Anthropogenic System Period
Anthropogenic System (Period)
the last system in the stratigraphic scale and the last period of the geological history of the earth, continuing into the present. The age of the Anthropogenic system (period) is estimated at from 600,000–1,000,000 years to 2,500,000–3,500,000 years.
History of the classification and subdivision of the Anthropogenic system (period). In the 1870’s the German scientist A. Werner designated the porous continental drifts, which correspond to the present Anthropogenic system, as an “alluvial formation.” In 1823 the British scientist W. Buckland subdivided this into two parts: the ancient diluvial (the deposits of the “deluge”) and the younger alluvial. (The modern term “alluvium” has a different meaning.) In 1825–29 the French scientist J. Desnoyers combined the diluvial and the alluvial into one system and called it the Quaternary. He counterposed the Quaternary system (period) to all the more ancient geological formations, which were subdivided at that time into primary, secondary, and tertiary. In 1832 the British geologist C. Lyell introduced the term “Pleistocene” to designate all the young marine deposits immediately preceding the deposits of the modern period. This term was later extended to include continental diluvial deposits. When it was established that the Pleistocene was a period of general cooling off of the climate, when thick ice sheets covered the land, the term “Ice Age” was also applied to it, and what had been called the alluvial became known as the postglacial period, the Recent epoch, or the Holocene. In the 20th century the term Quaternary system (period) has become an anachronism, inasmuch as all the other terminology of Desnoyers’s time has long since been rejected. Therefore, in 1922 the Russian geologist A. P. Pavlov suggested a new term—the “Anthropogenic system (period),” or “Anthro-pocene,” which corresponds to the names of the other systems of the Cenozoic era (Paleocene and Neocene) and which reflects the most important event in the history of the organic world of the Anthropogenic system (period)—the appearance and evolution of human beings and human society. The term “Anthropogenic system (period)” has been accepted in the USSR although geological services both in the USSR and abroad still use “Quaternary system (period).”
Outside the USSR the foundation of the Calabrian marine and Middle Villefranchian continental layers in Italy is regarded as the lower boundary of the Anthropogenic system. Many Soviet scientists include in the Anthropogenic system analogous deposits in the USSR—the Akchagyl and Apsheron layers, the majority of which follow the scale adopted in the USSR. According to this scale, these layers are in the Neocene (Upper Pliocene) and the boundary of the Anthropogenic system is drawn beneath the Baku layers (see Figures 1 and 2). These circumstances explain the differences in estimates of the age of the Anthropogenic system (period).
The Anthropogenic system is usually divided into four segments: Lower (eo), Middle (meso), Upper (neo) Pleistocene, and Holocene; these were introduced in 1932 by the Commission for an International Map of the Quaternary Deposits of Europe. There are differences of opinion about the magnitude of the Pleistocene subdivisions, and some scientists exclude the eo-Pleistocene from it altogether. However, even taking the maximum possible age for the Anthropogenic period, it is much shorter than any other geological epoch, and a much more detailed segmentation is required to reconstruct a cohesive chronicle of its events. Therefore, the Anthropogenic system is classified by climatostratigraphic and biostratigraphic methods, with segmentation based on the many sharp changes in climate. These changes are characterized by changes in types of deposits, buried soils, fossils of warmth- and cold-loving animals, and particularly the spores and pollen of terrestrial vegetation.
