Tuesday, March 18, 2008

Evidence of early life: Stromatolites and its presence in Jharkhand State of India.

Evidence of early life: Stromatolites and its presence in Jharkhand State of India.
By
Dr. Nitish Priyadarshi
Stromatolites have the distinction of being by far the oldest indicators of organized life on earth, ranging back over 3 billion years. They occur all on continents in rocks from middle Precambrian to Holocene age. Stromatolites are laminated limestone structure of simple to complex form commonly attributed to debris-binding and biochemical processes of benthonic blue-green, green, and possibly, red algae.
The stromatolites can withstand a wide range of physical and chemical conditions, they are found in fresh water, marine, and hyper saline environments. Most stromatolites originate in shallow water, but under conducive geochemical and geological conditions “deep water” stromatolites can grow down to 150m below water level. Observations of both modern and ancient stromatolites demonstrate the very shallow water origin. The close association of mud cracked limestones, flat-pebble conglomerates, and oolites further suggests very shallow waters.
They are best displayed and are by fore most abundant in the older rocks, especially the Precambrian and early Paleozoic. Their scarcity in the later Phanerozoic strata has been attributed to the destruction of the algal mats by grazing forms, particularly snails, and destruction of algal laminations by burrowing organisms. It is presumed that such organisms were not present in the Precambrian and absent only in later times when salinity or other environmental factors restricted or extinguished the usual biota.
Stromatolites are a very useful tool in evaluating the nature of early Earth as well as microbial metabolism and the development of the biogeochemical cycles. They indicate what past environments were like in an area.
Stromatolites were called “living rocks” by scientists before their exact makeup was understood, and they do resemble rocks, growing to well over three feet (one meter) high and being almost as wide. From a distance, a colony of stromatolites can look like a series of boulders scattered across the beach. However, stromatolites are actually laminar structures composed of many layers of material accumulated by the cyanobacteria that make up the stromatolite colony. In fossilized form, stromatolites have distinctive bands of material which are quite striking.
Stromatolites are formed by prokaryotic cyanobacteria, which have cells lacking a distinct nucleus. Prokaryotic bacteria are considered by some scientists to be the oldest and most primitive life form on Earth, with a well established fossil record stretching back for millions of years. The cyanobacteria which dominate stromatolites process many of the building blocks of life, including oxygen, carbon, and nitrogen.
Cyanobacteria use water, carbon dioxide, and sunlight to create their food. The byproducts of this process are oxygen and calcium carbonate (lime). A layer of mucus often forms over mats of cyanobacterial cells.
Prokaryotic bacteria form an important part of the world's biomass today, especially in the oceans. These bacteria thrive in extreme environments due to their relative lack of complexity. They are also very susceptible to being overwhelmed by more complex organisms, and are therefore often found in areas which other organisms cannot survive, or paired with other, more hardy forms of life. Because they play an important role in reducing carbon dioxide levels and emitting oxygen in exchange, many scientists have urged more research on these bacteria.
Precambrian stromatolite is the oldest of all fossils, and with much labor (cutting and polishing), it is most beautiful. The banding that commonly appears in stromatolite is a record of the growth patterns of colonies of microorganisms, principally photosynthetic cyanobacteria, but also other Eubacteria and the Archaeans. The colors that are often expressed are the result of the interaction of biological and sedimentary processes, together with subsequent chemistry and mineral exchange.
Oxygenation of the Atmosphere - a profound transformation of the bioshere
Regardless of when the cyanobacteria appeared, it is widely accepted that they comprised the predominant form of life on early earth for some two billion years, and were responsible for the creation of earth's atmospheric oxygen, consuming CO2 and releasing O2 by photosynthetic metabolism. Creation of the modern atmosphere is, of course, perhaps the most critical event in geological history that powered the Cambrian explosion and subsequent evolution of the aerobic forms of life, including all animals.
We will likely have no more than a sketchy understanding of the paleoenvironments in which stromatolites were formed in the deep Precambrian time, and only an incomplete understanding of the environments in the Paleozoic. Sound conjecture is possible if we examine the now rare environments that support stromatolitic growth during modern times. Cyanobacteria are found to be a primary organism in the formation of modern microbial carbonates. These prokaryotic bacteria (slang name is blue-green algae owning to pigmentation involved in photosynthesis) are now only found in areas where there is reduced grazing and burrowing by other organisms, and a low occurrence of macro-algae and plants. Environments where modern stromatolites are found typically are hypersaline, but also include areas of high alkalinity, low nutrients, high or low temperatures, and strong wave or current actions. The obvious pattern emerges that modern stromatolites tend to exist in areas that most other life forms consider less desirable or possibly intolerable. Thus, organisms producing modern stromatolites are generally limited to areas where organisms with which they have to compete and/or organisms that might use them for nutrients are not prevalent.
Banded Iron Formations (BIFS)
While not always recognized as such, Banded Iron Formations (BIFs) are another form of stromatolites, and again the cyanobacteria are the heroes that provided the source of oxidants for BIF formation. BIFs are massive, laterally extensive and globally distributed chemical sediment deposits that consist primarily of Fe-bearing minerals (iron oxides) and silica. Iron can occur naturally in two states. Reduced, or ferric iron, is soluble in water. In the Archaean oceans, prodigious ferric iron was released from the Earth's interior. The presence of free oxygen in the oceans would have oxidized the reduced (soluble ferrous) iron in solution to form oxidized (insoluble ferric) iron, which precipitated as iron oxide. Thus, banded iron layers are the result of oxygen released by photosynthetic organisms combining with dissolved iron in Earth's oceans to form insoluble iron oxides. The banding is assumed to result from cyclic peaks in oxygen production. It is unclear whether these were seasonal or followed some other cycle. It is assumed that initially the earth started out with vast amounts of iron dissolved in the world's seas.
Stromatolites in Jharkhand:
Iron ore groups (Archaean age)
of Jharkhand and bordering Orissa need pointed reference as they have the potential to constrain concepts of early evolution of life and also the age of the Iron Ore Group. These relate to the occurrence of palaeobiological remains and the extensive development of carbon phyllites that may have an organic carbon source.
These are found in the chert, jasper, haematite and dolomite beds in the iron-ore formations of the Noamundi-Joda area of Orissa bordering Jharkhand State. Good exposures of stromatolytic dolomite are also found at the base of the iron and manganese formations at Kasia and Belkundi. The stromatolites may be of the stratiform, nodular and columnar types.
Stromatolites have also been recorded Bachra coalfield in North Karanpura coalfield of Jharkhand state. It has been found in Talchir Formation (Permo-Carboniferous). The rock types of Talchir formation in order of superposition comprise tilloides and boulder beds, green shales and varvites with stromatolites. Stromatolites have been recorded by CMPDIL organization for the first time in this area.
They were the dominant life form on Earth for over 2 billion years. Today they are nearly extinct, living a precarious existence in only a few localities worldwide.
Reference:
Mahadevan,T.M.(2002) Geology of Bihar and Jharkhand. Geological Society of India,Bangalore, India.
Pettijohn, E.J.(1984). Sedimentary Rocks, 3rd Edition. CBS Publishers & Distributors, New Delhi.
· http://www.fossilmall.com/Stromatolite.htm
· http://www.fossilmuseum.net/Tree_of_Life/Stromatolites.htm

Dr.Nitish Priyadarshi
Geologist
Rch_nitishp@sancharnet.in
Nitish.priyadarshi@gmail.com
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