A 2200-Year Record of Permafrost Dynamics and Carbon Cycling in a Collapse-Scar Bog, Interior Alaska

Abstract

Recent high-latitude warming is increasing the vulnerability of permafrost to thaw, which is amplified by local disturbances such as fire. However, the long-term ecological effects and carbon dynamics are not well understood. Here we present a 2200-year record of pollen, plant macrofossils, testate amoebae, and apparent carbon (C) accumulation rates from two peat cores in a collapse-scar bog (thermokarst) near Fairbanks, Alaska. A black spruce ecosystem with low apparent C accumulation rates existed on the site during the first ~1500 years of the record. We identify two thaw events, which are linked to local fires. Permafrost aggraded rapidly following the first thaw, which we attribute to local vegetation feedbacks and a cooler climate. The second thaw event at 525 cal y BP is preceded by a stand-replacing fire, as evidenced by a drastic decline in Picea and an initial increase in Epilobium, Salix, and ericaceous shrubs, followed by a sustained increase in Populus. Locally, the forest does not recover for more than 100 years, and the site has remained permafrost-free for the last 500 years. Following thaw, average apparent C accumulation rates (60 to >100 g C m−2 y−1) are 5–6 times higher than average boreal C accumulation rates, indicating that peat C accumulation rates can remain substantially elevated for much more than a century following thaw. The low apparent C accumulation for the formerly forested, permafrost peat (<5 g C m−2 y−1) may suggest that C accumulation increases substantially following thaw, but it remains unknown whether deep peat C loss occurred immediately following thaw. Well-preserved Sphagnum peat dominates during this period of rapid accumulation, except for an interval from ~400 to 275 cal y BP which alternates between Sphagnum and vascular plant-dominated peat and wetter, minerotrophic conditions. A decline in Picea pollen during this interval and again ~100 cal y BP suggests a decrease in suitable substrate for tree growth likely attributable to thermokarst expansion on the collapse-scar margin. These findings suggest that the combined effects of fire and thermokarst will result in a long-term reduction of spruce ecosystems in interior Alaska.