| 05 March 2009
RECONSTRUCTION OF PALAEOENVIRONMENT
Most East African sites have a characteristic mixture of lacustrine and fluvial deposits but are limited in extent; The Olduvai Basin covers an area of about 3,600 km2, and Hadar only 38 km2. The broad perspective of the Turkana Basin makes it possible to see approximately 20 km east west since the fossil-rich sediments are exposed from the present eastern and western lake shores to at least 20 km inland. As a result, the Turkana Basin presents the opportunity to investigate an exceptional breadth of geological and environmental diversity. For example it is possible to trace palaeoenvironments from the ancestral Omo river channel with its gallery forests and swamps, across thorn bush and grassy floodplains strongly influenced by seasonal flooding, to the basin margins where an arid local climate prevailed and the landscape and sedimentation regimes were totally different. With the geographic extent of the Turkana Basin, this spatial perspective can be extended far to the north, as well as to the west and to the east. Size is a crucial aspect of the Turkana Basin because so many of the questions that are of interest have a spatial component. The badlands type of exposures at Turkana, make it possible to trace long distances across the palaeolandscapes following river channels soils and fossil distributions, and to ask important questions such as: How were habitats distributed across the landscape? How heterogeneous was the "mosaic" of habitats on a floodplain? How were the faunal assemblages and the hominids related to the different habitats.
GEOLOGICAL RECORD AND DATING
The Turkana Basin is situated in the Rift Valley. Its history is thus related to the geological forces that shaped the Gregory Rift. Fossiliferous exposures to the north, east and to the west of the modern lake preserve a considerable depth of time, extending from the late Oligocene beds at Losodok, approximately 27 million years old to the Galana Boi Beds dated between 10,000 and 4,000 years old. Of particular relevance to human evolution studies are the sediments of the Omo group deposits which document the time interval between just over 4 and 0.7 million years ago.
The fossils or fluvio/lacustrine sediments in which they occur can not be dated but instead are placed relative to dateable volcanic ash horizons. Feldspar crystals extracted from pumice stones in volcanic ash layers can be dated very accurately from the radioactive decay of Argon isotopes. These dated horizons have unique chemical compositions and other of volcanic tephra can be identified this way. These volcanic tephra are thus instrumental in identifying the ages of the fossil exposures. Fossils above a known horizon are younger and below are older than than the dated levels.
FAUNAL RECORD
The sediments in the Turkana Basin are exceptional in their preservation of fossils so that the biotic record is unusually good. This record is also unusual in that it has a temporal as well as a lateral component. Not only is it possible to demonstrate evolutionary trends within lineages, and within faunal assemblages, but also to follow biotic changes across the landscape and relate these to changing palaeoenvironments and palaeohabitats. The interpretation of faunas at many other sites documenting shorter time intervals and having less complete faunas is dependant on the evidence from the Turkana Basin, which provides an unparalleled record of faunal change through time.
ARCHAEOLOGICAL RECORD
The Turkana Basin has an exceptional archaeological record that compliments the evidence of the fossil hominins. The archaeology of the eastern and western shores of the lake are currently under investigation by American and French teams respectively. The Koobi Fora sites largely occur in two stratigraphic intervals. The earlier sites, dated from 1.9 to 1.8 million years, preserve artefacts of the Oldowan Industry. The artefact assemblages from the later sites, between 1.6 to 1.4 million years, although in general similar to the Developed Oldowan Industry, include some distinctive tools such as the "Karari" scrapers and are therefore termed the Karari Industry. The latest of these sites show signs of more careful shaping of tools and other characters attributable to the Acheulian Industry. On the western shores, the numerous sites are both laterally and temporally distributed, and include some of the oldest archaeological sites known. They are providing insight into the relationship between tool making techniques and the availability of raw materials as well as the evolution of stone knapping techniques.
