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Fossil, a trace of a long-past form of life and thus the first indication of the occurrence of unimaginably long ►stretches of time prior to the human era. Taphonomy is the science that deals with the processes involved in fossilization. Taphonomy distinguishes between different kinds of fossilization processes corresponding to the different kinds of existing fossils. Fossilized Bacterial Mats (Stromatolites) Stromatolites are the oldest known fossils and can be found in rock formations dating back up to 3.4 billion years (for example, in Western Australia). They consist of fine limestone laminations that produce cauliflower-shaped structures with conical, columnar, or wavy morphologies. Limestone deposits are mainly formed by the metabolic processes occurring in cyanobacterial mats (blue-green algae). Recent research indicates that other early life forms -- archaeans, eubacteria, and eukaryotes -- can also have formed stromatolites. The metabolic activities of the bacterial mats -- consumption of calcarious water, carbon dioxide, and sunlight -- produce oxygen and calcium carbonate. The latter produces the limestone lamination. Stromatolites played an important part in the development of life on this Earth; they formed reefs long before there were corals and substantially contributed to the release of basic oxygen into the (once oxygen-free) Earth's atmosphere. Even today, we can encounter living stromatolites in hypersaline waters that are low in oxygen -- for example, in the shallow lagoons of Western Australia. Fossilization of Vertebrates (Petrification) Fossilization can occur when a dead organism is embedded in a decomposition-resistant substrate before it has completely decomposed. This is why you can find fossils only in those specific locations on Earth that contain such a substrate: Airtight substrates that are washed ashore, such as mud and silt, are especially advantageous for fossilization of embedded organisms. Pure sand deposits (sandstone) rarely contain fossils as these are usually destroyed in the process of diagenesis. Diagenesis is the geological process that unconsolidated sediments (clay, silt, sand, limestone deposits) undergo to form sedimentary rocks. Continuous addition of new material builds up sediment layers. The increasing number of cover layers exerts more and more pressure on the bottom layers and raises their temperature, causing densification and dehydration. The unconsolidated material solidifies through dissolution of minerals (especially carbonates and limestone) and crystallization. Thus loose sand eventually turns into solid sandstone, just as limestone turns into chert through silicification and formation of quartz crystals. The fossilizing organism decays very slowly during the diagenetic process due to the exclusion of air; the result is a vug in the surrounding rock that, over time, fills up with infiltrating minerals or mineral precipitations -- calcite, dolomite, fluorite, hematite, silica deposits -- from infiltrating solutions. These produce rock or quartz cores as fossils.
Depending on the nature of the substance, the organic material migrating from the fossil into its surrounding rock environment may remain there and continue to provide the rock with carbon compounds. Sometimes, even residues of bone fragments consisting primarily of calcium compounds (calcium phosphates) remain. In rare cases, if the organism was embedded in bitumen or soft silt (which can produce extremely durable fossils), even soft tissue may be preserved. Petrified Wood Most petrified woods are products of silicification. The process begins upon embedding of the wood in sediment strata; in time, first the cell interstices and vugs and subsequently even the cell walls are filled up with silica compounds. Silica (silica dioxide) is a component of sedimentary rock, especially volcano ashes. In petrification, all organic material is gradually replaced by silica dioxide. Eventually, the silica crystallizes into quartz (chalcedony). Apart from silicification, which is one of the most common petrification processes, there are also other processes by which woods can be "fossilized"; these involve other kinds of minerals. Inclusions in Fossil Resin (Amber and Copal) Amber is fossilized plant resin that sometimes contains inclusions: insects, plant residues or small vertebrates (lizards). These are trapped in the sticky liquid and are subsequently enclosed by the flow of additional resin. Then begins the process of decomposition of the soft tissue in which muscles, adenoids, and body liquids escape through the body wall and cavities. This is why the environment of the inclusions often is of a milky-white color. Of the inclusions themselves there usually remains only a vug lined with carbon patina -- the remainder of the original organism. We seldom find tissue residues inside an inclusion. Fossil resins are the only kind of fossils whose age can be directly estimated independently of their locations. The formation of fossil resins takes about 10 million years, during which time the non-permanent components (oils) of the resin disappear and the hydrocarbon chains undergo polymerization. Oxygen and ultraviolet rays break fossil resin down over time, so that its maximum age is about 260 million years. Newer tree resin that is not yet fully fossilized is called copal; it is much lower-priced on the market. In copal, polymerization has set in but some non-permanent oils still remain. Just like regular fossil resin, copal can contain inclusions; these, however, date back no further than a few thousand to a few million years. Chemical methods (solvents, gas chromatograph) are commonly used to distinguish between amber (ancient fossil resin) and copal. Fossil Age Determination Usually it is not possible to determine the age of a fossil directly. However, fossil age can be derived from the age of the embedding sediment layers. There are several dozen methods available for doing this, some of which give us absolute and others only relative (comparative) results. The most important of these are: ► Varve chronology: Absolute age determination method for younger fossils embedded in sediment strata formed at the bottoms of lakes. Annual snow melt here produces a characteristic varve pattern in the sediments not unlike the pattern of tree rings. In various European lakes these varve patterns have been dated back as far as 75,000 years. ► Luminescence dating: Absolute age determination method in which the rock is irradiated with light rays of a certain wavelength and the resulting afterglow (luminescence) measured. Since the rock's luminescence capacity is reset by the sunlight and subsequently rebuilds only gradually, we can tell by measuring the residual luminescence when the rock was last exposed to sunlight. This method can be used to determine ages up to 500,000 years. ► Magnetostratigraphy: Relative age determination method that takes advantage of the geomagnetic field's sporadic reversals in polarity. About two thousand of these reversals occur every billion years. The polarities are imprinted on magnetic minerals (magnetite, hematite) during diagenesis. This produces a characteristic polarity reversal pattern in the sediment strata, which can be precisely analyzed in a way comparable to tree ring analysis up to an age of approximately 100,000 years. ► Global catastrophes: Relative age determination method relying on the lasting effects of catastrophes. Events such as volcano eruptions or meteorite impacts produce characteristic inclusions in the sediment. These serve to identify sediments with regard to their age. For example, an iridium layer that was produced by the impact of a large meteorite approximately 65 million years ago can be found in all sedimentary rocks of the time. ► Radiometric dating: Absolute age determination method relying on evidence of radioactive decay. Radioactive elements such as uranium decay over a precisely known ►half life period into daughter isotopes (lead). This is used to determine the date of origin of the mineral by measuring the mass ratios of parent and daughter isotopes in radioactive minerals. Depending on the isotopes used in the process, any of several dating methods may be used. The uranium-lead method provides the most precise results for zircon crystals as these do not consume any external lead, so that inaccurate measurements resulting from the presence of impurities are prevented. Radiometric dating produces results with an accuracy of up to one percent for ages up to 4.5 billion years.
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