Accuracy of cosmogenic ages for moraines

Abstract

Analyses of all published cosmogenic exposure ages for moraine boulders show an average age range of 38% between the oldest and youngest boulders from each moraine. This range conflicts with the common assumption that ages of surface boulders are the same as the age of the landform. The wide spread in boulder ages is caused by erosion of the moraine surface and consequent exhumation of fresh boulders. A diffusion model of surface degradation explains the age range and shows that a randomly sampled small set of boulders (n ϭ 3-7) will always yield a lower age limit for the moraine. The model indicates that for identical dating accuracy, six to seven boulders are needed from old and tall moraines (40,000 -100,000 yr, 50 -100 m initial height) but only one to four boulders from small moraines (20,000 -100,000 yr, 10 -20 m). By following these guidelines the oldest obtained boulder age will be Ն90% of the moraine age (95% probability). This result is only weakly sensitive to a broad range of soil erosion rates. Our analysis of published boulder ages indicates that Ͻ3% of all moraine boulders have prior exposure, and 85% of these boulders predate the dated moraine.

Investigating the glacial history of the northern sector of the Cordilleran Ice Sheet with cosmogenic 10Be concentrations in quartz

“…The consistent trend of increasing age with increasing boulder height on the northern moraine, together with the degraded nature of the moraine, indicates that the distribution in exposure duration is due to moraine degradation (DukRodkin et al, 1996;Putkonen and Swanson, 2003) and thus that the longest exposure duration of 16.2±1.5 ka may be nearest the true age of the deposit. The interpretation of the southern moraine is complicated by the fact that the cobble sample has a minimum exposure duration of 40.1±3.6 ka and clearly has a prior exposure history compared to the boulder samples from the same moraine.…”

“…The spread in ages may be due to an exhumation of boulders as the moraine degraded to its present subdued form. If exhumation is the cause of the disparate ages, the oldest apparent exposure age would be closest to the true age of the deposit (Putkonen and Swanson, 2003). Furthermore, the two datasets from the alpine moraines (YK36-38 and YK39-41) suggest that the older moraines have associated larger spreads in ages, which would be an expected consequence of moraine degradation (Duk-Rodkin et al, 1996).…”

On the reconstruction of palaeo-ice sheets: Recent advances and future challenges

“…If, on the other hand, a surface was not exposed before the last glaciation and has only been exposed during a part of the post-glacial time, the surface will yield exposure ages that are younger than the deglaciation age due to incomplete exposure. Several samples from a surface with an expected distinct deglaciation age commonly display scattered exposure ages indicating that the problem with prior and/or incomplete exposure is common (Putkonen and Swanson, 2003;Kaplan et al, 2007;Heyman et al, 2011).…”

First results of cosmogenic dated pre-Last Glaciation erratics from the Montoz area, Jura Mountains, Switzerland

“…Soil creep could have slid the smaller rock for some meters, but rotation would likely induce more dynamic movements toward the valley. Alternatively, a younger age of a boulder within a sample set can be explained by its post-depositional coverage with sediment causing a decrease of the production rate (Putkonen and Swanson, 2003). We exclude coverage of solely MO-04-03 because of the above described sample site characteristics.…”

“…In addition, it has been statistically shown by Putkonen and Swanson (2003) that in many cases older ages from a data set are more representative of the true deposition age than younger ones. According to this, the boulders were deposited on the Montoz anticline between 126 and 184 ka, a time span corresponding to Marine Isotope Stage (MIS) 6.…”