Sands of Thar: Key to Past Climates

Science Daily of 26 June, 2007 quoting United Nations University said, 'Desertification exacerbated by climate change, represents the greatest environmental challenge of our times'. Indeed deserts and desertification are a real challenge for the mankind. Indian scientists are not lagging behind in the study of the process of desertification and its implications. Here is a report.

Thar or the Great Indian Desert stretches beyond the political boundaries of India. It is located in western India and southeastern Pakistan. Within our country it lies mostly in the state of Rajasthan, and extends into parts of Punjab, Haryana and Gujarat. Thar is one of the most inhospitable places in the world. The beauty of Thar is that despite extreme aridity it does hold life.  

There have been myths about desert's origin due to human indulgence and also the desertification of Delhi and Mathura regions was ascribed to anthropogenic reasons.

In a review published by A.K. Singhvi of Physical Research Laboratory (PRL), Ahmedabad and an expert of Quaternary sediments and climates along with Amal Kar of Central Arid Zone Research Institute (CAZRI), the myth has been exploded in a publication of Indian Academy of Sciences in 2004. The report is very interesting because it brings forth new ideas about the past climates and process of desertification.

The Thar Desert in India is the easternmost extension of the vast Saharo-Arabian deserts in the horse latitudes says Singhvi. Subtropical latitudes between 30 and 35 degrees both north and south are known as the horse latitudes. The confused and choppy sea, terrible heat with high winds often delayed the ships sailing through these latitudes in the good old colonial days. Consequently the sailors used to dump cattle and horses en-route to lighten the weight of the ships. That is why the name horse latitudes.

Southwest monsoon dominates the desert. But the atmospheric dynamics and sinking of air masses in the region inhibit rain, despite high amounts of moisture in the atmosphere. Some parts of the desert are now vegetated and experience rainfall in excess of 500mm per annum. The presence of fossil dunes in these areas suggest that changes in the circulation patterns in the atmosphere causes rainfall, leading to vegetated cover, though in the past this area was as dry as the others.

In India the desert is located in Rajasthan between the foothills of Aravalli ranges in the north and international border with Pakistan in the west.

We all know that the rivers carry sediments to the sea. There layers after layers of these sediments are deposited for millions of years. These get compacted and converted to sedimentary rocks over a period of time. Such rocks when exposed at the surface, like the Himalayan rocks prove extremely useful for the geologists to work out the geological history of such areas. But in a desert the sands are the only source of this kind of study. The sands seem to be shifting all the time. Then how one is able to read and interpret the geological history? It is the layers of sands accredited painstakingly by the nature over long periods of time that the experts are able to read and decipher.

Singhvi says the naturally stabilized and free-forming high sand dunes, dunes formed around major obstacles, as well as the thick Aeolian sand sheet deposits are the best keepers of sufficiently long records. 

A desert is like a sea of sand. Aeolian records or rather say records obtained from the deposits of wind blown sand are some of the finest tools for adding to the information on global warming. Thus a study of deserts is useful for comparing the changes in climate over a period of time.

Wind activity over Thar Desert is at low ebb during the winters. From March onwards when the surface is dry and temperatures soar the summer winds associated with south west monsoon reach a maximum speed of 20km per hour or more as per the record of the meteorological stations spread across the Thar. Advancing summers further dry up the ground and whatever vegetation tried to grow in the winters is dry by May. Thus, the tiny or insignificant 'binders' of the soil those shrubs of desert become ineffective from May to July. This is the ideal time for winds to carry or move sands from place to place. The moving sand during this period can be termed as dynamic sand. The arrival of monsoon in July puts an end to this activity.

Kar developed a wind erosion index in 1993. It is based on threshold wind velocity i.e. velocity on an average day and potential evapo-transpiration for the period of strong sand-moving wind during the period of south west monsoon. As per that the maximum potential wind erosion occurs approximately southwest of Jaiselmer and decreases gradually towards east or northeast depending upon the increasing rainfall and decreasing wind speed. This index helped him in working out the fact that the sand dune formation in Thar Desert was governed by the strength of SW monsoon.

The sand dunes studied by these workers in Thar Desert are of the 'old system' and of the 'new system'. Dunes of the 'old system are 10 to 40 m high and are now stabilized with calcified roots of vegetation and also with the help of carbonate derived from the nodules present in them. Some times traces of soil formation are also visible on them. Such dunes therefore, can sustain good amount of moisture and also support natural vegetation. The dunes of the 'new system' range from 2 to 40 m. They are nothing but masses of moving sand formed under the present day arid conditions. Apart from these 'fossil dunes' are found much beyond the borders of Thar up to Rohtak, Sultanpur, Bandikui and Lalsot in the north-east and up to Idar-Langhnaj in the south. These areas receive up to 550 mm annual rainfall. The dunes are covered with vegetation and have developed gullies due to flow of water. Soil formation has also taken place.

With all details of the types of wind blown sand deposits of Thar available, Singhvi and his friends set in to establish a Luminescence dating of the sands of dunes. Such procedure is quite precise because with the help of state of the art equipment it is possible to work out the exposure to the Sun the particular sand grain had before its burial. A chronology of the deposits is thus developed.

Singhvi and his friends found that:

Wind has been blowing sands of Thar since past 150 thousand years. The activity is cyclic in order. The authors recommend a further probe for the still older sands. May be the desertification process started much earlier than the date established by them!

Prior to last glacial maxima, that is before 18000 years from now major accumulation of wind blown sand had taken place between 100 to 115thousand years, ~75 to ~55thousand years and between 30 and 25thousand years.

Last Glacial Maxima or the period during which the glacial activity on the Indian sub-continent had attained its peak occurred around 18000 years ago. This was a period of high aridity. The desert could have spread far and wide during this period. But it didn't happen. Because the wind activity during this period was not powerful enough to mobilize the sands conclude Singhvi and his colleague.

The sand movement and deposition after the Last Glacial Maxima again started around 16 thousand years ago. Major sand movement took place between 14 to 10thousand years ago when the sand spread beyond its eastern limit.

While Gujarat province was lucky to have escaped sand depositional activity beyond 10000 years before present, the core of the desert in Rajasthan experienced peak desertification 5000 years from now. Some parts of Rajasthan continued to have sand movement activities by the winds even up to 2000 years ago. Small portions of the desert experience it even today, says Singhvi.

Thar desert is not just a heap of burning sand. It supports life too. The desert needs a thorough probe recommend Singhvi and Kar. They feel that complete information about the desert will help the government plan its developmental activities. Areas which can be reclaimed will thus become large chunks of oasis on the margin of main desert. In addition study will also help in knowing the 'next move' of the desert much before it occurs. 


More by :  V. K. Joshi (Bijji)

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