But if you are going to be looking at time scales that are that old how do you get the dates? Where are the dates coming from and how is the measurement occurring? How does the fossil record work with the geologic time scale. The answer is that you use radioactive carbon dating to get the dates. But this is only the most current method. But other methods have also been used to date the fossil record. The Fossils Sequence Record It was the study of rock layers in England near the beginning of the 19th century that lead to the study of paleontology and from there to the study of fossils.
Early geologists, at the end of the 18th and early 19th century noticed how fossils appeared in certain sequences: This meant that the ones below were older than the ones on top. It took a canal surveyor circa , William Smith in England, who noticed that he could map out great tracts of rocks on the basis of their contained fossils.
The sequences he saw in one part of the country could be matched precisely with the sequences in another. This lead to the recognition of one of the principles of geology, i.
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The next step was for geologists began to build up the stratigraphic column. And this gave us the familiar list of divisions in the geologic time scale -- Jurassic, Cretaceous, Tertiary, and so on. Most importantly, it was recognized that each time-unit was characterized by the appearance of particular fossils. The scheme worked all round the world, without fail.
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The next observation occurred when geologists noted how fossils became more complex through time. At the oldest, or deepest layer of rock there was no record of fossils, but then they noticed that simple sea creatures were found at the next higher level, then more complex ones like fishes at the next higher level and so on. Next came life on land, then reptiles, then mammals, and finally humans. One fact was soon clear, no dinosaur record could be found to coincide with a human fossil record.
So it was apparent that there was some kind of 'progress' going on. The 'progress' shown by the fossils was a documentation of the grand pattern of evolution through long spans of time. The accuracy of the fossil record using the stratigraphy method has been well documented. The order of appearance in a sequence is well documented, but that is not all.
Phylogeny, mathematics, and other methods used to date fossils. Biologists now have at their disposal a variety of independent means to look at the history of life. Besides the order of fossils in the rocks, another method is the use phylogenetic trees. Phylogenetic trees are used to show how all the species of particular groups of plants or animals relate to each other.
They are drawn up mathematically, using lists of morphological external form or molecular gene sequence characters. What is noteworthy is that modern phylogenetic trees derive no input from stratigraphy, scientific comparisons between tree shape and stratigraphy can be used to confirm the fossil record.
The majority of test cases show good agreement, so the fossil record accordingly relates the same story as the molecules enclosed in living organisms. Techniques used for Absolute Dating All of this gets us to one of the most important physical techniques, radioactive Dating. Carbon dating in geology may be relative or absolute. One does relative dating by observing fossils sequences using the stratigraphical method. 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 in , and 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 mirror 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 acids 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 may 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, temperature , and 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 over time.
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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 radioactive substances such as uranium,.
The longer the radiation exposure, the more electrons get bumped into an excited state. With more electrons in an excited state, more light is emitted upon heating. The process of displacing electrons begins again after the object cools. Scientists can determine how many years have passed since a ceramic was fired by heating it in the laboratory and measuring how much light is given off.
Thermoluminescence dating has the advantage of covering the time interval between radiocarbon and potassium-argon dating , or 40, — , years.
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In addition, it can be used to date materials that cannot be dated with these other two methods. Optically stimulated luminescence OSL has only been used since It is very similar to thermoluminescence dating, both of which are considered "clock setting" techniques. Minerals found in sediments are sensitive to light. Electrons found in the sediment grains leave the ground state when exposed to light, called recombination. To determine the age of sediment, scientists expose grains to a known amount of light and compare these grains with the unknown sediment. This technique can be used to determine the age of unheated sediments less than , years old.
A disadvantage to this technique is that in order to get accurate results, the sediment to be tested cannot be exposed to light which would reset the "clock" , making sampling difficult. The absolute dating method utilizing tree ring growth is known as dendrochronology. It is based on the fact that trees produce one growth ring each year.
The rings form a distinctive pattern, which is the same for all members in a given species and geographical area. The patterns from trees of different ages including ancient wood are overlapped, forming a master pattern that can be used to date timbers thousands of years old with a resolution of one year. Timbers can be used to date buildings and archaeological sites. In addition, tree rings are used to date changes in the climate such as sudden cool or dry periods. Dendrochronology has a range of one to 10, years or more.
As previously mentioned, radioactive decay refers to the process in which a radioactive form of an element is converted into a decay product at a regular rate. Radioactive decay dating is not a single method of absolute dating but instead a group of related methods for absolute dating of samples.
Potassium-argon dating relies on the fact that when volcanic rocks are heated to extremely high temperatures, they release any argon gas trapped in them. As the rocks cool, argon 40 Ar begins to accumulate. Argon is formed in the rocks by the radioactive decay of potassium 40 K. The amount of 40 Ar formed is proportional to the decay rate half-life of 40 K, which is 1. In other words, it takes 1. This method is generally only applicable to rocks greater than three million years old, although with sensitive instruments, rocks several hundred thousand years old may be dated.
The reason such old material is required is that it takes a very long time to accumulate enough 40 Ar to be measured accurately. Potassium-argon dating has been used to date volcanic layers above and below fossils and artifacts in east Africa. Radiocarbon dating is used to date charcoal, wood, and other biological materials.