Fossils

Explain the correlation of rock strata is used to determine the relative age of strata

Involves matching layers of rock from different areas. Matching strata can be done by examining the rock itself and also by studying the fossils it contains. Rocks that contain the same fossils may be assumed to be of the same age.

Describe C-14 (radiocarbon) dating

Is based on the decay of the radioactive isotope of carbon, carbon-14 to nitrogen. Carbon-14 is produced in the upper atmosphere by the action of cosmic radiation on nitrogen at about the same rate at which ti decays. In the atmosphere there is a ratio of one carbon-14 atom to every million million (10¹²) atoms of the stable isotope carbon-12. With death, an organism's intake of carbon-14 ceases but the carbon-14 already in the tissues of the organism continues to decay at a fixed rate. By measuring the amount of radiation liberated by a sample, the ratio of carbon-14 to carbon-12 can be estimated.

Explain fluorine dating

It is based on the fact that when a bone is left in soil, fluoride ions, which are present in the soil replace some of the ions in the bone itself. All the fossil bones in a particular deposit should contain the same amount of fluoride, so that fossils that have been displaced can be detected. The older the fossil, the more fluoride it contains so that the relative ages can be established. It is not possible to decide absolute ages using this method since the concentration of fluorine in ground water varies from place to place and from time to time.

Describe what is studied in stratigraphy

It is the study of layers or strata.

List some limitations of this principle

It must be applied with care because distortions of Earth's crust do occur and a sequence of rock layers may be turned upside down. It is also possible for fossils or artefacts to be buried by animals or perhaps early humans some time after deposition of sediment. In this case, the specimen may be younger than some of the layers above it.

Define the term isotope is and explain how they are used in absolute dating

An isotope is different forms of the same element that have a different number of neutrons in the nucleus of their atoms. Isotopes are used in potassium-argon dating and radiocarbon or carbon-14 dating. They are important because each radioactive element decays at a constant rate which is unique to that element. These rates of decay or known so it you can measure the proportion of parent and daughter isotopes in rocks, you can calculate when the rocks were formed.

Define the term fossil

Any preserved trace left by an organism that lived long ago

Give the approximate age of the earth

4.543 billion years

Outline the conditions suitable for fossil formation

A quick burial of the material The presence of hard body parts An absence of decay organisms A long period of stability - the organism needs to be left undisturbed The Nature of the soil - In wet, acid soils, the minerals in the bone are dissolved and no fossilisation occurs. If the soil contains no oxygen, complete preservation of the soft tissues and bones of the animal may occur. Bones buried in alkaline soils produce the best fossils since the minerals in the bones are not dissolved. New minerals (lime, iron oxide) are deposited in the pores of the bone, replacing the organic matter that makes up about 35% by weight of bone. Bone becomes petrified (turned into rock) but the details of structure are still preserved. The dead organism needs to be buried quickly - As soon as organism dies, it is covered in sediment. On land, it can be buried by sand or volcanic ash. In oceans and lakes, it can be covered by sand and mud which are being carried by rivers or currents. A quick burial usually involves the dead organism being covered by sediments. The dead organism must not decay - Soft parts of organisms decay because scavengers and decomposers eat them. If they can be kept away from the dead organism, it may become a fossil. Decay can be prevented by having unsuitable conditions for decomposers. The dead organism must lie undisturbed - It takes a long time for a fossil to form. The sediments in which the dead organism is buried harden and become sedimentary rock. Disturbance of a dead organism must be prevented for a very long time if a fossil is to form.

Distinguish between relative and absolute dating

Absolute dating: Provide the actual age of the specimen in years Reliving Dating: Can tell whether one sample is older or younger than the other

List some limitations of c-14 dating

After about 70000 years, the quantity of carbon-14 left if negligible. For this reason the radiocarbon dating method cannot be used to date back more about 60000 years. A further limitation is that the material to be dated must contain an organic compound - contain more carbon. It was once assumed that the ratio of carbon-14 to carbon-12 in the atmosphere was constant, but it is not known that the amount of carbon-14 in the atmosphere varies.

Explain the principle of superposition

Assumes that in layers of sedimentary rock the layers at the top are younger than those beneath them. Any fossils or other materials found in the top layers will be younger than the material found lower down.

Explain who the geological time scale is organised

Earth's geological history has been divided into a geological time scale. The time scale consists of eras which are subdivided into periods, and the periods are further subdivided into epochs.

List the types of materials that can become fossilised

Footprints, burrows, faeces, impressions of all or part of an animal/plant, bones, teeth or shell.

Distinguish between fossils and artefacts

Fossils are the remains of living things. Artefacts are the objects that have been deliberately made by humans.

Explain what an index fossil is and how they are used in relative dating

Fossils that were only on Earth for a limited period of time. They are of great value in this correlation work because they are widely distributed making relative dating of strata more precise.

List 5 different forms of fossils

Mould fossils, cast fossils, fossil fuels, actual remains, petrified wood

List 3 types of absolute dating

Potassium Argon dating, Carbon-14 dating, tree ring dating

List some limitations of this technique

Potassium argon dating has limited usefulness; not all rocks are suitable for this method of dating and it can only date rocks older than 100 000 to 200000 years. At the earlier date of 100000 years, only 0.0053% of the potassium-40 in a rock would have decayed to argon-40. Such a small amount pushes the limits of detection devices currently in use. To determine the age of a fossil using this methods, there must be available suitable rock of the same age of the fossil. This occurs, for example, when rocks produced in volcanic eruptions bury bones. Despite these limitations, potassium argon dating is very useful in providing pushes in providing precise dates for those rocks for which is suitable.

Explain the significance of fossilised pollen grains in dating

Some fossil pollen grains are useful as index fossils but even if they cannot be used in this way, the presence of preserved pollen grains in a social or rock sample can enable a botanist to construct a picture of the type and amount of vegetation existing at the time the deposit was laid down. This data can be used to confirm or refute relative dates arrived at by other methods.

List 2 types of relative dating

Stratigraphy & Fluorine Dating

Describe some problems with the fossil record

The fossil record is very incomplete as the conditions for fossilisation do not always occur, or they occur at irregular periods of time. There are many gaps in the fossil record because organisms have not been preserved. There are also a very small proportion of the fossils that do exist have actually been discovered. Some are buried too deep in the ground to be found or they are in inaccessible places. Others may have been inadvertently destroyed by human activity like agriculture or industry.

Describe the potassium-argon technique of absolute dating.

The isotope potassium-40 is radioactive and decays to form calcium-40 and argon-40. Such decay takes place at an extremely slow but at a constant rate so by determining the amount of K-40 and argon-40 in a rock sample that age of the rock can be calculated.

Describe why AMS radiocarbon dating is superior to c-14 dating.

The normal method of radiocarbon dating requires at least 3 grams of organic material so that the rate of radioactive decay of carbon-14 in the sample can be measured. AMS can be used to date a sample as 100 micrograms. This technique involves breaking the sample up into constituent atoms so that the number of atoms of each isotope of carbon can be counted.

Describe some limitations of this technique

The particular conditions necessary for the use of the method do not occur often and timber is rarely preserved for more than a few thousand years

Describe tree ring dating

There are concentric rings on a surge of a cut tree trunk. Each ring represents one year's growth and the rings differ in width according to how favourable the growing season was.