9 Reconstruction of Past Environment
K. P. Rao
1. History of the Earth
In the 4.5 Billon years of long history, earth has undergone several changes. Both its mantle and the crust or the outer shells have experienced drastic alterations. Apart from the solid globe, the Earth also has a gaseous layer, known as atmosphere. Based on the changes that were occurring on the surface of the earth, its history is divided into several eras. Broadly, the Geological periods are classified as different eras – Pre-Cambrian, Cambrian, Palaeozoic, Mesozoic and Cenozoic. The Cenozoic is the most recent era, which started about 65 million years ago. Cenozoic is further divided into Tertiary and Quaternary periods. Tertiary has further five subdivisions – Pliocene, Miocene, Oligocene, Eocene, Plaeocene. Quaternary is subdivided into Holocene and Pleistocene. Quaternary period is very important for the study of human cultures. Quaternary period started about 1.8 million years ago, and is subdivided into Holocene and Pleistocene. Holocene is the most recent period, which is still continuing and has started about 10,000 year back.
In the long history of the Earth, the environment on the earth has not remained static. Several drastic changes in the environment were known. The last phase of the Pleistocene period experienced drastic alterations in the temperatures on the earth. Since the temperatures were dropping drastically, the Pleistocene period is also known as an Ice Age. In fact, there were four Ice Ages known as Gunz, Mindel, Riss and Wurm. Each of these Ice Ages lasted about 2 lakh years. During these Ice ages, thick ice sheets covered most surface of the earth. These ice ages were interspersed by warmer periods known as inter-glacials. The reasons for these altering climatic conditions are not known exactly. Several theories have been proposed to explain the changing climatic conditions. Some of these theories are mentioned below:
Sun Spots and Solar Winds
Sun spots are the darker and cooler areas on the surface of the Sun. As the Sun spots are cooler areas, they radiate less heat than the normal surface of the earth. The radiation received by the Earth in a way is influenced by the number and size of the Sun spots and the solar wind. Some scholars have proposed that the Sun spots and the solar winds were responsible for the climatic changes on the earth. But, the periodicity of the climatic cycles could not be explained satisfactorily by such theories.
Eccentricity of the Earth’s Orbit
Some scholars have proposed that due to some reasons the earth has drifted far off from the Sun in its orbit. They propose that probably, some heavenly body has come closer to the Solar system, which has exerted such gravitational pull that Earth and other planets were pulled off from their regular orbits around the Sun. As the distance from the Sun increased, the temperatures on the earth have dropped. This theory also cannot explain the periodicity of the Glacial and Inter-Glacial cycles of the Pleistocene period.
Cosmic Dust
Several regions of the Universe are engulfed by cosmic dust. Though the origin of this dust is not certain, there are regions where this dust is thickly concentrated. It is proposed that solar system has passed through such a region resulting in partial blockage of the radiation from the Sun. Due to this blockage, the temperature on the earth has come down considerably. They further argue that the dust forms into a spiral, as the gravitation pulls the dust particles together. The periodicity in the Pleistocene Ice ages is due to the passage of solar system through these spirals resulting in varied radiations in cyclic manner.
Precession of Earth’s Orbit
Precession is a change in the orientation of the rotational axis of a rotating body (Wikipedia). Precession is the wobble in a rotating device like a top. The gravitational pull by the Sun and the Moon causes precession in the Earth’s orbit. Axial precession is the rotation of a celestial body in the shaped of a cone. If we draw an imaginary line through north and South Pole, the line would trace a circle in the form of a cone. Earth’s precision takes about 22,000 years to complete one circle in its precession. The precession of the Earth influences the climate on the earth. It has been proposed that the cyclic Ice Ages on the earth are related to the precession of the Earth’s rotation.
2. Geomorphological Changes
The altering cold and warmer climatic conditions have brought several changes on the face of the earth during the Pleistocene period. These changes are studied in Pleistocene Geomorphology. These changes are not uniform throughout all the regions of the earth. We can detect discernable changes in the main climatic regions, that is Tropical and Temperate regions.
Tropical Pleistocene Geomorphology
In tropical regions (Between 23°30’ North and South Latitudes) the altering cold and warm periods have brought several changes in the rainfall pattern. During the warmer periods, there used to be heavy rainfall, which are known as fluvial periods. During the colder periods, the evaporation of the water was less and as a result, the rainfall was scarce and hence known as Anti-Fluvial periods. These altering rainfall patterns resulted in drastic changes in the landscape. During the fluvial cycles, the mineral sediments from the interior lands were transported by the rivers into the coastal regions. Thus the laterites, which mainly contain ferrous (iron) minerals were transported and deposited in the coastal regions. Large scale erosion of the landscape due to heavy rainfall during the fluvials resulted in exposure of the underlying rock cones, which now appeared as peaks in the landscape. Further, the height of the hills also has increased due to erosion of the sediments from the foot of the hills and the valleys.
