Understanding Relative and Absolute Dating Techniques in ANTH 102

ANTH 102 LAB 5 1 Lab 5: Relative and Absolute Dating Techniques List of Digital Components for this Lab (all are posted in the Lab 5 folder in Blackboard)  Lab 5 Digital Lab Packet ( this document )  Lab 5 Introduction Video  Lab 5 Online Quiz  Supplemental Files (alternate formats, transcripts, etc.) General Instructions Step 1. Carefully read through the Lab 5 Digital Lab Packet ( this document ) and watch the Lab 5 Introduction Video before doing anything else. You should complete this step before your lab meeting so you may ask any questions at that time. The Lab 5 Introduction Video covers concepts that you must understand before attempting the lab activities in the lab packet. Step 2. Complete Activities 1, 2, 3, and 4 on your own time, as instructed in the lab packet. As you complete the lab activities, you should write down your answers on a printout of the lab packet or (if you cannot print the packet) on a separate piece of paper. You will not turn in this physical copy, but you will use it to enter your answers in the next step. The questions in this packet are numbered sequentially from 1-39 to simplify this process for you. Step 3. To turn in your lab work, you must type your answers for Activities 1, 2, 3, and 4 into the Lab 5 Online Quiz and submit before the deadline. You must submit your work via the online quiz to receive a grade — we cannot accept submissions in other formats. The deadline for turning in Lab 5 is Sunday, October 1, at 11:59 P.M. Chicago time. Work turned in after this deadline will be considered late and will be docked points according to the policy in the course syllabus. Learning Outcomes This lab introduces fundamental concepts that will help students to achieve a number of course and General-Education learning outcomes, including:  Recognize, describe, and explain the nature of past historical events and their consequences for the present.  Use and understand scientific method to analyze ideas and obtain knowledge  Understand and apply the techniques archaeologists use in their research.

ANTH 102 LAB 5 2 Key Terms  Absolute date : A date expressed as a calendar date, or as a specific unit of scientific measurement, such as days, years, centuries, or millennia. This differs from relative dating in that, rather than comparing an artifact to other associated artifacts or sediment strata, absolute dating techniques date an artifact to a specific span of time.  Absolute Dating : techniques, including radiocarbon dating and dendrochronology, that determine the exact age of something by referencing a specific absolute time frame, such as a fixed calendrical system (e.g. this artifact was manufactured in 300 BC).  Chronology : a framework of time to show the order of events; a dated sequence of events  Cutting date : The year in which a particular tree was cut down for human use. Archaeologists can determine the cutting date for a tree-ring specimen if the bark or outermost growth ring is present on the specimen.  Dendrochronology : Tree-ring dating (dendron = tree; chronos = time; logos = the science of); dendrochronology relies upon establishing a timeline based upon patterns of tree-ring growth within a specific region and matching individual tree-ring samples to that master sequence.  Feature : an immovable layer, pit, or post in the ground having archaeological significance.  Law of Superposition : the geological principle stating that if no disturbance has occurred, each layer is older than the layers above it and younger than those below it. In other words, the deeper the layer, the older the layer.  Occupation surface : a stratum where human activities were actually carried out or the living floor where people lived (e.g. room floors, plazas, etc.).  Old wood problem: Refers to erroneously old dates arising from ancient peoples’ use of old, dead wood for campfires or as building materials. The dated wood could be decades or centuries older than the archaeological feature in which it was used, and from which it was recovered by an archaeologist.  Organic materials – materials that come from plants or animals that were once alive, such as wood, seeds, charcoal, bone, etc.  Radiocarbon dating: A method of absolute dating that measures the decay of the radioactive isotope of carbon ( 14 C, or radiocarbon). Materials older than 40,000 years old generally cannot be dated by this method, since they do not contain enough remaining radiocarbon.  Relative dating : technique that allows the chronological placement of one artifact, feature, or stratum relative to another (e.g. this burial pit is older than that burial pit), but does not necessarily assign it to a specific calendar age as in absolute dating. Stratigraphic analysis, drawing on the law of superposition, is one such technique.  Stratigraphy : a sequence of strata (singular stratum ), or layers in the ground, produced by the deposition of geological and/or cultural sediments; also the study of that sequence.

