5 Quaternary Period : Climate and Environment of the Indian Subcontinent
Ravi Korisettar
Introduction
In archaeology, particularly in prehistory, geological studies are an indispensable part of understanding our past. Our earth is 4.6 billion years old. Humans appeared for the first time on earth only about 3 million years ago. During this vast time scale, our earth has undergone many changes. These changes are reflected in the climatic, environmental, faunal and floral record. For example, in the faunal and floral record, we get fossilised remains of extinct animals and plants within the rock strata and we can observe major changes in the dominance of particular life forms during a particular period. In the geological record,major changes can also be seen in the activities of erosion, transportation, deposition, mountain building and other geological events, horizontal and vertical movements of continents, shrinking and expansion, the rise and fall of oceans and sea levles and mountain ranges over vast distances.
2.Objectives
In the chapter we will get to learn about the importance of geological ages, how time periods are subdivided, how these divisions are reflected in the geological and archaeological record, what are the main changes within each division, what caused them, which period is the most important in terms of human evolution and history, and the tools with which we study them. We will also see how these changes in the past affected man-land relationships.
3.The Geological Ages
A geological age is a phase/age of earth characterised by the introduction of a particular new floral, faunal life form and which lasted for a limited time after which it gave way to a new age. In other words this is described as first and last appearance ofa particular faunal and floral species in the geological strata. This helps in the chronological division of the earth‟s stratigraphy. The largest division of a geological time is known as an Eon lasting many hundreds of millions of years. An Eon includes two or more geological eras. An Era comprises two or more geological periods. The period is the basic unit of geological time in which a single type of rock system is formed, lasting tens of millions of years. Each Era is hundreds of millions of years in duration. An Epoch is a division of a geologic period. It is the smallest division of geologic time lasting several million years.
The history of the earth is divided into four eras or ages related to the evolution of life forms: Pre Cambrian, Primary (Palaeozoic), Secondary (Mesozoic), Tertiary and Quaternary (Cenozoic).
4. A Brief Idea of the Geological Ages
The Precambrian Era, beginning at 4.6 billion years ago saw the beginning of single celled and multi-celled organisms. The Palaeozoic Era (600-270 Mya) with Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian periods, saw the emergence of the first invertebrates, vertebrates, fish, amphibians, reptiles and trees emerging for the first time. The next Era, Mesozoic with Triassic, Jurassic and Cretaceous (225-135 Mya) had the first ever dinosaurs, mammals, birds, insects and flowering plants. The last era, the age of the mammals, is the Cenozoic, divided into Tertiary and Quaternary periods. Tertiary has five epochs, Palaeocene, Eocene, Oligocene, Miocene, and Pliocene lasting from 65 Mya to 5.1 Mya. Tertiary provides us with important evidence of expansion and speciation of mammals. Quaternary period is the most important in terms of human evolution. During the Pleistocene epoch beginning 2 Mya, the early humans and giant mammals become extinct. Holocene epoch, the most recent and continuing epoch, beginning 10,000 years ago witnessed the emergence civilizations.
The onset of the Cenozoic was marked by replacement of the Mesozoic large reptiles by mammals as dominant animals on earth, as well as widespread expansion of angiosperms in flora. The period is distinguished by a rich evolution of birds, gastropods and insects. The geologic strata covering the time period from Palaeocene to Holocene reveal the gradual transformation and modernization of the organic world. The faunal evidence and the gradual disappearance of coral reef building activity, pollen spectra of the Cenozoic strata revealed that moderate climatic fluctuations had occurred during the Pliocene and by the beginning of the Pleistocene climate had already become cooler.
Continental flora and fauna preserved in the Tertiary strata, along with coral reefs, were considered for reconstructing the evolution of temperature. Changes in flora and fauna were attributed to climate changes.
Covering the recent glaciations including the last great glacial retreat, this was a very dynamic period characterized by major changes in the environments (climate, earth‟s magnetic fields, global sea levels, migrations and extinctions of various flora and fauna) and the evolution of humans. The records of these changes can be observed in the fluvial and glacial sediments, in the ice sheets, in the stratigraphy, in the floral, faunal, humanoid remains preserved. These records provide valuable information on human history.
