PLEISTOCENE CLIMATIC CONDITIONS

Transcription

1 1 PLEISTOCENE CLIMATIC CONDITIONS The Pleistocene is a remarkable epoch in the geological calendar of the earth marked by the appearance of man and great climatic events. The high latitudinal areas of the earth as well as the high altitudinal areas of the tropics experienced intense cold climate with ice coverage called glacial period. These glacial periods were intervened by comparatively warm phases termed interglacial periods. The other areas of the tropical regions experienced high rainfall and humid periods called pluvial periods intervened by dry phases termed as interpluvial periods. These climatic phases corresponded to the Lower, Middle and Upper Pleistocene periods which have been tried to be shown in tabular forms. The effects of glaciation and pluviations were also noticed in the seas and oceans of the earth besides the occurrence of faunal and floral species peculiar to the epoch. The article tries to discuss these features of Pleistocene climate in the light of the author s understanding supplemented by relevant literature. The Pleistocene happens to be a significant period in Prehistory marked by the appearance of man and remarkable climatic oscillation.

2 2 The Pleistocene epoch is identified by the occurrence of Villafranchian fossils namely Bos (bovine), Equus (horse) and Elephas (elephant) in the terrestrial stratigraphy and calabrian formation or specific marine species in the seas. The International Conference of Geologist held in London in 1948 thus commented, The commission recommends that in order to eliminate existing ambiguities, the lower Pleistocene should include as its basal members in the type areas the calabrian formation (marine) together with its terrestrial (continental) equivalent, the Villafranchian. Different regions of the earth experienced the Pleistocene differently. There prevailed cold climatic conditions in the higher latitudes and altitudes of the earth marked by intense snowfall and glaciations. On the other hand, the tropical regions of the earth experienced high humidity and heavy rainfall. Most part of Northern Europe, Great Britain, North America, Scandinavia, Switzerland and almost all the Alps were covered with icesheets. It is estimated that during maximum glaciation almost 30% of the earth surface was covered by icesheet and the minimum temperature went down to -6º C (21º F). Even higher altitudes of mountains in the tropical region like Kilimanjaro, Mount Kenya, Ruwenzori range in East and Central Africa were also under the influence of glaciation. So intense was the snowfall that in some mountains the permanent snowline came below by almost 3500 ft. In Central Europe most of the mountain glaciers had glaciers during the Ice

3 3 Age; the glacial snowline therefore was depressed to an elevation of 3000 feet and less. In the west it lay lower than in the east, the mountains of the Wales produced a considerable glaciation in the same latitudes in which the Ural Mountains were not glaciated at all. Penck (1904) has stated that We find very small glaciers sometimes in high latitudes where the country is not mountainous, on the other side, a very considerable development of glaciers in lower latitudes for example, in the Himalayas- is due to the existence of a mountain chain. The Climatic condition of the development of glaciers can be best recognized by the elevation of the snowline above the sea and in late years one prominent task of glacial geology on the continent has been to determine the elevation of the snowline during the glacial period. Thus the snowline lies above the region of morainic accumulation along the regions where glaciers begin to melt called the tongue of the glaciers. The snowline of the glacial period descended towards the ocean and ascended towards the centre of the continent as it is the case with the present snowline. This points to the fact during the Great Ice Age Central Europe was in the neighbourhood of the sea. In the Alps the glacial snowline was more elevated in the Central part than in the peripheric region. It arched over the mountains. The same happens at present and there is a marked parallelism between the actual and the glacial snowline, the later lying from 3600 to 3900 feet below the former. In Southern Europe there are conspicuous irregularities in the elevation of the glacial snowline. This arrangement of the glacial snowline reveals to us some of the climatological circumstances of the glacial epoch. There is always a considerable depression of the snowline in mountainous regions which stretch across the direction of the prevailing winds. The very remarkable depression of the actual snowline along the western flanks of the southern peninsulas of Europe therefore indicates prevailing westerly

