Detection and spatiotemporal analysis of methane ebullition on thermokarstlake ice using high-resolution optical aerial imagery

[摘要] Thermokarst lakes are important emitters of methane, a potent greenhousegas. However, accurate estimation of methane flux from thermokarst lakes isdifficult due to their remoteness and observational challenges associatedwith the heterogeneous nature of ebullition. We used high-resolution (9–11 cm)snow-free aerial images of an interior Alaskan thermokarst lake acquired2 and 4 days following freeze-up in 2011 and 2012, respectively, to detectand characterize methane ebullition seeps and to estimate whole-lakeebullition. Bubbles impeded by the lake ice sheet form distinct whitepatches as a function of bubbling when lake ice grows downward and aroundthem, trapping the gas in the ice. Our aerial imagery thus captured asnapshot of bubbles trapped in lake ice during the ebullition events thatoccurred before the image acquisition. Image analysis showed that low-fluxA- and B-type seeps are associated with low brightness patches and arestatistically distinct from high-flux C-type and hotspot seeps associatedwith high brightness patches. Mean whole-lake ebullition based on opticalimage analysis in combination with bubble-trap flux measurements wasestimated to be 174 ± 28 and 216 ± 33 mL gas m⁻² d⁻¹ for the years 2011 and 2012, respectively. A largenumber of seeps demonstrated spatiotemporal stability over our 2-yearstudy period. A strong inverse exponential relationship (R² >  =  0.79)was found between the percent of the surface area of lake icecovered with bubble patches and distance from the active thermokarst lakemargin. Even though the narrow timing of optical image acquisition is acritical factor, with respect to both atmospheric pressure changes andsnow/no-snow conditions during early lake freeze-up, our study shows thatoptical remote sensing is a powerful tool to map ebullition seeps on lakeice, to identify their relative strength of ebullition, and to assess theirspatiotemporal variability.

[学科分类] 地球化学与岩石