Oceanic Water Regime in the Pleistocene

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During the Pleistocene, the ongoing altitudinal differentiation of the Earth's surface, expansion of the total land

Figure 6 Sea-level down-drop in the Mesozoic.

area, and gradual cooling of land surface were accompanied with increasing climatic contrasts and alteration of global cooling and warming epochs. As a result, a certain cyclicity of natural processes on the Earth's surface was typical for the period. Specific features of the evolution of natural conditions were reflected in the global water balance, particularly in the sea-level fluctuations.

The periodic occurrence of global cooling epochs was mainly induced by changes in solar radiation coming to the Earth's surface. They were a result of changes in the astronomic factors governing the position and movement of the Earth in space (the eccentricity of the Earth's orbit, the longitude of the perihelion, the angle of the Earth's rotational axis relative to the plane of its orbit, etc.).

The influence of the astronomical factors on the Earth's climate is well confirmed by changes of temperature conditions, which were reconstructed based on the analysis of bottom sediments from several areas of the World Ocean. It was found that the concentration of 18O isotope in carbonates deposited from water solutions depends on the temperature. Thus the changes of the oxygen-isotope composition along the bottom sediment columns made it possible to reconstruct the temperature variations over recent >400 x 103 years.

The general pattern of temperature variations suggests the alteration of prolonged cooling epochs accompanied with large continental glaciations of about 100 x 103yr cycle and the relatively warm periods which took place every 20 x 103yr. The results of this study point to the fact that rather high temperatures like those we observe on the Earth now occurred for just 5% of the time.

The analysis of elevations and age of sea terraces along the coasts in different parts of the world revealed that their age increases from the lowest up to the highest one, which is generally 100 m high. This could seemingly prove the regressive trend of sea-level changes during the Pleistocene (Figure 7).

The areas with unstable tectonic conditions could demonstrate even more considerable changes of sea level in relation to the shorelines. The study of marine sediments in Japan suggested the amplitude of sea-level fluctuations to be more than 200 m. This is indicative of very active recent vertical movements of the Earth crust in the area of

Figure 7 Regressive changes in sea-level during the Pleistocene.

Figure 7 Regressive changes in sea-level during the Pleistocene.

Japanese islands. Oceanic islands, particularly atolls, the majority of which have rather stable tectonic conditions, are good indicators of sea-level fluctuations in the Pleistocene. During the glacial-eustatic regressions, they came from under the seawater, while during the interglacial epoches they became atolls again. Because of their altitudes, atolls could probably escape the effects of geocratic regressions during the submergence of the ocean floor. It is thus possible to reconstruct the pattern of sea-level fluctuations in the Pleistocene, which puts away the influence of global tectonic evolution of the Earth's surface. General pattern of sea-level fluctuations for the oceanic islands and atolls suggests that there was probably no regressive submergence of the ocean floor during the Pleistocene and the uplifting of the continents was most likely.

Despite significant variations, all curves of the sea-level fluctuations during the Pleistocene show the gradual decrease averaging to c. 0.1-0.4 mm yr-1. A principal factor governing the sea-level trend is probably the globe-wide recent tectonic movements resulting from the Cenozoic uplifting of the main continental structures.

Changes of the global water balance also influenced the large-scale fluctuations of the sea level during the Pleistocene. During the epochs of global cooling, large continental ice sheets were formed that accumulated enormous masses of water. It is supposed that at 230-300 x 103yr BP the Middle Pleistocene glaciation could reach its maximum in Eastern Europe, western Siberia, and Northern America. Total volume of water accumulated in the ice sheet could exceed 60 x 10 km3, thus resulting in the sea-level down-drop by more than 100 m.

Correlation of R. Fairbridge's data on the sea-level dynamics (if the general tectonic trend is excluded) with the mean annual temperature scale reconstructed on the basis of the oxygen-isotope composition of foraminifers from the bottom sediments and the data on solar radiation income to the Earth's surface reveals the surprisingly close synchronism of radiation changes, variations of temperature conditions, and water-balance fluctuations in the oceans (Figure 8).

Fluctuations of the sea level could also be induced by isostatic compensation. Movement and concentration of significant amounts of water in certain areas transformed the isostatic load on the surface of the Earth's crust both on land and in the ocean. As a result, compensatory vertical changes should occur lasting for several hundreds to dozens of thousand years.

Water-balance calculations show that during rather long periods of climate warming and almost total reduction of ice sheets, the sea level could stay >60 m higher than at present. During such periods, the sea level became radically less variable too, since the powerful glacioeustatic factor was inactive. Apart from tectonic movements, sea level was considerably influenced by the changes of land water resources (in rivers, lakes, bogs, and under ground), particularly under warm and moist climatic conditions when the total amount ofwater on the continents increased.

At the beginning of the Late Pleistocene glaciation, the average level of the oceans decreased rapidly, though the rate of this process was a little bit lower, than that of the subsequent rise. It is likely that in the time interval from 80 to 70-65 x 103 yr, the level of the oceans lowered by 50-80 m. The ocean water volume decrease rate could account for 3-4 x 103km3yr~\ The mean rates of the sea-level down-drop were 7-8mmyr~\ Particularly high rates of the sea-level down-drop during the Late Eemian interglacial are illustrated by the data on ancient shorelines of the Ryukyu, the Barbados, and the Bermuda Islands. According to the majority of models, the largest Late Pleistocene ice sheets of the Northern Hemisphere could form during 10-15 x 103yr.

