Constructing the first D/H record of atmospheric methane covering the last two centuries

Over the last 200 years, the atmospheric CH4 burden has doubled in response to
increased emissions from various anthropogenic activities. As this trend is
liable to continue in the foreseeable future and CH4 is an important greenhouse
gas, we need to improve our understanding of the biogeochemical processes that
control CH4 emissions. In this context, records of variations in the
atmospheric loading of trace gases found in ice cores provide fundamental
boundary conditions for reconstructing historical emission records. One way to
refine our understanding of the cycling of bioactive trace gases and their
emission records is to use stable isotope tracers trapped in the ice cores. The
object of the current proposal is to measure the D/H ratio of CH4 (dDCH4)
trapped in shallow ice at Siple Dome, Antarctica.

A total of 20 individual samples from the SDM-E core were measured for dDCH4.
The dDCH4 values show a general trend towards lower values throughout the 19th
and early 20th centuries. During the 19th century, dDCH4 values were
-89.3±2.3" or 8" lower than present day values (-81.3" SMOW). Model results
with varying scenarios suggest a likely cause of the increasing dDCH4 values is
an increase in biomass burning methane. The preferred model result identifies
30% of the present day biomass burning as being anthropogenic in origin. The
remaining 70% is attributable to natural biomass burning.