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 occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay; the use of radiometric dating was first published in 1907 by Bertram Boltwood and is now the principal source of information about the absolute age of rocks and other geological features, including the age of fossilized life forms or the age of the Earth itself, can be used to date a wide range of natural and man-made materials. Together with stratigraphic principles, radiometric dating methods are used in geochronology to establish the geologic time scale. Among the best-known techniques are radiocarbon dating, potassium–argon dating and uranium–lead dating. 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 used to date archaeological materials, including ancient artifacts. Different methods of radiometric dating vary in the timescale over which they are accurate and the materials to which they can be applied. All ordinary matter is made up of combinations of chemical elements, each with its own atomic number, indicating the number of protons in the atomic nucleus. Additionally, elements may exist in different isotopes, with each isotope of an element differing in the number of neutrons in the nucleus. A particular isotope of a particular element is called a nuclide; some nuclides are inherently unstable. That is, at some point in time, an atom of such a nuclide will undergo radioactive decay and spontaneously transform into a different nuclide; this transformation may be accomplished in a number of different ways, including alpha decay and beta decay. Another possibility is spontaneous fission into two or more nuclides. While the moment in time at which a particular nucleus decays is unpredictable, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-life given in units of years when discussing dating techniques.
After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a "daughter" nuclide or decay product. In many cases, the daughter nuclide itself is radioactive, resulting in a decay chain ending with the formation of a stable daughter nuclide. In these cases the half-life of interest in radiometric dating is the longest one in the chain, the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter. Isotopic systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years to over 100 billion years. For most radioactive nuclides, the half-life depends on nuclear properties and is a constant, it is not affected by external factors such as temperature, chemical environment, or presence of a magnetic or electric field. The only exceptions are nuclides that decay by the process of electron capture, such as beryllium-7, strontium-85, zirconium-89, whose decay rate may be affected by local electron density.
For all other nuclides, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time. This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present; the basic equation of radiometric dating requires that neither the parent nuclide nor the daughter product can enter or leave the material after its formation. The possible confounding effects of contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created, it is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration. Precision is enhanced if measurements are taken on multiple samples from different locations of the rock body. Alternatively, if several different minerals can be dated from the same sample and are assumed to be formed by the same event and were in equilibrium with the reservoir when they formed, they should form an isochron.
This can reduce the problem of contamination. In uranium–lead dating, the concordia diagram is used which decreases the problem of nuclide loss. Correlation between different isotopic dating methods may be required to confirm the age of a sample. For example, the age of the Amitsoq gneisses from western Greenland was determined to be 3.6 ± 0.05 million years ago using uranium–lead dating and 3.56 ± 0.10 Ma using lead–lead dating, results that are consistent with each other. Accurate radiometric dating requires that the parent has a long enough half-life that it will be present in significant amounts at the time of measurement, the half-life of the parent is known, enough of the daughter product is produced to be measured and distinguished from the initial amount of the daughter present in the material; the procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate. This involves isotope-ratio mass spectrometry; the precision of
A fossil is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, exoskeletons, stone imprints of animals or microbes, objects preserved in amber, petrified wood, coal, DNA remnants; the totality of fossils is known as the fossil record. Paleontology is the study of fossils: their age, method of formation, evolutionary significance. Specimens are considered to be fossils if they are over 10,000 years old; the oldest fossils are around 3.48 billion years old to 4.1 billion years old. The observation in the 19th century that certain fossils were associated with certain rock strata led to the recognition of a geological timescale and the relative ages of different 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 and molds, authigenic mineralization and recrystallization, adpression and bioimmuration.
