Hello and welcome back. Please remember to keep up with the assigned reading, to complete all course activities before the posted due date, and to contact your TAs or myself with any questions. In this lecture, we will begin discussing archaeological dating methods. These are the methods we use to determine how old something is. These are very important for all excavations. If we don't know how old something is, we simply can't understand our site. Furthermore, the first question anyone asks when they find something is "how old is this?" Dating what we find is one of the most important components of archaeology. Before we begin discussing different dating methods, we first need to discuss dating conventions that are used in archaeology. There are three main dating conventions we use. The first is one that may seem familiar - this is called the BC/AD convention. You have probably heard of this convention before. BC means "Before Christ" and AD doesn't mean "after death" - a lot of people think it means that. It is Latin for "Anno Domani." This is a Latin phrase meaning "in the year of the Lord." Now, this convention uses the Julian or Gregorian calendars based on Medieval Christianity. This remains a popular convention used in mass media. Some anthropologists and some archaeologists do use it in certain contexts, but we consider it very ethnocentric. Many of the cultures we study existed before any historical Christ figure and many of these cultures likewise don't believe in Christianity. As a result, we have two other conventions that we do apply. Our second convention is similar. This is called the BCE/CE convention. BCE means "Before Common Era." CE equals "Common Era." This is based on the premise of a so-called "Common Era" which started in the year zero. In this way, it's similar to BC/AD, but it's a little little less ethnocentric. Many archaeologists do use this convention, but we still see it as somewhat ethnocentric because "Common Era" isn't well defined. Our third convention is the convention we call BP. BP stands for "Before Present." This convention states how old something is. For example, 400 BP is the same as saying 400 years old - in theory - however, BP doesn't mean our present today. The BP convention was developed in 1950, so BP calculates how many years before 1950. So there's still some issues with this convention. Several issues can arise and this includes citing older published data. Now, there is no clear cut answer to which dating convention to utilize. I just want to introduce these three different dating conventions so that you're aware of them because you may see them in different contexts throughout this course. I will try to stick with BCE/CE for most of the material that I'm going to present. All right, when we report dates as an archaeologist, we usually incorporate several different abbreviations, so let's go through some of these more common abbreviations. Here, the little lowercase "c." with a period typically means "Century" so here we might say the 4th Century BC or the 4th Century BCE even. We might see in lowercase "kya." Here, "K" means thousand years ago, so "kya" would mean "thousand years ago." So 300 kya would be mean 300,000 years ago. Likewise, "mya" means million years ago, so 63 mya really roughly translates to 63 million years ago. So now that we've covered these different conventions, let's discuss how we date archaeological materials. We can date artifacts, ecofacts, features, and layers in two manners. We can date things through what we call direct dating. This is where we directly date an artifact, a feature, an ecofact, or a stratum. Direct dating lets us date something directly - we're getting a date from that very particular thing that we want to date. So let's say for example we find some wood on our site. Here we have some Roman ships that were found outside Naples. These used to be in a harbor and they sank and then that harbor got silted in and today it looks like it's on the mainland, but it's actually the old harbor that just silted in and when they dug down, they found the ships. So let's say we want to date the wood itself. We can directly date the wood to see how old the wood is and then that would help us understand the age of the boat. Now we're not dating the ship in its entirety because that one board that we date may be much older, it might have been a reused timber, or it could be a repair that's a much more recent piece of wood. So that ship may have already been 50 to 60 years old when that particular timber was placed on it. This is why we need to be very careful with direct dating. We need to define what it is that we are dating and what it is that we are not dating. Now this differs from the second type of dating and that is indirect dating. Indirect dating is where we date something indirectly, so we may date something based on its association with something else. For example, we can date pottery that's found on a shipwreck. This is exactly what you're seeing. You have a photo of all these amphorae, these are basically wine and olive oil storage jars that were on a ship that sank and now they're on the bottom of the sea. So we could indirectly date the shipwreck because we could date the pottery that's found there. These different pots have different styles that changed over time much like cars have different styles that change over time and experts in cars can know what style car was associated with which year that that that model was manufactured. The same with pottery experts. There are pottery experts that understand what range of dates different styles of pots were manufactured. This will help us understand how old the pot is and then we can indirectly date the ship from that. So we can use the date of the pots to indirectly date the shipwreck itself. In this case, we're dating the pottery on the shipwreck, but not the ship, so we're indirectly dating the shipwreck. When it comes down