In every individual's life, there are certain events and occasions we hardly can forget. Some of them hold good experiences while some unfortunately are just so unpleasant. There are days we got embarrassed in public bus or in front of the whole class and there are days we gained entrance into the system we've always wanted to be part of. Some of these days that holds treacherous memories we tend to hardly forget them. Days and date keeping is very important in our lives. They make history and help keep the culture of people. Nations keep records of date. As days are important in our individual lives, they are also of scientific importance.
A sequence of events occurred over the years before we existed. Some are documented, majority of them lost in the encapsulated past. A few areas of science are devoted to the excavation of these treasures. This is essential as it exposes us to what things were and how they were. The bible which is one of the oldest history books claimed that at a point in time, giants walked on the surface of the earth. Science was above to prove this fact with the titanic skeletons like that of human all dating as far back as the bible recorded them.
Fossil of an extinct species- Seymouria
Licence: CC-SA-BY 4.0 -- Source: Wikimedia --- Author: Sanjay Acharya
Archaeology and paleontology has been able to prove a lot of historical writings as either right or wrong. Dating is not only important to history excavation, it is also of forensic importance. Forensic science is dedicated to solving legal cases using the knowledge of science. Time of death are being estimated with the knowledge of science.
So how do scientists predict ages and estimate time? They do this by utilising some well tested and reputed procedures. The most important and popular being relative dating and radiometric dating.
Relative dating is a method of determining the age of fossils, artifacts and biofacts. It doesn't necessarily determine the absolute or estimated age of an object- it can however measure the age of objects in comparison to another. The principle behind relative dating is based on stratification of rocks; sedimentary rocks are constantly being formed by pilling up of minerals and organic particles (detrius) which are subsequently cemented together over time. Sedimentation of living and non-particles to form sedimentary rocks often occurs in low-lying places such as lake bottoms, ocean flows and river flood plains. These rocks are typically stratified containing their oldest material at the base and the newest at the top. Therefore animal and human remains and every other biofact found at basal extreme of the rock is termed older than the subsequent materials above. Relative dating classes things as young or old.
Radiometric dating is a much more assured method of dating. It enable one to calculate the absolute or specific age of fossils, artifacts, bio-facts and rocks. This method is based on the knowledge of radioactive decay of chemical elements. It measures the ratio of the quantity of the naturally occurring radioactive isotope within the specimen to its decayed products.
Radioactive decay is a spontaneous disintegration of atoms of an element to produce an isotope of the element or further still, another element entirely. This is not a chemical reaction hence can not be altered with pressure, temperature or catalystic activities. It is more concerned with the nucleus of the atom rather than electron as it is in chemical reaction.
Radioactive decay occur at a constant and accountable rate since it it not affected by any change in environment.
Radioactive decay was first discovered by Pierre and Marie. Eversince then, it has been a useful tool in science. The use of radioactive dating was first published by Bertram Boltwood in 1907
Licence: Public Domain -- Source: Wikimedia
Radioactive dating is now a principal method of obtaining the absolute age of a wide range of both natural and man-made structures. Absolute dating is preferably referred to as chronological dating because the word "absolute" implies an false assurance of dating accuracy.
Chronological dating or absolute dating provides numerical age or range compared to relative dating which places events in order without providing any measure of the age between the events. In archaeology, absolute dating is usually based on the physical, chemical and life properties of the materials composing the specimen in study with a great dependency on radiometric dating.
Radiometric technique is instituted based on the known and constant radioactive decay rate of elements into their radiogenic daughter isotopes. Particular Isotopes are suitable for different application. Some have limitations such as not being able to estimated very much older ages, some have problem with purity estimation among others.
Radiocarbon is one of the most widely used radioisotope in estimating ages of organic remains- this includes bones, woods and animal remains. Carbon is a substantial components of living organisms. Cosmic radiations entering the earth's surface produces a type of radioactive carbon- C14. Cells of plants take up these carbon- 14 as part of carbon monoxide during cellular respiration. The carbon- 14 is then Incorporated into the structure of proteins, oils and carbohydrates which is later consumed by animals. The carbon- 14 has gained its way into the higher food chain- animals eat plants and predators feed on other animals.
Upon death of an animal, the intake of carbon- 14 ceases.
It takes 5, 730 years for half of the carbon 14 in an organism to decay into nitrogen- this is known as the half life. The half life of a radioactive element is the time taken for half of the total radio-isotope to decay into daughter isotopes. After another 5, 730 years, only one quarter of the original carbon 14 remains in the dead tissues of the organism. In another 5, 730 years, only one-eight of carbon 14 will be left. By measuring carbon 14 ratio to its newly formed nitrogen, scientists can predict dates of death of organic matters. However, the limitation that lies in using estimating date using carbon 14 is that it is impossible to measure ages of substances older than 50, 000 years. This is however possible by calculating for other radioactive isotopes such as potassium and uranium.
The radio-isotope used in potassium dating is potassium 40. Potassium- 40 decays into Argon-40 and has an half-life of 1.3 billion years, meaning way older samples can be dated. Just like the calculation of carbon-14, it takes another 1.3 billion years for the potassium to be reduced to quarter its initial value. Potassium-40 is commonly found in micas, tephra, clay minerals and evaporites. Uranium-lead dating is one of the most preferred radiometric dating technique. It is susually performed on zicron. These minerals have uranium and thorium atoms in its crytal structure and strongly repels lead, hence the lead found in these substances are radiogenic, that is they are of radiactive reaction origin. There are two uranium radio-isotope used in this study:
- 238U ---> 206Pb ( Uranium-238 which decay into lead-206. It has an half life of 4.47billion years)
- 235U ---> 207Pb ( Uranium-235 which decay into lead-207. It has an half life of 710 billion years)
Amino acid dating is another technique used in dating. It is not based on radioactive decay but on the dispersion of light by amino acid structures. With exception of glycine, all amino acids are optically active giving them the ability to rotate light either levorotatory or dextrorotary. Amino acids are a major constituent of tissues of living organisms. With only a few exceptions, the amino acids in organisms are kept in the L configuration. When an organism dies, the mechanism in place to keep all amino acid in L configuration all cease meaning these amino acids begin to change form to a D configuration. The ratio of D to L amino acid is then used to estimate how long an organism died.
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