Lithic Weathering: Post-Depositional Effects of Paleoenvironments on Lithic Materials

    While many past authors have simply referred to traces of weathering on lithic artifacts as "patina", it has become clear that specific environmental and material conditions produce unique weathering effects. The directors of ACS conducted an extensive examination of lithic weathering patterns through various analytical laboratory procedures and experimentation.

Magnified cross-section of a lithic flake showing the effects of 'dissolution', including an outer repricipitation zone, the dissolution zone, and an interior or core zone reflecting the original qualities of the chert raw material.

    Methods of chemical and physical examination in this study included scanning electron microscopy (SEM), electron microprobe analysis, X-ray diffraction, and thermal gravimetric analysis. Experimentation on fresh chert consisted of controlling and accelerating several environmental conditions (e.g. temperature, moisture, and pH) in order to reproduce the observed effects. Finally, an analysis was performed which attempted to correlate type and extent of weathering on artifacts from Paleolithic sites of the Near East with independent assessments of antiquity and paleoenvironmental setting.

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Scanning electron microscope (SEM) image at 3000- times magnification shows 'lepispheres' or accumulations of loosely bound silica particles in the dissolution zone of a weathered chert artifact.

    The study concentrated on dissolution, an invasive process which affects silicate materials subject to semi-arid and alkaline conditions. Chert artifacts which are exposed to these conditions at the surface for an extended period of time display a consistent whitening, and often with a clear, glossy surface. Daily heating and cooling pumps available alkaline moisture through capillary structures of chert material, the interior walls of which are dissolved. The pumping action further causes the reprecipitation of the dissolved silica at the surface of these artifacts where the solution is subject to evaporation.

    The common dependency on surface conditions for dissolution to take place has foiled past attempts to accurately predict the antiquity or paleoenvironments of chert artifacts based on its presence or extent, although the process is a useful indicator of site formation processes such as stratigraphic deflation. This study also identified observable qualities which can be used to distinguish dissolution and reprecipitation from other principal weathering forms, including desert varnish, mineral staining, encrustation, leaching, abrasion, and frost-fracturing. Finally, the observed redistribution of loosely bound silica particles resulting from fluctuations in heat has also helped to reveal the elusive and long-debated mechanisms associated with the culturally induced heat treatment of lithic artifacts.

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