Geochronology is the science of applying dates in the past to rocks. These rocks receive dates because they contain constituents that can be used as chronometers.
Geologic time[edit | edit source]
On the right is a geologic clock representation. It shows some of the major units of geological time and definitive events of Earth history. The Hadean eon represents the time before fossil record of life on Earth; its upper boundary is now regarded as 4.0 Ga (billion years ago). Other subdivisions reflect the evolution of life; the Archean and Proterozoic are both eons, the Palaeozoic, Mesozoic and Cenozoic are eras of the Phanerozoic eon. The two million year Quaternary period, the time of recognizable humans, is too small to be visible at this scale.
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 compresses the most recent eon. Therefore the second scale shows the most recent eon with an expanded scale. The second scale compresses the most recent era, so the most recent era is expanded in the third scale. Since the Quaternary is a very short period with short epochs, it is further expanded in the fourth scale. The second, third, and fourth timelines are therefore each subsections of their preceding timeline as indicated by asterisks. The Holocene (the latest epoch) is too small to be shown clearly on the third timeline on the right, another reason for expanding the fourth scale. The Pleistocene (P) epoch. Q stands for the Quaternary period.
|Segments of rock (strata) in chronostratigraphy||Time spans in geochronology||Notes to|
|Supereonothem||Supereon||1 total, four billion years or more (e.g. Precambrian)|
|Eonothem||Eon||4 total, half a billion years or more|
|Erathem||Era||10 defined, several hundred million years|
|System||Period||22 defined, tens to ~one hundred million years|
|Series||Epoch||34 defined, tens of millions of years|
|Stage||Age||99 defined, millions of years|
|Chronozone||Chron||subdivision of an age, not used by the ICS timescale|
Notations[edit | edit source]
- ALMA represent the Asian Land Mammal Age,
- b2k represent before AD 2000,
- BP represent before present, as the chart is for 2008, this may require an added -8 for b2k,
- ELMMZ represent the European Land Mammal Mega Zone,
- FAD represent first appearance datum,
- FO represent first occurrence,
- Ga represent Gegaannum, billion years ago, or -109 b2k,
- GICC05 represent Greenland Ice Core Chronology 2005,
- GRIP represent Greenland Ice Core Project,
- GSSP represent Global Stratotype Section and Point,
- HO represent highest occurrence,
- ICS represent the International Commission on Stratigraphy,
- IUGS represent the International Union of Geological Sciences,
- LAD represent last appearance datum,
- LO represent lowest occurrence,
- Ma represent Megaannum, million years ago, or -106 b2k,
- NALMA represent the North American Land Mammal Age,
- NGRIP represent North Greenland Ice Core Project, and
- SALMA represent South American Land Mammal Age.
"The term b2 k [b2k] refers to the ice-core zero age of AD 2000; note that this is 50 years different from the zero yr for radiocarbon, which is AD 1950 [...]."
Stratigraphy[edit | edit source]
Orbitally forced cyclicity[edit | edit source]
"Chemical and physical proxies from sedimentary rock sequences are frequently used for palaeoclimatic studies and for detecting orbitally forced cyclicity in marine Cenozoic sequences and calibrating recognized sedimentary cycles to time-periodicity."
"Spectral analysis of the [magnetic susceptibility (MS)] record reveals the presence of the complete suite of orbital frequencies in the precession, obliquity, and eccentricity (95–128 ka and 405 ka) bands with very high amplitude of the precession index cycles originating from [decimeter (dm)] dm-scale couplets."
"Ammonite zone duration estimates are made by counting the interpreted precession cycles, and provide an ultra-high resolution assessment of geologic time."
An alternative method of calibrating the used standard is astronomical tuning (also known as orbital tuning), which arrives at a slightly different age.
Cyclostratigraphy is the study of astronomically forced climate cycles within sedimentary successions.
Astronomical cycles (also known as Milankovitch cycles) are variations of the Earth's orbit around the sun due to the gravitational interaction with other masses within the solar system.
The main orbital cycles are precession with current main periods of 19 and 23 kyr, obliquity with main periods of 41 kyr, and 1.2 Myr, and eccentricity with main periods of around 100 kyr, 405 kyr, and 2.4 Myr.
