Deciphering organic matter degradation in the continuous permafrost zone of Alaska based on biomarker analyses.

Permafrost regions are highly vulnerable to global warming, as they warm much faster and store large amounts of organic matter (OM), which makes them key to the carbon-climate-cycle. Thermokarst processes, and coastal erosion strongly reshape permafrost landscapes. As thermokarst lakes (TL) have been shown to release organic carbon due microbial decomposition, drained thermokarst lake basins (DTLBs) can sequester OM again due to the potential to reaggregate permafrost in a cold climate, though this potential will slow or not occur in a warming climate. Therefore, understanding these processes is key to predict future potential greenhouse gas (GHG) emissions.  

This study investigates the OM characteristics in such a TL-DTLBs landscape on the Baldwin Peninsula, located in the continuous permafrost zone of Alaska. A multiproxy approach of biogeochemistry, hydrochemistry, sedimentology and n-alkane biomarker analysis was used to investigate (1) the paleoenvironment of the landscape and (2) the characterization of the OM by its quantity, source and quality in terms of its degradation state, which is critical for mineralization processes and potential GHG release upon permafrost thaw. Four sediment cores were collected in 2024 along a transect representing multiple thermokarst stages, from an undisturbed permafrost upland through a thermokarst lake and a recently drained thermokarst basin to a nearshore marine environment.  

Our findings show a continuous Pleistocene deposition in a strongly aeolian regime, with the oldest sediments of > 50 cal. ka BP in the drained lake basin (50 – 150 cm b.s.l.). The sediments are generally of coarse silt,  and show with high water contents, and organic-rich layers typical characteristics of late Pleistocene Yedoma, while the deep layers show signs of an ancient fluvial environment and early Holocene thermokarst processes. Also, the biomarker analysis support a common terrestrial origin of the OM, indicating a secondary marine infiltration for the marine site, as well as a slightly aquatic influences, especially in the deeper layers, resulting from ancient thermokarst processes and the lake / marine phases. The carbon quantity decreases significantly from the upland to the marine site, with higher preserved OM in the taliks than in the perennial frozen layers (e.g. CPI_{thermokarst lake} = 15.77 vs CPI_upland = 8.08 in median).  

Due the high ice amount and carbon quality, the studied deposits reveal a strong vulnerability to continued warming and thus constitute a high GHG release potential.