11.2 DATING FOSSILS

absolute dating

the actual age of a specimen in years

what is potassium-argon dating?

not aging the specimen, but the rock that the specimen is found within

how old are fossils that can be dated by potassium-argon dating?

100 000-200 000 years

why does potassium argon dating work?

• potassium is found abundantly within the earth’s crust and therefore in some rocks

• potassium argon dating is based on the decay of radioactive K isotopes to Ca-40, Ar-40

what is the half life of K-40

1.25 billion years

how do we tell the age of the fossil through potassium argon dating?

• can measure the amount of radiation the sample produces

• use this information to determine the amount of K-40 within the rock

• the age of the fossil can then be inferred

limitations of potassium argon dating:

• cant be done in all rock types

• can only date rocks older than 100 000-200 000 years

what is carbon 14 dating?

involves the actual specimen and based on the decay of the radioactive isotope C-14 into N-14

limitations of carbon 14 dating:

• beyond 60 000 years, the amount of c-14 is too low in the specimen

• sample must contain organic material (organic=contains carbon)

• amount of C-14 in the atmosphere varies (therefore need to know the amount of C-14 in the atmosphere at the time)

how old are the fossils that can be dated using carbon 14 dating?

up to 60 000 years

how does carbon 14 dating work?

• C-14 is a radioactive isotope that exists naturally in the atmosphere

• (ratio of C-14 to C-12= about 1 atom for every 1 trillion atoms of C-12)

• plants perform photosynthesis and incorporate carbon into their structure (both types)

• animals eat these plants and incorporate carbon into their tissue

• once the animal dies, it no longer takes in carbon

• C-14 in the organism decays

• measure the amount of radiation put out by C-14 within the sample and can then determine the ratio of C-14 to C-12

• use this to estimate age

how long is the half life for C-14?

5730 years

• after 1 half life, half of the sample will have decayed

what is relative dating?

used to determine whether a specimen is older or younger than another sample or the rock the specimen is found in is older or younger than the rock found elsewhere

what is relative dating based on

based on stratigraphy

principle of superposition

• in layers of sedimentary rock, layers closer to the surface are assumed to be younger than the layers below them

• fossils found in the top layers are assumed to be younger than those found in lower layers

considerations of principle of superstition

• earths crust can have movements that cause layers to be reordered

• fossils could either be buried/dug up by humans

correlation of rock strata

matching layers of rock from different areas. can be done by:

1. examining and matching the rock itself

2. studying and matching the fossils contained within different layers

   • rocks that contain the same fossils may be assumed to be of the same age

index fossils

• can be used in correlation of rock strata

what makes an index fossil?

1. must be widely distributed and abundant

2. present on earth for a limited, known period of time

   e.g. trilobites