7.2: Dating Exercise

Dating Exercise

Format: In person or online

One section of the fictional site stratigraphy that students will date. Art by Alexandra Broehl

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

1. 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.
2. 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.
3. 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.
4. 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).
5. 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):

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.

Image Attribution

The image of the stratigraphic profile was drawn by Alexandra Broehl.

Additional Documents

Below is the thought process behind the dates for each layer and sample questions to prompt students who are struggling:

1. Present layer.

2. 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.

3. 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?”

4. 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).
5. 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).

6. 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?”

7. 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?”

8. 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).

9. 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).
10. All that can be said about this layer is that it is older than Layer I, so estimated 330,000+ years ago.

Note cards with chronometric dating results:

Dating Exercise: Student Worksheet

You are an anthropologist who recently participated in an excavation at a site in Europe. Before your team can publish an article about your site, you need to determine the ages of the various strata and artifacts that your crew found.

A drawing of the site’s stratigraphic profile is included below, and information concerning the archaeological and paleontological specimens removed from each layer is available in the Descriptions of Strata document. Using this information, you will calculate the approximate date ranges for several specimens. You can use a mixture of chronometric and relative dating methods to accomplish this task. When using chronometric methods, you will send a sample off to a lab (i.e., the instructor) which will provide you with the results of the analysis. The lab can handle radiocarbon, potassium-argon, and argon-argon requests. When making your request, please submit in writing the specific method you would like to use for analysis (for example, radiocarbon dating), the object you would like dated, and the object’s position within the stratigraphic profile (i.e., the layer letter).

First, write the date range you obtain for each layer next to the stratigraphy below. Then, based on your stratigraphic analysis, please answer the questions on the next page.

Questions:

1. When were the fossilized footprints from Layer I made, and how do you know?

2. Approximately when were the skeletal remains in Layer C deposited? Briefly describe the process you used for dating the remains.

3. How old are the skeletal remains in Layer D? How do you know?

4. Approximately when did Neanderthals occupy this site? How did you reach this conclusion?

5. Approximately how many years ago were the Venus figurines from the site used? Briefly describe the process you used for dating them.

6. What is the date range for Layer H? How do you know?

7. What is the date range for Layer J? How do you know?

Descriptions of Strata

Layer A
This is the current surface, as well as the immediate subsurface, of the site.

Layer B
Fragments of historic glass and ceramics were found in this layer, but most were not sufficiently intact to contain diagnostic information. One intact glass bottle (a) was removed from Layer B. Characteristics of the bottle’s seams show that it was machine-made. Automated bottle-making machines were first used in the region around 1905. The bottle has a manufacturer’s mark from a company that shut down in 1920.

Layer C
Layer C yielded one human burial in an extended position (b). A cache of coins inside a jar (c) was associated with the skeleton. Several coins were too worn to discern any pictures or writing, but coins that were readable had the years 1727, 1733, 1734, and 1736. These types of coins were no longer used after 1750.

Layer D
This layer contained one human skeleton in a flexed position (d). Two bone harpoons (e, f) were in the grave, which were stylistically consistent with artifacts from nearby sites dated to the Magdalenian (ca. 17,000–11,000 years ago).

Layer E
Layer E included charcoal from a hearth that was likely used for cooking (g). This stratum also contained two clay Venus figurines (h, i). Venus figurines are usually dated around 21,000 to 26,000 years ago but can be found between 35,000 to 11,000 years ago.

Layer F
From Layer F, excavators removed several stone tools. Two of the tools (j, k) were diagnostic of the Mousterian industry, which is a technology used by Neanderthals between approximately 150,000 to 40,000 years ago. The fossilized remains of three extinct species were also found in Layer F. One species was a woolly rhinoceros (l), which is estimated to have existed from about 3 million years ago to 10,000 years ago. Another species was a straight-tusked elephant (m), which lived in the area approximately 400,000 to 50,000 years ago. The third species was a saber-toothed cat (n), estimated to exist 42 million years ago to 11,000 years ago.

Layer G
This stratum was made of volcanic rock and contained no cultural or paleontological material.

Layer H
The only significant find in Layer H was the fossilized cranium of an unidentified hominin (o).

Layer I
Layer I was made of volcanic rock and contained the preserved footprints of a bipedal hominin.

Layer J
This was the deepest stratum and contained the fossilized cranium (p) and femur (q) of an unidentified hominin(s). Charcoal (r) was present, but it is unknown whether the fire was intentional.