The effects of altering wetter and dryer cycles have altered the landscape drastically. During the fluvial periods the intense flow of water in the rivers also transported huge amounts of gravel, boulders, pebbles and silt, which was deposited in the valleys and river courses. The altering cycles of excessive and scarce rainfall resulted in terrace formation.
Terrace Formation
Terrace is a step-like level on the side of the river valleys. Terraces represent the former floor of the river. The increasing and decreasing flow of water in the rivers resulted in the terrace formation in the valleys. Initially, when the river course is formed due to tilting of the land consequent to tectonic effects or due to erosional action of the water, the river covers a very wide area. In course of time, due to the erosion of sediments by the water flow, a ‘degradational’ or ‘erosional’ terrace is formed. Similarly, if the velocity of the water in the river is not sufficient, it deposits the sediments in its bed and it results in ‘agradational terrace’. The altering cycles of fluvial and anti-fluvial activity of the Pleistocene period resulted in formation of several terraces. During the Pleistocene period, the sea levels were also altering considerably. This is due to accumulation of water in the form of ice especially in the temperate zones. The changing sea levels also effected the flow of the water in the rivers. When the sea level is low (during cold glacial period), the velocity of the water in the river was more, and consequently erosional activity. Similarly, when the sea levels rose (during warm inter-glacial period), the velocity of the rivers is reduced due to back pressure from the sea water obstructing the river flow, it resulted in aggradation in the coastal regions.
In the interior landscape, the situation is somewhat reversed. In the interior regions aggradations were takes place during the anti-fluvial phase (less rainfall), as the flow of water is less and the velocity of the river is less resulting in deposition of the sediments. During the fluvial period (wet phase) when there is a heavy flow of water, erosion or degradation takes place. During this phase, the earlier deposits are cut through resulting in formation of a terrace. Though, the material of the terrace is deposited during the agradational period, the actual terrace is cut during the erosional phase. Thus the erosional and depositional activity of the rivers results in the formation of several ‘steps’, ‘levels’ or ‘terraces’. The drastic alterations in the climate during the Pleistocene period resulted in the formation of terraces, particularly in the tropical regions. As a general rule, the higher terraces are the oldest and the lower terraces are the youngest.
Study of the terrace formation is important in archaeological studies, as the activities of the prehistoric man were concentrated more on the banks of the rivers and the valleys. The tools and other material found on the terraces could be easily dated, as the terraces have chronological value.
3. Temperate Pleistocene Geomorphology
Temperate region is the region falling between 23°30’ and 66°30’ North latitude and between 23°30’ and 66°30’ South latitude. Compared to the Tropical regions, these regions receives lesser solar radiation. Due to this these regions experience colder climatic conditions. The effects of the Pleistocene period on the geomorphology of the Temperate zones is entirely different from the Tropical zones.
Glacial Formations
During the Pleistocene period the temperate zones experienced drastic reduction in the temperatures, which resulted in heavy snow formation. Due to the heavy snow formation, these periods were known as ‘Glacial’ or ‘Stadial’ period. In fact, there were four Glacial periods, known as Günz, Mindel, Riss, Ürm. Ürm is the most recent glaciations. These four glacial periods were interspersed by three ‘Inter-Stadial’ or ‘Inter-Glacial’ periods. The inter-glacial periods are the warmer periods. During the glacial period heavy sheets of ice accumulated in some parts of the temperate zones. These ice sheets in vast areas reflected back the Sun’s radiation, which resulted in bring down the temperatures further. It is estimated that at places the ice was few miles thick. Because of this accumulation, lot of evaporated water got locked up in the temperate zones. This resulted in the lowering of the sea levels. The accumulated ice in the temperate zones brought several changes in the landscape in these regions.
One of the consequences of the falling temperature is that, some of the life forms of both animals and plants, that could not adapt to the changed climate perished. Water has the character of expanding as it cools down to 4° centigrade. Because of this property, the thick ice sheets exerted tremendous pressure on the rocks. Due to this pressure, many of the rocks have broken due to the ‘wedging’ effect. Big stones and boulders got embedded in the ice that formed in the channels. Since, the ice flows slowly following the slope, the embedded rocks scrubbed the bed of the river channel, which is known as ‘scourging effect’. Because of the scourging effect, the channels of the rivers formed ‘U’ shaped valleys. Further, big boulders embedded in the ice were transported to long distances over a period of lacks of years. All these effects changed the landscape drastically during the Pleistocene period.
4. Environmental Reconstruction
Since the life of the humans and their cultures are influenced greatly by the environmental conditions in different periods and different regions, it is essential in archaeology to reconstruct the past environment. Reconstruction of the past climate helps in understanding the overall cultural developments and subsistence strategies of the ancient communities. The study of the past climate is known as Palaeoclimatology. To reconstruct the past climate, several techniques are used, and here some of the important techniques are discussed.
