Sea level was 110 meters lower 18,000 years ago - VIDEO PDF Print E-mail
Friday, 01 May 2015 03:00

Did you know that 18,000 years ago, sea level was 110 meters lower. Why is so much to change the level of the ocean? A sharp drop in the ocean level was due to the extensive development of continental glaciation, when huge masses of water proved to be withdrawn from the ocean, and concentrated in the form of ice at high latitudes of the planet. From the glaciers slowly creep away in the direction of the middle latitudes in the northern hemisphere by land, to the south - the sea in the form of ice fields overlap shelf in Antarctica.

One of the curves showing the fluctuation in sea level over the past 18 000 years (the so-called eustatic curve). In the 12th millennium BC the sea level was approximately 65 meters lower than the present, and in the 8 millennium BC - Already at 40 m incomplete. Level rise occurs quickly, but unevenly. (According to N. Mёrneru, 1969)

It is known that in the Pleistocene, the duration of which is estimated at 1 million years old, there are three phases of glaciation, known in Europe, Mindel, Riss and Wurm. Each of them lasted from 40-50 thoUSAnd. 100-200 thoUSAnd. Years. They were separated by interglacial periods when the climate on Earth teplel significantly, approaching the modern. In some episodes, he became even warmer by 2-3 °, which led to the rapid melting of ice and the release of these vast spaces on land and in the ocean. Such abrupt changes in climate were accompanied by no less sharp fluctuations in sea level. In the epoch of maximum glaciation, he went down, as already mentioned, at 90-110 m, and in interglacial rose to the level of 10 + 4 ... 20 m to the present.

Pleistocene - is not the only period during which there were significant variations in sea level. In fact, almost all of them marked by geological epochs in the history of the Earth. The level of the ocean has been one of the most unstable geological factors. And it has been known for a long time. After submission of the transgressions and regressions of the sea developed in the XIX century. And how could it be otherwise, in many sections of sediment on the platforms and in the mountain-folded regions is clearly continental sediments are replaced by sea and vice versa. About sea transgression was judged by the appearance of remains of marine organisms in the rocks, but the regression - in their disappearance or appearance of coal, salt or Krasnotsvetov. The composition of faunal and floral complexes were determined (and determine so far), where it comes from the sea. The abundance of thermophilic forms indicating the invasion of water from low latitudes, the prevalence of the boreal organisms spoke of the transgression from the high latitudes.

In the history of each particular region stood a series of transgressions and regressions of the sea, as it was believed that they are caused by local tectonic events: the invasion of sea water associated with lowering the earth's crust, their care - with its uplift. When applied to areas of continental platform on this ground was not even a theory of oscillatory motions: cratons is lowered, the uplift a broad according to some mysterious internal mechanism. And each craton submitted its own rhythm oscillating movements.

Gradually it became clear that the transgressions and regressions often manifested almost simultaneously in different geological regions of the Earth. However, inaccuracies in the paleontological dating of certain groups of layers does not allow scientists to conclude on the global nature of most of these phenomena. This is unexpected for many geologists conclusion was made by American geophysicists P. Weil, Robert Mitchum and S. Thompson [1982], who studied the seismic sections of the sedimentary cover within the continental margins. A comparison of sections from different regions, often very distant from one another, helped to identify the association of many disagreements, breaks, or erosion accumulative forms to multiple time frames in the Mesozoic and Cenozoic. In the opinion of the researchers, they reflect the global nature of the fluctuations in the level of the ocean. The curve of such changes, built P. Weil et al., It allows not only to identify the era of high or low his state, but also to evaluate the course in the first approximation, their scale. Strictly speaking, this curve is generalized experience of many generations of geologists. Indeed, of the Late Jurassic and Late Cretaceous sea transgression or his receding at the turn of the Jurassic and Cretaceous, Oligocene, Miocene, later can be found in any textbook on historical geology. New was perhaps the fact that now these phenomena were associated with changes in the level of ocean waters.

The surprise was the magnitude of these changes. Thus, the most significant marine transgression, flooding in the Cenomanian and Turonian time most of the continent, was believed to be due to the rise of the level of ocean waters more than 200-300 m higher than at present. On the very same substantial regression occurred in the middle Oligocene, due to the fall in the level of 150-180 m below the present. Thus, the total amplitude of oscillation was in the Mesozoic and Cenozoic almost 400-500 m! What had caused such enormous fluctuations? On glaciers can not be attributed them, as for the late Mesozoic and Cenozoic climate of the first half of the planet was exceptionally warm. However, at least sredneoligotsenovy many researchers still associated with the start of a sharp cooling in high latitudes and the development of the Antarctic ice sheet. However, this one probably was not enough to reduce the level of the ocean just 150 m.

