What are Fossils?
Fossils are the preserved remains or traces of organisms that lived long ago e.g. footprints, faeces, impressions of an animal plant or bone, fossils can also include the area in which the fossil was found e.g. The rocks or plants
What are the two types of Fossils?
Type I - the remains of the dead animal, plant or the imprint left from remains
Type II - something that was made by the animal while it living that has hardened into stone, these are called trace fossils e.g. Footprinys
What is an artefact?
Objects that have been deliberately made by humans e.g. stone tools, beads, carvings, charcoal from cooking fires, cave paintings
How can fossils be preserved?
Parts of organisms can become fossilised when buried by drifting sand, mud, rovers, volcanic ash and other species. If buried rapidly fossils can be preserved and not suitable for the activity of decay organisms
How does soil impact fossilisation?
- Wet Acid Soils with oxygen–> minerals in bone are dissolved and no fossilisation occurs
- Wet Acid Soils without oxygen (peat bogs) –> complete preservation of bones, soft tissues and even skin and hair
- Alkaline soils —> minerals in bone are not dissolved & fossilisation occurs
- Alkaline soils with lime & iron oxide–> minerals replace organic matter of bone and therefore the burn turns to rock (becomes petrified)
What conditions are required for fossilisation?
- Anoxic environment (low oxygen)
- Quick burial event
- Presence of hard body parts
- Absence of decay organisms - e.g. bacteria decomposes the remains
- Long periods of stability (undisturbed)
What are the factors affecting fossilisation?
- Type of organism
- Hard parts fossilise better than soft
- Soil, pH, mineral content of soil
- Oxygen environment
- Weathering & erosion
How are tracks fossilised?
Imprints or impressions made in soft sediment such as sand and mud can become fossilised when covered quickly by sediments which dry our and harden and become buried deeper compacted and cemented together to form rock
What is relative dating?
Relative dating does not give an exact date of a fossil but can determine whether one fossil is older or younger than another
What is Stratigraphy?
A method of relative dating which is based on the layers or strata of sediments. The principle of super position states that the lowest strata in an area will be the oldest and the top layer will be the youngest. The law of correlation of rock strata involves matching layers of rock from different areas, rocks that contain the same fossils may be assumed the same age. These fossils are called index fossils
What are index fossils?
Fossils that are widely distributed and were only present on earth fir a limited period of time. They can be used to compare strata in different place such as countries, rock strata containing these fossils must be about the same age
Why are fossils not always found in every layer of strata?
Fossils are not always found in every layer of strata as erosion could of taken place, there could of been a low rate of existence at the time, fossilisation could not occur due to unfavourable conditions such as burial or environment
What is Absolute dating?
Absolute dating gives the exact age of a specimen in years. There are two main methods; potassium argon dating and carbon 14 decay
What is potassium argon dating?
Is a technique based on the decay of radioactive potassium to form calcium and argon. Potassium is a mixture of three different form of the isotope weights of potassium 39,40 & 41. The isotope potassium is radioactive and decays to form calcium 40 and argon 40 which decays at an extremely slow and steady rate, with a half late of 1.3 billion years. Determining the amounts of potassium 40 and argon 40 in a rock sample enables the age of the rock to be calculated.
What are the limitations of Potassium Argon Dating?
- Not all rock types are suitable
- Can only be used to date rocks older than 100 000 years old, if younger only 0.0053% of potassium in the rock would of decayed to argon 40 which is too small
- Only suitable for volcanic rock
What is Carbon.14 Dating?
Based on the decay of the radioactive isotope of carbon, carbon 14 to nitrogen. Carbon 14 is produced in the upper atmosphere by cosmic radiation on nitrogen at about the same rate at which it decays. When green plants use atmosphere CO2 in photosynthesis one atom in every million is incorporated into plant tissue is carbon 14. After an animal eats the plant the c14 becomes apart of the organisms tissue. With death an organism intake of carbon 14 ceases, but the carbon 14 already in the tissue of the organism continues to decay at a fixed rate. The half life of c14 is 5730. By measuring the amount of radiation liberated by a sample the ratio of c14 to c12 can be estimated and the age can be calculated. Requires at 3g of carbon to be measured
What are the limitations of radiocarbon dating?
- After about 60000-70000 years the quantity of c14 is minimal and therefore cannot be used to date rocks older than 60 000 years
- Material must contain 3g of organic material of carbon
- The amount carbon 14 - carbon 12 is not constant and therefore need to be treated with a degree of caution
- Great value in dating fossils more recent and artefacts - by dating the charcoal left by cooking fires the age of cave paintings near by can be calculated
What are some problems with the Fossil record?
- Fossils have not been preserved
- Very small proportion of fossils have been discovered
- Dating fossils can be problematic
- Unusual to find fossils of entire organisms
- Not all organisms have representations in the fossil record
- Those yet to be discovered - many not yet found
