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### Course: Cosmology and astronomy>Unit 4

Lesson 3: Measuring age on earth

# Carbon 14 dating 2

Carbon 14 Dating 2. Created by Sal Khan.

## Want to join the conversation?

• Wait! How do we know what the C14/C12 ratio within a specific tree ring was to start with?
• First you measure the ratio of C14 to C12 in the ring. Then you do the reverse calculation based on the known half-life of C14 to work out what the ratio was at the time the ring formed. The age of the ring is already known thanks to dendrochronology (dating by counting the rings, basically!). Bingo, you now know what ratio of C14 to C12 was the norm at that time.
• How can you tell how much Carbon something has? Or is there only Carbon 14..... I'm really confused.... Can I get some help?
• Carbon 14 dating uses the measurement of the ratio of carbon 14, out of all carbon atoms, within something. Since carbon 14 is a radioactive isotopes of carbon, it is not stable (meaning it does not "last" forever without turning into something else). Half of all carbon 14 within a collection of carbon atoms decays into carbon 12 atoms every ~5700 years (carbon 14's half-life). The most common isotope (think of isotope as a variation of an atom consisting of different number of neutrons) of carbon is carbon 12 (12 refers to the mass of the atom), since carbon 12 is one of the only stable isotopes of carbon. Carbon 14 is produced (mostly) by cosmic rays hitting the nitrogen within our atmosphere (as said within the last video), therefore something that does not interact with it's environment (dead or buried things) will not gain more carbon 14 after it stopped interacting with the environment. So by measuring the amount of carbon 14 something has that had not yet decayed, we can determine how long it's been since that "something" had interacted with the environment (how long it's been dead or buried)
• What is the record for longest living tree?
• The world's oldest recorded tree is a 9550 year old spruce in the Dalarna province of Sweden. It was planted during the last ice age
• At approx. , Sal makes the comment that after 50k - 60k years, carbon 14 dating isn't much help. Why is this so? Is it due to the amount of carbon 14 being minute enough to be unable to calculate a usable half-life? And how would this information be used to date fossils that are millions of years old? Thank you for your help.
• The half life of C14 is well known; it is 5,730±40 years. Most samples of organic material start out with a very small amount of C14. After around 54,000 years that original amount will have diminished by a factor of ~2^9 or ~500, which makes accurate measurement difficult. And for every ~6,000 years further back the remaining C14 decreases again by a factor of two.

C14 dating is not used to date fossils millions of years old. Uranium-lead or potassium-argon dating (and several other techniques as well) are used to date samples millions of years old.
• Sal mentioned that burning fossil fuels contribute to the amount of Carbon 14 in the atmosphere. Does nuclear testing affect it as well?
• Yes, I remember my teacher talking about it. There was an island used to test nuclear bombs, and the dating was messed up with too much carbon, making the test results show everything was younger than they actually are.
• Is carbon 14 constent?
• Carbon 14 is created by highly energetic cosmic rays hitting the
Earth's upper atmosphere. Since the rate of arrival of these cosmic rays has been reasonably constant, an equilibrium is reached between existing C14 decaying each year in the atmosphere versus the new C14 created by cosmic rays. Because we have organic material whose age we know independently, we can calculate the actual rate of creation of C14 from cosmic rays fairly accurately and calibrate our measurements. Then we can use our well-calibrated levels of C14 to determine the age of organic materials for which we don't have an independent way of knowing how old they are.
• what happens when all the carbon 14 has decayed then how do we measure the time
• We measure the ratio of C12 to C14 in the sample. That tells us the age, up to about 50,000 years or so. If there's no C14 left, that means the sample is too old to use this method. That's why the limit is around 50,000 years.
• At , Sal stated ... "since in recent times, we are burning more fossil fuels (carbon) we are altering the amount of Carbon 14 being produced".

If the creation of carbon 14 is a function of transforming Nitrogen, via cosmic radiation, into carbon 14, estimated at a very steady rate for tens of thousands of years, what possible effect would injecting greater amounts of carbon 12 into the atmosphere, by burning large amounts of fossil fuels, now and over the past 150 years?