Independent variations of CH₄ emissions and isotopic composition over the past 160,000 years

Möller, Lars; Sowers, Todd; Bock, Michael; Spahni, Renato; Behrens, Melanie; Schmitt, Jochen; Miller, Heinrich; Fischer, Hubertus (2013). Independent variations of CH₄ emissions and isotopic composition over the past 160,000 years. Nature geoscience, 6(10), pp. 885-890. Nature Publishing Group 10.1038/ngeo1922

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During the last glacial cycle, greenhouse gas concentrations fluctuated on decadal and longer timescales. Concentrations of methane, as measured in polar ice cores, show a close connection with Northern Hemisphere temperature variability, but the contribution of the various methane sources and sinks to changes in concentration is still a matter of debate. Here we assess changes in methane cycling over the past 160,000 years by measurements of the carbon isotopic composition delta C-13 of methane in Antarctic ice cores from Dronning Maud Land and Vostok. We find that variations in the delta C-13 of methane are not generally correlated with changes in atmospheric methane concentration, but instead more closely correlated to atmospheric CO2 concentrations. We interpret this to reflect a climatic and CO2-related control on the isotopic signature of methane source material, such as ecosystem shifts in the seasonally inundated tropical wetlands that produce methane. In contrast, relatively stable delta C-13 values occurred during intervals of large changes in the atmospheric loading of methane. We suggest that most methane sources-most notably tropical wetlands-must have responded simultaneously to climate changes across these periods.

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