APPLICATIONS OF OPTICALLY STIMULATED LUMINESCENCE DATING IN WISCONSIN

The lack of sufficient datable carbon has been a problem for re-constructing Wisconsin's de-glacial and Holocene histories. Optically stimulated luminescence (aka ‘OSL' or simply ‘optical') dating has recently been applied to late Quaternary sediments from eolian, fluvial, and lacustrine environments in Wisconsin. The OSL method provides an estimate for the last time sand grains were exposed to sunlight. Dating results are generally favorable for eolian dunes in Wisconsin where OSL is ideally suited because sunlight exposure is generally sufficient to zero out or ‘bleach' the luminescence signal from sand grains prior to their burial. Dunes provide an excellent opportunity to estimate minimum ages for glacial lake beds, outwash surfaces, and stream terraces. However, samples taken from within 2 m of the ground surface in some Wisconsin dunes are commonly younger than expected. We attribute these relatively young ages to the re-setting of the luminescence clock due to bioturbation rather than eolian processes. Results from fluvial and lacustrine environments are very promising in some settings, but clearly problematical in others. Some of these problems can be attributed to incomplete zeroing, or ‘partial bleaching', of the luminescence signal in sand grains prior to their burial. This phenomenon results in age over-estimations that are common in fluvial and lacustrine environments, particularly in proximal glacial environments. Some of the newer luminescence techniques, such as the single-grain dating method, can be used to test for and potentially mitigate problems with partial bleaching. Other problems can be attributed to inaccurately estimating the long-term dose rate or moisture content for a sample. Improved sampling strategies can reduce the effects of these latter errors.