Limited glacial erosion during the last glaciation in mid-latitude cirques (Retezat Mts, Southern Carpathians, Romania)
Résumé
The studied dataset from the Southern Carpathians permitted the quantification of a considerable amount of inherited 10 Be in the glacial boulders and bedrock samples in the cirque area. The samples from the glacial phases of largest extension display no signs of significant inheritance, and enabled the establishment of a deglaciation chronology in the southern valleys of the Retezat Mts. The timing of the maximum glacier extent (20.6 +0.8 / −1.3 ka) coincided with the Last Glacial Maximum, which was followed by five deglaciation phases during the Lateglacial having partly overlapping ages due to fast glacier retreat (at 18.4 +0.7 / −1.1 ka; 16.9 ± 0.9 ka, 15.8 +0.9 / −0.6 ka, 15.6 +0.8 / −0.8 ka and 14.4 ± 0.5 ka) but could be distinguished by the position of their terminal moraines. The currently available geochronological data do not support the assumption of any major glacial re-advance after Greenland Stadial 2.1a in the Retezat Mts. Given the lack of independent geochronological data, the amount of inherited cosmogenic nuclides is tentatively estimated by accepting the youngest cosmic ray exposure age(s) as the time of moraine deposition and abandonment by the glacier. The calculated amount of inherited 10 Be enables the estimation of a glacial erosion depth of 1.1-1.6 m for the bedrock samples and 1.4-1.8 m for the glacial boulders. The duration of the ice-covered and ice-free periods was adjusted in relation to independent paleoenvironmental and paleoclimatological data. The glacial denudation rate in the cirques was estimated at 19-28 mm/kyr and 24-33 mm/kyr for bedrock and boulders, respectively. The limited glacial erosion in the cirques during the last glaciation is attributed to frozen-bed conditions with no considerable glacial deepening during the more extended glacial phases. Only when warming led to the retreat of the glaciers to their cirques, they become steeper and shift to being warm-based and thus more erosive. However, the limited time spent under these conditions appears to be too short to remove material from the cirque floors in sufficient depth (>3 m) to reset the cosmogenic clock. This suggests that the development of the cirques must have taken place during several subsequent glacial phases, providing an indirect confirmation of repeated Quaternary glaciations in the Retezat Mts.
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