Absolute Dating

Geologists can compare layers of rock to decide which are older or younger, and which fossils represent animals that lived long ago or more recently. This process is called relative dating.

But relative dating does not give us a NUMBER. If we want to ask, "Yes, but WHEN did this rock layer form?", we need a different tool. When we try to measure the number of years that have passed since a rock formed (or since a piece of pottery was crafted, or since a tree died), we are trying to do absolute dating (the fancy word is time-measure: chronometry).

There are several techniques that can be used to assign a numeric age to a specimen. For our purposes we'll discuss two that are broadly applicable to fossil specimens; radiometric dating and luminescence dating.

The age ranges assigned by these methods are not the last word on how old a rock actually is! That's because different methods, or different laboratories, can give us conflicting dates. Thousands of scientists have spent the past decades improving the chemistry and technology for absolute dating.

Radiometric Dating
What are isotopes?


 * First, let's distinguish them from a similar-sounding word, ions.
 * Chemical bonds between atoms can shift the positions of their electrons, and an atom (or group of atoms) can gain or lose electrons in a break-up.
 * If we dissolve calcite mineral, the water will gain positively-charged calcium ions (Ca+2), and negatively-charged carbonate ions (CO3-2).
 * Nothing has changed in the nucleus of these atoms!
 * However many neutrons and protons they started with, it's the same now.
 * The outer electrons direct how the atom will interact with other ions.
 * The protons and neutrons in the nucleus direct how the atom will behave generally: its mass, strength, and how many electrons would help it be stable.
 * An isotope is a version of an atom that has a different number of items in its nucleus.
 * Many elements have stable isotopes.
 * Oxygen's atomic number is 8.
 * Your typical oxygen atom will have a nucleus with 8 protons and 8 neutrons. This isotope of oxygen has 16 items in the nucleus. We call it O16.
 * Some rare oxygen atoms have an extra neutron! This isotope of oxygen has 17 items in the nucleus. We call it O17.
 * Very rare oxygen atoms have two extra neutrons. This isotope of oxygen has 18 items in the nucleus. We call it O18.
 * These atoms will always behave like oxygen. The heavier isotope might make stronger bonds. An oxygen can join an ion and have a negative charge. But it will always be oxygen. All isotopes of oxygen are stable.
 * Some isotopes have stable AND radioactive isotopes.
 * Carbon's atomic number is 6.
 * Most carbon atoms have 6 protons and 6 neutrons in the nucleus. This isotope of carbon has 12 items in the nucleus. C12.
 * Some stable carbon atoms have an extra neutron. This isotope of carbon has 13 items in the nucleus. C13.
 * Some carbon atoms have two extra neutrons. This isotope of carbon has 14 items in the nucleus. C14.
 * Carbon 14 is radioactive. It is not stable.
 * Carbon 14 has 6 protons and 8 neutrons. Eventually, one of the neutrons can eject a tiny beta particle and swap to act like a proton. If the atom now has 7 protons and 7 neutrons, it just turned into ... a Nitrogen atom!

If you have a pile of C14 atoms and you check back in 6,000 years, about half of them will have turned into N14.

The half-life of carbon 14 is about 5,700 years. Here [link is a handy video explanation from Scientific American. ]

The time it takes for half of any amount of radioactive material to decay to a daughter element is referred to as a half life. Based on this time and the ratio of parent to daughter element in any given sample allows us to assign how much time has passed for that sample.

Carbon Dating
Carbon dating is the most well known of radiometric dating methods.

What: items with carbon in them. Sea shells, bones, wood, pottery, etc.

Good to measure: thousands of years

Half-life: 5730 years

This technique measures the ratio of radioactive isotope Carbon-14 to its stable isotope peer Carbon-12. The half-life of this process is 5730 years. We can get very precise and accurate dates on items that are a few thousand years old, and pretty good estimates for items that are up to 50-60 thousand years old. But! Eventually all the C14 in a given item will transform. That item is now "C14 dead". If you can't measure any C14, you can't estimate a date.

''ALSO weird! Some systems get goofy carbon concentrations, such as hot springs and tufa deposits. If you don't know how your C12, C13, and C14 SHOULD be mixing, you can't interpret measurements to tell time. There's a funny line in the movie Prometheus where the geologist gives a C14 date for a dead alien (spoiler) and Dr. Ritterbush is in the theater like, "Girl. You don't know the FIRST thing about C14 ratios on this planet. STOP."''

Argon-Argon Dating
What: minerals that form in igneous rocks and include some Argon in their structure

Good to measure: hundreds of millions of years

Half-life: 1.25 billion years

Geoscientists have made this method much more precise and accurate over the past 20 years by working together at labs around the world to get all the machines measuring it equally.

