Radio carbon dating
Overview.
Radio active isotopes.
Carbon dating.
Uranium radio active dating.
Overview.
Scientists have traced the evolution of life.
They have also traced the evolution of planet Earth,
since it was born.
How can we find out, at what period of time, a Egyptian queen lived?
How can we find out, at what period, an ancient rock was created?
To date anything from ancient times, scientists used a method called Radio Carbon dating.
Radio Carbon dating uses the properties of radio active isotopes.
Radio active isotopes.
All the elements we know of, have a unique atomic number.
The atomic number corresponds to the number of protons, in the element.
The nucleus of an atom also comprises, of neutrons.
The same element could have different number of neutrons.
For example, Carbon has 6 protons, but it could have 6, 7 or 8 neutrons.
Each of these variants, of the same element carbon is called isotopes.
Carbon 12 has 6 protons and 6 neutrons.
The isotope Carbon 13 has 6 protons and 7 neutrons.
The isotope Carbon 14 has 6 protons and 8 neutrons.
The most stable and abundant isotope, Carbon 12, is simply called as Carbon.
The other variants are called isotopes, Carbon 13, and Carbon 14.
Many elements in the periodic table, have such isotopes.
Typically one of the isotopes will be the most abundant, in nature.
For example Carbon 12 is the most abundant form of Carbon.
About 99% of the Carbon present in the atmosphere, is Carbon 12.
Some of the isotopes, will be stable, while others will be unstable, or radioactive.
For example, Carbon 12 is stable.
Carbon 14 is unstable or radioactive.
This means that Carbon 14 progressively decays, and becomes Nitrogen 14.
Carbon 14 emits beta particles.
One neutron becomes a proton, resulting in 7 protons and 7 neutrons.
Having 7 protons will change the atom from Carbon to nitrogen.
Carbon 14 now becomes nitrogen, with 7 protons plus 7 neutrons.
Radioactive isotopes decay to form other elements, over time.
The rate at which a particular radioactive isotope, decays, is fixed.
This rate is expressed as half life of the isotope.
If we start with 100% of a radioactive isotope,
the amount of time it takes to become 50% is called the half life.
For example, 100% of Carbon 14 decays, in 5730 years, to become 50%.
So, we say the half life of Carbon 14 is 5730 years.
This property of radioactive isotope, is fixed.
Every radioactive isotope will have a fixed half life.
Scientists have investigated and identified the half life of various radioactive isotopes.
Carbon 14 is extensively used in radioactive dating.
Uranium has a atomic number of 92.
The most abundant form of uranium is uranium 238.
More than 99.2% of the naturally existing uranium is uranium 238.
Uranium has multiple isotopes.
One of these isotopes is uranium 235.
0.72% of naturally existing uranium, is the uranium 235 isotope.
Uranium 235 isotope has a half life of 704 million years.
This means that half of the original uranium 235, will remain after 704 million years.
Uranium 235 is also used in radio carbon dating, specially in long term dating.
Carbon dating.
Carbon dating is one of the methods for radio carbon dating.
Carbon exists naturally in the atmosphere, as carbon dioxide.
The Carbon in the atmosphere has the abundant Carbon 12,
and also a small amount of the isotope, Carbon14.
The ratio of the amount of Carbon 14 and Carbon 12 is fixed.
This determines the starting point of the amount of radio active Carbon 14.
Atmospheric Carbon is absorbed by plants.
Typically the plants will synthesis carbohydrates from atmospheric carbon dioxide,
using photosynthesis.
These carbohydrates are food which are imbibed by animals and human beings.
As long as the living animal or human being is alive it will imbibe food,
including carbohydrates.
When the organisms dies, it no longer imbibes carbohydrates.
The total amount of Carbon becomes static at this point of death.
After death, the Carbon 12 in the body remains stable.
The Carbon 14 isotope however, will continue to decay.
Let us suppose that the remains of the human being,
who lived long ago, is excavated by archaeologists.
If the sample from the remains contains 50% of the Carbon 14,
that it should normally have,
the archaeologists would conclude that he lived 5730 years ago.
If the sample contains 25% of the Carbon 14,
they will conclude that he lived 11460 years ago.
We can easily plot a graph of the number of years,
versus the percentage of remaining Carbon 14.
By measuring the Carbon 14 content in the sample,
scientists are able to date, any organic material that lived long ago.
Carbon is an essential element, of all living organisms.
Plants, animals, and human beings, all have a significant Carbon content.
The remains of an organism that once lived, can be dated fairly accurately,
using Carbon 14 dating.
Scientists have developed sophisticated and elegant instruments,
to measure the Carbon 14 content, in a sample of the remains of a once living organism.
Samples which contain Carbon 14, emit beta radiation.
This beta radiation can be measured with the design instrument.