to dating something either directly or indirectly, we have two different types of methods. We can date things relatively or using absolute dating methods. Our relative dating methods give us a relative age of whatever we're dating. We don't get a date or a date range. Instead, we learn if something is older than or younger than something else. This allows us to order things into a sequence without specifying the actual age of something. For example, we could say that a covered wagon is very generally speaking older than a Jeep. That's just a very simple generalization. That is relative dating. In addition to relative dating methods, we also have those absolute dating methods. Absolute methods give us a calendar date for how old something is. Here, we don't get an exact age. Instead, we typically get a range of dates that are statistically determined. Now that we've introduced different dating conventions and broad methods, let's begin discussing individual dating methods. We'll start by discussing various relative dating methods first. We're going to begin with a discussion of stratigraphic sequencing. Stratigraphic sequencing is a relative dating method and it relies on understanding the stratigraphy or the layers that are found on a site. It employs the Law of Superposition to help us date materials. The Law of Superposition states that deeper strata are generally older. We use the Law of Superposition and a sites stratigraphy to to see what is older or younger than something else. We then determine the relative age of artifacts based on the stratum that they were found in. Very generally speaking, the deeper we go the older the stratum. Likewise, material found in the same stratum is generally of the same age. Now there are some limitations to using stratigraphic sequencing. We have to pay attention to various transformational processes. Strata can get mixed up by cultural processes and natural processes and we call these C and N transforms. They are cultural things and natural things that mix strata up. You can think of a natural thing that mixes strata being maybe a squirrel or a badger digging a hole and mixing up different soils together. Culturally, we have construction that moves soil around a whole lot and so we might get strata that get mixed together because of construction. Also we have to keep in mind that the stratigraphy that we see on a site indicates the age of deposition. Some artifacts may have been hundreds of years old when they were first deposited. This can then skew our interpretation of the relative age of the stratum. So in general, understanding stratigraphy is key to understanding any site. The second relative dating method that will be presented here is what we call sequence comparison. This is also sometimes called cross dating. We can use diagnostic artifacts whose date is known to determine the age of a stratum. This is something we do all the time. Usually we use coins to help us date how old a stratum is. So the basic principle of cross dating is that the deposit cannot be older than the youngest artifact found in it. Let's say we find a penny from 1856 in a stratum. That means that that stratum can't be older than 1856. We call this the "terminus post quem." It's a Latin phrase that means "date after which." It's basically the youngest date that a deposit can be. So think here about that penny. There are limitations to cross dating, for example we have to be very careful particularly when we're talking about coins because some coins are circulated for a very long period of time. Coins get curated as collectibles, they get forgotten in pockets and in drawers, and sometimes they get repurposed as jewelry and clothing adornments long after they are no longer circulated, so just because we have a coin that's dated to 1856 doesn't necessarily mean that that stratum is from 1856. That stratum could be from 1992 and the coin was curated and then thrown out later or dropped later. Our third relative dating method is seriation. Seriation is ordering artifacts or assemblages into a series. It helps us understand how a series can change over time. There are two different types of seriation. The first is known as stylistic seriation. This is ordering artifacts according to stylistic similarity. In other words, we're ordering artifacts by style. Here, the basic premise is that styles of artifacts will change gradually over time. For example, everyone could probably seriate these vehicles from the oldest to the newest. You could use your own knowledge and experiences to seriate the different styles and features of these cars. Go ahead and pause the video for a moment and give it some thought seriating these vehicles from oldest to newest then come back and continue the video. Now that you've seriated these vehicles, you likely came up with something similar to the oldest being G, A, F, B, C, E, and the most recent being D. Consider how you created that sequence. You likely observed different features on the cars including the mode of power, the presence or absence of a windshield, doors, body style, and wheels to develop a seriation. Archaeologists do the same, but with a wide variety of different artifacts. So let's do another quick exercise, but this time with shapes of pots we might find on a site. Go ahead and create a seriation of these pots from simplest to the most complex forms. Pause the video for a moment and give it some thought, then come back and continue the video. I hope that you've actually participated in this exercise. Now that you've given it some thought, you likely seriated these from simplest to most complex as A, H, B, E, G, D, F, and C. How did you come up with that seriation? Did you consider how different seriating these shapes was from seriating the cars? It's really just looking at shapes and trying to see how they change over time. So now we could even apply this to sorting decoration on different pots. They're the