 |
| Titel |
Boulder cosmogenic exposure ages as constraints for glacial chronologies |
| VerfasserIn |
Jakob Heyman, Arjen P. Stroeven, Jon Harbor, Marc W. Caffee |
| Konferenz |
EGU General Assembly 2010
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250044184
|
|
|
|
|
|
| Zusammenfassung |
| Cosmogenic exposure dating greatly enhances our ability to define glacial chronologies
spanning several global cold periods, and glacial boulder exposure ages are now routinely
used to constrain deglaciation ages. However, calculating an exposure age from a measured
cosmogenic nuclide concentration involves assumptions about the geological history of the
sample that are difficult to test and yet have a profound effect on the inferred age. Two
principal geological factors yield erroneous inferred ages: pre-depositional exposure (yielding
exposure ages that are too old) and post-depositional shielding (yielding exposure ages that
are too young). To evaluate the importance of these two problems we have compiled datasets
of glacial boulder 10Be exposure ages from the Tibetan Plateau (1099 boulders), the Northern
Hemisphere palaeo-ice sheets (613 boulders), and present-day glaciers (141 boulders). All
exposure ages have been recalculated with the CRONUS online calculator version 2.2
(http://hess.ess.washington.edu/) using the new 10Be half-life of 1.36 Ma. All boulders from
present-day glaciers have exposure ages 10 ka older than the deglacial age of
the surface. Boulders from the Tibetan Plateau have mainly been collected from
moraine ridges. We have organized them into boulder groups, each of which has
one deglacial age. The age spread of the Tibetan Plateau boulder group dataset is
significantly higher than the inheritance observed in the palaeo-ice sheet boulders. If this
spread is attributed to inheritance we would conclude that on the Tibetan Plateau
inheritance plays a much more prominent role than is seen in the palaeo-ice sheet areas.
Alternatively, a simple exponential post-glacial landform degradation model produces
exposure age distributions remarkably similar to the measured data, indicating that
post-depositional shielding is likely the dominant process producing spread among
boulder age distributions. Our analysis lends strong support to the argument that
post-depositional shielding is the most important geological process leading to
potential errors in cosmogenic exposure ages for glacial boulders older than a few
thousand years. The strong recommendation emerging from this analysis of global
10Be exposure ages is to interpret sets of dates from glacial settings in terms of
post-depositional shielding: i.e., that exposure ages represent minimum ages of deglaciation. |
|
|
|
|
|
|