The origin and stability of ground ice in the stable uplands of the McMurdo Dry Valleys remains poorly understood, with most studies focusing on the near-surface permafrost. The 2016 Friis Hills Drilling Project retrieved five cores reaching 50 m depth in mid-Miocene permafrost, a period when Antarctica transitioned to a hyper-arid environment. This study characterizes the cryostratigraphy of arguably the oldest permafrost on Earth and assesses 15 Myr of ground ice evolution using the REGO model. Four cryostratigraphic units were identified: 1) surficial dry permafrost (0–30 cm), 2) ice-rich to ice-poor permafrost (0.3–5.0 m) with high solute load and δ18O values (-16.2 ± 1.8‰) and low D-excess values (-65.6 ± 4.3‰), 3) near-dry permafrost (5–20 m) and 4) ice-poor to ice-rich permafrost (20–50 m) containing ice lenses with low solute load and δ18O values (-34.6 ± 1.2‰) and D-excess of 6.9 ± 2.6‰. The near-surface δ18O profile of ground ice is comparable to other sites in the stable uplands, suggesting that this ice is actively responding to changing surface environmental conditions and challenging the assumption that the surface has remained frozen for 13.8 Myr. The deep ice lenses probably originate from the freezing of meteoric water during the mid-Miocene, and their δ18O composition suggests mean annual air temperatures ~7–11°C warmer than today.

中文翻译：

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南极洲麦克默多干谷弗里斯山中中新世多年冻土的冰冻地层学

麦克默多干谷稳定高地地冰的起源和稳定性仍然知之甚少，大多数研究都集中在近地表永久冻土上。2016 年的 Friis Hills 钻探项目在中新世中期永久冻土中检索了 5 个深度达到 50 m 的岩心，这是南​​极洲过渡到超干旱环境的时期。这项研究描述了可以说是地球上最古老的永久冻土层的低温地层学特征，并使用 REGO 模型评估了 15 Myr 的地冰演化。确定了四个低温地层单元：1）表层干燥多年冻土（0-30 cm），2）具有高溶质负荷和δ的富冰至贫冰永久冻土（0.3-5.0 m）18O 值 (-16.2 ± 1.8‰) 和低 D 超值 (-65.6 ± 4.3‰), 3) 接近干燥的永久冻土 (5–20 m) 和 4) 贫冰到富含冰的永久冻土 (20–50 m) 含有低溶质负荷和 δ 的冰晶状体18O 值 (-34.6 ± 1.2‰) 和 D-超过 6.9 ± 2.6‰。近地表δ18地冰的 O 剖面与稳定高地的其他地点相当，这表明这种冰正在积极响应不断变化的地表环境条件，并挑战地表保持冻结 13.8 Myr 的假设。深冰透镜体可能起源于中新世中期大气水的冻结，它们的δ18O 成分表明年平均气温比今天高约 7–11°C。