Organic carbon (OC) reservoirs of global importance include the ocean, atmosphere, living terrestrial biomass, and terrestrial sediments. Recent studies suggest that 1400 to 1850 Gt of frozen OC are stored in northern high-latitude permafrost soils [McGuire et al., 2009; Tarnocai et al., 2009]. In the vast northern permafrost Yedoma region that remained unglaciated during the last ice age, alluvial floodplains, hill slopes, and polygonal lowlands [Strauss et al., 2012] accumulated OC in Yedoma deposits ≤50 m thick [Kanevskiy et al., 2011], while segregated ice (SEI) and massive wedge ice (WI) concurrently formed within the sediments (Figures 1, 2, and S1 in the supporting information (SI)). Yedoma region (Figure 1) permafrost-preserved OC, including ice-rich peat in thaw-lake basins and refrozen lacustrine deposits, is weakly decomposed [Schirrmeister et al., 2011] due to fast incorporation into permafrost from the seasonally thawed active layer. Once frozen, organic matter degradation ceases. Observations and models indicate warming and thawing permafrost in many regions [Romanovsky et al., 2010], an irreversible process on human timescales [Schaefer et al., 2011; MacDougall et al., 2012]. Once unlocked by thaw, permafrost organic matter decomposes and CO2 or CH4 are produced and released [Walter et al., 2006; Schuur et al., 2009; Mackelprang et al., 2011]. Projections suggest that greenhouse gas emissions from permafrost until 2300 (Representative Concentration Pathways 8.5 scenario) [Schneider von Deimling et al., 2012] are comparable in amount to all pre-2000 anthropogenic emissions, thus affecting the global carbon cycle and amplifying surface warming [McGuire et al., 2009; Koven et al., 2011; Schaefer et al., 2011; Burke et al., 2012; Ciais et al., 2012; DeConto et al., 2012; Schneider von Deimling et al., 2012; Schuur et al., 2013].
Figure 1. Location of the study sites. (a) Potential [after Romanovskii, 1993] and fragmented [Jorgenson et al., 2008; Grosse et al., 2013] area of Yedoma deposits in Arctic and sub-Arctic lowlands, including the studied thermokarst deposit areas. Sites are numbered; 1, Cape Mamontov Klyk; 2, Nagym Island; 3, Khardang Island; 4, Kurungnakh Island; 5, Bykovsky Peninsula; 6, Muostakh Island; 7, Buor Khaya Peninsula; 8, Stolbovoy Island; 9, Bel'kovsky Island; 10 and 11, Kotel'ny Island; 12, Maly Lyakhovsky Island; 13, Bol'shoy Lyakhovsky Island; 14, Cape Svyatov Nos; 15, Oyogos Yar; 16, Kytalyk; 17, Duvanny Yar; 18, Kitluk River; 19, Vault Creek tunnel; 20, Dalton Highway; 21, Itkillik River; 22, Colville River; and 23, Camden Bay. For site 18, only thermokarst deposit samples are available, resulting in 22 Yedoma and 10 thermokarst deposit sites. (b) A panchromatic Landsat-7 image of the wintery Svyatoy Nos and Shirokostan peninsulas (12 March 2012, extent of the white box shown in a), illustrating several granite domes mantled by Yedoma deposits and strong dissection by thermokarst depressions (Landsat data source: USGS EROS Data Center). The black box marks the location of the area shown in Figure 2b.
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Figure 2. Pictures of studied deposits. (a) Yedoma deposits with wedge ice (WI) exposed at the Itkillik River, Alaska (Figure 1, site 21, photo from J. Strauss ); (b) photo of a Yedoma-thermokarst landscape on Shirokostan Peninsula, Laptev Sea (black box in Figure 1b, photo from L. Schirrmeister ); (c) example of fossil root-bearing Yedoma deposits from Bykovsky Peninsula, Laptev Sea (Figure 1, site 5, photo from L. Schirrmeister ); and (d) example of segregated ice (SEI) (ice lenses and ice bands) at Buor Khaya Peninsula (Figure 1, site 7, photo from J. Strauss ).
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 Various factors could lead to quick Yedoma region OC mobilization, including active layer thickening [Schaefer et al., 2011], widespread thermokarst formation in icy substrates, or accelerated coastal erosion by a sea ice-free Arctic Ocean [Lantuit et al., 2012]. However, large uncertainties for the Yedoma region OC pool remain due to scarce data on OC spatial variability, ground ice content, bulk density (BD), and limited knowledge of Yedoma deposit thickness and spatial extent as well as scarce data on presence of other deposits in this region. Because >50% of the modeling-scenario spread in permafrost OC climate response is caused by uncertainties in the permafrost OC pool size [Burke et al., 2012], better size estimates of deep frozen OC pools are essential. The objective of this study is to assess the deep OC pool of the Yedoma region (SI, section S1.1.1; potential area of Yedoma-deposit distribution, including thermokarst deposit areas) to reduce quantitative uncertainty concerning thaw-vulnerable permafrost OC.
 The Yedoma region OC pool was previously estimated at 450 Gt based on mean 2.56 wt % OC content, mean 25 m deposit thickness, and an estimated 1,000,000 km2 coverage [Zimov et al., 2006]. However, recent studies [Grosse et al., 2013] (Figure 1) have shown that Yedoma deposits are fragmented by thermokarst processes and cover only part of the Yedoma region.