Abstract and keywords
Abstract (English):
We consider the patterns of existence of thermo-abrasion, thermo-denudation and submarine permafrost degradation in the coastal zone of the Laptev and East Siberian seas. The key goal is to assess their role in changing the permafrost conditions along the coastal zone of a few tens of kilometers wide.

Keywords:
thermo-abrasion, thermo-denudation, degradation of submarine (subsea) permafrost.
Text

 

 I. INTRODUCTION

The major cryogenic processes in the coastal zone of the Laptev and East Siberian  seas  are thermo-abrasion and thermo-denudation. They lead to the rapid retreat of the ice-rich shores and degradation of subsea permafrost. Degradation is a consequence of the transition of coastal sediments into submarine environment.

 

Fig. 1. Research area

The dynamics of the considerably extended shoreline was studied by the method of topographically accurate alignment of multitemporal and multiscale remote sensing data with the help of ScanEx Image Processor.  The assessment of coastal retreat rates and interpretation of landscapes along the coastline was carried out in a GIS program MapInfo Professional. Degradation of submarine permafrost was studied using mathematical modeling of its evolution and synthesis of data on distribution and depth of the subsea permafrost roof.

II. PERMAFROST CONDITIONS

 Continuous permafrost is developed onshore along the coastline of the Laptev and East Siberian  seas. Average annual temperatures of coastal sediments are -11÷-15 °C, and its thickness reaches 500-700 m [1]. Submarine permafrost thickness varies from 150-200 to 500-700 m [2].

Syncryogenic ice-rich silts of Late Pleistocene Ice Complex with high ice wedges at the top (30-40 m) of the cross-section are exposed along the coastline. Ice wedges are 5-8 m wide and penetrate the entire thickness of the Ice Complex. Bulk ice content of sediments reaches 80-95% [1]. High  coasts (up to 25-40 m) are composed of Ice Complex and often are represented by two bluffs. The lower one is destroyed by thermo-abrasion. The upper bluff is destroyed by thermo-denudation, and in fact, is separated from the lower one by a thermo-terrace 30-300 m wide. Thermo-abrasion and thermo-denudation are closely related to each other, activating the alternate retreat of both bluffs. The length of the coastline composed of the Ice Complex does not exceed 35% (for the Bol'shoy Lyakhovsky Island) and 20% (for the Oyogos Yar) of the total length of the eroded coastline.

Another relief-forming factor is the Holocene Alas Complex. It is comprised, from bottom to top, of the taber, lacustrine and boggy sediments with ice wedges. Ice wedges are 6-10 m height and up to 1.5-2 m wide. Ice content reaches 60-70%. Coasts composed of the Alas Complex are 8-12 m high. The marine and alluvial-marine coastal terraces height does not exceed 3-4 m.

The cliff structure depends on the areal distribution of neotectonic structures in the surrounding relief. The horst structures in the rift system of the Laptev Sea are usually represented by poor-ice sediments in the wave-breaking zone. Cliffs of the rift grabens are composed entirely of Ice and Alas Complexes. Here, the bottom of icy sediments goes underwater to a  level 3.5-10 m deep.

Horsts and rift grabens vary considerably in the geothermal flux density (q), which influences  the thickness and degradation rate of permafrost at the bottom. Characteristic values for horsts ​​are 45-53 mW/m2, and, presumably, for the graben they are 70-100 mW/m2 [2].

III. COASTAL EROSION

The Eastern Siberia has the highest  coastal retreat rate in the Northern Hemisphere. In recent years, due to a sharp drop in sea ice cover of the Arctic Ocean and rise of air temperatures, the retreat rates of the Laptev and East Siberian ice-rich shores have risen 1.3-2.9 times. Estimates for different years [3] have been made on the basis of comparison of the position of the Lyakhovsky Islands shoreline and the southern coast of Dm. Laptev Strait.  The data showed that during the 50-year period (1951-2000), 27.2 km2 of the Bol'shoy Lyakhovsky Island, 1.7 km2  of the Maly Lyakhovsky Island , and 12.4 km2 of the Oyogos Yar mainland have been removed. Moreover, over the past 13 years (2001-2013), the eroded 10.3 km2 area of the Bol'shoy Lyakhovsky Island and 6.5 km2 area of the Oyogos Yar must be added to the above estimates.  On average, the retreat rates for all the eroded coasts in the region have been estimated as 3.2 m2/year over the period up to year 2000, and 6.4 m2/year for the last 13 years.