Fossils vary in size from one-micrometre bacteria to dinosaurs and trees, many meters long and weighing many tons. A fossil preserves only a portion of the deceased organism that portion, mineralized during life, such as the bones and teeth of vertebrates, or the chitinous or calcareous exoskeletons of invertebrates. Fossils may consist of the marks left behind by the organism while it was alive, such as animal tracks or feces; these types of fossil are called trace ichnofossils, as opposed to body fossils. Some fossils are called chemofossils or biosignatures; the process of fossilization varies according to external conditions. Permineralization is a process of fossilization; the empty spaces within an organism become filled with mineral-rich groundwater. Minerals precipitate from the groundwater; this process can occur in small spaces, such as within the cell wall of a plant cell. Small scale permineralization can produce 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 details of the fossil. Some fossils consist only of skeletal teeth; this is a form of diagenesis. In some cases, the original remains of the organism dissolve or are otherwise destroyed; the remaining organism-shaped hole in the rock is called an external mold. If this hole is 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 mold formation. If the chemistry is right, the organism can act as a nucleus for the precipitation of minerals such as siderite, resulting in a nodule forming around it. If this happens before significant decay to the organic tissue 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. In some cases mineral replacement of the original shell occurs so 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. What remains is a carbonaceous film known as a phytoleim, in which case the fossil is known as a compression. 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 and impressions occur together. For instance, when the rock is broken open, the phytoleim will be attached to one part, whereas the counterpart will just be an impression.
For this reason, one term covers the two modes of preservation: adpression. 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, the isolation of proteins and evidence for DNA fragments. In 2014, Mary Schweitzer and her colleagues reported the presence of iron particles associated with soft tissues recovered from dinosaur fossils. 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. However, a older study based on eight taxa ranging in time from the Devonian to the Jurassic found that reasonably well-preserved fibrils that represent collagen were preser
A calendar is a system of organizing days for social, commercial or administrative purposes. This is done by giving names to periods of time days, weeks and years. A date is the designation of a specific day within such a system. A calendar is a physical record of such a system. A calendar can mean a list of planned events, such as a court calendar or a or chronological list of documents, such as a calendar of wills. Periods in a calendar are though not synchronised with the cycle of the sun or the moon; the most common type of pre-modern calendar was the lunisolar calendar, a lunar calendar that adds one intercalary month to remain synchronised with the solar year over the long term. The term calendar is taken from calendae, the term for the first day of the month in the Roman calendar, related to the verb calare "to call out", referring to the "calling" of the new moon when it was first seen. Latin calendarium meant "account book, register"; the Latin term was adopted in Old French as calendier and from there in Middle English as calender by the 13th century.
A calendar can be on paper or electronic device. The course of the sun and the moon are the most salient natural recurring events useful for timekeeping, thus in pre-modern societies worldwide lunation and the year were most used as time units; the Roman calendar contained remnants of a ancient pre-Etruscan 10-month solar year. The first recorded physical calendars, dependent on the development of writing in the Ancient Near East, are the Bronze Age Egyptian and Sumerian calendars. A large number of Ancient Near East calendar systems based on the Babylonian calendar date from the Iron Age, among them the calendar system of the Persian Empire, which in turn gave rise to the Zoroastrian calendar and the Hebrew calendar. A great number of Hellenic calendars developed in Classical Greece, in the Hellenistic period gave rise to both the ancient Roman calendar and to various Hindu calendars. Calendars in antiquity were lunisolar, depending on the introduction of intercalary months to align the solar and the lunar years.
This was based on observation, but there may have been early attempts to model the pattern of intercalation algorithmically, as evidenced in the fragmentary 2nd-century Coligny calendar. The Roman calendar was reformed by Julius Caesar in 45 BC; the Julian calendar was no longer dependent on the observation of the new moon but followed an algorithm of introducing a leap day every four years. This created a dissociation of the calendar month from the lunation; the Islamic calendar is based on the prohibition of intercalation by Muhammad, in Islamic tradition dated to a sermon held on 9 Dhu al-Hijjah AH 10. This resulted in an observation-based lunar calendar that shifts relative to the seasons of the solar year; the first calendar reform of the early modern era was the Gregorian calendar, introduced in 1582 based on the observation of a long-term shift between the Julian calendar and the solar year. There have been a number of modern proposals for reform of the calendar, such as the World Calendar, International Fixed Calendar, Holocene calendar, the Hanke-Henry Permanent Calendar.