The reason for these changes were tectonic reorganization entailing global redistribution of water masses in the ocean. Now we can offer a more or less plausible version to explain its water level fluctuations in the Mesozoic and early Cenozoic. Thus, analyzing the major tectonic events that occurred at the turn of the Middle and Late Jurassic; as well as early and late Cretaceous (which is associated with a long water-level rise), we find that these intervals were marked by the disclosure of major ocean basins. In the Late Jurassic was born and quickly expanded the western arm of the ocean, Tethys (Gulf Coast and Mid-Atlantic), and the end of the Early Cretaceous, and most of the Late Cretaceous epoch marked by the opening of the South Atlantic and Indian Ocean basins many.

How inception and spreading bottom in young oceanic trenches could affect the position of the level of water in the ocean? The fact that the depth of the bottom of them in the early stages of development is very low, no more than 1,5-2 thoUSAnd. M. The expansion of the area is due to a corresponding reduction in the area of ​​the ancient ocean basins, which have a depth of 5-6 thoUSAnd. m, and in the Benioff zone absorbed deep-bed areas abyssal basins. Squeezed out of the endangered ancient water basins raises the general level of the ocean, which is fixed in the ground sections of continents as a transgression of the sea.

Thus, the collapse of the continental megablocks must be accompanied by a gradual rise in sea level. That is what happened in the Mesozoic, during which the level rose to 200-300 m, and maybe more, though this rise was interrupted and epochs term regressions.

Over time, the bottom of the young in the process of cooling the oceans of the neocortex and the increase of its area (the law-Sorokhtina Slater) is becoming increasingly profound. Therefore, the following disclosure is much less influenced by the position of the level of ocean waters. However, it inevitably would lead to a reduction in the area of ​​ancient oceans, and even the complete disappearance of some of them from the face of the Earth. The geology of this phenomenon is called the "collapse" of the oceans. It is realized in the process of convergence of the continents and their subsequent impact. It would seem, slamming ocean basins should cause a new upsurge in the level of water. In fact, the opposite occurs. The point here is strong tectonic activity, which covers the converging continents. Orogenic processes in the band of the collision accompanied by a general uplift of the surface. At the edge of the continent tectonic activation seen in the collapse of the shelf and slope of blocks and their descent to the foot of the continental level. Apparently, these include lowering the bed and adjacent areas of the oceans, causing it becomes much deeper. The overall level of ocean waters falls.

As the tectonic activation - one-act event and covers a small period of time, then the decline in much faster than its increase in the spreading of young oceanic crust. This may explain the fact that the transgression of the sea on the continent are developing relatively slowly, whereas the regression usually occurs rapidly.

Map of possible flooding in Eurasia at different values ​​of the likely rise in sea level. The scale of the disaster (the expected during the XXI century sea level rise of 1 m) will be much less visible on the map and almost no impact on the lives of the majority of states. The increase in the coastal areas are the North and Baltic seas and southern China. (The card can enlarge!)

Now let's consider the question to mean sea level.

Surveyors producing leveling on the ground, determine the height of the "mean sea level". Oceanographers who study the sea level fluctuations, compare them with the marks on the bank. But, alas, the sea level is even "average long-term" - the value is not constant and is also not the same everywhere, and the sea shore in some places rise, others fall.

An example of modern subsidence of land may serve as the coast of Denmark and the Netherlands. In 1696 in the Danish city of Agger 650 meters from the shore was a church. In 1858, the remains of this church finally swallowed by the sea. Sea during this period occurred on land with a horizontal speed of 4.5 meters per year. Who is on the west coast of Denmark completed the construction of the dam, which is to block the further advance of the sea.

The same risk are low-lying coast of Holland. The heroic pages in the history of the Dutch nation - is not only a struggle for liberation from Spanish rule, but no less heroic struggle against the advancing sea. Strictly speaking, there is not so much the sea comes as retreats in front of him going down the land. This is evident from the fact that the average level of the water in full. Nordstrand in the North Sea from 1362 to 1962 rose by 1.8 m. The first frame (the mark of height above sea level) was made in the Netherlands on a large, specially set stone in 1682 from the XVII to the middle of XX century. lowering soil coast Holland occurred at an average rate of 0.47 cm per year. Now the Dutch are not only defending the country from the onset of the sea, but also win back land from the sea by building a grandiose dam.

However, there are places where the land rises above sea level. The so-called Fenno-Scandinavian board after the release of heavy ice ice age continues to rise in our time. The shore of the Scandinavian Peninsula in the Gulf of Bothnia is raised at a rate of 1.2 cm per year.

