Friday, July 20, 2012

Soil colour has been inferred as an indicator of past climate.

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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