Can we use cosmogenic isotopes to date stone artifacts?.

Abstract

Two chert artifacts from the region near Luxor, Egypt have yielded concentrations of cosmogenic (super 10) Be that allow calculation of nominal exposure ages of 326,000 and 304,000 years. Both artifacts are flakes that were collected atop limestone benches of the Eocene Thebes Formation which form cliffs along the west side of the Nile. The site is at elevation 240 m and is about 15 km from the Nile. Tools associated with these artifacts can be attributed to the Late Acheulean or early Middle Paleolithic (the transition has been suggested to have been on the order of 250,000-300,000 years ago). This area, where abundant chert nodules have weathered out, has been a collection, extraction, and fabrication site since the Early Paleolithic (since at least 400,000 years ago). Surface exposure dating records all periods of exposure. That means these ages represent composite ages, comprised of exposures both before and after working. But what fraction of the (super 10) Be concentration we have measured was acquired before the flakes were produced? Here we propose several approaches to deconvolute the different exposure periods and better approximate the real age of the artifacts. As there is no a priori reason that the two ages should agree with the typological ages of the artifacts, nor for the two independent ages to agree, these first results are especially exciting and intriguing.