How Old is that Fossil?
Objective: To determine the age of fossils using radioactive
dating.
Bell work: (from the Funky Monkey Lab)
1.
In the current Funky Monkey population, blonde-furred,
tailless Funky Monkeys are RARE. What environmental change
could cause this trait to become more abundant in the Funky
Monkey population?
Let’s Discuss!
© Getting Nerdy, LLC
Fossils are the solidified remains or imprints of onceliving organisms. The older a fossil is, the further down in
the rock layers it is. Organisms that resemble the living
creatures of today are mostly found in the top layers,
while more ancestral forms are found lower in the rock
layers.
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3
4
5
6
© Getting Nerdy, LLC
Most fossils are found in sedimentary rock, although they
can be found in many different types of substrates. Try a
couple of different places to see where fossils form the
best in this interactive website…
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3
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5
6
© Getting Nerdy, LLC
Whoa, dude… did
you know…?
Most all organisms that
have ever lived (99%) are
extinct.
Fossils provide a historical
sequence of life known as
the geologic time scale.
© Getting Nerdy, LLC
We can estimate the AGE of a
fossil using RADIOACTIVE
DATING:
- Rocks contain radioactive
chemicals that break down
- The Half-life is the amount of
time it takes for HALF of the
chemicals in a rock to decay,
or turn into another chemical
- So, we can analyze a sample
of the radioactive chemical
from a rock found near the
fossil, calculate its half-life,
and estimate how old the
fossil really is
© Getting Nerdy, LLC
N2 N
C14
2
C14 C
14
N2 C
C14N
2
14
N2
Nitrogen 14
Different chemicals have
different half-lives. For
our example, we are going
to use Carbon-14.
Carbon-14 is a radioactive
element, meaning that it is
an unstable substance. As
Carbon-14 decays, the
atoms change to another
more stable element
called Nitrogen-14. It takes
about 5,700 years for half
of the radioactive atoms in
a sample of Carbon-14 to
change to stable nitrogen.
This period of time is
known as the half-life of
Carbon-14.
Half-Life
Carbon
14
5,700 Years
Carbon 14
Lead 207
Uranium
235
713,000,000 Years
Uranium 235
Lead 209
Thorium
232
13,900,000,000 Years
Thorium 232
Bismuth 209
Plutonium
241
2,400,000 Years
Plutonium 241
© Getting Nerdy, LLC
All living things contain
Carbon-14. Plants take-in
Carbon Dioxide during
photosynthesis and
therefore also take-in
Carbon-14. When animals
eat the plants, the Carbon14 is transferred to the
animals. When a plant or
animal dies (they are no
longer taking in Carbon),
the Carbon within it begins
to change (decay) to
nitrogen. After a while,
most of the Carbon-14 will
decay to Nitrogen-14.
© Getting Nerdy, LLC
During Life
C14 Gain by
consuming plants
After Death
C14 Loss by Decay
What You Do:
1.Draw 100 dots in box ZERO (0). This represents the amount of C14
in the sample when the organism died (we start at the time of death
= ZERO).
2.5,700 years have passed - how many dots should I have left (think
HALF-Life)? Draw the new amount of dots in box ONE (1).
50 Dots remain
3.What would happen to box ONE (1) if another 5,700 years passed?
Would all the dots be gone? Draw the new amount of dots in box
TWO (2).
25 Dots remain
Continue drawing the new amount of dots in each box after a halflife has passed.
© Getting Nerdy, LLC
What You Do: (continued…)
4.Another 5,700 years have passed (box THREE (3)), how much C14 would be left?
12.5 dots remain
5.If something died 5,700 years ago how much of a percentage is left?
50% of the sample remains
6.If it has been 11,400 years, how much is left?
25% of the sample remains
7.How much is left after 17,100 years?
12.5% of the sample remains
8.Using boxes 0-5 above, how many total years have passed since this organism
died?
5,700 years x 5 half-lives = 28,500 years since death
9.What would be the amount left if another 5,700 years passed after box 5 (box
SIX (6))?
1.5625% of the sample remains
© Getting Nerdy, LLC
Notice how after a little
while it becomes so small
that we can hardly see it
anymore. That is the point
at which carbon dating is no
longer useful. So we need to
find something with a longer
half-life to date older things.
We use ones with VERY long
half-lives to date the Earth.
© Getting Nerdy, LLC
N2 N
C14
2
C14 C
14
N2 C
C14N
2
14
N2
What You Do: (continued…)
You have recently discovered the
fossilized remains of a woolly
mammoth.
On the small rectangular piece of
paper in front of you, draw your
best woolly mammoth. Be sure to
fill up the whole piece of paper.
This will represent all of the
Carbon-14 that was present in the
woolly mammoth when it died.
Cut the sample in half. Set one half
to the side and keep the remaining
half. Cut it again. Continue this
process of cutting your mammoth,
discarding half, again and again
until the box is so small that it is not
possible to make another cut.
Each time you make a cut, make a
mark in the box on your paper.
© Getting Nerdy, LLC
Keep Going
Until You Can
No Longer
Make a Cut in
Half!
III
III
What Happened? The Results:
1.What is the total number of times you were (practically) able to cut the
sample in half?
2.Each cut represents the half-life period of Carbon-14. What is the length
of time represented by each cut?
3.Multiply the number of cuts by the half-life period of Carbon-14. What is
the total amount of time represented by all of your cuts?
4.If an animal lived near and died in the LaBrea Tar Pits (found now in Los
Angeles, California) 40,000 years ago, could Carbon-14 be used to
determine when it died?
5.If an animal lived millions of years ago, could Carbon-14 be used when it
died? Why or why not?
6.If the initial amount of Carbon-14 in the mammoth was 8 grams, how
many half-lives have passed if the fossil sample has 1 gram left? How
many years have passed since the mammoth’s death?
© Getting Nerdy, LLC
7.
Which chemical(s) listed would
be useful for finding the age of
the Earth? Explain your answer.
Only Uranium would work well for
estimating the age of the Earth.
For example, if you have an
initial sample of 8 grams of
radioactive material, after 7
half-lives, only 0.125 grams of
the sample would remain, but
it is still enough to measure at
around 4.9 billion years of age.
Thorium’s half-life is too long
at 13.9 billion years – Earth is
only 4.6 billion years old! And
Plutonium’s half-life is limited
at 2.4 million years – the
sample would be too small to
measure accurately!
© Getting Nerdy, LLC
Half-Life
Uranium
235
Lead 207
713,000,000 Years
Uranium 235
Lead 209
Thorium
232
13,900,000,000 Years
Thorium 232
Bismuth 209
Plutonium
241
2,400,000 Years
Plutonium 241
Evolution of Whales
Objective:
To provide evidence for
evolution through tracing the history of
whales
Bell work:
The initial amount of Uranium227 in a
sample of rock is 16g and the half life
of U227 is 3,400 years. How much time
has passed if there are 2g of U227
remaining?
16 g/2 = 8 g
8 g/2 = 4 g
4 g/2 = 2 g
A total of 3 half lives have passed
3,400 years x 3 half lives = 10,200 years
© Getting Nerdy, LLC
N2 N
C14
2
C14 C
14
N2 C
C14N
2
14
N2