Glacial erosion and paleoclimate reconstruction
Zusammenfassung der Projektergebnisse
In order to reconstruct late glacial mountain climate in Corsica, we have mapped all latero-frontal moraines from present Mediterranean to montane elevation to the highest cirques. For advances in dating methodology, we assessed rates of postglacial granite erosion to extrapolate the effect of shielding by weathering of glacial features to obtain correct exposure ages. By means of exposure dating of glacial boulders and roche moutonnee and luminescence dating, we detected 4 glacier advances during the Last Glacial Maximum (LGM, MIS 2, 24-18 ka). We found some evidence for locally stronger glacier advances in the early and middle Wuermian, and for 2 much stronger pre-Wuermian glacier advances, likely reflecting Rissian age (MIS 6, 160 ka) and Mindel age (undated). The latter age is assessed on the base of a relative terrace stratigraphy and evidence of postglacial river incision rates. Late glacial moraines are generally well-preserved and reflect 3 advances during the Oldest Dryas (locally 1 more final small moraine ridge) and 2 advances during the Younger Dryas (locally 1 more). Holocene glaciations have not been mentioned before, whereas we present evidence of niche glaciers even in the Little Ice Age. Since latero-frontal moraines from the early to the late Wuermian, if existing, are situated quite close, differences of the equilibrium line altitude (ELA) during cold spells were small. A mapping of the ELA for the LGM in Corsica reflects local paleoclimate differentiation, controlled by relief. Surprisingly cool and wet conditions in southeastern Corsica (Bavella) caused extensive glaciation at low elevations and evacuation of debris out of extremely steep canyons which have never been suspected as to be formed by glaciers. Cold air came from the NW, N, and NE whereas moisture preferentially advected from easterly directions. The anomaly pattern both for annual average temperature and precipitation is much different from that of the Present. Gradients were somewhat greater in the LGM. LGM patterns of the ELA are grossly similar to that found during the late glacial, whereas local ELA data from Holocene cold spells are compatible with the present-day ELA. Discovery and dating of Holocene glaciers in Corsica is sensational from the local perspective and of major importance for Mediterranean-scale atmospheric circulation. An ELA depression of 600 to 700 m in the Little Ice Age (Maunder Minimum) as compared to the Present (climatic normal period 1960-1990) is more than twice as much as found in other Mediterranean mountains ranges including the Alps. A compilation of the Mediterranean-wide ELA distribution in the LGM supports stronger climate gradients as compared to the present. Stronger convection, driven by anomalously steep lapse atmospheric rates, appears to account for steeper precipitation gradients along the island margins. Relatively warm sea-surface temperatures anomaly in the central Mediterranean and off Iberia provided regional moisture sources. We explain a low ELA along the eastern margin of Corsica by more frequent Mediterranean cyclones, passing south of Corsica to advect both moist and cool air on their rear side. For methodological perspectives, post-glacial weathering rates of glacially polished rock surfaces for better shielding corrections have been quantified and found to range between 1 and 5 mm/ka for freshly polished roche moutonnee. Silicification after Alpine brittle deformation caused impressive hardening of granites. Soil downwearing in the range of 1.6 to 0.7 m since the LGM as constrained by weathering girdles underpin recommendations to avoid sampling of boulders smaller than 1.5 m. Glacial outburst floods destroyed moraines in most narrow valleys of Corsica and appear to be a usual phenomenon rather than an exception.
Projektbezogene Publikationen (Auswahl)
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(2005): Würmian maximum glaciation in Corsica: glacier extent, amplifying paleorelief, and mesoscale climate. Austrian Journal of Earth Sciences, 97: 68-81
Kuhlemann, J., Frisch, W., Székely, B., Dunkl, I., Danisik, M. & Krumrei, I.
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(2007): In situ 10Be-rosion rates in granites of subalpine Miocene paleosurfaces in the western Mediterranean (Corsica, France).- Int. J. of Earth Sciences
Kuhlemann, J., van der Borg, K., Danisik, M. & Frisch, W.
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(2008): Regional synthesis of Mediterranean atmospheric circulation during the Last Glacial Maximum. Science 321, 1338-1340
Kuhlemann, J., Krumrei, I., Rohling E., Kubik, P., Ivy-Ochs S., Kucera, M.
