Abstract
Abstract. Firn and polar ice cores enclosing trace gas species offer a unique archive to study changes in the past atmosphere and in terrestrial/marine source regions. Here we present a new online technique for ice core and air samples to measure a suite of isotope ratios and mixing ratios of trace gas species on a single sample. Isotope ratios are determined on methane, nitrous oxide and xenon with reproducibilities for ice core samples of 0.15‰ for δ13C–CH4, 0.22‰ for δ15N–N2O, 0.34‰ for δ18O–N2O, and 0.05‰ per mass difference for δ136Xe for typical concentrations of glacial ice. Mixing ratios are determined on methane, nitrous oxide, xenon, ethane, propane, methyl chloride and dichlorodifluoromethane with reproducibilities of 7 ppb for CH4, 3 ppb for N2O, 70 ppt for C2H6, 70 ppt for C3H8, 20 ppt for CH3Cl, and 2 ppt for CCl2F2. However, the blank contribution for C2H6 and C3H8 is large in view of the measured values for Antarctic ice samples. The system consists of a vacuum extraction device, a preconcentration unit and a gas chromatograph coupled to an isotope ratio mass spectrometer. CH4 is combusted to CO2 prior to detection while we bypass the oven for all other species. The highly automated system uses only ~ 160 g of ice, equivalent to ~ 16 mL air, which is less than previous methods. The measurement of this large suite of parameters on a single ice sample is new and key to understanding phase relationships of parameters which are usually not measured together. A multi-parameter data set is also key to understand in situ production processes of organic species in the ice, a critical issue observed in many organic trace gases. Novel is the determination of xenon isotope ratios using doubly charged Xe ions. The attained precision for δ136Xe is suitable to correct the isotopic ratios and mixing ratios for gravitational firn diffusion effects, with the benefit that this information is derived from the same sample. Lastly, anomalies in the Xe mixing ratio, δXe/air, can be used to detect melt layers.
Highlights
The analysis of atmospheric trace gases and their stable isotopic ratios on air archived in ice cores is fundamental to reconstructing and understanding the composition of the atmosphere of the past
This allows for the measurement of isotopic ratios of different isobaric species within a run, while previous studies measuring CH4 and N2O on a single sample either reserved a specific isotope ratio mass spectrometer (IRMS) for each species (Sapart et al, 2011) or held one species until the IRMS was preconditioned for the second species (Sperlich et al, 2013)
We described a multi-parameter device to measure a large suite of trace gas species and their isotopic compositions on a single ice core sample and demonstrated its performance
Summary
The analysis of atmospheric trace gases and their stable isotopic ratios on air archived in ice cores is fundamental to reconstructing and understanding the composition of the atmosphere of the past. To these traditional trace gases at ppm (CO2) or ppb levels (CH4 and N2O), CO and atmospheric trace gases at ppt level, like ethane, propane or methyl chloride, are currently being studied in firn (Aydin et al, 2011; Worton et al, 2012) and ice core studies (Saltzman et al, 2009; Wang et al, 2010; Verhulst et al, 2013). For CH4, these processes have been shown to occur in core sections affected by surface melting (NEEM community members, 2013) or are associated with sharp spikes in aerosol content from biomass burning (Rhodes et al, 2013) In both cases sample size is a critical parameter, and the combined knowledge from many gas species is crucial to identify the underlying processes affecting the archived atmospheric composition of the particular ice core sections
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