The 405 kyr eccentricity cycle helps correct chronologies in rocks or sediment cores when variable sedimentation makes them difficult to assign. Indicators of these cycles in sediments include rock magnetism, geochemistry, biological composition, and physical features like color and facies changes.
Argon–argon dating[edit | edit source]
The age of a sample is given by the age equation:
where λ is the radioactive decay constant of 40K (approximately 5.5 x 10−10 year−1, corresponding to a half-life of approximately 1.25 billion years), J is the J-factor (parameter associated with the irradiation process), and R is the 40Ar*/39Ar ratio. The J factor relates to the fluence of the neutron bombardment during the irradiation process; a denser flow of neutron particles will convert more atoms of 39K to 39Ar than a less dense one.
Cathodoluminescences[edit | edit source]
The inelastic scattering of the primary electrons in the crystal leads to the emission of secondary electrons, Auger electrons and X-rays, which in turn can scatter as well, which leads to up to 103 secondary electrons per incident electron.
These secondary electrons can excite valence electrons into the conduction band when they have a kinetic energy about three times the band gap energy of the material .
The primary advantages to the electron microscope based technique is its spatial resolution, where the attainable resolution is on the order of a few ten nanometers, while in a (scanning) transmission electron microscope, nanometer-sized features can be resolved.
An optical cathodoluminescence microscope benefits from its ability to show actual visible color features directly through the eyepiece, where more recently developed systems try to combine both an optical and an electron microscope to take advantage of both these techniques.
Cathodoluminescence performed in electron microscopes is also being used to study surface plasmon resonances in metallic nanoparticles. Surface plasmons in metal nanoparticles can absorb and emit light, though the process is different from that in semiconductors. Similarly, cathodoluminescence has been exploited as a probe to map the local density of states of planar dielectric photonic crystals and nanostructured photonic materials.
Chemostratigraphy[edit | edit source]
Def. the "study and dating of sedimentary strata by the analysis of trace elements and isotopic ratios" is called chemostratigraphy.
"Global changes in marine geochemistry, on scales between one thousand and one million years permit the detailed correlation of sedimentary sequences in different ocean basins. The condition is that the geochemical signals are at least approximately dated by biostratigraphy (and magnetostratigraphy, where applicable). Through mutual reenforcement of chemostratigraphy and biostratigraphy unusually high stratigraphic resolution can be obtained. The integration of chemostratigraphy and biostratigraphy opens new avenues for analyzing the record-producing system within the framework of systemic stratigraphy. This type of stratigraphy focuses on global change in sea level, climate, and general geologic setting. It attempts to identify the underlying causes of global stratigraphic signals by considering 1) changes in input of matter and energy ; 2) changes in spatial distribution of sediments ; and 3) temporary changes in the partitioning of materials between active geochemical reservoirs."
"Correlation based on fossils (biostratigraphy) generally is superior to correlation based on lithologic properties (lithostratigraphy) when the goal is chronologic equivalence of rock sequences."
"There is one type of chemical record which has a minimum of regional interference, namely that of the deep sea. It is here, therefore, that global geochemical signals are most easily detected."
"Carbonate dissolution cycles in the Indo-Pacific, Swedish Deep-Sea Expedition. Cores 45 (7°40'N, 106°21'W) and 59 (3°05'N. 133°06'W); stratigraphic record and numbering system from Arrhenius (1952). Core 153 (2°18'S, 55°33'E): record from Olausson (1960); numbers added by us. Note the striking similarities in these records, but also the difficulty in correlating older cycles without additional information."
The "the approach to stratigraphy [...] now provide the framework for Pleistocene history. It is to these chemostratigraphic (i.e. lithostratigraphic) sequences that the traditional subdivisions (Günz, Mindel, Riss, Würm; Nebraskan, Kansan, Illinoian, Wisconsin) must be correlated to have more than regional meaning."
"Chemostratigraphic signaIs such as carbonate cycles and isotope cycles appear closely linked to sea level changes, at least in the Neogene."
