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| Titel |
The geological significance of cosmogenic nuclides in sediment of large lowland basins |
| VerfasserIn |
H. Wittmann, F. von Blanckenburg |
| 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 |
250070213
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| Zusammenfassung |
| Understanding the links between sediment production in the source area, its transport through
large lowland basins, and the final delivery into the ocean is indispensable for assessing
long-term sediment delivery ratios with the ultimate aim to determine global Earth surface
mass fluxes. Challenges in predicting such sediment yields are 1) spatial- and temporal
scale-dependent variability of erosion in the source area; and 2) the quantification of sediment
incorporation into sinks such as floodplain deposits, and the timing of their release after
intermittent storage.
In the large Amazon basin, we have tackled these challenges with a combination of using
the in-situ-produced cosmogenic nuclides 10Be and 26Al and modern suspended sediment
fluxes. Concentrations of cosmogenic nuclides in river sediment yield basin-wide denudation
(weathering + erosion) rates. Sampled at some distance from the Andean front in the
upper Amazon floodplain, we show that 10Be-derived denudation rates provide a
spatially-averaged rate that sums both sediment production and the release of dissolved
weathering products of the Andes. With increasing distance from the high-relief
sources, local variability in denudation rates is averaged out, providing a mean
sediment production rate that also integrates over long time scales (a few kyr),
such that effects of human disturbances are minimized[1]. In the fine sand-sized
sediment fraction at the outlet of the Amazon basin, we find the same concentrations of
10Be as in the source areas[2], showing that their erosion rate is dominating the
sediment flux in the entire Amazon basin. The preservation of this Andean erosional
signal is possible because 10Be and 26Al concentrations are not modified during
sediment storage times if the duration of storage is 0.5 Myr) from the modern river. The modern river can
re-incorporate large amounts of these old deposits, and once corrected for decay, their
concentrations show that this sediment is sourced in cratonic shield areas of low denudation
rate[4].
Modern sediment fluxes, determined by river load gauging at the outlet of the Amazon
basin, record the rate of sediment export of the basin over ~10 yr time scales and agree with
our 10Be-derived rates within a factor of 2[2]. This agreement indicates that a sediment
delivery ratio is, at first order, constant from the Andes to the Amazon outlet, and net
sediment deposition is apparently not taking place. We can budget Andean sediment
production as 620 Mt/yr over the last ~5 kyr, and total export of sediment into the Atlantic
today at ~1000 Mt/yr.
[1] H. Wittmann, et al. (2009), EPSL, 288, 463-474.
[2] H. Wittmann, et al. (2011), GSA Bull., 123, 934-950.
[3] H. Wittmann, F. von Blanckenburg (2009), Geomorphology, 109, 246-256.
[4] H. Wittmann, et al. (2011), Geology, 39, 467-470. |
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