Temporal trends of deglacial and Holocene peatland initiation: a new look at old 14C dates

Peatlands play an important role in millennial-scale climate change and the C cycle because they are important C sinks as well as a substantial CH4 source. Chronologies of past peatland initiation are typically based on compiled basal peat radiocarbon (14C) dates and frequency histograms of binned calibrated age ranges; results from such compilations have suggested that northern peatlands were important drivers of abrupt, high-magnitude changes in deglacial and early Holocene atmospheric CH4 concentrations. However, this approach is problematic because poor quality 14C dates are commonly included and because frequency histograms of binned age ranges introduce chronological artefacts that bias the record of peatland initiation. Using a published compilation of 274 basal 14C dates from Alaska as a case study, we show that nearly half the 14C dates are inappropriate for reconstructing peatland initiation, and that the temporal structure of peatland initiation is sensitive to sampling biases and treatment of calibrated 14C dates. Revised chronologies of peatland initiation from Alaska (179 14C dates) and the circumpolar north (1516 14C dates), based on summed probability distributions of calibrated 14C dates, reveal that rapid increases in peatland initiation lagged abrupt rises in atmospheric CH4 concentration at the start of the Bølling-Allerød interstadial (Termination 1A) and the end of the Younger Dryas chronozone (Termination 1B), suggesting that northern peatlands were not the primary drivers of the initial increases in atmospheric CH4. The revised chronologies also reveal increases in northern peatland initiation during the late Holocene, which are broadly coincident with the late Holocene reversal and subsequent rise in atmospheric CH4 concentration. Our results demonstrate that subtle methodological changes in the synthesis of basal 14C ages lead to substantially different interpretations of temporal trends in peatland initiation, with direct implications for the role of peatlands in the C cycle.