Chuck Randall Smallwood ^1* Nicholas Hasson ² Jihoon Yang ³ Jenna Schambach ¹ Haley Bennett ¹ J. Bryce Ricken ¹ Jason Sammon ⁴ Monica Mascarenas ¹ Naomi Eberling ¹ Stephanie Kolker ¹ Wittney Mays ¹ Joshua Whiting ⁴ Katey Walter Anthony ² Philip R Miller ⁴

• ¹ Department of Environmental Systems Biology, Sandia National Laboratories, Albuquerque, New Mexico, United States
• ² Water and Environmental Research Center, University of Alaska Fairbanks, Alaska, Alaska, United States
• ³ Bioresource and Environmental Security, Sandia National Laboratories (DOE), Livermore, California, United States
• ⁴ Department of Biological & Chemical Sensors, Sandia National Laboratories, Albuquerque, NM, United States, Albuquerque, New Mexico, United States

Permafrost thaw increases the bioavailability of ancient organic matter, facilitating microbial metabolism of volatile organic compounds (VOCs), carbon dioxide, and methane (CH4). The formation of thermokarst (thaw) lakes in icy, organic-rich Yedoma permafrost leads to high CH4 emissions, and subsurface microbes that have the potential to be biogeochemical drivers of organic carbon degradation and greenhouse gas production in these systems. However, to better characterize and quantify rates of permafrost changes, methods that further clarify the relationship between subsurface biogeochemical processes and microbial dynamics are needed. In this study, we investigated four sites (two well-drained thermokarst mounds, a drained thermokarst lake, and the terrestrial margin of a recently formed thermokarst lake) to determine whether biogenic VOCs 1) can be effectively collected during winter, and 2) whether winter sampling provides more biologically significant VOCs correlated with subsurface microbial metabolic potential. During the cold season (March 2023), we drilled boreholes at the four sites and collected cores to simultaneously characterize microbial populations and captured VOCs. VOC analysis of these sites revealed "fingerprints" that were distinct and unique to each site. Total VOCs from the boreholes included >400 unique VOC features, including >40 potentially biogenic VOCs related to microbial metabolism. Subsurface microbial community composition was distinct across sites; for example, methanogenic archaea were far more abundant at the thermokarst site characterized by high annual CH4 emissions. The results obtained from this method strongly suggest that ~10% of VOCs are potentially biogenic, and that biogenic VOCs can be mapped to subsurface microbial metabolisms. By better revealing the relationship between subsurface biogeochemical processes and microbial dynamics, this work advances our ability to monitor and predict subsurface carbon turnover in Arctic soils.