Who did our ancestors mate with? Who did our ancestors mate with? "Dirt everywhere ... And I'm not talking about the weather ..."
One of the curves showing sea level fluctuations over the past 18,000 years (the so-called eustatic curve). In the 12th millennium BC. sea level was about 65 m below the present, and in the 8th millennium BC. - already at incomplete 40 m. The rise in level occurred quickly, but unevenly. (According to N. Mörner, 1969)
The sharp drop in ocean level was associated with the widespread development of continental glaciation, when huge masses of water were withdrawn from the ocean and concentrated in the form of ice in the high latitudes of the planet. From here, the glaciers slowly spread towards the middle latitudes in the northern hemisphere by land, in the southern hemisphere - by sea in the form of ice fields that overlapped the shelf of Antarctica.
It is known that in the Pleistocene, the duration of which is estimated at 1 million years, three phases of glaciation are distinguished, called in Europe the Mindelian, Rissian and Würmian. Each of them lasted from 40-50 thousand to 100-200 thousand years. They were separated by interglacial epochs, when the climate on Earth warmed noticeably, approaching the modern one. In some episodes, it even became 2-3° warmer, which led to the rapid melting of ice and the release of huge spaces on land and in the ocean from them. Such dramatic climate changes were accompanied by equally sharp fluctuations in ocean levels. During the epochs of maximum glaciation, it decreased, as already mentioned, by 90-110 m, and in the interglacial period it increased to +10 ... 4-20 m to the current level.
The Pleistocene is not the only period during which there were significant fluctuations in ocean levels. In fact, they marked almost all geological epochs in the history of the Earth. Ocean level has been one of the most unstable geological factors. And this has been known for quite some time. After all, ideas about the transgressions and regressions of the sea were developed back in the 19th century. And how could it be otherwise, if in many sections of sedimentary rocks on platforms and in mountain-folded areas clearly continental sediments are replaced by marine ones and vice versa. The transgression of the sea was judged by the appearance of the remains of marine organisms in the rocks, and the regression was judged by their disappearance or the appearance of coals, salts or red flowers. Studying the composition of faunistic and floristic complexes, they determined (and still determine) where the sea came from. The abundance of heat-loving forms indicated the intrusion of waters from low latitudes, the predominance of boreal organisms spoke of transgression from high latitudes.
In the history of each specific region, its own series of transgressions and regressions of the sea stood out, since it was believed that they were due to local tectonic events: the intrusion of sea waters was associated with the subsidence of the earth's crust, their departure - with its uplift. In application to the platform regions of the continents, on this basis, a theory of oscillatory motions was even created: the cratons either fell or rose in accordance with some mysterious internal mechanism. Moreover, each craton obeyed its own rhythm of oscillatory movements.
It gradually became clear that transgressions and regressions in many cases manifested themselves almost simultaneously in different geological regions of the Earth. However, inaccuracies in the paleontological dating of certain groups of layers did not allow scientists to come to a conclusion about the global nature of most of these phenomena. This conclusion, unexpected for many geologists, was made by the American geophysicists P. Weil, R. Mitcham and S. Thompson, who studied the seismic sections of the sedimentary cover within the continental margins. Comparison of sections from different regions, often very distant from each other, helped to reveal the confinement of many unconformities, breaks, accumulative or erosional forms to several time ranges in the Mesozoic and Cenozoic. According to these researchers, they reflected the global nature of ocean level fluctuations. The curve of such changes, constructed by P. Weil et al., makes it possible not only to single out the epochs of its high or low standing, but also to estimate, of course, in the first approximation, their scales. Strictly speaking, this curve summarizes the experience of geologists of many generations. Indeed, one can learn about the Late Jurassic and Late Cretaceous transgressions of the sea or its retreat at the turn of the Jurassic and Cretaceous, in the Oligocene, Late Miocene, from any textbook on historical geology. Perhaps what was new was that now these phenomena were associated with changes in the level of ocean waters.
The scale of these changes was surprising. Thus, the most significant sea transgression, which flooded most of the continents in the Cenomanian and Turonian times, was believed to be due to a rise in the level of ocean waters by more than 200–300 m above the present day. The most significant regression that took place in the middle Oligocene is associated with a drop in this level by 150-180 m below the modern one. Thus, the total amplitude of such fluctuations in the Mesozoic and Cenozoic was almost 400-500 m! What caused such grandiose fluctuations? You can’t write them off as glaciations, since during the late Mesozoic and the first half of the Cenozoic, the climate on our planet was exceptionally warm. However, many researchers still associate the Middle Oligocene minimum with the onset of a sharp cooling in high latitudes and with the development of the Antarctic ice sheet. However, this alone, perhaps, was not enough to lower the ocean level immediately by 150 m.
The reason for such changes was tectonic restructuring, which led to a global redistribution of water masses in the ocean. Now we can offer only more or less plausible versions to explain fluctuations in its level in the Mesozoic and early Cenozoic. Thus, analyzing the most important tectonic events that occurred at the turn of the Middle and Late Jurassic; as well as the Early and Late Cretaceous (with which the long rise of the water level is associated), we find that it is these intervals that were marked by the opening of large oceanic depressions. In the Late Jurassic, the western arm of the ocean, Tethys (the region of the Gulf of Mexico and the Central Atlantic), was born and rapidly expanded, and the end of the Early Cretaceous and most of the Late Cretaceous epochs were marked by the opening of the southern Atlantic and many basins of the Indian Ocean.
