UC San Diego

Gases preserved in ice cores provide a potential direct archive for atmospheric oxygen. Yet, oxygen-to-nitrogen ratios in ice cores (expressed as δO₂/N₂) are modified by a number of processes related to gas trapping and gas losses in the ice. Such complications have long hindered the use of ice core δO₂/N₂ to derive true atmospheric oxygen concentrations. Recently, a persistent decline in δO₂/N₂, observed in four different ice cores (GISP2, Vostok, Dome F, and EDC), is interpreted to reflect decreasing atmospheric O₂ concentrations over the late Pleistocene (Stolper et al., 2016). The rate of δO₂/N₂ change is -8.4±0.2 ‰/Myr (1σ). Using new measurements made on EDC samples stored at -50 °C and therefore free from gas loss, Extier et al (2018) confirms the decrease in δO₂/N₂ with a slope of -7.0±0.6‰/Myr (1σ).

Here, we present new δO₂/N₂ measurements made on 1.5-million-year-old blue ice cores from Allan Hills Blue Ice Areas, East Antarctica. We use argon-to-nitrogen ratios (δAr/N₂) in the ice to correct for the fractionations during bubble close-off and gas losses. In those processes, δAr/N₂ is fractionated in a fashion similar to δO₂/N₂ (Huber et al., 2006; Severinghaus and Battle, 2006). Paired δO₂/N₂-δAr/N₂ values measured from the same sample were classified into three different time slices: 1.5 Ma (million years old), 950 ka, and 490 ka. Between 950 ka and 490 ka, we observe a decline in δO₂/N₂ similar to that observed in the aforementioned deep ice cores. This observation gives us confidence in the validity of the Allan Hills blue ice δO₂/N₂ records. Between 1.5 Ma and 950 ka, however, there is no statistically significant trend in ice core δO₂/N₂. Our results show a surprising lack of variability from 1.5 to 0.95 Ma; even during the past ~0.9 Ma, the rate of decline was very slow.