Understanding Carbon Storage in Permafrost Peatlands: Examples from Thermokarst Landscapes in Interior Alaska

Abstract:

As the climate at high latitudes continues to warm, permafrost is becoming increasingly vulnerable to thaw, particularly in the zone of discontinuous permafrost where mean annual air temperatures hover close to zero. Permafrost thaw has dramatic consequences on local hydrology, ecology, and the carbon (C) balance of these ecosystems. The degree to which permafrost peatlands become C sources or sinks following permafrost thaw remains a topic of much debate, owing to the difficulty in accurately dating cores from collapse-scar bogs and in understanding the vulnerability of the formerly forested peat to decomposition immediately following thaw and inundation. Here we use a combination of a chronosequence, paleoenvironmental, and modeling approaches from a permafrost peat plateau interspersed with collapse-scar bogs in Interior Alaska to address how C storage is impacted on timescales from decades to millennia following permafrost thaw. Our initial results indicate that C stocks are lowest in young (years to decades) and intermediate (decades to centuries) collapse-scar bogs, but that net C stocks in old (centuries to millennia) bogs can be as high as in permafrost plateaus. Thawed forested plateau peat is subject to rapid decomposition in the years to decades following thaw, but that rate of loss slows over time. We hypothesize that this is because the labile C pool is preferentially consumed and quickly exhausted by soil microbes. Meanwhile, post-thaw collapse-scar bog peat accumulates rapidly at the surface, and our data show that on longer centennial to millennial timescales, this near-surface C may offset initial post-thaw losses at depth. Collapse-scar bog peat accumulation rates outpace the former-plateau peat loss in a matter of years to decades. Older landscapes (i.e., those that began accumulating peat many thousands of years ago), are subject to greater C loss following thaw than are younger landscapes(i.e., those that initiated only a couple thousand years ago), leading to a much longer period of net C loss from the ecosystem in older peatlands.