Past atmospheric methane concentrations show strong fluctuations in parallel to rapid glacial climate changes in the Northern Hemisphere 1,2 superimposed on a glacial-interglacial doubling of methane concentrations [3][4][5] . The processes driving the observed fluctuations remain uncertain but can be constrained using methane isotopic information from ice cores 6,7 . Here we present an ice core record of carbon isotopic ratios in methane over the entire last glacial-interglacial transition. Our data show that the carbon in atmospheric methane was isotopically much heavier in cold climate periods. With the help of a box model constrained by the present data and previously published results 6,8 , we are able to estimate the magnitude of past individual methane emission sources and the atmospheric lifetime of methane. We find that methane emissions due to biomass burning were about 45 Tg methane per year, and that these remained roughly constant throughout the glacial termination. The atmospheric lifetime of methane is reduced during cold climate periods. We also show that boreal wetlands are an important source of methane during warm events, but their methane emissions are essentially shut down during cold climate conditions.The atmospheric concentration of CH 4 , the second most important anthropogenic greenhouse gas, is determined by a balance between natural and anthropogenic CH 4 sources and sinks that is still debated. Photochemically induced oxidation in the troposphere and stratosphere, and uptake by methanotrophic bacteria in aerated soils, represent the most important sinks 9,10 . The dominating natural CH 4 sources comprise tropical and boreal wetlands, ruminants, and biomass burning 10,11 . These sources all differ in their carbon and hydrogen isotopic signature. In addition, a release of CH 4 from marine gas hydrates 12,13 and emissions from plants under aerobic conditions are currently debated 14,15 . Most probably all those sources and sinks were subject to palaeoclimatic changes, as reflected by CH 4 being as low as 360 parts per billion (10 9 ) by volume (p.p.b.v.) during the Last Glacial Maximum (LGM), compared with up to 725 p.p.b.v. in the preindustrial Holocene epoch 3,16,17 . Throughout the glacial period and during the last transition, CH 4 changed by up to 200 p.p.b.v. (refs 3, 18) in parallel with rapid climate changes. Using the interhemispheric CH 4 gradient in ice cores, an increase of high-latitude CH 4 sources in the Northern Hemisphere was derived for warm periods 3,17 . However, a more detailed quantitative source attribution is still missing.Such quantitative constraint on the sources can be derived from methane isotopic measurements on ice cores 6,7 , making use of the different isotopic signatures of the CH 4 sources and the different isotopic fractionation factors for the individual removal processes (Supplementary Table 1). In Fig. 1

Partially deuterated and various substituted flavin and thiaflavin model compounds have been synthesized. For the first time, high-resolution H, D, and 14N ENDOR and TRIPLE resonance experiments in fluid solutions have been performed on the paramagnetic derivatives of these compounds. Additionally, valuable information has been obtained about hyperfine anisotropies and molecular structures from ENDOR in rigid matrices. Solid matrix ENDOR studies of native flavoenzymes, namely, "Old Yellow Enzyme" (NADPH dehydrogenase), two flavodoxins, and a methanol oxidase are reported. The ENDOR matrix signals of the various flavoproteins are different in intensity, suggesting that the microenvironments are remarkably different. Applicabilities and limitations of the ENDOR technique in the studies of flavins and flavoenzymes are discussed.

Atmospheric methane (CH 4 ) records reconstructed from polar ice cores represent an integrated view on processes predominantly taking place in the terrestrial biogeosphere. Here, we present dual stable isotopic methane records [δ 13 CH 4 and δD(CH 4 )] from four Antarctic ice cores, which provide improved constraints on past changes in natural methane sources. Our isotope data show that tropical wetlands and seasonally inundated floodplains are most likely the controlling sources of atmospheric methane variations for the current and two older interglacials and their preceding glacial maxima. The changes in these sources are steered by variations in temperature, precipitation, and the water table as modulated by insolation, (local) sea level, and monsoon intensity. Based on our δD(CH 4 ) constraint, it seems that geologic emissions of methane may play a steady but only minor role in atmospheric CH 4 changes and that the glacial budget is not dominated by these sources. Superimposed on the glacial/interglacial variations is a marked difference in both isotope records, with systematically higher values during the last 25,000 y compared with older time periods. This shift cannot be explained by climatic changes. Rather, our isotopic methane budget points to a marked increase in fire activity, possibly caused by biome changes and accumulation of fuel related to the late Pleistocene megafauna extinction, which took place in the course of the last glacial.atmosphere | methane | megafauna | ice core | stable isotopes

Events Emissions at the Onset of Dansgaard-OeschgerThe following resources related to this article are available online at www.sciencemag.org