Evaluating Signal Attenuation and Gas Diffusion Impacts on Orbital Dating and Atmospheric Evolution of O2 Concentration in the Beyond EPICA Ice Core

Polar ice cores provide valuable insights into past environmental and climatic variability. The recently drilled Beyond EPICA ice core is believed to preserve the climate history of the past 1.5 million years (Ma). To effectively interpret the climatic information retrieved from this ice core, it is imperative to establish a precise chronology, assigning an age to each individual depth level. Dating old ice strongly relies on orbital tuning of the isotopic and elemental composition of atmospheric oxygen and nitrogen, extracted from air bubbles enclosed within the ice to variations in solar insolation. However, extensive layer thinning and enhanced vertical gas diffusion in the deep section of an ice core can substantially impact the signal preservation of these dating tools. Recently obtained data of δ¹⁸O of atmospheric O₂ (δ¹⁸O_atm) from the deepest 100 metres, possibly spanning from ~0.7 to 1.5 Ma, reveal a significant attenuation of the orbital signal amplitude prior to 1.1 Ma, challenging precise orbital dating. Diffusion effects however does not impede reconstruction of the long-term trend in O₂ atmospheric concentration and isotopic composition. While δ¹⁸O_atm remains stable throughout this period, the relative concentration of atmospheric O₂ distinctly decreases around 0.9 Ma. Although this long-term trend with time aligns with previous observations during more recent periods, the natural variation of O₂ concentration in the atmosphere potentially poses an additional difficulty for orbital dating accuracy in which the δ(O₂/N₂) orbital signal is driven by local insolation. We propose using δ(Ar/N₂) as a supplementary dating tool, as this ratio exhibits a similar relationship with local insolation while being independent of oxygen. Further, the new deep Beyond EPICA δ(Ar/N₂) record reveals a higher signal amplitude in the deepest section compared to δ(O₂/N₂) because Ar is expected to diffuse less than O₂, thereby enhancing the potential for orbital dating. Thus, integrating atmospheric δ¹⁸O, δ(O₂/N₂), and δ(Ar/N₂) can facilitate to establish a chronology for the Beyond EPICA ice core, provided that high-precision and high-resolution data are ensured.