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Hi! I was just listening to my voicemails. You might find this one interesting.
“Hello, professor. I would like to know more about dating. For example, when historians,
archaeologists, and also ? historians using the term carbon dating. I want to know other forms
of dating and when they can be applied. What is their range and accuracy? Thank you, professor.”
And thank you for your question. Since you mentioned carbon dating,
I am assuming you are referring to the dating of material remains. Yes, it seems lots of people
have heard about carbon dating, but not many other forms. So let’s go over the important ones.
It’s good to keep in mind that we have relative dating methods and absolute dating methods.
Relative dating involves organizing artifacts, deposits, and societies in
sequences. It determines whether something is older or younger than something else.
One way to determine relative dates of items is stratigraphy, that is, the study
of stratification. What’s that? Stratification is a process by which layers of natural and cultural
debris are laid down. A stratum of debris on top of another stratum indicates that the one on top
was deposited after. The basic principle is that artifacts that are in lower strata are earlier,
artifacts that are in a higher strata are later, and artifacts that are in the same
stratum are contemporaneous. So if you look at a succession of layers from the side, you are being
given a relative chronological sequence from the earliest on the bottom to the latest on the top.
Keep in mind that this does not mean that everything lower is earlier and everything
higher is later. Just because one item is on top of another, that doesn’t mean it is later. What
matters is what stratum they are in. If they are in the same stratum they are contemporary.
I have seen people say that if a building stone is underneath another building stone,
that must mean it is older. And yes, the one underneath was placed first,
but if they are found in the same stratum, they are contemporary.
Now you might say, wait, but if the strata have been tampered with
or messed up in some way? Ah yes, that happens. And archaeologists
know they can’t sequence material in disturbed strata. Fortunately,
by doing enough excavation the original strata and the disturbed strata can be differentiated.
Another matter to be concerned with is whether an artifact, like a ceramic pot, for example,
was old or new when deposited in the layer. If it was found in the remains of someone’s house, we
can understand it to have been in use at the time the house was lived in, but if it was found in
an ancient garbage dump, it would have been used earlier than the time the garbage dump was made.
Overall, however, stratigraphy is a great way to determine sequence. But by itself a sequence
is not anchored to a timeline. It needs an absolute dating method to do that.
Another form of sequencing is typological sequencing.
This is the arrangement of human creations: art, artifacts, buildings, etc. chronologically.
Distinctive shapes and decorations, basically what I am talking about is style,
change over time. Often this will be based on improvements in technology, but also just on
what happens to be fashionable. If I asked you to sequence car styles through time,
you could probably do it, though of course that is because you have a basic knowledge of it,
but the same can be done with artifacts, and stratigraphy also helps in this case. Artifacts
produced at the same time are often alike, and so sequencing them across archaeological
sites can be accomplished. Every time new artifacts are dug up, they can be compared to
a well-established sequence, and the relative time of their manufacture can be determined.
Pottery is probably the most valuable artifact to sequence, because its decorations change the most
rapidly, sometimes over periods of just a few decades. By using this method, archaeologists
can arrange artifact styles in serial order to indicate their relative chronology.
As for these relative dating methods, their range is unlimited. They can apply to any
material remains. Their accuracy increases the more material remains that are found.
Okay, so let’s talk about absolute dating, which gives us the age of an artifact in years. The most
commonly known is carbon dating, also known as radiocarbon dating, which you mentioned.
Carbon gets into both plants and animals. After it oxidizes, plants absorb it through photosynthesis,
and animals absorb it indirectly by eating plants (or eating other animals that eat plants).
Plants and animals absorb both carbon-12 and carbon-14,
the latter of which is very rare, and while they are alive, they generally have one trillion times more carbon-12
than carbon-14 in their bodies. After they die, an interesting change takes place. They retain
the carbon-12, but lose the carbon-14. Carbon-14 is radioactive and unstable, so it decays over
time, gradually converting back into nitrogen-14. We can measure how much carbon-12 and carbon-14
is in organic matter. It is therefore possible to measure how long a plant or
animal has been dead based on the ratio of carbon-14 to carbon-12 within it.
