Nicholas Drake

Ancient watercourses and biogeography of the Sahara explain the peopling of the desert

Evidence increasingly suggests that sub-Saharan Africa is at the center of human evolution and understanding routes of dispersal “out of Africa” is thus becoming increasingly important. The Sahara Desert is considered by many to be an obstacle to these dispersals and a Nile corridor route has been proposed to cross it. Here we provide evidence that the Sahara was not an effective barrier and indicate how both animals and humans populated it during past humid phases. Analysis of the zoogeography of the Sahara shows that more animals crossed via this route than used the Nile corridor. Furthermore, many of these species are aquatic. This dispersal was possible because during the Holocene humid period the region contained a series of linked lakes, rivers, and inland deltas comprising a large interlinked waterway, channeling water and animals into and across the Sahara, thus facilitating these dispersals. This system was last active in the early Holocene when many species appear to have occupied the entire Sahara. However, species that require deep water did not reach northern regions because of weak hydrological connections. Human dispersals were influenced by this distribution; Nilo-Saharan speakers hunting aquatic fauna with barbed bone points occupied the southern Sahara, while people hunting Savannah fauna with the bow and arrow spread southward. The dating of lacustrine sediments show that the “green Sahara” also existed during the last interglacial (∼125 ka) and provided green corridors that could have formed dispersal routes at a likely time for the migration of modern humans out of Africa.

Monitoring desertification and land degradation over sub-Saharan Africa

A desertification monitoring system is developed that uses four indicators derived using continental-scale remotely sensed data: vegetation cover, rain use efficiency (RUE), surface run-off and soil erosion. These indicators were calculated on a dekadal time step for 1996. Vegetation cover was estimated using the Normalized Difference Vegetation Index (NDVI). The estimation of RUE also employed NDVI and, in addition, rainfall derived from Meteosat cold cloud duration data. Surface run-off was modelled using the Soil Conservation Service (SCS) model parametrized using the rainfall estimates, vegetation cover, land cover, and digital soil maps. Soil erosion, one of the most indicative parameters of the desertification process, was estimated using a model parametrized by overland flow, vegetation cover, the digital soil maps and a digital elevation model (DEM). The four indicators were then combined to highlight the areas with the greatest degradation susceptibility. The system has potential for near-real time monitoring and application of the methodology to the remote sensing data archives would allow both spatial and temporal trends in degradation to be determined.

Preferential dust sources: A geomorphological classification designed for use in global dust-cycle models

Received 11 April 2011; revised 28 September 2011; accepted 4 October 2011; published 24 December 2011. [1] We present a simple theoretical land-surface classification that can be used to determine the location and temporal behavior of preferential sources of terrestrial dust emissions. The classification also provides information about the likely nature of the sediments, their erodibility and the likelihood that they will generate emissions under given conditions. The scheme is based on the dual notions of geomorphic type and connectivity between geomorphic units. We demonstrate that the scheme can be used to map potential modern-day dust sources in the Chihuahuan Desert, the Lake Eyre Basin and the Taklamakan. Through comparison with observed dust emissions, we show that the scheme provides a reasonable prediction of areas of emission in the Chihuahuan Desert and in the Lake Eyre Basin. The classification is also applied to point source data from the Western Sahara to enable comparison of the relative importance of different land surfaces for dust emissions. We indicate how the scheme could be used to provide an improved characterization of preferential dust sources in global dust-cycle models.

Production of Landsat ETM+ reference imagery of burned areas within Southern African savannahs: comparison of methods and application to MODIS