4 4 winds in the northern part of the Mediterranean sea during the Great Ice Age in the same way as the depression of the actual snowline on the coasts of Patagonia, New Zealand and the Alaska depends on the westerly winds there. Thus indication of westerly winds is found in the south of Alps while there are traces of westerly as well as easterly winds in the north of the Alps. The easterly winds are responsible for the deposits of loess in the north which is absent in the south. Although the great ice age is generally understood to be a single event however, there were climatic oscillations within it. In general ice age is generally marked by the occurrence of phases of intense cool with thick ice coverage known as glacial period intervened by phases of dry and warm period termed as interglacial. Glaciation also consists of phases of advance and retreats called stadial and interstadials. According to Richmond and Fullerton (1986) A major glacial event is a general glacial excursion, termed as glacial. Glacials are separated by interglacials. During a glacial, the glacier experiences minor advances and retreats. The minor excursion is a stadial; times between stadials are interstadials. Similarly the American Commission (1961) defines the fundamental unit of the geologic-climate classification as follows, A glaciation is a climatic episode during which extensive glaciers developed attained a maximum extent and receded. A stadial (stade) is a climatic episode representing

5 5 a subdivision of a glaciation during which a secondary advance of glaciers took place. An interstadial is a climatic episode within a glaciation during which a secondary recession or standstill of glaciers took place. These events are defined differently in various regions of the glacial range, which have their own glacial history depending on latitude, terrain and climate. The most widely referred glacial and interglaciation cycle is that recorded in the Alps. These sequences in Alpine regions have been identified by two geologists Penck and Bruckner (1909) on the basis of their studies of the ancient sedimentary and other deposits called moraines made by glaciers during its sliding movements. Thus four glacial sequences have been recorded from the Alps and these are the Gunz, Mindel, Riss and Wurm intervened by three interglacials namely Gunz- Mindel, Mindel-Riss and finally the Riss-Wurm. Corresponding glacial sequence recorded from United States of America are Nebraskan, Kansan, Illinoian and Wisconsinan while the interglacial episodes are Aftonian, Yarmouthian and Sangamonian respectively. The Pleistocene has been classified into three phases as Lower, Middle and Upper and the first formal use of the term Lower, Middle and Upper was done at the Second International Quaternary Association Congress in Leningrad in Geologist working in the Alps have estimated that the Gunz glaciation falls within the Lower Pleistocene started around 1.7 million

6 6 years ago; the Gunz-Mindel interglaciation and Mindel glaciation and Mindel Riss interglaciatios fall within the Middle Pleistocene having started around 4.5 lakhs years ago and finally the Riss glaciation, Riss-Wurm interglaciation and Wurm interglaciation are reported to occur within the Upper Pleistocene having started around 1 lakhs years ago. Some prehistorians prefer naming the different glacial periods in numerical sequence as First glacial period for Gunz, Second glacial period for Mindel, Third glacial period for Riss and Wurm as Forth glacial period. Similarly, the interglacials are termed as First, Second and Third inter glacial periods respectively. As mentioned earlier the effect of glaciation was noticed in the higher altitudes and latitudes of the earth during Pleistocene. The tropical regions of the earth experienced high precipitation and high humidity. In some regions of the tropics due to high precipitation the rivers and streams were saturated with water causing intense flood and sedimentation. These sediment deposits in the form of silt, sand and gravel were formed extensively in the floodplains of the rivers in the tropics. This period of flood were termed as pluvials and the period between two pluvials were termed as interpluvials. The interpluvials were periods of dry and lean season marked by receding volumes of water in the rivers and water bodies. Wayland (1930) a reputed geologist has defined pluvial period as, A period of geological significance during which rainfall was in general considerably heavier than in earlier times

7 7 and then it is today over an area sufficiently large to be of some account in world events is called a pluvial period. Four pluvial periods were traced on the basis of the geological evidences in East and South Africa and these are namely, 1. Kageran 2. Kamasian 3. Kanjeran and 4. Gamblian. These pluvials were contemporaneous to the glacial stages and thus Kageran occurred at the time of Gunz during the Lower Pleistocene while Kanjeran and Kamasian occurred at the time of Mindel and Riss during Middle Pleistocene while the Gamblian occurred corresponding to Wurm during the Upper Pleistocene. The deposition of the ice during the glacial period extended upto many thousand feet. This is estimated on the basis of the study of current deposition of the ice layers in Polar region (TC Sharma: 1974). Since the water of the seas and oceans condensed to form ice resulting the sea level to recede by about 400 ft to 500 ft. As a result the coastal regions in many areas of the earth expanded and some areas now submerged and separated by seas got connected. Thus the continents of Europe and Africa now separated by the Gibraltar were connected by land bridges. Similarly Alaska of North America and Siberia of Asia were

