Dating techniques are procedures used by scientists to determine the age of a specimen. Relative Dating methods in science methods tell 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 recent years, a few of these methods have undergone "Dating methods in science" 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. 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 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. 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 types are seriated by analyzing Dating methods in science 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 wide 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.
A limitation to this method is that it assumes all differences in artifact 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 Dating methods in science layers. 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 quickly, expanded rapidly over a large area, or suffered a mass extinction. In addition to providing rough absolute dates for specimens buried Dating methods in science 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 most 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 Dating methods in science 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 Dating methods in science 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 beyond the limits of radiocarbon 40, yearsand can be used in some places where Dating methods in science dates are unobtainable. Fluorine is found naturally in ground water. This water comes in contact with skeletal remains under ground. When this occurs, the fluorine in the water saturates the bone, changing the mineral composition.
Over time, more and more fluorine incorporates itself into the bone. By comparing the relative amounts of fluorine composition of skeletal remains, one can determine whether the remains were buried at the same time. A bone with a higher fluorine composition has been buried for a longer period of time. Absolute dating is the term used to describe any dating technique that tells how old a specimen is in years.
These are generally analytical methods, and are carried out in a laboratory. Absolute dates are also relative dates, in that they tell which specimens are older or younger than others.
Absolute dates must agree with dates from other relative methods in order to be valid. This dating technique of amino acid racimization was first conducted by Hare and Mitterer inand was popular in the s.
It requires a much smaller sample than radiocarbon dating, and has a longer range, extending up to a few hundred thousand years. It has been used to date coprolites fossilized feces as well as fossil bones and shells. These types of specimens contain proteins embedded in a network of minerals such as calcium. Amino acid racimization is based on the principle that amino acids except glycine, a very simple amino acid exist in two image forms called stereoisomers.
Living organisms with the exception of some microbes synthesize and incorporate only the L-form into proteins. When these organisms die, the L-amino acids are slowly converted into D-amino Dating methods in science in a process called racimization.
The protons are quickly replaced, but will return to either side of the amino acid, not necessarily to the side from which they came. This Dating methods in science form a D-amino acid instead of an L — amino acid. The rate at which the reaction occurs is different for each amino acid; in addition, it depends upon the moisture, temperatureand pH of the postmortem conditions. The higher the temperature, the faster the reaction occurs, so the cooler the burial environment, the greater the dating range.
The burial conditions are not always known, however, and can be difficult to estimate.
For this reason, and because some of the amino acid racimization dates have disagreed with dates achieved by other methods, the technique is no longer widely used. Cation-ratio dating is used to date rock surfaces such as stone artifacts and cliff and ground drawings.
It can be used to obtain dates that would be unobtainable by more conventional methods such as radiocarbon dating. Scientists use cation-ratio dating to determine how long rock surfaces have been exposed. They do this by chemically analyzing the varnish that forms on these surfaces. The varnish contains cations, which are positively charged atoms or molecules. Different cations move throughout the environment at different rates, so the ratio of different cations to each other changes Dating methods in science time.
By calibrating these ratios with dates obtained from rocks from a similar microenvironment, a minimum age for the varnish can be determined. This technique can only be applied to rocks from desert areas, where the varnish is most stable. Although cation-ratio dating has been widely used, recent studies suggest it has potential errors.
Many of the dates obtained with this method are inaccurate due to improper chemical analyses. In addition, the varnish may not actually be stable over long periods of time. Thermoluminescence dating is very useful for determining the age of pottery. Electrons from quartz and other minerals in the pottery clay are bumped out of their normal positions ground state when the clay is exposed to radiation.
This radiation may come from substances such as uranium.