Dusty Records of Past Climates

Ever been to land of snow capped mountains? Even in the sub-zero temperatures winds rage through the valleys and ridges, whistling and howling. Dust raised by these winds is carried far off. The phenomenon has been going on through the ages. How the dust is created, picked up by the wind and deposited in layers, and how these deposits are studied by the scientists to interpret the past environments makes an interesting reading. May be the following narrative will make the reader aware about the significance of the dust!  

Curiosity about the past has always been a human weakness. The climatic records hidden in the rocks, ice and tree rings have been attracting the experts since quite some time. An interpretation of these records helps in building a chronology of the past climatic events, which often comes handy for predicting the future climates too.

A newer dimension in these studies has been the interpretation of the past climates through glacial rock flour which is blown by the wind. The dust so raised is deposited elsewhere year after year. Thick deposits of dust or the Loess are found in China and Midwestern United States. However, may be less in thickness but Loess deposits are a hall mark of several European countries and also India.

Glaciers are like the nature's road rollers. Unlike the rivers they move in the reverse direction. Their movement though imperceptible to human eye, is powerful enough to pulverize the basement rocks to fine flour like powder. Hence the term rock flour. The rock flour deposits on the border of the glacier and once dry it is susceptible to wind erosion. Dust plumes generated by the katabatic winds move down the valley. The term Katabatik is derived from Greek Katabatikos, meaning down the hill. Such winds carry the rock flour to long distances at altitudes lower than the glacier to deposit it there. Night after night such storms rage and the dust spreads. The resultant Aeolian silt deposits or Loess are homogenous because winds can carry particles of a particular size only. Being fine the deposits remain porous. Many times carbonate content from some grains leaches and binds the deposit on the surface to remain steady even if exposed vertically.

In India Dr. R.K. Pant and his colleagues from the Physical Research Laboratory (PRL), Ahmedabad were the first to identify and describe Loess, palaeosol sequences from Kashmir valley in 1978. Palaeosol means the soil of the past. Such soils assume a great significance for the geologists. Apart from environmental foot prints they also sometimes preserve the pre-historic animals and remain under the scanner of the geologists for the foot prints of human ancestors! Pant and his colleagues found a blanket of silt over the Karewas of Kashmir. Karewas are the lake deposits found in Kashmir. These table topped sequences have the blanket of Loess over them. Mind you the loess are the wind blown sediments. A regular deposition of Loess must have been quite a windy affair for a number of years!

Subsequently in 1984 R.J. Williams and M.F. Clarke reported loess from Son Valley in Central India. Imagine the distances such fine grained sediments must have traveled with the winds! While searching for Loess Pant and his team were lucky to strike it once again at altitudes ranging from 1800 to 2500 m in the Central Himalayas. These patchy deposits of loess occur between Dhakuri in Bageshwar district in Pindar River basin and Chopta in Chamoli district in Alaknanda River basin of Uttarakhand. They published their findings in the Journal of Quaternary Science in 2005.

Pant and his colleagues subjected the samples of loess collected from the above area to various kinds of laboratory tests in the state of art laboratories at Wadia Institue of Himalayan Geology, Dehradun, PRL, Ahmedabad, Indian Institute of Geomagnetism, Navi Mumbai and Institut fur Geowissenschaften, Universitat Tubingen, Germany . The results were quite interesting. Environmental magnetic studies revealed that these fine sediments showed a higher magnetic content. The problem was that the anticipated source area of the glacial flour was a granite terrain and granites are poor in iron content. Pant and his colleagues conjecture that either the source of this flour was far away. They quote some researchers having obtained loess from the drill-cores taken from the sea floor. The other reason they give for the presence of magnetic content is due to oxidation of the rock flour after it was deposited as loess. Unless confirmed both explanations are valid. 

One of the most interesting ways to estimate the age and duration pf loess accretion and soil formation episodes was the use of Infrared Stimulated Luminescence (IRSL) and radiocarbon (14C) techniques. IRSL determines the time elapsed since the mineral grains constituting the loess were last exposed to Sunlight. As the fine sediments carried by the winds are deposited layer after layer, year after year, the sunlight to older sediments becomes dearer till they are completely covered. The IRSL ages ranged from 20 ' 4 thousand years (ka) for loess at 200 cm depth and 1' 0.3 ka for loess at 20 cm depth.

A detailed chronological history obtained from the loess by Pant and his colleagues indicates that first two events of loess deposition took place between 20 and 9ka and were separated by a phase of moderate weathering. Pedogenesis (conversion of loess to soil) was identified in the area by this team between 9 to more than 4ka. This was followed by the last phase of loess deposition between 4 to more than 1ka. The chronology of past 20,000 years clearly shows that there were periods of loess formation and pedogenesis coinciding with the changes in strength of Indian southwest monsoon. Much of the loess making and deposition process depended upon the monsoon. At present the region experiences 80% of precipitation through the southwest summer monsoon while the westerly disturbances contribute the remaining 20% during winters. In other words the region is highly influenced by the summer monsoon. During the inter-glacial periods the region was exposed to more sunlight and a low pressure belt over Tibetan Plateau was generated. This helped rain laden winds to travel northwest wards from Indian Ocean. During the glacial periods the sunlight penetration in the region became much less and this reduced the low pressure over the Tibetan Plateau, which and in turn facilitated southwards propagation of cold dry winds and enhanced wind activities over Indian sub-continent during weak monsoon.

One of the co-workers A.K. Singhvi has established that there was a global peak of loess accumulation around 17,000 years ago. The ages of loess accretion obtained with the help of IRSL studies also support this observation. During the past one thousand years the loess deposited in the last phase is being converted to soil.

The dust deposited thousands of years ago has given enough information about cold and hot spells on the Indian sub-continent. We are now in a warm phase. It can last for 3 to 4000 years, can we survive it?


More by :  V. K. Joshi (Bijji)

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