Global peatland evolution since the last deglaciation and its role in atmospheric methane

Peatlands play a critical role in global carbon storage and methane cycling. Despite much investigation of widespread peatland initiation since the last deglaciation, the subsequent global pattern of peatland evolution and its impacts on atmospheric methane remain poorly understood. Here, we integrated palaeoecological records from >120 peatlands worldwide to develop a global synthesis of peatland evolution. Our synthesis documents peatland initiation and (fen-to-bog) transitions based on vegetation community changes, stratigraphy, and reconstructed pH variations.

Our dataset reveals that peatland evolution has been continuous since ~15 ka BP (before present), with a maximum in the number of peatlands transitioning from fens to bogs during the early Holocene (10-7 ka BP). More than 50 % of peatlands completed this transition within 3,000 years of initiation, and ~75 % within 5,000 years, independent of climate state. This highlights the dominant role of autogenous peat accumulation processes in driving long-term peatland evolution. The peak in fen-bog transition coincided with a ~100 ppb decline in atmospheric methane concentrations and a ~2 ‰ depletion in methane carbon isotopes as recorded by the ice cores, possibly partly reflecting reduced methane emissions and a large-scale shift from acetoclastic to hydrogenotrophic methanogenesis due to the global fen-to-bog transitions. Supporting this, modern flux data from >130 fen and bog sites indicate that fen-bog transitions reduce methane emissions by ~50 %.

In tropical peatlands, limited palaeoecological data from key regions such as the Congo Basin, Southeast Asia, and the Amazon suggest that tropical peatland evolution occurred later than that of northern peatlands, primarily during 6-2 ka BP. Unlike the herbaceous fen to Sphagnum bog transitions typical for northern peatlands, these tropical transitions were characterized by a shift from herbaceous vegetation to tree-dominated swamps, or by changes in the dominant tree species within forested swamps. Consequently, these transitions may have enhanced or at least maintained methane emissions considering that tree-mediated methane transport can be as important as sedge-mediated transport. This later tropical shift may be one of the reasons why atmospheric methane did not continue to decline during the mid-to-late Holocene.