exact same shape of pot. Here, we could seriate these from simplest to most complex as B, G, E or D, C, F, H, and A. Seriation can be applied to styles that are physical styles, styles that are form, styles that are decoration, styles that are technological - there's all different kinds of stylistic seriation. The other type of seriation is what's known as frequency seriation. This one's a little different. Here, we're ordering sites or deposits according to relative frequencies of different artifact types, so we're looking at how frequent something is - how popular something is - on a site. The basic premise here is that the popularity or frequency of an artifact will come and go. Cell phones are a great example of this. When a new phone comes out, very few people have it right away. Over year one, its popularity grows. Over year two, its popularity wanes. By year three, maybe even year four, a few people still have and use that model. After five years, almost none of that model are still being used. So this forms a predictable pattern of introduction, growing popularity, waning popularity, and disappearance. Artifacts and styles might decrease in popularity due to the introduction of new forms. Ideally, the popularity or frequency distribution will form what we refer to as a battleship curve and you can see that with this middle pot on this image at the right. If you were to tilt that axis on its side, it would look like a battleship where you have the center of the ship being the highest point and that's because over time you will see that there's few people that have it and then as it gains in popularity, more people have it and then people start adopting a different style so there's fewer people using that style. So here you can see the light green pot on the left is very popular early on. We have time moving up and so the higher we go, the more time has elapsed. You see that that pot on the left is very popular and then all of a sudden right around 760 you have the introduction of that other style pot. So fewer people are using that first pot. People are then adopting more frequently that middle pot and then the other pot on the far right is introduced and people start liking that pot for whatever reason. So now fewer people are using the middle pot and more people are using the pot on the right. That is an excellent example of a frequency seriation. Now we must not inadvertently force our data to follow that curve. If we observe it archaeologically, we have empirical data for this and this is what we're seeing then we can clearly see that these different styles of pots and their use are tied back to human behaviors. Selective behaviors. Now seriation has a limitation. Seriation alone does not tell us what is earliest or what is latest. As a result, we must use another method along with seriation, so we can say that in general it's best to employ more than one relative dating method. In addition to relative dating methods, we also have absolute dating methods. Absolute dating goes beyond ordering strata and artifacts. These methods assign numerical age ranges to artifacts. Absolute dating methods are relatively new and they're highly scientific. Most were developed after 1950 and these methods are becoming increasingly sophisticated and expensive. Let's explore five common absolute dating methods. Here, the first is dendrochronology. This is commonly known as tree ring dating. If you think back to grade school, you may have had an exercise where you counted tree rings to see how old a log was. If you did that, you conducted dendrochronology. Let's explore how dendrochronology works. This method is based on differential growth of tree rings. Each year that a tree grows, it creates a new ring. The thickness of the ring depends on the environmental conditions where that particular tree was growing. We we can cut a log in half and we can see the rings in that profile. We then count the rings back through time and this is one of the first absolute dating methods that was ever developed. It's also one of the most accurate of absolute dating methods. In order to conduct dendrochronology, we need old logs, old bits of wood, and this will help us to form what we call a master chronology. This is a continuous chronology of tree rings from many different trees over time. It combines overlapping samples to create a continuous ring sequence. Master chronologies can extend back to hundreds, even thousands of years. The oldest sequence is from a site in Germany called Hohenheim. This sequence extends back more than 10,000 years. Here in the United States, the oldest sequence comes from the southwest and extends back about 8,500 years. Master chronologies are complicated. They're also species-specific. They're generated using wood from the same species. Different tree species may have different growth rings in the same period. This is why we have to have very species-specific master chronologies. They also help to determine the year a tree was cut down; however, keep in mind that they don't indicate the year the wood was used by humans. Furthermore, they're only used in certain parts of the world. Wood tends to preserve very poorly in much of the world, so we're limited in terms of where we can use dendrochronology to date sites. Despite its utility, dendrochronology faces several limitations. The technique does not work on all tree species. It only works for specific species with well-defined rings that vary due to minor climatic changes. Oak and pine are some of the best tree species to use. Furthermore, not all cultures used wooden beams. This method is useless if a culture didn't use wood for large things like beams on a house or beams on a ship. If they're only using wood for tiny little things, we're not going to get to see a lot of rings if the wood is even preserved. Finally, wood typically preserves poorly. If we don't have that wood preserved, then we can't do