The reason for the hydrate destabilization on the Arctic shelf can be both permafrost degradation as a result of the temperature increase, and the processes of penetration of seawater and salt ions contained in it into the layer of hydrate saturated frozen sediments. Today, the issues of salt transfer as a result of the interaction of cooled sea water with frozen hydrate-containing sediments are not sufficiently considered. In this regard, the experimental studying of the mechanism and hydrate dissociation parameters in frozen sediments as a result of salt migration is of particular interest for assessing the role of salt transfer in destabilization of intra-permafrost gas hydrate formations and methane emission on the Arctic shelf.

Since most hydrate deposits in the Arctic are permafrost-controlled, stability of permafrost is a key to whether hydrates are stable [ 31 ]. According to the permafrost thermobaric conditions of the Laptev Sea shelf, there are gas hydrates in the underwater permafrost. However, due to the absence of deep drilling, direct data on the presence of gas hydrate formations is not observed. Nevertheless, by indirect evidence, a number of researchers associate active gas shows with dissociation of gas hydrate formations [ 12 36 ].

ESAS sediments have not been considered a CHsource to hydrosphere or atmosphere because submarine permafrost, which underlies most of the ESAS, was considered to be continuous and to act as an impermeable lid [ 18 ], preventing CHescape through the seabed. However, multi-year data (2000–2018) showed extreme CHsuper-saturation of surface waters (up to 10,000% saturation), implying that about 90% of the total ESAS area serves as a source of CHto the atmosphere [ 12 ] and high air-to-sea bubble fluxes occur at numerous seepage sites [ 11 31 ]. Conservative estimation of CHebullition from the coastal ESAS areas yields an annual contribution of at least 9 Tg-CH, which increases annual atmospheric flux from the ESAS to 17 Tg-CH, on par with flux from the entire Arctic tundra [ 11 ]. That estimate does not include the non-gradual CHrelease discovered recently on the outer ESAS. Sustained CHrelease to the atmosphere from thawing Arctic subsea permafrost and dissociating hydrates were suggested to be positive and likely to be significant feedbacks to climate warming [ 32 33 ].

Under natural conditions, gas hydrates are formed and exist in the bottom sediments of the seas and oceans, as well as in the areas of permafrost distribution where there exist favorable temperatures, pressurization, and geochemical conditions. On the Arctic shelf, gas hydrates can be expected at depths of the sea of 250–300 m, as well as at shallower depths in the presence of underwater permafrost. Considering that the thickness of the submarine permafrost can reach several hundred meters, gas hydrate formations can be located both in the sub-permafrost and intrapermafrost horizons [ 17 20 ]. The possible existence of hydrates within the gas hydrate stability zone (GHSZ) in the East Siberian Arctic shelf (ESAS) was predicted in the late 1970s, when it was understood that the high-latitude, shallow ESAS has been alternately subaerial and inundated with seawater during glacial and interglacial periods, respectively. Submarine conditions foster the formation of permafrost and associated underlying hydrate deposits, whereas inundation with relatively warm seawater destabilizes the permafrost and hydrates [ 21 22 ]. Later, hydrates were found at shallower depths (around 20 m), and their existence within the entire permafrost body was attributed to a so-called “self-preservation phenomenon” [ 23 25 ]. These “relict” gas hydrate formations in permafrost soils could have formed earlier, when there were favorable thermobaric conditions. Subsequently, when thermobaric conditions changed, hydrate transferred to the metastable state due to the manifestation of the self-preservation effect [ 26 29 ]. Transition from the last glacial period to the current warm Holocene, accompanied by sea level rise that inundated the previously-exposed shelf area, started 5–12 years ago. According to modeling results, sufficient time has elapsed since inundation to cause permafrost/hydrate system destabilization, which is manifested by formation of taliks (areas of completely thawed sediments within a permafrost) in a certain fraction of the ESAS affected by fault zones, runoff from large rivers, and thermokarst [ 11 31 ].

Gas hydrates are crystalline clathrate compounds that are formed from gas (mainly methane in natural conditions) and water under certain temperature and pressure conditions [ 13 14 ]. An important characteristic of gas hydrates is a huge accumulation of gas in the clathrate structure-up to 160 volumes of gas in one volume of hydrate. As it is well known, methane is one of the most active greenhouse gases. In this regard, the dissociation of Arctic gas hydrates, accompanied by the active emission of methane into the atmosphere, can have a significant greenhouse effect and cause climate change [ 15 16 ].

The Arctic shelf is the most promising hydrocarbon production area. However, its development is associated with the solution of a number of problems that are associated with the conditions of relict permafrost and the presence of gas hydrates [ 1 4 ]. Of particular interest is the assessment of methane emissions during the shelf permafrost degradation and the hydrate decomposition due to heat and mass transfer processes. According to many researchers, dissociation of gas hydrate formations in bottom sediments makes the largest contribution to methane emissions on the Arctic shelf [ 5 12 ].

2. Methods

Experimental modeling of gas hydrate dissociation in frozen sediments as a result of salt migration included the following steps:

1) Preparation of hydrate containing sediment samples, using a pressure cell for artificial hydrate saturation;

2) Freezing of hydrate saturated samples in the pressure cell and transfer pore hydrates to a metastable state by reducing gas pressure to atmospheric at a negative temperature;

3) Extraction of frozen hydrate-saturated samples from the pressure chamber and their contact with a cooled NaCl solution at constant negative temperature and atmospheric pressure.

