Post-coring entrapment of modern air in some shallow ice cores collected near the firn-ice transition: evidence from CFC-12 measurements in Antarctic firn air and 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
Received: 05 January 2010 – Published in Atmos. Chem. Phys. Discuss.: 21 January 2010 Abstract. In this study, we report measurements of CFC-12 (CCl2F2) in firn
air and in air extracted from shallow ice cores from three Antarctic sites.
The firn air data are consistent with the known atmospheric history of
CFC-12. In contrast, some of 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 the presence of modern air
entrapped in the shallow ice core samples that likely contained open pores
at the time of collection. We propose that this is due to closure of the
open pores after drilling, entrapping modern air and resulting in elevated
CFC-12 mixing ratios. Our results reveal that open porosity can exist below
the maximum depth at which firn air samples can be collected, particularly
at sites with lower accumulation rates. CFC-12 measurements demonstrate that
post-drilling closure of open pores can lead to a change in the composition
of bubble air in shallow ice cores through purely physical processes. The
results have implications for investigations involving trace gas composition
of bubbles in shallow ice cores collected near the firn-ice transition.
Revised: 22 April 2010 – Accepted: 22 May 2010 – Published: 09 June 2010
Citation: Aydin, M., Montzka, S. A., Battle, M. O., Williams, M. B., De Bruyn, W. J., Butler, J. H., Verhulst, K. R., Tatum, C., Gun, B. K., Plotkin, D. A., Hall, B. D., and Saltzman, E. S.: Post-coring entrapment of modern air in some shallow ice cores collected near the firn-ice transition: evidence from CFC-12 measurements in Antarctic firn air and ice cores, Atmos. Chem. Phys., 10, 5135-5144, doi:10.5194/acp-10-5135-2010, 2010.