Dating methods Dating techniques are procedures used by scientists to determine the age of a specimen.
Relative dating methods read article only if one sample is older or younger than another sample; absolute dating methods provide a date in years. The latter have generally been available only since Many absolute dating techniques take advantage of radioactive decaywhereby a radioactive form of an element is converted into another radioactive isotope or non-radioactive product at a regular rate.
Others, such as amino acid racimization and cation-ratio dating, are based on chemical changes in the organic or inorganic composition of a sample. In what is absolute dating method years, a few of these methods have undergone continual refinement as scientists strive to develop the most accurate dating techniques possible.
Relative dating methods determine whether one sample is older or younger than another. They do not provide an age in years. Before the advent of absolute dating methods, nearly all dating was relative. The main relative dating method is stratigraphy.
Stratigraphy is the study of layers of rocks or the objects embedded within those layers. It is based on the assumption which, except at unconformitiesnearly always holds true that deeper layers were deposited earlier, and thus are older than more shallow layers.
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The sequential layers of rock represent sequential intervals of time. Although these units may be sequential, they are not necessarily continuous due to erosional removal of some intervening units.
The smallest of these rock units that can be matched to a specific time interval is called a bed. Beds that are related are grouped together into members, and members are grouped into formations.
Seriation is the ordering of objects according to their age. It is a relative dating method.
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In a landmark study, archaeologist James Ford used seriation to determine the chronological order of American Indian pottery styles in the Mississippi Valley. Artifact styles such as pottery types are seriated by analyzing their abundances through time. This is done by counting the number of pieces of each style of the artifact in each stratigraphic layer and then graphing the data.
A layer with many pieces of a particular style will be represented by a what is absolute dating method band on the graph, and a layer with only a few pieces will be represented by a narrow band. The bands are arranged into battleship-shaped curves, with each style getting its own curve.
The curves are then compared with one another, and from this the relative ages of the styles are determined.
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A limitation to this method is that it assumes all differences in link styles are the result of different periods of time, and are not due to the immigration of new cultures into the area of study. The term faunal dating refers to the use of animal bones to determine the age of sedimentary layers or objects such as cultural artifacts embedded within those layers.
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Scientists can determine an approximate age for a layer by examining which species or genera of animals are buried in it. The technique works best if the animals belonged to species that evolved what is absolute dating method, expanded rapidly over a large area, or suffered a mass extinction. In addition to providing rough absolute dates for specimens buried in the same stratigraphic unit as the bones, faunal analysis can also provide relative ages for objects buried above or below the fauna-encasing layers.
Each year seed-bearing plants release large numbers of pollen grains. This process results in a "rain" of pollen that falls over many types of environments. Pollen that ends up in lakebeds or peat bogs is the what is absolute dating method likely to be preserved, but pollen may also become fossilized in arid conditions if the soil is acidic or cool.
Scientists can develop a pollen chronology, or calendar, by noting which species of pollen were deposited earlier in time, that is, residue in deeper sediment or rock layers, than others. A pollen zone is a period of time in which a particular species is much more abundant than any other species of the time. In most cases, this also reveals much about the climate of the period, because most plants only thrive in specific climatic conditions.
Changes in pollen zones can also indicate changes in human activities such as massive deforestation or new types of farming. Pastures for grazing livestock are distinguishable from fields of grain, so changes in the use of the land over time are recorded in the pollen history.
The dates when areas of North America were first settled by immigrants can be determined to within a few years by looking for the introduction of ragweed pollen. Pollen zones are translated into absolute dates by the use of radiocarbon dating. In addition, pollen dating provides relative dates.