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?
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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!
