Cryoplanation terraces are prominent but enigmatic landforms found in present and past periglacial environments. Geomorphologists have debated for more than a century over processes involved in the formation of these elevated, step-like, bedrock features. Presented here are the first numerical surface exposure ages and scarp retreat rates from cryoplanation terraces in the Yukon-Tanana Upland (YTU) in Alaska, part of unglaciated eastern Beringia, obtained from terrestrial cosmogenic nuclides (TCN) in surface boulders. Ages comprise six 10Be TCN ages from two terrace treads near Eagle Summit and six 36 Cl ages from two treads on Mt. Fairplay. Based on these exposure ages, scarps at both locations were last actively eroding from 49 to 22.4 ka. Both locations exhibit time-transgressive development, particularly near scarp-tread junctions. Boulder exposure ages and distances between sampled boulder locations were used to estimate scarp retreat rates of 0.11 to 0.56 cm/yr. These numerical exposure ages presented here demonstrate that the cryoplanation terraces in the YTU are diachronous surfaces actively eroding during multiple cold intervals. With these results, hypotheses for cryoplanation terrace formation are discussed and evaluated for the YTU, including those based on geologic structure, nivation, and the influence of permafrost.

中文翻译：

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阿拉斯加育空-塔纳纳高地冰冻平台的宇宙成因 10Be 和 36Cl 年代学

冰冻平台是在现在和过去的冰缘环境中发现的突出但神秘的地貌。一个多世纪以来，地貌学家一直在争论这些高架、阶梯状基岩特征的形成过程。这里介绍的是阿拉斯加育空-塔纳纳高地 (YTU) 冰冻平台的第一个数字地表暴露年龄和陡坡退缩率，该平台是白令洲东部无冰川的一部分，从地表巨石中的陆地宇宙成因核素 (TCN) 获得。年龄包括六10从 Eagle Summit 附近的两个露台踏板和六个36Cl 年龄来自 Mt. Fairplay 的两个踏板。根据这些暴露年龄，这两个位置的陡崖最后从 49 到 22.4 ka 积极侵蚀。这两个地点都表现出时间超越的发展，特别是在陡坡-胎面交界处附近。巨石暴露年龄和采样巨石位置之间的距离用于估计 0.11 至 0.56 厘米/年的陡崖退缩率。这里提出的这些数字暴露年龄表明，YTU 中的冰冻平台是在多个冷间隔期间积极侵蚀的历时表面。根据这些结果，对 YTU 的冰冻平台形成假设进行了讨论和评估，包括基于地质结构、成因和永久冻土影响的假设。