[Chemistry Class Notes] on Carbon 14 Dating Pdf for Exam

Carbon 14 dating, also defined as radiocarbon dating, is a method of determining age and relies heavily on the decay of radiocarbon to nitrogen (otherwise called carbon-14). Carbon 14 dating is formed continuously in nature by the interaction of neutrons with nitrogen-14 in the atmosphere of Earth. The required neutrons for this particular reaction can be created by cosmic rays interacting with the atmosphere.

About Radiocarbon Dating

Radiocarbon, which is contained in ambient carbon dioxide molecules, reaches the biological carbon cycle by being consumed from the environment by green plants and then passed down the food chain to animals. Radiocarbon decays slowly in living organisms, and the amount lost is constantly replenished as long as the organism eats or breathes. However, after an organism dies, it stops absorbing carbon-14, resulting in a steady decline in the amount of radiocarbon in its tissues.

The half-life of carbon 14 is given as of 5,730.40 years, which means that half of the amount of radioisotope present at any given time will spontaneously disintegrate for the next 5,730 years. Since the Carbon-14 compound decays at a constant rate, calculating the amount of residual radiocarbon may be used to measure the date that an individual died.

The method of Carbon-14 dating was developed in about 1946 by the American Physicist named Willard F. Libby. It’s also proven to be a flexible method for dating archaeological specimens and fossils varying in age from 500 to 50,000 years. Also, the method is widely used by anthropologists, Pleistocene geologists, investigators, and archaeologists in related fields.

Dating Considerations

Atmospheric Variation

It was recognised in the early years of using the technique that it was dependent on the atmospheric 14C/12C ratio remaining constant over thousands of years. Many items that were dateable by other methods were checked to check the accuracy of this method; the testing results were in good agreement with the true ages of the objects.

However, over time, discrepancies began to appear between the well-known chronology for the radiocarbon dates of Egyptian artefacts and the oldest Egyptian dynasties. Neither the new radiocarbon dating method nor the pre-existing Egyptian chronology could be assumed to be accurate, but one of the third possibilities was that the 14C/12C ratio had changed over time.

Here, the question was resolved by the tree ring study: comparison of the overlapping series of tree rings has allowed the continuous sequence of tree-ring data construction that spanned 8,000 years.

Isotopic Fractionation

Photosynthesis is defined as the primary process where carbon moves from the atmosphere into living things. In the photosynthetic pathways, 12 C can be absorbed slightly more easily than 13 C that in turn is very easily absorbed than 14 C. The differential uptake of these three carbon isotopes leads to 13 C/12 C and 14 C/12 C ratios in plants that differ from the atmosphere ratios. This effect is called isotopic fractionation.

Reservoir Effects

The original exchange of the Libby reservoir hypothesis has assumed that the 14 C/12 C ratio present in the exchange reservoir is constant worldwide, but it has since been discovered that there are many causes of variation in the ratio across the reservoir.

Marine Effect

The CO2 present in the atmosphere transfers to the ocean by dissolving in the surface water as bicarbonate and carbonate ions. And, at the same time, the carbonate ions present in the water are returning to the air as CO2. This exchange mechanism transfers 14 C from the atmosphere to the ocean’s surface waters, where it takes a long time for the 14 C to percolate into the entire volume of the ocean.

The ocean’s deepest parts mix very slowly with the surface waters, and the mixing is noticed to be uneven. The major mechanism that brings the deep water to the surface is upwelling, which is quite common in regions that lie closer to the equator. Upwelling can also be influenced by factors such as the climate of the local ocean bottom and coastlines, the topography, and the wind patterns.

Hemisphere Effect

The southern and northern hemispheres have atmospheric circulation systems, which are sufficiently independent of each other that there is a noteworthy time lag in mixing between the two. The ratio of the atmospheric 14 C/12 C is lower in the southern hemisphere, with an apparent additional age of up to 40 years for the radiocarbon results from the south as compared to the north.

Material Considerations

It is more common to reduce the wood sample to just the cellulose component prior to testing, but since this reduces the sample’s volume to 20% of its original size and testing of the whole wood is often performed too. Often, charcoal can be tested, but it is likely to need treatment to remove the contaminants.