Cosmogenic dating

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If the paleosol is formed in a sedimentary unit such as till, this unit may also have been emplaced with unknown Be concentrations at different depths in the paleosol will show a linear relationship.

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Terrestrial cosmogenic nuclide surface exposure dating of moraine boulders and alluvial fan sediments define the timing of five glacial advances over at least the last five glacial cycles in the Ladakh Range of the Transhimalaya.No longer should it be considered a major player in postglacial sea-level rise.Until just 20 years ago, when pioneering work in accelerator mass spectrometry (Elmore and Phillips, 1987), cosmogenic isotope systematics (Lal, 1988), and geologic applications (Craig and Poreda, 1986; Kurz, 1986) hit the presses, such conclusions were unreachable because many hypotheses regarding rates and dates of glacial processes were simply unfalsifiable.Measurement of paired isotopes in glacial erratics has been done before (Davis et al., 1999; Marsella et al., 2000), but Mackintosh et al.use it as a powerful tool by which to reject data from two erratics that clearly had a history of prior exposure and substantial burial before they were abandoned by the retreating East Antarctic Ice Sheet.In areas where ice has been frozen to the bed, concentrations of cosmogenic nuclides in bedrock often tell a garbled story (Bierman et al., 1999).The chronologic gibberish results from intermittent exposure of the rock to cosmic rays; between periods of surface or near-surface exposure, radioactive isotopes are no longer produced but rather decay under a passive cover of non-erosive ice.Those who want to date deglacial events refer to the fraction of cosmogenic nuclides left behind from prior periods of exposure as and do their best to avoid it (Davis et al., 1999).For others, this inheritance is a treasure trove of information about the stability of landscapes under ice and the distribution of erosion as a function of landscape position (Briner and Swanson, 1998; Colgan et al., 2002; Marquette et al., 2004).Similarly, the decrease in ice thickness that Mackintosh et al.infer is several times less than that implied by ice sheet models, and small enough that it effectively precludes ice extending to the shelf margin.


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