Abrupt Climate Change by Harunur Rashid & Leonid Polyak & Ellen Mosley-Thompson

Author:Harunur Rashid & Leonid Polyak & Ellen Mosley-Thompson
Language: eng
Format: epub
Publisher: Wiley
Published: 2013-03-27T16:00:00+00:00

6. DISCUSSION

In this chapter, we have identified two challenges in the use of cosmogenic exposure dating to date moraines associated with abrupt climate changes such as the Younger Dryas. First, pristine boulders will tend to yield exposure dates that are younger than the true age of the moraine, if moraines lose significant thicknesses of material from their crests over time, and boulders erode once exhumed. Second, geomorphic processes likely impart an additional uncertainty of at least several percent in our present estimates of reference cosmogenic nuclide production rates. For moraines of late glacial to early Holocene age, these effects bias exposure dates by hundreds to thousands of years. Thus, these challenges limit our confidence in the ability of cosmogenic exposure dating to identify moraines associated with abrupt climate changes, which have time scales of a few years to a few hundred years.

Moreover, our results confirm prior suggestions that preferentially sampling tall boulders is a good strategy [Phillips et al., 1990; Gosse et al., 1995a, 1995b]. However, even exposure dates from tall boulders may underestimate the age of a moraine by thousands of years, depending on boulder height and the thickness of material lost from the moraine’s crest.

Our conclusions do not hold where geomorphic processes other than moraine degradation and boulder erosion affect exposure dates. In particular, inheritance might cause the exposure dates from tall boulders to overestimate the ages of moraines; boulders that fall onto the glacier from oversteepened valley walls will not be eroded in transport and thus may be larger than other clasts that have had most of their inherited nuclides stripped away by subglacial transport.

Moreover, we assume that the erosion rate for exposed boulders is the same for all the rocks on a moraine. However, we expect that boulders on real moraines will weather at different rates, depending on their lithology, position in the landscape, and size. Our models also assume slow, grain-by-grain erosion of moraine boulders, but the rapid loss of several centimeters of rock is also possible [Zimmerman et al., 1995]. In the future, we plan to update our models to represent this style of erosion, perhaps following Muzikar [2009].

Here we explain the lack of correlation between boulder height and apparent exposure time on the inner Titcomb Lakes moraine (Figure 4) with moraine degradation. Other explanations are also possible. In cases where inheritance dominates the scatter among exposure dates, there may be no relationship between boulder size and apparent exposure time. If all the boulders on a moraine are taller than the thickness of snow cover or the thickness of sediment that has been removed from the surface of a moraine, there will also be no correlation between boulder height and apparent exposure time. We believe that the geomorphic processes responsible for the scatter among exposure dates vary between moraines, and so the degree and sign of the correlation between boulder height and apparent exposure time probably also vary.

Our results imply that our ability to invert our models against collections of exposure dates depends on how the samples were chosen.

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