Holocene development and recent vegetation and carbon storage dynamics in polygonal permafrost peatlands of the Hudson Bay Lowlands, Canada

The Hudson Bay Lowlands peatland complex in Canada, the world’s second-largest peatland complex, is experiencing the impacts of climate change, potentially threatening its carbon sink capacity; however, the direction and magnitude of recent changes are uncertain, particularly in response to ongoing permafrost thaw, climate change, and isostatic rebound. In this project, we aim to document the vegetation changes and carbon (C) storage dynamics, in both the long- (millennial) and short-term (decadal to centennial), of polygonal permafrost peatlands in Wapusk National Park (WNP), located on the northwestern coast of Hudson Bay.

First, we aim to estimate the total C stored in peatlands in WNP and study the Holocene development history of polygonal permafrost peatlands. During 2023–2024, we collected twenty complete peat cores across the different land cover types and ecoregions within WNP. The twenty peat cores will be dated with radiocarbon (¹⁴C) and analyzed for total organic C. The peat cores collected from permafrost peat plateaus will also be examined for paleoecological and palaeoclimatological reconstructions, providing insights into major shifts in plant communities, climate, and permafrost dynamics during the Holocene. Preliminary results show that in WNP, peatland initiation and C accumulation connect to undergoing isostatic rebound. Peat accumulation began ca. 5705 cal. BP in the forest-tundra region and 2390 cal. BP in the coastal fen ecoregion. Permafrost peat plateaus store approximately 80.7 kg C m^-2, with an average long-term apparent rate of carbon accumulation (LORCA) of 26.1 g C m^-2 yr^-1. At several sites, spruce and larch forests preceded contemporary, lichen-dominated peat plateaus.

Second, we will explore why the numerous ponds within the permafrost peatlands are now being infilled by Sphagnum mosses, the most important genus for storing C as peat, and whether this is linked to recent permafrost thaw. Twenty surface peat monoliths were collected along transects at the edges of seven ponds and will be dated with coupled ¹⁴C and lead-210 (²¹⁰Pb) age-depth modeling and analyzed for total organic C, plant macrofossils, and diatom communities. Preliminary results from plant macrofossil analyses indicate infilling and a subsequent increase in Sphagnum cover over fen vegetation rather than thawing-induced subsidence of permafrost peat plateaus. However, this may differ between ecoregions (forest-tundra vs. subarctic peat plateau region), and the timing of the transitions from moss-sedge to Sphagnum peat will be verified with age-depth modeling and remote sensing techniques. These early results show that vegetation changes along the pond edges can significantly affect peatland C accumulation.