The micro-stratigraphy of weathering effects upon long-exposed Palaeolithic chert artifacts from central Arabia.

Examination by lOX optical- and Scanning Electron- microscopy of 91 chert tools found on surface sites in central Arabia disclosed a suite of weathering characteristics such as subsurface fabric changes (Weathering Fronts), loss of Silica (Corrosion Pitting), deposition on- and penetration of- fabric by externally sourced materials (Manganese) (Fe- Rock Varnish), and the effects of various edaphic, aeolian and biotic elements, which appeared to have an order to their occurrence. The tools were typo 1ogically classified as Mousterian and Rub' al Khali "Neolithic", and are common to the area. The various effects appeared in regular stratigraphic order on similar varieties of chert (mainly brown and white) from sites over an 80 kilometre distance, and suggested the tools had experienced a common sequence of changes probably caused by weathering. The "Neolithic" tools displayed characteristics only from the upper end of the matrix. Some effects were specific to particular types of stone. The flaking episodes were interspaced at various points within the combined sequence derived from all the tools. This allowed flaking episodes to be sorted into relative order which assigned the "Neolithic" pieces to two upper- and the Mousterian to four earlier- groups or sets. A speculative model, using the current understanding of the local palaeoc1imatic sequence, suggests the Mousterian may not only have remained late in Arabia but may have arrived late as well. It predicts some of the forms of weathering to be expected on Acheulean tools, and suggests at least two humid periods earlier than the Late Pleistocene Humid Period (ca 36,000 - 18,000 bp). Application of the system allows hitherto undateable surface finds and isolates to be assigned relative positions within a local stratigraphy, and may allow determination of earlier palaeoc1imatic sequences.