Methane is an important greenhouse gas that is emitted from multiple natural and anthropogenic sources. Atmospheric methane concentrations have varied on a number of timescales in the past, but what has caused these variations is not always well understood. The different sources and sinks of methane have specific isotopic signatures, and the isotopic composition of methane can therefore help to identify the environmental drivers of variations in atmospheric methane concentrations. Here we present high-resolution carbon isotope data (δ(13)C content) for methane from two ice cores from Greenland for the past two millennia. We find that the δ(13)C content underwent pronounced centennial-scale variations between 100 BC and AD 1600. With the help of two-box model calculations, we show that the centennial-scale variations in isotope ratios can be attributed to changes in pyrogenic and biogenic sources. We find correlations between these source changes and both natural climate variability--such as the Medieval Climate Anomaly and the Little Ice Age--and changes in human population and land use, such as the decline of the Roman empire and the Han dynasty, and the population expansion during the medieval period.

Abstract. We thoroughly characterized a system for thermal separation of organic carbon (OC) and elemental carbon (EC) for subsequent radiocarbon analysis. Different organic compounds as well as ambient aerosol filter samples were introduced into an oven system and combusted to CO 2 in pure O 2 . The main objective was to test which combustion times and temperatures are best suited to separate OC and EC. The final separation step for OC was combustion at 360 • C for 15 min. Combustion at this temperature proved enough to remove several organic test substances from the filter (including high molecular weight humic acid) but did not remove substantial amounts of EC. For isolation of EC, OC first needs to be completely removed from the filter. This was achieved by water extraction of the filter, followed by combustion of the water insoluble OC at 360 • C and combustion at an intermediate temperature step of 2 min at 450 • C. This last step removed the most refractory OC together with some EC. Finally, the remaining EC was combusted to CO 2 at 650 • C. The recovery of black carbon after the intermediate 450 • C step was approximately 80 %. Several tests provided strong evidence that OC was removed efficiently during the intermediate temperature step: (i) brown carbon, indicative of refractory OC, was removed; (ii) the fraction modern of EC did not decrease significantly if the temperature of the intermediate step was further increased. Based on tests with various organic compounds, we estimated that charred organic carbon could contribute 4-8 % to an elemental carbon sample that was isolated according to our method.

Stable carbon isotope analysis of methane (δ¹³C of CH₄) on atmospheric samples is one key method to constrain the current and past atmospheric CH₄ budget. A frequently applied measurement technique is gas chromatography (GC) isotope ratio mass spectrometry (IRMS) coupled to a combustion-preconcentration unit. This report shows that the atmospheric trace gas krypton (Kr) can severely interfere during the mass spectrometric measurement, leading to significant biases in δ¹³C of CH₄, if krypton is not sufficiently separated during the analysis. According to our experiments, the krypton interference is likely composed of two individual effects, with the lateral tailing of the doubly charged ⁸⁶Kr peak affecting the neighbouring m/z 44 and partially the m/z 45 Faraday cups. Additionally, a broad signal affecting m/z 45 and especially m/z 46 is assumed to result from scattered ions of singly charged krypton. The introduced bias in the measured isotope ratios is dependent on the chromatographic separation, the krypton-to-CH₄ mixing ratio in the sample, the focusing of the mass spectrometer as well as the detector configuration and can amount to up to several per mil in δ¹³C. Apart from technical solutions to avoid this interference, we present correction routines to a posteriori remove the bias

Abstract. N 2 O is currently the third most important anthropogenic greenhouse gas in terms of radiative forcing and its atmospheric mole fraction is rising steadily. To quantify the growth rate and its causes over the past decades, we performed a multi-site reconstruction of the atmospheric N 2 O mole fraction and isotopic composition using new and previously published firn air data collected from Greenland and Antarctica in combination with a firn diffusion and densification model. The multi-site reconstruction showed that while the global mean N 2 O mole fraction increased from (290 ± 1) nmol mol −1 in 1940 to (322 ± 1) nmol mol −1 in 2008, the isotopic composition of atmospheric N 2 O decreased by (−2.2 ± 0.2) ‰ for δ 15 N av , (−1.0 ± 0.3) ‰ for δ 18 O, (−1.3 ± 0.6) ‰ for δ 15 N α , and (−2.8 ± 0.6) ‰ for δ 15 N β over the same period. The detailed temporal evolution of the mole fraction and isotopic composition derived from the firn air model was then used in a two-box atmospheric model (comprising a stratospheric box and a tropospheric box) to infer changes in the isotopic source signature over time. The precise value of the source strength depends on the choice of the N 2 O lifetime, which we choose to fix at 123 years. The average isotopic composition over the investigated period is δ 15 N av = (−7.6 ± 0.8) ‰ (vs. air-N 2 ), δ 18 O = (32.2 ± 0.2) ‰ (vs. Vienna Standard Mean Ocean Water -VSMOW) for δ 18 O, δ 15 N α = (−3.0 ± 1.9) ‰ and δ 15 N β = (−11.7 ± 2.3) ‰. δ 15 N av , and δ 15 N β show some temporal variability, while for the other signatures the error bars of the reconstruction are too large to retrieve reliable temporal changes. Possible processes that may explain trends in 15 N are discussed. The 15 N site preference (= δ 15 N α − δ 15 N β ) provides evidence of a shift in emissions from denitrification to nitrification, although the uncertainty envelopes are large.

Nitrous oxide (N2O) is a strong greenhouse gas whose mole fraction in the atmosphere has increased over the industrial period. We present a new set of isotope measurements of N2O in air extracted from ice cores covering the last 3,000 years. For the preindustrial (PI) atmosphere, we find an average N2O mole fraction of (267 ± 1) nmol/mol and average tropospheric N2O isotopic values of δ15NavPI = (9.5 ± 0.1)‰, δ18OPI = (47.1 ± 0.2)‰, δ15NαPI = (17.8 ± 0.4)‰, and δ15ΝβPI = (1.2 ± 0.4)‰. From PI to modern times all isotope signatures decreased with a total change of δ15Nav = (−2.7 ± 0.2)‰, δ18O = (−2.5 ± 0.4)‰, δ15Nα = (−2.0 ± 0.7)‰, and δ15Νβ (−3.5 ± 0.7)‰. Interestingly, the temporal evolution is not the same for δ15Nav and δ18O. δ18O trends are relatively larger during the early part, and δ15Nav trends are larger during the late part of the industrial period, implying a decoupling of sources over the industrial period. Using a mass balance model, we determined the isotopic composition of the total average N2O source. Assuming that the total present source is the sum of a constant natural source and an increasing anthropogenic source, this anthropogenic source has an isotopic signature of δ15Navsource,anthrop = (−15.0 ± 2.6)‰, δ18Osource,anthrop = (30.0 ± 2.6)‰, δ15Nαsource,anthrop = (−4.5 ± 1.7)‰, and δ15Nβsource,anthrop = (−24.0 ± 8.4)‰. The 15N site preference of the source has increased since PI times, which is indicative of a relative shift from denitrification to nitrification sources, consistent with agricultural emissions playing a major role in the N2O increase.