Such ideas are mooted from time to time but have failed to gain traction because of the loss of continuity, massive upheaval in implementation, religious objections. A full calendar system has a different calendar date for every day, thus the week cycle is by itself not a full calendar system. The simplest calendar system just counts time periods from a reference date; this applies for Unix Time. The only possible variation is using a different reference date, in particular, one less distant in the past to make the numbers smaller. Computations in these systems are just a matter of subtraction. Other calendars have one larger units of time. Calendars that contain one level of cycles: week and weekday – this system is not common year and ordinal date within the year, e.g. the ISO 8601 ordinal date systemCalendars with two levels of cycles: year and day – most systems, including the Gregorian calendar, the Islamic calendar, the Solar Hijri calendar and the Hebrew calendar year and weekday – e.g. the ISO week dateCycles can be synchronized with periodic phenomena: Lunar calendars are synchronized to the motion of the Moon.
Solar calendars are based on perceived seasonal changes synchronized to the apparent motion of the Sun. Lunisolar calendars are based on a combination of both solar and lunar reckonings; the week cycle is an example of one, not synchronized to any external phenomenon. A calendar includes more than one type of cycle, or has both cyclic and non-cyclic elements. Most calendars incorporate more complex cycles. For example, the vast majority of them track years, months and days; the seven-day week is universal, though its use varies. It has run uninterrupted for millennia. Solar calendars assign a date to each solar day. A day may consist of the period between sunrise and sunset, with
Uranium–lead dating, abbreviated U–Pb dating, is one of the oldest and most refined of the radiometric dating schemes. It can be used to date rocks that formed and crystallised from about 1 million years to over 4.5 billion years ago with routine precisions in the 0.1–1 percent range. The dating method is performed on the mineral zircon; the mineral incorporates uranium and thorium atoms into its crystal structure, but rejects lead. Therefore, one can assume that the entire lead content of the zircon is radiogenic, i.e. it is produced by a process of radioactive decay after the formation of the mineral. Thus the current ratio of lead to uranium in the mineral can be used to determine its age; the method relies on two separate decay chains, the uranium series from 238U to 206Pb, with a half-life of 4.47 billion years and the actinium series from 235U to 207Pb, with a half-life of 710 million years. The above uranium to lead decay routes occur via a series of alpha decays, in which 238U with daughter nuclides undergo total eight alpha and six beta decays whereas 235U with daughters only experience seven alpha and four beta decays.
The existence of two'parallel' uranium–lead decay routes leads to multiple dating techniques within the overall U–Pb system. The term U–Pb dating implies the coupled use of both decay schemes in the'concordia diagram'. However, use of a single decay scheme leads to the U–Pb isochron dating method, analogous to the rubidium–strontium dating method. Ages can be determined from the U–Pb system by analysis of Pb isotope ratios alone; this is termed the lead–lead dating method. Clair Cameron Patterson, an American geochemist who pioneered studies of uranium–lead radiometric dating methods, is famous for having used it to obtain one of the earliest estimates of the age of the Earth. Although zircon is most used, other minerals such as monazite and baddeleyite can be used. Where crystals such as zircon with uranium and thorium inclusions do not occur, a better, more inclusive, model of the data must be applied. Uranium-lead dating techniques have been applied to other minerals such as calcite/aragonite and other carbonate minerals.
These types of minerals produce lower precision ages than igneous and metamorphic minerals traditionally used for age dating, but are more common in the geologic record. During the alpha decay steps, the zircon crystal experiences radiation damage, associated with each alpha decay; this damage is most concentrated around the parent isotope, expelling the daughter isotope from its original position in the zircon lattice. In areas with a high concentration of the parent isotope, damage to the crystal lattice is quite extensive, will interconnect to form a network of radiation damaged areas. Fission tracks and micro-cracks within the crystal will further extend this radiation damage network; these fission tracks act as conduits deep within the crystal, thereby providing a method of transport to facilitate the leaching of lead isotopes from the zircon crystal. Under conditions where no lead loss or gain from the outside environment has occurred, the age of the zircon can be calculated by assuming exponential decay of Uranium.
