Accounting for Subsidence Reveals 50% Deeper Permafrost Thaw and Double the Rate of Carbon Thaw in a Permafrost Warming Experiment
Abstract
Permafrost soils contain large stores of carbon in organic matter that are vulnerable to decomposition upon thaw. One common metric of permafrost thaw is active layer thickness, or the depth of the seasonally thawed layer of soil, as measured from the ground surface. When monitored over time, active layer thickness measurements assume that the ground surface remains static, despite the potential for ground subsidence due to loss of ice and organic matter volume from the soil profile upon thaw. Therefore, active layer thickness often underestimates the rate of permafrost thaw. Thaw penetration, the sum of active layer thickness and subsidence, can be used to track permafrost thaw and improve accuracy of calculations of available carbon and nutrients in the active layer. We combined measurements of subsidence, active layer thickness, and soil carbon and ice content in order to determine the drivers of subsidence and the potential impact of subsidence on carbon cycling in a permafrost warming experiment near the southern limit of permafrost in Alaska. Control plots subsided 10.8 cm over 9 years and experimental air and soil warming increased subsidence by five times. Subsidence resulted in wetter soil conditions in both treatments as the soil surface dropped closer to the water table and the areas of deepest thaw became inundated starting around 2016. Regardless of treatment, ice and organic matter loss drove 85-91% and 9-15% of subsidence, respectively. Thaw penetration was between 19% and 49% deeper than active layer thickness in control and warming plots, respectively, and the amount of thawed carbon within the active layer was between 37% and 113% greater over the course of the experiment. This shows that old carbon in permafrost is thawing more quickly than we realized, and this could contribute to increased carbon fluxes to the atmosphere and lateral transport of carbon in groundwater. The magnitude of this impact is currently uncertain at the landscape scale, due to limited subsidence measurements and uncertainty about the hydrologic impacts of subsidence on carbon fluxes. Therefore, it is necessary to quantify subsidence more broadly across the circumpolar region to determine the full extent of permafrost thaw and its feedback on the carbon cycle.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMB027...01R
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0428 Carbon cycling;
- BIOGEOSCIENCES;
- 0702 Permafrost;
- CRYOSPHERE