Emerging thermokarst lakes amplify benthic activity and carbon mobilization in a continuous permafrost landscape

Abstract Continuous permafrost zones are experiencing accelerated thaw due to Arctic warming, resulting in considerable greenhouse gas (GHG) emissions that significantly impact climate feedbacks. These permafrost thaw-induced landscape shifts promote the formation of thermokarst lakes, hotspots of biogeochemical activity, contaminant cycling and GHG production. Here, we present the first comprehensive assessment of dissolved organic matter (DOM) and terminal electron acceptors (TEAs) dynamics in two recently formed thermokarst lakes situated within tundra landscapes undergoing early peatland development driven by permafrost thaw in the Zackenberg valley, Northeastern Greenland. We conducted vertical profiles of physicochemical measurements, DOM properties, and TEAs throughout the water column, along with lake sediment characterization ( δ 13 C signature, FTIR characterization, C/N ratio). Additionally, a benthic flux chamber experiment was conducted to study biogeochemical differences at the sediment-water interface in both lakes. Analytical approaches included UV–visible and fluorescence spectroscopy, solid-phase extraction, FTIR spectroscopy, ICP-OES, isotope ratio mass spectrometer, and atomic absorption spectrometry. Our results reveal that the emerging lake showed higher molecular weight DOM and organic-rich sediments compared to the more mature thermokarst lake. Similarly, the younger lake exhibited stronger microbial activity in the sediment, with accentuated dissolved oxygen consumption nearly three times higher than the more mature lake, and a shift to reducing conditions after 30 min. Nevertheless, both lakes showed exceptionally high sediment oxygen demand, indicating strong benthic activity at both sites. As permafrost continues to thaw, the emergence and development of thermokarst lakes are likely to amplify greenhouse gas emissions in the Zackenberg valley. These findings represent a critical first step in understanding the shifting biogeochemical dynamics of lakes in a continuous permafrost region with high methane emissions.