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Cosmogenic exposure age dating

► An incomplete exposure model predicts Tibetan Plateau boulder exposure age scatter.► Incomplete exposure is typically more important than prior exposure.Be production rate is carefully calibrated, for example by correcting for partial attenuation and complete shielding effects.Also, the exposure assumptions must be justified through other means, for example by taking into account clear signs of surface erosion and information consistent with (or suggestive of) 100% shielding prior the exposure event to be dated.The precision is effectively the laboratory uncertainty.The accuracy is how far this probability density curve falls from a reference value.In the sciences, it is important to distinguish between precision and accuracy.If we use the analogue of a clock we can investigate this further.

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These factors may result in you having an exposure age that is both precise and inaccurate.Your wrist watch may measure time with a precision of one second.A stop watch may time your race with a precision of one hundredth of a second.Geological data frequently has difficulties in dating things, even if the analytical techniques can give very precise answers.In the figure below, the precision is the laboratory probability density curve produced when a material is dated.In fact, in cosmogenic nuclide dating, the greatest uncertainties come from geological problems that result in inaccurate ages.This usually results in ‘geological scatter’, where boulders on one landform give a range of exposure ages.The large global dataset demonstrates that, in the absence of other evidence, glacial boulder exposure ages should be viewed as minimum limiting deglaciation ages.► We present a comprehensive compilation (n = 2174) of boulder exposure ages.► Prior exposure is limited in boulders from recent glaciers and palaeo-ice sheets.However, if the clocks change and you forget to reset your wrist watch, then you have a very precise time but it is not very accurate – you will be an hour early or late for all of your meetings!See the difference between accurate and precise measurements in the bull’s eye figure below.

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