1.1. Arctic Overview
 The Arctic climate is warming dramatically, threatening catastrophic climate change through rapid mobilization of the vulnerable reservoirs of carbon sequestered by permafrost [IPCC, 2007]. Increasingly, feedbacks in the Arctic are recognized as contributing to climate change, including cycles associated with the powerful greenhouse gas methane CH4, whose atmospheric concentration has more than doubled since the preindustrial epoch [Houghton et al., 2001]. Sustained CO2 and CH4 release to the atmosphere from thawing Arctic permafrost is a positive and likely highly significant feedback to climate warming [Oechel et al., 1993; ACIA, 2005; Zimov et al., 2006]. Other possible feedbacks include destabilization of CH4 hydrate deposits underlying the Arctic seabed, very conservatively estimated to contain 2 × 103 Gt of CH4 [Makogon et al., 2007]. Because of the large uncertainties in Arctic budgets and poor understanding of complex feedback processes, predictions of future emission trends, while critical, are highly unreliable [McGuire et al., 2009].
1.2. Geohistory of the East Siberian Arctic Shelf
 The Arctic Ocean comprises ∼1.5% of the global ocean by volume and <5% of its surface area but receives ∼10% of all global river runoff [Aagaard et al., 1981]. The majority (87%) of particulate material delivered by riverine waters and from coastal erosion accumulates in the Arctic continental shelf [Stein and Fahl, 2004; Vetrov and Romankevich, 2004]. Annually, the East Siberian Arctic Shelf (ESAS) accumulates organic carbon equal to that accumulated over the entire pelagic area of the world oceans [Vetrov and Romankevich, 2004]. The sedimentary basins of the ESAS provide favorable conditions for CH4 origination and are predicted to contain a giant natural pool of hydrocarbons stored as oil, natural gas, and CH4 hydrates [Gramberg et al., 1983; Ginsburg and Soloviev, 1994].
 The ESAS is the largest (2.1 × 106 km2) and shallowest (mean depth < 50 m) continental shelf among the world oceans [Jakobsson, 2002]. Because of its shallowness, the ESAS has a unique geological history. During cold climate periods, when sea level was more than 100 m lower, the coastline was up to 1000 km further north, exposing the continental shelf; at that time, the entire area of the Siberian coastal accumulative plain was larger than today by a factor of 5 [Romanovskii and Hubberten, 2001; Romanovskii et al., 2005]. Freezing to as deep as hundreds of meters led to permafrost formation (sediments with a 2 year mean subzero temperature), which restricted upward CH4 migration. Trapped CH4 was then transformed into permafrost-associated hydrates [Soloviev et al., 1987; Kvenvolden, 1988]. Replacement of the cold epoch by the current warm Holocene epoch was accompanied by a sea level rise that led to permafrost inundation 5–12 kyr ago [Fleming et al., 1998]. This inundation caused the environmental thermal regime to warm by as much as 12°C; in response, the subsea permafrost started warming to achieve a new quasi-stationary equilibrium [Soloviev et al., 1987; Kvenvolden, 1988]. Numerical models predict destabilization about 5–10 kyr after inundation, depending on the duration of inundation relative to the duration of previous freezing [Soloviev et al., 1987]. However, observational data [Shakhova et al., 2005; Rachold et al., 2006; Shakhova and Semiletov, 2007] suggest that subsea permafrost is prone to more rapid destabilization than has been predicted based on numerical modeling [Romanovskii et al., 2005].
 After inundation, terrestrial permafrost-related hydrate deposits became Arctic shallow hydrate deposits. It has been recognized that destabilization of these deposits can lead to a large-scale enhancement of aqueous CH4 [Kvenvolden et al., 1993]. Kvenvolden et al.  conducted seasonal (summer and winter) measurements in the Alaskan Beaufort Sea Shelf over 2 years and found significant CH4 supersaturation in some samples (to 118 nM, n = 250, where n is the number of samples). Although consistent with the destabilization hypothesis, Kvenvolden et al.  estimated that the Arctic shelf contribution to the global CH4 budget was small: ∼0.1 Tg yr−1.
 The ESAS comprises ∼25% of the Arctic continental shelf and contains over 80% of existing subsea permafrost and shallow hydrate deposits (Figure 1), which are estimated to contain a total ∼1400 Gt carbon. This reservoir consists of ESAS hydrate deposits estimated to hold ∼540 Gt of CH4 with an additional 2/3 (∼360 Gt) trapped below as free gas [Gramberg et al., 1983; Soloviev et al., 1987]. Because the ESAS is an overlooked sibling to Siberian on-land permafrost that was submerged, terrestrial permafrost estimates are expected to apply to the ESAS. Thus, subsea permafrost is estimated to contain a further 500 Gt within a 25 m thick permafrost layer [Zimov et al., 2006]. Yet despite its reservoir's significance, the role of the ESAS in the modern carbon cycle has received little attention because the ESAS has not been explored until recently [Shakhova et al., 2005; Shakhova and Semiletov, 2007].
 In this study, we test the hypothesis that submerged permafrost destabilization is occurring through analysis of data collected from the ESAS in summer 2005 and 2006 and winter 2007 with respect to the ESAS geological history. Elevated aqueous CH4 supersaturation and atmospheric concentrations were found, which were best explained by CH4 release from seabed deposits. Data demonstrate that the CH4 that accumulated over millennia during ages with colder climate under and within largely impermeable subsea permafrost is, to some extent, imperfectly sequestered. On the basis of these findings, we discuss the global climate implications.