OSL DATING OF HIGH ELEVATION FAN DEPOSITS IN THE ANTARCTIC DRY VALLEYS: ANALYTICAL METHODS

In our efforts to evaluate high elevation fan deposits in the McMurdo Dry Valleys of Antarctica as archives of fine-scale climatic data we have been required to innovate many aspects of optically stimulated luminescence (OSL) dating methodology. Below 10-40 cm depth the fan deposits are ice-cemented. Our team developed a relatively easy and reliable method for collecting an undisturbed field sample for OSL dating from ice-cemented deposits. Similarly we developed a laboratory pre-processing method to remove light exposed portions of the ice-cemented field sample form the unexposed sample without melting or mixing. With these hurdles surmounted we faced greater challenges that required innovation in analytical methodology. Fan deposits in general can present a challenge for OSL dating as sediments can be delivered to the fan by a combination of water-, gravity-, and wind-driven processes. This in turn can result in a wide range of signal resetting among grains in a single fan sample and complex OSL equivalent dose distributions. It is well established among OSL professionals that simplistic measures of central tendency are inappropriate for deriving an accurate age from these types of deposits. In addition, OSL dating practioners applying the technique to Antarctic sediments have noted that Antarctic samples generally have very weak signal response or very low optical output. Despite these analytical challenges, we have been successful in making age calculations for greater than 90% of the fan samples collected; rejecting only two samples as non-responsive. This presentation will overview our field sampling and pre-processing procedures, but will primarily focus on the analytical methodologies that have allowed us to achieve these results.