Ancient remains: how scientists discover their age
Only in the past 50 years have archaeologists had any definite means of dating the finds that they use to study the history of ancient peoples. During the 19th and early 20th centuries, they relied mainly on soil stratification as a guide, estimating the age their finds, such as bones, pottery and timber, from their position in the soil layers. For example, anything found beneath a datable layer — such as a Roman floor — ought to be older than the floor.
Dating by carbon atoms
Since 1955, the chief means of dating means remains, such as bone, shells and plants, has been radiocarbon dating. It was developed by an American physicist. Willard F. Libby, while working at the University of Chicago. He was awarded the Nobel Prize for Chemistry in 1960. Radiocarbon is another name for a radioactive isotope of the clement carbon, which is present in all living things. An isotope is an atom with a different atomic mass from the commonest form of atom in the clement. Carbon generally has an atomic mass of 12, but radiocarbon has a mass of 14, so it is also called carbon-14 or C-14 for short. Plants and animals absorb carbon dioxide from the air as long as they live, taking in one C-14 atom to every million C-12 atoms. When they die, their C-14 atoms begin to disintegrate although their C-12 atoms do not. As the rate of decay of C-14 is known, and is not affected by external factors, the age of the remains can be calculated by counting the total number of carbon atoms and comparing the ratio of C-14 to C-I2 atoms. To count the atoms, a small sample of the remains is heated in a furnace to convert it to carbon dioxide gas. The gas is passed through a machine called a mass spectrometer, where an electron beam ionizes the atoms — that is, converts them to ions (electrically charged atoms). The ions are drawn through a series of magnetic fields. This separates them according to the ratio of their charge to their atomic mass, and the C-14 isotopes are fed to a detector plate, where they are counted.
The radiocarbon calendar
As radioactive substances decay, they give off particles, and the time by which they lose half their particles is known as their half-life. Carbon-14 has a half-life of about 5700 years. After two half-lives (about 11.400 years) only a quarter of the C-14 remains, after three half-lives (about 17.100 years), only one-eighth. Detection of progressively smaller amounts gets difficult, so radiocarbon dating cannot be used for remains much more than about 35,000 years old.
A recent development is the Accelerated Mass Spectrometer, which separates and detects atomic particles of different mass. It can establish dates with greater accuracy, using a much smaller sample than older carbon-dating methods. Accelerated mass spectrometry was used in 1988 to date the Turin Shroud, a relic kept in Turin Cathedral. Italy, and believed to be the shroud in which Christ was wrapped after his crucifixion. It has a faint imprint on both the front and the back —almost like a photo negative — of a bearded, long-haired man with injuries similar to those suffered by Christ. Three fragments of the shroud were radiocarbon tested independently by lab-oratories in England, the USA and Switzer-land. All produced a date between AD 1260 and AD 1390, proving that the shroud was of medieval origin, and could not have been Christ’s.
Dating by tree rings
The age of a felled tree can be calculated by counting its growth rings — one for every year of its life — which vary in width according to the weather and climate for the year. For example, narrow rings indicate restricted growth in very dry or cold conditions. The growth patterns are similar for individual tree species over a fairly wide area, and master patterns for various areas, which include trees felled at a known date, have been compiled for comparison. This method is known as dendrochronology, and it can be used for dating old timber, as long as a large enough simple of the timber is available. The growth rings are compared against a master pattern to establish the felling year. When archaeologist’s unearthed remains of dozens of wooden stakes buried 18ft (5.5m) below London’s Victoria Embankment in 1988, they were able to compare the growth rings to give a felling date of AD 665-710. This showed the stakes were probably part of the quayside of the Anglo-Saxon port of London. Until 1989, the only way of counting the number of tree rings and matching the patterns was to do it manually. Now Danish scientists have invented a scanner, similar to the electric eye which reads supermarket bar codes, to count the rings. The information is stored in a computer and analyzed automatically. Whereas researcher could study three sections a day, the scanner can read 30 samples. Dating by gas formation
The formation of some rocks that are more than 100,000 years old can be dated by measuring the amount of rare radioactive Argon gas they contain (in younger rocks there is too little to measure). This method, known as Potassium-Argon dating, was used to date the world’s oldest-known humanoid relic, a jawbone around 5.25 million years old found near Lake Baringo in Kenya in 1984. Potassium is the seventh most abundant element in the Earth’s crust, and its radioactive isotope (K40) emits the rare gas. Argon 40 (Ar40), as it decays. In rocks formed from molten lava, the gas produced before the lava solidified would have escaped. Any gas trapped in the rock structure has accumulated since formation. K40 has a half-life of about 1300 million years, so comparing the amount of K40 with trapped Ar40 dates the rock formation, and also any embedded fossils. Dating by light emission Rocks which have been exposed to naturally occurring radiation accumulate electrons within their minerals. The number of trapped electrons is a measure of the radiation dose. Heating frees the electrons, which makes the radiation dose zero. Clay is made of rock sediments, and when it has been fired in a pottery kiln, it can be dated by a process known as thermo luminescence. When the pot is reheated at temperatures of 300.600°C (about 570-1110°F), radioactive energy is released in the form of light. Scientists can calculate the age of the pot with a margin of error of about 10 per cent. Thermo luminescence can be used to detect fake antiques — ceramics or bronzes (which have a clay core). What was thought to be a Roman pottery lamp in the British Museum collection was discovered, after testing by thermo luminescence in the 1970s, to have been made about 1920.