The system of classification devised by A. Penck and E. Brückner in 1909 for the Alps and later expanded by B. Eberl and others is regarded as standard for the climato-stratigraphy of the Anthropogenic system. It distinguishes five stages of severe cooling—when the glaciers expanded considerably (glaciations)—separated by warmer periods— when the glaciers shrank to their present sizes or even smaller (interglacials). Two of the glaciations (the Danube and the Günz) are placed in the Villefranchian (Upper Pliocene), and the other three (Mindel, Riss, and Würm) are placed in a later period—that is, the Anthropogenic period in its minimal sense. The continental ice sheets of the middle latitudes, which covered enormous areas of the continents of the northern hemisphere at about the same time during the cold periods, are aligned with this Alpine classification. In most instances, alignments are approximate or questionable; therefore, there are many local classifications with their own names for the glaciations and interglacials. The glaciations, originally designated in the Alpine and other classifications, fall into stages of glaciation themselves. These are called the stadials (during which the glaciers greatly expanded, or advanced) and the interstadials (when they shrank just as greatly, or retreated, as a result of the relative warming up of the climate). Inasmuch as it is difficult to objectively distinguish the major interstadials from genuine interglacials, different researchers estimate different numbers of glaciations. There are even some who take an extreme point of view and support the so-called theory of monoglacialism, according to which there was only one Anthropogenic glaciation with many long and short stages. However, the great majority share the concept of a multitude of glaciations, or polyglacialism, which contends that there were between three and eight separate glaciations. Furthermore, some place the first continental glaciations in the Villefranchian period (Upper Pliocene); others believe that all the glaciations occurred later and belong in the Anthropogenic period only in its minimal sense. There is as yet no single generally recognized international stratigraphical scale for the Anthropogenic system, and for that reason various divisions—sometimes even mutually exclusive—are used. The most important of these are presented in the table comparing the major subdivisions of the Anthropogenic system (period) and summarizing the data on the absolute age of the deposits. Radioactive carbons are used to determine age for the past 30,000 to 25,000 years. Potassium-argon and other radiometric methods are used for the more remote past.
General description of the Anthropogenic system (period). The most graphic feature of the Anthropogenic period is the general cooling of the earth’s climate, with periods of sharp temperature decline. These declines were especially severe in the middle latitudes of the continents of the northern hemisphere, where extensive ice sheets appeared. The climate changed less sharply in most of the tropical and equatorial zones, where the conditions on land remained close to what they are at present. However, an analysis of the silts raised from the bottom of the world ocean reveals that even in the tropics the average temperature of the seawater fluctuated as much as 6°C; this is indicated by alternating layers containing larger or smaller quantities of cold- and warmth-loving species of foraminifers and by the change in the oxygen isotope content of their shells, of l6O and 18O.
The first waves of considerable temperature declines (the Danube and Günz) go as far back as the Villefranchian period (Upper Pliocene). Some scientists believe that not only did mountain glaciers expand at this time but that large continental glaciations appeared (the Nebraskan in North America, Lower Forestian in Poland, and the oldest glaciers in Lithuania and Byelorussia). However, the majority of scientists believe that the climate was warmer then than it is now, that the cooling phases were mild and brief, and that the aforementioned glaciations came later. At any rate, these cooling periods had an enormous effect on the organic world. Subtropical vegetation in the middle latitudes gave way to forests and steppes closely resembling those of today. Northern species of mollusks appeared in the seas of western and southern Europe; Neocene mammal forms became extinct, except for afew relic forms (mastodons of the Anancus genus and others). New, typically Anthropogenic mammal groups reached their high point of development—among them elephants (Elephantidae of the Archidiskodon genus), genuine bulls (subfamily Bovinae, Leptobos and Bison genera), and one-toed horses (genus Equus stenonis major).
From the end of the Villefranchian period (Upper Pliocene) until the end of the Pleistocene, the many waves of temperature decline were without a doubt already marked by the expansion of mountain glaciers everywhere and by extensive continental glaciations in Europe, northern Asia, and North America. These glaciations left large areas of moraines—that is, fluvioglacial and lake-glacial deposits. The glaciers reached their maximum proportions during the Middle Pleistocene—that is, in the Dnieper glaciation of the Eastern
European plain, which is usually aligned with the Rissian in the Alps, and with the Illinoian in the USA (see Figure 1). At that time the total glacial area of the earth was about triple the size it is today. Continental ice sheets descended as far as 48°30’ in the southern part of the Eastern European plain and to nearly 37° N latitude in the USA. In Europe, average annual temperatures fell by approximately 6–8°C. The area containing permafrost rocks extended to the south of France. The climate was no less severe during the Upper Pleistocene glaciations (Kalinin and Ostashkovo [Valdai], Würm, and Wisconsin), although a much smaller area was under ice.