Accurate production of regional burned area maps are necessary to reduce uncertainty in emission estimates from African savannah fires. Numerous methods have been developed that map burned and unburned surfaces. These methods are typically applied to coarse spatial resolution (1 km) data to produce regional estimates of the area burned, while higher spatial resolution (<30 m) data are used to assess their accuracy with little regard to the accuracy of the higher spatial resolution reference data. In this study we aimed to investigate whether Landsat Enhanced Thematic Mapper (ETM+)‐derived reference imagery can be more accurately produced using such spectrally informed methods. The efficacy of several spectral index methods to discriminate between burned and unburned surfaces over a series of spatial scales (ground, IKONOS, Landsat ETM+ and data from the MOderate Resolution Imaging Spectrometer, MODIS) were evaluated. The optimal Landsat ETM+ reference image of burned area was achieved using a charcoal fraction map derived by linear spectral unmixing (k = 1.00, a = 99.5%), where pixels were defined as burnt if the charcoal fraction per pixel exceeded 50%. Comparison of coincident Landsat ETM+ and IKONOS burned area maps of a neighbouring region in Mongu (Zambia) indicated that the charcoal fraction map method overestimated the area burned by 1.6%. This method was, however, unstable, with the optimal fixed threshold occurring at >65% at the MODIS scale, presumably because of the decrease in signal‐to‐noise ratio as compared to the Landsat scale. At the MODIS scale the Mid‐Infrared Bispectral Index (MIRBI) using a fixed threshold of >1.75 was determined to be the optimal regional burned area mapping index (slope = 0.99, r 2 = 0.95, SE = 61.40, y = Landsat burned area, x = MODIS burned area). Application of MIRBI to the entire MODIS temporal series measured the burned area as 10 267 km2 during the 2001 fire season. The char fraction map and the MIRBI methodologies, which both produced reasonable burned area maps within southern African savannah environments, should also be evaluated in woodland and forested environments.

Rethinking the dispersal of Homo sapiens out of Africa.

Current fossil, genetic, and archeological data indicate that Homo sapiens originated in Africa in the late Middle Pleistocene. By the end of the Late Pleistocene, our species was distributed across every continent except Antarctica, setting the foundations for the subsequent demographic and cultural changes of the Holocene. The intervening processes remain intensely debated and a key theme in hominin evolutionary studies. We review archeological, fossil, environmental, and genetic data to evaluate the current state of knowledge on the dispersal of Homo sapiens out of Africa. The emerging picture of the dispersal process suggests dynamic behavioral variability, complex interactions between populations, and an intricate genetic and cultural legacy. This evolutionary and historical complexity challenges simple narratives and suggests that hybrid models and the testing of explicit hypotheses are required to understand the expansion of Homo sapiens into Eurasia.

Mapping Vegetation, Soils, and Geology in Semiarid Shrublands Using Spectral Matching and Mixture Modeling of SWIR AVIRIS Imagery

Abstract Spectral matching and linear mixture modeling techniques have been applied to synthetic imagery and AVIRIS SWIR imagery of a semiarid rangeland in order to determine their effectiveness as mapping tools, the synergism between the two methods, and their advantages, and limitations for rangeland resource exploitation and management. Spectral matching of pure library spectra was found to be an effective method of locating and identifying endmembers for mixture modeling although some problems were found with the false identification of gypsum. Mixture modeling could accurately estimate proportions for a large number of materials in synthetic imagery; however, it produced high variance estimates and high error estimates when presented with all nine AVIRIS endmembers because of high noise levels in the imagery. The problem of which endmembers to select was addressed by implementing a mixture model that allowed estimation of the errors on the proportions estimates, discarding the endmembers with the highest errors, recomputing the errors, and the proportions estimates, and iterating this process until the mixture maps were relatively free from noise. This methodology ensured that the lowest contrast materials were discarded. The inevitable confusion that followed was monitored the using the maps produced by spectral matching. Spectral matching was more effective than mixture modeling for geological mapping because it allowed identification and mapping of the relatively pure regions of all the surficial materials that exert an influence on the spectral response. The maps of the different clay minerals were of considerable value for mineral exploration purposes. Conversely, spectral matching was less useful than mixture modeling for rangeland vegetation studies because a classification of all pixels is needed and abundance estimates are required for many applications. Mixture modeling allowed identification of both nonphotosynthetic and green vegetation cover and thus total cover. Though the green vegetation mixture map appears to be very precise, the nonphotosynthetic vegetation estimates were poor.

Hominin dispersal into the Nefud Desert and Middle palaeolithic settlement along the Jubbah Palaeolake, Northern Arabia.