5. Definition of Quaternary
The definition of the concept of Quaternary was initially based on the sediments and their fossil fauna in the Paris Basin. As mentioned above the Quaternary Era is a subdivision of the Cenozoic, beginning around 2 million years ago and after the Tertiary Period. The term „Quaternary‟ (the „„fourth‟‟) was first used in 1829 by Jules Desnoyers to refer to the river sediments in the Seine Basin (Paris) in France. This period is subdivided into two epochs: the Pleistocene (2 million years ago–10,000 years ago) and Holocene (10,000 years ago to the present) and forms the upper part of the Cenozoic Era, along with the Tertiary Period. The British geologist E. Forbes was the first to put the concepts Pleistocene and ice age as equal and the term Recent gradually came to be equated with Holocene. Some argue and consider Holocene as part of Pleistocene as it represents the ongoing interglacial stage.
The application of K-Ar and palaeomagnetic dating methods to the Cenozoic strata has revealed that the Tertiary Period lasted from 65 million years ago (Mya) to 2 million years ago, and marks the beginning of Quaternary. The Quaternary Period includes Pleistocene and Holocene epochs. The Pleistocene-Holocene boundary is placed at 10,000 years ago by varve chronology and radiocarbon dating. Developments in palaomagentic dating has helped in placing the beginning of the Quaternary Period is placed at 1.8 Mya, coinciding with the Olduvai Event, magnetic polarity subchron.
6. Quaternary Period and Ice Ages
Around the middle of the 18th century fossil bearing geological strata were subdivided into Primary (Palaeozoic), Secondary (Mesozoic), Tertiary and Quaternary (both are now included in Cenozoic). The French term Quaternaire was later adopted into English as Quaternary. The French geologists used this term in the context of sediments and fossil fauna and were not focusing on glacial theories. The glacial deposits do not contain fossil material, and if they have they are secondary, not useful in dating geological strata. Therefore geologists thought it best to include the non-consolidated glacial deposits (tills, glacio-fluvial sediments), in the youngest period, the Quaternary, i.e. the last part of the Cenozoic Era. Both Tertiary and Quaternary periods are important in the study of human evolution and past changes in climate and associated environments. The fossil (both flora and fauna) and other proxy records of climate change of these two periods provide well preserved evidence in a variety of geological contexts. The marine cores (deep sea cores) in particular provide the continuous record.
The British geologist Charles Lyell had played a leading role in organizing the geological strata belonging to these two periods. While the classical stratigraphical column was divided into five epochs Lyell added the Pleistocene Epoch to the sequence, that included the latest deposits, because Lyell‟s Pleistocene included modern time too. Both Tertiary and Quaternary periods are subdivided into epochs. The Tertiary Period has five epochs: Palaeocene, Eocene, Oligocene, Miocene and Pliocence and the Quaternary period has two epochs: Pleistocene and Holocene.
It is generally believed that Pleistocene was the period of intense ice ages and that end of the Pleistocene marked the end of the ice ages. Palaeoclimatology has emerged as a major field of climate science which takes into consideration a variety of proxy records to reconstruct paleoclimate changes. To date reconstruction based on analyses of proxy records from ocean cores have provided authentic and continuous record of climate in the past as well as helping in the prediction of climate change in the future. It is now well known that ice ages on earth go back to 13 million years ago and the Pleistocene ice age record is most complete. The 13 million years ago ice age is associated with the formation of the Antarctic ice sheet and the end of the ice age is marked by the retreat of towards the poles and mountains around 10,000 years ago. Most of human biocultural evolution took place during the last 2.5 million years across the old world, responding by adapting to changing climates during the Pleistocene and Holocene. Changing climates of Pleistocene simply means that ice ages were not a time of unvarying cold and dry conditions. These were periods of fluctuations from cold and dry conditions to warm and humid like to day. The emergence of Holocene climate is not end of the ice age, the ice sheets will certainly expand in the future. During cold and dry conditions glaciers expand and during warm and humid conditions glaciers retreat. Glacier expansion period is called glacial phase and retreat period is called interglacial. Alternating glacial and interglacial cycles are reconstructed for the last 2.5 million years, first based on the study of alpine river terraces and later loess-palaeosol stratigraphy and now with oxygen isotope studies of deep sea cores. Cyclicity of glacial and interglacial climates clearly suggests that the earth‟s climate will eventually witness another ice age. But one thing is sure these changes are caused by the variation in the relationship between the incoming solar heat from the sun to the earth. It is important to know that main source of heat on the earth is the sun and that glaciers reflect the sunlight, as they grow the amount of reflected heat also increases causing fall in temperatures and further expansion of glaciers. This causes the glacial conditions. The opposite happens when the glaciers begin shrink, and with decreasing albedo temperatures on the surface of the earth increase leading to warm and humid conditions.
