Absolute dating calculations

When an organism dies it ceases to replenish carbon in its tissues and the decay of carbon 14 to nitrogen 14 changes the ratio of carbon 12 to carbon Experts can compare the ratio of carbon 12 to carbon 14 in dead material to the ratio when the organism was alive to estimate the date of its death. Radiocarbon dating can be used on samples of bone, cloth, wood and plant fibers. The half-life of a radioactive isotope describes the amount of time that it takes half of the isotope in a sample to decay.

In the case of radiocarbon dating, the half-life of carbon 14 is 5, years. This half life is a relatively small number, which means that carbon 14 dating is not particularly helpful for very recent deaths and deaths more than 50, years ago. This is accomplished by comparison with a location where annual deposition continues. Consider the varve sequences at locations A, B and C. At A, yearly deposition continues, so the absolute ages of layers can be determined by counting down from the top.

At locations B and C, deposition stopped at some time in the past. Indeed, some of the top layers may have been removed by erosion. However, by pattern matching, five layers within the series at A can be correlated with five layers at the top of B. Similarly, by pattern matching, five layers in the series at B can be correlated with five layers at the top of C.

Since the ages of the layers at A are known by counting down from the top, layers at B that correlate with them can also be assigned ages. Then, the ages of the rest of the layers at B may be determined by counting down. In similar fashion, layers at C that correlate with layers at B may be assigned ages, and the rest of the layers at C may be assigned by counting down.

Using this method, ages of varves that formed tens of thousands of years ago may be determined. For example, varves close to forty thousand years old have been dated in Japan. Pattern matching is also used to date trees by examining growth rings dendrochronology. Ages up to 14, years have been determined in this fashion.

An archeologist finds a dried out, abandoned flood plain at location 'A'. He drills a hole and extracts a drill core that shows a series of layers of sediment one of which contains pottery fragment 'X'. The archeologist then contacts his colleague who is working in a nearby area location 'B' where there is a modern floodplain to which a layer of sediment is added every year.

He asks his colleague to extract and send him a drill core from location 'B', making sure to include and label the most recent layer, deposited in She does so, and also includes another drill core from a third location 'C', where she has recently worked. She tells him that location 'C', like location 'A', is also a dried out, abandoned floodplain. The first archeologist wants to know in what year the layer containing pottery fragment 'X' was deposited. In what year was the layer formed that contains pottery fragment 'X'?

My answer to Question 1: The layer containing 'X' was deposited in: Indeed, dating of lake sediments using varves was undertaken as early as Their disadvantage is that they are restricted to sites where annual deposition has occurred and the absolute age of at least one layer can be determined with confidence by some other means for example, by counting or by pattern matching with places where annual deposition continues through to today.

Places satisfying these requirements are relatively few. Another disadvantage is that over geologic time, preservation of such layers is limited. Absolute age determination by varve counting is only suitable for materials less than several tens of thousands of years old. These limitations are overcome in radiometric dating. Radioactive elements, such as certain isotopes of uranium, thorium, rubidium, potassium, carbon and others, have the property that over set periods of time, known as their 'half lives' which are different for each radioactive element , half of their atoms decay to form atoms of different elements.

For example, over the course of million years, half the atoms of the 'parent' element uranium U decay to form atoms of the 'daughter' element lead Pb Over the next million years, half of the remaining U atoms change to Pb, and so on. By comparing the ratios of U to Pb that are found in the material today, the time when the process started may be ascertained see table below. Examples of radioactive parent-daughter pairs and their half lives include: U - Pb 4. An error of that magnitude may be quite acceptable for such old rocks. After careful analysis, a geochronologist determines that an unweathered, unmetamorphosed mineral sample contains 7 trillion atoms of the radioactive element K and trillion atoms of its decay product A How many years ago was the sample formed?

The number of years ago that the sample formed is: It is important to choose a radioactive parent-daughter pair whose half life is appropriate for the age of the material being dated. On the one hand, the half life should be short enough so that a measurable amount of the daughter element has formed. On the other hand, if the half life is too short, the amount of parent element left may not be measurable.

