Constructing an isotopic record of atmospheric methane and nitrous oxide over the last century from South Pole firn air

Over the last 150 years, atmospheric CH4 and N2O concentrations have risen in
response to increased emissions from various anthropogenic activities. As this
trend is liable to continue in the foreseeable future, it is important that we
understand the biogeochemical processes that contribute to the emissions of
these two greenhouse gases. In this context, records of variations in the
atmospheric loading of trace gases found in ice cores and interstitial spaces
in the snow near the surface of ice sheets (firn air) provide fundamental
boundary conditions for reconstructing historical emission records. One way to
improve our understanding of the cycling of bioactive trace gases and their
emission records is to use stable isotope tracers, which have been recorded in
the ice cores and firn air.

The primary objectives of the proposed work were to develop records of the
delC13 and delD of CH4 as well as the delN15, delO18 and the isotopomer
composition of N2O trapped in firn air samples which were retrieved from South
Pole in January of 1996 and 2001. The data from these firn air samples was
used to reconstruct isotopic records of these gases throughout the 20th
century. With the atmospheric records in hand, I was able to assess the
relative contribution of individual sources with a higher degree of confidence.

delC13 of CH4
Using two firn air samplings in 1995 and 2001 we calculate that delCH4 (13) has
increased by an average of 0.06+/-0.02%/yr over the six years between
samplings. Our ice core results suggest the delC13 of atmospheric CH4 has
increased by 1.8+/-0.2% between 1820 AD and 2001 AD. The delCH4 (13) changes
in both data sets are the result of an increase in the relative proportion of
CH4 sources with elevated 13C/12C isotope ratios. One explanation for observed
trends involves a 16 Tg/yr increase in CH4 emissions associated with biomass
burning over the past two centuries [Sowers et al., 2005].

delN15 and delO18 of N2O
I measured the delN15 and delO18 of the firn air samples spanning the 20th
century. Our results suggest the delN15 and delO18 of atmospheric N2O have
dropped by 1.8% and 0.9%, respectively, during the last century. These data
support previous predictions of decreasing atmospheric isotope ratios that are
related to a ~30% increase in total N2O emissions that are primarily related to
agricultural activities [Sowers et al., 2002].

References:

Sowers, T., S. Bernard, O. Aballain, J. Chappellaz, J.-M. Barnola, and T.
Marik, Records of the d13C of atmospheric CH4 over the last two centuries as
recorded in Antarctic snow and ice., Global Biogeochemical Cycles, 19 (GB2002),
doi:10.1029/2004GB002408, 2005.

Sowers, T., A. Rodebaugh, N. Yoshida, and S. Toyoda, Extending records of the
isotopic composition of atmospheric N2O back to 1800 A. D. from air trapped in
snow at South Pole and the Greenland Ice Sheet Project II ice core, Global
Biogeochemical Cycles, 16 (no.4), doi:1029/2002GB001911, 2002.