One gram sample of a human being who just died,
emits 15 beta particles, in one minute.
This is considered as 100%.
That is we can say, that the person died 0 years ago.
If the excavated sample emits, 7.5 beta particles per gram,
we can say, the person died 5730 years ago.
If it emits 3.75 beta particles per gram,
we can say the person died 11460 years ago.
Similarly 1.87% will correspond to 22920 years.
Similarly .93% will correspond to 45840 years.
We can imagine that this trend can be easily plotted in a graph.
So any sample, with the age of 0 to 50 thousand years,
can be fairly accurately dated using Carbon 14 dating.
For periods more than 50 thousand years,
the amount of Carbon 14, is so small that it cannot be accurately measured.
Carbon 14 dating is used to date organisms, which are less than 50 thousand years old.
Human beings started to migrate out of Africa, in a significant way, about 50 thousand years ago.
These human beings migrated as far as South America and Australia.
We humans, the Homo sapiens have been around for about 200 thousand years.
Living organisms have been around for billions of years.
Scientists have excavated the fossils of many organisms, that lived hundreds of thousands,
millions and billion years ago.
Many organisms which die, get buried in the Earth, which eventually becomes rocks,
over a period of time.
The remains of these organisms, in rock is called fossils.
Scientists are able to date these organisms also.
The same principles of radio active dating is used,
but they use different elements for datings which are more than 50 thousand years old.
Uranium radio active dating.
To date samples which are very old, like more than 50 thousand years,
we need to use a radio active isotope, with a very long half life.
One of the widely used element for long term dating, is Uranium.
Uranium has a atomic number of 92.
Uranium 238 is the most abundant form of natural Uranium.
More than 99.2% of naturally occurring Uranium is Uranium 238.
Uranium 235 is one of the radio active isotopes of Uranium.
Uranium 235 has a half life of 704 million years.
This makes it very useful for long term dating.
The rocks on Earth that we find today formed a long time ago.
The Earth is about 4.5 billion years old.
Different rocks formed during different periods of the Earth's life time.
Geologists have studied the evolution of Earth, in different ways.
They have found ways to approximately know when a rock was formed.
Uranium radio active dating, provides a more accurate way of dating these rocks.
Organisms which lived very long ago, get buried in earth.
This eventually becomes rock, over thousands and millions of years.
The remains of these organisms are called fossils.
The age of the rock, can be used as an indirect means of dating the fossil.
If we date the rock, we can date the fossil found in that rock.
We can then relate it to other fossils, which existed earlier and later.
In this way we can relate different forms of life, and the time periods they existed.
Life evolved over billions of years.
Life evolved from simple unicellular organisms, to advanced animals like human beings.
During this period millions of species came into existence, and went extinct .
The currently living organisms, evolved from earlier existing species.
Charles Darwin was the first scientist to propose theory of evolution of life.
During his time the technology for radio active dating did not exist.
Since then scientists have access to Radio active dating technology.
They can also make use of the powerful DNA sequencing technology.
Scientists have collected millions of fossils, dating back to the earliest forms of life.
Using the latest technologies they have been able to map the entire tree of life.
In this way, we can trace the ancestors, of all living species.
We now know that apes were the ancestors of human beings.
Amazing as it may seem, we know that dinosaurs are the ancestors of birds.
The fascinating and complex web of life, that existed, and exists today,
have been mapped to a large extent.
As and when new fossils are found, they fill up the gaps, in the giant jigsaw puzzle.
Radio active dating plays an important role in this process.
Uranium 235 is one of the common radio active isotopes,
used for long term radio active dating.
Uranium 235 radio actively decays, to finally become lead 207.
Lead 207 is a stable element.
When rocks have to be dated, a sample of the rock is ground into a fine powder.
The heavier elements are progressively isolated from the lighter elements.
Using an instrument called mass spectrometer,
it is possible to determine the composition of the rock.
The rocks will have a small trace of uranium.
Some of this uranium will still exist as the isotope uranium 235.
Some of the uranium 235 would have decayed, to form lead 207.
We know that the half life of uranium 235 is 700 million years.
This means that after 700 million years, 50% of uranium 235 will remain.
The rest will be lead 207.
After 1.4 billion years, 25% of uranium 235 will remain.
After 2.8 billion years, 12.5% of uranium will remain.
We can plot this, and use it, to date any rock found.
This will be the date of the fossil, found in the rock.
This is the methodology used is dating very old fossils.
Uranium 238 itself is radio active.
It has a half life of 4.5 billion years.
Interestingly the age of the earth is also 4.5 billion years.
Uranium 238 was one of the original element cooked in the stars.
These elements coalesced to form the earth.
The entire geological period of earth's existence,
can be mapped using uranium 238 radio carbon dating.
Such is the power of radio active dating.