete geochemistry. Geochemical analysis confirms that TiO2 provides the main variation between Kalahari and Cape Coastal silcretes but the low levels of TiO2 in Kalahari samples are attributed to differences in host material as opposed to overriding climatic controls.

Drought, desiccation and discourse: missionary correspondence and nineteenth-century climate change in central southern Africa

This paper examines the role that representatives of the London Missionary Society in central southern Africa during the nineteenth century may have played in the development of geographical debates concerning the long-term desiccation of the African continent. Observations on climate included within missionary documents are used to reconstruct a chronology of intra-decadal climatic variability for the period 1815-1900. This reveals six drought periods and seven wet phases that affected large areas of the region, but identifies no evidence for progressive desiccation. The chronology is then used as a framework within which to view missionary perspectives on drought and desiccation. Major influences upon the development of desiccationist theory appear to include the prevalence of contemporary moral economic explanations of climatic variability, as well as the uptake and acceptance of indigenous understanding of climate change. Significantly, many of the key observations by eminent missionaries used as supporting evidence for progressive desiccation are identified as having been made during periods of severe drought. This is used to suggest that the most widely propagated evidence for desiccation may, therefore, simply be the end-product of periods of short-term drought rather than long-term climatic deterioration.

Dual mechanisms for the formation of fluvial silcretes in the distal reaches of the Okavango Delta fan, Botswana

Silcretes exposed within river-marginal or valley settings have been described in a number of studies, but few models have been suggested for the development of these ‘fluvial’ silcretes. An exception is that proposed by McCarthy and Ellery (Journal of Sedimentary Research, 1995, Vol. A65, pp. 77–90) to describe mechanisms of early stage near-surface silica diagenesis in the Okavango Delta, Botswana. This paper describes the characteristics and possible origins of massive surface and sub-surface silcretes at Samedupe and Boro Junction, beyond the distal margin of the Okavango Delta and further downstream than the sites described by McCarthy and Ellery. Morphological and petrological evidence from surface exposures and three sedimentary cores suggests that other modes of formation may also be applicable. A dual model of formation is proposed: surface silcretes are suggested to have developed by silica accumulation in seasonal pools remaining after the annual Okavango flood, whilst sub-surface horizons appear to have formed under conditions of varying pH associated with fluctuating groundwater levels beneath the channel floor. This model is reviewed in the context of the fluvial silcrete debate.