Upon comparison between remote sensing images taken in different years, the data showed a significant spatial heterogeneity of the studied retreating shores. As a result of the remote and land-based studies, 4 morphostructural fields [4] and 10 types and subtypes of the coasts have been distinguished, based on the structure of the onshore cross-sections [5]. According to this classification, the retreat rates change from the minimum (0-0.5 m/year) in areas of stable uplifts (e.g., the Kigilyakh Peninsula, the Cape of Shalaurov, the Cape of the Holy Nose) to the significantly higher values of up to 5-7 m/year observed during the period 1951-2000, and 10-13 m/year during the period 2001-2013 in areas with a tendency to subsiding. The maximum rates were observed in the negative-relief structures where the bottom of ice sediments was located below a sea level.

 

                                                            

 

Fig. 2. Change of the retreat rates for the eroded coastlines of the Bol'shoy Lyakhovsky Island: the southern coast of 70 km long (a), the western coast of 48 km long (b), the north-eastern coast of 32 km long (c) and Oyogos Yar of 107 km long (d). IC - segments of Ice Complex, al - segments of alluvial sediments, m - segments of marine and alluvial-marine sediments, no- symbol line there were allocated - segments of Alas Complex

The mechanism of the coastal erosion depends on the structure of the onshore cross-section. The first (block-type) mechanism is most typical for the coasts 8-12 m high, preferably occurring in the Alas Complex. Sea wave-cut coves are generated at a depth of 10-15 m at the base of the coastal cliff. The overlying sediments along the fractures crumble and are washed away by sea waves.

The second type of erosion is observed along the coasts of more than 15-20 m high, composed of Ice Complex. Thermal cirques with ice cliffs on its remote edge and thermo-terraces in the basement are found here (fig.3). Thermo-terraces are formed upon retreat of the ice cliffs at a rate exceeding the rate of thermo-abrasion. Such retreats are due to the processes of thermo-denudation of ice cliffs under the influence of air temperature, solar radiation and precipitation and resulting erosion of the coastal part of thermo-terraces. Comparison of multitemporal remote sensing data showed that thermo-denudation rate has been increasing by 1.7-1.9 times over the last 13 years. 

 

Fig. 3. The Shore Oyogos Yar in 2007. Photo by A. Dereviagin

The third type of erosion is characteristic of marine and alluvial-marine terraces. Here, thermo-abrasive coves are not developed, but instead, thawing ice sediments are sliding down to the water edge, where they are being eroded.

IV. DEGRADATION OF THE SUBMARINE PERMAFROST AT THE TOP

Submerged below the sea level sediment sequences are initially frozen. Firstly, its subsea erosion is initiated by thawing. Summer warming of bottom water up to 10-12 °C as well as salinization of the bottom sediments are contributing factors to permafrost degradation. As a result of salinization and seasonal thawing at depth interval 0-2 m, and perennial thawing at depth interval 2-7 m, thickness of the degraded permafrost on the submarine coastal slope is increasing from 1-2 to 30-40 m [6]. The first value 1-2 m is characteristic of a distance 0.1-0.2 km from the coast, the second value 30-40 m is observed at a distance 3-10 km offshore. Sea waves carry upslope a smaller portion of thawed sediments. Approximately 2/3 to 1/2 of sediment mass is deposited on the coastal slope [7].

V. PERMAFROST DEGRADATION FROM BELOW

Permafrost degradation from below begins one-two millennia later than the processes of the degradation on top. It takes place under the influence of geothermal flux. This process starts when a temperature profile of submarine permafrost corresponds to a temperature of the bottom water. According to the mathematical modeling permafrost 500-700 m thick had been developed over a period 1500-2000 years with geothermal flow (q) equal to 50 mW/m2 [2]. The degradation is observed where there is a reduction of permafrost thickness from below, and ice-bonded permafrost is transformed into ice-bearing permafrost (with unfrozen water).

 

VI. THE CONTRIBUTION OF CRYOGENIC PROCESSES IN PERMAFROST COASTAL DYNAMICS

Assessing the contribution of cryogenic processes in permafrost dynamics of the coastal zone was carried out by constructing a conceptual model. It is based on the data of permafrost thickness reduction, changing of permafrost state and estimation of coastal retreat rates over the last 5 thousand years when a sea level rise had stabilized on modern altitudes.