8 8 connected by land bridges. Such land links among different continents and countries of the world facilitated movement of prehistoric man and animals. On the other hand during interglacial periods the ice sheet melted to a great extent and as a result the level of water in seas and oceans enhanced which ultimately separated many countries and continents restricting movements of prehistoric man and culture. These changes in the level of ocean water due to the alternating glaciations and deglaciations of the northern hemisphere continents are designated as glacio-eustatic fluctuations (Karl Butzer: 1971). Butzer thus comments, These short term features are attributed to actual changes in the amount of ocean waters due to alternating glaciation and deglaciation of the northern hemisphere continents. These are designated as glacio-eustatic fluctuations. The range of possible glacio-eustatic sea levels depends on the calculation of how much water is still retained in solid state in glaciated areas like Greenland, Antarctica and others and how much more water was held in the glaciers that existed during the various Pleistocene glaciations. The activities of rivers were also influenced by the climatic oscillation during the glacial period. During glaciation

9 9 since the water condensed to form ice resulting in the reduction in the volume of the river water. On the other hand during interglacial period the volume of the river water increased due to melting of ice. Similarly the rivers were saturated with water in the tropical regions of the earth during pluvial due to high precipitation causing flood. On the other hand, during inter-pluvial due to drier climate the volumes of river water receded considerably. These fluctuation in the climate and affected the degradation and a gradational activities of the river leading to the formation of terraces in the river valleys. Four terraces are noticed in some rivers corresponding to the glacial or pluvial stages. The effects of glaciation were also noticed in the periglacial regions of the world particularly on the earth surface caused by wind. There prevailed an intense dry and cold climatic condition in the glaciated regions leading to high velocity wind. The glaciated regions remained barren free of vegetation during the interglacial phases with enough sand and other sediment components. The high velocity wind carried the sand particles and deposited them in the periglacial regions particularly along hill or valley slopes. These unstratified sand deposits are termed as loess in geological literature. As mentioned there are considerable deposits of loess in the northern Alps formed by the easterly wind. Originally the loess has been taken as a deposit of highly swollen rivers of the Ice Age. Then there was also the ingenious hypothesis on the origin of the loess as a continental deposit of a dry climate but the whole arrangement of the European loess is not consistent with its deposition in interior basins. However recent geological studies have indicated that loess is an Aeolian deposit and

10 10 that it is the river mud of the Pleistocene epoch blown off and redeposit by winds. There is always a certain relation between the accumulation of Pleistocene river gravels and river sands and the loess and along the Danube River in Austria similar indication has been recorded. Traces of fossil remains of prehistoric human and animals including Palaeolithic cultural remains are also found sometimes in the Loessic deposits. The flora and fauna of the Pleistocene were different from today as indicated by fossil evidences from different parts of the world. In Siberia animals like woolly mammoth, woolly rhinoceros, reindeer, cave lion etc. existed. These animals could adapt themselves to the adverse climatic conditions that prevailed during the Pleistocene. The dominant faunal species of North America were vast population of Proboscideans, Columbian mammoth, imperial mammoth and the mastodons besides short faced bear, saber toothed cat, American lion and the dire wolf. The plant varieties that existed during the Pleistocene also corresponded to the climate. Thus in Britain during the Ice ages the plants varied according to the climate ; in general during the warm periods Britain was covered with forests made up of both deciduous and coniferous trees while during the cold periods the land was covered with short grasses, mosses and lichens. The severe climatic changes during the ice age had major impacts on the fauna and flora. With each advance of the ice, large areas of the continents became totally depopulated, and plants and animals retreating southward in front of the advancing glacier faced tremendous stress. The most severe stress resulted from drastic climatic changes, reduced living space, and curtailed food and consequently during the end of the Pleistocene several animals including the mastodons and mammoths went extinct. The most remarkable event of the Pleistocene was the evolution of the Man in the earth and this is

11 11 corroborated by the fossil findings from different parts of the world. All the major stages of human e.g. the Australopithecus, Homo erectus, Neanderthal man besides modern man evolved during this period. The Neanderthal man who evolved in the European countries is known to have adapted initially itself to the cold climatic condition of the Pleistocene but consequently it could not thrive long against the adverse climatic conditions and became extinct. ***