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| Titel |
The Beryllium-10(meteoric)/ Beryllium-9 ratio as a new tracer of weathering and erosion rates |
| VerfasserIn |
F. von Blanckenburg, J. Bouchez, H. Wittmann, N. Dannhaus |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250070131
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| Zusammenfassung |
| A perfect clock of the stability of the Earth surface is one that combines a first
isotope the flux of which depends on the release rate during erosion, and a second
isotope produced at constant rate. The ratio of the meteoric cosmogenic nuclide
10Be to stable 9Be, suggested to serve as proxy for weathering and erosion over the
late Cenozoic [1], is such a system. We provide a quantitative framework for its
use.
In a weathering zone some of the 9Be, present typically in 2ppm concentrations in silicate
minerals, is released and partitioned between a reactive phase (adsorbed to clay and
hydroxide surfaces, given the high partition coefficients at intermediate pH), and into
the dissolved phase. The combined mass flux of both phases is defined by the soil
formation rate and a mineral dissolution rate – and is hence proportional to the
chemical weathering rate and the denudation rate. At the same time, the surface
of the weathering zone is continuously exposed to fallout of meteoric 10Be. This
10Be percolates into the weathering zone where it mixes with dissolved 9Be. Both
isotopes may exchange with the adsorbed Be, given that equilibration rate of Be is
fast relative to soil residence times. Hence a 10Be/9Be(reactive) ratio results from
which the total denudation rate can be calculated. A prerequisite is that the flux of
meteoric 10Be is known from field experiments or from global production models
[2]. In rivers, when reactive Be and dissolved Be equilibrate, a catchment-wide
denudation rate can be determined from both sediment and a sample of filtered river
water.
We have tested this approach in sediment-bound Be [3] and dissolved Be in water [4] of
the Amazon and Orinoco basin. The reactive Be was extracted from sediment by combined
hydroxylamine and HCl leaches. In the Amazon trunk stream, the Orinoco, Apure, and La
Tigra river 10Be/9Be(dissolved) agrees well with 10Be/9Be(reactive), showing that in most
rivers these two phases equilibrate. 10Be/9Be ratios range from 5 Ã 10-9 for the Brazilian
shield rivers to 2 Ã 10-10 for the Beni river draining the Andes, corresponding to denudation
rates of 0.01mm/yr for the shields and 0.5mm/yr for the Andes, compatible with
denudation rates from in situ-produced cosmogenic 10Be [3]. 10-50% of the 9Be
was mobilised from bedrock. Once delivered to the ocean, this riverine Be, be it
dissolved or reactive, will eventually drive 10Be/9Be ratios of ocean water and disclose
global denudation rates – at the present and in the sedimentary record from the
past.
[1] Willenbring and von Blanckenburg, Nature 465, 2010
[2] Willenbring and von Blanckenburg, Earth Science Reviews 98, 2010
[3] Wittmann et al., Geol Soc. Am. Bull., 123, 2011
[4] Brown, E. et al., Geochim Cosmochim Acta 56, 1992 |
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