Uranium-Lead Dating
What: minerals that form in igneous rocks and include some Uranium in their structure

Good to measure: hundreds of millions of years

Half-life: 710 million years

Instead of one parent-daughter pair (as in C and N), Uranium-238 to Lead-206 has LOTS of steps. The half-life of this method is 710 million years which makes it a useful middle ground between the above methods. The longer series of decay reactions adds to the precision of this method, because there are more parts to measure.

Here are recent observations using this technique on rocks made from volcanic ash.

The volcanic ash forms layers of rock:

After geologists (a hard-working team of graduate students, probably!) find the target minerals in their rock samples, they can check the minerals in a scanning electron microscope.

Geoscientists can zap the grains with very tiny lasers. This lets us count atoms in super-tiny areas of the mineral grains. This map shows mineral grains that have been sliced into microscope slides and lit up with a cathode ray, to show the layers. That's right, instead of the entire mineral forming instantaneously as a single crystal, these minerals can grow layers like rock candy. This means that each layer of crystal has its OWN formation date. By now, the lasers we use are so tiny, and our machines that tally atoms are so strong, we can get multiple dates from a single grain! This is good, because it gives us more data and allows us to estimate our precision and accuracy better. But it's also SO ANNOYING because OY geology is challenging!!

Luminescence Dating
What: minerals exposed to the sun as part of a sediment

Luminescence dating is a relatively new dating method which is often used as a second check on ages retrieved by other means. As grains within a sample sit after deposition, they slowly lose radiation that was accumulated while they were exposed to direct sunlight. By bringing these grains into a controlled environment (thermally or optically) this radiation can be released in such a way that we can measure how long the object has been buried.

Because of the nature of these methods it is less consistently retrievable in a sample than something like radiometric materials. Additionally the rate of decay in the sample is not as well constrained making it less accurate as a stand-alone method. For this reason it is often used to increase confidence in the numbers retrieved from another method.

What to Look For!
In this class, we will NOT memorize the exact dates assigned to geologic time intervals.

Instead of memorizing exact numbers, we want to:


 * remember the basic estimates:
 * The Triassic Period began about 250 million years ago.
 * The Jurassic Period began about 200 million years ago.
 * The Cretaceous Period began about 150 million years ago.
 * The Cenozoic Era began about 65 million years ago.


 * Understand WHY the absolute dates change as science progresses.

If we estimate the age of a rock layer or geologic time interval, that exact number keeps changing as we get better at absolute dating techniques.


 * In the 1990s, we estimated that the Jurassic Period began 208 million years ago.
 * Twenty years later, we estimated that the Jurassic Period began 202 million years ago.
 * In 2008, labs around the world coordinated to estimate that the Jurassic Period began 201 million years ago.

That sounds silly, but remember: we're trying to measure when a piece of ash fell to the ocean floor 201,000,000 years ago.

If we see a date assigned to a rock (or fossil, or human item), what should we look for?

Pay attention to Precision and Accuracy.


 * At archery lessons, Dr. Ritterbush always hit the same spot on the target. All her arrows hit within five inches of each other! Dr. Ritterbush is a very PRECISE archer!
 * Sadly, Dr. Ritterbush couldn't get the arrows to hit the bullseye. They would all be clustered tightly together at, like, the third ring and over to the left. Dr. Ritterbush is not yet a very ACCURATE archer!

Geoscientists publish absolute dates with estimates of the ERROR on that date.


 * The error on the date combines both precision and accuracy.
 * Chemists can tell you how good their machine is at getting a consistent answer. Maybe they run the same sample ten times, and the answer changes each time. We can measure the precision of the instrument. Low precision = the date is an estimate!
 * A geo-chemist can also run a bunch of samples from one rock layer. Each sample might give a slightly different isotope measurement. Technically, all of these answers are "right", but if they don't converge on one answer, it decreases the precision of our estimate.
 * Different methods can give us consistent, but disagreeing, values.
 * Throughout the 1990s and 2000s, Argon-Argon dating gave values consistently below Uranium-Lead dating.
 * Each method was increasingly precise, but they could not both be totally accurate!
 * We now estimate that the Jurassic Period began 201.38 +/- 0.31 million years ago.
 * That means our BEST GUESS is 201,380,000 years ago.
 * But it could have been 310,000 years earlier, or later!

Radiometric dating methods are constantly improving, thanks to efforts of scientists on every continent. Ten years from now, the exact dates we guess for geologic time, and the precision and accuracy of those estimates, will be different from today.

We are also always comparing absolute and relative dating. The methods often give different answers, so we have to try again to improve all of our methods.

Can you sense a theme here?

We need to be aware of the observations and interpretations that scientists make, rather than memorizing a list of facts.