Millennial-scale hard rock erosion rates deduced from luminescence-depth profiles

Abstract:

Optically stimulated luminescence (OSL) is a well-established Quaternary dating method that is conventionally used to determine the time when sedimentary grains were last exposed to daylight. Recently, a very different approach to this concept has helped develop a new technique to estimate the length of time a rock surface was exposed to daylight. When a rock surface is first exposed to daylight the charge population (and so the latent luminescence signal) trapped in its constituent minerals (e.g. quartz and feldspar) starts to decrease. This charge had accumulated due to previous exposure to natural ionizing radiation. As the surface is exposed to light for longer periods, the latent luminescence signal is reduced farther into the rock. In a rock surface which has been exposed to light for a prolonged period (decades to millennia), the remaining luminescence will be zero (fully bleached) at the surface and then increase, initially exponentially, before approaching saturation at a depth where charge detrapping due to light penetration is negligible compared to the rate of charge trapping due to the environmental dose rate. By modelling the characteristic shape of luminescence resetting with depth into rock surfaces, Sohbati et al. (2012) proposed a new surface-exposure dating technique based on OSL. Here we further develop the current model to include the effect of erosion rate on luminescence-depth profiles. By fitting the model to local known-age calibration samples, we first determine the site-specific resetting rates of the luminescence signal at rock surfaces. We then use the calibration values in a numerical model to derive the steady-state erosion rate for rocks of different mineralogy and different geological settings. The preliminary erosion rates obtained from glacial and landslide granite boulders from the Chinese Pamir Plateau are ~1 mm.ka^-1, whereas active streambeds of Permian sandstone in the Grabens district of Canyonlands National Park, Utah, are eroding at rates of >10 mm.ka^-1, consistent with the existing denudation rates derived from cosmogenic nuclides.