Extent and chronology of the Pleistocene permafrost in France : database of periglacial structures and OSL dating of sand wedges.

Abstract : During the Mid to Late Pleistocene, the land area affected by periglacial conditions expanded and contracted repeatedly over large surfaces in mid-latitude Western Europe. In such environments, permafrost or deep seasonal freezing of the ground formed typical features, which have been the subject of abundant research by geomorphologists. In particular, researchers attempted to reconstruct the maximal extent of Pleistocene permafrost based on field evidence. Although most reconstructions suggest that permafrost spread over part of France during the coldest periods of the Pleistocene, there is no agreement regarding the land surface affected. This is mainly due to the scarcity of field data used for mapping and to the questionable palaeoclimatic significance of certain periglacial features. In addition, permafrost modelling during the Last Glacial Maximum using Global Climate Models does not seem consistent with field data. To solve these issues, a database of Pleistocene periglacial features has been compiled from a review of academic literature and unpublished reports, the analysis of aerial photographs and new field surveys. Polygons, soil stripes, ice-wedge pseudomorphs, sand wedges and composite wedge pseudomorphs were included in the database together with their geographic coordinates, geological context, description and references. The distribution of the identified features was analysed with a GIS software and clearly indicates that large areas in France were affected by periglaciation, apart from the southwesternmost part of France and the Languedoc. Ice-wedge pseudomorphs do not extend south of 47°N which indicates that widespread discontinuous permafrost did not affect the land south of the Paris basin. The exclusive presence of sand wedges with primary infill between 45 and 47°N, mainly in the periphery of coversands, suggests that thermal contraction cracking of the ground occurred together with sand drifting in a context of deep seasonal frost or sporadic discontinuous permafrost, unfavourable for the growth of significant ground-ice bodies. However, the description of composite-wedge pseudomorphs below 47°N indicates that at least locally ice veins formed probably during exceptionally cold winters. To provide a chronological framework for thermal contraction cracking single-grain OSL measurements were performed on 33 samples taken in the sandy infilling of sand-wedges and composite-wedge pseudomorphs. Results suggest that multiple events were recorded within wedges. The extraction of the datasets using the Finite Mixture Model, which was developed to analyse statistically data comprising multiple components, allowed calculating 86 ages. These age estimates show that wedge activity in France occurred at least 11 times over the last 100 ka. The most widespread events of thermal contraction cracking occurred between ca. 30 and 24 ka (Last Permafrost Maximum) and are concomitant with periods of high sand availability (MIS 2). Although most phases of sand-wedge growth correlate well with known Pleistocene cold periods, the identification of wedge activity during late MIS 5 and the very beginning of the Holocene strongly suggests that sand-wedges do not only indicate permafrost but also deep seasonal ground freezing in the context of low winter insolation. The previously published young ages yielded by North-European sand-wedges likely result from poor record of periglacial periods concomitant with low sand availability and/or age averaging inherent to standard luminescence methods. This work allowed us to propose a map of the maximum extent of Late Pleistocene permafrost in France, which partially reconciles field data with palaeoclimatic simulations. The remaining discrepancies may be linked with a potential time lag between the Last Permafrost Maximum (c. 31–24 ka) and the Last Glacial Maximum (21 ka) and to the already identified warm winter bias of the models.
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Eric Andrieux. Extent and chronology of the Pleistocene permafrost in France : database of periglacial structures and OSL dating of sand wedges.. Earth Sciences. Université de Bordeaux, 2017. English. ⟨NNT : 2017BORD0615⟩. ⟨tel-02191524⟩

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