How could the initiation and spreading of the bottom in young oceanic basins affect the position of the water level in the ocean? The fact is that the depth of the bottom in them at the first stages of development is very insignificant, no more than 1.5-2 thousand meters. The expansion of their area occurs due to the corresponding reduction in the area of ancient oceanic reservoirs, which are characterized by a depth of 5-6 thousand meters. m, and in the Benioff zone, sections of the bed of deep-sea abyssal basins are absorbed. The water displaced from the disappearing ancient basins raises the general level of the ocean, which is recorded in the land sections of the continents as a transgression of the sea.
Thus, the breakup of continental megablocks must be accompanied by a gradual rise in ocean level. This is exactly what happened in the Mesozoic, during which the level rose by 200-300 m, and maybe more, although this rise was interrupted by epochs of short-term regressions.
Over time, the bottom of the young oceans in the process of cooling the new crust and increasing its area (the Slater-Sorokhtin law) became deeper and deeper. Therefore, their subsequent opening had much less effect on the position of the level of ocean waters. However, it inevitably had to lead to a reduction in the area of the ancient oceans and even to the complete disappearance of some of them from the face of the Earth. In geology, this phenomenon is called the "collapse" of the oceans. It is realized in the process of convergence of continents and their subsequent collision. It would seem that the collapse of the oceanic depressions should cause a new rise in the water level. In fact, the opposite happens. The point here is a powerful tectonic activation that covers converging continents. Mountain-building processes in the zone of their collision are accompanied by a general uplift of the surface. In the marginal parts of the continents, tectonic activation is manifested in the collapse of the blocks of the shelf and slope and in their lowering to the level of the continental foot. Apparently, these subsidence also cover the adjacent areas of the ocean floor, as a result of which it becomes much deeper. The general level of ocean waters is falling.
Since tectonic activation is a one-stage event and covers a short period of time, the level drop occurs much faster than its increase during spreading of the young oceanic crust. It is precisely this that can explain the fact that sea transgressions on the continent develop relatively slowly, while regressions usually begin abruptly.
Map of possible flooding of the territory of Eurasia at various values of the probable sea level rise. The scale of the disaster (with a sea level rise of 1 m expected during the 21st century) will be much less noticeable on the map and will have almost no effect on the life of most states. Zoomed in areas of the coasts of the North and Baltic Seas and southern China. (The map can be enlarged!)
Now let's look at the issue of MEAN SEA LEVEL.
Surveyors performing leveling on land determine the height above "mean sea level". Oceanographers who study sea level fluctuations compare them to the marks on the shore. But, alas, even the “average long-term” sea level is far from constant and, moreover, not the same everywhere, and the seashores rise in some places and fall in others.
The coasts of Denmark and Holland can serve as an example of modern land subsidence. In 1696, in the Danish city of Agger, a church stood 650 meters from the shore. In 1858, the remains of this church were finally swallowed up by the sea. During this time, the sea advanced on land at a horizontal speed of 4.5 m per year. Now on the western coast of Denmark, the construction of a dam is being completed, which should block the further advance of the sea.
The low-lying shores of Holland are exposed to the same danger. The heroic pages of the history of the Dutch people are not only a struggle for liberation from Spanish rule, but also a no less heroic struggle against the advancing sea. Strictly speaking, here it is not so much the sea that advances, but the sinking land recedes before it. This can be seen at least from the fact that the average level of full waters on about. Nordstrand in the North Sea from 1362 to 1962 rose by 1.8 m. The first benchmark (altitude mark) was made in Holland on a large, specially installed stone in 1682. soil subsidence on the coast of 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 reclaiming land from the sea, building grandiose dams.
There are, however, places where the land rises above the sea. The so-called Fenno-Scandinavian shield, after liberation from the heavy ice of the ice age, continues to rise in our time. The coast of the Scandinavian Peninsula in the Gulf of Bothnia is rising at a rate of 1.2 cm per year.
Alternate subsidence and rise of coastal land are also known. For example, the shores of the Mediterranean Sea fell and rose in places by several meters even in historical time. This is evidenced by the columns of the temple of Serapis near Naples; marine lamellar-gill mollusks (Pholas) have burrowed into them up to the height of human growth. This means that since the construction of the temple in the 1st c. n. e. the land sank so much that some of the columns were submerged in the sea, and probably for a long time, since otherwise the mollusks would not have had time to do such a great job. Later, the temple with its columns again emerged from the waves of the sea. According to 120 observation stations, the level of the entire Mediterranean Sea has risen by 9 cm in 60 years.
Climbers say: "We stormed a peak so many meters above sea level." Not only surveyors, climbers, but also people who are not at all connected with such measurements are accustomed to the concept of height above sea level. She seems unshakable to them. But, alas, this is far from the case. The ocean level is constantly changing. It is swayed by tides caused by astronomical causes, wind waves excited by the wind, and as changeable as the wind itself, wind revolvers and water surges off the coast, changes in atmospheric pressure, the deflecting force of the Earth's rotation, and finally, the heating and cooling of ocean water. In addition, according to the studies of Soviet scientists I. V. Maksimov, N. R. Smirnov and G. G. Khizanashvili, the ocean level changes due to episodic changes in the speed of the Earth's rotation and the displacement of its axis of rotation.
