The importance of northern peatland expansion to the late-Holocene rise of atmospheric methane

AbstarctWetlands have been considered as the most important natural source of the atmospheric methane concentration (AMC) prior to anthropogenic influences. According to ice cores, AMC varied significantly during the Holocene, the causes of which are not completely understood. In particular, the reasons for the increased AMC during the late Holocene (from 5 ka onwards) have been debated widely, including an anthropogenic explanation. Initially, this increase was associated with increased emissions from northern wetlands, but estimated peat initiation rates seem not to support the conclusion. Based on a new data set of 954 basal peat radiocarbon dates that accounts more properly for the horizontal growth dynamics of northern peatlands (by containing only sites with multiple basal dates per site), we show here that the most extensive lateral expansion of high-latitude peatlands occurred only after 5 ka, parallel with the rise of CH4 in the ice cores. Because this explosive increase in the extent of peatlands resulted in the formation of moist minerotrophic fen ecosystems that emit high amounts of CH4 for a long time since their formation, and because many Arctic peatlands have remained minerotrophic throughout their development, northern peatlands cannot be neglected when seeking cause(s) for the late-Holocene rise in CH4. A similar event in future could enhance climate change by causing a rapid shift in atmospheric greenhouse gas concentrations.