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| Titel |
LGM glaciers on Mount Lebanon? New insights from 36Cl exposure dating of moraine boulders |
| VerfasserIn |
Adrien Moulin, Lucilla Benedetti, Jérôme Van der Woerd, Ata Elias, Pierre-Henri Blard, Robert Finkel, Régis Braucher, Jérôme Lavé, Didier Bourlès, Mathieu Daëron |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250055792
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| Zusammenfassung |
| In situ-produced 36Cl has been used to determine the exposure age of boulders located below
the top of Mount Lebanon (3088 m asl) at the Cedars locality (2000 m asl, 34.25°N).
Morphological and geological evidence such as horseshoe- shaped cirques, U-shaped valleys
and coarse and chaotic Quaternary deposits suggest the presence of former glaciers. Detailed
field mapping allow identifying different moraine ridges. Sampling of the topmost
centimeters of surfaces of large, flat-topped, stable carbonate blocks, protruding from the
moraine crests have been performed. To constrain parameters such as possible inheritance in
moraine material or erosion of the glacial deposits, samples have also been collected along
a depth profile (0 to 1100 cm depth) in the center of a moraine ridge. 36Cl and
Cl concentrations have been measured in collected samples by isotope dilution
accelerator mass spectrometry at both CAMS-LLNL (US) and ASTER-CEREGE
(France). Those analysis have been completed by radiocarbon dating of charcoal
sampled in a pit deposit located a few meters from the depth profile between two
ridges.
The exponential decrease of the measured 36Cl concentrations as a function of depth along
the profile is best explained by a surface exposure age ranging between 18-20 ka, negligible
inheritance and an erosion rate ranging from 50 to 100m/Ma (density measured on site of
1.3 g/cm3 at 120 cm depth). Exposure ages yielded from surface samples spread
between 19 and ~130 ka when using the range of erosion rates deduced from the
depth profile. Radiocarbon ages range from 0 to 8.3 ka in stratigraphic order and in
agreement with vegetation and soil development after glacial retreat during the
Holocene.
To further constrain the paleoclimatic conditions (temperatures and precipitations) prevailing
during the formation of these glacial deposits, we used a numerical ice–flux model on a
high resolution (100 m) digital elevation model. Ice ablation is computed using a
positive-degree-day model. To account for the presence of a glacier at Cedars, preliminary
results suggest annual temperature decrease from 10 to 14°C, for x2 to x0.5 precipitation
change respectively, during the LGM.
Although the Quaternary deposits of Mount Lebanon have been interpreted as massive
rockfalls, questioning their glacial origin, our results favor emplacement at least 18 ka ago, in
agreement with the presence of a glacier during the LGM in this area. |
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