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### Course: AP®︎/College Biology>Unit 7

Lesson 5: Evidence for evolution

# Carbon 14 dating 1

Carbon-14 forms in the atmosphere through cosmic rays interacting with nitrogen-14, creating a small fraction of carbon-14 in living organisms. When an organism dies, its carbon-14 decays back into nitrogen-14 at a known rate, called a half-life (5,730 years). By measuring the remaining carbon-14 in fossils, scientists can estimate their age, helping us understand the history of life on Earth. Created by Sal Khan.

## Want to join the conversation?

• If the carbon14 used to date an organism, let's say an elephant is derived mainly from the plants ingested, and the elephant is dated by comparing it's ratio of carbon atoms to another elephant's ratio of carbon atoms, how do you account for the amount of plants the elephant actually ate? For illustrative purposes, let's assume elephant A ate mostly peanuts and a little grass and elephant B ate mostly grass. If they were both born at the same time, wouldn't Elephant A be dated older?
• The ratio between C14 and C12 is important not the absolute value of C14 atoms. And that ratio is reflected everywhere. It will be the same in the grass as it is in the peanuts.
• How accurately in time can one measure the decay of C-14? The video speak in terms of years, but can one get the time down to the month, day, hour?
• The best precision is probably on the order of several years. There are several sources of error. For example, the production rate of 14C varies over time with changes in the Earth's orbit. We can only estimate this factor by back-calculating orbital parameters. There is also a little bit of fractionation of C by organisms such that either 14C or 12C is preferentially incorporated into tissue. And then there are the analytical uncertainties, which include measurement errors and the uncertainty of the decay constant.
• Interesting video... but I don't understand how one could know the exact "amount of Carbon 14" that has accumulated in one given bone.
"Every living thing has a certain proportion of Carbon 14 in their tissue" ... how do we know what that proportion is?
Is it a fraction of another element that's not decaying?!
• All elements can have different numbers of neutrons in their nucleuses. We call them different isotopes, and name them by the sum of the number of protons and neutrons. All carbon has six protons -- that is what makes it carbon -- and most carbon has six neutrons, making carbon-12 the normal isotope of carbon. About 99% of natural carbon is carbon-12. There is also another stable, non-radioactive isotope of carbon with seven neutrons: carbon-13. It makes up about 1% of natural carbon. The type of carbon that we are interested in, because it is radioactive, is carbon with eight neutrons: carbon-14. It makes up about one part in a trillion of natural carbon.

Since natural carbon contains all these isotopes mixed together, anything that takes up carbon from natural sources will have the same mix of isotopes in its carbon. Now the key (and really cool) thing here is that carbon-14 is continuously produced in the atmosphere by cosmic rays. Neutrons produced by the cosmic rays hit nitrogen-14 (which makes up about 78% of the atmosphere) and knocks out a proton, turning it into carbon-14. This carbon-14 then reacts with oxygen to form CO2, and that is absorbed by plants and algae in photosynthesis. This is how it is mixed into the other natural carbon. Since living things continuously replace their atoms while they live, but stop when they die, the ratio of carbon-14 will be the same as the natural ratio as long as they live, but will start dropping when they die due to radioactive decay. This is the basis for radiocarbon dating.

The production of carbon-14 has been pretty constant for a long time, so the proportion of carbon-14 in natural carbon was pretty similar in the past. There have been changes however, which we account for with several calibration methods which you can read about here: https://en.wikipedia.org/wiki/Radiocarbon_dating#Calibration

Other more recent changes in the proportion of carbon-14 have been caused by nuclear tests (which produced a lot of carbon-14) and burning of fossil fuels (which releases a lot of old carbon, without any carbon-14, thereby reducing the proportion of carbon-14 in natural carbon).
• At "Neutron hits a N14 in just the right way, the neutron replaces the proton."
This sounds like a traumatic event, but having precise results at a constant rate.
1. Are the neutrons travelling at speed of light?
2. Is this demonstrated in particle accelerators?
3. Why aren't the electrons messed up?
• No, the neutrons are not travelling at the speed of light, since only photons do.
• Is it possible for a bone to have no Carbon 14 at all? Then how would they date it.
• Up to about 58,000 to 62,000 years (wikipedia "carbon dating") is the effective maximum of Carbon dating. Other methods to date the bone would include other isotopes, either in the bone or in surrounding material, or inference from the geological layer the bone is present in (usually via nearby volcanic layers, which tend to have nice datable isotopes in them).
• At . To get accurate dates we must be assuming that the neutron bombardment has always been constant in the past. After all we weren't recording any data on this just one half life ago. What if the neutron bombardment were greater or lesser in the past?
• Yes, this is an important point. The intensity of cosmic ray bombardment depends on the strength of the magnetic field of the Earth, which has varied in the past. The atmospheric 14C concentration is therefore not constant, and past atmospheric 14C concentrations must be calibrated using other methods, including dendrochronology and K/Ar dating.
• Is it possible for a bone to have no Carbon 14 at all? Then how would they date it? Would it be possible, or is there another way to do it?
• Radiometric dating using C14 is only effective to about 50,000 years so if the sample is much older than that there may not be detectable amounts of C14 in it. There are other isotopes that are used that have longer half-lives that can be used.
• if carbon fourteen is a thing is there carbon fifteen
• There are currently 15 known isotopes of carbon, from C-8 through C-22. Carbon-15 has a half life of about 2.5 seconds.