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

Methane is a strong greenhouse gas and large un- certainties exist concerning the future evolution of its atmo- spheric 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 at- mospheric CH4 mixing ratios in the future under the influ- ence of a changing climate. The aim of this study is to as- sess whether past atmospheric 13 C(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 dis- crepancies 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 multi- site inversion can be performed, which returns an isotope re- construction 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 high- accumulation ice core data. We conclude that with the current datasets and understanding of firn air transport, a high preci- sion reconstruction of 13 C 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 in- herent uncertainties in the method, which are the firn frac- tionation correction (up to 2 ‰ at individual sites), the Kr isobaric interference (up to 0.8 ‰, system dependent), inter-laboratory calibration offsets ( 0.2 ‰) and uncertain- ties in past CH4 levels ( 0.5 ‰).

[1]  J. Schmitt,et al.  On the interference of Kr during carbon isotope analysis of methane using continuous-flow combustion-isotope ratio mass spectrometry , 2013 .

[2]  J. Schmitt,et al.  On the interference of 86 Kr 2+ during carbon isotope analysis of atmospheric methane using continuous flow combustion - isotope ratio mass spectrometry , 2013 .

[3]  S. Houweling,et al.  Natural and anthropogenic variations in methane sources during the past two millennia , 2012, Nature.

[4]  T. Bromley,et al.  No inter-hemispheric δ13CH4 trend observed , 2012, Nature.

[5]  C. Buizert,et al.  Simultaneous stable isotope analysis of methane and nitrous oxide on ice core samples , 2011 .

[6]  V. Petrenko,et al.  The isotopic record of Northern Hemisphere atmospheric carbon monoxide since 1950: implications for the CO budget , 2011 .

[7]  S. Houweling,et al.  Interpreting methane variations in the past two decades using measurements of CH 4 mixing ratio and isotopic composition , 2011 .

[8]  D. Etheridge,et al.  A new multi-gas constrained model of trace gas non-homogeneous transport in firn: evaluation and behaviour at eleven polar sites , 2011 .

[9]  P. M. Lang,et al.  Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland , 2011 .

[10]  T. Röckmann,et al.  Continuous-flow isotope ratio mass spectrometry method for carbon and hydrogen isotope measurements on atmospheric methane , 2010 .

[11]  G. Brasseur,et al.  Long-lived halocarbon trends and budgets from atmospheric chemistry modelling constrained with measurements in polar firn , 2009 .

[12]  S. Houweling,et al.  Early anthropogenic CH4 emissions and the variation of CH4 and 13CH4 over the last millennium , 2008 .

[13]  Vincent R. Gray.  Climate Change 2007: The Physical Science Basis Summary for Policymakers , 2007 .

[14]  Andrew L. Rice,et al.  Stable isotope ratios in atmospheric CH4: Implications for seasonal sources and sinks , 2007 .

[15]  J. Barnola,et al.  CO2 isotopes as tracers of firn air diffusion and age in an Arctic ice cap with summer melting, Devon Island, Canada , 2007 .

[16]  Ilse Aben,et al.  Atmospheric constraints on global emissions of methane from plants , 2006 .

[17]  Paul Steele,et al.  Law Dome CO2, CH4 and N2O ice core records extended to 2000 years BP , 2006 .

[18]  D. Etheridge,et al.  Law Dome CO 2 , CH 4 and N 2 O ice core records extended to 2000 years , 2006 .

[19]  D. Etheridge,et al.  Unexpected Changes to the Global Methane Budget over the Past 2000 Years , 2005, Science.

[20]  J. Barnola,et al.  Records of the δ13C of atmospheric CH4 over the last 2 centuries as recorded in Antarctic snow and ice , 2005 .

[21]  Pieter P. Tans,et al.  CH4 sources estimated from atmospheric observations of CH4 and its 13C/12C isotopic ratios: 1. Inverse modeling of source processes , 2004 .

[22]  Pieter P. Tans,et al.  CH4 sources estimated from atmospheric observations of CH4 and its 13C/12C isotopic ratios: 2. Inverse modeling of CH4 fluxes from geographical regions , 2004 .

[23]  J. Schwander,et al.  The attenuation of fast atmospheric CH4 variations recorded in polar ice cores , 2003 .

[24]  E. Dlugokencky,et al.  Development of analytical methods and measurements of 13C/12C in atmospheric CH4 from the NOAA Climate Monitoring and Diagnostics Laboratory Global Air Sampling Network , 2002 .

[25]  W. Sturges,et al.  Changes in the global atmospheric methane budget over the last decades inferred from13C and D isotopic analysis of Antarctic firn air , 2001 .

[26]  J. Severinghaus,et al.  Thermal fractionation of air in polar firn by seasonal temperature gradients , 2001 .

[27]  Sander Houweling,et al.  The modeling of tropospheric methane: How well can point measurements be reproduced by a global model? , 2000 .

[28]  D. Etheridge,et al.  A history of δ13C in atmospheric CH4 from the Cape Grim Air Archive and Antarctic firn air , 1999 .

[29]  E. Dlugokencky,et al.  The isotopic composition of atmospheric methane , 1999 .

[30]  D. Etheridge,et al.  Atmospheric methane between 1000 A.D. and present: Evidence of anthropogenic emissions and climatic variability , 1998 .

[31]  J. Barnola,et al.  Reconstructing recent atmospheric trace gas concentrations from polar firn and bubbly ice data by inverse methods , 1997 .

[32]  D. Etheridge,et al.  Modeling air movement and bubble trapping in firn , 1997 .

[33]  P. Tans.  A note on isotopic ratios and the global atmospheric methane budget , 1997 .

[34]  B. Stauffer,et al.  The age of the air in the firn and the ice at Summit, Greenland , 1993 .

[35]  D. Raynaud,et al.  δ15N of N2 in air trapped in polar ice: A tracer of gas transport in the firn and a possible constraint on ice age-gas age differences , 1992 .

[36]  D. Raynaud,et al.  Elemental and isotopic composition of occluded O2 and N2 in polar ice , 1989 .

[37]  H. Craig,et al.  Gravitational Separation of Gases and Isotopes in Polar Ice Caps , 1988, Science.

[38]  H. Oeschger,et al.  Enclosure of Air During Metamorphosis of Dry Firn to Ice , 1985, Annals of Glaciology.

[39]  H. Craig,et al.  Methane: The record in polar ice cores , 1982 .