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

Abstract

Abstract. Thermokarst lakes are important emitters of methane, a potent greenhouse gas. However, accurate estimation of methane flux from thermokarst lakes is difficult due to their remoteness and observational challenges associated with the heterogeneous nature of ebullition. We used high-resolution (9-11 cm) snow-free aerial images of an interior Alaskan thermokarst lake acquired 2 and 4 days following freeze-up in 2011 and 2012, respectively, to detect and characterize methane ebullition seeps and to estimate wholelake ebullition. Bubbles impeded by the lake ice sheet form distinct white patches as a function of bubbling when lake ice grows downward and around them, trapping the gas in the ice. Our aerial imagery thus captured a snapshot of bubbles trapped in lake ice during the ebullition events that occurred before the image acquisition. Image analysis showed that low-flux A-and B-type seeps are associated with low brightness patches and are statistically distinct from highflux C-type and hotspot seeps associated with high brightness patches. Mean whole-lake ebullition based on optical image analysis in combination with bubble-trap flux measurements was estimated to be 174 ± 28 and 216 ± 33 mL gas m −2 d −1 for the years 2011 and 2012, respectively. A large number of seeps demonstrated spatiotemporal stability over our 2-year study period. A strong inverse exponential relationship (R 2 > = 0.79) was found between the percent of the surface area of lake ice covered with bubble patches and distance from the active thermokarst lake margin. Even though the narrow timing of optical image acquisition is a critical factor, with respect to both atmospheric pressure changes and snow/no-snow conditions during early lake freeze-up, our study shows that optical remote sensing is a powerful tool to map ebullition seeps on lake ice, to identify their relative strength of ebullition, and to assess their spatiotemporal variability.

Seasonal Sources of Whole‐Lake CH4 and CO2 Emissions From Interior Alaskan Thermokarst Lakes

“…To further characterize the sources of whole‐lake CH 4 emissions, we utilized an iterative four‐source mass balance model (equation ) to estimate the range of possible contributions from four key C reservoirs in GSL. The four 14 C end‐members were chosen based on the distinct seep types characterized by Lindgren et al () and Walter Anthony and Anthony () and previously surveyed in a comprehensive study ( n = 30). Seep categories included tiny seeps 120 ± 40 YBP ( n = 1, analytical error only), A and B seeps (combined in this plot) 2,710 ± 2590 YBP ( n = 5), C seeps 11,200 ± 3,250 YBP ( n = 3), and weaker hotspot seeps still susceptible to blockage by winter ice 13,740 ± 3,160 YBP ( n = 21).…”

“…For GSL only, annual ebullition rates were estimated by quantifiying the density of predefined ebullition seep types (A, B, C, and Hotspot) using ice-bubble surveys as in Walter Anthony and Anthony (2013). A fifth seep type, known as tiny seeps, which are small, widely dispersed, and unmerged in lake ice (Greene et al, 2014;Lindgren et al, 2016), were also quantified. Walter Anthony and Anthony (2013) also show the locations of these surveys on GSL, which covered aproximately 10% of the lake area.…”

Remote Sensing of Landscape Change in Permafrost Regions

“…In another type of remote sensing application, Lindgren et al . () used low‐altitude aerial photographs to map and quantify methane ebullition features in lake ice in interior Alaska and found that higher methane‐seep densities occurred along lake shores with higher erosion rates, signalling a direct linkage between permafrost thaw along thermokarst lake margins and decomposition of previously frozen soil carbon.…”

Methane fluxes from the sea to the atmosphere across the Siberian shelf seas

“…Determining annual sea‐air CH 4 flux is complicated by ice cover, which impedes and collects CH 4 emissions for much of the year and may contribute to enhanced flux at ice‐out. These processes have been observed in lakes [ Greene et al , ; Jammet et al , ; Lindgren et al , ].…”