Late Pliocene and Early Pleistocene CO2 and CH4 from ice cores from the Allan Hills, Antarctica

Currently, chronologically discontinuous ice cores from the Allan Hills Blue Ice Area (BIA), Antarctica, are our only direct insight into the atmospheric composition of periods beyond the continuous ice core record (800 ka BP). An accurate and precise greenhouse gas history beyond 800 ka would aid understanding of the mechanisms involved in the climatic transitions across the late Pliocene and early Pleistocene. Here we present carbon dioxide (CO₂) and methane (CH₄) results from a new core from the Allan Hills BIA (ALHIC1901). The bottom 25 m of ALHIC1901 contain 52 sampled depths with co-registered ⁴⁰Ar_atm dates (Shackleton et al. in prep), measurements of δD of ice, δ¹⁸O_atm, and concentrations of CO₂ and CH₄ in trapped air. Of these samples, 25 are older than the continuous ice core record, with ages from 821 ± 80 ka to 2700 ± 270 ka. The bottom meter contains ice from the Pliocene with ages from 2700 ± 270 ka to 4000 ± 400 ka. The carbon isotope ratio of CO₂ (δ¹³C-CO₂) was measured on 18 samples to examine the possibility of input of non-atmospheric CO₂ from oxidation of organic matter. Our results indicate that CO₂ and CH₄ levels were similar in the early Pleistocene to those found for the last 800 ka. A small decline of approximately 20 ppm is seen in CO₂ across the Pleistocene, and no secular trend is observed in CH₄. Pliocene-aged samples appear to contain a mixture of atmospheric CO₂ and CO₂ derived from respiration of organic matter at the glacier bed. Using an isotope mixing model we estimate that atmospheric CO₂ was lower than 350 ppm at ~3.1 Ma,