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Climate History Uncovered from Lake Sediments
|by V. K. Joshi (Bijji)|
News about climate change, melting of polar ice, retreating glaciers is galore in the newspapers these days. Everyday we read about the impact of global climate change. The word climate seems to have overtaken everything and everyone is curious about it. Is the rise in temperatures going to melt all the ice? Will it cause droughts? Will it affect the rainfall pattern? Such questions often haunt the mind.
The scientists on the other hand are busy trying to find the clues about the climates during the recent past. Beware the term 'recent' here is in geological terms-as it includes a span of past 25,000 years.
It is said 'present is a key to the past and the past is a window of the future'. Thus the scientists are trying all the keys to get the clues about the past climates.
Where one can get such clues? A student of past climates has to work like Sherlock Holmes and hunt. Some struck evidences of past climates in the stalagmites, others in the tree rings, while still others realized that the glacial lake sediments hold undisturbed records of climates.
For septuagenarian Dr. R.K. Pant, a student of archeology turned an avid earth scientist the arduous climb to Goting Lake in the higher Central Himalayas close to Tibet border meant a lot of excitement. And yes indeed it was. The samples of sediments collected from the lake were no ordinary mud they were actually storehouse of immense information about the climatic history of the bygone eras.
Nearly 20 million years ago the collision between the Indian and the Asian Plates produced the lofty Himalayas. Soon after they came in to being, the Himalayas became the climate controllers of a large part of the urban Planet. Since the rocks in the Himalayas have undergone lots of upheavals the climate change is poorly documented because of lack of preservation of the layers of rocks. The researchers on the past climates know that glacial lake sediments often contain undisturbed, well documented pages of the climate history.
Goting Lake is one such place where the researchers like Pant hoped to strike rich evidence for the past climates. It lies southeast of 7756 m high Mount Kamet and falls under Chamoli district of Uttarakhand. Earlier workers M.V.A. Sastry and V.D. Mamgain of Geological Survey Of India (GSI) ascribed the lake sediments to be of interglacial stage (Indian Minerals, V. 24-1970).
During the glacial phase everything remains frozen, there is hardly any discharge of melt-water and as such hardly any sediment get deposited. As the climate changes and the glacier retreats the volume of melt-water increases. The fine, glacial sediments then get deposited in the lakes formed in front of the glaciers due to blocking of the melt-water stream by the moraines or dammed by the ice. The alternate thick and thin sediment layers (Varves) of these deposits indicate the summers and winters of the past. Counting the layers is one of the ways of finding out about the past climates.
The counting of coarse and fine layers, though still considered most reliable often has problems. For example in a hostile terrain, it becomes physically impossible for a glaciologist to physically count the tin layers. The instrumentation in recent years has proved to be a boon to the researchers. The advantage is that once a sediment sample is collected and safely packed it can be brought and studied in detail in the comforts of a laboratory. Optically Stimulated Luminescence (OSL) dating is one such technique used to find out the age of the sediment by determining how long a mineral was exposed to day light. This makes turning the pages of climate history of inhospitable terrains comparatively easy.
The terrain is so tough that even the young trekkers find it difficult to reach Goting. It was perhaps the sheer grit to collect samples to unravel the evidences of the past climates treasured in Goting lake sediments, R.K. Pant (then at Wadia Institute of Himalayan Geology, Dehradun) despite his age made trips to Goting in 1997, 2000 and again in 2002. He was joined by scientists from PRL, Ahmedabad, Indian Institute of Geomagnetism, Navi Mumbai and Radiation Research Department, RisØ National Laboratory, Technical University of Denmark. They published their findings in the recent issue of the Journal of Asian Earth Sciences 34 (2009) 437-449.
The lake sediments at Goting preserve layer by layer a high resolution record of Indian summer monsoon variations they report. Using OSL dating techniques they found moderate monsoon conditions around 25 thousand years (ka) with a marginal decline between 25 and 23.5 ka. The strength of the monsoon again improved between 23.5 to 22.5 ka. Around 22 ka there was a sudden decrease of monsoon strength. This was a period of cooling as the impact of widespread glaciation was till on. That is why Pant says that decrease in monsoon strength at Goting corresponds with Last Glacial Maxima (LGM). Evidences from other sources show that the LGM was at its peak around 18 ka. Monsoon gained strength again but with low frequency fluctuations between 22 ka till 18 ka. Thereafter the monsoon decreased till 17 ka with a brief improvement around 16.5 ka. There was instability in monsoon after 16.5 ka and before 14.5 ka. Monsoon remained instable but with an improved strength till 13 ka.
The lake environment of the periglacial regions however, changed with the increase in rainfall frequency and occurrence. In addition there were low magnitude high frequency climate oscillations following the LGM. The ups and downs of the climate forced an increase in meltwater discharge and the serene glacial lakes soon vanished, to be replaced by deposition of gravels near the glacier fronts.
The warming episode had set in.
The data collected by Pant et al matches with the monsoon record of marine sediments on a millennial scale. It strengthens their view that Indian summer monsoon and higher Himalayan climate during the LGM to early Holocene (current chapter of the earth's history from 8000 B.C. till date) was closely coupled. The climatic instability in the higher northern latitudes was modulated by the position of the westerlies. However, on a centennial scale the abrupt changes in ice and snow cover have been ascribed by them to solar irradiance.
In a communication Pant cautions, 'despite well preserved evidences of past climates in the mud of Goting Lake further refinement is essential. The OSL data may have to be supplemented with the age old method of counting the 'varves' (fine and coarse layers of glacial sediments deposited annually). Lake sediments in the upper reaches of Mount Kamet need to be studied to decipher the centennial or millennial scale climatic fluctuations especially in view of the ongoing debate on global temperature rise and response of the Himalayan Glaciers.
The pages of climate history unraveled by Pant et al will be a boon for the contemporary climatologists to predict the future!
|More by : V. K. Joshi (Bijji)|
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