科研之友
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摘要
Understanding and interpretation of 'numbers' produced about the depositional age of an erratic boulder by cosmogenic nuclide surface-exposure dating is important in the construction of glacial chronology. We have sampled three 'Findlinge' (glacially transported boulders) located on the right-lateral margin of the Aare glacier at Moschberg, Grosshochstetten, southeast of Bern, with the aim of shedding light on this topic. The boulders have the same depositional, but different post-depositional histories: simple exposure; exhumation; and human impact. This sampling is specially selected for this study, since the boulders showing exhumation and human impact would not have been sampled in a regular surface-exposure dating application. We measured cosmogenic Be-10 concentrations and calculated apparent exposure ages that are 13.6 +/- A 0.5, 18.1 +/- A 0.8, and 7.5 +/- A 0.4 ka, respectively. The exposure age of the first boulder reflects exhumation. The apparent exposure age of 18.1 +/- A 0.8 ka (erosion-corrected exposure age 19.0 +/- A 0.9 ka) from the second boulder correlates well with the end of the Alpine and global last glacial maximum. The third boulder shows evidence of quarrying as it is surrounded by a rim of excavation material, which is also reflected by the 7.5 +/- A 0.4 ka apparent exposure age. We modeled the variation of Be-10 concentrations with depth down into the sediment in which the first (exhumed) boulder was once buried in, and down into the third (quarried) boulder. According to our modeling, we determined that the exhumed 'Findling' was buried in sediment at a depth of around 0.5 m, and around 2 m of rock was quarried from the third 'Findling'. Our results reveal the importance of sampling for surface-exposure dating within a well defined field context, as post-depositional impacts can easily hinder exposure-dating of surfaces.
- 出版日期2011-12