If only the upper 100 m of ocean water is heated by 10 °, the ocean level will rise by 1 cm. Heating by 1 ° of the entire thickness of ocean water raises its level by 60 cm. Thus, due to summer heating and winter cooling, the ocean level in middle and high latitudes subject to significant seasonal fluctuations. According to the observations of the Japanese scientist Miyazaki, the average sea level off the western coast of Japan rises in summer and falls in winter and spring. The amplitude of its annual fluctuations is from 20 to 40 cm. The level of the Atlantic Ocean in the northern hemisphere begins to rise in the summer and reaches a maximum by winter, in the southern hemisphere its reverse is observed.
The Soviet oceanographer A. I. Duvanin distinguished two types of fluctuations in the level of the World Ocean: zonal, as a result of the transfer of warm waters from the equator to the poles, and monsoon, as a result of prolonged surges and surges excited by monsoon winds that blow from the sea to land in summer and in reverse direction in winter.
A noticeable inclination of the ocean level is observed in areas covered by ocean currents. It is formed both in the direction of the flow and across it. The transverse slope at a distance of 100-200 miles reaches 10-15 cm and changes along with changes in the speed of the current. The cause of the transverse slope of the surface of the current is the deflecting force of the Earth's rotation.
The sea also reacts noticeably to changes in atmospheric pressure. In such cases, it acts like an "inverted barometer": more pressure - lower sea level, less pressure - higher sea level. One millimeter of barometric pressure (more precisely, one millibar) corresponds to one centimeter of sea level.
Changes in atmospheric pressure can be short-term and seasonal. According to the studies of the Finnish oceanologist E. Lisitsyna and the American J. Patullo, level fluctuations caused by changes in atmospheric pressure are isostatic in nature. This means that the total pressure of air and water on the bottom in a given section of the sea tends to remain constant. Warm and rarefied air causes the level to rise, while cold and dense air causes it to fall.
It happens that surveyors are leveling along the seashore or overland from one sea to another. Arriving at the destination, they discover a discrepancy and begin to look for an error. But in vain they rack their brains - there may not be a mistake. The reason for the discrepancy is that the level surface of the sea is far from 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. Lisitsyna, is about 30 cm. Between the northern and southern parts of the Gulf of Bothnia at a distance of 65 km, the level changes by 9.5 cm. the difference in level between the sides of the Channel is 8 cm (Creese and Cartwright). The slope of the sea surface from the English Channel to the Baltic, according to Bowden's calculations, is 35 cm. The level of the Pacific Ocean and the Caribbean Sea at the ends of the Panama Canal, which is only 80 km long, varies by 18 cm. In general, the level of the Pacific Ocean is always slightly higher than the level of the Atlantic. Even if you move along the Atlantic coast of North America from south to north, a gradual rise in level by 35 cm is found.
Without dwelling on the significant fluctuations in the level of the World Ocean that occurred in past geological periods, we will only note that the gradual rise in the level of the ocean, which was observed throughout the 20th century, averages 1.2 mm per year. It was caused, apparently, by the general warming of the climate of our planet and the gradual release of significant masses of water, bound until that time by glaciers.
So, neither oceanologists can rely on the marks of surveyors on land, nor surveyors on the readings of tide gauges installed off the coast in the sea. The level surface of the ocean is far from an ideal equipotential surface. Its exact definition can be reached through the joint efforts of geodesists and oceanologists, and even then not earlier than at least a century of material of simultaneous observations of the vertical movements of the earth's crust and sea level fluctuations in hundreds, even thousands of points is accumulated. In the meantime, there is no "average level" of the ocean! Or, which is the same thing, there are many of them - each point has its own coast!
Philosophers and geographers of hoary antiquity, who had to use only speculative methods for solving geophysical problems, were also very interested in the problem of ocean level, although in a different aspect. We find the most specific statements on this subject from Pliny the Elder, who, by the way, shortly before his death while observing the eruption of Vesuvius, rather presumptuously wrote: “There is currently nothing in the ocean that we could not explain.” So, if we discard the disputes of the Latinists about the correctness of the translation of some of Pliny's arguments about the ocean, we can say that he considered it from two points of view - the ocean on a flat Earth and the ocean on a spherical Earth. If the Earth is round, Pliny reasoned, then why does the water of the ocean on the other side of it not drain into the void; and if it is flat, then for what reason does the ocean waters not flood the land, if everyone standing on the shore can clearly see the mountainous bulge of the ocean, behind which ships hide on the horizon. In both cases he explained it this way; water always tends to the center of the land, which is located somewhere below its surface.
The problem of ocean level seemed unsolvable two thousand years ago and, as we see, remains unresolved to this day. However, the possibility is not ruled out that the features of the level surface of the ocean will be determined in the near future by means of geophysical measurements made with the help of artificial earth satellites.
Gravity map of the Earth compiled by the GOCE satellite.
These days …
Oceanologists re-examined the already known data on the rise in sea level over the past 125 years and came to an unexpected conclusion - if for almost the entire 20th century it rose much more slowly than we previously thought, then in the last 25 years it has grown at a very fast pace, according to article published in the journal Nature.
