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Pollen Reveals Mysteries of Past Climate
|by V. K. Joshi (Bijji)|
Weather and climate form an integral part of routine human conversation. Other animals do not have a spoken language, therefore it not possible to comment with certainty that about their conversation on climate! Humans with the passage of time turned agrarians from hunter and gatherers. Naturally climate became a topic of daily discussion.
The contemporary world has suddenly realized the importance of climate and its impact on the very survival of the human race. Hence the climate now is a topic of major discussion in the international forums. What will the future climate be like is a question that the experts who gathered at Cancun recently may be able to answer. But if we accept the Hutton’s theory of uniformitarianism, which says that the nature’s forces have always acted upon the Universe in the manner they do so today, since times immemorial, then the present climate has a lot to reveal about the past climates. It has been a human weakness to know about the past and in situations where the past can influence the present or future, the inquisitiveness to know about the past becomes deeper.
Like Sherlock Homes trying to unravel the clues to murder, the paleoclimatologists use several tools to unearth the past climates and build a sequence of events. Various tools like geology, geomorphology, paleontology, dendroclimatology etc have been used from time to time in different places.
Knowing about the climate through past 10000 years has all the more fascinated the scientists, because this is the period which has influenced the man-kind the maximum. Areas that were richly inhabited by man and other animals naturally gain more interest than those areas which were kind of desolate in the past. Central Narmada valley in India was one such area where the oldest human ancestors roamed around with a variety of other mammals. Some parts of the valley near Hoshangabad are rich in Paleolithic tools. A very look at the site says that ‘this was perhaps a factory of stone implements!’
Not merely hominid remains, the Narmada valley is a house to a diverse mammalian fauna and a variety of vegetation. It is this large variety of vegetation which has left its remnants in the form of palynomorphs or ‘pollen fossils’ which have been studied by Poonam Verma and M.R. Rao of the Birbal Sahni Institute of Paleobotany, (BSIP) Lucknow and reported recently in the Current Science. In fact the reconstruction of past climates by identifying the vegetation with the help of pollen fossils is an interesting science. These scientists of the BSIP have tried to reconstruct the vegetation scenario of a part of Narmada valley during the past 12000 years.
The pollen however represent only the existence of the plants to which they belonged to. Rest of the deductions is made on the basis of the type of the vegetation and the environments that supported their luxuriant growth. In other words the pollen acts as a proxy for the plants. Materials like pollen or tree rings used for reconstructing the past climates are called as proxies.
Present day swamps are one place where luxuriant vegetation is found. Thus going by the dictum, present is a key to the past; Poonam and Rao selected Kusumelli Swamp in Sehore district of Madhya Pradesh. Incidentally this is a perennial swamp of about three kilometer periphery and is mainly fed by rainwater runoff from the Vindhya foothills close by and also by the seepage from the hills. It is important to know that under the microscope at times it becomes very difficult to discern between the present day pollen and the fossil pollen. In addition on the surface often there are chances of the recent pollen ‘leaking’ into the sediments containing the fossil pollen. Therefore in order to avoid any mix up, paleonologists use a hand operate machine, which cuts through the rocks and collects samples from a depth. The samples that Poonam and Rao collected were from a depth of 1.5 m, considered to be a safe depth to avoid contamination.
Once identified the fossil pollen can be categorized in to the plants they belonged to. Poonam and Rao could establish that the pollen fossils belonged to trees, shrubs and ferns, and algal and fungal remains. But this identification does not help in deciphering the climate. However, it is but natural that the pollen fossils which dominated the scene must have constituted the majority of plants! In other words the scientists were narrowing down the gap by grouping the pollen fossils in to those derived from the trees and those derived from vegetation other than trees.
The jigsaw puzzles was fitting into a shape now and on the basis of the frequency of occurrence of these broad two types of pollen fossils Poonam and Rao were able to establish three zones of vegetation corresponding to climate fluctuations.
Their zone I included trees which shed leaves annually and require moisture to survive. These trees were growing between 11900 to 8500 years before present (BP) and included the Indian Teak. Not only that apart from a variety of trees there were a large variety of shrubs growing in the area. The type of vegetation indicated that between 11900 to 8500 years BP the intense summer heat led to increased rain and moisture which also water-logged the area. Water-logging is further confirmed by the presence of pollen fossils of water plants.
Climate never remains the same. From hot and humid it turned to cool and dry climate, as evidenced by the reduction of a number of large tree varieties and increase of Savannah type trees and grasses between 8500 to 7000 years BP. These were placed in zone II by Poonam and Rao. There are certain varieties of plants such as Artemisia which proliferate in extreme cold and dry climates only. Increased count of their pollen dated to be occurring 8200 Years BP coincided with short lived global cooling event, an event which is well documented in Africa, China, India and Pakistan
From 7000 to 5100 years BP the climate took another turn. This is evidenced by pollen fossils of trees that grow in dry climates and annually shed their leaves. These were grouped in zone III by Poonam and Rao. However, there were varieties of plants that required more moisture too. Hence the climate was not truly dry. Sudden increase of water plants in this zone also proves that the climate was not as dry as indicated by the trees. There was enough moisture to keep the pond filled up. This was a period of climatic optimum and has been globally witnessed say the researchers.
From the foregoing it is apparent that the climate does change, but not in the manner perceived by us. We take the minor shifts in climate as a major change. Whereas in reality a climate change takes long time and stays on for more than a thousand years. Climatologists say that the earth entered a warm phase some 18000 years ago, at the end of the Ice Age. But since then there have been phases of extreme cold, lasting for as much as 400 years, extremely wet phases lasting for three centuries and as we read in this narrative even cold a dry phases lasting for couple of hundred years.
The climate changes as worked out by Poonam and Rao took place from 11900 to 5100 years BP. Those were the days when man had not even become a farmer and smoke belching industries were no where around. Yet the climate changed. It clearly shows that climate is controlled by several factors other than human interference. But yes the human factor which has started pumping obnoxious gases in to the atmosphere is bad as it affects the everyday climate and needs to be curbed globally.
Images (c) Gettyimages.com
|More by : V. K. Joshi (Bijji)|
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