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For septuagenarian earth scientist Dr R.K. Pant, the arduous trek to Goting Lake (3,820 m above sea level) in the central Himalayas was fraught with excitement. Pant knew that sediments from the lake could be a treasure trove of information on the climatic history of the region.
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A satellite picture of a retreating Himalayan glacier leaving behind moraines. (Courtesy NASA)

Nearly 20 million years ago the collision between the Indian and the Eurasian land masses — the Indian and the Asian plates — produced the lofty Himalayas. Soon after they came into being, this chain of mountains became the climate controller of a large part of the region. However, scientists have found it difficult to study the geological evidence of climate change here since Himalayan rock layers have undergone too many upheavals and hence are not well preserved.

But researchers know that glacial lake sediments often contain undisturbed, well-documented pages of a region’s climatic history. Goting Lake, which lies southeast of the 7,756-metres-high Mount Kamet in the Chamoli district of Uttarakhand, is one such water body. In fact, earlier scientists such as M.V.A. Sastry and V.D. Mamgain of the Geological Survey of India determined in the 1970s its sediments to be of interglacial stage. That is why Pant was hopeful that the sediments would yield vital information on climate change in the Himalayas.

During the glacial phase everything remains frozen, but as it gets warmer and the glacier retreats a rush of melt water is released. Lakes form in front of the retreating glacier when the melt water streams are blocked by moraines or glacial debris. Fine glacial sediments are deposited in these lakes and the alternating thick and thin sediment layers (varves) of these deposits indicate the summers and winters of the past. Counting the layers is one way of finding out about past climates.

However, though this is still considered to be a reliable method, the counting of layers has its problems. For example, in a hostile terrain, it becomes impossible for a glaciologist to physically count the layers. But in recent years, new and improved dating techniques such as Optically Stimulated Luminescence (OSL) have made the job of scientists much easier. OSL can be used to find out the age of the sediment by determining how long a mineral was exposed to sunlight.

Pant, then at the Wadia Institute of Himalayan Geology, Dehra Dun, made trips to Goting in 1997, 2000 and 2002. He was joined by scientists from the Physical Research Laboratory, Ahmedabad, the Indian Institute of Geomagnetism, Navi Mumbai, and the Radiation Research Department, RisØ DTU National Laboratory for Sustainable Energy, Technical University of Denmark. They published their findings in the Journal of Asian Earth Sciences recently.

Pant and his team report that the sediments at Goting preserve a high-resolution record of Indian monsoon variations. Using the OSL dating technique, they found that the region witnessed moderate monsoon conditions around 25,000 years ago, with a marginal decline taking place around 23,500 years ago. Monsoons were once again on an upswing till about 22,500 years ago, followed by a sudden decline about 500 years thereafter.

Pant says that the decrease in monsoon strength at Goting corresponds with the Last Glacial Maxima (LGM), that is, around 18,000 years ago when the glaciers of the subcontinent were at their maximum extent. The monsoon decreased still further, with a brief improvement around 16,500 years ago. According to Pant, the monsoon remained unstable, but with an improved intensity, all the way till about 13,000 years ago.

The data collected by Pant and others match with the monsoon records gathered from marine sediments. It also reinforces the view that during the LGM and right up to the early Holocene (the current chapter of the earth’s history that commenced from 8000 BC), the Indian monsoon and the climate of the higher Himalayas were closely linked. The climatic instability in the higher northern latitudes was, in fact, determined by the strength or otherwise of the monsoon winds.

However, Pant advocates a more detailed study of his findings. “Despite well preserved evidence 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. Lake sediments in the upper reaches of Mount Kamet need to be studied to decipher the centennial or millennial scale climatic fluctuations….”

No doubt further study will throw up greater insights. But Pant’s findings already represent a big step forward in the process of mapping the pattern of climate change in the higher reaches of the Himalayas.

Sources: The Telegraph (Kolkata, India)

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