A group of researchers came to such conclusions after analyzing data on fluctuations in the levels of the seas and oceans of the Earth during the tides, which are collected in different parts of the world using special tide gauge instruments over the course of a century. The data from these instruments, as scientists note, are traditionally used to estimate sea level rise, but this information is not always absolutely accurate and often contains large time gaps.
“These averages do not correspond to how the sea actually grows. Tide gauges are usually located along the banks. Because of this, large areas of the ocean are not included in these estimates, and if they are included, then they usually contain large "holes", - the words of Carling Hay from Harvard University (USA) are quoted in the article.
As another author of the article, Harvard oceanologist Eric Morrow, adds, until the early 1950s, mankind did not systematically observe sea levels at the global level, which is why we have almost no reliable data on how quickly the world's ocean in the first half of the 20th century.
sources
http://ria.ru/earth/20150114/1042559549.html
http://www.okeanavt.ru/taini-okeana/1066-mif-o-srednem-urovne.html
http://www.seapeace.ru/oceanology/water/68.html
http://compulenta.computerra.ru/zemlya/geografiya/10006707/
Here we examined, and also tried to find out where it is. See what happens and here is the information The original article is on the website InfoGlaz.rf Link to the article from which this copy is made -MOSCOW, February 17 - RIA Novosti. Scientists have found inclusions of the human genome in the oldest samples of Neanderthal DNA from the Altai, which tell us that the first people entered Asia already 100 thousand years ago, long before the migration of the Cro-Magnons to Europe, according to an article published in the journal Nature.
"Over 100 thousand years ago, modern people left Africa for the first time in their history. These Cro-Magnons met and began to contact and interbreed with a group of Neanderthals, which, it seems, then migrated to the south of modern Siberia, taking with them the genes of Homo sapiens," — said Antonio Rosas (Antonio Rosas) from the National Museum of Science of Spain in Madrid.
Rozas and his colleagues, led by the famous paleogeneticist Svante Paabo of the Institute for Evolutionary Anthropology in Leipzig, Germany, came to this surprising conclusion by "resurrecting" and studying the genome of Neanderthals, whose remains were recently found in Denisova Cave in Russia and elsewhere. Altai.
Analyzing this genome and comparing it with the DNA structure of the Denisovans, the authors of the article noticed something unusual - the genetic material of the Altai Neanderthals contained a small number of mutations in about 5% of the genes unique to the modern inhabitants of Africa, which were not in the genome of their "neighbors" in the cave .
Why is it important? In previous years, Paabo and his colleagues showed that the inhabitants of Africa did not interbreed with the "European" Neanderthals and in their genome there is absolutely no trace of the DNA of the natives of Europe, which is in the genes of all other inhabitants of the Earth. The presence of "African" mutations, therefore, indicates that the Altai Neanderthals could have contacted them, leaving their DNA to the Cro-Magnons or having received mutations from them.
Scientists tried to figure out which of these two scenarios happened by comparing how these DNA sections were arranged in about five hundred modern Africans. A fairly small number of differences in the structure of these genes indicated that the second hypothesis was correct - Neanderthals contacted ancient people in the distant past, whose genetic "traces" remained in the genome of Homo neanderthalensis from Altai.
These contacts, as the calculations of the authors of the article show, occurred unexpectedly and frighteningly long ago - at least 100 thousand years ago, tens of thousands of years before the alleged start of the mass migration of Cro-Magnons from Africa to Europe and the time of the first, as previously thought, contacts with Neanderthals. Apparently, humanity left Africa at least twice - 210-100 thousand and 65-60 thousand years ago.
Surprised by this discovery, Paabo and his colleagues tested whether there are similar "human" parts of the genome in the DNA of European Neanderthals, whose remains were found in Croatia and Spain, in the famous Sidron Cave. As it turned out, there were no such traces in their genomes, which suggests that such contacts occurred either in the Middle East, where Neanderthals lived in the Pleistocene, or already in Altai.
What genes did Neanderthals inherit from our ancestors? The preliminary data of the authors of the article indicate that the Cro-Magnons "gave" Neanderthals some of the genes associated with the functioning of the liver, as well as the FOXP2 gene, which is responsible for the development of the vocal apparatus and the ability to articulate speech.
All these genes, even if they were useful for Neanderthals, apparently were poorly compatible with their Neanderthal variants - all critical parts of the Altai Neanderthal DNA, like the female X chromosome, do not contain "African" inclusions, which indicates their negative impact on the survival of offspring from mixed marriages.
On the whole, as Paabo and his colleagues acknowledge, such an unexpected discovery is not too surprising. It fits well with the latest paleogenetic and paleontological findings - in particular, in October 2015, scientists found that the first people could have entered China as early as 110-80 thousand years ago. In principle, nothing prevented them from making contact with Neanderthals as they colonized Asia, the scientists conclude.
By the middle of the Middle Holocene, broad-leaved species in the territory of the Moscow region reached their maximum distribution and abundance. It was the time of the Holocene "climatic optimum". The climate was characterized not only by higher temperatures, but also by higher humidity.
M. I. Neishtadt
In recent decades, paleoclimatology has received powerful research tools - pollen spore analysis and radiocarbon dating. The first one makes it possible to reliably determine the composition and ecological conditions of plant communities of past epochs, the second one, with sufficient accuracy, to date the time of these epochs in absolute terms.
The use of new research tools in the layer-by-layer study of continental deposits over the past 20,000 years has revealed an unusually wide and striking spectrum of climatic changes. The results of these studies are especially valuable, since they relate to a time as close as possible to ours.
