Mother Nature's clocks | Bangkok Post: learning

THE CHINGCHOK Hunter

Although the deck of this article says that it is about dating really old things, that doesn't quite do it justice. This article is about dating the oldest things on earth, even earth itself!

Available methods

There are several methods for dating really old things. Dendrochronology, or tree-ring dating, is one common method. Radiometric dating is the general term applied to: (1) radiocarbon dating, which uses the naturally-occurring radioisotope carbon-14; (2) potassium-argon dating, which uses an isotope of potassium; and (3) uranium-lead dating, which uses uranium and actinium to determine age. Each method has its own advantages.

Background science

Each atom is made up of three subatomic particles: protons, electrons and neutrons. An element contains only one type of atom and is therefore the simplest form of any substance.

If you look around you now, you won't see many elements. You may have gold or silver jewellery. You may use a pencil, the "lead" in which is actually pure carbon, as is the diamond that may be on your finger, but most substances are combinations of elements chemically bonded together in compounds.

But back to atoms. All elements have a set number of protons. The number of protons determines what the element is. For example, oxygen has eight protons, and only eight. Nitrogen, the element before oxygen on the periodic table, has seven protons; and fluorine, the element after oxygen, has nine protons.

In the atoms of elements, the number of electrons is equal to the number of protons because protons have a positive charge and electrons have a negative charge, and so they are balanced. Electrons determine how elements react as they can lose, gain or share electrons when chemically reacting.

For carbon dating purposes, we are only concerned with protons and neutrons. Oxygen's atomic number is 8, which represents the number of protons (and electrons), but it has a mass number of 16. As electrons have virtually no mass, the mass of an atom is comprised of the number of protons plus the number of neutrons that make up the nucleus of the atom. Therefore, the number of neutrons in an atom of oxygen is 16 (its mass number) minus 8 (its proton number), leaving 8 neutrons.

Unstable isotopes

The number of neutrons can change in an atom, without changing the atom's chemical properties. For example, chlorine has an atomic number of 17, and therefore has 17 protons and 17 electrons. But chlorine's mass number can be 35, meaning it has 18 neutrons, or 37, meaning it has 20 neutrons.

Chlorine 35 and chlorine 37 are isotopes, meaning they are different atoms of the same element due to having different numbers of neutrons. And these isotopes exist in virtually exactly the same proportions throughout the earth, having a set ratio of chlorine 35 to chlorine 37.

Isotopes can be stable or unstable, and this is where the clocks come into the picture. Unstable isotopes are called radioactive, and they decay at precise rates over a period of time that is unique to each different radioactive isotope. And they are all precisely known. Such is the expertise in this area of physics.

Decay can take place in various ways, but basically, protons can turn into neutrons, neutrons into protons, or alpha radiation can occur where protons and neutrons are ejected from an atom. Either way, because the proton number changes, so does the element.

The durations of decay can range from milliseconds to billions of years, and these durations are recorded as half-lives. The decay is exponential, which essentially means the isotope decays at a set proportion of how much of the isotope is left, which is where the term half-life comes from.

So, for the isotope potassium 40, which decays to argon 40, it has a half-life of 1.26 billion years, meaning, if you start with 100g of potassium 40, after 1.26 billion years you will have 50g of potassium 40 and 50g of argon 40.

And after another 1.26 billion years, you will have 25g of potassium 40 and 75g of argon 40, and a further 1.26 billion years will result in half of the 25 remaining grammes decaying, resulting in 12.5g, and so on, hence the exponential half-life decay.

Radiometric dating

Radiometric clocks (apart from carbon) only work when the radioactive isotopes are found in igneous rocks due to the nature of their development, as the atoms in them are formed when the rocks solidify from molten lava or magma.

So, when scientists analyse the chemical compositions of igneous rocks, they can study the ratio of potassium 40 (present at formation) to argon 40 (not present at formation) and can then deduce how much time has elapsed since the rock's formation, thereby using it as a clock.

The potassium-argon clock is just one of many radiometric clocks, which is helpful because it allows dates to be cross-checked. They are accurate to within about 1 percent. Therefore, the older the date, the greater the margin of error, but its approximation is still a good guide for dating fossils and geological formations.

Special case

Carbon is a special element in all respects, and not least in the radiometric dating field. It exists in three isotopes: carbon 12, the most common form; carbon 13, another stable form although it is extremely rare; and carbon 14, a radioactive isotope.

The great thing about carbon is that it constantly enters into food webs via plants that use it for photosynthesis. A plant doesn't care whether it uses carbon 12 or 14, as it takes in the carbon in the same standard ratio found in the atmosphere.

The vast majority of food we eat is either plant material or comes via plants, as herbivores use plants for food. Therefore, carbon 12 and carbon 14 are passed along the food chain relatively quickly. When animals or plants die, they stop the cycle of carbon through the food web.

This allows the ratio of carbon 12 to carbon 14 in a dead organism's remains to be compared because the carbon 14 slowly decays into nitrogen 14 with a half-life of 5,730 years. This relatively quick half-life only allows carbon dating to be accurate for specimens of up to about 60,000 years old.

The radioactive dating system is a highly complex, yet suitably accurate, one. It enables us to say, with absolute confidence, that the earth is between 4 and 5 billion years old, that dinosaurs became extinct about 65 million years ago, and that the Turin shroud was created in about 1200 to 1300 AD and is therefore not the image of Jesus Christ, who lived over 2,000 years ago. What an amazing scientific tool.

Dave Canavan has an MSc in Behavioural Ecology and is the Head of Secondary at Garden International School. Dave is fascinated by science and loves animals. Contact him at davidc@gardenbangkok.com.