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

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

An old Rock

My General Biology class is discussing means by which we can date objects. This comes from a question by Ernst Mayr, “What kind of world do we live in?” The basis of this question is, how old is the planet? and how constant is it?

In biology these questions have notable importance because how we explain life’s processes requires that we know how much time these processes are operating over. One would be limited in their explanation of life if we know the world to be young – say, only 1 million years old. Whereas, the current estimate of 4.5 billion years, including ~3.5 billion years with life of some kind, allows for much slower processes to operate.

I discussed one of these measurements, radiometric dating, in an earlier post.  Take a look at how this is done and be sure that you understand the practice problem presented and can work out a similar problem on your own for the quiz.

Posted by on April 9, 2013 in Uncategorized

## How Old (Con’t)

From 1912 until about 1953, biologists interested in human evolution were being duped.

One hundred years ago, Charles Dawson presented his new find, a transitional fossil of an organism that plainly appeared part human and part ape, bearing a number of hallmarks of being a ‘missing link’ between modern man and early ancestors.. The fossil, found in Piltdown, England and was dubbed Eoanthropus dawsoni and was accepted as a welcome addition to the record of humanity’s existence.

It was just what was expected.

The Piltdown Gang

Expectations make fertile soil for a hoax.  Darwin’s work predicted such a find would be made. The question was merely, who would find it? What would it look like? And how famous would this make the man who discovered it?

“Sir Arthur Keith, famous British paleontologist, spent more than five years piecing together the fragments of what he called a ‘remarkable’ discovery. He said the brain case was ‘primitive in some respects but in all its characteristics distinctly human.'”1

Over time, When the skull fragments of E. dawsoni, commonly called Piltdown man, were examined, doubts were raised as to whether it represented a single organism or several, which just happened to become mixed together in the unearthing. But these doubts took decades to culminate into action.

The best way to address the question was to determine whether the several pieces of skull were at least contemporaries of one another. They could still be a jumble, but it was a start. To assess the age of the fragments, fluorine dating was done. This method is used to determine the amount of time that a sample has been buried underground. The principle is that groundwater contains fluorine and the longer a sample remains buried, the more fluorine will become absorbed into the sample. This testing confirmed that the samples could still have come from the same source, but that they were both considerably more recent that initially suggested.2

Following this analysis, Carbon dating gave a more accurate age of the samples themselves indicating that they were both quite recent, but not from the same organism. Once this data came in, the house of cards fell and a number of other observations came to light confirming the hoax.

What does this teach us?

1. Science is difficult business. When everyone is working honestly, it is difficult. When people are willfully trying to subvert the process, it can take years to remedy. (I immediately think of the damage done by Andrew Wakefield’s fraudulent 1998 Lancet paper and subsequent work that undermined the public’s trust in vaccines)

2. Science is self-correcting. Again, this can take time, but eventually, mistakes are worked out and our understanding of the world gradually improves.

3. People are people. With an obvious prize, people sometimes make their own luck.

4. No single experiment will always give accurate data. Extra-ordinary claims require extra-ordinary evidence.

References

1. “The Piltdown Man Discovery: Unveiling of a Monolith Memorial”Nature 142, 196-197 (30 July 1938) | doi:10.1038/142196a0

2. “Relative Dating of the Piltdown Skull” Kenneth P. Oakley, Advancement of Science 1950

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Posted by on December 20, 2012 in Uncategorized