Consider climate change in the following major stages.
20,000 years ago, 67% of the area of the continental glaciers of the globe was concentrated in the Northern Hemisphere. Today it is only 16% (Table 1). At that time, the European ice sheet occupied all of Scandinavia, Finland, the Baltic Sea, including the Skagerrak Strait. Its southern edge overlapped the territory of Berlin, Plock (Poland) and came close to Orsha, Smolensk, Rzhev, Rybinsk reservoir. Even more extensive was the North American Glacier. It covered the entire northern part of the continent. Its southern edge approached almost closely to the territory of the cities of Cincinnati, Pittsburgh and New York.
Over the past 20,000 years, the area of all continental glaciers in the Northern Hemisphere has decreased by 24.5 million km 2, i.e. by 91%. Of the remaining 2.3 million km 2, only one Greenland glacier occupies almost 1.8 million km 2.
The current volume of continental ice is estimated at 24-27 million km3. If they completely melted, the level of the World Ocean could rise, according to formal calculations, by 65-70 m. The volume of continental ice during the period of maximum glaciation increased by 16 million km 3, which lowered the ocean level by 45 m. climate change is extremely slow (see Table 1), then we have the right to believe that the increase in ice was mainly for the formation of continental glaciers in the Northern Hemisphere. In accordance with this, the average thickness of the ice cover was 650 m. The maximum thickness was approximately the same and in the same areas as during the Dnieper glaciation. On the periphery, the power decreased to several tens of meters, or even simply disappeared.
In the central region of glaciation, the ice temperature, as our calculations show, was approximately -10°C, i.e., much higher than the ice temperature of Greenland, which is equal to -28°, and even more so of Antarctica with its -50, -60°.
Such a high temperature of ice in the Central Region was essential. Being warmer, it naturally reacted to warming and cooling faster than the ice sheets of Greenland and Antarctica.
The decrease in the level of the World Ocean by 45 m due to the increase in continental ice caused the drying of a significant part of the continental shelves. The Bering, Chirikov, and Spanberg straits became so shallow that the water exchange between the Polar Basin and the Pacific Ocean practically ceased, and with it, the marine advection of heat from the Pacific Ocean to the Arctic basin ceased.
18,000 years ago began warming and the associated retreat of the ice sheets. The retreat was not monotonous. It was interrupted by stops during periods of warming decline and thrusts into previously liberated territories during cooling (Fig. 6).
What are the reasons for such profound and relatively rapid changes in continental ice sheets? It turns out that insignificant but stable changes in the heat balance of the surface layer of the ocean are enough to significantly affect natural processes. This is clearly seen in the example of sea ice. The English climatologist C. Brooks believes that an increase in temperature on the Earth's surface by only 1 ° C would be sufficient to bring the entire ice cover of the Polar Basin into an unstable state.
Thermal processes are especially effective at the border of melting and freezing of water. Phase transformations (water, snow, ice) within one degree are accompanied by major changes in the absorption of solar radiation by the sea surface.
It is estimated that as a result of the destruction of sea ice per unit area of the Polar Basin, solar radiation heat is absorbed eight times more than is required to reduce the thickness of continental ice at a rate of 0.5 m per year.
Over the past 18,000 years, the warming in the middle Holocene has been especially significant. It covered the time from 9000 to 2500 years ago, culminating in the period 6000-4000 years ago, that is, when the first pyramids were already being erected in Egypt. It should be noted that the time of the ascending branch of warming is dated differently: according to Gross, up to 7500 years ago, after which the culmination phase began, which lasted up to 4500 years ago, and according to M.A. Lavrova, up to G000 years ago, followed by the phase of the most magnificent flourishing of marine life, which lasted until 4000 years ago (Fig. 7).
The most exciting questions of the stage under consideration are whether the Arctic Basin was ice-free during the period of the optimum climax and what was the reaction of climatic conditions on the continents in connection with this.
Many scientists believe that during the period of the climatic optimum, the Arctic Basin was free of ice. C. Brooks substantiates his statement about the ice-free Arctic basin by the fact that there were no ice on Svalbard, there was a relatively rich flora and warm-water mollusks lived, and also by the fact that the temperature of the open Arctic basin and its coasts was higher than modern. An increase in the temperature of surface waters and the surface air layer by 2-2.5° (which is quite enough for the complete elimination of drifting ice in the Polar Basin) has been well proven by a number of independent studies carried out using different methods.
The permafrost on the continents, circumpolarly covering the Arctic Basin, during the period of its warming was strongly degraded. So, in the north and north-west of Siberia, the thawing depth reached 200-300 m. Mountain glaciers were significantly reduced, and in some places they completely disappeared.
How did the climate react to the disappearance of ice in the Arctic basin?
Vegetation zones have moved circumpolarly towards the pole. On the Eurasian continent, the displacement reached 4-5° latitude in the west and 1-2° in the east. Separate vegetation belts have moved their northern borders by 1000 km. Forests came close to the coast of the Barents Sea, and oak, linden, hazel reached the shores of the White Sea. There is evidence to suggest that the tundra and forest-tundra zones disappeared completely on the European mainland. In the northern part of Asia, the remains of woody vegetation were found only 80 km from Cape Chelyuskin, and peat bogs were found on Novaya Zemlya. In Ukraine, in conditions of a favorable, more humid than now, climate, agriculture was first developed. It has been established that the Middle Dnieper region was completely covered with forest. Forests along the river valleys descended to the Black, Azov and Caspian Seas, and broad-leaved species spread quite densely in the space from Saratov to the lower reaches of the Volga region. Favorable climatic conditions are also evidenced by the fact that the Tripoli and Lower Danube tribes have all the main grain crops known today, as well as large and small cattle.
