Tag Archives: fossils

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…


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, Imagecombustion 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!


I hope these answers help you to understand the concept here and how to calculate answers for some basic problems.

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Posted by on April 15, 2013 in Uncategorized


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New (to me) Podcast

ImageI just discovered a podcast that I thought some of you might be interested. It’s called paleocast and can be found at:

Check it out and let me know what your thoughts are on their work.

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Posted by on March 25, 2013 in Uncategorized


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What kind of planet do we live on?

This is the last week of my Fall 2012 General Biology class and we have finally gotten to the material I look forward to all year.

Throughout the semester we build towards an understanding of the central dogma (DNA –> RNA –> Protein). Early on we are introduced to this idea and are given the basics that DNA is the information that is required to build cells; which are made of these proteins as well as some other biomolecules. It’s easy to have a protein-centric view of the cell because these molecules provide structure to the cell and accomplish many of the functions of the cell by acting as receptors, enzymes, signaling molecules, etc. (This is an over-simplification, but one I can live with)

Last week we focused on the details of transcription and translation and how these molecular processes read information from DNA to make an mRNA ‘message’ that leaves the nucleus and goes to the cytoplasm where translation of this message results in the production of a protein with the specified amino acid (AA) sequence.

This segued into what happens when there are errors or mutations along the way. Because the AA sequence of the protein is determined directly from what is encoded in the DNA, changes is DNA may have direct consequences on the protein. Another central idea I teach is ‘Form Dictates Function.’ Because the form of a protein is determined by the AA sequence, changes in sequence mean changes in form and therefore changes in function.

So, how does this relate to my opening question, ‘What kind of planet do we live on?’

A Molecular View of The Central Dogma

A Molecular View of The Central Dogma

The process illustrated by the central dogma is fairly faithful. Most often the proteins are NOT mutated and are made just as the DNA directs and they function as expected. However, once in a while, mutations come in and hit the DNA and there is suddenly a DNA change leads to a change in the RNA, that leads to a change in the AA sequence, that leads to a new folding of the protein. This may provide a benefit (very rarely) or may cause a problem (more commonly). If it confers a benefit, this provides an advantage to the bearer of these new proteins and they may be more successful living longer and leaving more children. If it is detrimental, then the bearer may not live as long and may have few, if any, children.

But this is a SLOW process.

If the world was 4000 years old, this process could not conceivably explain the diversity of life on Earth. But if the world is 4.5 BILLION years old, that may be enough time. Darwin struggled with this idea and it was not until he witnessed the wide world during his voyage aboard the HMS Beagle that he started to see evidence that the world was much older than he once suspected.

Andean Fossils

Marine Fossils from the Andean Mountains – Similar to what Darwin found in the same place.

One of the first suggestions that Darwin say about the history of the Earth was the presence of sea fossils high in the Andes Mountains. How could this be? These could only make sense if the mountains were not always mountains, but the Earth changed over time.

With the idea of ‘deep time’, meaning that the Earth is very, very old – on the order of Billions of years – Darwin’s idea of mutations accumulating over time as fuel for evolution becomes plausible.

What kind of planet do we live on?

A very old , changing planet

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Posted by on December 4, 2012 in Uncategorized


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