Can the carbon isotopic composition of methane be reconstructed from multi-site firn air measurements?

Abstract

Methane is a strong greenhouse gas and large uncertainties exist concerning the future evolution of its atmospheric abundance. Analyzing methane atmospheric mixing and stable isotope ratios in air trapped in polar ice sheets helps in reconstructing the evolution of its sources and sinks in the past. This is important to improve predictions of atmospheric CH4 mixing ratios in the future under the influence of a changing climate. The aim of this study is to assess whether past atmospheric 13C(CH4) variations can be reliably reconstructed from firn air measurements. Isotope reconstructions obtained with a state of the art firn model from different individual sites show unexpectedly large discrepancies and are mutually inconsistent.We show that small changes in the diffusivity profiles at individual sites lead to strong differences in the firn fractionation, which can explain a large part of these discrepancies. Using slightly modified diffusivities for some sites, and neglecting samples for which the firn fractionation signals are strongest, a combined multisite inversion can be performed, which returns an isotope reconstruction that is consistent with firn data. However, the isotope trends are lower than what has been concluded from Southern Hemisphere (SH) archived air samples and highaccumulation ice core data.We conclude that with the current datasets and understanding of firn air transport, a high precision reconstruction of 13C of CH4 from firn air samples is not possible, because reconstructed atmospheric trends over the last 50 yr of 0.3–1.5‰ are of the same magnitude as inherent uncertainties in the method, which are the firn fractionation correction (up to 2‰ at individual sites), the Kr isobaric interference (up to 0.8 ‰, system dependent), inter-laboratory calibration offsets (0.2 ‰) and uncertainties in past CH4 levels (0.5 ‰)

An automated GC-C-GC-IRMS setup to measure palaeoatmospheric δ<sup>13</sup>C-CH<sub>4</sub>, δ<sup>15</sup>N-N<sub>2</sub>O and δ<sup>18</sup>O-N<sub>2</sub>O in one ice core sample

“…8). However, we found a disagreement between IMAU Kr and the mean of four CIC samples (three Eurocore, one NEEM) during the δ 13 C-CH 4 excursion reported by Sapart et al (2012) at 964 AD, where the IMAU Kr sample is more enriched in δ 13 C-CH 4 by ∼ 1 ‰. Between 900 and 1000 AD, three IMAU Kr samples agree well, which rules out that the disagreement is based on one outlier.…”

“…The strongest of these CH 4 artefacts (+78 ppb) was detected in a depth of 272.1 m, which is close to the samples of Sapart et al (2012) from 964 and 1009 AD (269.5 m and 278.3 m, respectively) but even closer to the depth of the one NEEM sample that was measured at CIC for this intercomparison (272.8 m). Note that the samples of Rhodes et al (2013) were taken from a shallow core at NEEM, while the discussed NEEM samples are taken from the main core.…”

“…Comparison to published ice core data and established systems 3.11.1 δ 13 C-CH 4 We measured a total of 13 EUROCORE and NEEM ice core samples for δ 13 C-CH 4 and compared our results with NEEM ice core data that were recently published by Sapart et al(2012) as shown in Fig. 8.…”

“…Brenninkmeijer et al (2003) describe how the isotope fractionation of specific source and sink processes affect the integrated isotopic composition of the respective trace gases in the atmosphere. In an inverse approach, ice core isotope records of CH 4 and N 2 O provide distinct constraints on biogeochemical processes that can be linked to the variability observed in the CH 4 and N 2 O mixing ratios on decadal to millennial timescales (Sowers et al, 2003(Sowers et al, , 2005Sowers, 2006Sowers, , 2009Ferretti et al, 2005;Schaefer et al, 2006;Bock et al, 2010b;Melton et al, 2011a;Sapart et al, 2012). Because ice core records of CH 4 and N 2 O isotopic composition indicate the natural response of specific greenhouse gas sinks and sources to palaeoclimate changes, this information is of great interest to global warming predictions.…”

Online technique for isotope and mixing ratios of CH<sub>4</sub>, N<sub>2</sub>O, Xe and mixing ratios of organic trace gases on a single ice core sample

“…Here, our [CH 4 ] results agree well with those of Buiron et al (2011) as well as with a highresolution data set using the continuous-flow analysis system, with a precision of 15-20 ppb (Schüpbach et al, 2011). Finally, we compare results from a firn air sample (canister name "FA23", depth 76 m, termed "firn air NEEM" in Tables 4 and 5) from the NEEM 2008 EU hole that has also been investigated by IMAU and the Centre for Ice and Climate, Copenhagen (CIC) (see Sapart et al, 2013). Our δ 13 C-CH 4 value of −49.49 ± 0.04 ‰ agrees well with data from IMAU (Kr-corrected value, −49.69 ± 0.03 ‰) and CIC (no Kr issue, −49.52 ± 0.13 ‰).…”