A number of foreign researchers - W. Fitzgerald, O. Bernard, F. Morett, R. Capo-Rey, R. V. Fairbridge and others - unanimously note that the hydrography and vegetation of the Sahara bear clear imprints of climate volatility. Everywhere you can see lifeless wadis, dried-up lakes, where, obviously, there was water quite recently. The striking contrast between the ruins of settlements in North Africa and the bare landscape that now surrounds them speaks of a recent change in moisture.
An interesting fact is that in the Cenozoic, the Sahara reached its greatest aridity and the greatest distribution precisely in the Quaternary time - during the period of the greatest cooling of our planet, including the northern polar latitudes.
Even in the late glacial period, due to the predominance of northeasterly winds, the upper reaches of the Nile received little water from the Abyssinian plateau. The Nile did not reach the Mediterranean Sea, just as the Emba River does not reach the Caspian today in dry seasons. “The present hydrographic regime of Northeast Africa,” states Fitzgerald, “did not arise until the end of the last glaciation of northern Europe, probably around 12,000 BC. e., that is, not earlier than the disappearance of the main masses of ice in the northwestern part of Europe, the fall in ice cover in the Arctic Ocean and the increase in the temperature of the surface waters of the North Atlantic.
In the period V-III millennium BC. e. in various points of the Sahara, the Arabian and Nubian deserts, a much more humid climate was noted. The distribution of man and animals was wider. The elephant, hippopotamus and rhinoceros disappeared in the Sahara at the end of the third millennium BC. e. Further drying up of the Sahara led to the departure of nomadic tribes from it.
The famous polar explorer V. Yu. Vize established a connection between the decrease in the ice cover in the Arctic and the increase in the level of lakes in Africa, including Lake Victoria, the source of the Nile. The connection is so stable that it allowed the author to draw a very curious conclusion - a person who monitors the level of lakes can judge the state of ice in the Arctic seas.
The absence of ice in the Arctic Basin during the culmination of the Middle Holocene Optimum had a favorable effect on the climate of the entire planet. Throughout Europe, from the Iberian Peninsula to the Volga, as already noted, thermophilic forest vegetation prevailed. People were engaged in fishing and hunting, hoe farming developed. In the mountains, the border of the forest lay higher than now. “It must be emphasized,” wrote K. K. Markov, “that after the end of the ice age in Central and North Asia there are no signs of a systematic drying out of the climate. After the disappearance of the last ice sheet on the Russian Plain, the climate becomes generally more humid” 1 . “The state of the vegetation of Central Asia,” E. P. Korovin noted in turn, “in the epoch immediately after the glaciation is characterized by the progressive development of plant formations of the mesophilic warehouse. In connection with the retreat of glaciers, the general warming and humidification of the mountain climate, boreal flora, which developed in the middle latitudes of Siberia soon after its liberation from ice cover, opened up in Central Asia.
In Inner Alaska and Yukon, the absolute age of peat deposits is determined to be 5,000 years. In the north-west of Canada, 64° 19' north latitude and 102° 04' west longitude, hornwort was found in sediments that are 5400 years old. The northern limit of the modern distribution of the hornwort reaches only 59° 14' north latitude. On the eastern slope of the Rocky Mountains of Colorado, the age of peat overlying the deposits of the last glaciation is 6170 + 240 years. In the Lake Michigan basin, the climate was warmer and wetter 3,000 years ago than it is today.
In the region of the San Rafael lakes (Southern Chile), the climatic changes of the Late Pleistocene chronologically coincide with the climate fluctuations established in other areas of the Southern Hemisphere (Tierra del Fuego, Patagonia, Tristan da Cunha, New Zealand, Hawaiian Islands). In the Andes (39° south latitude) the interglacial climate was wetter than the present; the main waves of climate change are synchronous in both hemispheres. The dry periods of Tierra del Fuego and Patagonia are synchronous with the boreal, subboreal and modern periods of Europe. In Australia and New Zealand, the population was engaged in agriculture. The South African Kalahari Desert 6000-7000 years ago had a wetter climate than it does today.
The extinction of the culmination of the climatic optimum of the Middle Holocene began 4000 years ago. Approximately 3000 years ago, the restoration of the ice cover of the Arctic Basin began.
The time of 2500 years ago, according to the scheme of division of the Holocene by M. I. Neishtadt, is the boundary between the middle and late Holocene. Since that time, a more intense cooling has been recorded. However, after about a thousand years, somewhat later than 500 AD. e. a new warming began and, as Brooks stated, "Arctic ice has entered a stage of semi-sustainable existence." This stage dominated until about 1200. Brooks characterizes the semi-stability of the Arctic ice as a state when they completely disappear in summer and are restored in winter in an insignificant volume.