The technique is considered accurate up to about 50,000 years. Why? Going back further,
the amount of carbon-14 becomes too miniscule to be measured. By the way, dates ascertained from
radiometric measurements have chosen a neutral international system for dating, and that is
using a date BP (before present). But because the present year changes with each passing year,
they have decided to use a set year, 1950, to count back from. So any time you see a date BP,
know that it means before 1950. I expect they are going to have to change this soon.
Then there is tree-ring dating, also known as dendochronology. In archaeology it is used two
ways: as a method of absolute dating and as a way to calibrate dates calculated
through carbon dating. Here is the reasoning behind tree ring dating: Generally speaking,
a tree produces one ring per year. The more a tree grows in a particular year,
the larger the space between the ring it makes and the one from the year before.
Narrow growth rings are caused either by old age or by lower rainfall and less sunlight. Felled
trees in a certain region have been compared with each other, and their tree rings have been matched
up (this ring in this stump matches with this ring in that stump), so that we now have existing
rings corresponding to every year starting in the present and going back thousands of years. This is
not global. It’s just in certain areas, but in those areas, if we find, say, a wooden object,
timber used in a house, for example, we can see when that tree was chopped down and date it.
Tree-ring dating also can be linked with carbon dating, because we can carbon date trees.
Thermoluminescence dating can determine the time that pottery, baked clay, burnt stone,
and even in some cases burnt soil were heated to a temperature over 500 degrees Celsius.
Minerals, when they are exposed to radiation, accumulate energy at a uniform rate over time,
When minerals are heated to a sufficient temperature, they release all the energy
and their radiation clock starts over. By reheating samples from these objects,
scientists measure the thermoluminescence released and hence date the firing of the material.
Have you ever heard people say, “You can’t carbon date stones.”
True. Ah, but you can optical date them. Optically stimulated luminescence dating,
is similar to thermoluminescence dating, and sometimes gets confused with it. I’ve gotten
them mixed up before. But instead of dating when an object was exposed to heat, this dates
when an object was last exposed to light. In other words, we can tell when it was buried.
Thermoluminescence and optically stimulated luminescence are
considered accurate up to about 100,00 years ago.
Obsidian hydration dating can be used for both relative dating and absolute dating. If a culture
using stone tools knew about obsidian, it’s almost certain that they would have used it, because
obsidian can create very sharp edges and breaks in predictable ways. Obsidian accumulates a rind over
time, and when you break it, the newly exposed surface will start accumulating a new rind.
So by measuring the rind, you theoretically can see how long it has been since the break.
Since a break made by human hands is usually when a tool is being made, this can be useful to know.
Relative dating is straightforward: By measuring the thickness of the rind, you can easily
determine if one artifact in a certain area is older than another. And if you know the rate at
which water diffuses into the glass to create the rind for that particular chunk of obsidian (the
rate is not uniform throughout the world, because it is affected by temperature), you can determine
the absolute age of the object. This technique is considered accurate up to about 500,000 years ago.
Some other forms of dating used, but not as commonly, are amino-acid racemization,
considered accurate to about 100,000 years ago, which dates bone and shell by the rotation of
amino acids, which can indicate the date of death of a living organism,
and archaeomagnetic dating, considered accurate to about 10,000 years ago, which can date the
firing of clay by looking at how iron particles in it have been affected by earth’s magnetic field.
There are many long-term dating methods, which are used for the study of prehistory,
that I have not mentioned, because I am sticking with the ones that apply mostly
to the study of ancient history. But if you are interested in the dating of older stuff,
prehistoric archaeologists would be the ones to talk to about that.
All the dating methods I mentioned are archaeological dating methods, that is,
they help us to date material remains, but you know, there are also historical dating methods,
employed by ancient historians like myself, to determine the timing of events. And yes,
archaeological and historical dating methods go hand in hand, but maybe in another video
I will tell you about these historical dating methods, because they are interesting too! But
somebody has to ask me the question first, so if you’d like to ask me that,
or any other ancient history question, you can do that at speakpipe.com/DavidMiano.
You might like my little e-booklet, Why Ancient History Matters. It's designed to persuade people
that the subject is important, even in the modern world. You might also wish to use it
to help spread the word, so feel free to share it with someone you know. It's free for anyone who
wants it. I've left a link in the description box below the video for you to grab a copy.
Catch you later.