ProjectA1

Revision as of 17:27, 10 June 2009

Project A1:
Tectonic geomorphology and climatic influence on Plio-Pleistocene hominin environments in southern Africa

French pi: G. King
South African pi: J. Lee-Thorpe

Project Participants:

• Prof. Geoffrey King Tectonique Laboratory, Institut de Physique du Globe de Paris, France
• Prof. Julia Lee-Thorp Division of Archaeological, Geographical and Environmental Sciences, University of Bradford, UK, and Africa Earth Observation Network, South Africa.
• Prof Frederick Fluteau Paleomagnetique Laboratory, Institut de Physique du Globe de Paris.
• Dr Sally Reynolds Anatomical Sciences, University of Witwatersrand, South Africa & Tectonique Laboratory, Institut de Physique du Globe de Paris, France
• Dr Woody Cotterill, Department of Geological Sciences and Department of Molecular and Cell Biology, University of Cape Town, South Africa.
• Dr Laurie Barrier Tectonique Laboratory, Institut de Physique du Globe de Paris, France

Background

A central research area in the study of human evolution is assessing the role and extent that climatic shifts and habitat changes have played in the emergence of our own species, Homo sapiens. South Africa has highly fossiliferous deposits from this important temporal period, starting with the appearance of Australopithecus africanus at sites such as Taung, Makapansgat and Sterkfontein at around 3Ma . From these and other sites, it is possible to build up a semi-sequential, and highly informative record of the faunal, floral and hominin changes, up until the earliest members of our own species appear at sites such as Hofmeyr (Grine, Bailey et al. 2007) and evidence of behavioural modernity at Blombos Cave.

Although the sites have been subject to much excavation and site-specific analysis, broader interpretations of regional environmental conditions are often simply extrapolatedfrom site-specific datasets, based on the standard assumption that these individual sites are representative of larger geographic areas. This appears defensible for large-scale patterns, such as climatic cooling and aridification of Africa (e.g. deMenocal 2004), which precipitated the spread and intensification of C4 grasses across certain regions of southern Africa, but other patterns influence smaller regions in specific, and important ways. One such influence is tectonic activity, which can alter drainage, sedimentation, topography and vegetation on much smaller scales. Furthermore, tectonic activity causes uplift and subsidence, resulting in localised regions of steep, uplifted topography (kloofs, steep valleys, river gorges) and also areas of subsidence (fertile plains, swamps and lakes). This results in the creation and maintenance of topographically influenced „mosaic‟ environments, which King and Bailey (2006) have argued to be important, preferred landscapes for early and later hominins. Figure 1 shows schematically how active, normal faulting creates these landscape features.

In this project, we propose to bring together several multi-disciplinary techniques to explore 1) constraints on hominin landscape use and range sizes, derived from a combination of geomorphological reconstructions versus strontium isotope sourcing information, based on a study already underway in the Sterkfontein Valley and 2) to characterise the neotectonic control on landscape features at long-occupation sites in particular Sterkfontein and Makapansgat, and extending this to other important hominin sites in southern Africa which show clear signs of tectonic changes to the geomorphology, such as Taung, Florisbad and Boomplaas Cave.

Makapansgat Valley sites – an illustration of the approach to be taken at Sterkfontein, Taung, Florisbad, Boomplaas Cave

The methodology can be illustrated for the Makapan Valley region, where a series of caves in a single valley have been exploited by hominins from at least 3 Ma to the present day. These sediments contain pre-stone tool-using Australopithecus africanus, while later deposits boast a complete cultural sequence (Early, Middle, Later Stone Ages as well as Iron Age tool industries), preserved with archaic Homo heidlbergensis and our own species Homo sapiens. Later, the steep, rugged terrain around the Historic Cave (in the same valley) was exploited by the Ndebele tribespeople during their uprising against the Boers in 1845, because the protection it afforded. In the same manner, earlier hominins may have found relative safety from large, cursorial predators. This feature, combined with sufficient food andsources of water, could explain why this site was repeatedly used by hominins over millions of years.

Figure 2 provides a simplified interpretation of the region around the site to illustrate the tectonic features to be examined. Figure 2a shows a Landsat Image draped over SRTM3 digital topography. Figure 2b interprets some important features of the image. Two faults (white) can be identified that have a controlling influence on the regional morphology and hydrology. In the background subsidence has occurs at the edge of the large plain. This causes the river to form braided channels and the river sediments are now exploited for farming. In the middle distance a medium-sized plain results from a normal fault that down-drops the valley and uplifts and back-tilts the ridge. Continued activity on the fault causes the river to hug the escarpment. Approaching the site, a second fault showing slope steepening at the interface between the hills and the plain (evidence of ongoing tectonic activity) creates some smaller backfilled valleys (Figure 2b) that can be see in more detail in Figure 2c. For the immediate site region this produces features commonly observed close to other sites of long-term occupation (King and Bailey 2006). Figure 2d shows the flat depositional plain that results from refilling an earlier valley with sediment and Figure 2e viewed towards the hills from the site shows the river down-cutting that produced the severe topography that prevented Boers from storming the Ndebele stronghold.