EGU2020-12753, updated on 12 Jun 2020
https://doi.org/10.5194/egusphere-egu2020-12753
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Oxygen-to-nitrogen ratios in 1.5-million-year-old ice cores from Allan Hills Blue Ice Areas: implications for the long-term atmospheric oxygen concentrations

Yuzhen Yan1,2, Michael Bender1,3, Edward Brook4, Heather Clifford5, Preston Kemeny6, Andrei Kurbatov5, Sean Mackay7, Paul Mayewski5, Jessica Ng8, Jeffrey Severinghaus8, and John Higgins1
Yuzhen Yan et al.
  • 1Department of Geosciences, Princeton University, Princeton, NJ, USA
  • 2Department of Earth, Environmental and Planetary Sciences, Rice University, Houston, TX, USA (yuzhen.yan@rice.edu)
  • 3School of Oceanography, Shanghai Jiao Tong University, Shanghai, China
  • 4College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR, USA
  • 5Climate Change Institute, University of Maine, Orono, ME, USA
  • 6Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
  • 7Department of Earth and Environment, Boston University, Boston, MA, USA
  • 8Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA

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.

How to cite: Yan, Y., Bender, M., Brook, E., Clifford, H., Kemeny, P., Kurbatov, A., Mackay, S., Mayewski, P., Ng, J., Severinghaus, J., and Higgins, J.: Oxygen-to-nitrogen ratios in 1.5-million-year-old ice cores from Allan Hills Blue Ice Areas: implications for the long-term atmospheric oxygen concentrations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12753, https://doi.org/10.5194/egusphere-egu2020-12753, 2020

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Presentation version 1 – uploaded on 05 May 2020
  • CC1: Comment on EGU2020-12753, R.Valli Divya, 06 May 2020

    Why blue ice cores was used in this study?

    • AC1: Reply to R.Valli Divya, Yuzhen Yan, 06 May 2020

      Hi R.Valli, thanks for the question. The reason for using blue ice cores is that the continuous ice core record currently stops at 800 thousand years (ka). Only blue ice provides a discontinuous record beyond 800 ka. In our case, we measured O2/N2 ratios in samples dating back to 1.5 million years old, hence allowing us to infer the atmospheric oxygen concentrations back then.

      • CC2: Reply to AC1, R.Valli Divya, 07 May 2020

        Thank you for the explanation.

        Regards,

        Divya