ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-5135-2010 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 Aydin M. 1 Montzka S. A. 2 Battle M. O. 3 Williams M. B. 6 De Bruyn W. J. 4 Butler J. H. 2 Verhulst K. R. 1 Tatum C. 5 Gun B. K. 1 Plotkin D. A. 3 Hall B. D. 2 Saltzman E. S. 1 Department of Earth System Science, University of California, Irvine, California, USA Earth System Research Laboratories – Global Monitoring Division, National Oceanic and Atmospheric Administration, Boulder, Colorado, USA Department of Physics and Astronomy, Bowdoin College, Brunswick, Maine, USA Schmid College of Science, Chapman University, Orange, California, USA Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, USA ICF International, San Francisco, California, USA 09 06 2010 10 11 5135 5144 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/10/5135/2010/acp-10-5135-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/5135/2010/acp-10-5135-2010.pdf

In this study, we report measurements of CFC-12 (CCl<sub>2</sub>F<sub>2</sub>) 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.

 References % vor jede Referenz Albert, M. R., Shultz, E. F., and Perron, F. E.: Snow and firn permeability at Siple Dome, Antarctica, Ann. Glaciol., 31(1), 353–356, 2000. Anklin, M., Schwander, J., Stauffer, B., Tschumi, J., and Fuchs, A.: CO<sub>2</sub> record between 40 and 8 kyr B.P. from the Greenland Ice Core Project ice core, J. Geophys. Res., 102, 26539–26545, 1997. Aydin, M., Williams, M. B., Tatum, C., and Saltzman, E. S.: Carbonyl sulfide in air extracted from a South Pole ice core: a 2000 year record, Atmos. Chem. Phys., 8, 7533–7542, doi:10.5194/acp-8-7533-2008, 2008. Aydin, M., Williams, M. B., and Saltzman, E. S.: Feasibility of reconstructing paleoatmospheric records of selected alkanes, methyl halides, and sulfur gases from Greenland ice cores, J. Geophys. Res., 112, D07312, doi:10.1029/2006JD008027, 2007. Aydin, M., Saltzman, E. S., De Bruyn, W. J., Montzka, S. A., Butler, J. H., and Battle, M.: Atmospheric variability of methyl chloride during the last 300 years from an Antarctic ice core and firn air, Geophys. Res. Lett., 31, L02109, doi:10.1029/2003GL018750, 2004. Aydin, M., DeBruyn, W. J., and Saltzman, E. S.: Preindustrial atmospheric carbonyl sulfide (OCS) from an Antarctic ice core, Geophys. Res. Lett., 2, 1359, doi:10.1029/2002GL014796, 2002. Battle, M., Bender, M. L., Sowers, T., Tans, P. P., Butler, J. H., Elkins, J. W., Ellis, J. T., Conway, T., Zhang, N., Lang, P., and Clarke, A. D.: Atmospheric gas concentrations over the past century measured in air from firn at the South Pole, Nature, 383, 231–235, 1996. Bender, M. L.: Orbital tuning chronology for the Vostok climate record supported by trapped gas composition, Earth Planet. Sci. Lett., 204, 275–289, 2002. Bender, M. L., Sowers, T., and Lipenkov, V.: On the concentrations of O<sub>2</sub>, N<sub>2</sub>, and Ar in trapped gases from ice cores, J. Geophys. Res., 100, 18651–18660, 1995. Bender, M. L., Sowers, T., Barnola, J.-M., and Chappellaz, J.: Changes in the O2/N2 ratio of the atmosphere during recent decades reflected in the composition of air in the firn at Vostok station, Antarctica, Geophys. Res. Lett., 21(3), 189–192, 1994. Brook, E. J., Sowers, T., and Orchardo, J.: Rapid variations in atmospheric methane concentration during the last 110,000 years, Science, 273, 1087–1091, 1996. Butler, J. H., Battle, M., Bender, M. L., Montzka, S. A., Clarke, A. D., Saltzman, E. S., Sucher, C. M., Severinghaus, J. P., and Elkins, J. W.: A record of atmospheric halocarbons during the twentieth century from polar firn air, Nature, 399, 749–755, 1999. Clerbaux, E., Cunnold, D. M., Anderson, J., Engel, A., Fraser, P. J., Mahieu, E., Manning, A., Miller, J., Montzka, S. A., Nassar, R., Prinn, R., Reimann, S., Rinsland, C. P., Simmonds, P., Verdonik, D., Weiss, R., Wuebbles, D., and