That is N N o w = N. N O r i g is the number of uranium atoms - equal to the sum of uranium and lead atoms U + P b measured now. Λ = λ U is the decay rate of Uranium. T is the age of the zircon; this gives U = e − λ U t, which can be written as P b U = e λ U t − 1. The more used decay chains of Uranium and Lead gives the following equations: These are said to yield concordant ages, it is these concordant ages, plotted over a series of time intervals, that result in the concordant line. Loss of lead from the sample will result in a discrepancy in the ages determined by each decay scheme; this effect is referred to as discordance and is demonstrated in Figure 1. If a series of zircon samples has lost different amounts of lead, the samples generate a discordant line; the upper intercept of the concordia and the discordia line will reflect the original age of formation, while the lower intercept will reflect the age of the event that led to open system behavior and therefore the lead loss. Undamaged zircon retains the lead generated by radioactive decay of uranium and thorium until ver
Electron paramagnetic resonance
Electron paramagnetic resonance or electron spin resonance spectroscopy is a method for studying materials with unpaired electrons. The basic concepts of EPR are analogous to those of nuclear magnetic resonance, but it is electron spins that are excited instead of the spins of atomic nuclei. EPR spectroscopy is useful for studying metal complexes or organic radicals. EPR was first observed in Kazan State University by Soviet physicist Yevgeny Zavoisky in 1944, was developed independently at the same time by Brebis Bleaney at the University of Oxford; every electron has a magnetic moment and spin quantum number s = 1 2, with magnetic components m s = + 1 2 and m s = − 1 2. In the presence of an external magnetic field with strength B 0, the electron's magnetic moment aligns itself either parallel or antiparallel to the field, each alignment having a specific energy due to the Zeeman effect: E = m s g e μ B B 0, where g e is the electron's so-called g-factor, g e = 2.0023 for the free electron, μ B is the Bohr magneton.
Therefore, the separation between the lower and the upper state is Δ E = g e μ B B 0 for unpaired free electrons. This equation implies that the splitting of the energy levels is directly proportional to the magnetic field's strength, as shown in the diagram below. An unpaired electron can move between the two energy levels by either absorbing or emitting a photon of energy h ν such that the resonance condition, h ν = Δ E, is obeyed; this leads to the fundamental equation of EPR spectroscopy: h ν = g e μ B B 0. Experimentally, this equation permits a large combination of frequency and magnetic field values, but the great majority of EPR measurements are made with microwaves in the 9000–10000 MHz region, with fields corresponding to about 3500 G. Furthermore, EPR spectra can be generated by either varying the photon frequency incident on a sample while holding the magnetic field constant or doing the reverse. In practice, it is the frequency, kept fixed. A collection of paramagnetic centers, such as free radicals, is exposed to microwaves at a fixed frequency.
By increasing an external magnetic field, the gap between the m s = + 1 2 and m s = − 1 2 energy states is widened until it matches the energy of the microwaves, as represented by the double arrow in the diagram above. At this point the unpaired electrons can move between their two spin states. Since there are more electrons in the lower state, due to the Maxwell–Boltzmann distribution, there is a net absorption of energy, it is this absorption, monitored and converted into a spectrum; the upper spectrum below is the simulated absorption for a system of free electrons in a varying magnetic field. The lower spectrum is the first derivative of the absorption spectrum; the latter is the most common way to publish continuous wave EPR spectra. For the microwave frequency of 9388.2 MHz, the predicted resonance occurs at a magnetic field of about B 0 = h ν / g e μ B = 0.3350 teslas = 3350 gausses. Because of electron-nuclear mass differences, the magnetic moment of an electron is larger than the corresponding quantity for any nucleus, so that a much higher electromagnetic frequency is needed to bring about a spin resonance with an electron than with a nucleus, at identical magnetic field strengths.
