| Popis: |
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. |
