Triple Isotopes of Oxygen as a Proxy for Carbon Dioxide in Altered Early Pleistocene Ice Core Samples

Traditional ice cores provide invaluable insights into changes in past atmospheric greenhouse gas levels, like carbon dioxide (CO 2 ), and their relationship to climate over the last 800,000 years. Recently, ice as old as 6 million years (Ma) has been retrieved from the Allan Hills Blue Ice Area (BIA), Antarctica. However, CO 2 levels in samples older than 3 Ma are anomalously elevated by post‐depositional processes. In this work, we explore the viability of using triple isotopic composition of atmospheric oxygen (Δ 17 O‐O 2 ) as an independent approach for reconstructing atmospheric CO 2 from such altered samples. Atmospheric Δ 17 O‐O 2 and CO 2 are dynamically linked via photochemically driven isotope exchange reactions in the stratosphere. While Δ 17 O‐O 2 is also modulated by global biospheric productivity (GPP), its observed covariation with CO 2 on glacial‐interglacial timescales indicate that Δ 17 O‐O 2 , to first order, is affected by atmospheric CO 2 levels. We employ this covariation to reconstruct atmospheric CO 2 concentrations from both pristine and altered ice samples from the Allan Hills BIA back to 2.7 Ma. We find the Δ 17 O‐O 2 accurately reproduces pristine CO 2 values to within 18 ± 15 ppm (1σ) and confirms recent direct observations that long‐term mean values remained below 300 ppm for the entire Pleistocene epoch. Additionally, our results exhibit a secular ∼20 ppm decline in mean CO 2 from the early to mid‐Pleistocene transition (2.7–1.2 Ma), followed by stabile values across the MPT. Finally, we obtain first‐order estimates of early‐mid Pleistocene (2.4–0.8 Ma) GPP using paired Δ 17 O‐O 2 and CO 2 measurements in pristine samples.