HR: 1340h
AN: A53B-1160    [Abstracts]
TI: A 70 ka Record of Methyl Chloride From a Siple Dome, West Antarctica Ice Core
AU: * Verhuls, K
EM: kverhuls@uci.edu
AF: UC Irvine, Dept. of Earth System Science, Irvine, CA 92697, United States
AU: Aydin, M
EM: maydin@uci.edu
AF: UC Irvine, Dept. of Earth System Science, Irvine, CA 92697, United States
AU: Williams, M B
EM: mbwilliams@arc.nasa.gov
AF: Bay Area Env. Res. Inst./NASA Ames, Moffett Field, Moffett Field, CA 94035, United States
AU: Saltzman, E S
EM: esaltzma@uci.edu
AF: UC Irvine, Dept. of Earth System Science, Irvine, CA 92697, United States
AB: Methyl chloride (CH₃Cl) is most abundant naturally occurring halocarbon in the atmosphere, with a modern mean mixing ratio of 550 parts per trillion (ppt). Prior studies in Antarctic ice and firn suggest that reconstruction of a paleoatmospheric record may be possible (Williams et al., 2007), but there is also some evidence for in situ production in glacial polar ice with high dust content (Saito et al., 2007). In this study, CH₃Cl was extracted and analyzed in 42 samples from the Siple Dome, West Antarctic ice core covering the past 70 ka. Over the whole record, CH₃Cl mixing ratios ranged from 407 to 819 ppt. Glacial core samples from 30-70 ka had a mean of 455∓46 ppt, significantly lower than the Holocene mean of 532∓61 ppt in this core. There are no data from the period between ~25-33 ka. Four data points around the last glacial maximum between 21-25 ka are higher, with a mean level of 625∓141 ppt. It is unclear whether these higher CH₃Cl mixing ratios reflect higher atmospheric levels or in situ production within the ice. The observation of lower CH₃Cl levels in glacial ice compared to interglacial ice is in apparent contradiction with earlier measurements from the Dome Fuji, East Antarctica ice core, which showed increased CH₃Cl levels in glacial ice compared to interglacial ice (Saito et al., 2007). That core showed a strong correlation between CH₃Cl and Ca⁺² levels, implicating in situ production related to glacial dust. At Siple Dome, Ca⁺² levels are generally lower, and there is no correlation with CH₃Cl in the data set as a whole. However, the increased levels of CH₃Cl at the last glacial maximum may indicate in situ production in samples with the highest dust loads.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0322 Constituent sources and sinks
DE: 0325 Evolution of the atmosphere (1610, 8125)
DE: 0365 Troposphere: composition and chemistry
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting