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| Titel |
Intra-catchment variability and significance of catchment-averaged denudation rates from 10Be concentrations in stream sediments: a 10Be-budget of the Etages catchment, French Western Alps |
| VerfasserIn |
Romain Delunel, Peter van der Beek, Julien Carcaillet, Didier Bourlès |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250039788
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| Zusammenfassung |
| As most of the European Alps, The Ecrins-Pelvoux massif (French Western Alps) was
extensively glaciated during Quaternary glaciations, leading to strong rejuvenation of its
morphology. The massif therefore provides a suitable area to study the efficiency of erosion
processes in relief evolution on postglacial timescales. Denudation rates inferred from in-situ
produced 10Be concentrations in stream sediments, obtained from 12 catchments throughout
the Ecrins-Pelvoux massif, have been recently shown to correlate with mean catchment
elevation in the absence of significant relationships with other morphometric parameters
(Delunel et al., in press). We have proposed that the present-day denudation of
Ecrins-Pelvoux massif climatically driven trough increasing frost-controlled processes with
elevation, providing a mechanistic link for the inferred feedback between uplift, elevation and
denudation rates observed in the European Alps (Wittmann et al., 2007; Champagnac et al.,
2009). However, cosmogenic isotope measurements of stream sediments do not allow
distinguishing the intrinsic spatial variability of denudation within a catchment.
Therefore, we have sought to verify our previous conclusions on a smaller scale within a
single catchment, from exhaustive measurements of 10Be concentrations carried by
quartz fraction of different sources feeding the high-altitude stream sediment routing
system.
We focus our current study on the Etages catchment, a high-elevation hanging tributary of
the Vénéon valley (western part of the Ecrins-Pelvoux massif) underlain by homogenous
granitic bedrock. This 14 km2 catchment presents elevations ranging from 1600 m to ~3600
m (mean catchment elevation ~2700 m), within the altitudinal range where frost-controlled
processes are most efficient in the Western Alps (Delunel et al., in press). This catchment also
hosts a small cirque-glacier, remaining from the Little Ice Age glacial advance. We have
collected 19 samples on the most representative morphologic features resulting from
active high-altitude erosion processes, in order to extract their in-situ produced 10Be
signatures.
10Be concentrations measured from these samples vary strongly from 0.1Ã105
to 6.3Ã105 atoms g-1. Beryllium content of glacial materials varies from 0 (i.e.
undistinguishable from procedural blanks) close to the present-day glacier position to
0.8Ã105 atoms g-1 toward the LIA moraines. Scree-slope materials collected at different
levels within the catchment have slightly higher 10Be concentrations (0.4 to 0.7Ã105 atoms
g-1). Regolith material collected close to the highest crests carries much higher
concentrations (1.4 to 4.2Ã105 atoms g-1). Bare rocks that deliver the material to form
regolith are also characterized by relatively high 10Be concentrations, ranging from 4.2 to
6.3Ã105 atoms g-1. Finally, stream sediments collected at the outlet of the Etages catchment
carry a 10Be concentration of only 0.2Ã105 atoms g-1. These 10Be concentration
measurements at a single catchment scale thus suggest the processes within a catchment to be
more complex than our initial interpretation based on catchment-wide erosion rates on the
massif scale.
Therefore, we performed numerical modelling based on the spatial variability of erosion
rates within this catchment in order to explain how the wide range of initial 10Be
concentrations evolves toward those measured within downstream sediments. Although
glacial material with low 10Be content cannot be rejected as contributing to the stream
sediment signature, results obtained from this model suggest the significant role of
bedrock gorges that rapidly incise the area between hanging tributaries and main
trunk valleys (Valla et al., 2010) in the 10Be signature carried by stream sediments.
Our study hence shows that combining cosmogenic isotope measurements with
numerical modelling may help to significantly improve our understanding of which
processes govern the relief evolution in high elevation areas on post-glacial timescales. |
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