Glacier expansion causes erosion and deposition of eroded material in the form of a variety of drift deposits, erratics, morains, till, glacio-fluvial terraces, that were first documented by Penk and Buckner in 1909. They reconstructed four glacial-interglacial cycles during the Pleistocene, later revised to five.
Glacier movement results in the formation of soil which is known as loess (yellow calcareous silt) normally preserved in the periglacial areas as a windblown sediment. However, there are many other formations on land reflecting on the advance and retreat of glaciers, which are not mentioned here, but the oceans also experienced rise and fall of sea levels as a result of contraction and expansion of glaciers, much of the water flowing into the oceans was land locked. During the glacial phases the sea levels were much lower than today. The land that is presently and the sea bottom were exposed during the glacial phases and the existence of land bridges connecting continental areas facilitated movement of animals and humans in the past. Glacial and interglacial conditions also caused changes in the vegetation on land, extinction of animal populations and so on. Palynology, the study of pollen preserved in soil formations, also helps in the reconstruction of past climate changes.
7. Why and How Climate Change Occurs
The development of ice age concept has its roots in attempts by geologists who tried to correctly explain the processes behind the formation of glacial tills, erratic, glacio-fluvial sediments and morainal accumulations. In 1935 Charles Lyell presented the drift theory (as those days such deposits were referred to as drifts) suggesting that large accumulation of tills and morains were the result of their transportation to their place of occurrence by floating icebergs. However, by 1940 he abandoned the drift theory under the influence of new ideas expressed by contemporary Scandinavian and French geologists. Since those deposits were found far beyond the present extent of modern glaciers geologists concluded they were indicative of former extension of glaciers, and that glaciers expanded and retreated several times in the past resulting in the occurrence of glacio-fluvial deposits along the path of the glaciers. Further it was obvious to geologists that glacier expansion and retreat were governed by changing climatic conditions in the past.
Early scholars who believed in a single ice age advocated monoglacialistic conception. However, there were also advocates of two or more glaciations separated by interglacial stages with climate as congenial as today. The polyglacialistic conception postulated the occurrence of recurrent glacial and interglacial episodes during a complex ice age. As knowledge of the complexity of the ice age developed it became necessary to divided ice ages into inferior stadials, marking stages of climatic deterioration and glacial advance, and interstadials, subordinate stages of moderate warming and limited ice sheet.
Theoretical models on why climate change takes place have been proposed since early part of the 20th century. Though we are not required to go into the details of these the most commonly accepted model is that of Milutin Milankovitch. This model is based on the observation that (a) “the angle of earth‟s axis of rotation increases and decreases over a period of 41 000 years”; (b) “this angle is responsible for the changing length of night and day from summer to winter” suggesting that sunlight reaching the surface of the earth decreases and increases at different latitudes during the course of 41,000 year cycle; (c) the earth‟s orbit which is elliptical also changes over a 100,000 year cycle, the point to be noted is the distance between sun and earth varies during the years (causing seasonal conditions); and “the point along the earth‟s orbit at which the solstices or equinoxes occur varies through two superimposed cycles of 19,000 and 23,000 years. If the orbits were circular, this variation would make no difference, this means that sometimes the (northern hemisphere‟s) long summer days will fall near the sun, while at other times they will occur far from the sun”.
