HR: 1340h
AN: A53B-1164    [Abstracts]
TI: Methane isotope records from Antarctic firn air
AU: * Sowers, T A
EM: sowers@geosc.psu.edu
AF: Earth and Environment Systems Institute, 237 Deike Bldg, Dept. of Geoscience, University Park, PA 16802, United States
AU: Battle, M
EM: mbattle@bowdoin.edu
AF: Dept. of Physics and Astronomy, 8800 College Station Bowdoin College, Brunswich, ME 04011-8488, United States
AB: The earth's climate is directly influenced by changes in the atmospheric loading of trace "greenhouse" gases. Methane is an important greenhouse gas whose atmospheric concentration has doubled over the last century as a result of various anthropogenic activities. Understanding the methane cycle in the recent past is a compelling scientific problem because increasing methane levels in the modern atmosphere will contribute to predicted future warming. The only source of air that allows us to study the long-term cycles of these gases is found in the trapped gases in ice cores and the interstitial pore spaces in firn from the central portions of Greenland and Antarctica. Firn air samples, retrieved from the South Pole in 1995 and 2001, were analyzed for δ¹³CH₄. In December of 2005, another suite of firn air samples was recovered from the new US deep coring site located along the west Antarctic divide (WAIS D) which were analyzed for both δ¹³C and δ D of CH₄. In general, replicate flasks were sampled from each depth from two separate drillings each year. The precision of the δ¹³C and δ D analyses is generally better than ± 0.2‰ and 2‰, respectively, with surface values in good agreement with NOAA (INSTAAR) biweekly flask measurements from South Pole. In all cases, δ¹³C and δD values decrease gradually with depth below the convective zone. However, these isotopes and CH₄ mixing ratios decline sharply below the "lock-in" depth. The measured firn air profiles are primarily the result of three factors. First, tropospheric changes drive compositional changes in the near-surface convective layer that are subsequently mixed downward due to bulk air-movement within the shallow firn. Secondly, gases are mixed below the convective zone solely by molecular diffusion, leading to the gradual downward propagation of the compositional changes in the convective layer. Finally, differential gravitational settling causes the heavier atmospheric constituents (both elemental and isotopic) to accumulate deeper in the firn relative to the lighter gases. We use δ¹⁵N of N₂ measurements to identify the depth of the convective zone and to correct for gravitational settling. We developed a physically based air transport model to account for the diffusive properties of the firn that allows us to test various historical records spanning the 20^th century in a forward modeling exercise. Preliminary results suggest atmospheric δ¹³C and δ D values near the start of the 20^th century were 1.8‰ and 11‰ lower than present day values, respectively. The increasing isotope trends throughout the last century are primarily the result of increased CH₄ emissions with elevated isotope signatures (e.g. fossil fuel and biomass burning).
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0315 Biosphere/atmosphere interactions (0426, 1610)
DE: 0325 Evolution of the atmosphere (1610, 8125)
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting