10.4 4.2.5 fossils  (Page 2/4)

Film fossils form where leaves have left black or brown marks, made of carbon, on rock surfaces. A film fossil of a leaf is shown in Figure 4.

Fossil dating

Radiometric dating

Radiometric dating is sometimes used when scientists want to date fossils. Some elements are unstable and break down into lighter elements over time. Suchsubstances are said to be radioactive. The original element is called the parent and the element it changes into is called the daughter. For example,uranium-238 decays into lead-206. What is the data which scientists use in this method, and the assumptions they might make when drawing conclusions aboutfossil dates?:

Data: A rock has a certain ratio of the parent to daughter elements, e.g. a certain ratio of uranium to lead.
Conclusions: In radiometric dating, the ratio of parent to daughter elements is compared. From this the age of the rock is concluded.
Assumptions: A scientist has to make many assumptions in order to conclude the age of a rock from ratios between elements. These assumptions include:

The rock must have had only parent, and no daughter, elements when the rock formed.
The rate at which the parent element changed into the daughter element is known from the rate this occurs today.
For example, if a rock is found containing both uranium-238 and lead-206, it is assumed that all this lead came from the uranium by radioactive decay and thatthe rate at which that happened is the same as the rate observed today.

Carbon dating

Carbon dating is the only type of radiometric dating which can be done directly on fossils. However, carbon dating gives young ages, and so is generally notaccepted, except for dating organisms which have been dead for less than about 30 000 years.

How does carbon dating work? All organisms incorporate carbon atoms into their bodies when they are alive. This stops at death. There are two isotopes (forms)of carbon. The most common isotope is C12. The other isotope, C14, is radioactive. The proportion C12:C14 in the atmosphere is known. Both carbonisotopes combine with oxygen to form CO2 and both are incorporated into plants during photosynthesis. Therefore both carbon isotopes form part ofanimals’ food. The organism dies. The C12 in the dead body does not break down radioactively, but the C14 in the body does break down radioactively. Wecan measure the rate at which C14 breaks down radioactively over time today. Scientists compare the ratio of C12:C14in the body to the ratio of C12:C14inthe atmosphere. This is the data used. The scientists may assume that the C12:C14 ratio at the time of death was the same as the atmosphere’sC12:C14ratio, and that the rate at which C14broke down in the past was the same as it is today. From this they may conclude the age of the rock.

Dating fossils by dating rocks

For fossils which are believed to be older than 30 000 years, carbon dating cannot be used, so other radiometric dating methods are used. However, thesemethods can only be done on igneous rocks (rocks formed from volcanoes).Igneous rock cannot contain fossils. Fossils are only found in sedimentary rock. Inorder to conclude the date of a fossil the following process is followed: