The silent carbon pool: Cryoturbic enriched organic matter in Canadian High Arctic semi-deserts

Abstract

High Arctic soil organic carbon (SOC) is an important component in the global C cycle, yet there is considerable uncertainty in the estimates for the polar deserts and semi-deserts that dominate these regions. Some of this uncertainty in SOC estimates arises from the cryoturbic processes including diapirism that structure polar desert soils. Diapirism occurs when the top, viscous layer of permafrost is deformed during freezing and ejected upwards into the soil profile forming a distinct diapiric soil patch or diapir. The diapiric is often nutrient rich relative to the surrounding soil; plants seek out and forage for nutrients in these patches creating a localized mixture of old carbon ejected from the permafrost and new carbon. Here we investigate how the subsurface SOC-rich patches in frost boils resulting from diapirism contribute to overall SOC storage in these environments. We quantify the rates of diapirism and fine-scale distribution of SOC in 560 frost boils at two Canadian high Arctic polar desert sites differing in parent material (dolomite versus granitic) with strikingly different plant surface communities. Though total soil organic carbon content did not differ between the dolomite and granite polar semi-deserts, SOC was being stored differently. The dolomitic site had greater SOC content below 10 cm reflecting the more common occurrence of subsurface SOC patches (46% of frost boils) compared to the granitic site (30%). When a subsurface patch of SOCC (SOC expressed on a m−2 basis) was present in a frost boil the boil contained nearly double the SOCC compared to frost boils without subsurface patches (11 ± 6.3 kg SOC m−2 compared to 6.4 ± 3.6 kg SOC m−2). Diapirism occurs in only 35% of all frost boils, but these diapiric patches represent an important, yet heterogenous, pool of SOC in polar semi-deserts. We upscale from these data to generate an improved estimate of SOC stored in the active layer of High Arctic polar semi-deserts of 8.14 ± 0.45 Pg SOC.