A. N. Salyuk

Extensive Methane Venting to the Atmosphere from Sediments of the East Siberian Arctic Shelf

Bubble, Bubble, Warming and Trouble Vast quantities of methane are stored in ocean sediments, mostly in the form of clathrates, but methane is also trapped in submerged terrestrial permafrost that was flooded during the last deglaciation. There is thus concern that climate warming could warm ocean waters enough to release methane cryogenically trapped beneath the seabed, causing even more warming. Shakova et al. (p. 1246; see the Perspective by Heimann) report that more than 80% of the bottom water, and more than 50% of the surface water, over the East Siberian Arctic Shelf, is indeed supersaturated with methane that is being released from the sub-sea permafrost, and that the flux to the atmosphere now is as great as previous estimates of that from the entire world ocean. Methane emissions from this region of sub-sea permafrost are comparable to previous estimates for the world ocean. Remobilization to the atmosphere of only a small fraction of the methane held in East Siberian Arctic Shelf (ESAS) sediments could trigger abrupt climate warming, yet it is believed that sub-sea permafrost acts as a lid to keep this shallow methane reservoir in place. Here, we show that more than 5000 at-sea observations of dissolved methane demonstrates that greater than 80% of ESAS bottom waters and greater than 50% of surface waters are supersaturated with methane regarding to the atmosphere. The current atmospheric venting flux, which is composed of a diffusive component and a gradual ebullition component, is on par with previous estimates of methane venting from the entire World Ocean. Leakage of methane through shallow ESAS waters needs to be considered in interactions between the biogeosphere and a warming Arctic climate.

The East Siberian Arctic Shelf: towards further assessment of permafrost-related methane fluxes and role of sea ice.

Sustained release of methane (CH4) to the atmosphere from thawing Arctic permafrost may be a positive and significant feedback to climate warming. Atmospheric venting of CH4 from the East Siberian Arctic Shelf (ESAS) was recently reported to be on par with flux from the Arctic tundra; however, the future scale of these releases remains unclear. Here, based on results of our latest observations, we show that CH4 emissions from this shelf are likely to be determined by the state of subsea permafrost degradation. We observed CH4 emissions from two previously understudied areas of the ESAS: the outer shelf, where subsea permafrost is predicted to be discontinuous or mostly degraded due to long submergence by seawater, and the near shore area, where deep/open taliks presumably form due to combined heating effects of seawater, river run-off, geothermal flux and pre-existing thermokarst. CH4 emissions from these areas emerge from largely thawed sediments via strong flare-like ebullition, producing fluxes that are orders of magnitude greater than fluxes observed in background areas underlain by largely frozen sediments. We suggest that progression of subsea permafrost thawing and decrease in ice extent could result in a significant increase in CH4 emissions from the ESAS.

Current rates and mechanisms of subsea permafrost degradation in the East Siberian Arctic Shelf

The rates of subsea permafrost degradation and occurrence of gas-migration pathways are key factors controlling the East Siberian Arctic Shelf (ESAS) methane (CH4) emissions, yet these factors still require assessment. It is thought that after inundation, permafrost-degradation rates would decrease over time and submerged thaw-lake taliks would freeze; therefore, no CH4 release would occur for millennia. Here we present results of the first comprehensive scientific re-drilling to show that subsea permafrost in the near-shore zone of the ESAS has a downward movement of the ice-bonded permafrost table of ∼14 cm year−1 over the past 31–32 years. Our data reveal polygonal thermokarst patterns on the seafloor and gas-migration associated with submerged taliks, ice scouring and pockmarks. Knowing the rate and mechanisms of subsea permafrost degradation is a prerequisite to meaningful predictions of near-future CH4 release in the Arctic.