The Arabian Peninsula is a key region for understanding hominin dispersals and the effect of climate change on prehistoric demography, although little information on these topics is presently available owing to the poor preservation of archaeological sites in this desert environment. Here, we describe the discovery of three stratified and buried archaeological sites in the Nefud Desert, which includes the oldest dated occupation for the region. The stone tool assemblages are identified as a Middle Palaeolithic industry that includes Levallois manufacturing methods and the production of tools on flakes. Hominin occupations correspond with humid periods, particularly Marine Isotope Stages 7 and 5 of the Late Pleistocene. The Middle Palaeolithic occupations were situated along the Jubbah palaeolake-shores, in a grassland setting with some trees. Populations procured different raw materials across the lake region to manufacture stone tools, using the implements to process plants and animals. To reach the Jubbah palaeolake, Middle Palaeolithic populations travelled into the ameliorated Nefud Desert interior, possibly gaining access from multiple directions, either using routes from the north and west (the Levant and the Sinai), the north (the Mesopotamian plains and the Euphrates basin), or the east (the Persian Gulf). The Jubbah stone tool assemblages have their own suite of technological characters, but have types reminiscent of both African Middle Stone Age and Levantine Middle Palaeolithic industries. Comparative inter-regional analysis of core technology indicates morphological similarities with the Levantine Tabun C assemblage, associated with human fossils controversially identified as either Neanderthals or Homo sapiens.

Remotely sensed dune celerity and sand flux measurements of the world's fastest barchans (Bodélé, Chad)

Quantifying sand flux with field measurements is an expensive and time-consuming process. We here present an alternative approach using the COSI-Corr software package for Earth surface deformation detection. Using pairs of ASTER satellite images, we detected dune migration in the Bodele depression of northern Chad over time intervals of one month to 6.5 years. The displacement map can be used to automatically distinguish dunes from interdunes, which is a crucial step towards calculating sand flux. We interpolated a surface between the interdune areas and subtracted it from a digital elevation model, thus obtaining dune heights and volumes. Multiplying height with celerity yields a pixel-by-pixel estimate of the sand flux. We applied this method to large diatomite dunes in the Bodele, confirming that these are some of the worlds fastest moving barchans. Plotting dune height against inverse celerity reveals sand flux at the dune crest of > 200 m(3)/m/yr. Average dune sand flux values for the eastern and western Bodele are 76 and 99 m3/m/yr, respectively. The contribution of the dunes to the total area-averaged sand flux is 24-29 m(3)/m/yr, which is similar to 10% of the saltation flux determined by previously published field measurements. Citation: Vermeesch, P., and N. Drake ( 2008), Remotely sensed dune celerity and sand flux measurements of the worlds fastest barchans ( Bodele, Chad), Geophys. Res. Lett., 35, L24404, doi: 10.1029/2008GL035921.

The climate-environment-society nexus in the Sahara from prehistoric times to the present day

The Sahara is a key region for studies of archaeology, human-environment interaction, global biogeochemical cycles, and global climate change. With a few notable exceptions, the region is the subject of very little international scientific research, a fact that is remarkable given the Saharas proximity to Europe, the developmental issues facing its growing population, the regions sensitivity to climate change and the Saharas potential for influencing global climate through the export of airborne mineral dust. This article seeks to address human-environment interaction in the Sahara from an interdisciplinary perspective, focusing on the implications of Saharan environmental variability and change for human populations both within and outside of the region on timescales ranging from decades to millennia. The article starts by addressing past climatic changes and their impacts on human populations, before moving on to consider present day water resources and rainfall variability in their longer-term context; the possibility of a ‘greening’ of the southern Sahara as suggested by some climate models is also discussed. The role of the Sahara as the worlds largest source of airborne mineral dust is addressed in some detail, as are the impacts of dust on climate, ecosystems and human health, as well as the implications of future changes in climate for dust production and the role of the Sahara in the Earth system. The article ends with a discussion and synthesis that explores the lessons that may be learnt from a study of the physical and social sciences in the Sahara, in particular focusing on what the signature of past environmental and socio-cultural changes can tell us about human responses and adaptations to climatic and environmental change – a matter of great relevance to researchers and policy makers alike in the context of anthropogenic climate change or ‘global warming’.

Reflectance spectra of evaporite minerals (400-2500 nm): applications for remote sensing

Abstract The spectral response of evaporite minerals is evaluated to determine those minerals that can be identified and mapped by remote sensing. The vast majority of evaporite minerals have diagnostic spectra due to the vibration of H2O, CO3, HCO3, NH4, NO3 bonds. Only the anhydrous Cl and SO4 salts do not contain any diagnostic features and cannot be distinguished from each other. Many of the hydrous salts exhibit unusual spectral behaviour. Large grain size samples exhibit numerous well developed absorption features at wavelengths 1500 nm. The spectral diversity exhibited by evaporite minerals suggests numerous applications in spectroscopy and remote sensing can be realized. These applications are investigated.