In this state, the area of drifting sea ice in the Southern Hemisphere during the cold season reaches 22 million km 2, in February it is reduced to 4-6 million km 2, i.e. by 80%. In the Arctic Ocean, the total area of drifting ice reaches 11 million km 2 in winter, and in summer, by the end of melting, it can decrease to 7 million km 2, i.e., by one third. If, however, the balance of drifting ice in the Northern Hemisphere includes the ice of the Bering and Okhotsk seas that completely disappear in summer and the volume of ice melting from the ice cover of the Arctic Ocean by about 20%, then it can be seen that the volume of sea ice in northern latitudes is half as much by the end of summer, than at the end of winter.
According to later data by V.S. Nazarov, the annual growth and melting of sea ice on the whole globe is 37,000 km 3 with an annual carryover of 19,500 km 3 . In other words, every year 67% of the sea ice on our planet is renewed. Consequently, if sea ice is unstable at the present time, then they were all the more unstable in the early Middle Ages, when summer temperatures were 1-2 ° higher than modern ones.
L. Koch studied the dynamics of the ice cover of the North Atlantic over the past millennium. The research results are presented in fig. 8. Low ice coverage at high latitudes reduced the strength of storms and the number of storm days. Asturian fishermen of that time may have engaged in whaling there.
The ice cover has also decreased in the Antarctic polar latitudes. Even in the middle of the 7th century. n. e. Polynesians, in particular Wi-Te-Rengina, sailed in Antarctic waters, despite the primitive ship and navigation technology of that time. At the same time, during the years of J. Cook's voyage (1772-1775), the ice cover, judging by the description of him and his companions, significantly exceeded the modern one.
In the region of Iceland and South Greenland from 900 to 1200 the climate was milder; no sea ice was observed in these areas. In the southwest of Greenland, there were Scandinavian colonies with a strikingly high level of pastoralism. When excavating a cemetery near Cape Farwell, located in the modern permafrost zone, archaeologists found that at the time when the burials were made, the permafrost should have thawed in the summer, since coffins, shrouds and even corpses were pierced by plant roots. In an earlier period, the ground had to thaw to a considerable depth, since the coffins were lowered relatively deep at the most ancient burials. Subsequently, these horizons ended up in the permafrost zone, and later burials were located closer and closer to the surface.
In the Alps, the glaciers were shrinking greatly. According to Italian scientists, from the VIII to the XIII century. the climate was more conducive to agriculture than from the thirteenth to the middle of the sixteenth century, when droughts were more frequent. This also applies to our forest-steppe south, where in the IX-X centuries. large flourishing cities, arable farming with a plow "ralo", almost all types of livestock known to us testify to the high level of development of Kievan Rus.
On the territory of the modern Tatar ASSR in the X century. Ibn Fadlan observed that the Bulgarians, who occupied this territory, developed agriculture with the cultivation of wheat. Cultivated wheat and other peoples that were part of the Volga Bulgaria. This is confirmed by the Russian chronicles. On the other hand, it is known for certain that from the 14th to the 19th centuries. wheat was not sown in this area due to the harshness of the climate.
A large number of historical and archaeological evidence shows that in Central Asia in the VIII-XII centuries. moisture was sufficient to occupy almost the entire interfluve of the Amu-Darya and Syr-Darya with irrigated agricultural crops. According to Arab historians, the cat could run from Samarkand to the Aral Sea on the roofs of houses. Not only the deserts of Central Asia, but even the largest desert on Earth, the Sahara, reacted to the decrease in ice cover in the Arctic basin by some decrease in its aridity.
From the 13th century n. e. cooling again. It manifested itself most fully in the period 1550-1850. In this tercentenary, severe winters become more frequent. The mountain glaciers of Scandinavia, the Alps, Iceland, and Alaska have grown. In a number of areas, they blocked settlements and cultivated lands. According to P. A. Shuisky, in the XVIII-XIX centuries. the advancement of glaciers in places reached "the maximum size since the last ice age ..."
The pack ice entering the Greenland and Norwegian Seas from the Arctic Basin melted more slowly, which affected the ice blockade of Greenland. Greenlandic colonies founded in the 10th century. and flourished before the blockade, began to lose touch with the metropolis, fall into decay and in the middle of the XIV century. ceased to exist.
Despite some periods of warming and the associated retreat of glaciers, in general, the period under consideration was so cold that it was called the "Little Ice Age". The high latitudes were chilled out, and the ice cover of the polar seas increased. In the North Atlantic, sea ice reached its greatest development during the post-glacial period, for example, in the years from 1806 to 1812, ships rarely managed to penetrate above 75 ° north latitude.
Radiocarbon studies of plant remains taken from under 47 meters of ice in northwestern Greenland have shown that, less than 200 years ago, the area's glaciers continued to advance vigorously. At the climax of the cold snap, the snow boundary dropped to sea level, which naturally created favorable conditions for the revival of ice sheets that had disappeared during the previous warm period.
At the time of the Fram's drift, the conditions for the formation of a more cohesive and thicker ice sheet were more favorable than they are now. Arctic explorers in the past have often reported thick 4-6 meter "paleocrystalline" drifting ice. Encountering such ice is rare these days, as it is a product of a colder climate.
The high ice cover of the Polar Basin has always given rise to a restless regime of the atmosphere. Its direct consequence was lean famine years, the frequency of which increased markedly.
