Fossil dating methods wikipedia

Radiometric dating , radioactive dating or radioisotope dating is a technique used to date materials such as rocks or carbon , in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay. Together with stratigraphic principles , radiometric dating methods are used in geochronology to establish the geologic time scale. By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change. Radiometric dating is also used to date archaeological materials, including ancient artifacts.

Radiocarbon dating

A fossil from Classical Latin fossilis ; literally, "obtained by digging" [1] is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons , stone imprints of animals or microbes , objects preserved in amber, hair, petrified wood , oil, coal, and DNA remnants. The totality of fossils is known as the fossil record.

Paleontology is the study of fossils: The development of radiometric dating techniques in the early 20th century allowed scientists to quantitatively measure the absolute ages of rocks and the fossils they host. There are many processes that lead to fossilization, including permineralization , casts and molds, authigenic mineralization , replacement and recrystallization, adpression, carbonization , and bioimmuration.

A fossil normally preserves only a portion of the deceased organism, usually that portion that was partially mineralized during life, such as the bones and teeth of vertebrates , or the chitinous or calcareous exoskeletons of invertebrates. Fossils may also consist of the marks left behind by the organism while it was alive, such as animal tracks or feces coprolites. These types of fossil are called trace fossils or ichnofossils , as opposed to body fossils.

Some fossils are biochemical and are called chemofossils or biosignatures. Permineralization is a process of fossilization that occurs when an organism is buried. The empty spaces within an organism spaces filled with liquid or gas during life become filled with mineral-rich groundwater. Minerals precipitate from the groundwater, occupying the empty spaces.

This process can occur in very small spaces, such as within the cell wall of a plant cell. Small scale permineralization can produce very detailed fossils. For permineralization to occur, the organism must become covered by sediment soon after death, otherwise decay commences. The degree to which the remains are decayed when covered determines the later details of the fossil. Some fossils consist only of skeletal remains or teeth; other fossils contain traces of skin , feathers or even soft tissues.

This is a form of diagenesis. In some cases, the original remains of the organism completely dissolve or are otherwise destroyed. The remaining organism-shaped hole in the rock is called an external mold. If this hole is later filled with other minerals, it is a cast. An endocast , or internal mold , is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail or the hollow of a skull.

This is a special form of cast and mold formation. If the chemistry is right, the organism or fragment of organism can act as a nucleus for the precipitation of minerals such as siderite, resulting in a nodule forming around it. If this happens rapidly before significant decay to the organic tissue, very fine three-dimensional morphological detail can be preserved. Nodules from the Carboniferous Mazon Creek fossil beds of Illinois, USA, are among the best documented examples of such mineralization.

Replacement occurs when the shell, bone or other tissue is replaced with another mineral. In some cases mineral replacement of the original shell occurs so gradually and at such fine scales that microstructural features are preserved despite the total loss of original material. A shell is said to be recrystallized when the original skeletal compounds are still present but in a different crystal form, as from aragonite to calcite.

Compression fossils , such as those of fossil ferns, are the result of chemical reduction of the complex organic molecules composing the organism's tissues. In this case the fossil consists of original material, albeit in a geochemically altered state. This chemical change is an expression of diagenesis. Often what remains is a carbonaceous film known as a phytoleim, in which case the fossil is known as a compression.

Often, however, the phytoleim is lost and all that remains is an impression of the organism in the rock—an impression fossil. In many cases, however, compressions and impressions occur together. For instance, when the rock is broken open, the phytoleim will often be attached to one part compression , whereas the counterpart will just be an impression. For this reason, one term covers the two modes of preservation: Because of their antiquity, an unexpected exception to the alteration of an organism's tissues by chemical reduction of the complex organic molecules during fossilization has been the discovery of soft tissue in dinosaur fossils, including blood vessels, and the isolation of proteins and evidence for DNA fragments.

Based on various experiments that studied the interaction of iron in haemoglobin with blood vessel tissue they proposed that solution hypoxia coupled with iron chelation enhances the stability and preservation of soft tissue and provides the basis for an explanation for the unforeseen preservation of fossil soft tissues. Carbonaceous films are thin coatings which consist predominantly of the chemical element carbon. The soft tissues of organisms are made largely of organic carbon compounds and during diagenesis under reducing conditions only a thin film of carbon residue is left which forms a silhouette of the original organism.

Bioimmuration occurs when a skeletal organism overgrows or otherwise subsumes another organism, preserving the latter, or an impression of it, within the skeleton. Sometimes the bioimmured organism is soft-bodied and is then preserved in negative relief as a kind of external mold. There are also cases where an organism settles on top of a living skeletal organism that grows upwards, preserving the settler in its skeleton. Bioimmuration is known in the fossil record from the Ordovician [18] to the Recent.

Paleontology seeks to map out how life evolved across geologic time. A substantial hurdle is the difficulty of working out fossil ages. Beds that preserve fossils typically lack the radioactive elements needed for radiometric dating. Radioactive elements are common only in rocks with a volcanic origin, and so the only fossil-bearing rocks that can be dated radiometrically are volcanic ash layers, which may provide termini for the intervening sediments.

