Records of the δ¹³C of atmospheric CH₄ over the last 2 centuries as recorded in Antarctic snow and ice

Abstract

[1] Methane is one of the important greenhouse gases accumulating in the atmosphere today. The increased loading over the past 2 centuries is thought to be the result of increased anthropogenic emissions. Here we present records of the d 13 C of CH 4 in firn air from the South Pole and in trapped bubbles in a short ice core from Siple Dome, Antarctica, that help constrain historical emissions of various sources throughout the last 2 centuries. Using two firn air samplings in 1995 and 2001 we calculate that d 13 CH 4 has increased by an average of 0.06 ± 0.02%/yr over the 6 years between samplings. Our ice core results suggest the d 13 C of atmospheric CH 4 has increased by 1.8 ± 0.2% between 1820 A.D. and 2001 AD. The d 13 CH 4 changes in both data sets are the result of an increase in the relative proportion of CH 4 sources with elevated 13 C/ 12 C isotope ratios. One explanation for observed trends involves a 16 Tg/yr increase in CH 4 emissions associated with biomass burning over the past 2 centuries.

Interlaboratory comparison of <i>δ</i><sup>13</sup>C and <i>δ</i>D measurements of atmospheric CH<sub>4</sub> for combined use of data sets from different laboratories

“…Sowers et al, 2005;Sowers, 2010). The overall measurement reproducibility including every step for ice core measurements was evaluated to be 0.3 for δ 13 C-CH 4 and 3 ‰ for δD-CH 4 (Sowers, 2010).…”

“…Later, a method based on a continuous-flow gas chromatography isotope ratio mass spectrometry (GC-IRMS) technique combined with combustion and pyrolysis furnaces became available (Merritt et al, 1995;Burgoyne and Hayes, 1998;Hilkert et al, 1999), which dramatically reduced time and effort in the laboratory and likewise the amount of sample air required (now typically 100 mL STP ). Such systems are now used in most laboratories worldwide to acquire δ 13 C-CH 4 and δD-CH 4 data in the current and past atmosphere (Rice et al, 2001;Miller et al, 2002;Sowers et al, 2005;Ferretti et al, 2005;Morimoto et al, 2006;Fisher et al, 2006;Umezawa et al, 2009;Brass and Röckmann, 2010;Sperlich et al, 2013;Schmitt et al, 2014;Bock et al, 2014;Brand et al, 2016;Röckmann et al, 2016). Although these systems use a similar measurement principle, they vary in the use of pre-concentration of CH 4 in sample air, GC separation and combustion/pyrolysis, data corrections and in the specific IRMS instrument among laboratories (see Schmitt et al, 2013, Sect.…”

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

“…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.…”

Methane and nitrous oxide in the ice core record

“…Etheridge et al 1998) as well as firn air and ice cores (Ferretti et al 2005;Sowers et al 2005) show methane became isotopically heavier during the twentieth century (values increasing from K49‰ to just above K47‰). This implies a shift in the balance of sources towards those that are isotopically heavy (fossil and pyrogenic) in comparison with the isotopically light biogenic sources.…”