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Environment
Climate History
Uncovered from Lake Sediments
by VK
Joshi
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.
Melting Glacier in Swiss Alps and in the
Himalaya Region, Uttaranchal, India
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, RisŲ 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!
March 15,
2009
Image under license with
Gettyimages.com
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