American biologists seem to have figured out why 100-74 thousand years ago humanity passed through the "bottleneck" - its population declined sharply. Blame it on bacterial infections that kill people in infancy. Our ancestors managed to cope with them only by losing two genes that were "treacherous" activities.
It is possible that the influence of bacteria and viruses on the evolution of mammals and, in particular, humans, is also significant. At least, according to scientists, they once helped us overcome the so-called "bottleneck". Let me remind you that this is the name given to a situation in which, for a variety of reasons, the population size and, accordingly, its genetic diversity are sharply reduced. It is believed that this happened to people about 100-74 thousand years ago.
Then, according to anthropologists, the human population, which was already approaching a million, suddenly dropped sharply to 10-20 thousand individuals. And, most interestingly, almost all other representatives of the genus died out in parallel Homo, except for Neanderthals, and possibly Denisovans. It is still not entirely clear why this suddenly happened, although there were many versions. The most popular of them are the extinction of large ungulates, which people hunted, as well as the consequences of the eruption of the Toba supervolcano on the island of Sumatra.
The last version, despite the fact that it is often stated on the pages of popular science books, is the weakest reasoned. Yes, of course, the eruption that happened 77.5 thousand years ago was powerful - the volcano threw out 800 cubic kilometers of ash alone. Nevertheless, anthropologists found traces of human settlements in East and South Asia after this event. That is, even the people who lived near the volcano did not die out immediately. And besides, the decline in the number of the human race was not sharp - it began about 100 thousand years ago, that is, long before the eruption itself. So it could only hasten the extinction of many human groups, but clearly was not its cause.
As for the hypothesis that the "bottleneck" effect was associated with the extinction of large ungulates that were hunted by people, there is also some kind of temporal discrepancy here. It is known that several such extinctions occurred during the Quaternary period, but not one of them occurred at the indicated time. And besides, during the previous extinctions of large ungulates, no decrease in the number of people was observed - but in theory, it should have been if these two processes are interconnected. In addition, anthropological data show that 100 thousand years ago, people still mainly hunted small animals, not large ones.
So why did the number of people in those distant times decline so much? Biologists from the University of California at San Diego (USA) believe that the reason for this was epidemics caused by bacterial infections. However, according to their data, already after several tens of thousands of years, people were able to defeat these diseases - due to the fact that the genome of our ancestors got rid of two "traitor" genes that collaborated with harmful microorganisms.
The researchers were interested in two immune genes, Siglec-13 and Siglec-17. These DNA sequences help the immune system decide which immune cells to send to fight the pathogen. Both of these genes are known to be active in chimpanzees and gorillas, but they do not work in humans, since Siglec-17 is “turned off” as a result of a mutation, and its counterpart at number 13 is completely cut out of the genome.
After the scientists synthesized the proteins encoded by these genes, they discovered an interesting thing - both proteins prevented antibodies from binding to antigens on the membranes of group B streptococci ( Streptococcus) and Escherichia coli K1 ( Escherichia coli). It turns out that if these proteins are present in immune cells, they will no longer recognize these bacteria as a potential threat to the body. And this led to disastrous consequences. The fact is that the aforementioned B streptococci and E. coli K1 are aggressive microbes that are very dangerous for newborns. After they enter the body, the child most often dies, and only modern medicines, which had not yet been invented 100 thousand years ago, can save him.
So, it turns out that about 400 thousand years ago, representatives of the genus Homo first "got acquainted" with these diseases. (Perhaps this was due to a change in diet or habitat.) The result was a sharp decline in the population of our ancestors, since most of them simply died in infancy. However, natural selection also did not doze off - according to genetic analysis, the "traitorous" genes Siglec-13 and Siglec-17 began to turn off between 400 and 270 thousand years ago, that is, even before modern man broke away from the branches of Neanderthals and Denisovans.
Thus, somewhere between 270-265 thousand years ago, a certain population was formed, which consisted of individuals devoid of these molecular "traitors". However, the infection also did not doze off. As people spread beyond Africa, she traveled with them. As a result, the number of all groups, except for the one that did not have genes dangerous to health, steadily decreased. The most intense extinction occurred just 100-74 thousand years ago. By the way, it is quite possible that it was the Toba eruption or the reduction in the number of animals that people ate that spurred him on, because in a stressful situation, any disease becomes more dangerous.
However, even for a population whose members had already rid themselves of harmful genes, it took time for this change to become the general norm. Therefore, it is not surprising that the last traces of the Siglec-17 gene are found in some people. However, there are none on the planet right now. If suddenly a reverse mutation occurs, which "turns on" this section of DNA during the development of the child, then he will not live up to a year.
Our planet is over 4.5 billion years old. When she first appeared, she looked completely different. What was in ancient times on the territory of modern Russia, and how it has changed over the years - in the book "Ancient Monsters of Russia".
3000 million years ago
For the first million years of its life, Earth was like hell. There were constant acid rains, hundreds of volcanoes erupted. There were many more asteroids. Endless meteor showers shaped the planet - crashed into it and became part of it. Some meteorites reached the size of modern cities.
Once the Earth collided with another planet, one part of which joined us, and the second flew into orbit and over the years turned into the modern Moon.
Illustration from the book
3 billion years ago, a day lasted only 5 hours, and there were 1500 days in a year. Once every 50 hours there was a lunar eclipse, and every 100 hours - a solar eclipse. It certainly looked very beautiful, but there was still no one to admire the natural phenomena.