At the time of the glaciers, sharp changes occurred in climatic and vegetation zones on the land, not only because of the temperature decline but also because of changes in the general atmospheric circulation under the influence of the huge continental ice sheets. The climate became drier in the adjoining regions, and a wide periglacial zone appeared with a distinctive terrain that combined characteristics of the tundra and the steppes. The forest belt of the temperate zone was pushed southward, shrank, and in some places disappeared altogether, so that the periglacial tundra-steppe directly met the true steppes, located farther to the south. Loesses and loesslike rocks were formed in this extensive arid zone, and it was inhabited by “glacial” fauna, including the now extinct mammoth (Mammuthus) and woolly rhinoceros (Coelodonta antiquitatis); tundra animals such as the musk-ox, reindeer, arctic fox, lemming, and white partridge, which sometimes existed as far south as the foothills of the Crimea and the Northern Caucasus; and distinctly steppe and forest-steppe animals, such as the horses, saiga, bison, and big-horned deer (Megaloceros giganteus). Still farther to the south, in the subtropical and tropical desert and semidesert zone, the glaciations corresponded to very wet climatic periods, known as pluvials, during which large freshwater and saltwater lakes (now dried out) were formed. Water-loving animals such as the hippopotamus and the crocodile penetrated even into the interior of the Sahara at that time.
During the interglacials, zonal divisions resembling those of the present were restored, and the climate was sometimes even warmer than it is today. For example, during the Mikulino (Eemian) interglacial in Europe broad-leaved forests grew at the latitude of Vologda, now an area of taiga. In the desert and semidesert zone the interglacials were marked by dry climatic phases known as “arids.”
Enormous quantities of water, taken from the oceans and returned to them after the ice melted, were expended in the formation of the continental glaciers. This caused general, so-called eustatic, fluctuations of the sea level. During glaciations the sea level fell to 85–120 m below its present level, according to some estimates, and a large part of the continental shelf of northern Eurasia and North America dried up (as indicated by sunken river valleys, ancient coastlines, and so forth found on the bottoms of the outlying seas). The British Isles were joined to the European continent several times, and a land bridge between Asia and America appeared from time to time where the Bering Strait is today. Some North American animals (for example, the reindeer) reached Eurasia via that bridge, while some Eurasian animals (for example, mammoths) reached North America the same way. Over this bridge, no more than 20,000–30,000 years ago, human beings came to America.
During the interglacials the sea level again rose to nearly the height that it is today, or even a little higher; as a result of this, some of the low-lying outskirts of the continents were under water. However, in some places—for example, in the northeastern European part of the USSR and in the northern part of the West Siberian Plain—the advance of the sea onto dry land was the result of local tectonic depressions and it sometimes coincided with glaciations. Hence, glacial marine deposits as well as continental moraines are found there.
Climatic changes and lesser fluctuations of sea level have occurred in the postglacial, or Holocene, period (that is, within the last 10,000 years). Thus Europe had its warmest climate about 5,000–6,000 years ago (the so-called postglacial climatic optimum), followed later by some temperature decline.
Vigorous tectonic movements of the earth’s crust have occurred during the Anthropogenic period, especially powerful in the Alpine folded belt of Eurasia, in the zone of young folded structures on the periphery of the Pacific Ocean, in the mountain systems of Central Asia, and in the southern Asian part of the USSR (Tian’-Shan’, Altai, Saiany, and so forth). During much of the Pleistocene period volcanic action was more intensive than it is now on the periphery of the Pacific Ocean and in the Caucasus, Iceland, East Africa, and some other areas.
The history of the appearance and development of man is connected with the Anthropogenic period.
Despite changes in land and sea outlines, they were not very different in the earlier part of the Anthropogenic period from what they are today, and thus continental Anthropogenic deposits predominate on the land surface today. Several genetic types are distinguished according to the dynamics of accumulation, structure, and composition. In addition to the aforementioned glacial deposits and loesses, alluvial, proluvial, lake deposits, eolian sands, and eluvium are widespread. In various parts of the continents they are of varying importance. All of these deposits serve as the substratum of modern soils and are important objects of engineering and geological exploration. They are the chief source of many building materials (sand, gravel, brick clays, and so forth) and peat, and are connected with the beds of noble metals, diamonds, and other useful minerals. Specific theoretical problems and the unique methods of studying Anthropogenic deposits have given rise to an independent science known as Quaternary geology.
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E. V. SHANTSER