According to Milankovich, all the climatic cyclical changes in Quaternary were a result of variations of different orbital parameters of the earth and the modified insulation received by the earth. The three important parameters that played an important role in the climatic changes were the earth‟s eccentricity, axial tilt, and its orbital precession. A brief explanation is given below.
Eccentricity – The shape of the earth‟s orbit changes from more elliptical to less elliptical in a cycle of 100,000 years. This indirectly affects the solar radiation received by the earth‟s surface, as depending on the elliptical range, the sun-earth distance gets determined.
Axial tilt – Axial tilt is the inclination of the Earth’s axis in relation to its plane of orbit around the Sun that oscillates periodically about 41,000 years. Growth of ice sheets is believed to happen with warmer winters where in precipitation would lead to more snowfall. Lesser Axial tilt thus would have resulted in differential distribution in the Equatorial and Polar Regions too.
Precession – slow wobble of earth on its axis is known as precession, which has a periodicity of 23,000 years. Greater seasonal contrasts occur due to longer distance of Earth from Sun with Northern Hemisphere having winter and vice versa.
Some earth scientists also suggest that massive volcanic eruptions can also lead to global level climate changes. The volcanic dust (ash) emerging from supervolcanic eruptions cause reduced sunlight reaching the surface and consequently causing drastic reduction in surface temperatures. The impact of such dramatic changes is of high magnitude causing biological bottlenecks and landscape changes on a large scale. The most recent volcanic winter experienced on the surface of the is said to have been caused by the Toba supervolcanic eruption of 74,000 years ago. It means that there is no single explanation of ice ages, however, the cyclicity of ice ages is not debated.
8. How Many Glacial and Interglacial Cycles
During the first half of the 20th century four glacials and three interglacials were recognized by the classical Alpine and North European Pleistocene ice age models. Alpine glacial stages were identified by sediments formed during the both glacial and interglacial climate. By the 1970s new stratigraphy of Pleistocene was developed: the deep sea sediments containing foraminifers were found sensitive to temperature changes and responding by changing the coiling direction. Further the isotopic composition of oxygen (O18 and O16) in the calcareous shells of foraminifers showed significant variation through time and helped construct isotopic stratigraphy. This variation was controlled by variation in the land-locked ice. The three Pleistocene subdivisions are climatostratigraphic systems based on sequences of strata of geomorphic features with specific climatic signatures. Comparative study of these three stratigraphies revealed discontinuities in land based stratigraphies and it was recommended to discontinue the use of classical terminology in all interregional correlations and to base the chronostratigraphic subdivisions on the O18 record of deep sea cores. The record obtained from deep sea cores was found to be the most complete as it came from uninterrupted deposition and therefore it was recommended as the standard.
The ratio of oxygen isotopes in foraminiferal tests provides the most complete record of global climatic changes. It is ideally suited for a standard with whcihall other records can be compared. The fluctuation in benthic (sea bottom) and planktonic (floating in sea) foraminifera is totally controlled by fluctuations in the total amount of ice deposited on land. In the early day this variation in isotope composition in the deep sea sediments was used to sub-divide the deep sea sediments into numbered stages. This came to be referred to as oxygen isotope stratigraphy and now widely used in palaeoclimate reconstructions the world over. Isotopic stratigraphy is subdivided into numbered stages from top to bottom inorder of increasing age. The odd numbered stages are relatively depleted in O18 and correlate with warm intervals and the opposite is true with even numbered stages, corresponds with cold intervals characterized by increased landlocked ice. These stages also correspond with rise and fall in sea levels during the Pleistocene.
