Reconstructed Arctic biome and soil distributions: implications for the late Quaternary permafrost subsystem

As part of a larger project exploring permafrost distribution in the Arctic and Beringia region, we are updating BIOME4 model outputs and pollen maps for 21ka (ka = thousand years ago), 6ka, and 0ka time periods. Vegetation is a key control for permafrost distribution as it can affect the surface conditions controlling permafrost in high latitudes. BIOME4 is a coupled biogeography and biogeochemical model that simulates the global equilibrium distribution of biomes. BIOME4 was initially run with two historical climatologies for the 20th century, one of which is a more current climatology not used in previous BIOME4 runs, to obtain a general 'present day' view of the biome distributions. The updated climatology shows advances in the tree line in northern Alaska, but both climatologies generally agree on evergreen and deciduous taiga/montane forest locations. Using PMIP3/CMIP5 global circulation model climate and soil data as well as prescribed carbon dioxide concentrations, we ran the BIOME4 model for each time period. 21ka reconstructions based on the BIOME4 model output show various types of tundra widespread across the region, as far south as 40°N. This differs from present day where modeled tundra is generally limited to northern areas poleward of 70°N, the southernmost extent being ~60°N. However, when compared to real-world data provided by the updated BIOME 6000 global palaeovegetation map, preliminary 21ka tundra biomes differ in coverage. The model places shrub tundras in the focus region where we know graminoid or steppe type tundra existed. The discrepancy appears to arise from soil moisture content that was not changed in the BIOME4 runs for the different time periods, implying that soil water content may be very important in obtaining correct biome distributions in the Arctic. In particular, the modeling results suggest that moisture may be a critical feature determining the distribution of shrubby vs. herbaceous tundra. We know from lake level studies that available moisture was much reduced during the 21ka time period.