Palaeoclimatology, the study of climates during the geological past, is one of the most topical areas of research in the geosciences 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 climate 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.
Understanding our climate history in the geological past is also important for climatologists trying to construct accurate numerical computer models of our present climate system to use for predicting future climate change.
Basic information about past climates comes from understanding how climate influences certain sedimentary systems, floras and faunas on earth today and extrapolating this information back to interpret geological evidence.
The formation of some rock types and minerals are directly influenced by aspects of climate. Some of the most useful are coals, evaporates, glacial deposits, Bauxite and carbonates. I am presenting only a brief resume of rocks and minerals as a paleoclimatic indicators in Jharkhand State of India.
Coal is a compact stratified organic rock composed largely of metamorphosed plant remains mixed with a variable but subordinate amount of inorganic material. The coal-forming plant material may have accumulated where it grew or may have been transported to the depositional site by water or wind. The accumulation of peat requires a humid climate to support a rich growth of vegetation and a high water table to permit prolonged accumulation of plant material in a reducing environment. In other words, the presence of coal, initially formed from the accumulation of plant material as peat, is generally taken to indicate warm and wet humid climates ideal for lush plant growth, and where the rainfall is higher than the rate of evaporation, such as in equatorial regions.
In the past, the most abundant coal deposits were formed during the Carboniferous when large subsiding continental areas were situated in low latitudes and experienced hot and humid climates. The great Carboniferous forests were composed of the pithy-stemmed clubmosses and lycopods, such as Lepidodendron, Siggillaria and Calamites, which grew to giant sizes in the hot wet conditions and formed thick layers of peat as they collapsed into waterlogged swamps. The disappearance or decrease in size of these water loving plants at the end of the Carboniferous marked the onset of much drier conditions in low-latitude regions during the Permian.
A discussion on depositional environment of Permian peat swamps phases may well be preceded by the remarks that, Karharbari, Barakar and Raniganj ( these are all different coal formation stage) stages of Lower Gondwanas of Jharkhand State of India, were climatically controlled. The climate during Karharbari period was rather cold as evidenced by flora and by possible effects of glaciation in Talchir series. On the contrary, climate during Barakar and Raniganj commenced with coal and humid climate gradually becoming warmer and humid as evidenced by flora and coal composition.
Evidence for glaciation and the presence of thick ice sheets can be obtained from a variety of sources. The most convincing are striated pavements, that is surfaces of bedrock with grooves scratched by debris frozen into the base of moving ice glaciers. The orientation of ice movement and therefore in some cases the position of glacial centres can also be determined. Till or tillites can also provide information about ice passage. The term till was originally applied in Scotland to a stiff, hard clay subsoil, generally impervious and unstratified, often containing gravel and boulders. In present usage, the word till means a clastic glacial deposit, usually poorly sorted and nonstratified, and derived from glacial drift; it consists of a heterogeneous mixture of rock and mineral fragments of varied lithologic composition, size and shape. If till is indurated by cementation or metamorphism, it is called tillite.
Evidences of tillites are found in different coalfields area of Jharkhand State.
Bauxite is not a product of normal weathering in temperate regions, it is almost entirely lacking from soils formed there. It is however, a constituent of laterite soils formed in tropical and subtropical regions. They have mostly been formed in the late Mesozoic or Tertiary time under climate conditions different from those that prevail in the same places today. Possibly some of the conditions that affected the change from the age of reptiles to the age of mammals likewise promoted the formation of bauxite.
The conditions necessary for bauxite deposits are humid tropical or subtropical climate. It has been shown that a temperature above 20 degree C favors chemical processes by which SiO2 goes into solution and Fe2O3 and Al2O3 remain behind. The wet season of the tropics is one of formation of Al2O3 and Fe2O3, the dry season one of leaching of silica away from these oxides. This may supply the answer to why a tropical climate is necessary for bauxite formation.
Bauxite are found in Lohardaga and Netharhat areas of Jharkhand State.