Factors Governing Water Regime Changes

Global changes of the ocean regime during the Earth's history can be reconstructed in detail by analyzing the fragments of preserved sedimentary rocks that were formed as a result of water action. Such an important indicator as the areas of marine sediments makes it possible to restore spatial-temporal dynamics of water regime of the oceans and reconstruct the total area of seas, the relative altitude of the mean sea level, and the average depth of the oceans. It is well known that during billions and millions of years major changes of the oceans were caused mainly by tectonic processes and evolution of the continents.

Different interrelations of such factors as the increase of the total volume of water and steady deepening of the oceans combined with the development of tectonic processes and sedimentation resulted in global marine transgressions and regressions.

The distribution of sedimentary rocks over the continents testifies that numerous transgressions of the sea took place (Figure 3) due to the development of geosynclinal processes, thus leading to the reduction of ocean area in the geological past. The reduction of ocean area

Figure 3 Transgressions of the sea leading to reduced ocean area.

against the background of continuous degassing could be explained only by changes in the relief of the Earth's surface, that is, gradual rising of the continents and deepening of ocean depressions.

During the Phanerozoic, the gradual reduction of the ocean area and the increase of its level were interrupted by large marine transgressions and regressions. The oceans could then cover more than 50% of the present-day land area and the rate of change of the sea level could reach more than 10ml0~6yr. On an average, transgress and regress phases altered every 60-70 x 106yr. A clear concurrence was revealed between the largest regress phases and the periods of more intensive tectonic activity and orogenic processes. Large glaciations could also influence the ocean phases change.

During the geological history of the Earth, horizontal movements ofthe lithosphere plates could probably result several times in the consolidation of the Earth's crust and formation of 'supercontinents'. It is supposed that such 'supercontinents' were formed at least 3 times during the Phanerozoic; they were called Gondvana (570440 x 106yr), Pangea (280-200 x 106yr), and Laurasia (160-100 x 106yr). Lesser Earth heat flux through the consolidated blocks of the continental crust could probably cause the increase of temperature and expansion of the underlying mantle. The consequent rise of continental blocks above the ocean floor and increase of the ocean size could lead to considerable lowering of the mean sea level. The amplitude of sea-level fluctuations from the consolidation of 'a supercontinent' to its disintegration is estimated at about 500 m.

The average rate of bottom sediment accumulation calculated according to the age parameters of different marine ground layers makes about 1 x 10_ mm per year. The rate of sedimentation can change from 0 in the areas of high bottom erosion up to 1mmyr_1 in delta areas. The deep-sea (4500-5000 m) carbonate clay accumulates with an average rate of 1-10 mm per 100 years. At large depths, the sedimentation rate changes from 0.01 mm per 1000 years up to 0.5 mm per year. At present the larger part (~36%) of sediments - nearly 21.3 x 10 t of suspended matter per year - comes to the ocean with the runoff. The coastal erosion is the second important source of sediments. Detailed evaluation of this process suggests that the volume of solid matter can amount to 16.7 x 109 tyr~ , or about 28% of the total.

An important role in filling the oceanic depressions belongs to the eolian processes. Based on the assessment of continental shelf deposits and Quaternary sediments of abyssal plains, the rate of eolian deposition is estimated at 1 up to 80 m 10~6 yr. At present the input of eolian matter in the oceans is about 11 10 tyr~ (18% of the total).

Other processes closely related to the continental crust evolution (input of dissolved matter, volcanic sedimentation, etc.) are also important for filling of the ocean. The increasing amplitude of relief augmented the role of sedimentation. Thus the average rate of sedimentation in the Holocene could not be taken as standard for the whole geological history of the Earth. The available data suggest considerably higher rates of sedimentation in the Quaternary period with particular variations during glacial epochs. During the decay of the last Upper Pleistocene ice sheet, the inflow of suspended matter into the oceans could be 15 times greater than at present.

Total volume of accumulated matter can be estimated using the volumetric weight of solid marine deposits. Density of sediments is estimated at 1.5-2.7 gcm~3. Bearing in mind the compaction ofsediments, the average annual filling of the oceanic depressions should be about 30 km3, thus leading to the sea-level rise by 83 mm 10~3yr. Without spreading and subduction of the Earth crust and changes of relief, the complete filling of the oceans would take c. 45-50 x 106yr. Data on the recent rate ofsedimentation in the oceans show that it is an order higher than that of sea transgressions of the geological past. Therefore it is possible to suggest that during inter-orogenic periods with no essential deformations of the Earth crust, the general sea-level rise was mainly the result ofsedimentation processes. At the same time, changing water exchange conditions were also of importance.

During the recent 150 x 106yr, c.225 x 10 km3ofsedi-ments was removed from the ocean floor by subduction processes. Thus the average rate of this process is probably about 1.5 x 106 km3 every 106yr, or 1.5km3yr_1. It is worth noting that during the geological history the intensity of this process could vary depending on tectonic and volcanic activity; the balance of matter could also vary considerably through time. Under the present-day rate of sedimentation in the oceans (30 km yr~ ), the equilibrium of the oceanic depressions size could be preserved if the rate of subsidence is at least 12 cmyr-1 (the total area of subsidence being 60 x 103 km x 4 km).

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