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Atmospheric methane variability: Centennial-scale signals in the Last Glacial Period

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Rhodes, RH 
Brook, EJ 
McConnell, JR 
Blunier, T 
Sime, LC 


In order to understand atmospheric methane (CH4) biogeochemistry now and in the future, we must apprehend its natural variability, without anthropogenic influence. Samples of ancient air trapped within ice cores provide the means to do this. Here we analyze the ultrahigh-resolution CH4 record of the West Antarctic Ice Sheet Divide ice core 67.2–9.8 ka and find novel, atmospheric CH4 variability at centennial time scales throughout the record. This signal is characterized by recurrence intervals within a broad 80–500 year range, but we find that age-scale uncertainties complicate the possible isolation of any periodic frequency. Lower signal amplitudes in the Last Glacial relative to the Holocene may be related to incongruent effects of firn-based signal smoothing processes. Within interstadial and stadial periods, the peak-to-peak signal amplitudes vary in proportion to the underlying millennial-scale oscillations in CH4 concentration—the relative amplitude change is constant. We propose that the centennial CH4 signal is related to tropical climate variability that influences predominantly low-latitude wetland CH4 emissions.



methane, ice cores, centennial variability, Last Glacial Period, atmospheric composition, paleoclimate

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Global Biogeochemical Cycles

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European Commission Horizon 2020 (H2020) Marie Sk?odowska-Curie actions (658120)
This study was funded by the U.S. National Science Foundation (NSF) grants 0944552, 1142041, and 0968391 to E.J.B. and 0839093 and 1142166 to J.R.M. A European Union Horizon 2020 Marie Curie Individual Fellowship (grant 58120, SEADOG) provided partial support for R.H.R. This work also benefitted from funding to X.F. from the French RPD COCLICO ANR program (ANR-10-RPDOC-002-01), the INSU/LEFE project IceChrono, and the Ars Cuttoli foundation and additionally from the UK Natural Environment Research Council (NERC) grant NE/P009271/1 awarded to L.C.S. Grateful thanks to B. Tournadre for help in Fletcher Promontory ice core analysis. The authors appreciate the support of the WAIS Divide Science Coordination Office at the Desert Research Institute, Reno, NV, USA, and University of New Hampshire, USA, for the collection and distribution of the WD ice core (NSF grants 0230396, 0440817, 0944348, and 0944266). We are grateful to all participants in the field effort led by K. Taylor. The NSF Office of Polar Programs also funded the Ice Drilling Program Office and Ice Drilling Design and Operations group, the National Ice Core Laboratory, Raytheon Polar Services, and the 109th New York Air National Guard.