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Dating methods

Introduction

Dating methods have been used by scientist to determine the age of plants/animals. Dating methods can either be relative or absolute. Relative methods only tell us if substance A is older than substance B.  On the other hand, absolute dating methods tell us the age in terms of number of years a given plant/ animal has taken. Absolute dating methods usually take advantage of radioactive capabilities to determine age of a given material. Here I am going to focus on radiocarbon dating (also known as carbon dating or carbon 14 dating). 

Radiocarbon dating is based on the frequent creation of radiocarbon by interaction of cosmic rays and atmospheric nitrogen. Radioactive carbon dioxide is then the output of the combination of the resultant radioactive carbon and atmospheric oxygen. This carbon dioxide is incorporated into plants through a process called photosynthesis. Radiocarbon is then acquired by animals which eat on plants. Animals and plants stop exchanging carbon with the environment when they die. The carbon levels stars to decrease gradually upon the death of the plant or animal, a process known as radioactive decay. The age of a dead animal or plant can therefore be determined by measuring the amount of radiocarbon present in the animal or plant. The more carbon is detected the younger the age. 

 
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How radiocarbon dating works

Naturally carbon exists as two stable; the non reactive isotopes (carbon-12, and carbon-13) and the radioactive isotope, (carbon 14). The half life of carbon 14 is expected to reduce in the next thousands of years to come. Carbon 14F it is still being produced by cosmic rays from the lower stratosphere the troposphere. Neutrons from cosmic rays strike nitrogen 14 atoms to produce carbon-14 and other products. Below is the nuclear reaction;

 n + 147N → 146C + p

p represents proton and n represent neutron 

The carbon 14 then quickly combines with the atmospheric oxygen to form carbon dioxide. The carbon dioxide diffuses into the oceans and is taken up by plants through photosynthesis. Animals then eat up the plants and thus carbon is evenly distributed in the entire biosphere. During the lifetime of a plant or animal, it contains the same amount of carbon as the environment because of the frequent carbon exchange. When an animal dies the carbon exchange stops, the acquired amount of carbon starts to decrease. Since carbon decays at a known rate, using the available amount of carbon found, the time since the stop of carbon exchange can be measured. This is radiocarbon decay and is governed by the following equation;

N=N0e-( 0.693t/)

Where N0 is the original number of isotopes at time t=0 (when the animal in question died). N is the number of atoms left after time t. λ is a constant of a given isotope. λ is the reciprocal of mean life that is the average time an atom will take before undergoing radioactive decay. These calculations assume that the level of carbon 14 in the atmosphere has remained even over time.

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Frequent availability of carbon the atmosphere is due to the availability of carbon exchange reservoir. These are the biosphere and the oceans. The amount of carbon available in a reservoir depends on how long it takes for the generated carbon 14 to completely mix with them and how much carbon they can store. This affects the carbon-14 to carbon -12 ratios. Carbon-14 generated in the atmosphere (which accounts for 1.9 % of the entire carbon -14 given out) takes about seven years to mix completely. If the ratio of carbon-14 to carbon -12 is low, it means that carbon -14 has decayed more and this implies that carbon -14 is older. Oceans only take a few years to dissolve carbon-14. The deep waters have old carbon and take about 1000 years before they circulate back to the surface waters. The amount of carbon in an animal or plant is directly proportional to the amount in its environment. Due to the different carbon ratio, sea animals have a carbon ratio that differs from the ration of animals on land. This may be challenging when estimating the age of those animals that live both is water and on land. For instance if an animal spends it last days in water yet naturally it spends most of the time on land, then during radiocarbon dating, the ratio in the atmosphere will be used instead of those in the ocean. This will give incorrect results.

Date Range

The normal radiocarbon rating is about 30,000 to 50,000 years. This is because the half life (the period after which half of a given sample will have decayed) of carbon 14 is about 5,730 years. But with the advancement in technology the range can be increased to 70,000 years.

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Materials

Radiocarbon dating is used to date biological materials such as wood, charcoal, linen, bone, etc. Bones and human skeletons can also be dated using the carbon dating technology. Oil and coal, the ejected carbon dioxide interfered with the carbon14and carbon 12 ratios. This is why materials dated in the 20th century give an older age than their actual age (prone in highly industrialized areas).

Limitations of radiocarbon dating

The sources of error in radiocarbon calculations are mainly; atmospheric variation of carbon-12 and carbon -12, isotopic fractionation, contamination, and variation of carbon -14 and carbon -12 in different parts of the reservoir. Since this method is based on the fact that atmospheric variations remained the same over thousands of years, other methods of dating we used to verify its accuracy. There were discrepancies meaning that one or either of the methods was wrong, but a third reason would be necessary. Another effect is from the above ground nuclear testing. Such testing released a lot of neutrons thus creating carbon 14. This affects the carbon-14 to carbon-12 ratios. 

Example

The different carbon isotopes (carbon -12, carbon -13 -carbon-14) are absorbed differently by plants. The rate of carbon-12 absorption is higher than the rate at which carbon -14 is absorbed, thus the ratio represented by the plants in not necessarily the ration in the atmosphere. Since photosynthesis is the main process through which carbon moves from the atmospheric to plants and animals an effect called isotopic fractionation is formed. This is more complicated in marine organisms whose photosynthesis details are dependent on temperatures. Carbon dioxide has low solubility in water at low temperatures, and photosynthesis under these temperatures is also reduced.

Carbon dioxide in the atmosphere is transferred to the ocean when dissolved in the upper waters. The carbon 14 dissolved in the water takes a long time to reach the deep waters and the mixing is done unevenly. The mixing of the deep-waters with the surface waters is done through a process called upwelling. This process is fastest at the equator and is dependent on topography. Water in the deep-sea contains old carbon-14 and that is why shells and marine mammals such as whales tend to show ages spanning thousands of years ago. Freshwaters rise from aged rocks and carbon acquired here wills show a reduced carbon-14 to carbon 12 ratios. Rives that pass through limestone beds tend to have more carbonate minerals in their waters. Carbon from volcanic eruptions also affects the ration in the atmosphere.

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