HR: 1340h
AN: A53B-1159    [Abstracts]
TI: C2-C4 alkanes measured in a South Pole ice core: Are atmospheric histories of light hydrocarbons preserved in Antarctic ice?
AU: * Williams, M B
EM: mbwilliams@arc.nasa.gov
AF: Bay Area Environmental Research Institute / NASA- Ames Research Center, 245-5 Moffett Field, Moffett Field, CA 94035, United States
AU: Aydin, M
EM: maydin@uci.edu
AF: Department of Earth System Science, Croul Hall University of California, Irvine, Irvine, CA 92697, United States
AU: Saltzman, E S
EM: esaltzma@uci.edu
AF: Department of Earth System Science, Croul Hall University of California, Irvine, Irvine, CA 92697, United States
AB: Non-methane hydrocarbons play a significant role in global atmospheric photochemical system, but we have little knowledge about their atmospheric variability on long time scales. In this study, we analyze ethane (C₂H₆), propane (C₃H₈) and n-butane (C₄H₁₀) in a South Pole ice core, to examine the potential for using polar ice cores to reconstruct atmospheric histories of these gases. Air was dry- extracted from 124 ice core samples from the SPRESSO core, a 295 m core drilled in 2002 near South Pole as part of the ITASE campaign. The mean gas ages estimated for these samples range from 150 B.C.E. to 1720 C.E. The average mixing ratios for ethane, propane, and n-butane are 798±417 ppt, 234±89 ppt and 117±48 ppt, respectively. Point to point variability generally lies within the estimate of analytical uncertainty. These measurements demonstrate that 1) recoverable amounts of light hydrocarbons exist in polar ice, and 2) there is no evidence of down-core trends to suggest systematic loss or production with time. However, the hydrocarbon levels in this ice core are considerably higher than those in modern air over Antarctica (by factors of 2, 7, and 10 respectively for ethane, propane, and n-butane). The ice core data are not normally distributed, but appear to have a lower limit with superimposed variability. These "baseline levels" are roughly 300 ppt (C₂H₆), 80 ppt (C₃H₈), and 40 ppt (C₄H₁₀) and are consistent with modern ambient air and firn air levels measured at South Pole. A working hypothesis to explain these results is that alkane levels in ice reflect a combination of two components: 1) entrapped air possibly recording the atmospheric histories of these gases, and 2) some source of alkane contamination that is generated at or near bubble close-off, but does not continue at depth. It is unlikely that the elevated alkane levels reflect contamination during storage, extraction, or analysis.
DE: 0365 Troposphere: composition and chemistry
DE: 0490 Trace gases
DE: 0724 Ice cores (4932)
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting