1. How Old is that Fossil?
Objective: To determine the age of fossils using radioactive dating.
Bell work: (from the Funky Monkey Lab)
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!
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2. Fossils are the solidified remains or imprints of once- living 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. 1 2 3 4 5 6
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3. Most fossils are found in sedimentary rock, although they can be found in many different types of substrates. In what conditions do you think fossils could best be created? 1 2 3 4 5 6
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4. 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.
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5. 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 N2 N2
C14
C14
C14 N2
C14
C14 N2 N2
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6. Carbon 14
5,700 Years
Bismuth 209
Plutonium 241
Uranium 235
Thorium232
713,000,000 Years
13,900,000,000 Years
2,400,000 Years
Lead 207
Lead 209
Thorium 232
Nitrogen 14
Half-Life
Different chemicals have different half-lives. For our example, we are going to use Carbon 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.
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7. C14 Gain by consuming plants
During Life
After Death
C14 Gain by consuming plants
C14 Loss by Decay
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 Carbon-14 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.
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8. What You Do:
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).
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
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 half- life has passed.
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9. 5,700 years x 5 half-lives = 28,500 years since death
What You Do: (continued…)
Another 5,700 years have passed (box THREE (3)), how much C14 would be left?
12.5 dots remain
If something died 5,700 years ago how much of a percentage is left?
50% of the sample remains
If it has been 11,400 years, how much is left?
25% of the sample remains
How much is left after 17,100 years?
12.5% of the sample remains
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
What would be the amount left if another 5,700 years passed after box 5 (box SIX (6))?
1.5625% of the sample remains
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10. 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. N2 N2
C14
C14
C14 N2
C14
C14 N2 N2
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11. Keep Going Until You Can No Longer Make a Cut in Half!
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.
Keep Going Until You Can No Longer Make a Cut in Half! II I
III
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12. What Happened? The Results:
What is the total number of times you were (practically) able to cut the sample in half?
Each cut represents the half-life period of Carbon-14. What is the length of time represented by each cut?
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?
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?
If an animal lived millions of years ago, could Carbon-14 be used when it died? Why or why not?
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?
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13. 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 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!
Bismuth 209
Plutonium 241
Uranium 235
Thorium232
713,000,000 Years
13,900,000,000 Years
2,400,000 Years
Lead 207
Lead 209
Thorium 232
Half-Life
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