Himenkov, A. and Sheshin, Y.

MASSIVE ICE: ORIGIN AND APPROCHES TO CLASSIFICATION

Introduction

A characteristic feature of Arctic coasts is the presence of ice formations referred to as massive ice. Their thickness reaches tens of meters, the extension - hundreds of meters. The ice formations are located mostly on Yamal and Gidan peninsulas in Western Siberia, but they are also found in the area stretching from Kolguev island and Yugorski peninsula on the west and up to Chukotka peninsula and Alaska on the east. There are gas deposits, settlements, and communication means in the areas of massive ice. For example, the town of Amderma on Yugorski peninsula is practically fully located above a congestion of massive ice. The Bovanenkovo gas field is located on Yamal peninsula in a zone of distribution of most massive ice deposits (thickness exceeds 100 meters). Many such examples could be presented. Catastrophic destruction of earth layers occurs during thawing of massive ice caused natural reasons and human activity.  The extent of damage is revealed by the numerous lake basins that extend up to several kilometers in the central part of Yamal peninsula. Massive ice could still be found in the steep coasts of the basins towering 20-30 meters above ground. Massive ice origins and distribution should be extensively questioned in areas of intensive Northern development on the background of global warming. 

History and Approaches to Study

The study of underground massive deposits of ice is already more than 40 years old. For the first time the term "massive deposits" was used by P.A. Shumski [12] for description of the structural forms of injected ice. Thus given term was connected with the concrete mechanism of origin from the very beginning. In the consequence, this term has become broader and now does not carry a genetic meaning. The term has come to represent all monolithic bodies of underground ice of a various origin and morphology. As time has shown, this expansion of meaning caused numerous discussions about the origin of the underground ice.

The task of the authors consists of providing a solution for the problem of formation of massive ice based on definition of the conditions of water accumulation (in various forms) in local zones of soil mass, and based on definition of genetic mechanisms. The formation of certain genetic type of deposits and their freezing are on the foreground of such approach.

In the publication, we consider massive ice only in marine deposits of Arctic Region. There they are distributed extensively and have the largest sizes. In the region the major factor determining sedimentation and permafrost formation is fluctuation of sea level. The low negative temperatures are also a constant factor. As it was shown by Y.K.Vasilchuk [1] and others, "the syngenetic frozen deposits were created continuously through time including the period of the Holocene optimum in Northern Eurasia". The massive ice is basically distributed in areas where marine deposits are dominant. The freezing of marine deposits is connected to their transition in subaerial position.  We shall consider change of geocryological conditions at this transition, and then look at processes of ice formation.

Geocryological Conditions of Massive Ice Formation

The coasts of the Arctic seas are transitive zones between two basic areas of cryolithozone - sea (subaqueous) and continental (subaerial) [9]. Shown below are the basic types of massive ice that correspond to the major zones of the Arctic coasts:

Buried ice: deep-water subaqueous conditions;

Injected ice: zone of transition from subaqueous to subaerial position;

Segregated ice: final phase (freezing of residual lakes).

The first area somewhat depends on the zone climatic factors; the temperatures of the soils are influenced by depth of sea, salinity of waters, sea currents, a river drain, etc. The deposits here are mainly in the cool condition, their temperatures range from positive to 0⁰C and below to -1,8⁰C.  In the deposits, the ice either occurs as separate crystals and forms small congestions of ice, or it is included in the structure of deposits as buried ice. Buried frozen soils are can be encountered in areas of sea subaqueous cryolithogenesis. Temperature conditions in these deposits are influenced by the same factors.

On depths exceeding the thickness of fast ice, temperature of ground deposits is determined by a mineralization of seawater. Shallow water, as known, has a smaller mineralization factor and higher temperature gradient due to summer heating. At increase of depth the mineralization of further down waters is increased, and temperature is accordingly lowered down to -1,8 [4,13].