Cosmogenic radionuclide dating[edit | edit source]
Earth is constantly bombarded with primary cosmic rays, high energy charged particles — mostly protons and alpha particles. These particles interact with atoms in atmospheric gases, producing a cascade of secondary particles that may in turn interact and reduce their energies in many reactions as they pass through the atmosphere. This cascade includes a small fraction of hadrons, including neutrons. When one of these particles strikes an atom it can dislodge one or more protons and/or neutrons from that atom, producing a different element or a different isotope of the original element. In rock and other materials of similar density, most of the cosmic ray flux is absorbed within the first meter of exposed material in reactions that produce new isotopes called cosmogenic nuclides. At Earth's surface most of these nuclides are produced by neutron spallation.
Using certain cosmogenic radionuclides, how long a particular surface has been exposed, how long a certain piece of material has been buried, or how quickly a location or drainage basin is eroding can be determined. The basic principle is that these radionuclides are produced at a known rate, and also decay at a known rate. Accordingly, by measuring the concentration of these cosmogenic nuclides in a rock sample, and accounting for the flux of the cosmic rays and the half-life of the nuclide, it is possible to estimate how long the sample has been exposed to the cosmic rays.
The cumulative flux of cosmic rays at a particular location can be affected by several factors, including elevation, geomagnetic latitude, the varying intensity of the Earth's magnetic field, solar winds, and atmospheric shielding due to air pressure variations.
Rates of nuclide production must be estimated in order to date a rock sample. These rates are usually estimated empirically by comparing the concentration of nuclides produced in samples whose ages have been dated by other means, such as radiocarbon dating, thermoluminescence, or optically stimulated luminescence].
The excess relative to natural abundance of cosmogenic nuclides in a rock sample is usually measured by means of accelerator mass spectrometry. Cosmogenic nuclides such as these are produced by chains of spallation reactions. The production rate for a particular nuclide is a function of geomagnetic latitude, the amount of sky that can be seen from the point that is sampled, elevation, sample depth, and density of the material in which the sample is embedded. Decay rates are given by the decay constants of the nuclides. These equations can be combined to give the total concentration of cosmogenic radionuclides in a sample as a function of age.
The two most frequently measured cosmogenic nuclides are beryllium-10 and aluminum-26. These nuclides are particularly useful to geologists because they are produced when cosmic rays strike oxygen-16 and silicon-28, respectively. The parent isotopes are the most abundant of these elements, and are common in crustal material, whereas the radioactive daughter nuclei are not commonly produced by other processes. As oxygen-16 is also common in the atmosphere, the contribution to the beryllium-10 concentration from material deposited rather than created in situ must be taken into account.
10Be and 26Al are produced when a portion of a quartz crystal (SiO2) is bombarded by a spallation product: oxygen of the quartz is transformed into 10Be and the silicon is transformed into 26Al. Each of these nuclides is produced at a different rate. Both can be used individually to date how long the material has been exposed at the surface. Because there are two radionuclides decaying, the ratio of concentrations of these two nuclides can be used without any other knowledge to determine an age at which the sample was buried past the production depth (typically 2–10 meters).
Chlorine-36 nuclides are also measured to date surface rocks. This isotope may be produced by cosmic ray spallation of calcium or potassium.
Dendrochronology[edit | edit source]
Electron spin resonances[edit | edit source]
Fission track dating[edit | edit source]
Geomagnetic Polarity Time Scale[edit | edit source]
Ice cores[edit | edit source]
Lichenometry[edit | edit source]
Magnetostratigraphy[edit | edit source]
Marker horizons[edit | edit source]
Optically stimulated luminescences[edit | edit source]
Paleomagnetic dating[edit | edit source]
Palynology[edit | edit source]
Potassium–argon dating[edit | edit source]
The method most commonly used to date the primary standard is the potassium-argon dating technique.
Radiocarbon dating[edit | edit source]
During its life, a plant or animal is in equilibrium with its surroundings by exchanging carbon either with the atmosphere, or through its diet. It will therefore have the same proportion of 14
C as the atmosphere, or in the case of marine animals or plants, with the ocean. Once it dies, it ceases to acquire 14
C, but the 14
C within its biological material at that time will continue to decay, and so the ratio of 14
C to 12
C in its remains will gradually decrease. Because 14
C decays at a known rate, the proportion of radiocarbon can be used to determine how long it has been since a given sample stopped exchanging carbon – the older the sample, the less 14
C will be left.
