A detailed wrietup on radio dating techniques.
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Radio dating the past - Opening lid on secrets
- The story: Dating techniques are procedures used by scientists to determine the age of rocks, fossils, or artifacts. Relative dating methods tell only if one sample is older or younger than another; absolute dating methods provide an approximate date in years. The latter have generally been available only since 1947. Many absolute dating techniques take advantage of radioactive decay, whereby a radioactive form of an element decays into a 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.
- Radioactive decay: Such decay occurs in unbalanced atoms called radionuclides (elements that emit ionizing radiation). When it decays, a radionuclide transforms into a different atom - a decay product. The atoms keep transforming to new decay products until they reach a stable state and are no longer radioactive. The majority of radionuclides only decay once before becoming stable. The most stable form of an element is the most common in nature. All elements have an unstable form. Unstable forms emit ionizing radiation and are radioactive. There are some elements with no stable form that are always radioactive, such as uranium.
- Radiocarbon dating: Radiocarbon dating is a method that gives age estimates for carbon-based materials that originated from living organisms Carbon is one of the many chemical elements on Earth. Carbon, along with hydrogen, nitrogen, oxygen, phosphorus, and sulfur, is a building block of biochemical molecules of life (fats, proteins, hormones etc.).
- All carbon atoms have a nucleus containing six protons, and 99% of these also contain six neutrons. The 6 proton + 6 neutron atoms are said to have a mass of 12 and are referred to as "carbon-12"
- What about other 1%? Their nuclei contain either seven or eight neutrons in addition to six protons. They have masses of 13 and 14 resp. and are referred to as “carbon-13” and “carbon-14.” (isotopes). 14C varies from 0.0000000001% (one part per trillion) down to zero
- C12 and C13 are stable, but C14 is not. The rate at which Carbon-14 decays is constant. In any given mass of C-14, half of them will decay in 5730 years. So in any dead body, after 5730 years only half remains. After another 5730 years only a quarter remains. This process continues until no 14C remains C-14 dating is applicable to organic and some inorganic materials (not metals).
- Carbon 14 is continually being formed in the upper atmosphere due to effect of cosmic ray neutrons on nitrogen 14 atoms. It is rapidly oxidized in air to form carbon dioxide and enters the global carbon cycle. Plants and animals assimilate carbon 14 from carbon dioxide throughout their lifetimes. When they die, they stop exchanging carbon with the biosphere and their carbon 14 content then starts to decrease at the standard rate.
- Maximum dating possible is upto 50 -70,000 years and minimum is 100 yrs. Other methods can date to much older periods. K-Ar method, Uranium dating, etc.
- Various methods:
- Relative dating - it determines if one sample is older or younger than another but does not provide an age in years. It can be of many types. (i) Stratigraphy is the study of layers of rocks or the objects embedded within those layers. Assumption is that deeper layers were deposited earlier, and thus are older, than more shallow layers. The sequential layers of rock represent sequential intervals of time. (ii) Seriation is the ordering of objects according to their age. Example: finding the chronological order of American Indian pottery styles in the Mississippi Valley. (How was it done? By analyzing their abundances through time, by counting the number of pieces of each style of the artifact in each stratigraphic layer and then graphing the data) (iii) Faunal Dating is the use of animal bones to determine the age of sedimentary layers or objects such as cultural artifacts embedded within those layers. Works best if the animals belonged to species, which evolved quickly, expanded rapidly over a large area, or suffered a mass extinction. (iv) Pollen Dating (Palynology) - Each year seed-bearing plants release large numbers of pollen grains, resulting 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, and a pollen chronology can be made by noting which species of pollen were deposited earlier in time (residue in deeper sediment or rock layers, than others).
- Absolute dating - it tells how old a specimen is in years, and is carried out in a laboratory. Absolute dates must agree with dates from other relative methods in order to be valid. It can be of many types. (i) Amino Acid Racemization requires a 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 specimens contain proteins embedded in minerals such as calcium. (ii) Cation-Ratio dating dates 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 radio-carbon dating. The principle is that the cation ratio (K+ + Ca2+)/Ti4+ decreases with increasing age of a sample. (iii) Thermoluminescence dating is 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. It covers the time interval between radiocarbon and potassium-argon dating, or 40,000 – 2,00,000 years. (iv) Tree-Ring dating is dendrochronology, and based on the fact that trees produce one growth ring each year. Narrow rings grow in cold and/or dry years, and wide rings grow in warm years with plenty of moisture. The rings form a distinctive pattern, which is the same for all members in a given species and geographical area. It has a range of 1 - 10,000 years or more. (v) Radioactive Decay dating is a group of related methods for absolute dating of samples, including Potassium-Argon Dating, Radiocarbon Dating and Uranium Series Dating.
- Potassium-Argon Dating - When volcanic rocks are heated to extremely high temperatures, they release any argon gas trapped in them. As the rocks cool, argon-40 (40Ar) begins to accumulate. Argon-40 is formed in the rocks by the radioactive decay of potassium-40 (40K). The amount of40Ar formed is proportional to the decay rate (half-life) of40K, which is 1.3 billion years. In other words, it takes 1.3 billions years for half of the40K originally present to be converted into40Ar.
- Uranium Series Dating - Uranium series dating techniques rely on the fact that radioactive uranium and thorium isotopes decay into a series of unstable, radioactive “daughter” isotopes; this process continues until a stable (non-radioactive) lead isotope is formed. The daughters have relatively short half-lives ranging from a few hundred thousand years down to only a few years. The “parent” isotopes have half-lives of several thousand million years. This provides a dating range for the different uranium series of a few thousand years to 5,00,000 years. Uranium series have been used to date uranium-rich rocks, deep-sea sediments, shells, bones, and teeth, and to calculate the ages of ancient lake beds.
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