For example, for the field of 3350 G shown at the right, spin resonance occurs near 9388.2 MHz for an electron compared to only about 14.3 MHz for 1H nuclei. (For NMR spectroscopy, the corresponding resonance equation is h ν = g N μ N B 0 where g N and
History of Earth
The history of Earth concerns the development of planet Earth from its formation to the present day. Nearly all branches of natural science have contributed to understanding of the main events of Earth's past, characterized by constant geological change and biological evolution; the geological time scale, as defined by international convention, depicts the large spans of time from the beginning of the Earth to the present, its divisions chronicle some definitive events of Earth history. Earth formed around 4.54 billion years ago one-third the age of the universe, by accretion from the solar nebula. Volcanic outgassing created the primordial atmosphere and the ocean, but the early atmosphere contained no oxygen. Much of the Earth was molten because of frequent collisions with other bodies which led to extreme volcanism. While the Earth was in its earliest stage, a giant impact collision with a planet-sized body named Theia is thought to have formed the Moon. Over time, the Earth cooled, causing the formation of a solid crust, allowing liquid water on the surface.
The Hadean eon represents the time before a reliable record of life. The following Archean and Proterozoic eons produced the beginnings of life on Earth and its earliest evolution; the succeeding eon is the Phanerozoic, divided into three eras: the Palaeozoic, an era of arthropods and the first life on land. Recognizable humans emerged at most 2 million years ago, a vanishingly small period on the geological scale; the earliest undisputed evidence of life on Earth dates at least from 3.5 billion years ago, during the Eoarchean Era, after a geological crust started to solidify following the earlier molten Hadean Eon. There are microbial mat fossils such as stromatolites found in 3.48 billion-year-old sandstone discovered in Western Australia. Other early physical evidence of a biogenic substance is graphite in 3.7 billion-year-old metasedimentary rocks discovered in southwestern Greenland as well as "remains of biotic life" found in 4.1 billion-year-old rocks in Western Australia. According to one of the researchers, "If life arose quickly on Earth … it could be common in the universe."Photosynthetic organisms appeared between 3.2 and 2.4 billion years ago and began enriching the atmosphere with oxygen.
Life remained small and microscopic until about 580 million years ago, when complex multicellular life arose, developed over time, culminated in the Cambrian Explosion about 541 million years ago. This sudden diversification of life forms produced most of the major phyla known today, divided the Proterozoic Eon from the Cambrian Period of the Paleozoic Era, it is estimated that 99 percent of all species that lived on Earth, over five billion, have gone extinct. Estimates on the number of Earth's current species range from 10 million to 14 million, of which about 1.2 million are documented, but over 86 percent have not been described. However, it was claimed that 1 trillion species live on Earth, with only one-thousandth of one percent described; the Earth's crust has changed since its formation, as has life has since its first appearance. Species continue to evolve, taking on new forms, splitting into daughter species, or going extinct in the face of ever-changing physical environments; the process of plate tectonics continues to shape the Earth's continents and oceans and the life they harbor.
Human activity is now a dominant force affecting global change, harming the biosphere, the Earth's surface and atmosphere with the loss of wild lands, over-exploitation of the oceans, production of greenhouse gases, degradation of the ozone layer, general degradation of soil and water quality. In geochronology, time is measured in mya, each unit representing the period of 1,000,000 years in the past; the history of Earth is divided into four great eons, starting 4,540 mya with the formation of the planet. Each eon saw the most significant changes in Earth's composition and life; each eon is subsequently divided into eras, which in turn are divided into periods, which are further divided into epochs. The history of the Earth can be organized chronologically according to the geologic time scale, split into intervals based on stratigraphic analysis; the following four timelines show the geologic time scale. The first shows the entire time from the formation of the Earth to the present, but this gives little space for the most recent eon.
Therefore, the second timeline shows an expanded view of the most recent eon. In a similar way, the most recent era is expanded in the third timeline, the most recent period is expanded in the fourth timeline; the standard model for the formation of the Solar System is the solar nebula hypothesis. In this model, the Solar System formed from a large, rotating cloud of interstellar dust and gas called the solar nebula, it was composed of hydrogen and helium created shortly after the Big Bang 13.8 Ga and heavier elements ejected by supernovae. About 4.5 Ga, the nebula began a contraction that may have been triggered by the shock wave from a nearby supernova. A shock wave would have made the nebula rotate; as the cloud began to accelerate, its angular momentum and inertia flattened it into a protoplanetary disk perpendicular to its axis of rotation. Small perturbations due to collisions and the angular momentum of other large debris