Fig. 6.1. Section of the cryogenic weathering crust (according to Sh.Sh. Gasanov): 1 - bedrock; 2 - fractures resulting from stress release; 3 - fine-grained material with montmorillonite-hydromica clay minerals; 4 - debris of little altered bedrock; 5 - vein ice in the bedrock fractures grading towards the top into ice-cement; 6 - structure-forming segregation ice; 7 - golets ice or structure-forming ice-cement.
these rates vary within a wide range. Thus, for example, according to data provided by V.L. Sukhodrovskiy, the maximum rate of weathering reaches: on the bench of the basalt escarpment of Franz-Josef Land 0.05 mm yr "on the bench slopes of limestones and schists of Scandinavia 0.04-0.15 mm yr"1, while on limestone benches of Spitsbergen island up to 5 mm yr-1.
In general, owing to the predominance of mountain-folded regions in the permafrost regions, the coarse-grain share is over 50% of the eluvium and it is represented by gravel, rock debris and blocks. Sandy and aleurolite fractions can be 40%, on lowlands 70-80%. Pelite and argillaceous fractions rarely exceed 15% making up 3-5% on the average. The particular composition of eluvium formations suggests that the latter belong to a specific type of eluvium - cryoeluvium.
As is known, loose products of weathering and solutions formed in eluvium not only migrate within the limits of catchments to accumulate in the final water bodies, but also become concentrated in transit. As regards the stages of transport and accumulation of sedimentary material in areas of permafrost, wide differences are observed in quantitative indices of sediment transport and accumulation compared to noncryolithic regions. This is associated with the existence of quite distinct processes of transportation resulting from various kinds of freezing processes and in the formation of specific genetic types of continental sediments with respect to chemical-
mineral composition, and cryogenic structure and the kind of freezing -whether syngenetic or epigenetic.
The matter resulting from weathering is conveyed for considerable distances to the final runoff water bodies, in the form of true and colloidal solutions, suspended solids of rudaceous material and as mudflows and similar forms of displacement. In the course of such displacement within the catchments weathering products are differentiated, i.e. the components of sedimentary material are separated one from another. At least three aspects of the matter of differentiation are to be considered: a) differentiation of weathering products into mechanical and chemical components; b) differentiation of weathering products by composition and content of chemical elements as they move towards the final runoff water bodies (chemical differentiation); c) differentiation of weathering products by grain size and mineral composition of granular deposits encountered over catchment areas (mechanical differentiation).
Initially, the separation of materials usually occurs in the foothills and lower slopes of catchment areas such as mountain uplands, hills, terraces etc. In the regions of perennially frozen ground such differentiation is more or less limited to the seasonally thawing layer. Note the extremely low rate of displacement of fine clastic material within the limits of watersheds as compared with the migration rate of pore solutions and runoff waters containing chemically dissolved and mechanically suspended substances. Owing to this, rudaceous material is separated from dissolved matter in the course of transport. The main volume of fragmented material remains on the slopes and lower slope regions, while dissolved and mechanically suspended matter is leached into rivers and water bodies. The mechanical composition of the deposits - talus, alluvium, prolluvium - is associated exactly with this process as is the formation of predominantly chemical-biogenic deposits in the final runoff water bodies.
Mechanical and chemical denudation is controlled by both climatic zonation and structural-tectonic and geomorphological features of a region. The ratio of mechanical to chemical denudation varies between mountain rivers and those flowing on plains: in mountain rivers mechanical denudation prevails, while on plains - chemical. The analysis of such types of denudation in rivers of the Euro-Asian continent, conducted by I.D. Danilov (6), showed the predominant significance of dissolved matter over sediment flow in allogenic rivers of Siberia and in rivers of the Arctic, sub-Arctic and northern temperate zones.
The predominance of dissolved matter over suspended solids in rivers flowing on plains is conditioned by low gradients of terrain, the gentle nature of the summer rainfall and the existence of dense and persistent sod cover. Accordingly, the mechanical denudation is slowed down in conditions of tundra landscapes. However, in general, the intensity of both mechanical and chemical denudation in the permafrost regions is lower by a factor of ten than in the warm climate regions.
At the stage of transportation weathering, products are differentiated by grain size and by composition of minerals in the granulometric ranges. Differentiation is a function of distance of material displacement and the geologic-geographic conditions for separation of materials in the course of transport.
