Copy of Lab 5 - How Do Archaeologists Know How Old Something Is_ (Part 2)

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ARCH 2800 - Archaeological Science Dr. Kara Fulton Lab 5 - How Do Archaeologists Know How Old Something Is? (Part 2) Your Team Name: Your Team Motto: Remember: this is a team assignment, so anything you write here should be the voice of the team (i.e., use “we” NOT “I”) Tip: As you read through the introductory material in this lab, highlight important information as you go. Resource : How to Use a Highlighter to Improve Your Grades. This lab will have you practice with two different types of absolute dating techniques: dendrochronology and radiocarbon dating. Dendrochronology Dendrochronology, sometimes called “tree-ring dating,” is one of the most effective absolute dating methods archaeologists can use in temperate regions of the world. The method is based on the fact that trees in temperate and arctic regions are dormant during the winter and then grow the rest of the year. This creates a distinct ring of growth every year. The rings vary in thickness, color, and texture depending on that year's environmental conditions such as temperature and precipitation. This means one ring equals one year of growth. So, by counting and comparing the tree rings with numerous samples, dendrochronology can determine the age of the tree when it was felled (Figure 1). Trees start out small, and each year, they grow by one ring. By counting the rings from the middle of the trunk out to the end, a scientist can tell how old a tree is. One tree ring is added each year. Different trees of the same species will have the same pattern of rings for each year they are alive at the same time. For instance, in a year where there might have been a drought, the ring on each tree alive that year might be relatively narrow (i.e., not much growth) as opposed to a year where there was optimal rain available (i.e., the ring might be thicker). By matching up, or “cross-dating,” the trees and connecting events where a date is known (such as the year the tree was cut down), sequences of rings associated with specific calendar dates can be reconstructed, sometimes far back into prehistory (Figure 2c). In some regions of the world, scientists have put together tree-ring sequences that go back for several thousands of years; these are called master sequences . By matching wood samples from archeology sites to regional master sequences, archeologists can tell how old a site is, sometimes to the specific year it was occupied. One region of the world where there is significant master sequence is the Southwestern United States (Figure 2a-c). A. E. Douglass, an astronomer, was the first to pioneer tree-ring dating in the Southwest by creating master sequences from recently felled yellow pines near

ARCH 2800 - Archaeological Science Flagstaff, Arizona, and preserved wooden house beams from prehistoric Pueblo structures. Assuming the Puebloans used the trees around the time they died, archeologists can absolutely date a house structure. While tree-ring counting is scientifically sound, archeologists do have to be careful about such assumptions -- can you think of a scenario in which such an assumption might not work? Figure 1. Growth of tree rings (a) and overlapping tree-ring sequences (b). Important : Note that cut dates can occur on the left or the right side of dendrochronology samples depending on how they’re prepared, so pay attention to the details of what you’re looking at! For example, in Figure 1, above, the cut date is on the right side of the sample. However, in the video tutorial below as well as the samples you’ll work with in Q1 and Q2, the cut date is on the left side of the sample.

ARCH 2800 - Archaeological Science Figure 2. Pueblo house with wooden beams (a); example of a bristlecone pine (left) and Giant Sequoia (right) (b); illustration of cross-dating house beams to living and dead trees (c). The Scenario You are an archeologist working in the Southwestern United States. You have recently excavated five Pueblo house structures at one site and found that the structural beams have all been well- preserved. You are interested in knowing how old each structure is and the order the houses might have been built (one of your research questions: Are certain houses older than others?). You select one beam from each house to match up to a master sequence for the region to determine the specific cutting date of each beam. Assuming the inhabitants immediately used the cut beams when constructing their house, you'll be able to determine the age of each house structure using dendrochronology. Working off the master sequence initially, match up the five wooden beams A-E, which represent five Pueblo hose structures, respectively. Once they are matched, determine the cutting date for each wooden beam and the age of each house structure by counting the rings. First, watch the following tutorial which will help you complete this activity: Dendrochronology: Walk-through activity Q1. Next, make a copy of this Google Slide file: Dendrochronology Samples . In the file, you have one master sequence and five wood samples (labeled A-E). Drag the samples so that they align correctly with the master sequence (all rings on each sample should match up with either another sample and/or or the master sequence, creating a continuous timeline). When you’re done, download the slide showing the entire finished sequence as an image (File > Download > PNG image) and paste the image below. Note: Your finished sequence should be aligned in a similar manner to Figure 1b (above). Remember to pay attention to the location (left vs. right) of cut dates.

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