How to date a fossil (without spending a fortune for dinner and flowers) Have you wondered how the age of fossils are determined? There are several different methods scientists use to determine age of fossils. Sometimes, it is possible to determine age directly from the fossil. Many times however, fossils are to old to have their age directly measured. Instead, age can be determined from radioactive elements occuring within rock found in association with the fossils.
Radiometric (or radioactive) dating
This method is based upon radioactive decay. The spontaneous release of energy and/or particles from the nucleus of an unstable atom (referred to as the parent ) into a stable atom (referred to as the daughter ) is radioactive decay. This rate of decays occurs at a specific and constant rate. The age of a rock can be determined by measuring the amount of the daughter product and adding that to the amount of the remaining parent material.
There are four standards necessary for elements to be useful in radometric dating.
1. The numbers of parent atoms and daughter atoms must be measurable.
2. The parent element must decay rapidly enough to produce measurable amounts of the daughter element, but measurable amounts of the parent element must also be present in the sample.
3. Little or no daughter element must have been present in the sample when it was formed.
4. The sample used must have been chemically isolated from outside chemical changes.
These systems meet the standards listed above. Half-life refers to the length of time required for 50% of the parent material to decay into the daughter product.
Uranium 235 to Lead 207 (half-life = 710,000,000 years)
Uranium 238 to Lead 206 (half-life = 4,500,000,000 years)
Thorium 232 to Lead 208 (half-life = 14,000,000,000 years)
Rubidium 87 to Strontium 87 (half-life = 47,000,000,000 years) - this is the most common system used for dating rocks older than 100 million years.
Potassium 40 to Argon 40 (half-life = 1,300,000,000 years) - this method is very often used to date rock less than 60 million years old.
Carbon 14 to Nitrogen 14 (half-life = 5,570 years)--- There are 3 forms (isotopes) of carbon occuring in nature: Carbon 12 (accounts for 99%), Carbon 13 (accounts for 1%), and Carbon 14 (accounts for less than 1%). While alive, plants and animals incorporate these isotopes of carbon into their tissues at the ratio found in the atmosphere. Upon death, the Carbon 14 in their tissues begins to decay. By measuring the remaining amount of Carbon 14, the age of the fossil can be determined. This method can be used to date material ranging in age from a few hundred years to about 50,000 years. The use of Carbon 14 permits the determination of age directly a fossil. For fossils greater than 50,000 years old, the age of the fossil is found indirectly by determing the age of the rock associated with the fossil. Carbon 14 dating has a dating range of several hundred years before present to 50,000 years before present.
Fission-track dating
Fission-track dating is based on the presence of Uranium 238 and Uranium 235 in the sample to be tested. These two uranium isotopes always occur in the same ratio in nature. Uranium 238 will undergone spontaneous decay or fission. Each time this happens, a tiny damage track is created in the surrounding material. Etching with acid enlarges the tracks allowing them to be seen under a microscope and counted. However, Uranium 235 does not undergo spontaneous fission. Uranium 235 can be induced to undergo fission by irradiating the sample with high energy neutrons in a nuclear reactor. By counting the number of induced tracks and knowing the neutron dose, the uranium content can be determined. From the ratio of natural fission tracks to induced fission tracks and knowing the half-life of Uranium 238 (half-life = 4,500,000,000 years), the sample's age can be determined.
Paleomagnetism
At the time of their formation, iron-bearing rocks and sediments may acquire a natural remnant magnetism . This primary magnetism aligns parallel to the existing magnetic field of the Earth. In a sense, a rock becomes a compass capturing its orientation to the Earth's magnetic field in its structure.
The orientation of the magnetic field of the Earth at any point on Earth is specified by two measurements: declination (direction) and inclination (plunge). The inclination varies from horizontal at the equator to vertical at the poles. Today, the magnetic field is directed downward in the northern hemisphere and upward in the southern hemisphere. Earth's magnetic field periodically reverses its polarity. During the time of reversed polarity, a compass needle would point south. These reversals make excellent markers in the geologic record because they global in extend. The age of these reversals can be determined by radiometric dating. The age of a fossil can be determined by correlating the position of the strata of rock where it was found and where a reversal occurs.
Amino-acid dating
Amino-acid dating is based upon the principle that amino acids which make up proteins change when an organism dies. The proteins produced by an orgamism when it is alive almost entirely consists of amino acids in a "left-handed" configuration. After death, amino acids begin to invert to their "right-handed" configuration. This process is called racemization . In fossils, an equilibrium ratio is eventually reached. The time needed to reach this equilibrium depends mainly upon temperature and secondarily on the species of the organism. Once the absolute date for a region is determined using radiometric dating and the temperature history of a region is established, amino-acid dating can be used to determine the age of a fossil. In the example of marine mollusks, the ratio for the amino acid isoleucine increases from nearly zero in modern shells to an equilibrium value of 1.30 +/- 0.05. At 10 degrees centigrade, it takes about 2 million years to reach equilibrium. At minus 10 degrees centigrade, it takes 20 million years to reach equilibrium.
