With special reference to Jharkhand State
of India
By
Dr. Nitish Priyadarshi.
Palaeoclimatology, the study of climates during the
geological past, is one of the most topical areas research in the geoscience at
present.
The threat of future climate change caused by higher levels
of greenhouse gases, which would drastically alter many aspects of our
environment, has prompted much research to try to understand how our complex
climate system works. Only by understanding how climate has evolved over
million of years can we identify important cycles with a frequency in excess of
the short climate records we possess. These climate cycles have the potential
to have a profound effect on our environment.
The determination of past climate parameters by the use of paleohydrologic
conditions is an important phase of paleoclimatology. The present climate of
any local area, on any continent, and around any lake basin, depends on the
same factors which controlled Pleistocene climate. The study of present
meteorological conditions is then a perfect application of the present being
the “key to the past”.
The present climate for any area of the earth is controlled
by innumerable and diverse variables, the same variables that undoubtedly
controlled the paleoclimate of any particular area during any period of earth
history. The earth’s temperatures and climates are basically controlled by the
amount of solar radiation and the inclination of the earth to the sun.
Many methods exist which help in determining paleoclimatic
conditions. The most popular probably concern the study of sand dunes, coal
measures, soil studies, and spores and pollen although considerable attention
is also devoted to the fossil plants, paleohydrologic conditions, and to the
chemistry of lacustrine sediments.
Soil studies:
Much of the history of ancient lake basins, and particularly
their past extent, may in many cases be quickly determined by soil studies.
Climate influences soil formation primarily through effects
of water and solar energy. Water is the solvent in
which chemical reactions take place in the soil, and it is essential
to the life cycles of soil organisms. Water is also the principal medium for
the erosive or percolative transport of solid particles. The rates at which
these water-mediated processes take place are controlled by the amount of
energy available from the sun.
On a global scale, the integrated effects of climate can
readily be seen along a transect from pole to Equator. As one proceeds from the
pole to cool tundra or forested regions, polar desert soils give way to
intensively leached soils such as the Podzols (Spodosols) that exhibit an
eye-catching, ash-coloured E horizon indicative of humid, boreal climates. Farther
into temperate zones, organic matter accumulates in soils as climates become
warmer, and eventually lime (calcium carbonate) also begins to accumulate
closer to the top of the soil profile as evapotranspiration increases. Arid
subtropical climate then follows, with desert soils that are low in organic
matter and enriched in soluble salts. As the climate again becomes humid close
to the Equator, high temperature combines with high precipitation to create red
and yellow tropical soils, whose colours reveal the prevalence of
residual iron oxide minerals that are resistant to leaching losses because
of their low solubility.
The presence of specific minerals can also affect soil
color. Manganese oxide causes a black color, glauconite makes the
soil green, and calcite can make soil in arid regions appear white.
The development of a soil type depends greatly on climate,
parent material, topography and time. Therefore, because parent material, time,
and topography are well known for the Plistocene soils they are indicators of
paleoclimate even though climate intensity still exists as an important
variable. Basically pedalfer soils indicate temperate, forested areas, pedocal
soils warm, dry grasslands, and the laterites a tropical environment.
Certainly red to yellow soils, because of their
high concentration of iron oxides, suggest a warm, humid, oxidizing climate,
and light gray to white, calcified soils indicate a warm, dry climate.
Red soils have been extensively developed in Singhbhum,
Ranchi , Hazaribag, and Santhal Paraganas
districts in Jharkhand State of India . The pH of the soils vary
from 5 to 6.8. They are acidic in nature. The Jharkhand plateau consists of
gneisses and schists. Many of these gneisses and schists contain a large
proportion of biotite and hornblende and as they are highly ferruginous, the
soils derived from them are deep red.
The red soils usually drain off quickly and can hardly retain moisture
for any length of time.
Laterites are soil types rich
in iron and aluminium, formed in hot and wet tropical areas.
Nearly all laterites are rusty-red because of iron oxides. They develop by
intensive and long-lasting weathering of the underlying parent
rock. Tropical weathering (laterization) is a prolonged process of chemical
weathering which produces a wide variety in the thickness, grade, chemistry and
ore mineralogy of the resulting soils. The majority of the land areas with
laterites was or is between the tropics of Cancer and Capricorn.
Laterites are formed from the leaching of
parent sedimentary rocks (sandstones, clays, limestones); metamorphic
rocks (schists, gneisses, migmatites); igneous
rocks (granites, basalts, gabbros, peridotites); and mineralized
proto-ores; which leaves the more insoluble ions, predominantly iron
and aluminium. The mechanism of leaching involves acid dissolving the
host mineral lattice, followed by hydrolysis and precipitation of
insoluble oxides and sulfates of iron, aluminium and silica under the high
temperature conditions of a humid sub-tropical monsoon climate. An
essential feature for the formation of laterite is the repetition
of wet and dry seasons. Rocks are leached by percolating rain
water during the wet season; the resulting solution containing the leached ions
is brought to the surface by capillary action during the dry season.
Laterite soils are found in the ‘Pat’ region of west Ranchi and south Palamau in Jharkhand
State of India . The typical red colour is
due to a high percentage of iron oxides. The soils are generally poor in
nitrogen, phosphorous, potassium and organic matter, the pH ranging between 4.5
to 6.0.
The location of the Jharkhand state is just near the Tropic
of Cancer which has imparted to it a typical tropical climate. The average
temperature is 22 degree C. As the area is situated in a zone of transition
between Arabian Sea branches and Bay of Bengal
branches of south-west monsoon a moderate rainfall of 1200 to 1400 mm. is
experienced.
However, caution must be exercised when using soils for the
deduction of past climate events because development results from the intricate
variability of innumerable factors such as topography, groundwater, time,
rainfall, and temperature to mention only a few. Soil colour has been inferred
by many investigators (Simonson, 1954; Carter, 1956) as an indicator of either
past climate or soil age, the reds and yellows indicating a warm, dry climate
with the soil becoming redder with age. Such generalizations may be adequate
for local areas and restricted use, but are obviously dangerous for proper
scientific evaluation.
Reference:
Carter, G.F.,1956. On soil color and time. Southern J.
Anthropol., 12: 295-324.
Simonson,R.W., 1954. Identification and interpretation of
buried soils. Am. J. Sci.,252:705-722.
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