One section of the fictional site stratigraphy that students will date. Art by Alexandra Broehl
Format: In person or online
Author: Kristen A. Broehl and Stephanie J. Cole
Time needed: 90-120 minutes
Learning Objectives
Differentiate between chronometric and relative dating methods
Recognize scenarios when different dating methods are useful, including materials and date ranges appropriate for chronometric techniques
Analyze a stratigraphic profile by combining chronometric and relative methods
Supplies Needed
Note cards (in person)
Student worksheet
Descriptions of Strata document
Readings
King, Sarah S. and Lee Anne Zajicek. 2019. Chapter 7: Understanding the Fossil Context. Explorations.
Introduction
To complete this activity, students will use a stratigraphic profile and supplementary descriptions to estimate the dates for each stratum at the site and the assemblages within them. The goal of the activity is to help students recognize which methods are most appropriate for dating different strata. When a layer requires chronometric dating, students submit a request to a “lab” (i.e., the instructor) and must use the data they receive along with cultural or biostratigraphic information to produce the most precise dates possible.
Steps
Make photocopies of the note cards document provided in the Additional Documents section. Make sure to copy enough note cards, so that each group can receive results from each test. Also be sure to copy extra error note cards, so there are enough to return to students if they request tests that are not appropriate (e.g., requesting radiocarbon dating on rock or for materials older than 50,000 years). Cut out the note cards and keep them in separate stacks for easy distribution.
Create groups comprising approximately three to five students. Provide each student with the student worksheet and each group with at least one copy of the Descriptions of Strata document.
Ask each group to review the stratigraphic profile and descriptions to identify materials within each layer that they can date with relative or chronometric (radiocarbon, potassium-argon, or argon-argon) techniques. Students should use this information to complete the student worksheet.
For materials that the students want to date chronometrically, they must submit a written request to the “lab” (i.e., the instructor) that includes the material to be dated (e.g., bone, volcanic rock, etc.), the letter of the layer the material comes from, and the chronometric method they want to use. As groups submit chronometric dating requests to the “lab,” the instructor gives them the appropriate note card with the results of their request. If you return an error result, ask a question that will prompt the group to think about why their request is problematic (see Tips and Suggestions below).
Review the dates of each layer as a class, discuss challenges, and address questions.
Review Questions
Using examples from the activity, what are the pros and cons of chronometric dating? What are the pros and cons of relative dating?
What are some limitations associated with radiocarbon and potassium-argon (or argon-argon) dating?
What obstacles did you face while trying to date the assemblages at this site?
Adapting for Online Learning
If this is an in-person lab, rank how adaptable to online learning it is (mark in bold): Easy to adapt.
This lab exercise is easily adaptable to online course formats. In synchronous classes, students can work independently or in Zoom breakout groups to complete the activity. When students are ready to submit a sample for dating to the lab (i.e., the instructor), they can message the instructor individually with their request (via email, private Zoom chat, etc.), and the instructor can message back with the results.
For asynchronous course formats, instructors can provide students with the chronometric dating results along with the strata descriptions. In this case, the activity is focused more on accurately combining the relative and chronometric data to estimate the dates of each stratum rather than recognizing when to use chronometric versus relative techniques.
Alternatively, instructors could provide students with the stratigraphic profile image and strata descriptions and ask them to write a short “proposal” to the site’s director requesting funds for chronometric dating. The proposal should describe (1) the current estimated dates of each layer, including details about how they found the date ranges for each layer, and (2) the chronometric dating tests for which they need funding, including the material(s) within the specific layers they want to date, which chronometric methods they would use for each material and why, and how the chronometric results could improve the dating of relevant layers.
Tips and Suggestions
Be sure to remind students what +/- means in a date (e.g., 5,000 +/- 500 years ago) since many students are not familiar with this notation. See the Additional Documents section below for the thought process behind the dates of each layer and sample questions to prompt students that are struggling with dating certain strata. The note cards needed for in person instruction can be found here as well.
Additional Documents
Below is the thought process behind the dates for each layer and sample questions to prompt students who are struggling:
Present layer.
The earliest date for the deposit is 1905 since that’s the earliest time a machine-made bottle would have been produced. The end date is unknown; even though the company shut down in 1920, the bottle could have been deposited long after it was initially purchased.
The dates of the coins show that the layer is only a few hundred years old, so the deposit is too young to radiocarbon date the bone. The coins are associated with the skeleton and therefore represent a single deposition event. Therefore, the coins provide the earliest year for the deposit (1736), based on the date of the most recent coin. The coins were out of use after 1750, so they were likely deposited before that date.
a. If students request a radiocarbon date for the skeleton, the instructor can ask: “What is the date range for when radiocarbon can be used? Is the skeleton at least 500 years old?”
b. If students put 1727 as the early date (the earliest coin date), ask: “In the year 2020, can you have a penny from 2010? Can you have a penny from 2030? So, could the coins have been deposited before 1736?”
Based on the harpoons, the skeleton is relatively dated to the Magdalenian, which indicates that the skeleton is within the proper date range for radiocarbon dating. Radiocarbon dating the skeleton yields a result of 12,900 +/- 300 years ago (12,600–13,200 years ago).
The relative dates of the Venus figurines show that the layer is within the range for radiocarbon dating and charcoal is available. Radiocarbon dating the charcoal gives a result of 28,000 +/- 400 years (27,600–28,400 years ago).
The dates of the Mousterian tools provide an approximate range for this layer of 40,000–150,000 years. The woolly rhinoceros and saber-toothed cats are not helpful in decreasing the range because they lived throughout that time frame. However, the presence of the straight-tusked elephant shows the layer is older than 50,000 years ago. Initially, the dates cannot be further narrowed, but dating the next layer leads to an upper estimate of 140,000 years ago.
a. If students request radiocarbon dates of the fossils, the instructor can ask: “What is the date range when radiocarbon dating can be used? Is this layer less than 50,000 years old?”
This layer is made of volcanic rock and the dates of the previous layer show it is likely older than 100,000 years, so it can be dated chronometrically with potassium-argon or argon-argon methods. The test returns dates of 160,000 +/- 20,000 years ago (140,000–180,000 years ago).
b. If students indicate they don’t know how to date this layer, the instructor can ask questions such as: “What material makes up Layer G? Do we have any methods to date volcanic rock?”
Initially, only the more recent date can be estimated based on the upper limit of Layer G (180,000 years ago). After dating Layer I, students will have the other date for this layer (270,000 years ago).
Layer I is made of volcanic rock, so it can be dated chronometrically with potassium-argon or argon-argon methods. The test returns dates of 300,000 +/- 30,000 years ago (270,000–330,000 years ago).
All that can be said about this layer is that it is older than Layer I, so estimated 330,000+ years ago.
