Heterogeneity in Permafrost Organic Matter Molecular Composition With Depth and Ramifications Upon Thaw in the Western Siberian Lowlands

Abstract

Northern circumpolar permafrost represents a globally significant soil organic carbon (C) reservoir. The reintroduction of this previously frozen C into the contemporary C cycle with accelerated northern climate change has raised concerns over a potential permafrost C warming feedback. Additionally, the chemically distinct molecular composition of permafrost organic C has previously been linked to its high biolability upon thaw. Despite this, few studies to date have characterized trends in the molecular composition and hence potential reactivity of this frozen C with depth in permafrost, limiting understanding of potential feedback evolution with forecast 21st century thaw trajectories. Here, we examine depth‐dependent trends in dissolved organic matter (DOM) molecular composition within frozen peat cores collected from the Western Siberian Lowlands (WSL), the largest permafrost peatland system in the world. We observe a strong positive correlation between dissolved organic C (DOC) concentration and high‐energy aliphatic compound relative abundance within WSL permafrost peat ( ρ  = 0.81, p  < 0.01), with distinct “zones of enrichment” containing between 1.5‐4x higher DOC concentrations and ∼10 percentage point higher aliphatic DOM contribution versus surrounding permafrost peat horizons found to be present within core depth profiles. These observations, coupled with previous regional findings documenting nutrient and trace element enrichment within these depth ranges, suggest that these localized zones of enrichment may act as biogeochemical hotspots within the depth profile of WSL permafrost peat. As such, solute mobilization from these zones may drive non‐linear trajectories (specifically hot moments) of regional and global thaw impacts from WSL permafrost peatlands with continued warming.