Fen to bog transitions in high latitudes: what conditions lead to permafrost aggradation?
Abstract:Northern high-latitude peatlands accumulated an estimated 436 Gt of carbon over the Holocene. Vegetation changes, such as the succession from fen to bog species, are often clearly visible in peat profiles and can be caused by organic matter accumulation or by changes in regional climate. Most peatlands developed during the early Holocene as fens under a climate that was warmer than today due to a summer insolation maximum. Subsequent transition to bogs facilitated permafrost aggradation during the mid- to late-Holocene. Teasing out permafrost aggradation in peat cores remains a challenge, as they often resemble dry bogs. However, in many locations permafrost aggradation can be assumed especially if thermokarst is evident later in the peat record (i.e., an abrupt transition from dry bog or plateau peat to wet Sphagnum riparium or even fen peat). We used a database of existing peat core records from around the northern high latitudes to determine transition of fen to bog from plant macrofossils and determined permafrost aggradation from both plant macrofossils and physical peat properties to improve constraints on methane emissions from northern peatlands throughout the Holocene.
Here, we examine the spatial and temporal trends of the fen to bog transition and permafrost aggradation in the northern high latitude regions by compiling a database of existing records of macrofossil assemblages and peat properties (carbon, nitrogen, and bulk density). We find that the timing of the fen-to-bog transition varied throughout the northern high latitudes, from 5200 yr BP in Alaska and Western Canada to < 1000 yr BP in Eastern Canada and Siberia. Similarly, the first occurrences of permafrost aggradation varied across the high latitudes, ranging from 4000 yr BP in Western Canada to the Little Ice Age in southern regions and parts of Western Siberia. The spatial and temporal differences in the fen to bog transition and permafrost aggradation suggest that methane emissions differed considerably across northern high latitudes throughout the Holocene. Identifying controls of the fen-to-bog transition and permafrost aggradation in the northern high latitudes has important implications for both carbon sequestration and methane emissions from northern peatlands to the atmosphere throughout the Holocene.