Friday, February 20, 2015

Question from a Reader: Dating the fossils

I just have a few questions in which I would like some of your insight. I have a strong background in physics and mathematics however I only have a basic understanding of geology/paleontology.

In terms of radiometric dating of the fossils (or of rocks in the same layers as the fossils) I was wondering if you knew exactly how this was commonly done. Once a fossil is found does the paleontologist/anthropologist...
a) sample a single rock in the same layer of the fossil and simply take the age of that rock and attribute it to the fossil?
b) sample multiple rocks in the same layer as the fossil (with these rock ages agreeing to within a small range) and attribute something like the average age of these rocks to the fossil?
c) sample multiple rocks in the same layer as the fossil - with a large range of ages resulting - and attribute to the fossil the most 'expected' age (out of the selection of rocks) in terms of satisfying the evolutionary model?

If the answer is a) or b) I would say this would perhaps support the evolutionary model. If radiometric dating was truly unreliable then it is unlikely it would be able to consistently date fossils to the same era of absolute time. What I mean is if the fossils were really approximately the same age (as in the creation flood model) then there should not be an upward progression of rock ages as we move 'up' the geologic column- as almost the entire geologic column should be roughly the same age. As the dating methods are supposed to be unreliable, and are simply dating rocks all from roughly the same time, shouldn't we expect a more random distribution of rock ages and certainly not an upward progression?? The only scenario in which this is possible (the way I currently see it) is c).

This has really bugged me over the last few months and it would be great to get an answer from someone who knows what they're talking about :)


A Brief Introduction

The ages assigned to fossils by paleontologists are frequently stated as matter-of-fact in news reports with little to no explanation of how they were obtained.  Interestingly, even some young-earth creationists accept these dates and reason that dinosaurs could never have existed because their bones are dated to before the creation of the world.  Children's books, schools, and television programs have so firmly connected dinosaur bones and other fossils with a multimillion-year history that the two are practically inseparable in today's culture.

To be clear, most young-earth creationists (like me) believe that dinosaurs were real live animals that were created by God on Day 6 of Creation Week approximately 6000 years ago.  According to this view, most fossils that we find in the ground, including dinosaur bones, were buried by Noah's Flood, a worldwide event (or series of events) that took place around 2300 BC.  How, then, do paleontologists date these fossils to tens or hundreds of millions of years earlier?

Some History

Artist's depiction of the hypothetical geologic column, with associated
ages and fossils.  Credit: Ray Troll.  Image source.
Back when fossils were first being cataloged by the scientific community, early paleontologists quickly realized that there was some sense of order to the fossils within the rock layers.  That is to say, fossils are not just randomly distributed throughout the rocks.  They can be concentrated in certain layers and absent in others, and certain fossils may only be found in the same layers as another kind of fossil.  Scientists began naming these groups of fossils and rocks to better keep track of them.  While numerous naming schemes were used, eventually the scientific community settled on a particular set of names (some of which you may recognize, such as "Jurassic" and "Cretaceous").  Originally, these names were used to refer to specific packages of rocks and the fossils they contained (for example, "Jurassic" referred to the rocks and fossils found in the Jura Mountains of France and Switzerland), but as the timeline of the history of the world was being constructed, the names became more closely associated with the time periods in which the rocks were deposited.  Thus, a rough outline of geologic history was put together, allowing fossils to be assigned a spot on the timeline based on the rocks in which they were found and on the other fossils with which they were associated.  This is known as "relative dating."  However, numerical ages were not assigned until the introduction of radiometric dating.

Using Radiometric Dating

Our friend Marco specifically asked about the use of radiometric dating for fossils, which gives a numerical age rather than a relative one.  For an overview of how radiometric dating works and some potential problems with it, see my previous posts on the subject.  For our intentions here, we must keep in mind that non-carbon radiometric dating theoretically only gives the age at which crystals cooled from their original liquid state (as magma or lava), which is irrelevant to the fossils and to most of the rocks they are found in.  Carbon dating is typically not used because fossils are thought to be much older than what this method can measure.

In order to use radiometric dating to date a sediment layer, geologists must find a layer that was deposited at about the same time that it cooled.  Buried lava flows would be convenient, as these flow directly onto the rocks below them as they cool, but they are not very common.  Instead, geologists look for layers of ash from large volcanic eruptions.  Ash is spread much farther than lava, so it is relatively common in the rock record.  Ash is also deposited within hours or days of when it is first ejected from a volcano and cools into very tiny crystals.  So, if a fossil is found in a rock layer with abundant ash, then the ash is dated and the age is assigned to the fossil.  If there is no ash in that layer, then layers above or below may be dated to get a range.  If there is no ash in the vicinity, the layer may be stratigraphically correlated with another ash-bearing layer somewhere else.  Stratigraphic correlation can sometimes be a dubious and subjective process, but I will not attempt to detail it here.

