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We pioneer a technique of surface-exposure dating based upon the characteristic form of an optically stimulated luminescence (OSL) bleaching profile beneath a rock surface, which evolves as a function of depth and time. As a field test of this new method, the maximum age of Barrier Canyon Style rock art in Canyonlands National Park, Utah is constrained. The natural OSL signal from quartz grains is measured from the surface to a depth of >10 mm in three samples of the Jurassic Navajo Sandstone. Two samples are from talus with unknown daylight exposure histories; one sample is now buried, and no longer light exposed. A third sample has a known exposure duration of 80 years. The OSL-depth profile of this sample is used to estimate material-dependent and environmental parameters, to then calibrate a model fit to corresponding data for the unknown samples, and so to estimate exposure ages. The shielded surface of one of the talus samples has rock art on it that fell from the Great Gallery panel ~800 years ago, as determined in related research. Our results indicate that the prior daylight exposure of this pigmented block while in place in the alcove wall was ~700 years in duration, constraining the episode of time when the rock art must have been created. These are the first credible estimates of exposure age based on luminescence bleaching profiles. The robustness of this novel OSL method is its ability to establish both ongoing and prior exposure times, at decadal to millennial timescales or longer, even for material subsequently buried. This has considerable potential in many archaeological, geological and geohazard applications.

Reconstructions of climatic and environmental conditions can contribute to current debates about the factors that influenced early human dispersal within and beyond Africa. Here we analyse a 200,000-year multi-proxy paleoclimate record from Chew Bahir, a tectonic lake basin in the southern Ethiopian rift. Our record reveals two modes of climate change, both associated temporally and regionally with a specific type of human behavior. The first is a long-term trend towards greater aridity between 200,000 and 60,000 years ago, modulated by precession-driven wet-dry cycles. Here, more favorable wetter environmental conditions may have facilitated long-range human expansion into new territory, while less favorable dry periods may have led to spatial constriction and isolation of local human populations. The second mode of climate change observed since 60,000 years ago mimics millennial to centennial-scale Dansgaard-Oeschger cycles and Heinrich events. We hypothesize that human populations may have responded to these shorter climate fluctuations with local dispersal between montane and lowland habitats.