Thus, K-Ar dating would not be appropriate for a material that is 50, years old, as hardly any daughter element would have formed. Similarly, C dating is not be appropriate for materials older than about 70, years as the amount of the parent element left becomes too small to be measured accurately. Radiometric dating depends on certain assumptions. The most fundamental assumption is that the half life of a parent-daughter pair does not change through time.

Experimentally and theoretically, that assumption seems justified. Also, successful cross-checking of ages using different dating techniques on the same sample supports the constancy of half lives. For example, C dates may be checked against ages determined through varve counting. A second assumption is that the system is closed.

How Does Radiocarbon Dating Work? - Instant Egghead #28

That is, no parent or daughter material has been added to or lost from the material being dated. Such addition or subtraction may occur if the material mineral or rock has been weathered or metamorphosed. Therefore, material to be dated must be carefully examined to determine whether such processes may have taken place. Because the dating method depends upon comparing the ratio of parent to daughter element, the assumption must be made that the amount of daughter element initially present be zero or else be determinable.

Igneous rocks and highly metamorphosed rocks are the best candidates for radiometric dating because for them, for reasons that won't be discussed here, it can relatively easily be determined whether the initial amount of daughter element present was zero or, if it wasn't zero, what was the initial amount. The 'age' of an igneous rock refers to the time when the magma or lava from which it formed cooled below a certain temperature. A useful material for dating that time is the mineral zircon, a minor but common constituent of igneous rocks.

How is radiometric dating used to estimate absolute age

As magma or lava solidifies, the elements zirconium Zr , silicon Si and oxygen O link together to form zircon crystals. If uranium U atoms are in the vicinity, they may be incorporated into the zircon in place of Zr atoms.


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This substitution is possible because the size and charge of the U is similar to that of Zr. When people talk about carbon fixation, they're really talking about using mainly light energy from the sun to take gaseous carbon and turn it into actual kind of organic tissue.

Absolute dating

And so this carbon, it's constantly being formed. It makes its way into oceans-- it's already in the air, but it completely mixes through the whole atmosphere-- and the air.

And then it makes its way into plants. And plants are really just made out of that fixed carbon, that carbon that was taken in gaseous form and put into, I guess you could say, into kind of a solid form, put it into a living form.

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That's what wood pretty much is. It gets put into plants, and then it gets put into the things that eat the plants. So that could be us. Now why is this even interesting? I've just explained a mechanism where some of our body, even though carbon is the most common isotope, some of our body, while we're living, gets made up of this carbon thing.


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Well, the interesting thing is the only time you can take in this carbon is while you're alive, while you're eating new things. Because as soon as you die and you get buried under the ground, there's no way for the carbon to become part of your tissue anymore because you're not eating anything with new carbon And what's interesting here is once you die, you're not going to get any new carbon And that carbon that you did have at you're death is going to decay via beta decay-- and we learned about this-- back into nitrogen So kind of this process reverses. So it'll decay back into nitrogen, and in beta decay you emit an electron and an electron anti-neutrino.

I won't go into the details of that. But essentially what you have happening here is you have one of the neutrons is turning into a proton and emitting this stuff in the process. Now why is this interesting? So I just said while you're living you have kind of straight-up carbon And carbon is constantly doing this decay thing. But what's interesting is as soon as you die and you're not ingesting anymore plants, or breathing from the atmosphere if you are a plant, or fixing from the atmosphere.

And this even applies to plants.

Carbon 14 dating 1

Once a plant dies, it's no longer taking in carbon dioxide from the atmosphere and turning it into new tissue. The carbon in that tissue gets frozen. And this carbon does this decay at a specific rate. And then you can use that rate to actually determine how long ago that thing must've died.

How to calculate absolute dating

So the rate at which this happens, so the rate of carbon decay, is essentially half disappears, half gone, in roughly 5, years. And this is actually called a half life. And we talk about in other videos. This is called a half life. And I want to be clear here. You don't know which half of it's gone. It's a probabilistic thing. You can't just say all the carbon's on the left are going to decay and all the carbon's on the right aren't going to decay in that 5, years. So over the course of 5, years, roughly half of them will have decayed. Now why is that interesting?