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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

Abstract. The temperature sensitivity of soil organic carbon decomposition is critical for predicting future climate change because soils store 2-3 times the amount of atmospheric carbon. Of particular controversy is the question, whether temperature sensitivity differs between young or labile and old or more stable carbon pools. Ambiguities in experimental methodology have so far limited corroboration of any particular hypothesis. Here, we show in a clear-cut approach that differences in temperature sensitivity between young and old carbon are negligible. Using the change in stable isotope composition in transitional systems from C3 to C4 vegetation, we were able to directly distinguish the temperature sensitivity of carbon differing several decades in age. This method had several advantages over previously followed approaches. It allowed to identify release of much older carbon, avoided un-natural conditions of long-term incubations and did not require arguable curve-fitting. Our results demonstrate that feedbacks of the carbon cycle on climate change are driven equally by young and old soil organic carbon.