Figure 8 R. Fairbridge's data showing synchronism of radiation changes, variations of temperature conditions and water - balance fluctuations in the ocean.

Figure 8 R. Fairbridge's data showing synchronism of radiation changes, variations of temperature conditions and water - balance fluctuations in the ocean.

During the maximal stage of the last glaciation (18 x 103yr BP), the temperature of superficial oceanic waters was on an average 2.3 °C below the present-day one. It is supposed that the maximum fall of seawater temperature (by more than 10 ° C) occurred in the North Atlantic. Significant changes took place on the Earth's surface as a whole (Figure 9).

Evolution of natural conditions within the shelf area and the coast of the Black Sea during the latest large regression (about 18 x 103 yr BP) when the sea level was 90 m below the present-day one is illustrated by a paleogeographical scheme (Figure 10). It represents the extensive areas of terraced slopes, watersheds, and river valleys formed during the Middle Pleistocene. It is also possible to trace the fragments of marine terraces of the earlier transgressions. Wide occurrence of turbidity flows and landslides was probably typical for the period under discussion and the paleochannels are clearly detectable on the sea bottom.

The statistical analysis of more than 600 most reliable datings of the Pleistocene coastal deposits suggests that they were mainly formed 70-90, 110-140, 200-230, as well as 300-370 and 500-600 103yr BP.

In the Early Pleistocene, the sea level (if the recent tectonic factor is excluded) could hardly exceed +20-55 m, in the Middle and Late Pleistocene +10-12 m (Figure 11). At the same time, the presence of sea basins in the areas which were exposed to tectonic, glacioeustatic, and other deformations could contribute to much more significant rise of the sea level. For example, during the Mikulinian

Interglacial, the paleosea occupied a significant part of the northern east European plain (at present the elevations of the area are +80-100 m, and up to +120 m in some places). Some authors even suggest the possibility of interconnection between the Arctic Basin and the Baltic Sea.

Sea-Level Fluctuations in the Holocene

Use of the isotope methods for the analysis of the geological history of the Earth gives an opportunity to construct more objectively the curves of sea-level change in the latest geological past, that is, for the recent 30-35 x 103yr BP. The latest regression of the oceans by 100-130 m took place at 15-20 x 103yr BP (see Figure 9). It was the result of global water cycle disturbances caused by the changes in thermal conditions on the Earth's surface that the transformation of the global water balance occurred.

At c. 16 x 103yr BP, the rapid rise of the sea level began due to input of large amounts of water from the thawing ice sheets to the oceans. The rate of sea-level rise in this period was approximately 10 mm yr" ; during particular periods, it could exceed 20-25 mm, and sometimes even 50mmyr"1 (Figure 12).

According to the results of the isotope study of bottom sediments, the process of ice water input in the oceans, and thus the decrease of ice sheets, became sharply accelerated from 16 to 13 x 103 yr BP and from 10 to 7 x 103 yr BP.

160 120 80 40 0 40 80 120 160

160 120 80 40 0 40 80 120 160

160 120 80 40 0 40 80 120 160

Figure 9 Changes during last glaciation leading to maximum fall of sea water temperature.

160 120 80 40 0 40 80 120 160

Figure 9 Changes during last glaciation leading to maximum fall of sea water temperature.

Figure 10 Paleogeographical scheme showing large regression within the shelf area and coast of Black sea.

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Figure 11 Sea levels during barly, Middle, and Late Pleistocene.

Glaciers were the main factors that influenced these changes; their volume could change depending on temperature variations and the ratio between ablation and accumulation. Mountain glaciers that experienced both augmentation and decline during the Holocene are the most indicative in these terms. Changes of glacier masses resulted in their advance and retreat with approximately 2 x 103 yr period. The mass of mountain glaciers is relatively small, so they are just one of the indicators of ice volume changes. It is the fluctuations of the water balance of the Antarctic and Greenland ice sheets that contributed significantly to sea-level fluctuations during the Holocene.

The rise of the global sea level caused the rapid inundation of coastal lowlands, which in combination with the large input of sediments resulted in the formation of large accumulative features, such as bars, spits, sand barriers, etc. The most intensive formation of coastal barriers took place under the relatively lower rate of sea-level rise. These forms are partially preserved and provide many details of the sea-level rise in particular seas. Generalized data on such complexes all over the World Ocean suggest their obviously uneven distribution in terms of depth. Most of them are concentrated at depths of 20-25, 55-60, 80, 100, and 120 m below the present-day sea level.

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Figure 12 Sea-level changes in the Holocene.

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The review of extensive data made it possible to identify particular zones of the World Ocean with principally different trends of changes of sea level during the Holocene. The models of uneven spreading of ice water and the development of glacial-isostatic and hydro-isostatic processes were elaborated, as well as of the uneven spreading of water from the thawing glaciers caused by the deformations of the Earth's body. If the difference between these zones is generalized, it becomes clear that in the tropical belts of Northern and Southern Hemispheres sea level was several meters above the present-day one during the Middle Holocene. In the equatorial zone, the sea level was progressively on the rise. The areas of the Late Pleistocene glaciation have been experiencing the general down-drop of the sea level till now. In the adjacent areas, a relative uplifting of land gave way to its submergence in the Middle and Late Holocene.

During the recent 104 years, the highest sea level was 5 m above its present-day one. This occurred during the climatic optimum of the Holocene. This period was marked by global climate warming by 2.5 °C, which resulted in the decline of ice volume and the relevant recharging of the oceans.

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