Vegetation succession, peat and carbon accumulation in a peatland affected by permafrost thaw in Nunavik, Northern Quebec, Canada

Permafrost peatlands are vulnerable to warming, yet the net effect of thaw-induced carbon (C) release vs accumulation due to increased primary productivity is still unclear. Even tough there is an abundance of climate and greenhouse gas emissions projections, the permafrost C feedbacks remain largely uncertain. This project documents the timing and trajectory of vegetation succession following permafrost thaw in a thermokarst landscape near Kangiqsualujjuaq, Nunavik (Québec, Canada). The main objectives are to i) reconstruct vegetation succession following permafrost thaw, ii) quantify peat and C accumulation since thaw supported by ¹⁴C and ²¹⁰Pb chronologies, and iii) assess small scale variability within the site.

In the field, short peat cores (n= 23) have been collected from the edges of four thermokarst ponds along outward transects perpendicular to each side of the pond. Metal boxes of 30 to 50­-cm length were used to collect subsurface samples, as the focus of the study was to reconstruct recent vegetation succession. In each site, surface vegetation surveys were also conducted. On each core, loss on ignition (LOI) at 1‑cm interval was performed to quantify organic matter and estimate organic C content (50% of organic matter mass). Plant macrofossils analyses at a minimum of 4-cm interval was realized to reconstruct historical plant succession and hydrological variations in the peat. High resolution chronologies using ¹⁴C and ²¹⁰Pb dating will support estimations of the successional changes in relation with climate warming and permafrost thaw. The chronologies will be used to compute age-depth models, as well as peat and apparent C accumulation rates.

Diachronic analysis of aerial photographs highlighted changes in thermokarst features in the palsa site, where numerous ponds have been infilled with vegetation between 1964 and 2021, while new ones have formed. Preliminary results show that recent accumulation and related C content is variable between the vegetated pool edges. Vertical C content follows similar but offset paths according to position within transects. Peat layers with higher C content were associated with qualitatively greater decomposition and found deeper in cores closest to the pond. The reconstruction of peat and C accumulation rates using ¹⁴C and ²¹⁰Pb chronologies (to come), will support improved understanding of the vegetation successions and related C dynamics.

Incorporation of spatiotemporal heterogeneity in C accumulation might present challenges in C budget modeling. This study emphasizes the significance of empirical data in documenting small scale ecological processes under the scope of remote sensing and modeling. Results will contribute to the evaluation of the responses of high-latitude peatland ecosystems to climate warming. Additionally, documentation of recent changes in peat accumulation environments in northern latitudes can support conservation decision making, as protection of C stocks and sinks is increasingly recognized as a natured-based solution in global warming mitigation.