OIS5 has been subdivided into five substages, 5a-e in order of increasing age. Substage 5e has lowest isotopic ratio of the five. It corresponds to minimum volume of ice and is considered to represent the last interglacial. Similalry stage 7 is has been divided into &a-c. The O18 record shows eight completed glacial cycles during the last 0.7 million years of normal geomagnetic polarity. The ice volume oscillation as recorded by O18 ratio within each glacial cycle seem to generally follow a sawtooth pattern, progressing from early minimum to a late maximum. Apparently the glaciers have grown in steps interrupted by temporary regressions. Thereofre the marine record is considered the most complete and provide the most accurate information on Pleistocene climates.
9. Loess-Palaeosol Stratigraphy
Loess is a yellow calcareous, porous, windblown silt characteristic of Pleistocene sequences of all land based Pleistocene stratigraphy anywhere in the world. Thickest deposits are found in the modern states of Czech Republic, Slovakia and Austria. As mentioned earlier loess is extensively found in the periglacial areas, which were never glaciated in the past. The best exposures are found along the rivers. Bulk of this deposit is of glacial age. Largest are found in China, followed by China and Ukrain. These deposits have been subject extensive research as part of glaciology. Loess is a continuous deposit and with help of magnetostratigraphy loess deposits can be correlated with deep sea sequence. They are also correlated with terraces of rivers flowing through formerly glaciated areas, particularly around Prague, Brno, and Nita in the Czech Republic and Austria. When the Alpine and Fenno-Scandinavian ice sheets reached their maximum extent loess accumulated in this region. During the interglacials climate was Atlantic type, similar to the present one, or warmer and wetter. In the loess profile glacial and interglacial cycles are recorded by repeated alternation of loess and forest soils. The loess glacial environment is reconstructed on the basis of gastropod fauna (papilla fauna and columella fauna as these animals survived the cold conditions. Their presence also indicates cold loess-steppe environment. Soils interstratified with loess and hillwash sediments record intervals of dense vegetation. The quality of soil reflects the type, density and duration of vegetational cover on land and indirectly on climate. These soils contain warm loving land snails, particularly Banatica.
The climate subdivisions of the Pleistocene based on deep sea isotope stratigraphy and loess-palaeosol stratigraphy have shown to be the standard in comparison with the classical ice age models and that the latter has numerous gaps. As the Pleistocene gross climate changes were globally synchronous it recommended to refer to deep sea isotopic stratigraphy as a standard for the purpose of correlation.
10. Quaternary Environment
Quaternary period was a period of great activity. It was during this time changes conducive to human evolution began to appear. The Great Ice Age or the Glacial Age began in the Pleistocene epoch. This was an important global event that had enormous effect all over the earth. Glaciation refers to the formation of glaciers and their effects on landscape. Interglacial means when the great ice sheets melted completely from temperate lowlands and interstadial refers to the periods when the ice sheets only contracted. There were also regions where instead of glaciation, pluvial stages occurred.
There were four main glaciations in North America – Nebraskan (the earliest), Kansan, Illinoisan and Wisconsin while the interglacials were Aftonian, Yarmouth and Sangamo.
Northern Europe witnessed five glacials and four interglacials. The glacial periods were named as Eburon, Menapian, Elsterian, Saale and finally Weichsel. The interglacials were Waal, Cromerian, Holstein, and Eemian.
The Alpine glaciation saw five phases beginning with Donau, Gunz, Mindel, Riss and Wurm with interglacials in between Donau-Gunz, Gunz-Mindel, Mindel-Riss and Riss-Wurm.
During the Pleistocene, associated with Gauss Matuyama geomagnetic reversal, a series of momentous climatic events occurred. The northern latitudes and mountainous areas were subjected on four successive occasions to the advances and retreats of ice sheets (known as Günz, Mindel, Riss and Würm in the Alps). In northern latitudes throughout the Pleistocene era, the characteristics of animal and vegetal populations were determined by the advance and retreat of the glaciers. These frequently covered large parts of Europe, Asia and North America with impenetrable ice sheets, locking up huge quantities of seawater and reducing average temperatures by 10-12°c and ocean levels by over 450 feet – far below the modern times.