Freezing of water results in a layer of water that is saltier and accordingly denser. These waters, when falling downwards, lower temperature of deposits. In the summer, the thawing of ice establishes steady stratification of seawaters. Fresh, warm, and consequently less dense waters are formed in the top layer; more salty, cold, and therefore more dense waters are formed in the bottom layer.

The self-regulation of natural systems is interesting to us because "sea water - sea deposits" system is based on the mechanics of vertical circulation of sea waters that provide stability of this system and its parameters (temperature, mineralization, material structure, water content of deposits, etc.). According to  Shpolanskya [13], steady negative temperatures in the bottom layer of water could lead to permafrost formation in the subaqueous position at depths of sea of 35 - 100 m. Ice could be kept buried in sea deposits at such conditions.

The supporters of the glacial hypothesis of the origin of massive ice consider the possibility of the existence of widespread glacial coverings in areas of modern Arctic seas. Formation of massive ice as the result of burying of glacial ice by sea deposits is not considered in theory, but it follows from the natural conditions of Arctic Region. An important question is what portion of the deposits they occupy.  From our point of view, for the explanation of the whereabouts of the origin of the deposits it is not necessary to involve models of grandiose changes of palaeogeography. The considered massive ice could still be formed in modern conditions. Glaciers are widely distributed in the Arctic Region at the present time.  Evidence of numerous specific landforms suggests that they were widely distributed in the past. A downfall of summer temperatures by 2-3 degrees would be enough reach it.  When travelling down the Arctic seas, the icebergs are influenced by currents. When moving in shallow water, the icebergs are processed by seawaters and buried by deposits.  In our belief, the reasons for the theory of the presented mechanism of massive ice formation are as follows:

1. The massive ice does not make any important geological horizon. The formations appear as separate inclusions in the homogeneous deposits with well-expressed stratification, marine fauna, and marine type of salinity.

2. The massive ice appears in sediment at all geomorphological levels.

3. The massive ice is unevenly distributed within the limits of given geomorphological level. For example, in the western part of Yamal peninsula the massive ice is found throughout the deposits more often than in eastern part of the peninsula. One of the possible reasons for such occurrence could be that the sea currents in this part of the Arctic Region are directed from west to east; consequently the ice bodies will be transferred by currents to the western part of Yamal.

The possibility of existence of massive ice in the fresh sea deposits poses a problem for the Arctic sea sedimentogenesis.  It is traditionally believed that the ice formation stops processes of diagenesis and inhibits life in the sea. In the case considered the existence of ice formations does not stop diagenesis processes.

The reasons for existence of ice and its forms differ in subaqueous sea deposits. The ice can be formed as separate crystals (at negative temperatures the local ice formation appears even in the saline deposits); deeper down ice is formed in hashing waters. Ice formation occurs in zones of penetration of fresh continental waters into the saline sea deposits with a negative temperature gradient (for example, at mouths of the rivers running into the Arctic sea). The ice is an authigenic material in sea deposits, so this problem appears to be more broad and important than the problem of burying.

Massive Ice and Coastal Processes

Origin of coastal permafrost is usually connected to a regressive cycle of sea sedimentation. The coast is put out forward into the sea; and a completely new element - coastal bar [3] - occurs there. Formation of bars and lagoons is an obligatory stage of the development of "coastal raisings".

The deposits of clay are found extensively throughout the lagoons. The salinity of waters and deposits in the lagoons depends on their closeness to the sea, and also on the gradient of atmospheric precipitation and evaporation. For the Russian Arctic coast, the summer evaporation factor is about 10-20 mm, and precipitation factor is about 100-200 mm. For the most part of year the reservoirs are covered by ice; fresh thawed water gets into the reservoirs in spring.  Therefore, the salinity of water in the lagoons is decreased at that time. The depths of the lagoons vary from several meters up to several tens of meters up. The lagoons stretch several kilometers in length, commonly having a width of about a hundred meters and depth of up to 5 m. The transition from subaqueous to subaerial conditions is accompanied by significant redistribution of waters and their accumulation in local zones [8]. These processes are caused by two factors, the first one being diagenetic processes and the second freezing.

