OPTICALLY STIMULATED LUMINESCENCE DATING (OSL) IN AWKWARD PLACES: A STORY OF THE LAST LAURENTIDE ICE SHEET ADVANCE IN SOUTH-CENTRAL INDIANA

A glacial deposit section along a river bend in south-central Indiana was recently ‘rediscovered’. The exposure consists of sorted glaciofluvial sediments (sand, gravel, and fossiliferous silt), sandwiched between two tills. The lower and upper till units were respectively interpreted as associated to the maximum ice extent and an ice reoccupation during the Late Wisconsinan.

In an attempt to constrain the timing of these events, two samples for optically stimulated luminescence dating (OSL) were retrieved from the glaciofluvial sandy deposit. Radiocarbon dating was also performed on the fossiliferous silt. Difficulties were encountered during the OSL dating, most notably partial bleaching. Partial bleaching, if not properly taken into account, produces an OSL age overestimation. In addition, it was found that these sediments suffer from uranium disequilibrium (prominent radium 226 excess). This violates a key requirement for OSL dating.

The current hypothesis is that the radium excess originated from groundwater flowing through these sediments. In addition, groundwater flow would likely be enhanced during the Holocene. This significantly and negatively impacts any attempt to perform OSL dating. We will present a way to model the time variation in radium 226 for the purpose of obtaining a more accurate timing for these geological events.