Collaborations for Week Four

Welcome to the Week 4 lab. One of the first questions that people commonly ask about an artifact or feature is

“How old is it?” So in this lab we are going to learn about some of the most commonly used archaeological dating

techniques, and their strengths and weaknesses. First of all, there are two main categories of archaeological dating

methods – relative dating and chronometric, or absolute, dating. Relative dating tells us if something is younger,

older, or the same age as something else. There are three types of relative dating – stratigraphy, seriation, and cross-

dating. Stratigraphy is the geologic or archaeological layering of deposits over time. There are two laws of

stratigraphy. The first law is the Law of Superposition. This means that in any sequence of undisturbed strata, or

layers, the strata above will always be younger than the strata below. The other law related to relative dating is the

Law of Association. This means that all inclusions, like artifacts or ecofacts, found within an undisturbed stratum

were deposited at the same time as the formation of the strata. In other words, all of the material in each strata are

contemporaneous. The second type of relative dating technique we are going to learn about is seriation. Seriation is based on the observation that styles change gradually over time. Seriation is the arrangement of artifacts in chronological order. For example, we see in this photo that this particular type of ceramic storage container used by the Moche of South America changed gradually over time. And the third kind of relative dating is cross-dating. This is based on the idea that a particular kind of artifact found in an undated context will have a similar date to one found in a dated context. Sometimes this method makes use of what we call index fossils. An index fossil is a fossil of an extinct species that only existed for a relatively short period of time and then went extinct. If we know the date of that fossil from one site, we then can be pretty sure that if we find the same fossil in another context that does not have a date, we know it was from the same time frame. Next, let’s talk the other major kind of archaeological dating method – absolute, or chronometric,

dating. Chronometric dating is dating that provides an age, expressed in units such as years. But before we start

looking at the various chronometric dating methods, we need to keep in mind the following four questions that must

be asked about each technique:  What material is being dated?  What event is really being dated?  What is the appropriate time range for the method?  What are the potential problems associated with the method? As we examine some of the most commonly used chronometric dating methods, we are going to try and answer

these questions. The first method we are going to cover is dendrochronology, more commonly known as tree-ring

dating. Dendrochronology was developed in the early 1900s by Andrew E. Douglass, an astronomer at the Lowell

Observatory in Flagstaff, Arizona. He noticed that the patterns of growth rings in particular species of trees will

vary depending on yearly rainfall patterns. In years with lots of rain, the rings will be much wider than years of

drought. This method will only work on certain species of trees that are sensitive to climatic fluctuations, like

juniper and ponderosa pine. A regional growth-ring sequence is then developed, starting with the present and

working back in time. In some parts of the world, tree-ring sequences have been developed that go back over 9,000

years! So let’s think back to the questions. What material is being dated? Wood. What event is being dated? The year the

tree was cut. What is the time range for this method? Up to 10,000 years, depending on the region. And what are

the potential problems? Well, the wood may have been reused in construction, especially in regions of scarcity. So

the wooden beam might be older than the construction of the feature. Older, weaker beams might have been

replaced with newer and stronger beams, so the wood might be younger than the construction. K/Ar Thermoluminescence

Now we will look at another category of chronometric dating called radiometric dating techniques. Radiometric

dating is a chronometric technique that is based on the decay of unstable radioactive isotopes into stable elements

such as argon, potassium, carbon and uranium. The most commonly used radiometric technique is radiocarbon

dating. This method was developed in 1949 by a chemist named Willard Libby, who was researching the effects of

cosmic rays on the earth’s atmosphere. This method is used for dating organic materials, and is based on the

constant rate of decay of the radioactive isotope Carbon-14. Let me tell you how radiocarbon dating works! When cosmic rays from the sun enter the earth’s atmosphere, they collide with atoms and create neutrons. These

neutrons then collide with Nitrogen-14 atoms, knocking a proton particle off of the atom, which then creates a

different kind of atom – a radioactive form of carbon known as Carbon-14. These carbon-14 atoms combine with

oxygen atoms to create carbon dioxide, which is then absorbed by plants through photosynthesis. Animals and

people then eat the plants, ingesting carbon-14 atoms throughout their lifetimes. Once a plant or animal dies, they

no longer are taking in the radioactive carbon-14 (and don’t worry, the amount of radioactive c-14 we ingest is so

small it does not present a health risk!). After death, the radioactive carbon starts to decay back into the non-

radioactive nitrogen-14 atom. We know the rate of radioactive decay. For carbon-14 it takes 5,730 years for half

of the radioactive c-14 to decay into non-radioactive n-14. This is known as the half life- the amount of time it takes

for radioactive atoms to decay into stable, non-radioactive isotopes. So let’s think about those questions again! What material is being dated? Radiocarbon dating is dating anything that is carbon-based, which means anything

that was once alive. This includes things like wood, charcoal, paper, body tissue, shell, and bone. What event is really being dated? Radiocarbon dating dates the time of death of the once living organism. What is the appropriate time range for this method? Up to about 50,000 years ago, although newer technology

might get dates even further back What are the potential problems associated with radiocarbon dating? Contamination of the samples. For example,

handling samples with your bare hands or drops of sweat falling on the sample will contaminate it with modern

carbon traces. Some environments, like areas with high limestone content (which contain large amounts of carbon)

can affect the dates.

Another type of chronometric dating is Potassium Argon dating, which is also based on radioactive

decay. Potassium Argon dating is used to measure the age of certain kinds of volcanic rocks that contain

potassium. This method determines when the rock was last in a molten state, and is based on the decay rate of

radioactive potassium-40 into argon gas. Potassium-40 decays into argon gas at a steady rate. The half-life of

potassium is 1.3 billion years. As potassium decays into argon gas, the argon gas is trapped in the crystal structure

of the rock. When the rock is melted (during a volcanic occurrence) the gas escapes and the buildup of argon starts

again once the rock is cooled. This method can be used on rock deposits that range from 100,000 years to 4.5 billion

years ago, and is most often used for dating hominid fossils by bracketing fossils between dateable volcanic layers. So let’s go back again to the questions: What material is being dated? Volcanic rock. What event is really being dated? The date of the volcanic eruption What is the appropriate time range for this material? Between 100,000 and 4.5 billion years What are the potential problems? The potassium and argon must both stay put in the rock over geologic time. In other words, the gases cannot escape from the rock.

Thermoluminescene

Thermoluminescence is a method that is used on crystalline materials like ceramics or glass. These kinds of

materials trap electrons that are trapped in the material. The electrons accumulate over time, and then when the

material is heater above a certain critical temperature ( for example, 400-500 degrees Farenheit for ceramics) the

electrons are released as light energy. The intensity of the light emission is measured and is then correlated to the

length of time that the radiation was built up. The greater the release in energy, the older the material. This method

can be used on materials between 1,000 and 500,000 years old. Let’s apply the four questions once again to this method. First, what material are we dating? Ceramics or

glass. What event is really being dated? The last time the glass or ceramic was heated over the critical

temperature. What is the time frame for this method? Between 1,000 and 500,000 years. What are the potential

problems with this method? The ceramic or glass might have been heated over the critical temperature by exposure

to a natural fire, like a forest fire, instead of when it was last heated by man. So to sum things up, remember that there are two main categories of dating – relative and chronometric. And when

thinking about chronometric dating, be sure to use the four questions to evaluate the best application of the methods

and the potential problems that might be associated with the methods. Now you will go on to do the Lab Worksheet

and apply the methods you learned here today!