Rock types as indicators of ancient climate.
by
Dr. Nitish Priyadarshi
The
Earth’s climate has changed dramatically over the eons, as the atmosphere
continuously interacts with oceans, lithosphere, and biosphere over a wide
range of timescales. Efforts to place recent climate observations into a
longer-term context have been stimulated by concern over whether the twentieth
century global warming trend is part of natural climate variability or linked
to increasing anthropogenic inputs of greenhouse gases into the atmosphere. The
ability to decipher past climates has expanded in recent years with an improved
understanding of present climatic processes and the development of more
sophisticated analytical tools.
Scientists know the Earth's average temperature has increased approximately 1°F since 1860. Is this warming due to something people are releasing into the atmosphere or natural causes? Many people today are quick to blame the greenhouse effect for global warming, but the temperature increases may have a natural cause, for example, from elevated volcanic activity. Gases in the earth’s atmosphere which trap heat, and cause an increase in temperature cause the greenhouse effect. Carbon dioxide (CO2), water vapour, and other gases in the atmosphere absorb the infrared rays forming a kind of blanket around the earth. Scientists fear that if humans continue to place too much carbon dioxide in the atmosphere, too much heat will be trapped, causing the global temperature to rise and resulting in devastating effects. Some scientists speculate that natural events like volcanic eruptions or an increase in the sun's output, may be influencing the climate. Perhaps the temperature rise is a natural trend that is part of a long-term cycle. Obviously, using only the weather data scientists have collected in the past 140 years will not be sufficient to answer these questions, so scientists use paleoclimactic studies to determine if today’s warming climate has occurred anytime in the past. Through past climate studies, scientists can predict what future climates and trends may occur.
Paleoclimate studies focus on both determining the climate
states of the earth during the geological past and understanding how the
climate system worked to produce those ancient environments. The careful
observation, collection and interpretation of geological evidence from as many
independent sources as possible is the most reliable way of determining details
of ancient climate states. For many geological periods, paleoclimatic data are
still new, and more rigorous methods for interpreting the data that we already
have are needed ( Francis, 1998). The most exciting aspect of this geological
research is that we can study unique environments that existed on earth in the
past that are no longer present in our modern world, such as forests and
dinosaurs living in warm climates in the polar regions and the extreme
environments inn continental interiors on supercontinental landmasses such as
Pangaea.
Over Earth history, the climate has changed a lot. For example, during the Mesozoic Era, the Age of Dinosaurs, the climate was much warmer and carbon dioxide was abundant in the atmosphere. However, throughout the Cenozoic Era (65 Million years ago to today), the climate has been gradually cooling. How do we know about past climates? Geologists use proxy indicators to understand past climate. A proxy indicator is a biological, chemical, or physical signature preserved in the rock, sediment, or ice record that acts like a “fingerprint” of something in the past . Thus they are an indirect indicator of something like climate.
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 research to try to understand how our complex climate system works. Only by understanding how climate has evolved over millions 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. Earth's climate has shifted dramatically and frequently during the last few million years, alternating between ice ages, when vast glaciers covered northern Europe and much of North America, and interglacials — warm periods similar to the present. Geoscientists use "proxies", or indirect means to reconstruct climates that existed long before the invention of thermometers, barometers, or other meteorological instruments: tree rings, pollen grains, animal and plant fossil assemblages. 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.
Minerals also furnish important clues about ancient climates. At Earth's surface, minerals interact closely with water and the atmosphere. Most useful are those deposited under relatively narrow climatic ranges or within specific environmental settings. These include evaporites, low temperature minerals such as ikaite and hydrohalite, minerals of residual soils (e.g., in bauxites or laterites), and some clay minerals like kaolinite.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.
Evaporites
Evaporite minerals form by evaporation of seawater or lakes
in narrow basins, rift valleys (like the East African Rift Valley), and coastal
lagoons under extremely hot and dry climates. Their distribution closely
matches that of deserts. As water evaporates from the basin, salts precipitate
in a sequence usually starting with carbonates, sulfates, and ending with the
more soluble chloride salts. Typical evaporite minerals include gypsum,
anhydrite, halite (rock salt), borax, and nitrates, such as saltpeter or niter (potassium
nitrate). Major deposits of rock salt occur in the Gulf Coast, the Austrian
Alps, the Dead Sea, upstate New York, Michigan, and elsewhere.
Climate
Clues from Soils and Sediments
Clay
minerals form by the chemical break-down of rocks near Earth's surface. The
detritus is removed by water erosion and accumulates in lakes, estuaries, and
the sea. Clays also occur in terrestrial soils and airborne dust. The types of
clay minerals and their relative abundances are closely related to climate,
although the composition of the source rocks can also influence their
development. Kaolinite, for example, is created by intense chemical weathering
in warm, humid climates where silica is leached out, leaving soils enriched in
alumina. Chlorite and illite, on the other hand, tend to form in soils
dominated by mechanical weathering, both in colder, often formerly glaciated
regions, but also in hot, dry climates.
Bauxite.
Bauxite is a residual soil that forms by intense chemical
weathering of rocks in wet, tropical climates where the average rainfall is 60
inches/year. The extreme leaching destroys most silicates and even resistant
minerals such as quartz, leaving insoluble aluminum minerals, such as gibbsite,
and boehmite, with lesser quantities of diaspore, kaolinite, and iron oxides. Laterite and bauxite peaks were coeval with times of global high warmth
and precipitation, elevated atmospheric carbon dioxide, oceanic anoxia,
exceptional fossil preservation, and mass extinction.
Coal.
Just as evaporates indicate hot and
dry climates, occurrences of coal in the geologic record suggest hot and wet
climates. The presence of coal, initially formed from the accumulation of
plant materials 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 a more
important factor than temperature, as are high water tables and waterlogged
swamps which are required to preserve the peat.
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
climate.
Carbonates.
Coral reefs provide paleoclimatologists with important proxy data. Coral reefs have been a part of the Earth's oceans for millions of years and are very sensitive to changes in climate. Scientists can use indicators from corals to study weather conditions from the past hundreds or even thousands of years to determine trends in climate. Corals form skeletons by extracting calcium carbonate from the ocean waters. When the water temperature changes, calcium carbonate densities in the skeletons also change. Coral formed in the summer has a different density than coral formed in the winter. This creates seasonal growth rings on the coral (like rings on a tree). Scientists can study these rings to determine the temperature of the water, and the season in which the coral grew. By using these growth bands, scientists can date the coral samples to an exact year and season.
Glacial deposits.
Varves
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 the 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. For example, studies of Carboniferous and Permian glacial deposits in the Southern Hemisphere enabled Crowell and Frakes ( 1975) to reconstruct the location of ice centres over Gondwana continents and to determine the direction of movement of ice lobes from these centres.
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, particularly for the Permian from which sequences they were originally described.
Reference:
Crowell, J.C. , Frakes, L.A. 1975. The Late Palaeozoic glaciation. In Campbell, K.S. (ed.) Gondwana Geology. Australian National University Press, Canberra, 313-331.
Francis, J.E. 1998. Interpreting Palaeoclimates. In Peter, D. and Matthew R.B. (ed.) Unlocking the Stratigraphical Record. Advances in Modern Stratigraphy. John Wiley & Sons, 471-490.
https://www.giss.nasa.gov/research/briefs/gornitz_08/
https://www.marine.usf.edu/pjocean/packets/sp00/sp00u1le4.pdf
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