ANTH 102 LAB 5 3 Lab Overview Stratigraphy and Chronology Stratigraphy is the study of vertical sections or layers of the earth, called strata (singular stratum ), which show the relative positions of layers of cultural and geological deposits. From the study of these layers we can determine the relative chronologies, or order of events, at archaeological sites and obtain a better understanding of the processes that created the archaeological deposits (see Figure 1). Stratigraphy also reveals different episodes of natural or human occupation activity and can be used to help date finds and features from various contexts at the site. Relative dating is a technique used with stratigraphic sequences to determine the approximate age of an artifact, feature, or layer in relation to another. This approach is useful when an absolute (calendar) date cannot be obtained through techniques such as carbon dating or dendrochronology. Absolute dating is discussed later in this lab packet. Figure 1. Soil stratigraphy in a Middle-Ages church site in Szeged, Hungary [Photo: Bill Parkinson] Four general principles are useful in determining relative dates : 1. At an undisturbed site, artifacts found in the same stratum will most likely date to the same human occupation layer. This relationship assumes that the layer was deposited quickly with no mixing. 2. A single deposit can only be as old as the youngest (most recent) artifact found within it, again assuming there was no mixing and it was deposited all at once. 3. The law of superposition (see Figure 2) holds that artifacts found in deeper strata will be older than artifacts found in strata closer to the modern surface, as long as the site is relatively undisturbed and no geological folding or uplift has taken place. Thus, stratigraphic sequences provide a relative chronology of geological layers, along with their associated

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ANTH 102 LAB 5 4 artifacts and features, from older at the bottom to younger at the top. Note that while the law of superposition is a powerful dating tool, the date at which a deposit was laid down does not necessarily reflect the manufacture date(s) of the artifacts found within it. Those artifacts could have been made or used any time before the deposit was created (e.g. we could discard million year-old Acheulean handaxes into a modern landfill). Artifacts in a given deposit may have been created and used thousands of years before they were deposited in a particular stratum and then eventually found by archaeologists. Therefore, archaeologists must rely on contextual evidence. 4. Any cut into a sediment or stratum must take place after that sediment or stratum has been deposited. Figure 2: Stratigraphy and the Law of Superposition Site Formation Processes and Disturbances Archaeological sites are rarely completely undisturbed. Abandoned sites can experience significant modification through the actions of people and/or natural processes, which can distort or destroy evidence of the past. Examples of such disturbances include human occupation activity and waste disposal; human construction of structures, wells, storage pits, and burial graves; natural soil formation; natural deposition or removal of sediment or material via wind, gravity, running water, or volcanic eruption; and natural disturbances by tree roots, freeze-thaw cycles, rodent burrows, and geological folding or uplift. These processes may disturb a site’s stratigraphy and make interpretation of the strata difficult on either side of the disturbance.

ANTH 102 LAB 5 5 Absolute Dating Archaeologists often wish to go beyond a relative chronology and assign absolute dates within a standard calendar system to artifacts, strata, and archaeological sites. Archaeologists have a range of methods to obtain absolute dates on artifacts or sediments, depending on the material and the date range (see Table 2.2 on page 49 of your textbook for a list of techniques, their ranges of accuracy, and the specific materials that can be dated). These techniques and the resulting dates are important for reconstructing and explaining human actions in the past. In this lab, we will focus on radiocarbon dating and dendrochronology . Radiocarbon Dating A (Very) Basic Overview of How Radiocarbon Dating Works The radiocarbon clock is based on the known decay rate of the unstable isotope of carbon, namely 14 C. Radiocarbon, or 14 C, is formed when nitrogen ( 14 N) is struck by cosmic rays in the atmosphere. The C then combines with oxygen to create a radioactive form of carbon dioxide. 14 C enters an organism (plant or animal) through the food chain. Once it dies, however the organism is no longer taking in new 14 C to replace the 14 C that has decayed. If we know the rate of decay, and if we can determine how much 14 C is left in a specimen, we can calculate how much time has passed since it died — in other words, we can date it. In fact, we do know the rate of decay: We know how fast 14 C decays based on its half-life . The half-life of 14 C is 5,730 years. This means that half of any amount of 14 C will decay to 14 N after 5,730 years. Materials and Limitations of Radiocarbon Dating Radiocarbon dating can only be used for materials that were once alive, also known as organic materials . Datable materials include wood, charcoal, bone, seeds, paper/parchment, shells, fabrics, hair, leather, coprolites, antler/horn, and eggshells, to name a few. However, there exists the so-called old wood problem : occasionally, people in the past scavenged pieces of old, dead wood that had been lying around on the ground — for decades or centuries, even — for use in campfires or as building material. Obviously, this can produce erroneously old dates for the associated features. What the archaeologist is interested in is the date when the tree was cut down and used. However, when an archaeologist radiocarbon dates a tiny fragment of charcoal, he or she may actually be dating a section from the innermost part of a tree, which could have been formed many years before the tree was finally cut and used. Again, this would produce an erroneously early date. For this reason, archaeologists generally prefer to date remains of annuals — plants that grow and die within a single year, such as seeds and grasses. An Important Note Present in radiocarbon dating is defined as AD 1950 – not the current calendar year. “Before Present” (i.e., “before 1950”) is often abbreviated “BP”. Introduction to Dendrochronology Dendrochronology is the study of the annual growth rings of trees as a dating method. It is based on the principle that trees develop annual rings of growth, with wider rings generally produced when there is good rainfall or climatic conditions, and narrower rings during drier or colder years. All of the trees of one species in a particular region will exhibit a similar sequence of wider and narrower growth rings, other things being equal. Ultimately, an archaeologist will

