Geometry of Floating Vegetation Mats on the Margins of a Thermokarst Lake, Northern Seward Peninsula, AK

Thermokarst lakes are complex, dynamic systems that have the potential to be an important positive feedback to the global carbon cycle. Thermokarst lakes in yedoma are especially important due to the higher percentage of labile carbon present in newly formed thaw-bulbs. Permafrost thaw underneath expanding lakes and erosion of permafrost along lake margins result in tapping old Pleistocene and Holocene carbon stocks. This organic carbon is decomposed to methane in anaerobic environments in lake bottoms and thaw bulbs and released to the atmosphere. Understanding mechanisms of lake-margin expansion is essential to predictions of the growth of thermokarst lakes and their greenhouse gas emission potential in the future. Floating vegetation mat features around the margins of thermokarst lakes have recently been identified on the Seward Peninsula. These features are an indicator of rapid lake expansion (up to 2.5m/yr) and represent a recent, widespread mode of thermokarst lake expansion in the region. These vegetation mats have similar spectral properties as the surrounding tundra surface and are therefore not identified in automated remote-sensing-based lake mapping approaches. Since the floating vegetation mats are essentially part of the thermokarst lake, discounting their surface area underestimates permafrost degradation associated with thermokarst lake expansion as well as methane emission potentials. In this study, we have determined the geometry of the floating vegetation mats in a typical second generation thermokarst lake on the Northern Seward Peninsula. Ground penetrating radar determined the thickness of the floating mat and the water column. Aerial photography was used to calculate lateral spatial extent of the mats along the 3.4 km perimeter of the lake. Vegetation mats were 0.5 - 0.8 m thick floating on 0.5 - 0.9 m of water. The thickness of the mat varies little over distances up to 60 m. Initial temperature measurements indicate that the thick organic mats act as insulation, such that a thaw bulb can develop beneath the mats despite the shallow water depth (less than 1m). The insulating quality of the mats also warms adjacent permafrost further promoting lateral expansion. Understanding the geometry of these vegetation mats has implications for determining expansion rates of thermokarst lakes and correctly estimating carbon release potential from remote sensing and aerial photography studies.