First Δ¹⁷O of O₂ Data from Beyond EPICA Ice Core across the Mid-Pleistocene Transition (1.2 to 0.7 million years)

Within the framework of the Beyond EPICA project, the oldest continuous Antarctic ice core recovered to date provides, for the first time, direct measurements of the atmospheric composition over (at least) the past 1.2 million years (hereafter Ma), through the analysis of the air bubbles trapped in the ice. This interval encompasses the Mid-Pleistocene Transition, marked by a shift in the dominant climatic periodicity from ~40 ka to ~100 . Interestingly, preliminary results show that this time interval also witnesses a long-term decline in the atmospheric dioxygen (O₂). Despite being a period of major reorganization of the Quaternary glacial–interglacial climate system, the processes that drove the MPT remain debated. Several hypotheses have been proposed, involving changes in ice-sheet dynamics, ocean circulation or the radiative forcing of atmospheric CO₂. Because atmospheric CO₂ and O₂ are tightly coupled through biogeochemical processes, investigating the evolution of atmospheric O₂ across the MPT may provide additional constraints on the mechanisms underlying this fundamental climatic shift. Here, we focus on the concentration of the triple isotopic composition of O₂ (Δ¹⁷O(O₂)), a proxy for global biosphere productivity when interpreted together with CO₂ concentration. We present the evolution of Δ¹⁷O(O₂) including the first continuous records from the deepest section of the Beyond EPICA ice core (from 2400 m to 2507 m). This record has been obtained as a by-product measurement when analysing the O₂, N₂ and Ar elemental and isotopic composition hence without O₂ purification. Although characterized by relatively low analytical precision so far, the dataset offers comparatively high temporal resolution. The new Δ¹⁷O(O₂) record also confirms previously reported trends over the last 0.7–0.8 Ma, supporting its robustness for investigating both long-term changes and glacial-interglacial variability. We compare the long-term evolution of Δ¹⁷O (O₂) with the observed decrease in atmospheric O₂, and examine the amplitudes of glacial–interglacial variations in Δ¹⁷O (O₂) relative to those of CO₂ over the past 1.2 Ma. It appears that variations in the Δ¹⁷O (O₂)–CO₂ relationship during specific glacial – interglacial cycles could reflect changes in the biosphere productivity warranting further investigation.