The initial stage of differentiation takes place within the limits of watersheds giving rise to various types of slope deposits. Thus, the processes of gravitational denudation are intense on slopes (talus and rock falls); gradually, they are replaced by movement of boulders caused by freezing water which pushes them apart and outwards, subsequently leading to downslope movement as the cementing ice thaws out. This is the origin of the rock stream belt widely developed in the permafrost regions which is represented by rock debris flows and boulder fields. Fine debris cones are formed on the margin of this belt. Further down the slope a qualitatively new process starts to develop intensely, this process being one of the leading among those developing in the permafrost regions - namely, solifluction - giving rise to the zone of solifluction deposits. Since solifluction movement of soils develops at gradients of about 2-3°, it is evident that such displacement forms will be found at the foot of slopes accompanied by further separation of these deposits by grain size. Along with differentiation of weathering products by grain size and mineral composition in the course of their movement towards the final water bodies there is also chemical differentiation. Certain chemical elements become more mobile and are intensely leached by the surface and ground water; other elements, on the contrary remain practically immobile and, retained within the limits of watersheds and slopes, begin to become concentrated.
Thus, for an area of permafrost (the Aldan upland) the migration of a series of chemical elements was determined by I.B. Nikitina with respect to fissure-ground water, soil-ground water, surface water and rills. It was shown that in the permafrost regions in both oxidizing and reducing conditions the mobility of F, Fe, Ti, Cu, Ni, Zr, Ag, Mo and a number of other elements increased substantially as compared with their mobility in the noncryogenic regions.
Flowing water plays an important role in the transportation and accumulation of sedimentary material. This is contributed to by the activity of intermittent and continuous watercourses as well as by sheet erosion caused by meltwater and rainfall, which account for short-distance transportation of enormous amounts of sedimentary material. Continuous watercourses make possible concentration of the rudaceous material arriving from a vast area with long transportation distances from the place of formation. Therefore, this mode of transportation is considered to be the main one in the formation of sedimentary deposits.
A predominant role in the formation of river valleys is played by down-cutting and lateral erosion. In the permafrost regions the processes of lateral erosion prevail over downcutting both in mountain and plain rivers, which predetermines meandering of rivers and formation of ox-bow lakes - a typical process for plains and flat territories of the North. The frozen state of the banks and high content of ice (up to 80%) in the soils that compose such banks, promote intensification of lateral erosion owing to the thermal-mechanical impact exerted by water flow and solar radiation on frozen soils. Among manifestations of such impact are collapse of thawed soils into rivers and formation of numerous thermal-abrasion and thermal-erosion niches in banks with the cutting of ice-rich soils by the flow or by waves. At the same time downcutting (as compared with lateral erosion) is hampered by the intense inflow of talus-solifluction material from slopes, which overloads river beds causing meandering and cutting of banks. Small rivers can be filled up and cease to exist temporarily, forming the so-called 'spoonful alluvium' consisting of poorly sorted and graded slope deposits. And, finally, downcutting in the permafrost regions can be effectively hampered by the processes of bottom ice formation in shallow watercourses.
Comparative study of alluvium in warm humid regions and in the permafrost regions identified a qualitative distinction of the northern-rivers alluvium, which is characterized by a wide range of grain size and heterogeneity, by the increased content of heavy mineral fraction (pyrite, arseno-pyrite, gold, etc), often by a two-member layer of alluvium, by salinized floodplain deposits and the like. Associated with these characteristics and with further cryogenic transformation, fluvial sediments acquire typomorphic features of composition and structure not typical of other zones. This was the reason for the defining of an independent geographic origin of the alluvium by E.M. Katasonov, Y.A. Lavrushin and other investigators.
Sheet erosion of fine soil in the permafrost regions involves a variety of specific features of rainfall and meltwater transportation. Downslope movement of fine particles is facilitated by drizzling summer rains and thawing of snow cover. Downhill wash of sandy-silty and silty-clay soils increases substantially with high ice content and with schlieren cryogenic structures, which greatly reduce the structural strength of thawing soils. On slopes with rudaceous deposits sheet erosion of fine earth usually grades into its suffo-sion. The processes of downhill creep and washout may lead to the formation of a rather thick layer of deposits at the foot of slopes and on flat sites, known as talus trains, perluvium, dells and the like.
Of importance is the activity of intermittent fluvial flows in the area of permafrost or the action of steady runoff of rainfall and snowmelt. This process of linear erosion, which is called thermal erosion, contributes greatly, along with sheet erosion, to the formation of talus sediments and plays a significant role in the transportation of weathering products over the slopes.