There are also alternate lowering and raising coastal land. For example, the Mediterranean Sea raised and lowered by several meters in some places even in historical times. This is indicated by columns of the Temple of Serapis, near Naples; marine elasmobranch shellfish (Pholas) pierce them moves to the height of a man. This means that since the construction of the temple in I. n. e. land dropped so much that part of the column was immersed in the sea, and probably for a long time, as otherwise the shellfish did not have time to do such a great job. Later, the temple with its columns came out again from the waves of the sea. According to the 120 observation stations, in 60 years the level of the entire Mediterranean Sea rose to 9 cm.

Climbers say: "We stormed the peak altitude so many meters." Not only surveyors, mountaineers, but also people who are not associated with these measurements are used to the concept of altitude. It seems to them unshakable. But, alas, it is not so. The level of the ocean is constantly changing. His sway the tides caused by astronomical factors, wind waves generated by the wind and choppy as the wind itself, wind revolvers and sleeves water off the coast, changes in atmospheric pressure, biasing force of Earth's rotation, finally heating and cooling of ocean water. In addition, studies of Soviet scientists I. Maximov, NR Smirnov and GG Khizanashvili, sea level changes due to occasional changes in the Earth's rotation and moving the axis of rotation.

When heated to 10 ° only the upper 100 meters of ocean water, ocean levels will rise by 1 cm. The heating by 1 ° the entire thickness of the ocean water level raises it by 60 cm. Thus, due to the summer heating and winter cooling sea level in the middle and high latitudes subject to marked seasonal fluctuations. According to the observations of the Japanese scientist Miyazaki, mean sea level on the western coast of Japan in the summer of rises and falls in the winter and spring. The amplitude of its annual fluctuations - from 20 to 40 cm. The level of the Atlantic Ocean in the Northern Hemisphere begins to rise and reaches a maximum in the summer to winter in the southern hemisphere, there is a reverse its course.

Soviet oceanographer AI Duvanin distinguish two types of fluctuations in the level of the oceans: the zone as a consequence of the transfer of warm water from the equator to the poles, and the monsoon as a result of prolonged ebbs and surges excited by monsoon winds that blow from the sea to dry land and in the summer reverse winter.

Notable slope sea level observed in the areas covered by ocean currents. It is produced both in the flow direction and perpendicular to it. The transverse slope at a distance of 100-200 miles reaches 10-15 cm and varies with changes in the rate of flow. The reason for the cross-slope of the surface flow - deflecting force of the earth's rotation.

Sea markedly reacts to changes in atmospheric pressure. In such cases, it acts as a "inverted barometer": More pressure - below sea level, less pressure - above sea level. One millimeter of barometric pressure (more precisely - one millibar) corresponds to the height of one centimeter the sea level.

Changes in atmospheric pressure may be transient and seasonal. According to the Finnish research oceanographer E. Lisitsyna and American - John. Patullo, fluctuations caused by changes in atmospheric pressure are isostatic character. This means that the total pressure of air and water to the bottom of the sea in this area tends to remain constant. The heated and air is rarefied level rise, cool and dense - a decrease.

It happens that the surveyors are leveling along the coast, or by land from one sea to another. Arriving at the destination, they discover inconsistencies and begin to look for the error. But in vain they are scratching their heads - mistakes can not be. The reason for discrepancies in the level of the sea surface is far from the equipotential. For example, under the influence of the prevailing winds between the central part of the Baltic Sea and the Gulf of Bothnia the average difference in level, according to E. Lisitsyn - about 30 cm. Between the northern and southern part of the Gulf of Bothnia at a distance of 65 km level is changed to 9.5 cm. Between the sides of the Channel difference in the level of - 8 cm (Crease and Cartwright). The slope of the sea surface from the Channel to the Baltic Sea, estimated Bowden - 35 cm. The level of the Pacific Ocean and the Caribbean Sea on the ends of the Panama Canal with a length of only 80 km, varies by 18 cm. In general, the level of the Pacific Ocean is always somewhat above the level of the Atlantic. Even if you move along the Atlantic coast of North America from north to south, reveals a gradual rise in the level of 35 cm.

Without dwelling on the considerable fluctuations in global sea level that occurred in the past geological periods, we only note that the gradual rise in sea level, which was observed during the XX century., Equal to an average of 1.2 mm per year. It is caused, apparently, general warming of the planet and the gradual release of significant amounts of water, chained up to this time the glaciers.

So, no oceanographers can not rely on the mark on the land surveyors or surveyors - readings gauges installed off the coast of the sea. Level the surface of the ocean is far from ideal equipotential surface. By its exact definition can be reached through joint efforts of surveyors and oceanographers, and even then not before, as will be accumulated at least a century of material simultaneous observations of the vertical movements of the crust and sea level fluctuations in the hundreds, even thoUSAnds of items. And while the "average" of ocean, no! Or what is the same, a lot of them - at every point its shores!

Philosophers and geographers of antiquity, which had to use only speculative geophysical methods of solving problems is also very interested in the problem of sea level, though in a different aspect.



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