Yokouchi, Y.: Long-lived compounds, in Scientific Assessment of Ozone Depletion: 2006, Global Ozone Research and Monitoring Project – Report No 50, edited by: W. M. Organization, World Meteorological Organization, Geneva, 2006. Conway, T. J., Andrews, A. E., Bruwhiler, L., Crotwell, A., Dlugokencky, E. J., Hahn, M. P., Hirsch, A. I., Kitzis, D. R., Lang, P. M., Masarie, K. A., Michalak, A. M., Miller, J. B., Novelli, P. C. Peters, Tans, P. P., Thoning, K. W. Vaughn, B. H., and Zhao, C.: Carbon cycle greenhouse gases, in Climate Monitoring and Diagnostics Laboratory Summary Report No. 27 2002–2003, Chapter. 2, edited by: Schnell, R., Buggle, D. A.-M., and Rosson, R., Natl. Oceanic and Atmos. Admin., Boulder, Colo., 32–57, 2004. Courville, Z. R., Albert, M. R., Fahnestock, M. A., Cathles IV, L. M., and Shuman, C. A.: Impacts of an accumulation hiatus on the physical properties of firn at a low-accumulation polar site, J. Geophys. Res., 112, F02030, doi:10.1029/2005JF000429, 2007. Craig, H., Horibe, Y., and Sowers, T.: Gravitational separation of gases and isotopes in polar ice caps, Science, 242, 1675–1678, 1988. Delmas, R. J.: A natural artifact in Greenland ice core CO<sub>2</sub> measurements, Tellus, 45B, 391–396, 1993. Etheridge, D. M., Steele, L. P., Francey, R. J., and Langenfelds, R. L.: Atmospheric methane between 1000 A.D. and present: Evidence of anthropogenic emissions and climatic variability, J. Geophys. Res., 103(D13), 15979–15993, 1998. Etheridge, D. M., Steele, L. P., Langenfelds, R. L., Francey, R. J., Barnola, J.-M., and Morgan, V. I.: Natural and anthropogenic changes in atmospheric CO<sub>2</sub> over the last 1000 years from air in Antarctic ice and firn, J. Geophys. Res., 101(D2), 4115–4128, 1996. Etheridge, D. M., Pearman, G. I., and Fraser, P. J.: Changes in tropospheric methane between 1841 and 1978 from a high accumulation-rate Antarctic ice core, Tellus, 44B, 282–294, 1992. Haan, D. and Raynaud, D.: Ice core record of CO variations during the last two millennia: atmospheric implications and chemical interactions within the Greenland ice, Tellus, 50B, 253–262, 1998. Ikeda-Fukazawaa, T., Fukumizu, K., Kawamura, K., Aoki, S., Nakazawa, T., and Hondoh, T.: Effects of molecular diffusion on trapped gas composition in polar ice cores, Earth Planet. Sci. Lett., 229, 183–192, 2005. Lüthi, D., Le Floch, M., Bereiter, B., Blunier, T., Barnola, J.-M., Siegenthaler, U., Raynaud, D., Jouzel, J., Fischer, H., Kawamura, K., and Stocker, T. F.: High-resolution carbon dioxide concentration record 650,000–800,000 years before present, Nature, 453, 379–382, doi:10.1038/nature06949, 2008. MacFarling Meure, C., Etheridge, D. M., Trudinger, C., Steele, P., Langenfelds, R., van Ommen, T., Smith, A., and Elkins, J.: Law Dome CO<sub>2</sub>, CH<sub>4</sub> and N<sub>2</sub>O ice core records extended to 2000 years BP, Geophys. Res. Lett., 33, L14810, doi:10.1029/2006GL026152, 2006. Martinerie, P., Raynaud, D., Etheridge, D. M., Barnola, J.-M., and Mazaudier, D.: Physical and climatic parameters which influence the air content in polar ice, Earth Planet. Sci. Lett., 112, 1–13, 1992. Mischler, J. A., Sowers, T. A., Alley, R. B., Battle, M. O., McConnell, J. R., Mitchell, L., Popp, T., Sofen, E., and Spencer, M. K.: Carbon and hydrogen isotopic composition of methane over the last 1000 years, Global BioGeoChem. Cy., 23, GB4024, doi:10.1029/2009GB003460, 2009. Montzka, S. A., Aydin, M., Battle, M., Butler, J. H., Saltzman, E. S., Hall, B. D., Clarke, A. D., Mondeel, D., and Elkins, J. W.: A 350-year atmospheric history for carbonyl sulfide inferred from Antarctic firn air and air trapped in ice, J. Geophys. Res., 109, D22302, doi:10.1029/2004JD004686, 2004. Petit, J. R., Jouzel, J., Raynaud, D., Barkov, N. I., Barnola, J.