In polar and severe continental climatic conditions wind-borne transportation of fine earth also occurs widely, both in winter and in summer. Most very fine particles are blown out by strong winter winds on sites with disturbed or missing vegetation cover and where frozen soils become less cohesive due to ice sublimation. The volume transported in winter is a function of wind velocity, intensity and depth of sublimation. The transporting capacity of wind is one three-hundredth of that of water and, therefore, only sand or finer particles can be blown. However, the content of these moderately fine particles is rarely as much as 10%, while the clay content is generally insignificant. This is associated with the fact that ice sublimation in sandy soil leads to reduced cohesion between particles which can be easily carried away by wind. In frozen clayey soils, however, cohesion is not reduced after sublimation, but is increased.
Modifications of sedimentary material within the permafrost regions involve not only temperature and cryohydration weathering of hard rocks (magmatic, metamorphic and cemented) but also exogenous processes, freezing proper included, which cause breakup of the frozen soils into separate aggregates. Such processes include frost cracking, thermal abrasion, thawing of sheet deposits of structure-generating ice and recurring ice wedges, etc. At present the role and particular share of each process in transportation and accumulation is not evaluated, but it is evident that indices should be defined for this purpose. Such indices can include the following: the area of development in the permafrost regions or region of the given freezing-geological process, the volume of material and distance of transportation, rate of movement, intensity of occurrence of different accumulation forms, etc.
Thus, for example, in glaciated areas transportation by glaciers prevails over other types. Most widely developed in the permafrost regions is down-slope transportation by solifluction, gravitational, rock stream and thermal erosion processes. As regards distance of transportation, the predominant role is shared by the fluvial and eolian types. The highest displacement rate is typically in aeolian movement, and also in mudflows, slumps and rivers. Such disturbance of the mass balance by the transportation of weathering products from catchment areas leads to more intense flattening of terrain and more rapid shaping of the smooth profiles of slopes. This is likely to bring about the formation of flatter and smooth outlines of macro- and meso-relief in the permafrost regions with less difference in relative elevations.
Sedimentary material arriving in the final water bodies includes the dissolved and colloidal substances and suspended solids mixed with rudaceous material. As a result of mixing with sea and lacustrine water the colloids and weakly soluble salts are precipitated, clastic material and suspended solids are disseminated and other processes and phenomena also take place. All these processes result in the formation of sediments which are characterized by a specific chemical-mineral and granulometric composition, structure, properties and regular concentrations of ores. The final product is determined by the peculiarities of the sedimentary differentiated material arriving from land and having a specific regime and conditions of sedimentation in water bodies. For example, the Arctic basin has, typically, low positive temperatures, sometimes even negative ones, and is characterized by the occurrence of ice cover on the surface throughout the year. Such a regime hampers wave propagation and is favourable for the precipitation of fine-grain material even at shallow locations. Apart from this, floating ice in icebergs in water bodies in the permafrost regions makes possible spreading of rudaceous material over long distances with sporadic deposition (even in deep water areas), thus giving nonuniformity of basin deposits with their typical nonsorted nature across the section.
The specific nature of hydrochemical and hydrobiological regimes of the final water bodies in the permafrost regions is revealed by the intense accumulation of carbon dioxide in the bottom layer, poor access of oxygen and other gases to the bottom sediments, in the predominantly neutral and low alkaline water environment, etc. The salinity of the water of the Arctic ocean reaches 35%o diminishing from west to east down to 20%o owing to fresh water inflow from the Siberian rivers. Lacustrine water is brackish or fresh. In basin sedimentation chemical precipitation usually prevails over the biogenic.
It is difficult to determine the rate of accumulation of sediments in recent water bodies as knowledge of the intervals during the process is lacking. The maximum rate can be as high as tens of millimetres per year. In Lake Onega (Onezhskoye Ozero) this rate is 2-5 mm yr~ The recent deep-set silts are being accumulated at a rate of 0.02-0.06 mm yr-1 on the average. For ancient geosynclines the average rates of sedimentary strata accumulation amount to a hundredth or tenth of a millimetre per year, while for ancient platforms - a hundredth or thousandth of a millimetre per year.
Sedimentary material arriving in the final water bodies has been transported by rivers, glaciers, icebergs, wind, and it also results from thermal abrasion of the banks, the action of sea currents, transfer of ash material and a number of other processes. In this way accumulation of morainic material similar to that of continental moraines occurs in the bottom of the Bering Sea, Barents Sea, Kara Sea, etc., Bukhta Provideniya, Zaliv Kresta and other northern water areas. This material is characterized by coarse-grained sediments, high content of boulders locally prevalent, and homogeneity of petrographic composition. Simultaneously, over the whole water area of the Arctic basin transportation of rudaceous material by icebergs promotes deposition of coarse-grained material. The concentration of icebergs and the pattern of their distribution over the water surface are dependent upon ice content, the rate of drift and thawing, bottom relief, bank structure and the like.