dendrochronology. Remember that wood is best preserved in extreme environments such as arid regions in the American southwest or waterlogged sites such as in Hohenheim, Germany. The oldest preserved wooden artifacts - just in case you were wondering - come from Germany at a site called Schoningen. Here, we have wooden spears that were recovered and these have been dated to about 400,000 years ago. They had sharpened ends that were then hardened in a fire. Not only is the utility of dendrochronology limited, but we also have to think about limitations in our interpretations. Reused wood can be much older than the construction date. So we can think about a house. We might have timbers in the house that are much older than the house itself - they may have recycled wood from somewhere else or a feature or a structure could have been used long after its construction. So just because that house was built doesn't mean people only lived in it for that short period of time. We might have houses that were constructed and used for three or 400 years. Likewise, older timbers could have been replaced by newer timbers and these newer timbers might be younger than the construction date. You may have a house that's 400 years old and a repair is needed and they put a brand new timber in there. If the archaeologist dates that one timber, they might not understand the chronology of the real house. Despite these limitations, dendrochronology is a very important and accurate dating method. This brings us to our second dating method. This is called obsidian hydration. This is a direct method employed to date obsidian artifacts. Obsidian is a type of volcanic glass that's formed during volcanic eruptions. When obsidian forms, it's inherently dry due to the high temperatures inside the volcano. As a result, exposed surfaces very, very slowly absorb water over time. We can measure the thickness of what we call a zone of hydration on the surface of that artifact. If we know the rate of hydration per year, we can calculate the age of the break. In this case, we're calculating the age of the break of the stone itself - the obsidian. As a result, we can tell when that artifact was manufactured. Despite its utility, we do have several limitations. Obsidian hydration can only calculate dates on materials back about 8,000 years. We can't use this method on artifacts older than about 8,000 years ago and furthermore, absorption rates vary based on the type of obsidian that the artifact's made from and of course the depositional environment. We need to consider soil temperature, humidity, and other variables that may affect that rate of absorption. Overall, this method is a good way to date materials from the same region. We have three more absolute dating methods that we're going to discuss here and these three are very technical methods that are frequently referred to as radiometric methods or radiometric techniques. These methods measure the rate of radioactive decay of unstable isotopes. Unstable isotopes decay into stable isotopes. In order to use radiometric techniques, we must first know the quantity of radioactive isotopes when the decay began and this is computed indirectly. We also need to know the quantity of radioactive isotopes that are currently present and we need to know the rate of decay. This rate of decay is something called the half-life. This is a period of time after which the unstable isotope, also known as the radioactive isotope, has decayed into a stable isotope. So a half-life is a period of time and over that amount of time that very specific amount of time, half of the unstable isotope will have decayed into a stable isotope. So let's say that we have a fictitious unstable isotope - isotope ABC - and it has a half-life of 100 years. This means after 100 years, half of the unstable ABC isotope will decay into stable XYZ isotope. This is another hypothetical here. After another 100 years, half of the remaining ABC isotope will decay into XYZ. Consequently, we can measure how much ABC is in a sample and we can compare that with how much should naturally be there. This is all calculated via physics. In theory, with a half-life, you never get to a true zero. You're always halving that amount that's remaining over. Over that half-life, whatever that half-life may be, and every radioactive isotope has a different half-life. This is all again calculated via physics. So now that we have introduced radiometric techniques, let's explore each of the three different radiometric methods that we're going to talk about in this class. The first is one that most people have heard of before. This is called radiocarbon dating or maybe just carbon dating. This is the most important absolute dating method for archaeology. Every year, small amounts of carbon 14 are formed from nitrogen 14 in the atmosphere. Cosmic radiation creates neutrons that replace one of the nitrogen's protons. When plants photosynthesize, they absorb both carbon 12 and carbon 14. Carbon 14 is a little more rare. Animals then eat the plants. This means that animals are ingesting the carbon 12 and carbon 14 isotopes and throughout the life of the plant or the animal, the ratio of carbon 12 to carbon 14 is constant because they're constantly eating - they're taking in both 12 and 14 - that's a constant ratio. Now carbon 12 is a stable isotope. Carbon 14 is not. So once the plant or the animal dies, they're no longer ingesting any new carbon 12 and carbon 14 and so the clock begins to tick on that carbon 14 because remember, that's an unstable isotope. It wants to decay from carbon 14 into nitrogen 14. So we can then calculate how much carbon 12 is there to figure out how much carbon 14 would have been present at the time of death. This process then counts the beta particle emission from that carbon 14 so we can figure out how much should be there. We can figure out how much is left and that helps us understand how much time has