-M., Basile, I., Bender, M., Chappellaz, J., Davis, M., Delaygue, G., Delmotte, M., Kotlyakov, V. M., Legrand, M., Lipenkov, V. Y., Lorius, C., Pepin, L., Ritz, C., Saltzman, E. S., and Stievenard, M.: Climate and atmospheric history of the past 420,00 years from the Vostok ice core, Antarctica, Nature, 399, 426–436, 1999. Saito, T., Yokouchi, Y., Aoki, S., Nakazawa, T., Fujii, Y., and Watanabe, O.: Ice-core record of methyl chloride over the last glacial-Holocene climate change, Geophys. Res. Lett., 34, L03801, doi:10.1029/2006GL028090, 2007. Saltzman, E. S., Aydin, M., Tatum, C., and Williams, M. B.: 2,000-year record of atmospheric methyl bromide from a South Pole ice core, J. Geophys. Res., 113, D05304, doi:10.1029/2007JD008919, 2008. Schwander, J., Barnola, J. -M., Andrie, C., Leuenberger, M., Ludin, A., Raynaud, D., and Stauffer, B.: The age of the air in the firn and ice at Summit, Greenland, J. Geophys. Res., 98, 2831–2838, 1993. Schwander, J.: The transformation of snow to ice and the occlusion of gases, The Environmental Record in Glaciers and Ice Sheets, edited by: Oeschger, H. and Langway, C. C., Wiley, New York, 53–67, 1989. Schwander, J., Stauffer, B., and Sigg, A.: Air mixing in firn and the age of the air at pore close-off, Ann. Glaciol., 10, 141–145, 1988. Severinghaus, J. P. and Battle, M. O.: Fractionation of gases in polar ice during bubble close-off: New constraints from firn air Ne, Kr and Xe observations, Earth Planet. Sci. Lett., 244, 474–500, 2006. Severinghaus, J. P., Grachev, A., and Battle, M.: Thermal fractionation of air in polar firn by seasonal temperature gradients, Geochem. Geophys. Geosys., 2, 2000GC000146 ISSN: 1525-2027, 2001. Sowers, T., Alley, R. B., and Jubenville, J.: Ice core records of atmospheric N<sub>2</sub>O covering the last 106,000 Years, Science, 301, 945–948, 2003. Sowers, T., Bender, M. L., and Raynaud, D.: Elemental and isotopic composition of occluded O<sub>2</sub> and N<sub>2</sub> in polar ice, J. Geophys. Res., 94, 5137–5150, 1989. Sturges, W. T., McIntyre, H. P., Penkett, S. A., Chappellaz, J. A., Barnola, J.-M., Mulvaney, R., Atlas, E., and Stroud, V.: Methyl bromide, other brominated methanes and methyl iodide in polar firn air, J. Geophys. Res., 106, 1595–1606, 2001a. Sturges, W. T., Penkett, S. A., Barnola, J.-M., Chappellaz, J., Atlas, E., and Stroud, V.: A long-term record of carbonyl sulfide (COS) in two hemispheres from firn air measurements, Geophys. Res. Lett., 28, 4095–4098, 2001b. Sturrock, G. A., Etheridge, D. M., Trudinger, C. M., and Fraser, P. J.: Atmospheric histories of halocarbons from analysis of Antarctic firn air: Major Montreal Protocol species, J. Geophys. Res., 107(D24), 4765, doi:10.1029/2002JD002548, 2002. Trudinger, C. M., Etheridge, D. M., Sturrock, G. A., Fraser, P. J., Krummel, P. B., and McCulloch, A.: Atmospheric histories of halocarbons from analysis of Antarctic firn air: Methyl bromide, methyl chloride, chloroform, and dichloromethane, J. Geophys. Res., 109(D22), D22310, doi:10.1029/2004JD004932, 2004. Van der Veen, C. J., Mosley-Thompson, E., Gow, A. J., and Mark, B. G.: Accumulation at South Pole: Comparison of two 900-year records, J. Geophys. Res., 104(D24), 31067–31076, 1999. Walker, S. J., Weiss, R. F., and Salameh, P. K.: Reconstructed histories of the annual mean atmospheric mole fractions for the halocarbons CFC-11, CFC-12, CFC-113, and carbon tetrachloride, J. Geophys. Res., 105, 14285–14296, 2000. Weis, R. F., Bullister, J. L., Gammon, R. H., and Warner, M. J.: Atmospheric chlorofluoromethanes in the deep equatorial Atlantic, Nature, 314, 608–610, 1985. Williams, M. B., Aydin, M., Tatum, C., and Saltzman, E. S.: A 2000 year atmospheric history of methyl chloride from a South Pole ice core: Evidence for climate-controlled variability, Geophys. Res. Lett., 34, L07811, doi:10.1029/2006GL029142, 2007.