Coals are best indicators of ancient climate.
By
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. 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 is directly influenced by aspects of climate. Some of the most useful are coals, evaporates, glacial deposits and carbonates. I am presenting only a brief resume of coal as a paleoclimatic indicators.
Coal- climatically sensitive rock:
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. However, rainfall is more important factor than temperatures, as are high water tables and waterlogged swamps (mires) which are required to preserve the peat.
Coal seams are composed of genetic coal types which are determined to a certain extent by the character of the particular type of vegetation. A careful analysis of all the available data on geochemical, palynological and petrological constituents of the coals reveals that there existed distinctive types of vegetation associated with different peat types. The character and relation between the miospore assemblage and petrographic type reflect particular environment, topography and climatic conditions.
Pollen and spores commonly retain their morphological characteristics through all stages of coal formation. They bear specific relationship to the original geological and botanical setting.
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 Lepidodnedron, Sigillaria 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. Extensive forests dominated by glossopterid plants lived on all southern continents and their remains form extensive and some economically important and coal deposits today.
A discussion on depositional environment of Permian peat swamp phases may well be preceded by the remarks that, based on different analysis and support from geological setup, Karharbari, Barakar, and Raniganj Stages of Lower Gondwanas of India were climatically controlled. The climate during the 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 cool and humid climate gradually becoming warmer and humid as evidenced by flora and coal composition. Humidity seems to have recurred in some part of Raniganj Stage also.
In the early period of the Permian, coal formation took place under the relatively cold, humid, shallow water deposition mainly from arborescent vegetation.
Chemistry of Coal-bed and paleoclimate:
The chemical arguments for the interpretation of paleoclimate from coal beds come principally from the work of different geologists. Their work was partly in response to studies purporting to show that high-sulfur coals were influenced by marine sedimentation. They argued that peat that forms economic coal cannot form in seawater because ash and sulfur enrichment is too great there. Thus they concluded that all economic coals were originally freshwater peats. They further concluded that, if all economic coal beds were derived from freshwater peats, ash content must be indicative of climate, and they proposed the following model, which predicts three types of peat:
1. anaerobic (permanently waterlogged) peat with pH less than 4.5, which would give rise to low-ash, low-sulfur, vitrinite-rich coals;
2. anaerobic with pH greater than 4.5, which would give rise to high-ash, high-sulfur, liptinite-rich coals; and
3. intermittently aerobic peat, which would give rise to low-sulfur, moderately high-ash, inertinite-rich coals.
Boron element in coal as a Paleosalinity Indicator:
The concentration of boron in Australian and Canadian coals was determined in order to assess the variation of boron in coal with respect to rank, age, geological setting and the degree of paleosalinity of the coal forming environment. The boron content of seams is sensitive to the environment of deposition and may show the variation in the same seam laterally due to changes to the environment of deposition and /or the enrichment of boron by secondary source.
It is proposed that the following ranges of values for boron in coal indicate the degree of marine influence during the early stages of coalification:
1. up to 50 ppm (parts per million) boron- coal formed in a freshwater environment.
2. 50 to 110 ppm boron – coal formed in a mildly brackish water environment.
3. greater than 110 ppm boron- coal formed in a brackish water environment.
Coal petrography and paleoclimate:
Vitrinite-rich coal beds are generally regarded to have been deposited in wet conditions, usually meaning high water tables, especially if the coal beds have clay partings and inclusions of syngenetic pyrite . Inertinite-rich coal beds are generally regarded to have been deposited in dry conditions, usually meaning relatively low or fluctuating water tables.
