Quantitative analysis of diurnal CO2 flux variations above an alkaline playa

• Published in: Vadose zone journal 2023-11, Vol.22 (6), p.n/a
• Main Authors: Doucet, Anne‐Martine, Jones, Frances, Raymond, Katherine E., Dipple, Gregory, Black, Thomas Andrew, Ladd, Bethany, Mayer, Klaus Ulrich
• Format: Article
• Language: English
• Subjects: Air temperature; Alkalinity; Arid regions; Arid zones; Atmosphere; Bicarbonates; Carbon dioxide; Carbon dioxide concentration; Carbon dioxide emissions; Carbonate minerals; Carbonates; Climate change; Diurnal; Dynamic stability; Earth surface; Emissions; Exchanging; Fluxes; Gas sampling; Groundwater; Mineralization; Minerals; Moisture content; Monitoring; Monitoring methods; Oscillations; Playas; Pore water; Precipitation; Sediments; Surface stability; Tortuosity
• ISSN: 1539-1663; 1539-1663
• DOI: 10.1002/vzj2.20292

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 (CO2) emission rates and characterize processes governing the CO2 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 CO2 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 CO2 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 CO2 exchange is predominantly governed by changes in CO2 solubility in alkaline porewater related to diurnal temperature fluctuations and variations in CO2 concentrations in ambient air above the ground surface. Even though CO2 concentrations exceed 8000 ppmv at 1‐m depth, CO2 emissions to the atmosphere were found to be minimal, likely due to high moisture contents, low connectivity, and tortuosity, limiting upward CO2 migration. These findings provide insights into CO2 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. Core Ideas Magnesium‐carbonate playas forming from CO2‐rich groundwater show no net emission of CO2. Surficial CO2 exchange is found to be dominated by CO2 solubility from diurnal cycling in temperature. Diurnal fluctuations involve daytime CO2 emissions and CO2 uptake during the night. Radiocarbon signature of CO2 indicates mixing of atmospheric CO2 with a geologic source of carbon from the groundwater. Core Ideas Magnesium‐carbonate playas forming from CO2‐rich groundwater show no net emission of CO2. Surficial CO2 exchange is found to be dominated by CO2 solubility from diurnal cycling in temperature. Diurnal fluctuations involve daytime CO2 emissions and CO2 uptake during the night. Radiocarbon signature of CO2 indicates mixing of atmospheric CO2 with a geologic source of carbon from the