Quantitative analysis of diurnal CO 2 flux variations above an alkaline playa

The alkaline playas at Atlin, BC, provide a unique opportunity for studying the carbonate–bicarbonate system and carbonate mineral stability at the Earth's surface. In this study, dynamic closed chambers (DCCs) and pore‐gas sampling were used to directly quantify carbon dioxide (CO 2 ) emission rates and characterize processes governing the CO 2 exchange across the playa‐atmosphere interface. Data were collected at the Atlin site continuously over 27 days in 2020 and 14 days in 2021. Results indicate minimal net exchange of CO 2 across the playa‐atmosphere interface during the monitoring periods, with average fluxes over the two periods of −0.03 and 0.09 µmol m −2 s −1 in 2020 and 2021, respectively. However, distinct diurnal oscillations of CO 2 fluxes were measured with average daytime fluxes of 0.15 ± 0.34 µmol m −2 s −1 (2020) and 0.15 ± 0.19 µmol m −2 s −1 (2021) and nighttime fluxes of −0.24 ± 0.31 µmol m −2 s −1 (2020) and 0.04 ± 0.18 µmol m −2 s −1 (2021). These observations, supported by reactive transport modeling, indicate that CO 2 exchange is predominantly governed by changes in CO 2 solubility in alkaline porewater related to diurnal temperature fluctuations and variations in CO 2 concentrations in ambient air above the ground surface. Even though CO 2 concentrations exceed 8000 ppmv at 1‐m depth, CO 2 emissions to the atmosphere were found to be minimal, likely due to high moisture contents, low connectivity, and tortuosity, limiting upward CO 2 migration. These findings provide insights into CO 2 flux dynamics in alkaline arid regions and show promise for the application of the DCC method for monitoring ex situ carbon mineralization at sites with enhanced mineral weathering. Magnesium‐carbonate playas forming from CO 2 ‐rich groundwater show no net emission of CO 2 . Surficial CO 2 exchange is found to be dominated by CO 2 solubility from diurnal cycling in temperature. Diurnal fluctuations involve daytime CO 2 emissions and CO 2 uptake during the night. Radiocarbon signature of CO 2 indicates mixing of atmospheric CO 2 with a geologic source of carbon from the groundwater.