Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 10 1 2010 10.5194/acpd-10-1631-2010 http://www.atmos-chem-phys-discuss.net/10/1631/2010/ http://www.atmos-chem-phys-discuss.net/10/1631/2010/acpd-10-1631-2010.html http://www.atmos-chem-phys-discuss.net/10/1631/2010/acpd-10-1631-2010.pdf 1631 1657 2010-01-21 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 M. Aydin maydin@uci.edu S. A. Montzka M. O. Battle M. B. Williams W. De Bruyn J. H. Butler K. R. Verhulst C. Tatum B. K. Gun D. A. Plotkin B. D. Hall E. S. Saltzman 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 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. 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.: \chemCO_2 record between 40 and 8 kyr BP 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, 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, \doi10.1029/2006JD008027, 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, \doi10.1029/2003GL018750, 2004. Aydin,~M., DeBruyn,~W J., and Saltzman,~E S.: Preindustrial atmospheric carbonyl sulfide (OCS) from an Antarctic ice core, Geophys. Res. Lett., 29, 9, 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 \chemO_2, \chemN_2, and Ar in trapped gases from ice cores, J Geophys. Res., 100, 18651–18660, 1995. Bender,~M L., Sowers,~T., Barnola,~J.-M., Chappellaz,~J.: Changes in the \chemO_2/\chemN_2 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, in: Global Ozone Research and Monitoring Project – Report No 50, edited by: World Meteorological 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., Rosson,~R., National Oceanic and Atmospheric Administration, Boulder, Colorado, 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, \doi10.1029/2005JF000429, 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 \chemCO_2 measurements, Tellus, 45B, 391–396, 1993. 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 \chemCO_2 over the last 1000 years from air in Antarctic ice and firn, J Geophys. Res., 101, D2, 4115–4128, 1996. 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. Sc. 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, \doi10.1038/nature06949, 2008. 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. Sc. 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, \doi10.1029/2009GB003460, 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, \doi10.1029/2004JD004686, 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, \doi10.1029/2006GL028090, 2007. Saltzman,~E S., Aydin,~M., Tatum,~C., and Williams,~M B.: 2000-year record of atmospheric methyl bromide from a~South Pole ice core, J Geophys. Res., 113, D05304, \doi10.1029/2007JD008919, 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, in: 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. Sc. 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. Geophy. Geosy., 2, 1048, doi:10.1029/2000GC000146, 2001. Sowers,~T., Alley,~R B., and Jubenville,~J.: Ice core records of atmospheric \chemN_2O 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 \chemO_2 and \chemN_2 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. 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, \doi10.1029/2004JD004932, 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, 31,067–31,076, 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, \doi10.1029/2006GL029142, 2007.