How is radioactive decay used to date fossils?

Question

How is radioactive decay used to date fossils?

Thanks! =)

Answer 1

There are traces of Carbon-14 in the fossils.  Carbon-14 is an isotope of carbon that decays at a certain rate over time.  If you know how much carbon-14 is left compared to what it breaks down to,  then you know a relative date

Answer 2

I'm not positive but I know that radioactive decay is when something decays and the atoms within that thing change into radiation.  I'm pretty sure that scientists are able to date the fossiles by the amount of radioactive decay that the fossil has given off over the millions of years.

Answer 3

It depends on what kinds of fossils you're trying to date.

Let's start with carbon-14.  C-14 is an isotope of carbon that is made of six protons and eight neutrons.  That's not a stable arrangement, for reasons that would take too long to fully describe.  A nucleus of C-14 will eventually decay into another nucleus, nitrogen-14, which IS stable.  Nobody can predict when one particular C-14 nucleus will decay, but you can predict the probability.  A single nucleus of carbon-14 has a 50% chance of decaying in the next 5730 years.  That means that half of them WILL decay in that time, and half won't.  For that reason, 5730 years is called the HALF-LIFE of C-14.

Cabon-14 is made in the atmosphere by high-energy radiation coming from outer space.  It is incorporated into plants and animals via the carbon cycle, where it slowly decays back into N-14.  As long as the plant or animal is still living, it constantly replenishes the amount of C-14 in its body.  When that plant or animal dies, it stops taking up new C-14 and the C-14 left in the body starts to decay.  5730 years after the organism died, only half of the original amount of C-14 will remain, and 5730 years after THAT (or 11,460 years total since the organism died), the amount of C-14 remaining will only be 1/4 of what it was when the organism died.

You can make a pretty simple chart, then, showing how much C-14 should be left after any amount of time.  All you have to do is figure out how much C-14 the organism had in its body at the moment of death, and scientists work that out by comparing ancient organisms to those alive today.  There are many corrections that have to be made to account for changing conditions, but it paints a pretty accurate picture of the age of an artifact from a once-living organism.

There's a problem, though.  Compared to the age of the Earth, 5730 years is pretty short, and the amount of C-14 in an organism is pretty small to start with.  That means that after a few half-lives, there isn't enough C-14 left to get an accurate date.  C-14 can therefore only be used to date things that are less than 40,000 to 70,000 years old.  C-14 is no good for dating dinosaur fossils then, since the last non-avian dinosaur died out 65 million years ago.

How do you find the dates of fossils older than 70,000 years, then?  Usually, you try to determine the age of the rock layer in which the fossils are buried, and then reason that the fossils must be about the same age as that rock.  You can't use radiometric dating to find the age of fossils directly.  Here's why:

Rocks are formed when liquid magma cools and hardens.  These types of rocks are called igneous rocks.  When an igneous rock cools, that starts a radiometric clock, so to speak.  Long-lived radioactive isotopes that got trapped in the cooling rock begin to decay at a constant rate, and if you can find an igneous rock that hasn't been worn away by millions and millions of years of weathering, you can predict pretty accurately when that rock first cooled.

Not so with sedimentary rock.  When igneous rocks get worn down by water and wind, their particles may eventually coalesce and re-harden to form a new kind of rock called a sedimentary rock.  Because sedimentary rocks are made of parts of much older igneous rocks, you can't use radiometric dating to figure out when the rock actually formed; only when its parts were formed.

When an organism is fossilized, it is buried in sediment and its miinerals are slowly turned into sedimentary rock.  You can see, then, why there would be a real problem using radiometric dating to figure out how old that T. rex femur you just dug up is.

That doesn't mean that radiometric dating isn't helpful in paleontology; volcanoes were erupting millions of years ago just as they are now (and in some cases, even more so).  If you can find some rocks that were thrown out by volcanoes in the same layers where you found your fossils, you CAN use radiometric dating to determine their age.  Again, inferring that everything in the same layer is roughly the same age, you simply transfer the age of the volcanic rocks to the age of the sedimentary rocks, which is the approximate age of the fossils.

When dating rocks that are millions and millions of years old, you can't use C-14, but you can use other radioactive isotopes like uranium-235 or potassium-40.  Both of these have extremely long half-lives, in the hundreds of millions or billions of years, so they can be used to get an accurate impression of the ages of these ancient rocks.

Keep in mind that paleontologists rarely have to rely on just ONE piece of evidence to figure out the ages of things.  In most cases, there are MANY lines of evidence pointing to a certain age, and the wise paleontologist considers all the available evidence before making an informed decision.

I hope that helps.  Good luck!

Answer 4

Radiocarbon dating is used for organic remains up to about 50 or 60 thousand year. After that radio-isotope comparison (Potassium-Argon) dating methods are used for fossils by looking at the igneous rocks surrounding the sedimentary rocks in which they are found.

http://en.wikipedia.org/wiki/Radiocarbon...

http://www.suite101.com/article.cfm/pale...