Incremental dating
Encyclopedia
Incremental dating techniques allow the construction of year-by-year annual chronologies, which can be temporally fixed (i.e., linked to the present day and thus calendar
or sidereal
time) or floating.
Archaeologists use tree-ring dating (dendrochronology
) to determine the age of old pieces of wood. Trees usually add growth rings on a yearly basis, with the spacing of rings being wider in high growth years and narrower in low growth years. Patterns in tree-ring growth can be used to establish the age of old wood samples, and also give some hints to local climatic conditions. This technique is useful to about 9,000 years ago for samples from the western United States using overlapping tree-ring series from living and dead wood.
The Earth's orbital motions (inclination of the earth's axis on its orbit with respect to the sun, gyroscopic precession of the earth's axis every 26,000 years; free precession every 440 days, precession of earth orbit and orbital variations such as perihelion precession every 19,000 and 23,000 years) leave traces visible in the geological record. These changes provide a long-term sequence of climatic events, recorded as changes in the thickness of sediment layers (known as "varve analysis"—the term "varve
" means a layer or layers of sediment. Typically, varve refers to lake or glacial sediment), as temperature induced changes in the isotopic ratios for oxygen isotopes in sediments, and in the relative abundance of fossils. Because these can be calibrated reliably over a period of 40 million years this provides an alternate verification to radiometric dating
in cases where sufficient record exists to provide a reliable trace.
Polarity reversals in the Earth's magnetic field have also been used to determine geologic time. Periodically, the magnetic field of the earth reverses leaving a magnetic signal in volcanic and sedimentary rocks. This signal can be detected and sequences recorded, and in the case of volcanic rocks, tied to radiometric dates.
Another technique used by archaeologists is to inspect the depth of penetration of water vapor into chipped obsidian
(volcanic glass) artifacts. The water vapor creates a "hydration rind" in the obsidian, and so this approach is known as "hydration dating" or "obsidian dating
", and is useful for determining dates as far back as 200,000 years.
Calendar
A calendar is a system of organizing days for social, religious, commercial, or administrative purposes. This is done by giving names to periods of time, typically days, weeks, months, and years. The name given to each day is known as a date. Periods in a calendar are usually, though not...
or sidereal
Sidereal time
Sidereal time is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky...
time) or floating.
Archaeologists use tree-ring dating (dendrochronology
Dendrochronology
Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...
) to determine the age of old pieces of wood. Trees usually add growth rings on a yearly basis, with the spacing of rings being wider in high growth years and narrower in low growth years. Patterns in tree-ring growth can be used to establish the age of old wood samples, and also give some hints to local climatic conditions. This technique is useful to about 9,000 years ago for samples from the western United States using overlapping tree-ring series from living and dead wood.
The Earth's orbital motions (inclination of the earth's axis on its orbit with respect to the sun, gyroscopic precession of the earth's axis every 26,000 years; free precession every 440 days, precession of earth orbit and orbital variations such as perihelion precession every 19,000 and 23,000 years) leave traces visible in the geological record. These changes provide a long-term sequence of climatic events, recorded as changes in the thickness of sediment layers (known as "varve analysis"—the term "varve
Varve
A varve is an annual layer of sediment or sedimentary rock.The word 'varve' is derived from the Swedish word varv whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as Hvarfig lera on the first map produced by the Geological Survey of Sweden in...
" means a layer or layers of sediment. Typically, varve refers to lake or glacial sediment), as temperature induced changes in the isotopic ratios for oxygen isotopes in sediments, and in the relative abundance of fossils. Because these can be calibrated reliably over a period of 40 million years this provides an alternate verification to radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
in cases where sufficient record exists to provide a reliable trace.
Polarity reversals in the Earth's magnetic field have also been used to determine geologic time. Periodically, the magnetic field of the earth reverses leaving a magnetic signal in volcanic and sedimentary rocks. This signal can be detected and sequences recorded, and in the case of volcanic rocks, tied to radiometric dates.
Another technique used by archaeologists is to inspect the depth of penetration of water vapor into chipped obsidian
Obsidian
Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth...
(volcanic glass) artifacts. The water vapor creates a "hydration rind" in the obsidian, and so this approach is known as "hydration dating" or "obsidian dating
Obsidian hydration dating
Obsidian hydration dating is a geochemical method of determining age in either absolute or relative terms of an artifact made of obsidian....
", and is useful for determining dates as far back as 200,000 years.
Techniques
Techniques of incremental dating include:- DendrochronologyDendrochronologyDendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...
- Sampling Ice coreIce coreAn ice core is a core sample that is typically removed from an ice sheet, most commonly from the polar ice caps of Antarctica, Greenland or from high mountain glaciers elsewhere. As the ice forms from the incremental build up of annual layers of snow, lower layers are older than upper, and an ice...
s - LichenometryLichenometryIn archaeology, palaeontology, and geomorphology, lichenometry is a geomorphic method of geochronologic aging that uses lichen growth to determine the age of exposed rock: lichens are presumed to increase in size radially at specific rates as they grow...
- Paleomagnetic datingPaleomagnetismPaleomagnetism is the study of the record of the Earth's magnetic field in rocks. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of...
- VarveVarveA varve is an annual layer of sediment or sedimentary rock.The word 'varve' is derived from the Swedish word varv whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as Hvarfig lera on the first map produced by the Geological Survey of Sweden in...
s - SpeleothemSpeleothemA speleothem , commonly known as a cave formation, is a secondary mineral deposit formed in a cave. Speleothems are typically formed in limestone or dolostone solutional caves.-Origin and composition:...
s - AcanthochronologyAcanthochronologyAcanthochronology is the interdisciplinary study of cactus spines or Euphorbia thorns grown in time ordered sequence . Physical, morphological or chemical characteristics and information about the relative order or absolute age of the spines or thorns is used to study past climate or plant...