Cosmogenic exposure dating
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Theoretical ¹⁰Be pyroxene production rates were also determined, based on the composition of the Murimotu pyroxene.
We observe a pattern of progressively more restricted glaciation during the last five glacial cycles, likely indicating a progressive reduction in the moisture supply necessary to sustain glaciation.
This work shows that production rates for ¹⁰Be in pyroxene are both empirically and theoretically 8-27% lower than in quartz.
The ³He/¹⁰Be ratio in the Murimotu pyroxene is 34.5 ± 9.9; this is indistinguishable from global ³He-pyroxene/¹⁰Be-quartz production ratios.
But there are other ways in which a rock can become exposed, as for example when a glacier erodes the sediment covering bedrock: when the glacier melts, the bedrock will be exposed.
In the article on radiocarbon dating we have already introduced one cosmogenic isotope, Si and which has a half-life of 717,000 years.
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.
The glacial stages that have been identified are: the Indus Valley glacial stage, dated at older than 430 ka; the Leh glacial stage occurring in the penultimate glacial cycle or older; the Kar glacial stage, occurring during the early part of the last glacial cycle; the Bazgo glacial stage, at its maximum during the middle of the last glacial cycle; and the early Holocene Khalling glacial stage.This method successfully removes the meteoric component of ¹⁰Be in pyroxene, allowing only the concentration of in situ produced ¹⁰Be to be measured.Additionally, production rates for ¹⁰Be in pyroxene have been determined empirically for New Zealand using cross-calibration with measured ³He concentrations and an independent radiocarbon age of the Murimotu debris avalanche in the central North Island, New Zealand of 10.6 ± 1.1 ka.The rate at which they do so will depend on a number of factors, including: If we take all the relevant factors into account, and calculate, estimate, or simply measure the amount of cosmic rays a given rock is exposed to per year, and if we measure the quantities of the cosmogenic isotopes in a sample of the rock, then we can figure out how long the rock has been exposed.The quantity of the relevant isotopes in the rock will not simply grow without limit with longer and longer exposure to cosmic rays; rather they will tend towards a maximum (a secular equilibrium): the point at which the cosmogenic cosmogenic production of unstable isotopes is equaled by their destruction by decay.This diagram, showing thinning of an ice sheet from the Last Glacial Maximum (LGM) to present day, helps to visualise how this works: When an ice sheet thins, rocks transported within it (erratics – shown here as red circles) are left perched on mountainsides – known as ‘nunataks’ when surrounded by ice – at a range of heights above the modern ice sheet surface.