The equation governing the decay of a radioactive isotope is:
where N0 is the number of atoms of the isotope in the original sample (at time t = 0, when the organism from which the sample was taken died), and N is the number of atoms left after time t. λ is a constant that depends on the particular isotope; for a given isotope it is equal to the reciprocal of the mean-life – i.e. the average or expected time a given atom will survive before undergoing radioactive decay. The mean-life, denoted by τ, of 14
C is 8,267 years,[note 1] so the equation above can be rewritten as:
The sample is assumed to have originally had the same 14
C ratio as the ratio in the atmosphere, and since the size of the sample is known, the total number of atoms in the sample can be calculated, yielding N0, the number of 14
C atoms in the original sample. Measurement of N, the number of 14
C atoms currently in the sample, allows the calculation of t, the age of the sample, using the equation above.
The half-life of a radioactive isotope (usually denoted by t1/2) is a more familiar concept than the mean-life, so although the equations above are expressed in terms of the mean-life, it is more usual to quote the value of 14
C's half-life than its mean-life. The currently accepted value for the half-life of 14
C is 5,730 ± 40 years. This means that after 5,730 years, only half of the initial 14
C will remain; a quarter will remain after 11,460 years; an eighth after 17,190 years; and so on.
The above calculations make several assumptions, such as that the level of 14
C in the atmosphere has remained constant over time. In fact, the level of 14
C in the atmosphere has varied significantly and as a result the values provided by the equation above have to be corrected by using data from other sources. This is done by calibration curves (discussed below), which convert a measurement of 14
C in a sample into an estimated calendar age. The calculations involve several steps and include an intermediate value called the "radiocarbon age", which is the age in "radiocarbon years" of the sample: an age quoted in radiocarbon years means that no calibration curve has been used − the calculations for radiocarbon years assume that the atmospheric 14
C ratio has not changed over time.
Calculating radiocarbon ages also requires the value of the half-life for 14
C. In Libby's 1949 paper he used a value of 5720 ± 47 years, based on research by Engelkemeir et al. This was remarkably close to the modern value, but shortly afterwards the accepted value was revised to 5568 ± 30 years, and this value was in use for more than a decade. It was revised again in the early 1960s to 5,730 ± 40 years, which meant that many calculated dates in papers published prior to this were incorrect (the error in the half-life is about 3%).[note 2] For consistency with these early papers, it was agreed at the 1962 Radiocarbon Conference in Cambridge (UK) to use the “Libby half-life” of 5568 years. Radiocarbon ages are still calculated using this half-life, and are known as "Conventional Radiocarbon Age". Since the calibration curve (IntCal) also reports past atmospheric 14
C concentration using this conventional age, any conventional ages calibrated against the IntCal curve will produce a correct calibrated age. When a date is quoted, the reader should be aware that if it is an uncalibrated date (a term used for dates given in radiocarbon years) it may differ substantially from the best estimate of the actual calendar date, both because it uses the wrong value for the half-life of 14
C, and because no correction (calibration) has been applied for the historical variation of 14
C in the atmosphere over time.
The term "conventional radiocarbon age" is also used. The definition of radiocarbon years is as follows: the age is calculated by using the following standards: a) using the Libby half-life of 5568 years, rather than the currently accepted actual half-life of 5730 years; (b) the use of an NIST standard known as HOxII to define the activity of radiocarbon in 1950; (c) the use of 1950 as the date from which years "before present" are counted; (d) a correction for fractionation, based on a standard isotope ratio, and (e) the assumption that the 14
C ratio has not changed over time.
Tephrochronology[edit | edit source]
Tephrochronology requires accurate geochemical fingerprinting (usually via an electron microprobe). An important recent advance is the use of LA-ICP-MS (i.e. laser ablation ICP-MS) to measure trace-element abundances in individual tephra shards. One problem in tephrochronology is that tephra chemistry can become altered over time, at least for basaltic tephras.
This technique relies upon the difference between the specific gravity of the microtephra shards and the host sediment matrix. It has led to the first discovery of the Vedde ash on the mainland of Britain, in Sweden, in the Netherlands, in the Swiss Lake Soppensee and in two sites on the Karelian Isthmus of Baltic Russia. It has also revealed previously undetected ash layers, such as the Borrobol Tephra first discovered in northern Scotland, dated to c. 14.4 cal. ka BP, the microtephra horizons of equivalent geochemistry from southern Sweden, dated at 13,900 Cariaco varve yrs BP and from northwest Scotland, dated at 13.6 cal. ka BP.
