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| Titel |
The U-series comminution approach: where to from here |
| VerfasserIn |
Heather Handley, Simon Turner, Juan Afonso, Michael Turner, Paul Hesse |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250102013
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| Publikation (Nr.) |
 EGU/EGU2015-1290.pdf |
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| Zusammenfassung |
| Quantifying the rates of landscape evolution in response to climate change is inhibited by the
difficulty of dating the formation of continental detrital sediments. The ‘comminution age’
dating model of DePaolo et al. (2006) hypothesises that the measured disequilibria between
U-series nuclides (234U and 238U) in fine-grained continental (detrital) sediments can be used
to calculate the time elapsed since mechanical weathering of a grain to the threshold size (Â50
µm). The comminution age includes the time that a particle has been mobilised in transport,
held in temporary storage (e.g., soils and floodplains) and the time elapsed since final
deposition to present day. Therefore, if the deposition age of sediment can be constrained
independently, for example via optically stimulated luminescence (OSL) dating, the residence
time of sediment (e.g., a palaeochannel deposit) can be determined.
Despite the significant potential of this approach, there is still much work to be done
before meaningful absolute comminution ages can be obtained. The calculated recoil
loss factor and comminution age are highly dependent on the method of recoil
loss factor determination used and the inherent assumptions. We present new and
recently published uranium isotope data for aeolian sediment deposits, leached
and unleached palaeochannel sediments and bedrock samples from Australia to
exemplify areas of current uncertainty in the comminution age approach.
In addition to the information gained from natural samples, Monte Carlo
simulations have been conducted for a synthetic sediment sample to determine
the individual and combined comminution age uncertainties associated to each
input variable. Using a reasonable associated uncertainty for each input factor and
including variations in the source rock and measured (234U/238U) ratios, the total
combined uncertainty on comminution age in our simulation (for two methods of recoil
loss factor estimation: weighted geometric and surface area measurement with an
incorporated fractal correction) can amount to ± 220-280 ka. The modelling shows
that small changes in assumed input values translate into large effects on absolute
comminution age. To improve the accuracy of the technique and provide
meaningful absolute comminution ages, much tighter constraints are required on the
assumptions for input factors such as the fraction of alpha-recoil lost 234Th and
the initial (234U/238U) ratio of the source material. In order to be able to directly
compare calculated comminution ages produced by different research groups, the
standardisation of pre-treatment procedures, recoil loss factor estimation and assumed input
parameter values are required. We suggest a set of input parameter values for such a
purpose. |
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