It was found that the concentration of rudaceous material drastically increased on sites with higher temperature seawater associated with intense ice thawing. In such conditions even continuous horizons of coarse sediments can be formed. Thus, for example, on the area from Greenland to Spitsbergen the layers of sediments contain much stone-sized material.
An important role in the delivery of sedimentary material from land to the bottom of water bodies is played by thermal abrasion of banks. The volume of sediments resulting from bank degradation constitutes about 20% of the whole volume conveyed by Arctic rivers. Virtually the whole of this material is deposited in the water surface of coastal seas giving rise to new accumulative forms of bank relief, causing the expansion of spits, shoals and islands. Only a fine-grained suspension is carried away into the ocean.
The rate of retreat of banks and sea coasts in the permafrost region is estimated in single figures and tens of metres per year. Thus, lowland banks on the northern margin of the Yana river delta retreat at a rate of 16-20m yr ~1, on the Laptev Sea coast not usually more than 6 m yr ~1. The thawing rate of ice-rich banks in the Central Yakutia reaches 7-10m yr-1, while thermokarst lakes in the Anadyr tundra migrate at a rate of 10m yr-1. Predominant average values of coastal retreat of continents and big islands range between 2 and 6m yr-1. Banks of water bodies composed of non-frozen soils retreat at a rate of 3-10 m yr ~1.
Thus, annual values of rate of retreat of both frozen and unfrozen banks can be similar. This suggests that the thermal abrasion rate of the banks composed of ice-rich fine-grained soils, under comparable conditions, exceeds by 3-4 times the abrasion rate of banks composed of unfrozen soils of similar composition. Because the Arctic seas are for a greater part of the year, compared to the southern ones, covered with ice which hampers the development of thermal abrasion in this period, the total annual effect for coast retreat is similar in both cases.
Sedimentary fine-grained material arriving in the final water bodies is subject to considerable displacement and spreading owing to winds, tides, perennial oceanic and marine currents as well as by the movement of icebergs, coastal and pack ice, etc. Distribution of sediments by mechanical composition in the northern seas is in conformity with Strakhov's outline and is characterized by the replacement of terrigenous material (sand-aleurolite) in the coastal zone by chemical-biogenic-carbonate material and in deep water by the siliceous material.
Organic matter is mainly carried into the marine basins with continental run-off and, in addition, arrives as sediments resulting from the life cycle of plankton and benthos. In the rivers of the North organic matter content can be as high as 70% of the whole amount of the dissolved and colloidal matter. However, the major portion of this organic matter does not move as far as to the sea and is precipitated in river mouths and shelves.
For the recently formed bottom sediments of the Arctic seas, oxide-ferriferous-manganese concretions, for the formation of which bacteria are responsible, are very typical. Their concentrations are more or less confined to areas of occurrence of brown silts and to the warm Atlantic waters. Iron is brought into marine and oceanic basins of the North with continental runoff, dissolved or suspended in rudaceous material and organic compounds. Iron content in the sediments of the Arctic basin makes up 3% on the shelf to 10% in basins, while in the Bering Sea it is as high as 11 %. Iron is present as colloidal Fe(OH)3 and ferriferous-organic compounds.
The granulometric, chemical-mineral composition and structure of the bottom sediments of seas and water bodies of the permafrost regions are characterized by a variety of specific features dictated by the peculiarities of sedimentation and regime of the final water bodies of runoff In seas and water bodies where there is long-term ice cover fine-grain sediments prevail while silt accumulation is already possible at a depth of 10-15 m (Laptev and East-Siberian Seas). If the ice regime is less severe, more coarse sediments accumulate at such depths (Chukchi and Bering Seas).
Grain-size distribution and structure of bottom sediments of fresh-water closed and semi-closed basins in the permafrost regions is influenced greatly by the duration of a persistent ice cover which diminishes wave action, disturbing the usual hydrodynamic sorting of the basin deposits and allowing the cyclic nature of the delivery of sedimentary material to be more apparent leading to the formation of banded-laminated sediments.
A very typical feature of shallow fresh-water basins of the cryolithozone (lakes and bogs of the tundra, the north of Tyumenskaya Oblast' [Tyumen region], the European part of Russia etc.) is also the accumulation of sediments rich in organic matter, mainly peaty sapropel and sapropel peaty silts.
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