DATING ALPINE GLACIAL EVENTS, AN EPISTLE IN RECOGNITION OF THE ‘POPE’ (Invited Presentation)

For more than four decades, Professor Jerry Osborn (JO) has contributed greatly to our understanding of alpine glaciation, first in mountain ranges throughout the western United States, Alberta, British Columbia, Yukon; and more recently in Argentina. JO and his collaborators have refined methods for dating alpine glacial events, notably the use of exposure dating techniques to improve the resolution of Late Glacial and Holocene glacial chronologies. In this presentation, I discuss some of the issues in establishing robust glacial chronologies that have surfaced in my discussions with the ‘Pope’, as my colleagues and I fondly refer to JO.

Although a variety of relative dating techniques have been used in the past to differentiate alpine glacial events, none is currently able to provide the precision required for use in paleoclimate studies. Their limitations are of two types. First are limitations inherent in the techniques themselves. For example, there are uncertainties in all radiocarbon ages, and these ages must be ‘calibrated’, which commonly increases the uncertainties in the true age. Although surface exposure and OSL dating methods are not subject to the second problem, they have uncertainties that are a consequence of laboratory processing and measurement techniques used to determine the ages.

Second are a suite of limitations that stem from real-world applications of the dating techniques. For example, OSL dating of glacial events is commonly indirect – it is applied to sediments with an assumed relationship to glaciation, rather than glacial deposits themselves. Radiocarbon ages on detrital plant material are maxima for the event of interest. And even radiocarbon ages on woody material in ‘growth position’ in glacier forefields only tell us the approximate time the glacier advanced past a particular point on the ground. Similarly, radiocarbon ages on plants in growth position within lateral moraines only indicate the time that ice thickened enough to deposit sediment at a specific elevation above the modern valley floor or glacier surface. Boulder surface exposure ages on a moraine date its abandonment, which may happen long after the glacier starts to build it. In effect, radiocarbon and surface exposure dating yield two different types of chronological information, and in both instances the ages are not the time of initiation or the culmination of a glacial.