Much of France remained unglaciated during the Late Quaternary and was subjected to repeated phases of periglacial activity. Numerous periglacial features have been reported but disentangling the environmental and climatic conditions they formed under, the timing and extent of permafrost and the role of seasonal frost has remained elusive. The primary sandy infillings of relict sand-wedges and composite-wedge pseudomorphs record periglacial activity. As they contain well-bleached quartz-rich aeolian material they are suitable for optically stimulated luminescence dating (OSL). This study aims to reconstruct when wedge activity took place in two regions of France; Northern Aquitaine and in the Loire valley. Results from single-grain OSL measurements identify multiple phases of activity within sand wedges which suggest 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) which 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 Younger Dryas strongly suggests that these features do not only indicate permafrost but also deep seasonal ground freezing in the context of low winter insolation. These data also suggest that the overall 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 with standard luminescence methods.

中文翻译：

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法国中部和南部沙楔 OSL 测年晚更新世热收缩裂缝的年代学

在第四纪晚期，法国的大部分地区仍然没有冰川，并且经历了多次冰缘活动。已经报道了许多冰缘特征，但解开它们形成的环境和气候条件、永久冻土的时间和范围以及季节性霜冻的作用仍然难以捉摸。残存沙楔和复合楔假形体的原始砂质充填物记录了冰缘活动。由于它们含有充分漂白的富含石英的风成物质，因此它们适用于光学受激发光测年 (OSL)。本研究旨在重建法国两个地区发生楔形活动的时间；阿基坦北部和卢瓦尔河谷。单粒 OSL 测量结果确定了沙楔内的多个活动阶段，这表明法国的沙楔活动在过去 100 ka 中至少发生了 11 次。最普遍的热收缩开裂事件发生在大约 30 和 24 ka（最后的永久冻土最大值）伴随着高砂可用性时期（MIS 2）。尽管沙楔生长的大多数阶段与已知的更新世寒冷时期密切相关，但在 MIS 5 晚期和新仙女木期间对楔活动的识别强烈表明，这些特征不仅表明多年冻土，而且还表明在低温环境下深部季节性地面冻结。冬季日晒。