Thermokarst Ponds Drive Landscape-Level Carbon Greenhouse Gas Fluxes in a Dynamic Ice-Rich Arctic Ecosystem Over Decadal Time Scales

Abstract:

A widespread occurrence across the circumpolar Arctic is the degradation of ice wedges and formation of water-filled thermokarst troughs and ponds. Establishment of aquatic vegetation and evolution of these ponds over time leads to organic matter accumulation that halts further degradation and promotes increased vegetation productivity, a process that results in ice wedge stabilization. The degradation-stabilization process occurs over many decades and is highly spatially variable resulting in a mosaic of undegraded, actively degrading, and stabilizing features across the landscape. To understand how carbon greenhouse gas fluxes in these dynamic landscapes change with time we measured growing season carbon dioxide (CO₂) and methane (CH₄) fluxes across a chronosequence of five ice-wedge degradation and stabilization stages at a 6.6-ha study area near Prudhoe Bay, Alaska, and paired our results with areal changes in these stages determined by imagery analysis from 1949-2018. CO₂ fluxes (respiration and net uptake) and CH₄ fluxes were greatest from the shallow vegetated stabilizing thermokarst troughs, whereas deep degraded unvegetated thermokarst ponds were moderate emitters of CO₂ and CH₄. Fluxes across all stages were significantly correlated with aboveground graminoid biomass. We estimate that seasonal fluxes in CO₂-equivalents increased by 14% from 1949-2018. Most of that increase occurred after 2009 coincident with a steady increase in deep degraded unvegetated thermokarst ponds, highlighting the importance of these features in landscape-level greenhouse gas emissions.