Dating by magnetic field
A compass needle points to magnetic north rather than the true north marked on a map. But the Earth’s magnetic field shifts from time to time, and the changes do not have a constant pattern. For example, 1500 years ago the deviation from true north was 50 per cent greater than at present. Some 5500 years ago it was only about 40 per cent. Iron-oxide particles in molten rocks and in clays align with the Earth’s magnetic field, and when the rock sets or the clay is fired in a kiln, the particles are fixed in the position they were pointing at the time of heating. Scientists can measure the directions with a magnetometer. This has led to a dating technique known as thermoremanent magnetism. The magnetic field of a rock can be compared with a date chart of the changes in the Earth’s magnetic field. The chart has been compiled from comparison with sites of known dates and the Potassium-Argon dating of rocks.
Reading the past from pollen grains
Microscopic grains of pollen are helping scientists to reconstruct the world’s past. The tiny grains can explain how the environment has been changed by man, and how the climate fluctuated thousands of years ago. One oak tree releases more than 100 million pollen grains into the air every year. Sonic smaller plants are even more prolific — the common sorrel of waysides and woodlands emits an incredible 400 million grains annually. Most windborne pollen ends up on the ground and decays in the soil in the presence of oxygen. But some falls into lakes or bogs, where it is preserved because peat deposits and the sediment at the bottom of lakes contain no oxygen. Some of the grains last for many thousands of years and fossilize. As new layers of sediment are formed, they trap pollen from plants growing at the time. This fossil pollen provides a ‘book’ that enables palaeobotanists — scientists who study ancient plant life — to build up a picture of the vegetation, and hence the climate, of the past few thousand years. Pollen grains vary in size from 15 to 50 thousandths of a millimeter across, and have individual structures varying from plant to plant that can be identified under the microscope. The grains’ tough outer walls are preserved because they contain a decay-resistant protein. Fossil pollen is counted by taking a core sample (with a hollow cylindrical drill) from an organic deposit such as a peat bog. Then specimens are taken at regular intervals throughout the depth of the deposit and dated by radiocarbon dating. The amount of pollen recovered in this way is very large. Samples taken have ranged from 20,000 grains per cubic centimeter from deposits made 11,000 years ago, to 650,000 grains per cubic centimeter a few thousand years later. From this huge quantity a representative sample of about 1000 grains is analyzed, and the proportions of the various plants are calculated. Scientists can see, for example, in what way plants colonized the northern lands after the last Ice Age about 12,000 years ago. One of the first trees found was juniper, which thrives in cold climates. As the weather became warmer it was replaced by birch, then oak and elm. A change to a moister climate brought alder. It is also possible to see how people have influenced the vegetation by cutting down forests and growing crops. Pollen analysis carried out in 1987 on sediments from the Sea of Galilee (Like Kinnereth), in northern Israel, showed that oak forests were cleared about 5000 years ago to make way for olive trees, grown for their fruit and oil. In the 3rd century AD the number of olive trees declined when the Jews left Palestine.
Means of Dating
Ancient remains: how scientists discover their age