Ground-ice features and depth of peat across a mire chronosequence, NW Alaska

Morphology and accumulation rates of peat vary over 10(2)-10(3) y in permafrost peatlands because of interactions between vegetation, permafrost and geomorphology. Measurements of peat depth, active-layer depth, and distributions of ground-ice features are reported here for a 4400 y peatland chronosequence in swales of the Espenberg beach-ridge plain, NW Alaska, with swale ages constrained by 55 radiocarbon assays. Permafrost aggrades to 1.5 m deep within 500 years after swales are isolated from the sea. Continuous peat layers form by 600 y, followed by initially rapid and then slower rates of peat accumulation, reaching depths of 1.2 m by 1500 y but only 1.9 in at 4400 y. Frost blisters occur in peatlands 600-1200 y old, where peat does not extend the full depth of the active layer, facilitating injection of water under hydrostatic pressure through unfrozen sand during early winter. Palsas and peat ridges first occur in large numbers after approximately 1200 y and persist thereafter. Peat ridges at Espenberg are sinuous, elongate palsas whose planform and initiation is attributed to cooling-induced tension fractures.