Miocene and Pliocene ice and air from the Allan Hills blue ice area, East Antarctica

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.

Plain-text abstract

We report the existence of remarkably old ice (6 million years) from the Allan Hills region of East Antarctca. The ice at this particular site is remarkable because it contains pressurized air bubbles that contain air samples that have been sealed from the rest of the atmosphere for far longer than those found anywhere else in the world ("old ice" usually contains "old air"). Dates for these samples of old air can be determined because certain radioactive particles present in all air decay at a certain known rate to certain other known particles. Measuring the ratio of the decayed particles to undecayed particles allows us to make age calculations since that air was sealed off from the atmosphere. The ice at the bottom of the core contained very little air bubbles, so has not been dated using this technique. However, using other techniques, we think it could be older than about 6 million years—possibly dating back to when the Antarctic Ice Sheet was in its very beginning stages.