Sea Level in the Mesozoic and Cenozoic

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The ongoing process of land expansion and increase of its average elevation accompanied by the reduction of the ocean area and increase of its average depth was typical for the Cenozoic. For example, the Tethys Sea, which separated Europe and Asia from Africa, became dry during the Alpine orogeny.

Long-continued subsidence of the ocean floor could be of particular importance for the development of sea regressions. Thus, the present-day occurrence of shallow sea facies and evaporates at the depth of 1.5-2 km in different parts of the World Ocean, particularly in the Atlantic Ocean, and the presence of flat-top underwater mountains (gayots), sometimes with old coral structures, at the depth of about 1300 m suggest the considerable subsidence of the ocean floor during the Mesozoic and Cenozoic.

By analyzing the depth of coral sediments on Pacific atolls, as well as the indirect data on the heat flow for the islands of different age, one can suppose that the subsidence of the oceanic depressions floor could accelerate during the Cenozoic. The rate of this process during the last 1-3 x 106yris estimated at about 0.15-0.23 m 10~3yr while the average rate for 15-25 x 106yr is 0.03-0.04 m 10~3 yr.

During particular periods of the Cenozoic and probably at the beginning of the Late Riphean, c. 1.1 x 10 yr BP, there could have been rhythmic fluctuations of sea level with the characteristic time from 40 to hundreds of thousands of years and the amplitude of dozens of meters. These fluctuations could be largely related to the gla-cioeustatic processes.

The increasing volume of oceanic depressions, general rise of the land, and other factors caused the overall decrease of the relative sea level during the Cenozoic, thus leading to the considerable deepening of river valleys. A large Sarmatian Sea (lake) was formed in the south of Eastern Europe and its level was several hundreds meters above the ocean level. The lake was from time to time connected to the ocean through the Mediterranean Sea and drained.

During the sharp fall of the sea level, the Mediterranean Sea intercepted the flow of the Danube River and several other large rivers of Europe. In combination with tectonic processes, this led to the disintegration of the vast lacustrine-marine water body and formation of three individual basins (Caspian, Black Sea, and Pannonian) at c. 7-8 x 106yr BP. The levels of these seas were several hundred meters below the present-day one. In the course of such profound transformation of water balance, the sea level could fall by 0.27 myr-1. When the Mediterranean Sea was linked with the ocean again and the ocean level became higher, the rivers' flow was redirected to the Black Sea. The rapid rise ofits level by several meters during 100 years resulted in the short-term reintegration of the Black Sea and the Caspian Sea (the so-called Pontic Basin). At c. 5 x 10 yr BP, these seas were definitely disintegrated.

At the same time, a deep regression (probably down to 300 m and more) took place at the Arctic coast of Eurasia.

The Arctic Basin was completely isolated and the area of ice cover increased considerably. After the Early Pliocene transgression of the oceans, a pronounced sea down-drop occurred in the middle of the Pliocene (3.7-3.3 x 106yr BP). Overdeepening of river valleys in the north of the east European platform, west Siberian lowland, and Far East regions was 200-300 m.

Since the Early Pliocene, periodic fluctuations of sea level in isolated and semienclosed seas occurred along with the above-discussed irregular changes. For the Black Sea, the periods of such fluctuations were 40-50 and c. 200 x 103 years and their amplitude amounts to 20-25 m. Periodic oscillations could be correlated with the global water-exchange processes and, probably, with changing intensity of the continental glaciation and amount of water resources. The decrease in the total area of oceans by 15% during the Cenozoic contributed to the differentiation of global climatic conditions and deceleration of water-exchange processes.

In the Early Cenozoic, total area of the oceans was much larger than at present, mainly due to the inundation of vast areas of the continental platforms and wide occurrence of geosynclinal seas. Data of historical geology, geological maps, and other sources of information allow estimating the area of the Late Mesozoic sea at about 416 x 106km2, that is, 55 x 106km2 more than at present. The average depth of the oceans was about 3 km.

One should take into account the formation of the Antarctic ice shield about 40 x 106yr BP (Eocene-Oligocene). As a result, c. 24 x 106 km2 of water was withdrawn from the global water cycle for a long period

T (x 106 yr)

causing the decrease of sea level by more than 60 m and a certain reduction of the oceans' area.

During the Cenozoic, there was a relatively high synchronism between the large sea regressions and periods of high tectonic activity, such as Austrian (95 x 106yr BP), Danubian (25 x 106yr BP), and Attic (9 x 106yr BP).

It is worth noting that during the Cenozoic 43% of the ocean floor area subsided to the depth of more than 1 km, and 13% to more than 2 km. This is well confirmed by the data of deep-sea drilling within the atolls, which were submerged by approximately 1300 m.

Besides the deepening of the seafloor, sea-level fluctuations could be caused by a wide range of factors. In the Mesozoic, the oceanic depressions were 230 km smaller than at present. Their growth could lead to the sea down-drop by more than 600 m. In addition, a certain amount of water was accumulated in the Cenozoic ice shields of Greenland and Antarctic. However, the sea-level rising factors were also active at the same time. The oceans were supplied with the juvenile water, bottom sediments and volcanic material were accumulated, geosynclinal and shelf seas were drained, and the amount of water on the land decreased, particularly within the enclosed water bodies and the underground aquifers.

As a result, there was a general trend toward sea-level down-drop by about 200 m (Figure 6).

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