Abstract.The recently reported finding that plant matter and living plants produce significant amounts of the important greenhouse gas methane under aerobic conditions has led to an intense scientific and public controversy. Whereas some studies question the up-scaling method that was used to estimate the global source strength, others have suggested that experimental artifacts could have caused the reported signals, and two studies, one based on isotope labeling, have recently reported the absence of CH 4 emissions from plants. Here we show -using several independent experimental analysis techniques -that dry and detached fresh plant matter, as well as several structural plant components, emit significant amounts of methane upon irradiation with UV light and/or heating. Emissions from UV irradiation are almost instantaneous, indicating a direct photochemical process. Longtime irradiation experiments demonstrate that the size of the CH 4 producing reservoir is large, exceeding potential interferences from degassing or desorption processes by several orders of magnitude. A dry leaf of a pure 13 C plant produces 13 CH 4 at a similar rate as dry leaves of non-labeled plants produce non-labeled methane.

Summary• The observation that plants produce methane (CH 4 ) under aerobic conditions has caused considerable controversy among the scientific community and the general public. It led to much discussion and debate not only about its contribution to the global CH 4 budget but also about the authenticity of the observation itself. Previous results suggested that methoxyl groups of the abundant plant structural component pectin might play a key role in the in situ formation process of CH 4 . Here, this effect is investigated using an isotope labelling study.• Polysaccharides, pectin and polygalacturonic acid, with varying degrees of trideuterium-labelled methyl groups in the methoxyl moieties, were investigated for CH 4 formation under UV irradiation and heating.• A strong deuterium signal in the emitted CH 4 was observed from these labelled polysaccharides.• Results clearly demonstrate that ester methyl groups of pectin can serve as a precursor of CH 4 , supporting the idea of a novel chemical route of CH 4 formation in plants under oxic environmental conditions.

Almost a decade after methane was first reported in the atmosphere of Mars there is an intensive discussion about both the reliability of the observations--particularly the suggested seasonal and latitudinal variations--and the sources of methane on Mars. Given that the lifetime of methane in the Martian atmosphere is limited, a process on or below the planet's surface would need to be continuously producing methane. A biological source would provide support for the potential existence of life on Mars, whereas a chemical origin would imply that there are unexpected geological processes. Methane release from carbonaceous meteorites associated with ablation during atmospheric entry is considered negligible. Here we show that methane is produced in much larger quantities from the Murchison meteorite (a type CM2 carbonaceous chondrite) when exposed to ultraviolet radiation under conditions similar to those expected at the Martian surface. Meteorites containing several per cent of intact organic matter reach the Martian surface at high rates, and our experiments suggest that a significant fraction of the organic matter accessible to ultraviolet radiation is converted to methane. Ultraviolet-radiation-induced methane formation from meteorites could explain a substantial fraction of the most recently estimated atmospheric methane mixing ratios. Stable hydrogen isotope analysis unambiguously confirms that the methane released from Murchison is of extraterrestrial origin. The stable carbon isotope composition, in contrast, is similar to that of terrestrial microbial origin; hence, measurements of this signature in future Mars missions may not enable an unambiguous identification of biogenic methane.

Abstract. Methane and nitrous oxide are important greenhouse gases which show a strong increase in atmospheric mixing ratios since pre-industrial time as well as large variations during past climate changes. The understanding of their biogeochemical cycles can be improved using stable isotope analysis. However, high-precision isotope measurements on air trapped in ice cores are challenging because of the high susceptibility to contamination and fractionation.Here, we present a dry extraction system for combined CH 4 and N 2 O stable isotope analysis from ice core air, using an ice grating device. The system allows simultaneous analysis of δD(CH 4 ) or δ 13 C(CH 4 ), together with δ 15 N(N 2 O), δ 18 O(N 2 O) and δ 15 N(NO + fragment) on a single ice core sample, using two isotope mass spectrometry systems. The optimum quantity of ice for analysis is about 600 g with typical "Holocene" mixing ratios for CH 4 and N 2 O. In this case, the reproducibility (1σ ) is 2.1 ‰ for δD(CH 4 ), 0.18 ‰ for δ 13 C(CH 4 ), 0.51 ‰ for δ 15 N(N 2 O), 0.69 ‰ for δ 18 O(N 2 O) and 1.12 ‰ for δ 15 N(NO + fragment). For smaller amounts of ice the standard deviation increases, particularly for N 2 O isotopologues. For both gases, small-scale intercalibrations using air and/or ice samples have been carried out in collaboration with other institutes that are currently involved in isotope measurements of ice core air. Significant differences are shown between the calibration scales, but those offsets are consistent and can therefore be corrected for.