Sea-Ice and Iceberg Sedimentation in the Ocean

Until recently, investigations of the sedimentation processes were restricted to the study of marine and continental deposits within continental sequences. Only during the last decades, geologists started investigations of the seas and oceans which occupy the largest part of the Earth’s surface (about 2/3). The latter represent a natural laboratory of diverse sedimentation processes occurring throughout the world — from poles to equator. Besides significant expanding of the spread of work and appearance of possibilities to introduce the com-parative-lithological method, new original instruments and methods have been involved into the study, thus resulting in development of new lines of investigations of sedimentation process and geochemistry.

Since 1837, due to the works of L. Agassiz in Switzerland, the glacial theory has started gaining recognition. In Russia its development is associated with the names of K.F. Rul’e, G.E. Shchurovskii, B.F. Shmidt, A.P. Pavlov, P.A. Kropotkin, and after the Revolution — with the names of S.V. Obruchev, I.P. Gerasimov, K.K. Markov, M.P. Grichuk, A.A. Velichko, and others. Not only continental glacial boundaries have been studied but also lithology of glacial bodies (Rukhina 1960; Aseev 1974; Moreny 1989, and others), stratigraphy, morphology, erosional and accumulation, etc. Study of the continental glacial process has been carried out very actively but it is still far from being completed.

All types of glacial-marine sedimentation are associated with and depend on processes within the cryosphere of the earth. This is the zone where the atmosphere, hydrosphere and lithosphere interact with each other and which is characterized by negative or zero temperatures and the presence of water in solid phase.

Cryosphere is one of the Earth’s spheres where negative temperatures are dominant and water is mainly in solid or supercooled state. The author considers the lithosphere to not only occupy cold regions within land but cold regions within the hydrosphere (the World Ocean in high latitudes).

Most part of sedimentary material within the area of sea ice sedimentation is supplied to the seas by rivers in the form of suspensions, colloids and solutions. The data on liquid river runoff together with the data on composition of suspensions and dissolved forms of elements and suspended and dissolved specific discharge are given in Table 5.1. River lengths and distribution of permafrost in river basins are given in Fig. 5.1.

The development of the notion of oceanic glacial process has been hampered by the lack of data on the processes of sediment incorporation, transportation and deposition by ice. Visual observations of ice with incorporated sedimentary material were the most important. The necessary data have been obtained only quite recently due to further progress in investigation technique and field explorations in inaccessible zones of glacial sedimentation, i.e. high latitudinal regions of the Northern and Southern Hemispheres. These investigations were supplemented by laboratory experiments. The first models have been put forward. So, processes of sedimentation in the areas covered with ice during most part of the year have become worth studied only quite recently. Understanding of these processes is based on the data obtained with the help of sputniks, research icebreakers and helicopters, drilling, study of fluxes of sedimentary material of all kinds (aerosols,cryosols andhydrosols), isotopic methods, etc.Nevertheless, this work is still far from being completed. There still remain a lot of “white spots”.

Glacial conditions of the Bering Sea are extremely severe. Its northern part is free from ice only during two months (July–August), while the Bering Strait is ice-covered all the year round. Ice reaches its maximum distribution in April, when ice edge is situated along the line: Bristol Bay — Pribylov Islands — along 57–58° N parallel — Komandorskie Islands — along Kamchatka coasts to its southernmost point. Warm waters entering the central parts of the sea via the straits between the Aleutian Islands shift the ice boundary to the north.

As shown above, the process of sedimentation under extreme, for the Earth’s low temperature conditions is distinguished by several unique features, which have been partly discussed above.

The complicated process of transformation of solid glacier bedrock into loose sediments is still far from being completely understood. At the moment three main approaches exist to solve this problem: (1) direct observations of the glacier activity (glaciohydrological investigations); (2) theoretic conclusions based on the laboratory experiments and models; (3) geomorphological investigations of both landforms appeared due to erosional activity of glaciers (fjords, troughs and other erosional forms) and accumulative landforms (different types of moraines, etc.).

Bottom sediments affected by “ice mountains” or icebergs occupy even larger areas in the World Ocean than the sea ice-rafted deposits. Only in the Antarctic are icebergs spread over the area of 60-65 million km², i.e. the area is 5 times as great as the Arctic Ocean (Table 10.1). The second large area of iceberg-rafted deposits is situated in the Northern Hemisphere (Davis Strait, western and eastern coasts of Greenland, regions stretching southward from Greenland to Newfoundland). The area of iceberg-rafted deposits in this part of the ocean is 1.41 million km².

A special feature of Antarctica is the fact that only 2% of its surface are accessible to direct geological observation, while 98% of it are covered with ice. Bedrock outcrops, separate mountains and small rocks (nunataks) rise above ice. Large shelf areas are also constantly covered with ice. Floating ice shelves exist there.

Greenland, the largest centre of the modern glaciation of the Northern Hemisphere, is situated in the North Atlantic. Its white ice cap rising above the sea level to the heights of 2–3 thousand metres is a source of numerous icebergs. Modern glaciation also exists on Spitsbergen, Franz Joseph Land, Novaya Zemlya and arctic Canada (see Table 3.1).

Study of the process of accumulation of marine deposits related to the various types of sea ice activity opens up new possibilities to solving the opposite problem usually arising during field investigations: how to reconstruct the origin of the beds exposed in the section or a series of sections, environmental conditions of their accumulation and their further evolution. Study of the modern processes allows solving this problem not on the basis of controversial guess-works but on the basis of comparative-lithologic method and understanding of real processes and their geologic consequences.

In accordance with the most common definition of “facies” the author considers them to represent an unity of sedimentation environment and specific features of bottom sediments related to it (including petrographic, mineralogic, structural-textural peculiarities and those related to biogenic component and initial stages of diagenesis, etc.). According to G. F. Krasheninnikov (1971), facies are considered as a sedimentary complex which differs from neighbouring deposits of the same stratigraphical level.

In the previous chapters we have already described the mechanism of sea ice and iceberg sedimentation in the World Ocean for both: the present epoch and the period of the last glaciation. How long in the Earth’s past has this mechanism been operating? Is sea ice and iceberg sedimentation typical of the Cenozoic epoch only?

The main types of sedimentogenesis appearing under different combinations of environmental factors, composition and properties of sedimentary matter along with sedimentary rock basins, cryogenic facies and the main time relevant features of this process have been discussed above. All this establishes the main cryogenic formations based on the synthesis of all these data.

Due to deep-sea drilling, geophysical and marine geological investigations new data on sedimentation under low temperatures have been obtained in high latitudinal regions of the Earth that are difficult of access.