elapsed. We can then use the half-life of carbon 14 to calculate how much time has passed. Now the half-life of carbon 14 is 5,730 years. This is all calculated with physics. All radiocarbon dates are reported using BP - that before present convention - and this technique can be used on any organic specimen. So carbon 14 can only be used on things that are organic. This means wood, charcoal, seeds, leather, bone collagen, and anything else with carbon in it. Charcoal is the most frequent material that's studied. It's important for archaeologists because it's pretty ubiquitous at sites. Before electricity, people burnt wood and other materials for heat and for light and for cooking, so we find a lot of charcoal on our sites and this makes it really nice for helping us to have something that we can date to help us understand our site, but remember that we can only apply carbon 14 dating to organic materials, so we cannot use it on stone. Despite its utility, radiocarbon dating has several limitations. It only works on organic samples and those samples have to date between 500 years old and about 40 or 50,000 years old. This is we have a limited time frame due to the short half-life of carbon 14. If not enough time has elapsed, if that object is less than 500 years old, there really hasn't been enough carbon 14 decaying into nitrogen 14 for us to detect it. Likewise, once we get past about 40 or 50,000 years ago, there's not enough carbon 14 left to get an accurate sense of how much time has elapsed. So we have this really interesting time frame which is kind of the golden zone for carbon 14 dating. We also are limited by the risk of contamination. Newer or in many cases even older carbon 14 isotopes can be introduced through groundwater and through smoking. A lot of archaeologists used to smoke and so a lot of these older samples that are stored in museums and stored in repositories may have been contaminated by the archaeologists smoking pipes and smoking cigarettes on site and ashing and having that ash mix with the carbon sample and introducing you know essentially brand new carbon at that point in time. So as a result, our samples must not come into contact with other organics and we usually wrap our samples in foil because the foil, especially if it doesn't have oils on it, will not contaminate our sample. Likewise, the dates are only meaningful if the contexts from which they came are meaningful. Mixed contexts are no good. This means that if we find a stratum, a layer that has a material that is 4,000, 3,000, 2,000, and 1,000 years old, if it's all mixed together, we really don't want to get a carbon date off of that because it's not going to tell us too much. Likewise, if we find a site and we're on the surface, we're walking around in a cornfield and we find a brand new site, if we find charcoal up on the surface, it might be really old, it might not be really old. We're not sure. It's not worth saving in order to try to date it. We need to have really good contexts, clean contexts. A very, very particular pit that hasn't been disturbed would be an excellent context. So again, we have to understand our cultural transforms and our natural transforms to see how things may have affected that site in the past because carbon dating can be very expensive. We don't want to spend hundreds of dollars on a sample just to learn that the carbon is only like 400 years old, 500 years old. I've done that. It's a huge disappointment. Our next method is called accelerator mass spectrometry or AMS. Now this is a newer version of carbon 14 dating. It does not count the beta particle emission from carbon 14. Instead, it counts the carbon 14 atoms themselves. As a result, we can date much smaller samples and it doesn't take too long to conduct either. Now theoretically, this method can date materials that go back as far as 100,000 years ago, so that's much better - it's twice as old as radiocarbon dating. Of course there are still limitations. Again, it only works on organic materials, samples can get easily contaminated, and we should only date materials from very good contexts. Likewise, AMS can be very expensive. In some cases, it costs $1,000 per sample, so we're very limited in terms of how many dates we can get because of the cost of the analysis itself. Finally, our last absolute dating method is what's known as potassium-argon dating, also known as K-Ar dating because K is the atomic symbol for potassium and Ar for argon. Again, this is somewhat similar to carbon 14 dating. This method measures the decay of unstable potassium 40 into stable argon 40 gas. This method is particularly good for volcanic deposits. During different volcanic eruptions, argon gas is released. These volcanic rocks cool after eruption and potassium 40 immediately begins to decay into argon 40 gas. This new argon gas gets trapped in the rock. We can then use the known rate of decay to calculate the age of the eruption. We're not calculating the age of the formation of the artifact itself, but of the eruption. So potassium 40 has a very long half-life. As a result, we can date very old deposits - several billion years old in fact. Similar to the other absolute dating methods we discussed, potassium argon dating has some limitations. It can only be used to date volcanic sediments, so it's good to date the volcanic layers that things are found in, but not the artifacts themselves and it's not good for dating anything younger than about 100,000 years ago, so it kind of fills the gap. It lets us go back further because we can go back more than 100,00 years ago, but it has to be a volcanic sediment and we're not dating artifacts themselves - instead we're dating the layers that contain these volcanic sediments. All right, that's it for this lecture. Please keep up with the assigned reading, complete all course activities before the posted due date, and reach out to me via email with any questions. Thank you.