Role of Ecosystem-Driven Permafrost in Soil Development and Ecosystem Fragmentation During the Holocene Evolution of a Boreal Lowland Landscape

Abandoned floodplains are complex landscapes in boreal permafrost regions due to diverse biogeomorphic effects that are highly modified by permafrost aggradation and degradation. We compiled biophysical properties for 150 cores at 135 sites with depths up to ~4 m on the Tanana Flats in central Alaska and developed depth profiles for bulk density, moisture, organic carbon, pH, stable isotopes, radiocarbon age, and thaw strain. We found strong associations among biophysical components but also highly variable soil properties and ecological histories. We developed a conceptual model of the transition pathways and biophysical drivers among ecosystem types. Across sites, fluvial deposition was active from 9820 to 4290 14C YBP, eolian silt and sand deposition prevalent from 3780 to 1890 YBP, and peat accumulation from 5330 YBP to present. Due to flat topography, water impoundment in depressions, groundwater, and ecological feedbacks, ecosystem-driven permafrost has a complex history of repeated aggradation and degradation resulting in highly fragmented landscapes. Recent thermokarst features had ages ranging from 20 to 920 YBP in collapse-scar bogs and from 50 to 250 YBP in collapse-scar fens. Abandoned floodplains with rapid thermokarst provide saturated environments for robust organic accumulation, with mean soil carbon stocks in the top 3 m being similar among thermokarst bogs (133 kg/m2), fens (108 kg/m2) and permafrost plateaus (115 kg/m2). This study contributes needed information on how permafrost dynamics and ground-ice characteristics influence landscape evolution and on the vulnerability of villages built on abandoned floodplains, infrastructure, and military use of training lands in boreal lowlands. Landscape fragmentation, complex palaeoecological histories, high spatial variability, and limited deep sampling of thermokarst features, however, restrict interpretation of how permafrost loss is affecting the soil carbon balance in an area where most permafrost will disappear in this century.