ANTH 102 LAB 5 6 compare the growth patterns of a tree sample to the patterns of known-age trees, and then determine the last year the sample tree grew. A tree-ring master chronology for a region is built by matching portions of tree-ring sequences from various known-age living trees and historical wood sample of known age, extending the known chronology back in time. An archaeological sample consisting of a portion of a tree of unknown age can be compared to the established master chronology sequence in order to date the archaeological sample. Master sequences for tree-ring widths have been developed for various parts of the world. Figure 3. How archaeologists can construct and use a dendrochronology master chronology sequence. What archaeologists are most interested in is the cutting date for a particular specimen — the year in which it was cut down for human use. To determine the cutting date for a specimen, the outermost growth ring must be preserved. The outermost ring can be identified by the presence of bark and other indicators. If the outermost ring is not present, the archaeologist cannot be certain exactly when the tree was cut down, although they may be able to determine that it was cut down after a certain date. Also, remember that trees may be cut down long before they are actually used in construction. A piece of wood might be cut and stored for years before it was used in a structure or ship, for instance. Archaeologists must take these potential complications into account when interpreting tree-ring dates from archaeological sites. Interpreting Tree-Ring Dates from Archaeological Sites Tree-ring datable roof beams are often preserved in arid climates, such as the cliff dwellings of the northern American Southwest. Archaeologists are sometimes able to reconstruct when a pueblo room was built or repaired based on the tree-ring dates recovered from preserved roof elements. If a room yields many roof beams that were all cut in a single year, archaeologists will often infer that this is the year the room was constructed . There are several potentially complicating factors, however: Ancient people would sometimes scavenge some beams from an earlier (older) structure and reuse them if they didn’t have enough newly -cut beams to complete a new roof. Also, years after a structure was built, people might have needed to repair or remodel a section of the roof, and often they would use pieces of newly-cut wood to do so.

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ANTH 102 LAB 5 7 Occasionally, people visited a site several times over the course of a couple of years before actually building a dwelling there. During this time, they might cut down a number of trees and stockpile them for future use in construction at the site. Remember, too, that a particular roof could have been built any time after the latest-dated beam that is found as part of that roof. Nevertheless, archaeologists try to make sense of the various combinations of dates in terms of construction and related processes, like scavenging/reuse , repair/remodeling, and stockpiling . Often, more than one interpretation is possible, and sometimes it is difficult to evaluate which is correct without independent, corroborating evidence.

ANTH 102 LAB 5 8 Activity 1: Reading Stratigraphy Task: You have excavated a Native American site just outside of Chicago. The stratigraphy is represented on the diagram below. Examples of the artifacts recovered from each pit, stratum, and feature are provided for you to study. Your task is to understand how all the different Strata (A through E), Pits (#1 through #5), and Features (#1 through #4) create the relative chronology of the site in order to answer the questions 1 – 22. Questions 1. What pits are associated with stratum A, if any? A. Pit 1 B. Pits 3 and 4 C. Pit 5 D. None 2. What features are associated with stratum A, if any? A. None B. Feature 1 C. Feature 2 D. Feature 3

