Chapter 16 Flashcards
Fossil
Any preserved trace left by an organism that lived long ago.
Examples: footprints, burrow, bones, shells and teeth.
Artefact
Objects that have been deliberately made by humans.
What is an index fossil?
Index fossils are fossils that only exist for a short period of time (appearing in one layer) that need to be widespread.
Example - pollen grains (determines type of vegetation), trilobites.
What are the best soil types for preservation of fossils?
Absence of oxygen, dry, cold, salty and alkaline conditions (opposite of acidic, high pH)
Relative dating?
Comparison between a fossil/artefact (can’t give age) -> no numbers.
Examples: stratigraphy, fluorine dating.
Absolute dating?
The actual age of the fossil is given (in years).
Examples: carbon dating, potassium argon and dendrochronology.
Why is relative dating used when absolute dating has good methods?
Absolute dating has limitations:
- Potassium-argon (Not all rocks suitable).
- Carbon-14 (Only useful for material up to 70,000 years old).
- Dendrochronology (Timber rarely preserved).
Radioisotope = Carbon 14 and potassium argon dating,
What are it’s methods?
The principle is that atoms decay with a known half life.
Comparing the amount of carbon (5370 years - half life) or potassium that has decayed.
Carbon 14 dating/lifecycle
Carbon 14 is unstable with a known half life.
It exists at a known ratio with carbon 14 that we keep up by continually eating plants/animals.
After death and fossilisation, we no longer maintain that ratio.
Carbon 14 decays without being replenished.
What is a half life?
Length of time it takes for half the carbon to decay.
AMS radiocarbon dating?
Can be used to date a sample as small as 100 micrograms.
Dendrochronology?
Each ring of tree = year of growth.
Marker ring = weather conditions causing notably different rings.
Thicker rings = good year, vice versa
Once we know the year that a marker ring happened, we can use older, dead trees to figure out the year an older marker point happened.
Does superposition always apply?
No because of digging and tectonic plate movement
Movement in the earths crust
Explain how index fossils can be used to compare strata from different locations.
Because index fossils are widespread and occur for only short periods of time - you know that the strata came from the time of the index fossil and therefore you can relatively date the ones on either side.
Usefulness of pollen grains
Index fossil
Type of environment
Food types (vegetation)
Fluorine dating?
More fluorine it contains, the older the fossil
Some fossils maybe older than others and contain less fluorine, depending on the ground water (one can contain more fluorine)
Limitation: you can compare the two fossils if they’re from the same area
Explain how Carbon-14 determines the age of fossil remains
Form of absolute dating - meaning it can give an exact age
Every 5730 years, that’s how long the fossil has been there for half its lifetime
Can be passed on in food chain, absorbed by plants which are ate by animals and we then consume animals
Limitations of carbon-14 dating
Can only date once living things - cannot date rocks etc
Can only date up to 70,000 years old - will eventually be NO carbon left
Phylogenetic trees
Shows where each animal has evolved from and showing how closely related we are to a particular species
Limitations of potassium argon dating
Not all rocks suitable (rocks produced in volcanic eruptions)
Useful for rocks between 100,000-200,000 years or older
Must have rock same age as a fossil
Carbon 14 limitations
70,000 or younger
Must contain carbon
Carbon varies in atmosphere
Requires 3 grams of carbon
Dendrochronology limitations
Timber rarely preserved
Only can date 9,000 years or younger
Only useful for wood
Potassium argon dating
Based on the decay of radioactive potassium to form - calcium and argon
Correlation of rock strata
Matching the layers of rick from different areas to form a time scale
Principle of superposition
Layers at the top are younger than those beneath them
