Recent increase in peatland carbon accumulation in a thermokarst lake basin in southwestern Alaska

• Thermokarst lake existed for ~ 2300 years prior to peatland establishment ~ 1600 AD.
• Apparent C accumulation rate increased 10 fold following peatland establishment.
• C accumulation rate reduced after transition to a Sphagnum-dominated peatland.
• Higher C accumulation rate after shift to warmer temperatures in the late 1970s.
• Average peatland hydrology did not fluctuate substantially since lake drainage.

Drained thermokarst lake basins cover large areas of northern high latitude lowlands. However, the importance of peat accumulated in these drained basins to carbon (C) cycling is poorly understood. Results are presented here from a permafrost sediment core aimed at investigating site history, lake drainage, apparent carbon accumulation, and paleohydrology in a thermokarst basin on the Yukon Kuskokwim Delta in southwestern Alaska. The results show that a thermokarst lake existed for about 2300 years before it drained and a peatland established in the early 1600s. Average apparent C accumulation rate (CAR) was 12 gC/m2/yr over the 2300 years before lake drainage, but increased to 120 gC/m2/yr following peatland initiation. After transitioning to a Sphagnum-dominated peatland at ~ 1700 AD, CAR reduced gradually before more than doubling from the mid-1970s to the early 1980s. This abrupt increase in CAR was possibly in response to a Pacific climate regime shift in the late 1970s, a transition characterized by a nearly 1.5 °C increase in regional average annual temperature. Even after considering the differential decay of recent peat, there was an increase in C accumulation after 1977. Reconstructed water-table depths from subfossil testate amoebae show little change throughout the past three centuries, but there was a pronounced shift in testate amoeba composition after the 1977 temperature change (e.g., absence of Phryganella acropodia type and increase in Hyalosphenia elegans). If the results from peatlands in other drained thermokarst lake basins are consistent with the results from this study, then the C accumulated in these vegetated drained basins under current warming might help offset some of the C released from thawing permafrost and thermokarst features in northern regions.