Carbonate characteristics of waters of the Arctic Ocean continental slope

Carbonate characteristics of the water mass of the deepwater part of the Arctic Ocean (AO) in the continental slope area were determined, and the range and reasons of their variability during summer-fall season were revealed. The AO water area is a meaningful sink for atmospheric carbon dioxide. The warm intermediate Atlantic waters (AW) are also undersaturated with carbon dioxide relative to its content in the atmosphere. While these waters move along AO continental slope, the value pCO2 in the AW core decreases to 8–10 μatm (mainly, due to drop in the water temperature). The potential absorption capacity of the AO deepwater basin is estimated at approximately 48 Tg of carbon (without sea ice taken into account). Joint analysis of carbonate and hydrological parameters showed that near-bottom waters formed on the shallow shelf of the Laptev Sea, which is rich in inorganic and organic carbon of terrestrial and marine genesis, take part in formation of halocline waters of the AO. They are modified due to interaction with AW penetrating to the shelf and are transferred to the deepwater AO segment, where they occur in the halocline according to their density. Transformed near-bottom waters of the Laptev Sea shelf, similar to waters of the halocline of Pacific origin in the eastern sector of the AO, are traced above the continental slope in Amundsen Basin on the basis of higher CO2 concentrations.

The current state of submarine island relicts on the East Siberian shelf

The assessment of the current trend in natural processes against the background of climatic fluctuations is impossible without comprehensive studies of regions where these trends are most prominent. The East Siberian shelf is one of the key regions within the context of the mentioned problem. Periglacial lithogenesis within the shelf determines a wide development of permafrost with high ice content [12, 15]. This situation is responsible for an extremely high degree of instability of the coastal–shelf cryolithozone zone to the thermal and hydrodynamic impact. This fact is evidenced by the reliably established fact of the destruction of several relatively large islands and their transition to the subaqueous state in the last 50–270 yr. The positions of some of them (for instance, the Semenov Shoal, and the Figurin, Diomede, Mercury banks, among others) are known on the shelf of the region [1, 3, 10, 14, and others] (Fig. 1).

Anthropogenic factor and methane emission on the East-Siberian shelf

Results of data analysis, based on measurement of atmospheric concentrations of methane in the shallow part of the East Siberian shelf (ESS) are presented in this work. It was shown that methane emission in the atmosphere is determined not only by natural factors, but is also sensitive to anthropogenic influences, like the engine mode of a ship. It was determined that the hydraulic impact, which occurs when starting a ship’s engine after drifting through a shallow, can induce a great methane outbreak in the atmosphere. The power of these “short-lived” sources can exceed the power of any one deep-water mud volcano. In the shallow parts of the ESS, the anthropogenic factor can be one of the important factors effecting methane outbreaks in the atmosphere.

Comparison of calculated and measured CO2 fluxes between the ocean and atmosphere in the southwestern part of the East Siberia Sea

The interaction between the degradation of land and underwater permafrost, which is a storage of enormous resources of organic matter [1, 2], and the emission of the final product of its decomposition ( CO 2 ) into the atmosphere is of special interest in global warming manifested most strongly in the Arctic region. Insufficient attention has been paid thus far to investigation of the carbonate system in the seas of the Eastern Arctic with the widest and shallowest shelf in the World Ocean. Previously, it was commonly accepted that the Arctic seas serve as sinks of atmospheric CO 2 [3] in the summer‐autumn period due to low water temperature and high seasonal productivity. However, our previous investigations demonstrated that the southwestern part of the East Siberia Sea (ESS) is an important source of carbon dioxide for the atmosphere in summer. It is assumed that high partial CO 2 pressure is formed due to the influence of the river discharge and destruction of labile organic matter transported to seawater as a result of the destruction of the coastal ice complex [2‐6]. Up to the present time, there is no commonly accepted opinion about the preference of applying one or another method for calculating CO 2 fluxes in the ocean‐atmosphere system, in particular, in the polar regions. Based on the study of the ESS, this work presents the first results of the comparison of calculated values obtained by different algorithms with the fluxes measured by the micrometeorological method. The optimal method for calculating the CO 2 fluxes in the ocean‐atmosphere system is chosen for the study region. This work is based on the materials of the expedition in the southwestern ESS in September 2005 (Fig. 1). In these expeditions, CO 2 fluxes between the ocean and the atmosphere in the eastern arctic region were measured synchronously and calculated for the first time. The methods of measurements and calculation of elements of the carbonate system and the CO 2 fluxes between seawater and the atmosphere are described in detail in [5]. The mean CO 2 concentration in the atmosphere (372 µ atm), hourly and daily mean wind velocities ( U ) at a height of 10 m, and quadratic and cubic dependence of gas transport ( k ) on wind velocity were used in the calculations [7, 8]. The peculiarities of the micrometeorological method of measuring CO 2 fluxes are described in [6, 9].