Using space borne Synthetic Aperture Radar (SAR) to detect superseeps in Alaskan lakes

Ebullition (bubbling) is often the dominant form of methane (CH4) emission from Arctic lakes. Understanding the dynamics of CH4 ebullition in these lakes is important to the global atmospheric CH4 budget and climate models. Lake CH4 ebullition bubbles generally originate from either ecologic or geologic sources. Ecologic CH4 is produced through anaerobic microbial decomposition of organic matter within lake sediments and the talik - a thawed zone beneath lakes in permafrost regions. Emissions from these seeps can be quantified and scaled based on existing field-based and remote-sensing methods. The other type of ebullition has not been well quantified, yet emits gas at a much higher rate than ecologic seeps. Geologic CH4 seeps originate from microbial, thermogenic, or a combination of both processes altering buried organics in ancient sedimentary basins. Bubbling rates of geologic seeps are strong enough to maintain holes in thick (>1 m) lake ice – creating winter traveling hazards in the Arctic and sub-Arctic. While ecologic CH4 seeps produced in surficial sediments have modern to Holocene radiocarbon (14C) ages and those produced deeper in the talik have Pleistocene to early Holocene 14C ages, geologic CH4 seeps are often 14C-depleted due to the large contribution of carbon from fossil sources. Quantification and upscaling of geologic CH4 seepage is challenging because CH4 accumulations are distributed beneath complex, site-specific geologic and cryospheric settings. Previously, geologic seeps were studied through aerial surveys and ground truthing of open holes in winter lake ice along a north-south Alaskan transect. However, this is not efficient for quantifying these “superseeps” on a larger scale. Therefore, a remote sensing approach is needed. This work aims to detect superseeps using space borne Synthetic Aperture Radar (SAR). Engram et al. (2013) showed that L-band SAR backscatter correlates with roughness caused by stratigraphically-layered ecologic CH4 bubbles trapped during freeze-up – the greater the ebullition, the stronger the backscatter. Using this correlation, we developed a new method that identifies superseeps as perennial backscatter anomalies in lake ice on a landscape scale. Results from three regions in Alaska will be presented and compared to other methods of studying superseeps.