Many floral and faunal extinctions and migrations and also disappearance of archaic forms of humanoids were characteristic features of the times. Climatic and environmental changes became more steady towards the end of Pleistocene and favourable for new forms of life and ways of adaptation. Warmer climates brought with it a new more steady way of settled life n Holocene.
11. Quaternary Studies
The main glacial advances have been accurately dated and their associated meteorological, geographical botanical and biological developments have been confidently established by a variety of techniques from the study of sediments deposited in lakes and the analysis of pollen zones to the use of radiocarbon and potassium argon decay times.
Isotopic stratigraphy and Oxygen Isotope studies
Quaternary climate change can be reconstructed using Deep Sea and Ice core analysis, which compares the ratio of 18O to 16O. Being temperature dependent, 16O evaporates more during the glacial periods leaving the heavier 18O. Both these isotopes are absorbed by the foraminifera and so the amount of 18O and 16O can be calculated from their shells on their death. Thus, this long record of climate change is preserved through the foraminifera and the ratio of absorbed oxygen isotopes gives us a chronology.
Just as in deep sea, ice cores on land preserve more of 18O as it is the heavier isotope and gets evaporated less during the interglacials. An examination of ice cores on land also thus gives us clues to the palaeoclimate.
Paleomagnetic Studies
It has been found that the earth‟s magnetic field has been changing its polarity in the past and one gets a complete sequence of these reversals of the magnetic poles. During these polarity changes, positions of earth‟s magnetic north and magnetic south are reversed. The earth has witnessed both normal (direction of field same as present direction) and reverse (direction of field was opposite the present direction) polarity. When the earth‟s polarity flipped about 2.5 mya Matuyama Magnetic Reversal was the result. Before the 2.5 mya, the polarity was normal called Gauss, normal as of today. During this long reversal period (Matuyama), two events of normality occurred. The first is called the Olduvai event, is dated to 1.9 mya, and marks the Plio-Pliestocene.
Volcanic rocks preserved the magnetic reversals in them and this could be used to create a database of magnetic reversals in the past. The first estimate of the timing of the magnetic reversals was made by Motonori Matuyama in the 1920s. He observed that the rocks with reversed fields were all of Early Pleistocene age or older.
Discussion
The importance of Quaternary studies can hardly be understated especially in Human Evolution. It was the time of great changes that finally gave way to the present living conditions. The Great Ice Age was a period of extreme climates. Vegetation, floral, faunal and hominoid species were entirely different. From pollen analysis, deep sea and ice core sheet analysis, stratigraphical, palaeomagnetic and oxygen isotopic analysis, fossil studies we get enormous information about the Quaternary period. This information not only provides us with a chronology, but also climatic and environmental conditions. These studies provide us with clues to answering why and how extinctions, migrations, and evolution happened. It makes us understand the adaptive mechanisms at work, which helped some species survive the extreme living conditions during that time.
It makes us understand how man-land relationships changed over time, how nature-dependent man came to master the same nature.
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Web links
- http://www.indiana.edu/~geol105/images/gaia_chapter_4/milankovitch.htm
- http://www.britannica.com/science/Quaternary
- https://en.wikipedia.org/wiki/Stadial
- https://en.wikipedia.org/wiki/Interglacial
- https://en.wikipedia.org/wiki/Brunhes%E2%80%93Matuyama_reversal
- http://australianmuseum.net.au/the-geological-time-scale
- http://www.enchantedlearning.com/subjects/dinosaurs/glossary/Geologictimeperiods.shtml)
Bibliography
- Barbara A. Maher and Roy Thompson Ed. 1999 Quaternary Climates, Environments and Magnetism. Cambridge University Press, UK
- Clive Gamble 1999 The Palaeolithic Societies of Europe (Cambridge World Archaeology) Second Edition
- Eric Delson, Ian Tattersall, John A. Van Couvering Ed. 2000 Encyclopedia of Human Evolution and Prehistory, Second Edition. Garland Publishing, New York
- John Lowe and Mike Walker 2014 Reconstructing Quaternary Environments Third Edition. Routledge Upinder Singh 2009. A history of Ancient and Early Medieval India. Pearson, India