Permafrost Origin Ice Content

Permafrost is a natural geological formation. There have been several periods of cooling during the geological evolution of the Earth which caused the formation of frozen strata in the Earth's crust. Polar migration and continental drift give evidence of ancient freezing in various sites, including those that occur nowadays in equatorial latitudes. The most ancient freezing period during the Proterozoic (more than 2 billion years ago) corresponded to the Huronian glaciation. This was followed by Riphean, Vendian and Paleozoic glaciations. The permafrost underlying the Antarctic ice sheet (the age of the ice which permanently "flows" to the surrounding ocean doesn't exceed a few hundreds of thousands of years) is probably the relict of Oligocene-Miocene glaciation that began 25 million years ago. The modern global distribution of permafrost (fig.2) started to form during the same period (i.e. during the Miocene). The most ancient permafrost relics of the last glaciations correspond to the upper Pliocene - Pleistocene (1.8 - 0.7 million years).

150° 18tf 15(f 120" 90" 6ff 30" 0= 30" SO" 90'; 120" 1 SO" 180° 150:

150° 18tf 15(f 120" 90" 6ff 30" 0= 30" SO" 90'; 120" 1 SO" 180° 150:

Figure 2. The world permafrost map: 1 - permafrost: a) under glaciers, b) in glaciers, c) in mountains, d) in plains; 2 - seasonally frozen ground: a) in wet climatic zone, b) in dry climatic zone; 3 - no freezing [Geocryology of the USSR: European territory of the USSR / Ed. E.D.Ershov. - Moscow: Nedra, 1988, p.15].

Figure 2. The world permafrost map: 1 - permafrost: a) under glaciers, b) in glaciers, c) in mountains, d) in plains; 2 - seasonally frozen ground: a) in wet climatic zone, b) in dry climatic zone; 3 - no freezing [Geocryology of the USSR: European territory of the USSR / Ed. E.D.Ershov. - Moscow: Nedra, 1988, p.15].

Frozen soils and rocks are regarded as geological formations which are characterized by negative temperatures and which contain non-freezing interfacial water and the ice that cements granular material and fills pores and cracks. Ground of any genesis, mineral composition, grain size (clastic, sandy, clay soils) and lithification (massive and fissured bedrocks) can be found in the frozen state. When ice is inter-granular it is considered a specific rock-forming mineral included in rock texture and structure descriptions, whereas ice bodies (such as various buried ices, wedge and segregation ices, massive ice beds) are regarded as mono-mineral geological bodies. Occasionally cryotic, but iceless, rocks are referred to as cryotic dry rock or dry permafrost.

Figure 3. Photographs showing surficial landscapes in permafrost regions. In (a) arrows highlight ground ice layers at the Yugorskii peninsula, while in (b) thermokarst and thermoerosion in picryogenic ice-rich soils at the Yugorskii peninsula. Photos by A.I.Kizyakov.

The frozen strata are subdivided into syngenetic and epigenetic according to the time of their formation versus the time of accumulation of the mineral and biogenic components. For the most part the frozen strata have had an epicryogenic origin, i.e. previously accumulated or formed units had frozen during a cooling epoch. The ice content inside this ground shows an approximate correlation with the water content present before freezing. In some cases conditions are created for the redistribution of moisture throughout the section as a result of moisture addition to the freezing front throughout the aquifers. In these cases ice beds from tens of centimeters to tens of meters are formed inside the freezing rock strata (fig. 3).

Syngenetically frozen strata have frozen in parallel with mineral and/or biogenic rock accumulation. They include recent Upper-Pleistocene and Holocene clay and silt deposits of fluvial and littoral plains, shelf deposits (for example, in Yakutia), aeolian-alluvial-deluvial slopes of arctic rock masses, and depressions of large northern lakes. The moisture content in this ground, freezing while accumulated, is essentially unlimited. The ice accumulates in abundance within so-called "cryostructures" and often forms wedge ices. Their total volume can reach 80% and more (fig.4).

Figure 4. Photographs showing thermokarst in ice wedges at the Bolvanskii Peninsula, with (a) giving an areal view from a helicopter while (b) shows a cross-section of one of the ice wedges. Photos by M.Z.Kanevsky.

To a lesser extent the state difference (frozen/unfrozen) affects bedrock, and their cryogenesis is always epigenetic. Ice within fissures and cavities do not significantly influence rock strength, however permeability is changed appreciably, with frozen fractured saturated stratum being transformed into an aquiclude. The presence of spatially restricted taliks leads to active water exchange and enhanced leaching across them . The cryotic dry rocks only occur in the uppermost part of a geological section, (typically only 1-3 m below surface in granular rocks) and thus are of interest with respect to CO2 sequestration only as an upper aquiclude above a CO2 underground storage facility. In this case most attention should be paid to the chemically active residuals, because under the conditions of talik water exchange and in the presence of dissolved CO2 fast reactions can take place which could lead to increased porosity and enhanced CO2 escape out of the storage.

Pebbly, gravelly and sandy ground (except silty sands) in frozen or unfrozen states don't vary greatly from each other in ice content, but the difference in conductivity and strength is rather considerable. In the frozen state it is a strong foundation which can take an almost unlimited construction load and it is a stable aquiclude providing an effect barrier to gas and water migration even at moderate thicknesses of the frozen horizon.

The moisture content in frozen clastic and sandy ground is usually slightly higher than in thawed ground. However at an aquiclude's boundaries (for instance, thawed aquicludes - clays) massive ice beds up to several tens of meters thick (fig.3) can be generated while freezing sandy strata. In such places the total thickness of ice layers can reach 80% of the overall frozen strata thickness within near-surface zones which are intersected by constructions. If human engineering works affect the massive ice beds, they may initiate the potentially disastrous geocryological processes of thermokarst and thermoerosion, thereby destroying landscapes as well as nearby engineering infrastructure.

Silty and clayey ground attract the special attention of geocryologists, constructors and other experts, as they can accumulate significantly greater quantities of water during both epi- and syncryogenic freezing than in their thawed state. This is the result of their mineral and granulometric composition which ensures the abundant migration of moisture to the freezing front. Lenticular, layered, reticulate or honeycomb cryostructures are formed, and the mineral ground-mass is dissected by an ice net and layers. Their thickness and spaces between layers range from sub-millimetre to several centimetres. The ground-mass is also divided into polygons by macrocracks and filled syngenetic ice wedges. Several meters thick wedges can form a multilayer structure having a total thickness of tens of meters. Air filled pores are essentially absent and ice content can reach more than 80%. In this case thermokarst and thermoerosion lead to disastrous results.

Thus the frozen rock is of no interest as a material which can host the carbon dioxide. The ice-saturated ground is gas and water impermeable, explaining why it is impossible to create conditions for solution or chemical absorption (for example, as gas hydrate) of carbon dioxide in the frozen rocks. The cryotic dry rocks are extremely rare and are of no practical value. Therefore, it is reasonable to examine permafrost not as the enclosing medium for CO2 sequestration but rather as a unit which can enhance the isolation of CO2 in deep geological units.

+1 0

Post a comment