Miocene and Pliocene Ice and Air from the Allan Hills Blue Ice Area, East Antarctica. PNAS. S. Shackleton et al. 2025 – A. Kurbatov, C. Introne and others

 www.pnas.org/doi/10.1073/pnas.2502681122 with DOI number 10.1073/pnas.2502681122

Paper reports the discovery of ice, that is up to 6 million years old. Isotopic temperatures from this ice indicate progressive cooling over the Pliocene; enigmatic basal ice from the Miocene is characterized by even warmer temperatures and may reflect a relic from the adolescent days of the Antarctic ice sheet. This archive opens up the possibility of reconstructing Earth’s climate and its largest ice sheet during periods when Earth’s climate was warmer and sea-level was higher.

Significance

Antarctic ice cores provide the most direct archive of Earth’s atmosphere and its largest ice sheets. We report the discovery of ice, dated by its deficit in ⁴⁰Ar compared to the modern atmosphere, that is up to 6 million years old. Isotopic temperatures from this ice indicate progressive cooling over the Pliocene; enigmatic basal ice from the Miocene is characterized by even warmer temperatures and may reflect a relic from the adolescent days of the Antarctic ice sheet. This archive opens up the possibility of reconstructing Earth’s climate and its largest ice sheet during periods when Earth’s climate was warmer and sea-level was higher.

Abstract

Antarctic ice cores provide a unique archive of Earth’s atmosphere and its largest extant ice sheet. The oldest continuous ice core extends back 800 ky, though discontinuous ice cores from the Allan Hills blue ice area (BIA) have been shown to preserve snapshots of ice and air back to at least 2.7 million years ago (Ma). Here, we provide snapshots of putatively Miocene and Pliocene ice and air from shallow ice cores drilled in the Allan Hills BIA. The ice, dated using the deficit in ⁴⁰Ar in ancient air compared to the modern atmosphere, is stratigraphically complex. Nevertheless, surface temperatures inferred from water isotopes correlate with sample age and indicate 12 ± 2 °C of cooling in Antarctica between 6 Ma and the late Pleistocene. Basal ice is nearly devoid of gases and remains to be dated with existing methods. This undated ice is characterized by an isotopic temperature 5 ± 1 °C warmer than the oldest dated (6 million year old) sample. We speculate that this ice reflects surface snowpack or permafrost that was preserved by the growth of the East Antarctic ice sheet in the Middle to Late Miocene.