THE ISOTOPIC SIGNATURE OF ICE AGE TERRESTRIAL METHANE

Atmospheric methane (CH₄) concentrations varied between ca. 350 and 700 ppb over the last 400 000 years. Between the Last Glacial Maximum (LGM, ca. 21 000 yr BP) and pre-industrial Holocene (to ca. 1850), CH₄ concentrations increased nearly 100% and more closely parallel the fast variations of polar temperature records than any other measured gas. Recent analysis of ice core CH₄ concentrations and carbon isotopic composition (d¹³CH₄) indicate that CH₄ emissions from wetlands and fires drove prehistoric changes in ice-core CH₄, but conflict as to the location of wetlands, magnitude of burning, and the environmental controls on CH₄ emission. Wetlands and fires are the most important contributors to the global CH₄ isotope mass balance because wetlands have the highest emission rates of all natural sources and biomass burning has a unique isotopic signature. To investigate the effect of climate change on natural CH₄ emissions I used the BIOME4 global vegetation model coupled with simple algorithms for determining wetland area based on topography and soil moisture, and CH₄ emissions based on ecosystem carbon turnover in wet soils and intensity of biomass burning. Observed climate drove the vegetation model for a present-day control experiment; a paleoclimate scenario was provided by coupled AGCM/mixed-layer ocean model simulations at 1000-year intervals from the LGM to present. LGM wetland area was 15% larger than present, but CH₄ emissions were 24% less, and ca. 2‰ more depleted in ¹³C because of the increase in tropical wetland area relative to northern wetlands. CH₄ emissions from fires were slightly greater at the LGM due to tropical aridity. The relative prevalence of C₄ grasslands at the LGM affected a shift in d¹³CH₄ from fires by up to 10‰. Wetland CH₄ emissions did not change drastically during the deglaciation because new wetland areas formed as ice sheets retreated, while other wetland areas were flooded by rising sea-level. Rapid changes in atmospheric CH₄ concentrations over the last 21 ka cannot be completely attributed to climate change on millennial time-scales, however much of the isotopic shift in CH₄ emissions may be due to the increased importance of boreal wetlands in the Holocene.