ANTH 102 LAB 5 9 3. What pits are associated with stratum B1, if any? A. Pit 1 B. Pits 4 and 5 C. Pit 3 D. None 4. What features are associated with stratum B1, if any? A. None B. Feature 1 C. Feature 2 D. Feature 4 5. What pits are associated with stratum B2, if any? A. Pit 1 B. Pit 4 C. Pit 5 D. None 6. What features are associated with stratum B2, if any? A. None B. Feature 1 C. Feature 2 D. Feature 3 7. What pits are associated with stratum C1 if any? A. Pits 2, 3 and 5 B. Pits 4 and 5 C. Pit 3 D. None 8. What features are associated with stratum C1, if any A. None B. Feature 1 C. Feature 2 D. Feature 3 9. What pits are associated with stratum C2, if any? A. Pit 2 B. Pits 3 and 4 C. Pit 5 D. None

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ANTH 102 LAB 5 10 10. What features are associated with stratum C2, if any? A. None B. Feature 1 C. Feature 2 D. Feature 3 11. What pits are associated with stratum C3, if any? A. None B. Pit 1 C. Pit 2 D. Pit 3 12. What features are associated with stratum C3, if any? A. None B. Feature 1 C. Feature 2 D. Feature 3 13. What pits are associated with stratum D1 if any? A. Pit 1 B. Pits 2 and 3 C. Pit 4 D. None 14. What features are associated with stratum D1, if any? A. None B. Feature 1 C. Feature 2 D. Feature 3 15. What pits are associated with stratum D2, if any? A. Pit 2 B. Pits 3 and 4 C. Pit 5 D. None 16. What features are associated with stratum D2, if any? A. None B. Feature 1 C. Feature 2 D. Feature 3

ANTH 102 LAB 5 11 17. What pits are associated with stratum E, if any? A. Pit 1 B. Pits 2 and 3 C. Pit 4 D. None 18. What features are associated with stratum E, if any? A. None B. Feature 1 C. Feature 2 D. Feature 3 19. Which was created first, Feature 4 or Pit 4? A. Feature 4 B. Pit 4 C. They are the same age D. It’s impossible to know 20. Which was created first, Feature 2 or Feature 1? A. Feature 1 B. Feature 2 C. They are the same age D. It’s impossible to know 21. Which was created first, Pit 2, Pit 3, or Pit 5? A. 5 is older than 2 and 3 B. 3 is older and 2 and 5 C. They are the same age D. I t’s impossible to know 22. Object 1 was found in in stratum D2 and Object 2 was found at the very bottom of Pit 1. Which object is older? A. Object 1 B. Object 2 C. They are the same age D. It’s impossible to know

ANTH 102 LAB 5 12 Activity 2: Using Stratigraphy Task: The relative age of an artifact can tell you whether it is older, younger, or relatively similar in age to other artifacts within the same excavation , but it can’t give you a precise age. Archaeologists can determine the relative date of artifacts by comparing the positions (e.g., above, below) of the strata in which the artifacts are found. Using the stratigraphy chart and supporting information below, answer questions 23 – 26. What we know:  Stratum 4 was formed between AD 500 and 700.  Stratum 1 was formed between AD 1600 and 1800.  Objects: A – copper axe; B – ceramic vessel; C – gold pendant; D – ceramic cup; E – obsidian flake Stratigraphy: Questions 23. List the artifacts in order from youngest (most recent) to oldest. A. A, C, D, E, B B. C, A, D, B, E C. E, B, D, A, C D. B, E, D, A, C

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ANTH 102 LAB 5 13 24. Which two artifacts are relatively similar in age? A. A and C B. D and E C. C and E D. B and D 25. Looking at what dates we currently have above, what is the relative date of the gold pendant (C)? A. After AD 1600 B. Between AD 700 and 1600 C. Before AD 500 D. Between AD 1600 and 1800 26. Based on the style of the gold pendant (C), we know it dates to approximately AD 900. How does this change the relative date of the copper axe (A)? ______________________________________________________________________________ ______________________________________________________________________________