The dehydration of clay deposits is due to migration of porous water. Diagenetic waters create pressure on contact with various layers. The small inclination of sandy layers results in accumulation of waters and their upward movement [14].

As it has been previously stated, the freezing begins in the subaqueous position and under fast ice that is in contact with the very bottom of the sea. The depth of the sea where frozen soils occur will vary according to the temperature gradient.  At temperatures of -10⁰C and below the frozen soils rise to a depth of about 1,5 m. At air temperatures of  -5⁰C  and -6⁰C the frozen soils are formed only in the subaerial position.

During regression of the sea, the sandy islands, bars, and sandbanks are formed.  Porous waters are under permafrost pressure. It was observed that subpermafrost waters were under pressure at the point when freezing of an underwater bar at meter capacity of a frozen layer occurred [2].

In the central part of sandy banks, islands, and bars the freezing of sand protrudes to a large depth. The thickness of permafrost decreases towards the sea and near the coast; in subaqueous conditions frozen strata disappears. The pressure of the subpermafrost waters is proportional to the thickness of permafrost.  Waters are squeezed out from the center of the formatting permafrost.

As the closeness to the sea decreases and there are closed lagoons present, the area of permafrost is increased. At the final stage there are only unfrozen sites in the residual lakes. At this point the differentiation of porous waters in a strata of deposits reaches the maximum. On sites that are already frozen and combined by sand, the water content is minimal, and in unfrozen sites the water content of deposits is at maximum. Right at the end of the existence of a lake mode the freezing starts from top. Growing hydrostatic pressure in strata of freezing deposits reaches the size of geostatic pressure. At this exact moment a blowout occurs, the layer of water and all strata above is supported by pressure in this layer.

The accumulation of water in strata of sea deposits and its allocation as lenses or layers does not contradict a natural development of the sea coastal zone. The presence of layers of water of thickness up to 0,5 m is characteristic feature of fresh sea silts on the depth of about 15 m [10,15]. The pressure of water in frozen soils of thickness of 12 m is about 1,05 kg /sq. cm, and when thickness reaches 22 m the pressure increases to 3 - 3,5 kg /sq.cm.

One more group of processes is attributed to viscous current of generated frozen soils and ice.

The third area of the Arctic sea cryolithogenesis is an area of freezing of residual lakes.

Gradually losing the connection with the sea, the lagoons pass into the mode of fresh lakes. After they are filled by deposits they dry up or break up into the series of fine lakes or convert to bogs. The probability of formation of massive ice is increased at the expense of water migration to the front of the freezing (segregated ice).

The results of the mechanism are well presented, but the mechanism itself is not studied in depth. Experience suggests that the salinization of soils of up to 1% considerably reduces redistribution of water content at the freezing point of clay soils.

Conclusion

The formation of massive ice is considered as a consecutive change of processes divided into stages. Our experience of study of massive ice suggests that the stages could be traced in their structure.

There are still different points of view concerning the origin of massive ice. Such discord could be attributed to several reasons:

1. In spite of the fact that geocryology as a science has existed for about a hundred years, the reliable techniques of a quantitative estimation of growth of ice crystals have not been created.

2. The problem of reconstruction of changes in massive ice has not been solved. Under the influence of varying loadings, the ice changes in structure, morphology, gas, and salt structure, but quantitatively the character of these changes is not described.

3. There is lack of precise mechanical and physical characteristics of deposits containing massive ice.

4. The analysis of change of superficial conditions is not developed at the point of transition of deposits from subaqueous conditions to subaerial position.

5. Properties of frozen saline soils require further study.

The existing classification actually reduces the large variety of processes carried out in massive ice to abstract models. It seems to us that it would be more perspective to consider formation of the ice as a part of sedimentation and freezing of the certain genetic type of deposit. In conclusion it is necessary to note that the formation and destruction of massive ice is connected to cardinal transformation of natural environment.

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