Atmospheric methane variability: Centennial‐scale signals in the Last Glacial Period. Issue 3 (22nd March 2017)

• Record Type: Journal Article
• Authors: Rhodes, Rachael H.; Brook, Edward J.; McConnell, Joseph R.; Blunier, Thomas; Sime, Louise C.; Faïn, Xavier; Mulvaney, Robert
• Is Part Of: Global biogeochemical cycles. Volume 31:Issue 3(2017:Mar.)
• Page Start: 575
• Page End: 590
• Publication Date: 2017-03-22
• Subjects: methane -- ice cores -- centennial variability -- Last Glacial Period -- atmospheric composition -- paleoclimate; Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1002/2016GB005570
• Languages: English
• ISSNs: 0886-6236

Abstract

Abstract: 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. Key Points: Novel centennial‐scale atmospheric methane variability observed in Last Glacial to early Holocene of WAIS Divide ice core Methane variability characterized by recurrence intervals within broad 80–500 year range and mean peak‐to‐peak amplitudes of 16 ppb Amplitude of centennial‐scale signal in stadial and interstadial periods is proportional to underlying millennial‐scale CH4 concentration Plain Language Summary: Using a new method to measure methane concentrations of ancient air trapped in ice cores, we have detected variability in atmospheric methane concentration on centennial time scales in the Last Glacial Period for the first time. We know these signals represent past changes in atmospheric methane because they appear in several ice core records. We propose that changes in methane emissions from tropical wetlands are responsible. How this new variability might be related to similar signals found in the late Holocene ice core records and the instrumental record of atmospheric methane is an open question.