Arctic-boreal lake methane ebullition estimation using ground penetrating radar

Arctic-boreal lakes are a globally significant source of atmospheric methane (CH4), but there are large uncertainties in quantifying lake CH4 emissions that limit the accuracy of climate change projections. Ebullition is the dominant pathway for lake CH4 emissions, and the most challenging to estimate due to its spatial and temporal variability. Previous research has estimated lake CH4 fluxes based on year-round bubble trap measurements and direct observations of gas bubbles trapped in ice and, recently, synthetic aperture radar (SAR) remote sensing was combined with direct observations to provide regional estimations of lake CH4 fluxes. However, the visual survey approach can be limited because the gas bubbles must be visible on the lake ice surface, which can be difficult if there is snow cover or white ice. Therefore, there is a need for a quantitative method for estimating CH4 flux in between the scales of visual surveys and SAR remote sensing. Here we show that this gap can be filled using ground penetrating radar (GPR) to acquire data non-invasively and estimate the volumetric gas content (VGC) trapped in lake ice. The VGC can be converted to CH4 flux rates based on the number of days between initial ice formation and the field measurement and using the average CH4 concentration from gas samples. GPR measurements were conducted on 12 lakes in Alaska, including ten lakes within the North Slope (Utqiagvik, Teshekpuk, and Oumalik), and two lakes near Fairbanks. Ice thicknesses in drill holes were measured in order to calibrate GPR for ice properties. The comparison of GPR (~ 1 x 1 m) and SAR (12.5 x 12.5 m) estimated CH4 flux suggests that GPR may be capturing some local higher CH4 fluxes that SAR averages to lower values considering its much larger sampling volume. This study contributes to an improved understanding of lake ice formation and CH4 fluxes from Arctic-boreal lakes that could be used to refine atmospheric carbon models. Furthermore, a better estimation of lake ice properties and CH4 emissions is essential to project the impacts of climate warming and determine the best management strategies.