Reducing uncertainty in cosmic-ray exposure dating

Reconstructing the behaviour of former ice sheets is vitally important to understand how the Earth’s cryosphere has changed over the longer term (~103–105 years), and to refine numerical model predictions of future ice-sheet behaviour. Cosmic-ray exposure-dating techniques allow us to place past glacier fluctuations into a firm chronological context on millennial timescales and with a relatively high level of precision (<10% uncertainty). By deriving absolute ages from in situ terrestrial glacial deposits, Earth scientists can now directly ascertain the timing of glacial events than rather by proxy (such as by radiocarbon dating organic material growing after or prior to a glacial advance). As a result of its general applicability, the number of cosmic-ray exposure studies has risen sharply over the past decade with research now undertaken on all 7 continents on surfaces spanning the last 25 million years (Dunai et al., 2010.
One major area of uncertainty stems from our currently weak understanding of the geological processes that cause apparent exposure ages to differ from the true age of the landform – a major factor when attempting to determine ages with precision. Put simply, what causes the scatter? Previous studies (e.g. Putkonen & Swanson, 2003; Balco, 2011) suggest that the four main causes of geological scatter are rooted in the assumptions governing the dating technique, namely: 1) that the rocks being sampled have no prior cosmogenic-nuclide inheritance; 2) that the rocks have not been disturbed since deposition; 3) that the rocks have not been buried/covered; and 4) that the rock surface is the original surface.
Using a carefully designed sampling methodology in combination with state-of-the-art analytical facilities (at SUERC) this PhD project will aim to assess and quantify the relative components of geological scatter in cosmic-ray exposure dating on glacial boulders deposited since the Last Glacial Maximum (ca. 25 ka BP), using NW Scotland as an exemplar deglaciated landscape.
This project will focus on Beryllium-10 concentrations found in quartz in the NW Highlands of Scotland – where the glacial history is already established and the landscape has seen little or no human interference. It is hoped that this PhD project will provide important new results to refine the uncertainties within this widely used geochronological technique; and in doing so generate a valuable resource for the global cosmogenic nuclide user community.
For further information, or informal enquiries, contact Dr Tom Bradwell [Email Address Removed]
or Dr Derek Fabel [Email Address Removed]