Post-coring entrapment of modern air in polar ice cores collected near the firn-ice transition: evidence from CFC-12 measurements in Antarctic firn air and shallow ice cores
1Department of Earth System Science, University of California, Irvine, California, USA
2Earth System Research Laboratories – Global Monitoring Division, National Oceanic and Atmospheric Administration, Boulder, Colorado, USA
3Department of Physics and Astronomy, Bowdoin College, Brunswick, Maine, USA
4Schmid College of Science, Chapman University, Orange, California, USA
5Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, USA
6ICF International, San Francisco, California, USA
Abstract. In this study, we report the first measurements of CFC-12 (CCl2F2) in air extracted from shallow ice cores along with firn air CFC-12 measurements from three Antarctic sites. The firn air data are consistent with the known atmospheric history of CFC-12. In contrast, the ice core samples collected near the firn-ice transition exhibit anomalously high CFC-12 levels. Together, the ice core and firn air data provide evidence for presence of modern air entrapped in shallow ice core samples. We propose that this is due to closure of open pores after drilling, entrapping modern air and resulting in elevated CFC-12 mixing ratios. Our measurements reveal the presence of open porosity below the depth at which firn air samples can be collected and demonstrate how the composition of bubble air in shallow ice cores can be altered during the post-drilling period through purely physical processes. These results have implications for investigations involving trace gas composition of bubbles in shallow ice cores.