Science, Art, Litt, Science based Art & Science Communication

How the age of ancient things is determined

You are on a journey into wilderness. Suddenly you find something that looks very ancient. But you don't know how old it is. How do you determine its age? Catch a scientist who can do that! Easy!

Scientists have developed more than a dozen methods for determining the age of fossils, human artifacts, and the sediments in which such evidence is found. These methods can date objects millions of years old. What’s more, the methods can be tested against one another to provide a highly reliable record of the past.

Until this century, relative dating was the only technique for identifying the age of a truly ancient object. By examining the object's relation to layers of deposits in the area, and by comparing the object to others found at the site, archaeologists can estimate when the object arrived at the site. Though still heavily used, relative dating is now augmented by several modern dating techniques.

Geologists study rock layers and the fossils in them to determine relative age. Using relative and radiometric dating methods, geologists are able to answer the question: how old is this fossil? (1) They have established a set of principles that can be applied to sedimentary (sedimentary rock is one of the three main rock groups (along with igneous and metamorphic rocks) and is formed in four main ways: by the deposition of the weathered remains of other rocks (known as 'clastic' sedimentary rocks); by the accumulation and the consolidation of sediments; by the deposition of the results of biogenic activity; and by precipitation from solution) and volcanic rocks that are exposed at the Earth's surface to determine the relative ages of geological events preserved in the rock record. For example, in the rocks exposed in the walls of the Grand Canyon (Figure below) there are many horizontal layers, which are called strata. The study of strata is called stratigraphy, and using a few basic principles, it is possible to work out the relative ages of rocks. In the Grand Canyon, the layers of strata are nearly horizontal. Most sediment is either laid down horizontally in bodies of water like the oceans, or on land on the margins of streams and rivers. Each time a new layer of sediment is deposited it is laid down horizontally on top of an older layer. This is the principle of original horizontality: layers of strata are deposited horizontally or nearly horizontally. Thus, any deformations of strata must have occurred after the rock was deposited.

Image result for grand canyon

The principle of superposition builds on the principle of original horizontality. The principle of superposition states that in an undeformed sequence of sedimentary rocks, each layer of rock is older than the one above it and younger than the one below it (Figure above). Accordingly, the oldest rocks in a sequence are at the bottom and the youngest rocks are at the top.

Once it was possible to measure the ages of volcanic layers in a stack of sedimentary rock, the entire sequence could be pinned to the absolute time scale. In a given landscape, for example, if a gray ash layer was found to be 73 million years old, this means that fossils in rock layers below the tuff are older than 73 million years, and those above the tuff are younger. Fossils found embedded within the ash, are the same age as the ash: 73 million years old.

Sometimes sedimentary rocks are disturbed by events, such as fault movements, that cut across layers after the rocks were deposited. This is the principle of cross-cutting relationships. The principle states that any geologic features that cut across strata must have formed after the rocks they cut through .

The principles of original horizontality, superposition, and cross-cutting relationships allow events to be ordered at a single location. However, they do not reveal the relative ages of rocks preserved in two different areas. In this case, fossils can be useful tools for understanding the relative ages of rocks. Each fossil species reflects a unique period of time in Earth's history. The principle of faunal succession states that different fossil species always appear and disappear in the same order, and that once a fossil species goes extinct, it disappears and cannot reappear in younger rocks.

Fossil species that are used to distinguish one layer from another are called index fossils. Index fossils occur for a limited interval of time. Usually index fossils are fossil organisms that are common, easily identified, and found across a large area. Because they are often rare, primate fossils are not usually good index fossils. Organisms like pigs and rodents are more typically used because they are more common, widely distributed, and evolve relatively rapidly.

Using the principle of faunal succession, if an unidentified fossil is found in the same rock layer as an index fossil, the two species must have existed during the same period of time . If the same index fossil is found in different areas, the strata in each area were likely deposited at the same time. Thus, the principle of faunal succession makes it possible to determine the relative age of unknown fossils and correlate fossil sites across large discontinuous areas.

It wasn't until well into the 20th century that enough information had accumulated about the rate of radioactive decay that the age of rocks and fossils in number of years could be determined through radiometric age dating.

