## The age of things – radiometric dating (again)

15 Apr

The most famous ship that didn’t sink

I feel the need to re-iterate my explanation of carbon dating. We had a recent quiz which was entirely based on carbon dating, that my students have cleverly manipulated into a warning flare to alert me to the fact that we need to slow down and be sure that everyone’s aboard before the ship goes sailing away.

Earlier, I posted this example of radiocarbon dating as a simplified exercise in determining how long it has been since a carbon-based organism was alive.

Here, I’m going to just walk through our recent quiz:

You are involved in an archaeological dig site of prehistoric humans.  You find some samples from several of the people you suspect lived in the site and do radiometric dating.

1. Assuming an original steady state ratio of   1 part 14C: 100 parts 12C, and a half-life of Carbon-14 of 5700 years, how old is the site if your samples have a 14C:12C ratio of 1:800?

What’s your starting point? —- an original steady state ratio of   1 part 14C: 100 parts 12C. This means, when this archeological site had people living in it, they had a 1:100 carbon ratio. This comes from their continued input of carbon sources from their environment all containing that 1:100 ratio.

what’s the endpoint? —- your samples have a 14C:12C ratio of 1:800. Over time, the radioactive carbon in these remains has decayed, so there is less 14C over time. However, the amount of 12C remains constant because this isotope of carbon is stable.

how do we get from 1:100 –> 1:800? the half-life of Carbon-14 of 5700 years. Every 5700 years, half of the 14C decays. After the first half-life (5700 years), the original ratio of 1 part 14C: 100 parts 12C changes. Now we have 0.5 part 14C: 100 parts 12C   –or–  1 part 14C: 200 parts 12C.

follow this for two more half-lives…

1:200 becomes…

1:400, which becomes…

1:800.

How many half-lives is that?

count the arrows (each is a half-life)  1:100 –> 1:200 –> 1:400 –> 1:800

3 half-lives x 5700 yrs / half-life =    17,100 years.

2. in 5700 another years, what will the 14C:12C ratio be?

Tack on one more half-life…   1:800  –> 1:1600

3. What is another method that you might employ to determine the age of this site ?

Here, I was asking for any answer that is consistent with the number of mechanisms we discussed that have been used to estimate time. I announced during the quiz that you could give any answer here, it did not matter whether that method was consistent with dating a sample of this approximate age.

Many of you chose dendrochronology – the means of using a daisy-chain of tree rings to walk back through time. This would require that someone has done the background work for this in the area and a sample of wood from the site from which tree rings could be identified… perfect.

You could have said, look at the geological strata if the site. – or mentioned that Paleomagnetism may also enable some reference for dating of rocks (despite the fact that these methods are likely out of scale for a timeperiod of 17,000 years.)

Extra Credit –

1. During the dig, one of your students falls down a well and is left for dead. Given the increased carbon in the atmosphere due to burning fossil fuels, if a future archeologist were to try to date this student’s remains assuming the original ratio of isotopes given in question #1, would this scientist overestimate or underestimate the time since your student died?

This question requires you to remember that fossil fuels are the result of very ancient carbon sources. because of their age, they are entirely depleted of 14C. When these fuels are burned, combustion results in CO2 (all of which is  12C) entering the atmosphere. This would skew the 14C:12C ratio in favor of 12C. Therefore, out student would have a ratio of greater than 1:100 – perhaps 1:200 as his baseline at time of death. would this scientist overestimate or underestimate the time since your student died? They would overestimate – in fact, the student appears to be 5700 years old right away.

2. What would the atmospheric carbon ratio be today if these scientists thought that your student died at the same time as the other prehistoric humans?

1:800 – i.e. the student’s ratio of carbon isotopes would have to be the same as those found in the prehistoric remains. Quite a co-incidence!

Posted by on April 15, 2013 in Uncategorized