Methane release and coastal environment in the East Siberian Arctic shelf

Abstract

In this paper we present 2 years of data obtained during the late summer period (September 2003 and September 2004) for the East Siberian Arctic shelf (ESAS). According to our data, the surface layer of shelf water was supersaturated up to 2500% relative to the present average atmospheric methane content of 1.85 ppm, pointing to the rivers as a strong source of dissolved methane which comes from watersheds which are underlain with permafrost. Anomalously high concentrations (up to 154 nM or 4400% supersaturation) of dissolved methane in the bottom layer of shelf water at a few sites suggest that the bottom layer is somehow affected by near-bottom sources. The net flux of methane from this area of the East Siberian Arctic shelf can reach up to 13.7 × 10⁴ g CH₄ km^− 2 from plume areas during the period of ice free water, and thus is in the upper range of the estimated global marine methane release. Ongoing environmental change might affect the methane marine cycle since significant changes in the thermal regime of bottom sediments within a few sites were registered. Correlation between calculated methane storage within the water column and both integrated salinity values (r = 0.61) and integrated values of dissolved inorganic carbon (DIC) (r = 0.62) suggest that higher concentrations of dissolved methane were mostly derived from the marine environment, likely due to in-situ production or release from decaying submarine gas hydrates deposits. The calculated late summer potential methane emissions tend to vary from year to year, reflecting most likely the effect of changing hydrological and meteorological conditions (temperature, wind) on the ESAS rather than riverine export of dissolved methane. We point out additional sources of methane in this region such as submarine taliks, ice complex retreat, submarine permafrost itself and decaying gas hydrates deposits.

Introduction

During the warm stages the Arctic is considered to be a source of atmospheric methane (CH₄), ensuring the existence of the inter-polar CH₄ gradient (8–10%); this gradient decreases to a practically negligible value during glacial epochs (IPCC, 2001). Northern terrestrial ecosystems are the first candidate thought to play a key role in production of atmospheric methane but their impact is not enough to explain the highest concentration of CH₄ above the Arctic. As is already known, the Arctic region is considered to contain a huge amount of organic carbon buried not only inland, but also within Arctic Ocean sedimentary basin (called “Arctic carbon hyper pool”, Gramberg et al., 1983). A significant portion of organic carbon withdrawal occurs over the Siberian shelf (Fahl and Stein, 1998, Bauch et al., 2000).

The ESAS represents the broadest and shallowest shelf in the World Ocean yet it is the least explored. Under the influence of the riverine discharge of the Siberian rivers, the ESAS acts as an “estuary” for the Arctic Ocean (Semiletov et al., 2000). Studies by Rogers and Morack (1980) on sub-sea permafrost and sea level history lead to the inference that offshore permafrost may persist beneath any part of the Arctic shelves inshore from about the 90 m isobath. It was considered until recently that, due to slightly negative annual temperatures within the water column and the lid type coverage of shelf sediments by sub-sea permafrost, old organic carbon buried on the Siberian Arctic shelf is completely preserved from being involved in the modern carbon cycle. Nevertheless, there is the geological model of degradation of permafrost submerged by seawater, performed for the ESAS, which allows complete degradation of sub-sea permafrost under the impact of geothermal heat flow in fault zones and under the channels of large rivers (Romanovskii and Hubberten, 2001). Permafrost contains a huge amount of ancient organic matter that might be involved in current biogeochemical cycling due to thawing of the upper permafrost and restoration of the activity of viable methanogens (bacteria that produce methane as a metabolic product) preserved in permafrost (Rivkina et al., 1998). The mechanism which allows old organic carbon to be involved in the modern cycle is one of the subjects of this paper.

The contribution from all marine methane sources is assumed to range between 5 and 20 Tg CH₄ yr^− 1 (IPCC, 2001). However, there are a number of published papers pointing at substantial underestimation of the contribution of aquatic environments; which is likely to grow as climate change leads to warming of the planet (Hovland et al., 1993, Lammers et al., 1995, Judd et al., 2002, Damm et al., 2005). Indeed, geological methane, generated by microbial decay and the thermogenic breakdown of organic matter, migrates towards the seabed to be released through natural gas seeps. The total annual contribution of marine geological sources to the atmosphere is estimated as 16–40 Tg of methane, which has never been taken into consideration in estimations of the global methane budget (Judd et al., 2002). It is known that temperature and pressure conditions in wide areas of the seafloor allow sub-sea methane accumulation in the form of methane gas hydrates, which might decay if disturbed. It was estimated by Kvenvolden and Grantz that a total amount of 10¹⁵ m^− 3 of methane (540 Gt of carbon) exists in sediments of the offshore Arctic Basin (Kvenvolden, 1988). In addition, previously unknown species of methane hydrates, located outside oil and gas producing areas, have been found within the pores of near-surface continuous permafrost in northeastern Siberia (Chuvilin et al., 2000, Rivkina et al., 2004). Upon thawing this interpore methane can also contribute to the atmospheric methane budget.

Recent environmental changes may result in a major carbon export to the Arctic Ocean due to the retreat of coastal ice complexes, permafrost thawing, increasing river runoff, and an increase of coastal erosion in the northern regions (Semiletov et al., 2000, Peterson et al., 2002). A hypothesized climate-change-driven increase in methane emission from the Arctic Ocean could dramatically alter not only the regional budget, but also the global cycle.