 Received data of permafrost degradation rate were used for assessment [8]. They show its dependence on the coast position in the certain morphological structure. The rate of permafrost degradation from below in the rift grabens at q = 70-100 mW/m2 is 2.2-3 cm/year; in the horsts it equals 1.5 cm/year at q = 50 mW/m2. The rate of permafrost thawing on the underwater coastal slope at the north-western end of the Muostakh Island in the Ust-Lensky graben (in the Buorhaya Bay) was 14-18.5 cm/year during the period of 30 years (1983 - 2013/2014.). Here bottom of Ice Complex goes under the sea level at 10 m. Thawing of poor-ice sediments underlying the Ice Complex is going 2.5-3 times slower (6 cm/year), what, in our opinion, corresponds to the horst conditions.

Data of the most extended borehole profile (11.5 km) near the Cape of the Mammoth Tusk (fig. 3) were used for paleogeocryological reconstructions. The bottom of the Ice Complex submerged under the sea level by 3.5 m. The thickness of thawed permafrost in the cross-section in the borehole C-2, located on the isobath 6 m, is 29 meters deep. If we consider that 1/3 of the sediment mass was eroded by waves, the thickness of thawed permafrost might have reached 35 m, and thus average rate of thawing equals 1.4 cm/year. 

 

Fig.4. The borehole profile next to the Cape of the Mammoth Tusk on the Anabar-Olenek near-shore zone and the distribution of temperature and salinity in borehole C-2 [by 6].

The cross-section in the borehole C-2 includes a layer of plastic-frozen sand and clay sediments (fig. 3). Temperatures of sediments in the borehole (-1.1÷-1.4 °C) expose gradientless distribution along the cross-section and correspond to a temperature of bottom water. Considering the distance from the coast and average coastal retreat rate (4.5 m/year), M.N. Grigoriev determined that permafrost at borehole C-2 area submerged under the sea level 2540 years ago [6].

The specified time period was estimated using mathematical modeling and paleogeographic data. It showed that permafrost from the borehole C-2 was land-based not less than 2000 years ago. During this period, the temperature of permafrost sequences 700 m thick assumes a value corresponding to the temperature of the bottom water. Since part of the permafrost sequence had been transformed into to the plastic-frozen state, an estimate of 2,500 years appears to be a reasonable value.

Paleogeographic data were used in the calculation of the coastal area submerged over the last 5 thousand years. In particular, we applied oxygen isotope GISP2 method [9], palynological data [10] and data on complexes of diatoms and foraminifera in the sediments of the Laptev Sea [11]. The results of calculation showed that the coastline in the western part of the cold-water Laptev Sea 5000 years ago was located 25 km away from its current position. In the rift grabens, where the bottom of Ice Complex was found at 10 m below modern sea level, the coastline could be found even further, while in the horsts it could be found closer. Earlier estimates obtained on the basis of the observed rates of coastal retreat suggested 30-36 km difference from the present position [12].

VII. CONCLUSIONS

  1. High-rate retreat of the coastline of Eastern Siberia is caused by the predominance of ice-rich Quaternary sediments in its structure.
  2. The rate of coastal retreat is influenced by the following factors: tectonic structures of the territory, expressed in certain morphological structures that define permafrost geology in the coastal cross-section; the duration of the ice-free period, the strength and direction of winds, snowdrift transfer; shore orientation relative to the direction of prevailing tides, storms and currents, solar radiation. The current climate warming,  likely driving  the reduction of sea ice cover in recent decades has led to an increase in coastal retreat rate 1.3-2.9 times in comparison with the second half of the  ХХ century.
  3. Top-down degradation of submarine permafrost  formed from the  subaerial permafrost as a result of  sea-level rise, is due to the summer radiative warming of bottom water and thawing of the submarine permafrost on the isobaths 2-7 m, as well as salinization of the sediments on isobaths 0-2 m. The average degradation rate for periods within the first thousand years does not exceed 1-2 cm/year. The highest recorded thickness of the thawed sediments as a result of the coastal retreat is more than 50 m. During the period of 5 thousand years coastline has retreated up to 25-30 km.
  4. The rate of degradation of submarine permafrost from below depends on the density of the geothermal flow and equals 1.5-3 cm/year. The greatest reduction in permafrost thickness submerged under the sea level as a result of coastal retreat, has been 45-90 m during the last 5 thousand years.
  5. The rates of the coastal retreat and permafrost degradation are higher in the negative-relief structures compared to the positive ones.
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