Some elements have forms (called isotopes) with unstable atomic nuclei that have a tendency to change, or decay. For example, U-235 is an unstable isotope of uranium that has 92 protons and 143 neutrons in the nucleus of each atom. Through a series of changes within the nucleus, it emits several particles, ending up with 82 protons and 125 neutrons. This is a stable condition, and there are no more changes in the atomic nucleus. A nucleus with that number of protons is called lead (chemical symbol Pb). The protons (82) and neutrons (125) total 207. This particular form (isotope) of lead is called Pb-207. U-235 is the parent isotope of Pb-207, which is the daughter isotope.

Many rocks contain small amounts of unstable isotopes and the daughter isotopes into which they decay. Where the amounts of parent and daughter isotopes can be accurately measured, the ratio can be used to determine how old the rock is.

After scientists learned that the nuclear decay of radioactive elements takes place at a predictable rate, they realized that the traces of radioactive elements present in certain types of rock, such as hardened lava and tuff (formed from compacted volcanic ash), could be analyzed chemically to determine the ages, in years, of those rocks.

Radiocarbon dating involves determining the age of an ancient fossil or specimen by measuring its carbon-14 content.

Carbon dating is the determination of the age or date of organic matter from the relative proportions of the carbon isotopes carbon-12 and carbon-14 that it contains. The ratio between them changes as radioactive carbon-14 decays and is not replaced by exchange with the atmosphere.

Carbon dating is a variety of radioactive dating which is applicable only to matter which was once living and presumed to be in equilibrium with the atmosphere, taking in carbon dioxide from the air for photosynthesis.

What are C-12 and C-14?

Carbon 12 is the commonest natural carbon isotope, of mass 12. It is the basis for the accepted scale of atomic mass units. It is the more abundant carbon of the two stable isotopes, amounting to 98.93% of the element carbon.

Cosmic ray protons blast nuclei in the upper atmosphere, producing neutrons which in turn bombard nitrogen, the major constituent of the atmosphere. This neutron bombardment produces the radioactive isotope carbon-14. The radioactive carbon-14 combines with oxygen to form carbon dioxide and is incorporated into the cycle of living things.

The carbon-14 forms at a rate which appears to be constant, so that by measuring the radioactive emissions from once-living matter and comparing its activity with the equilibrium levelof living things, a measurement of the time elapsed can be made.

Presuming the rate of production of carbon-14 to be constant, the activity of a sample can be directly compared to the equilibrium activity of living matter and the age calculated.

Carbon-14 decays with a half-life of about 5730 years by the emission of an electron of energy 0.016 MeV. This changes the atomic number of the nucleus to 7, producing a nucleus of nitrogen-14. At equilibrium with the atmosphere, a gram of carbon shows an activity of about 15 decays per minute.

The low activity of the carbon-14 limits age determinations to the order of 50,000 years by counting techniques. That can be extended to perhaps 100,000 years by accelerator techniques for counting the carbon-14 concentration.

Here of some of the well-tested methods of dating used in the study of early humans (2):

Potassium-argon dating, Argon-argon dating, Carbon-14 (or Radiocarbon), andUranium series. All of these methods measure the amount of radioactive decay of chemical elements; the decay occurs in a consistent manner, like a clock, over long periods of time.
Thermo-luminescence, Optically stimulated luminescence, and Electron spin resonance. All of these methods measure the amount of electrons that get absorbed and trapped inside a rock or tooth over time.
Paleomagnetism. This method compares the direction of the magnetic particles in layers of sediment to the known worldwide shifts in Earth’s magnetic field, which have well-established dates using other dating methods.
Biochronology. Since animal species change over time, the fauna can be arranged from younger to older. At some sites, animal fossils can be dated precisely by one of these other methods. For sites that cannot be readily dated, the animal species found there can be compared to well-dated species from other sites. In this way, sites that do not have radioactive or other materials for dating can be given a reliable age estimate.
Molecular clock. This method compares the amount of genetic difference between living organisms and computes an age based on well-tested rates of geneticmutation over time. Since genetic material (like DNA) decays rapidly, the molecular clock method can’t date very old fossils. It’s mainly useful for figuring out how long ago living species or populations shared a common ancestor, based on their DNA.