Consequently, palaeontologists rely on stratigraphy to date fossils. Stratigraphy is the science of deciphering the "layer-cake" that is the sedimentary record. If a fossil is found between two layers whose ages are known, the fossil's age is claimed to lie between the two known ages. However, fossils of species that survived for a relatively short time can be used to match isolated rocks: For instance, the conodont Eoplacognathus pseudoplanus has a short range in the Middle Ordovician period.

Such index fossils must be distinctive, be globally distributed and occupy a short time range to be useful. Misleading results are produced if the index fossils are incorrectly dated. However, this is difficult for some time periods, because of the problems involved in matching rocks of the same age across continents. It is also possible to estimate how long ago two living clades diverged, in other words approximately how long ago their last common ancestor must have lived, by assuming that DNA mutations accumulate at a constant rate.

These " molecular clocks ", however, are fallible, and provide only approximate timing: Organisms are only rarely preserved as fossils in the best of circumstances, and only a fraction of such fossils have been discovered. The transition itself can only be illustrated and corroborated by transitional fossils, which will never demonstrate an exact half-way point. The fossil record is strongly biased toward organisms with hard-parts, leaving most groups of soft-bodied organisms with little to no role.

These formations may have resulted from carcass burial in an anoxic environment with minimal bacteria, thus slowing decomposition. Stromatolites are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms , especially cyanobacteria. Stromatolites were much more abundant in Precambrian times. While older, Archean fossil remains are presumed to be colonies of cyanobacteria , younger that is, Proterozoic fossils may be primordial forms of the eukaryote chlorophytes that is, green algae.

One genus of stromatolite very common in the geologic record is Collenia. The earliest stromatolite of confirmed microbial origin dates to 2. A discovery provides strong evidence of microbial stromatolites extending as far back as 3. Stromatolites are a major constituent of the fossil record for life's first 3. The most widely supported explanation is that stromatolite builders fell victims to grazing creatures the Cambrian substrate revolution , implying that sufficiently complex organisms were common over 1 billion years ago.

The connection between grazer and stromatolite abundance is well documented in the younger Ordovician evolutionary radiation ; stromatolite abundance also increased after the end-Ordovician and end-Permian extinctions decimated marine animals, falling back to earlier levels as marine animals recovered. Factors such as the chemistry of the environment may have been responsible for changes.

While prokaryotic cyanobacteria themselves reproduce asexually through cell division, they were instrumental in priming the environment for the evolutionary development of more complex eukaryotic organisms. Cyanobacteria as well as extremophile Gammaproteobacteria are thought to be largely responsible for increasing the amount of oxygen in the primeval earth's atmosphere through their continuing photosynthesis. Cyanobacteria use water , carbon dioxide and sunlight to create their food.

A layer of mucus often forms over mats of cyanobacterial cells. In modern microbial mats, debris from the surrounding habitat can become trapped within the mucus, which can be cemented by the calcium carbonate to grow thin laminations of limestone. These laminations can accrete over time, resulting in the banded pattern common to stromatolites. The domal morphology of biological stromatolites is the result of the vertical growth necessary for the continued infiltration of sunlight to the organisms for photosynthesis.

Layered spherical growth structures termed oncolites are similar to stromatolites and are also known from the fossil record. Thrombolites are poorly laminated or non-laminated clotted structures formed by cyanobacteria common in the fossil record and in modern sediments. The Zebra River Canyon area of the Kubis platform in the deeply dissected Zaris Mountains of southwestern Namibia provides an extremely well exposed example of the thrombolite-stromatolite-metazoan reefs that developed during the Proterozoic period, the stromatolites here being better developed in updip locations under conditions of higher current velocities and greater sediment influx.

Index fossils also known as guide fossils, indicator fossils or zone fossils are fossils used to define and identify geologic periods or faunal stages. They work on the premise that, although different sediments may look different depending on the conditions under which they were deposited, they may include the remains of the same species of fossil. The shorter the species' time range, the more precisely different sediments can be correlated, and so rapidly evolving species' fossils are particularly valuable.

The best index fossils are common, easy to identify at species level and have a broad distribution—otherwise the likelihood of finding and recognizing one in the two sediments is poor. Trace fossils consist mainly of tracks and burrows, but also include coprolites fossil feces and marks left by feeding. Many traces date from significantly earlier than the body fossils of animals that are thought to have been capable of making them.

Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour in this case, diet rather than morphology. They were first described by William Buckland in Prior to this they were known as "fossil fir cones " and " bezoar stones. A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group.

Because of the incompleteness of the fossil record, there is usually no way to know exactly how close a transitional fossil is to the point of divergence. These fossils serve as a reminder that taxonomic divisions are human constructs that have been imposed in hindsight on a continuum of variation. Microfossil is a descriptive term applied to fossilized plants and animals whose size is just at or below the level at which the fossil can be analyzed by the naked eye.