The objects of study were sandy samples:

- Quarts fine sand (sand 1) - 16,31,37, Silty sand (predominantly quartz composition) sampling during drilling operations on the Laptev Sea shelf (Buor-Khaya Bay in the area of the Muostakh island) (sand 2) ( Table 1 ), where active gas emission was registered, and according to some indirect data there is a probability of the existence of natural hydrate formations in submarine permafrost [ 11 38 ].

The initial values of salinity of sandy samples are given in Table 2

4 —99.98%), and creation of conditions for uniform saturation of the sediment pore space with gas hydrate [ The method of obtaining frozen hydrate-containing samples included sediment samples (twins) preparation of a cylindrical shape (about 3 cm in diameter and 6–9 cm long) with a given moisture content (14–16%), and placing them in a pressure cell, sealing and vacuuming the pressure cell with samples, filling the pressure cell by hydrate-forming gas (CH—99.98%), and creation of conditions for uniform saturation of the sediment pore space with gas hydrate [ 23 39 ]. Several samples were prepared (about 5–6 pcs.) at the same time, which had similar values of water content, density, and hydrate saturation. After hydrate saturation, which lasted at least 1 month, hydrate saturated sediment samples were frozen at the temperature of −7 ± 1 °C. As a result, the residual pore moisture in the samples that did not transfer to the hydrate became frozen out. Subsequently, the gas pressure in the cell, which was at a negative temperature, was dropped to 0.1 MPa, converting the frozen pore hydrate to a metastable state. The frozen hydrate-saturated samples were then taken out. The samples had a massive ice–hydrate texture with pore hydrate contents uniformly distributed over the sample height [ 23 40 ].

For the obtained frozen hydrate-containing samples, prior to their contact with the salt solution, the initial physical parameters (moisture content, density, hydrate content) were determined. Then, the hydrate-containing samples were brought into contact and cooled up to the experiment temperature NaCl solution at a temperature below zero. The experiments were carried out at temperatures from −2.5 °C to −4 °C and concentrations of salt solution from 0.1 to 0.4 N (accordingly to the natural Laptev Sea water concentration, which equals 10–34‰ [ 41 ]). The duration of the tests depends on the experimental conditions and ranged from several tens of minutes to several days. For control, one of the samples was stored at a given negative temperature, but was not brought into contact with the salt solution. These control data showed that during the experiment the change in the hydrate content of sediment samples under self-preservation conditions was insignificant, and the main decrease in hydrate saturation of other samples was caused by salt migration.

During the experiments, the dynamics of the interaction of frozen hydrate-containing samples with a salt solution over time, as well as the effect of the concentration of the NaCl contact solution and the ambient temperature on the hydrate dissociation processes in the porous media of sediments were studied. In experimental modeling, hydrate-containing samples were removed from contact solution at certain time intervals. A number of parameters were determined for each sample, characterizing the content of moisture, salts and gas hydrate along the sample length separately in nine-ten 7–10-mm-thick layers of the samples after interaction with the salt solution. This made it possible to trace the dynamics of saline front migration, thawing, and dissociation of gas hydrate formations in frozen sediment samples.

39, G , cm3/g) was found as G = ( V 2 − V 1 ) ∗ T m s , V 2 − V 1 )—change in the volume of liquid in the gas collector tube (cm3); T —temperature correction; m s —the mass of the sediment sample (g). Gas contents were estimated by measuring the volume of gas released (with 2–3 times repeatability) as the samples were thawing in a saturated NaCl solution. Samples with residual hydrate content were put in a special glass tube filled with brine (saline solution), and as a result of gas hydrate dissociation and methane release volume of the brine in the glass tube was changed. And by the volume difference, it was possible to estimate gas content in studied samples. The obtained values were used to estimate hydrate content and hydrate coefficient [ 23 40 ], assuming a hydrate number of 5.9 for methane hydrate [ 19 42 ]. Specific gas content (, cm/g) was found aswhere: ()—change in the volume of liquid in the gas collector tube (cm);—temperature correction; m—the mass of the sediment sample (g).

H , wt.% of sample weight) was determinate for each interval as H = m g ∗ 7.64 ∗ 100 % , (2) m g is a specific gravity of methane in gas hydrate form (g/g i.e., grams of gas in per gram of sediment) calculated from the specific gas content ( G) for pure methane. The weight gas hydrate content (, wt.% of sample weight) was determinate for each interval aswhereis a specific gravity of methane in gas hydrate form (g/g i.e., grams of gas in per gram of sediment) calculated from the specific gas content (for pure methane.

h , u.f.) is given by K h = W h W , (3) W h is the percentage of water in a hydrate form (wt.% of sample weight) and W is the total amount of water or moisture content (wt.%) [ The fraction of water converted to hydrate or the hydrate coefficient (K, u.f.) is given bywhereis the percentage of water in a hydrate form (wt.% of sample weight) andis the total amount of water or moisture content (wt.%) [ 19 43 ].

The samples on which the moisture content was determined were further used for the interval determination of the content of salt ions that migrated from the contact salt solution. First of all, water extractions of salt were made from samples, and then analysis of the Na+ ion concentration was carried out by the method of water extracts on a flame photometer PFP-7 (Jenway).