Pleistocene to late Miocene ice core records of climate and atmospheric composition from Allan Hills, Antarctica

Under the auspices of the NSF Center for Oldest Ice Exploration and previous projects, multiple seasons of shallow ice core drilling in the Allan Hills Blue Ice Area have yielded ice samples as old as 6.7 Ma, and numerous younger samples beyond the current 800 ka limit of the traditional ice core record. The complex stratigraphy of the existing cores does not allow continuous time series. Instead, “snapshots” have been created by dating over 300 individual samples (with more coming) using the deficit in ⁴⁰Ar compared to modern air, and analyzing those samples for a range of environmental parameters. Parallel efforts are using continuous flow analysis, continuous electrical conductivity measurements, geophysical observations and surface transects to further understand the preservation and stratigraphy of environmental records in this unique region. This presentation will review recent results including evolving data sets from ice cores collected in the last two years. Primary observations include 1) long term Antarctic cooling of up to ~12 ˚C over the last 6 Ma based on the stable isotopic composition of the ice; 2) long-term mean ocean cooling over the last 3 Ma based on atmospheric noble gas ratios, with a prominent period of cooling coincident with the Plio-Pleistocene transition (~2.7 Ma) and steady temperatures across the mid-Pleistocene transition (1.2-0.8 Ma); 3) atmospheric CO₂ concentrations of less than 300 ppm in pristine ice back to 2.7 Ma, corroborated by similar levels reconstructed from ice samples affected by respiration near the glacier bed based on corrections using carbon isotopes or independent constraints based on mass independent fractionation of isotopes in O₂; 4) moderate atmospheric methane levels back to 3 Ma (generally less than 600 ppb) with evidence for biologically produced methane in samples near the glacier bed; 5) age reversals and inclined layering from 3 dimensional electrical and chemical measurements, and evidence for both pristine glacial ice and interactions with the glacier bed that alter the ice chemistry. These emerging new Antarctic ice core records are enhancing scientific understanding of Plio-Pleistocene climate and planetary evolution. Ongoing efforts in method development, ice coring, and geochemical analysis will continue to provide new insights from this enigmatic and challenging ice archive.