Stable Isotope Analysis
Isotopes are the atoms of a chemical element with the same number of protons but with different number of neutrons. For example Carbon12, Carbon13, and Carbon14 are the isotopes of Carbon. Whereas, the Carbon12 and Carbon13 are stable isotopes, Carbon14 is a radioactive isotope. Whereas, the atom of a radioactive isotope disintegrates over time, the stable isotopes will not disintegrate. Hence, for estimating the original quantities of different elements, the stable isotopes are assessed. Usually, samples collected from cave stalactite, stalagmite and the water sample collected from ancient aquifers are used for stable isotope analysis.
Dendroclimatology
The technique of dendroclimatology uses the changes in the growth pattern of the tree rings due to climate changes. It is well known that the thickness of the tree rings reflect the good and bad rainy seasons. During good rainy period, the trees form thick rings while during the drought conditions, the rings formed would be thinner. Thus the tree rings establish a record of good and bad rainy seasons. There are several species of trees that survive for few thousands of years, and consequently, they have a permanent climatic record of that many years. Scientists also have developed techniques to synchronize rings of recent trees with rings of trees that have died long back. Using such synchronization, we can reconstruct tree ring patterns for several thousands of years. We have to remember that, it is not essential to cut a tree or a truck to extract rings. Core drillings of few centimetre widths are sufficient to provide tree ring data.
Ice Cores
Ice formed in the mountain glaciers and the polar ice caps contain layers of ice that has formed over several millennia. Pauses in the ice formation results in layering, which helps in establishing the chronology of these layers. These layers are virtual records of past environmental conditions. Different methods are used to extract information from these layers.
1. Trapped air within the ice is compressed into tiny bubbles due to weight of the layers formed in the later years. This trapped air is a valuable source to understand the composition of the air in the past environment. Stable isotope analysis of the air gives us clues about the temperatures. For example O18 evaporates at higher temperature than O16 (normal Oxygen). Thus, in hotter climatic conditions the ratio of O16 to O18 increases, indicating hotter ocean surfaces.
2. Changes in the thickness of the layering of the ice help in determining the changes in the precipitation or temperature.
3. Pollen preserved in the ice layers helps in determining the environment. The species of plants to which the pollen belongs indicate the kind of environment existing in the region. The pollen helps in detecting the precipitation, temperature and the species of the plants.
Rock and Sediment formations
The rock and sediment formations in the layers of the Earth including under the sea floor gives us definite clues about the past environment. The rock layers also contain fossilised flora and fauna which help in understanding the past environment. Study of the rocks helps us understand the environment that existed millions or billions of years ago.
Stalagmite and Stalactite Formations
Stalagmite and stalactite formations occur because of calcium and other soluble minerals present in the water that drips from the cave sealing. Over a long period, the dripping water forms cones that hang from the sealing of a cave, which are known as stalactite. Similarly, the deposits that rise from the floor due to these drippings are known as stalagmite. By studying the layers of these formations, we can determine the chronology. The consolidating mineral layers embed various gases and trace elements that help in understanding the past environment. For example, the stable isotope estimation of Oxygen16 and Oxygen18 is carried out to determine whether the climate was cold or hot.
Pollen Analysis
Pollen, which is microscopic powdery material produced by all flowering plants is a very useful material in reconstructing the past environment and subsistence of the ancient communities. Study of the Pollen is known as Polynology. Each and every plant species have their own unique pollen, which can be detected from the shape, size and the pattern of the pores on the pollen shell. The pollen shell survives in the nature for a very long time. Pollen is collected from the archaeological deposits, sedimentary deposit layers and the ice cores, which can be dated on the basis of the layering present in them. The pollen and the fossilized pollen collected from the sediments, rocks and the ice cores help us in understanding the prevailing environment in a particular period.
Palaeontology
Palaeontology is the study of the life forms that existed in the past. Both plants and animals adapt to the climatic conditions in several ways. Such adaptations result in noticeable physical changes. For example, the mammoths were having thick fur on their bodies to protect from the severe cold during the Pleistocene period. Similarly, the Neanderthals had thicknose to warmup the air they breathe. Study of the physical changes in the life forms reveals the climatic conditions in which they were living.
- Summary
Earth has a long history stretching back to more than 4.5 billion years. This long past is divided into various geological periods. The most geological period, Cenozoic is further divided into 7 periods, and the last two periods are the Holocene and the Pleistocene. Holocene period is the present geological period, which commenced about 10,000 years ago, and is a warmer climatic period. On the other hand, the previous period was known as Pleistocene period is a colder period and was known as Ice Age. The changing climatic conditions have tremendous influenced the geomorphology of different regions of the world, and consequently influenced the human cultures. Hence, reconstruction of the past environment is very important in archaeological study of the cultures. Various methods are used to reconstruct the past climatic conditions. Methods like stable isotope analysis, dendroclimatology, pollen analysis, palaeontology and study of ice cores, rock sediment formations, and stalagmite and stalactite formations are used in the reconstruction of the past climate.
Web links
- https://en.wikipedia.org/wiki/Pleistocene
- https://www.livescience.com/40311-pleistocene-epoch.html
- https://www.britannica.com/science/Pleistocene-Epoch
- http://www.bbc.co.uk/nature/history_of_the_earth/Pleistocene