Study of sediments reveal ancient climatic changes.
DR. NITISH PRIYADARSHI
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.
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. It is obviously not possible to check the accuracy of models that are predicting the future so climatologists must turn to the past to see if their models can accurately simulate ancient climates. It is therefore the role of the geoscientist to collect as many data as possible from the rocks.
By studying palaeoclimatic changes in the past we are able to evaluate the various causes that led to global cooling or warming and are able to evaluate the full potential of greenhouse gases- a powerful source of climate changes, and compare their effects on the present climate with that of the past. This would enable us to say for sure if the consumption of the present fossil fuel reservoir, the main source of carbon dioxide, is likely to affect climatic changes in the near future or not.
Historic records tell us that abrupt climatic changes have occurred during the last 2000 years. Palaeoclimatic studies would tell us if such abrupt climatic changes are expected to occur in the near future. By scientific study of past climate, it will be possible to anticipate climate surprises in the future.
Detailed study of sedimentary rocks and their enclosed fossils has made possible estimates of such climatic factors as wind directions, rainfall, atmospheric and oceanic temperatures, and the effects of atmospheric changes. The most obvious palaeoclimate determinations are the recognition of ice ages, and the hot dry periods.
Estimates of climatic conditions become less and less reliable as they are projected further and further back in time. Thus, Pleistocene climates are relatively well known whereas climates for Lower Palaeozoic periods are probably little better than intelligent guesswork.
It is becoming increasingly apparent that some ancient environments cannot be found on earth today, such as the presence of warmth-loving vegetation and animals living near the poles. In these cases it is vital to carefully interpret all potential sources of environmental information to reconstruct these unique situations from primary data.
The formation of some rock types is directly influenced by aspects of climate. Some of the most useful are coals, evaporates, glacial deposit and carbonates.
The presence of coal, initially formed from the accumulation of plant material as peat, is generally taken to indicate warm 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 marine realm, carbonate sediments are often used as indicators of warm ocean waters. Carbonate sediments of Bahamian type (including reef-building hermatypic corals, some algae and ooids) are important indicators of warm marine seas.
A different suite of carbonate also form today in cool temperate waters. These are composed of benthic foraminifera, red algae, mollusks and bryozoa. These carbonate deposits form in much higher latitudes under cooler conditions. Identification of the carbonate constituents is therefore important to distinguish between cool-water and warm-water carbonates for palaeoclimatic interpretation.
Evaporites, such as anhydrite, gypsum and halite, are used as guide to aridity in the past. Their formation requires evaporation rates to exceed precipitation, at least seasonally, and to exceed water inflow into the evaporating basin.
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 surface of bedrock with grooves scratched by debris frozen into the base of moving ice glaciers.
Glacial tillites can provide information about ice passage but, in the absence of other glacial features, tillites can sometimes be hard to distinguish from other diamictites, such as debris flow deposits, which may have formed under totally different conditions. Ice-rafted dropstones and varves indicate that ice formed, at least seasonally, and produced dumps of ice-carried debris or seasonal lake sediments. In addition, glendonite nodules have also been used as evidence for cold climates.
Aeolian sediments and red beds:-
The distribution of red beds and Aeolian sediments (sediments deposited after transport by wind) can also provide some indication of controlling climatic parameters. Aeolian deposits can provide important on prevailing wind directions. The term aeolianite is used to describe all consolidated sedimentary rocks which have been deposited by wind and are cemented by calcium carbonate. These are widely found in India (Miliolite and Chaya rocks), the Persian Gulf coast, the Arabia, Australia, South Africa, Mediterranean coast and Madagascar.
Red beds were once considered as classic indicators of desert conditions. However, it is now believed that the main factor governing their formation is not solely aridity but the seasonal nature of rainfall. Alternating wet and dry periods govern the mobilization and precipitation of iron minerals. Therefore reddening can occur in a range of environments, from those which are generally arid with a short season of rainfall to those which are seasonally very wet.
There are several other climatically sensitive sediments that have been used to determine climate. For example certain clay minerals tend to form under specific climate settings. Bauxite are limited to tropical and subtropical settings with high rainfalls.
Lake deposits, especially those laid down in closed lakes (i.e. lakes with inlets but no outlets), are among the most useful source of information about palaeoclimate in many areas of tropics and subtropics. Their sediment rather provide rather continuous stratigraphic sequence, which often contain datable materials and allow chronological determination of climatic changes. Pollen grains in the lake sediments are well preserved and their analysis tell us about the kinds of plants growing at the time the sediments were deposited. Inferences can be made about the climate based on the types of plants found in each layer. The flora present in the lignite, peat and several layers of carbonaceous soil in the Karewa lake of Kashmir (India) has revealed a history of alternating dry glacial and humid interglacial conditions.
Lake level records provide reliable information on climatic oscillations, particularly of the major changes in hydrology. For example lake level records from Central Africa show that its major lakes like Chad, Naivash, Malawi, Chilwa got almost dried up during greater part of the Little Ice Age (1700-1830 A.D.)
All these methods of studying rocks and sediments has been utilized to great advantage in working out the climatic variations in the geological past by different geological organizations and even archaeological organizations.