ANTH 102 LAB 5 14 Activity 3: Radiocarbon Dates on a Maya Site Part A Task : While excavating, a team uncovered the remains of six different houses, each of which contained a number of artifacts. Below is a list of what each house contained. Use this list to answer questions 27 – 29. House 1  13 ceramics  4 animal bones  5 lithics House 2  4 ceramics  3 seeds House 3  21 lithics  6 animal bones House 4  1 piece of charcoal  45 ceramics House 5  37 ceramics  16 lithics House 6  23 ceramics  5 building stones Questions 27. Which materials found at the site would be suitable for radiocarbon dating (list all suitable materials)? ______________________________________________________________________________ ______________________________________________________________________________ 28. For which houses could you obtain radiocarbon dates? Explain. ______________________________________________________________________________ ______________________________________________________________________________ 29. How many samples from House 4 could be sent for radiocarbon dating? Explain. ______________________________________________________________________________ ______________________________________________________________________________

ANTH 102 LAB 5 15 Part B Task : Excavations at a Maya site uncovered five samples that could be sent for radiocarbon dating. Use this to answer questions 30 – 31. Remember that radiocarbon dates must be translated into calendar years (BC/AD). Remember that for BP (Before Present) the present is AD 1950 , not the current calendar year. Additionally, you must consider possible date ranges for each sample. For example, for a sample with the date of 1710 ± 40 cal. BP, the steps would be: 1950 – 1710 = 240 240 – 40 = 200 (this is the lower end of your date range) 240 + 40 = 280 (this is the upper end of your date range) Your date range = AD 200-280 or AD 240 ± 40 Also remember that BC switches to AD at the year 0. Results of radiocarbon analysis Sample 1 : 1,475 ± 41 cal. BP Sample 2 : 1,572 ± 55 cal. BP Sample 3 : 572 ± 24 cal. BP Sample 4 : 1,623 ± 59 cal. BP Sample 5 : 1,540 ± 52 cal. BP Chronology of Maya region Period Dates Terminal Classic AD 830-950 Late Classic AD 550-830 Early Classic AD 250-550 Late Preclassic 350 BC – AD 250 Middle Preclassic 1000-350 BC Early Preclassic 2000-1000 BC 30. Based on these radiocarbon dates, during which period was the site most likely inhabited? A. Terminal Classic B. Late Classic C. Early Classic D. Late Preclassic E. Middle Preclassic F. Early Preclassic 31. What might explain the more recent date of Sample 3? Explain in 1-2 sentences. ( Hint: Refer back to page 5 of the digital lab packet or the introduction video.) ______________________________________________________________________________ ______________________________________________________________________________

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ANTH 102 LAB 5 16 Activity 4: Using Dendrochronology in the American Southwest Task Our team is working in the American Southwest, at the Pueblo site of Snake-in-my-boot Ranch. The site consists of eight rooms that were built sometime between AD 1450, when the site was first established, and AD 1500, when the site was depopulated. Due to the arid climate and good state of preservation, we have recovered a number of wooden roof beams from five of the rooms. These have been analyzed at the Tree Ring Lab in Tucson, which provided cutting dates for each of the beams. The dates from the tree ring samples have been listed below for each room. Based on the tree-ring data, we can estimate the construction date of the rooms. Use this information to answer questions 32 – 39. Room 1 (7 beams) Beams (x6) – AD 1450 Beam (x1) – AD 1455 Room 2 (0 beams) Room 3 (10 beams) Beams (x3) – AD 1457 Beams (x7) – AD 1460 Room 4 (0 beams) Room 5 (12 beams) Beam (x 1) – AD 1465 Beam (x 1) – AD 1475 Beams (x 10) – AD 1486 Room 6 (12 beams) Beams (x 10) – AD 1486 Beams (x 2) – AD 1490 Room 7 (0 beams) Room 8 (10 beams) Beams (x2) – AD 1495 Beams (x8) – AD 1500 Questions ( Refer back to pages 5-7 of the digital lab packet for more help with this activity.) Provide the most likely date for the initial construction of each room. Remember! If a room yields many roof beams that were all cut in a single year, archaeologists will often infer that this is the year the room was constructed. 32. Room 1 ____________ 33. Room 3 ____________ 34. Room 5 ____________ 35. Room 6 ____________ 36. Room 8 ____________

ANTH 102 LAB 5 17 37. What possible explanation can you think of to explain why no beams were recovered from Rooms 2, 4, and 7? ______________________________________________________________________________ ______________________________________________________________________________ 38. If Room 5 was built in 1486, what process might account for the two earlier beams? ______________________________________________________________________________ ______________________________________________________________________________ 39. If Room 6 was built in 1486, what processes might account for the two later beams? ______________________________________________________________________________ ______________________________________________________________________________

LAB 5 UPDATED 23'

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