Microfossils may either be complete or near-complete organisms in themselves such as the marine plankters foraminifera and coccolithophores or component parts such as small teeth or spores of larger animals or plants. Microfossils are of critical importance as a reservoir of paleoclimate information, and are also commonly used by biostratigraphers to assist in the correlation of rock units.

Radiometric dating, radioactive dating or radioisotope dating is a technique used to date Together with stratigraphic principles, radiometric dating methods are used in geochronology to establish the geologic time scale. Among source of information about the ages of fossils and the deduced rates of evolutionary change. With the exception of the radiocarbon method, most of these to date speleothems, corals, carbonates, and fossil bones.

Chronological dating , or simply dating , is the process of attributing to an object or event a date in the past, allowing such object or event to be located in a previously established chronology. This usually requires what is commonly known as a "dating method". Several dating methods exist, depending on different criteria and techniques, and some very well known examples of disciplines using such techniques are, for example, history , archaeology , geology , paleontology , astronomy and even forensic science , since in the latter it is sometimes necessary to investigate the moment in the past in which the death of a cadaver occurred.

Geochronology is the science of determining the age of rocks , fossils , and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is provided by tools such as palaeomagnetism and stable isotope ratios.

Radiocarbon dating also referred to as carbon dating or carbon dating is a method for determining the age of an object containing organic material by using the properties of radiocarbon , a radioactive isotope of carbon. The method was developed in the late s by Willard Libby , who received the Nobel Prize in Chemistry for his work in It is based on the fact that radiocarbon 14 C is constantly being created in the atmosphere by the interaction of cosmic rays with atmospheric nitrogen.

Chronological dating

In the field of Geology, dating is an important term as it is a technique through which evaluation regarding the age and period about the fossil, remains, the archaeologists do valuables and artifacts. At first, there were not many methods of dating were available, but now with advancement in the technology, we mainly have two types of the techniques to ascertain ages of ancient belongings. Relative Dating and Absolute Dating are two types of such techniques which are under practice to determine the age of the fossils, objects or civilizations. The relative dating is the technique in the Geology through which the age is determined with relation to the other objects. In other words, we can say that in relative dating the archaeologist determines that which of the two fossil or the artifacts are older.

Difference Between Relative Dating vs. Absolute Dating

Carbon dating , also known as radiocarbon dating, is a scientific procedure used to date organic matter. It depends upon the radioactive decay of carbon 14 C , an unstable isotope of carbon which is continually synthesized in the upper atmosphere by cosmic rays. Plants take up atmospheric 14 C for as long as they live, through the process of photosynthesis. Animals take up atmospheric 14 C indirectly, by eating plants or by eating other animals that eat plants. Measuring the proportion of 14 C as opposed to 12 C remaining in a sample then tells us how long ago the sample stopped taking up 14 C — in other words, how long ago the thing died. Carbon dating has a certain margin of error, usually depending on the age and material of the sample used. Carbon has a half-life of about years, so researchers use the process to date biological samples up to about 60, years in the past. Beyond that timespan, the amount of the original 14 C remaining is so small that it cannot be reliably distinguished from 14 C formed by irradiation of nitrogen by neutrons from the spontaneous fission of uranium, present in trace quantities almost everywhere.

Absolute dating is the process of determining an age on a specified chronology in archaeology and geology.

A fossil from Classical Latin fossilis ; literally, "obtained by digging" [1] is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons , stone imprints of animals or microbes , objects preserved in amber, hair, petrified wood , oil, coal, and DNA remnants. The totality of fossils is known as the fossil record.

Absolute dating

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Carbon dating

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Dating Fossils – How Are Fossils Dated?

Сьюзан рассеянно подняла на нее глаза, безучастная к царившему вокруг нее безумию. Все в комнате дружно повернули головы. Диаграмма чем-то напоминала бычий глаз. В центре находился красный кружок с надписью БАЗА, вокруг которого располагались пять концентрических окружностей разной толщины и разного цвета. Внешняя окружность была затуманена и казалась почти прозрачной.

- У нас имеется пять уровней защиты, - объяснял Джабба.  - Главный бастион, два набора пакетных фильтров для Протокола передачи файлов, Х-одиннадцать, туннельный блок и, наконец, окно авторизации справа от проекта Трюфель.

Radiometric dating

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Извините, меня нет дома, но если вы оставите свое сообщение… Беккер выслушал все до конца. Где же. Наверняка Сьюзан уже начала волноваться. Уж не уехала ли она в Стоун-Мэнор без. Раздался сигнал, после которого надо было оставить сообщение. - Привет, это Дэвид.  - Он замолчал, не зная, что сказать .

Кровать застонала под его весом.  - Простите. Беккер вытащил из вазы, стоявшей на столике в центре комнаты, розу и небрежно поднес ее к носу, потом резко повернулся к немцу, выпустив розу из рук. - Что вы можете рассказать про убийство. Немец побелел. - Mord. Убийство.

FOSSILS: how fossils are dated
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