Organic Matter Characteristics in a changing Thermokarst Landscape: Insights from the Baldwin Peninsula, Alaska

Permafrost degradation is accelerating in ice-rich Arctic lowlands, reshaping thermokarst landscapes and mobilising formerly frozen organic carbon. As permafrost thaws, microbial decomposition speeds up, producing greenhouse gases (GHG) and altering carbon cycling. GHG emissions are closely linked to the microbial degradability of organic matter (OM), which depends on both physical accessibility and substrate quality. This study investigates OM characteristics in five landscape units on the Baldwin Peninsula, Alaska, representing successive stages of thermokarst-driven transformation: undisturbed Yedoma upland, thermokarst lake, semi-drained lake basin (SDL), drained lake basin (DLB), and a near-shore marine deposit as an end-member. Focusing on the uppermost meter of sediment, we applied a multi-proxy approach including n-alkane biomarker analysis, elemental composition (TOC, C/N), δ¹³C stable isotopes, radiocarbon dating, and sedimentological characterisation. We addressed two main questions: (1) How does OM quality vary across these landforms? (2) What is the potential for future OM decomposition and mobilisation? Our findings show that OM in thermokarst-influenced units (lake, SDL, DLB) is more strongly degraded than in upland deposits. The DLB represents a refrozen talik, while the SDL retained an unfrozen talik at the time of coring; OM degradation was highest in the SDL. These results highlight the role of talik dynamics in shaping OM decomposition potential and underscore the vulnerability of carbon stored in yedoma uplands under future thermokarst activity. Our study contributes to understanding permafrost carbon dynamics and the climate sensitivity of Arctic lowland systems.