Saline Permafrost and Cryopegs as Potentially Important Sources of CO2-Assessing Organic Carbon Mineralization Potentials on the Alaskan Coastal Plain.

Thermokarst lake and drained lake basin (DLB) dynamics are intensifying across the Alaskan Arctic Coastal Plain. Thawing, drainage, and erosion expose surface and deep sediments (> 1 m) to aerobic conditions, with saline deposits being particularly vulnerable due to freeze-point depression. As organic carbon mineralization remains poorly constrained, we determined potentials with an aerobic one-year long incubation at 10°C in permafrost upland, lake talik, lake cryopeg, and refrozen saline DLB sediments. We linked CO2 production to biochemical, hydrochemical, and microbial factors, and assessed carbon alteration via repeated n-alkane analyses. After 382 days, average CO2 production was 7.0 ± 0.4 mg C g-1 dry weight (DW), with DLB surface peat yielding the most (40.5 ± 2.7 mg C g-1 DW), and carbon-poor cryopeg deposits (1.4 ± 0.1 mg C g-1 DW) and refrozen saline permafrost (1.5 ± 0.02 mg C g-1 DW) the least. Total organic carbon (TOC) was the main driver of CO2 production, while age, nitrogen content, electrical conductivity, water content, pH, and microbial abundance also correlated significantly with CO2 production. Normalizing production to TOC contents, saline permafrost and cryopeg sediments showed similar CO2 production to active layers, stressing the importance of potentially carbon-rich saline deposits. TOC normalization revealed that carbon characteristics (δ13C, alkane content, ACL) also significantly influenced CO2 production. The n-alkane based quantification of carbon alterations during the incubation further contributes to the understanding of carbon cycling at the molecular level. n-Alkane contents increased on average by 153% and the carbon preference index (CPI) rose from 13.2 to 15.8, likely due to newly produced alkanes, preferential degradation, and desorption processes. This indicates strong responses of the carbon pool and raises questions about the reliability of the CPI as a degradation proxy. Altogether, our study highlights the overlooked role of salinity in CO2 production from Arctic coastal plains which could substantially shift carbon balances.