Large uncertainty in permafrost carbon stocks due to hillslope soil deposits

Northern circumpolar permafrost soils contain more than a third of the global soil organic carbon pool (SOC). The sensitivity of this carbon pool to a changing climate is a primary source of uncertainty in simulation-based climate projections. These projections, however, do not account for the accumulation of soil deposits at the base of hillslopes (hill toes) and the influence of this accumulation on the distribution, sequestration, and decomposition of SOC in landscapes affected by permafrost. Here we combine topographic models with soil profile data and topographic analysis to evaluate the quantity and uncertainty of SOC mass stored in perennially frozen hill toe soil deposits. We show that in Alaska this SOC mass introduces an uncertainty that is > 200% the state-wide estimates of SOC stocks (77 Pg C) and that a similarly large uncertainty may also pertain at a circumpolar scale. Soil sampling and geophysical imaging efforts that target hill toe deposits can help constrain this large uncertainty.