Thermoluminescences[edit | edit source]
Thermoluminescence (TL) research was focused on heated pottery and ceramics, burnt flints, baked hearth sediments, oven stones from burnt mounds and other heated objects.
TL can be used to date unheated sediments.
TL dating of light-sensitive traps in geological sediments of both terrestrial and marine origin became more widespread.
Uranium–lead dating[edit | edit source]
Zircons contain trace amounts of uranium and thorium (from 10 ppm up to 1 wt%) and can be dated using several modern analytical techniques. Because zircons can survive geologic processes like erosion, transport, even high-grade metamorphism, they contain a rich and varied record of geological processes. Zircons are usually dated by uranium-lead (U-Pb), fission track, cathodoluminescence, and U+Th/He techniques.
Imaging the cathodoluminescence emission from fast electrons can be used as a pre-screening tool for high-resolution secondary-ion-mass spectrometry (SIMS) to image the zonation pattern and identify regions of interest for isotope analysis. This is done using an integrated cathodoluminescence and scanning electron microscope.
Detrital zircon geochronology, i.e., zircons in sedimentary rock can identify the sediment source.
Zircons from Jack Hills in the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, have yielded U-Pb ages up to 4.404 billion years, interpreted to be the age of crystallization, making them the oldest minerals so far dated on Earth.
The oxygen isotopic compositions of some of these zircons have been interpreted to indicate that more than 4.4 billion years ago there was already water on the surface of the Earth. This interpretation is supported by additional trace element data, but is also the subject of debate. In 2015, "remains of biotic life" were found in 4.1 billion-year-old rocks in the Jack Hills of Western Australia. According to one of the researchers, "If life arose relatively quickly on Earth ... then it could be common in the universe."
Uranium-thorium dating[edit | edit source]
Varves[edit | edit source]
Phanerozoic[edit | edit source]
The Phanerozoic eon includes the Paleozoic, Mesozoic, and Cenozoic.
Cenozoic[edit | edit source]
Mesozoic[edit | edit source]
Paleozoic[edit | edit source]
Precambrian[edit | edit source]
Hypotheses[edit | edit source]
- Each time frame or span of time in geochronology has at least one dating technique.
- Late Jurassic and Upper Jurassic are different time frames.
- The overall size of—or efficiency of carbon export from—the biosphere decreased at the end of the Great Oxidation Event (GOE) (ca. 2,400 to 2,050 Ma).
See also[edit | edit source]
- Argon–argon dating
- Cosmogenic radionuclide dating
- Geochronology/Dye 3
- Electron spin resonance
- Fission track dating
- Geochronology/Geomagnetic Polarity Time Scale
- Geochronology/Ice cores
- Geochronology/Ice cores/Black ice
- Geochronology/Ice cores/Brittle ice
- Geochronology/Ice cores/Clear ice
- Geochronology/Ice cores/Firn
- Geochronology/Ice cores/Firns
- Geochronology/Ice cores/Quiz
- Geochronology/Ice cores/Sea ice
- Marker horizons
- Geochronology/Optically stimulated luminescence
- Paleomagnetic dating
- Potassium–argon dating
- Geochronology/Radiocarbon dating
- Geochronology/Tephra layers
- Uranium–lead dating
- Geochronology/Uranium-thorium dating
- Geochronology/Dye 3/Quiz
- Geochronology/Radiocarbon dating/Quiz
- Geochronology/Tephra layers/Quiz
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[edit | edit source]
- Global Boundary Stratotype Section and Point (GSSP) of the International Commission on Stratigraphy
- WiscAr Geochronology Laboratory, University of Wisconsin-Madison
- UC Berkeley press release: "Precise dating of the destruction of Pompeii proves argon-argon method can reliably date rocks as young as 2,000 years"
- New Mexico Geochronology Research Laboratory
- Argon Isotope Facility of the Scottish Universities Environmental Research Council
- Open University Ar/Ar and Noble Gas Laboratory
- Argon Laboratory / Australian National University
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