ИСТИНА

Yedoma deposits are one of the significant carbon pools in the Northern Hemisphere with estimates of ~ 80 to 130 gigatonnes of organic carbon (Strauss et al. 2013). The substantial volume of ground ice (up to 90 vol%) makes it highly vulnerable to potential thaw.The methane pool of Yedoma in Northern Yakutia is estimated as trace levels (Rivkina et al., 2007), but Brouchkov and Fukuda (2002) report high methane concentrations in gas bubbles of ice wedges in central Yakutia in the Aldan river valley of eastern Siberia. The site, located 325 km upstream from the mouth of the River Lena is a result of recent river erosion up to ~0.7 m per year with a Yedoma surface up to 50 m above river level. The exposure has three main units: Late Pleistocene Yedoma ca. 40,000 to 15,000 years BP, overlying Middle and Early Pleistocene sands and clay (1 to 0.1 Ma frozen at the time of formation) and late Neogene (Miocene and Pliocene), mostly sands, likely frozen since the middle Pliocene, or perhaps Early Pleistocene (Brouchkov, Fukuda, 2002). The Yedoma deposits are characterized by ice-rich, dark-grey and brown loam and silts penetrated by ice wedges with a total thickness of 7-10 m. The ground cryostructures variy from suspended at ice wedges to structureless, while organic detritus (wood) and turf inclusions are locally present. In the studied deposits the methane distribution has been measured by “headspace” (Rivkina et al., 2007) and bubble degassing (Brouchkov and Fukuda, 2002). Maximum concentrations were found in the Early Pleistocene alluvial sand (up to 0.2 ml/kg); at the same time, in Yedoma deposits methane (n=10) content does not exceed 0.001-0.006 ml/kg, in the ice wedges (n=5) – 0.001-0.016 ml/kg. Thus, we can speak about an absence of even trace amounts of methane in Yedoma. This result is in good agreement with data about methane distribution in Yedoma of Northern Yakutia (Rivkina et al., 2007), where methane is absent too. It should be noted, that our data conflict with previous investigations of the Central Yakutia which described high methane concentrations in ice wedge gas bubbles (Brouchkov and Fukuda, 2002). The methane distribution through the vertical exposure does not show any systematic trend with depth that would suggest a deep subsurface source or penetration downward from the surface. In general, geological cross-sections of exposure methane-free units (Yedoma) appear to be sandwiched between methane-containing layers.