Previous studies investigating organic‐rich tundra have reported that increasing biodegradation of Arctic tundra soil organic carbon (SOC) under warming climate regimes will cause increasing CO2and CH4emissions. Organic‐poor, mineral cryosols, which comprise 87% of Arctic tundra, are not as well characterized. This study examined biogeochemical processes of 1 m long intact mineral cryosol cores (1–6% SOC) collected in the Canadian high Arctic. Vertical profiles of gaseous and aqueous chemistry and microbial composition were related to surface CO2and CH4fluxes during a simulated spring/summer thaw under light versus dark and in situ versus water saturated treatments. CO2fluxes attained 0.8 ± 0.4 mmol CO2m−2h−1for in situ treatments, of which 85 ± 11% was produced by aerobic SOC oxidation, consistent with field observations and metagenomic analyses indicating aerobic heterotrophs were the dominant phylotypes. The Q10values of CO2emissions ranged from 2 to 4 over the course of thawing. CH4degassing occurred during initial thaw; however, all cores were CH4sinks at atmospheric concentration CH4. Atmospheric CH4uptake rates ranged from −126 ± 77 to −207 ± 7 nmol CH4m−2h−1with CH4consumed between 0 and 35 cm depth. Metagenomic and gas chemistry analyses revealed that high‐affinity Type II methanotrophic sequence abundance and activity were highest between 0 and 35 cm depth. Microbial sulfate reduction dominated the anaerobic processes, outcompeting methanogenesis for H2and acetate. Fluxes, microbial community composition, and biogeochemical rates indicate that mineral cryosols of Axel Heiberg Island act as net CO2sources and atmospheric CH4sinks during summertime thaw under both in situ and water saturated states. Arctic mineral cryosols oxidize atmospheric CH4under saturated and dry statesCO2emissions result of aerobic heterotrophy and governed by water saturationPermafrost thaw increases cumulative carbon loss of soils up to a factor of 3