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UBC Theses and Dissertations

Processes governing carbon dioxide exchange between the atmosphere and hydromagnesite-magnesite playas in Atlin, British Columbia Doucet, Anne-Martine


The hydromagnesite-magnesite playas in Atlin, BC provide a unique opportunity for studying the carbonate-bicarbonate system and carbonate mineral stability at the Earth’s surface. Based on analysis of pore water samples and mineralogical data, Power et al. (2014) concluded that the playas degas CO₂ from Mg-HCO₃-rich groundwater and in-situ carbonate mineral precipitation, but CO₂ emissions were not quantified directly. In this thesis, eddy covariance (EC) and dynamic closed chamber (DCC) systems were co-located to directly quantify rates and characterize processes governing the CO₂ flux across the playa-atmosphere interface. Data were collected continuously over 27 days in 2020 and 14 days in 2021. The results from the DCC method show distinct diurnal oscillations of CO₂ fluxes, with average daytime fluxes of +0.15±0.34 μmol mˉ² sˉ¹ (2020) and +0.15±0.19 μmol mˉ² sˉ¹ (2021) and nighttime fluxes of -0.24±0.31 μmol mˉ² sˉ¹ (2020) and +0.04±0.18 μmol mˉ² sˉ¹ (2021) (positive upward and negative downward). Fluxes measured via the DCC method indicate minimal net exchange of carbon across the playa-atmosphere interface during the monitoring period. These observations imply that DCC-measured CO₂ fluxes are governed predominantly 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. However, EC measurements show a continuous positive flux averaging +1.38±0.62 μmol mˉ² sˉ¹ (2020) and +1.07±0.43 μmol mˉ² sˉ¹ (2021). The net CO₂ flux measured by EC was attributed to a source undetected by the DCCs with possible contributions from soil respiration at the playa margins or surrounding forests, or directly released via preferential pathways from a deeper source within the playa. The use of two complementary flux measurement methods revealed that CO₂ fluxes vary as a function of scale and location at this site. These findings provide insights on CO₂ flux dynamics in sparsely vegetated arid and semi-arid regions and the application of these methods for monitoring and verification of ex-situ carbon mineralization at sites with enhanced mineral weathering. The complex interactions between minerals, fluids and the atmosphere make Atlin an ideal site to test complementary methods for measuring CO₂ fluxes.

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