A Jurassic Example

Let's see how this process is done with an actual example, which may address Marco's more specific questions.  In the western United States there is a package of rocks called the Morrison Formation that contains numerous dinosaur bone beds.  The dinosaurs found there, including Allosaurus, Stegosaurus, and Camarasaurus, along with numerous other animal and plant fossils, include it in the Jurassic or possibly Early Cretaceous rocks.  A few attempts were made to radiometrically date the layer, but there was no clear consensus until a paper was written in 1998 by Bart Kowallis and others.  This paper is the one cited when paleontologists ascribe an age to a fossil found in the Morrison Formation.

The authors noted that previous dating attempts had been made and conveniently recorded them in a table, which I have reproduced in graph form below.

The orange bars represent the span of probable ages given by each result, such that a long bar indicates a very imprecise result, whereas a short bar indicates a much more precise assignment.  The authors gave various reasons not to trust many of these results, mostly claiming that the minerals used for dating may have been "altered" at some point in their history.

For this study, the authors collected 76 ash-bearing rock samples from seven different locations around Utah and Colorado.  Twenty-eight of these samples had enough datable minerals, though only thirteen were actually submitted for dating.  One sample experienced technical difficulties while dating, and the results of three others were just never reported in the paper, leaving the dates of nine samples to work with.  Each sample was broken down and dated multiple times using the argon-argon method, which is an improved version of the often-erroneous potassium-argon method.  Plotting each individual reported measurement gives the following distribution:

Nearly all the dates fall on the same line, but there is obviously a group that stood out from the rest.  All of these discordant measurements came from the same sample, and they were blamed on "contaminants" by the authors.  The authors noted that this sample contained numerous crystals that did not resemble those of any other sample, so the claim of contamination is not unfounded.  It is, however, still interesting.  After removing the "contaminated" measurements from the data and zooming in to a more reasonable scale, we find this distribution of dates:

I color-coded the data so you can see which measurements came from which sample.  The data appear to "ascend" merely because the results were reported in the order of youngest to oldest for each sample.  The light blue and purple samples appear multiple times in the graph because their argon content was additionally measured using other techniques.  I should also note that the light blue and light green samples were taken from a lower layer than the others, so their older age is to be expected.  The measurements of each sample were averaged to give an age for each sample, which led the authors to report that the Brushy Basin Member (the name of the specific layer) was deposited between 150.3±0.3 and 148.1±0.5 million years ago.  Considering the fossils found within the Morrison Formation and the dates ascribed to layers above and below, the authors concluded that the Morrison Formation is 155 to 148 million years old.  Thus, any bones found in the Morrison Formation are now given an age in that range.


So, we can see that multiple samples are dated several times in order to establish the age of a layer, which rules out Marco's option (a).  Whether option (b) or (c) better represents what is going on here may depend on whom you ask.  Do the ages of these rocks agree within a small range?  All the dates given for the Brushy Basin Member, aside from the contaminants, fall within about 5.5 million years of each other.  Considering we're working on a scale around 150 million years, that's not bad.  However, these were all calculated using the Ar-Ar method.  How do these dates line up with the other methods?

Here I've combined the results from this study (red) with the previous radiometric dates (black for Rb-Sr, blue for K-Ar, and orange for previous Ar-Ar).  The shaded area represents the final age range of the Morrison Formation given by this study.  Obviously, not all is happy and concise, and the authors were quick to discredit the previous dates near the beginning of their paper.  It seems, then, that the ages given to Morrison dinosaurs rely almost entirely on the Ar-Ar method.  I do not deny that most radiometric techniques are highly precise, but I question their accuracy.  If multiple methods were used to arrive at the same date for the formation, old-earth geologists would have a much stronger case.  Yet, I've seen very very few studies in which this occurred.  At the same time, I've seen multiple studies by creationists (and even a few by evolutionists) in which multiple radiometric methods yielded very different results on the same rocks.  All told, there may just not be enough data yet to determine how well the differing methods agree with each other.

A final note

I would also quickly like to address Marco's discussion of the "upward progression" of the rock ages.  While there are reported exceptions, there does indeed seem to be a trend for earlier ages assigned to rocks higher in the geologic column.  This is little surprise, though, as even creationists believe that "upper" rocks are younger than "lower" rocks.  However, where evolutionists see millions or billions of years between these layers, creationists see minutes, hours, days, or sometimes years.  If accelerated nuclear decay can be blamed for the apparent long ages, then this upward progression is perfectly reasonable.  If accelerated decay did not happen, I think there is still enough unknown to merit further investigation without abandoning the creationist model.

Thanks, Marco, for your question!

If you have a burning question you would like to have answered, please send it to, or just leave it in the comments!


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