LESSON 7 OUTLINE

ASSIGNMENT: Radiocarbon Dating and the Age of Saharan Groundwater

In this exercise, you will determine the apparent age of water sampled from springs, wells, and oases in the Sahara desert.

You will find a form in the Lesson 7 folder in ANGEL called The Age of Saharan Groundwater (Submit your MS-Excel file here). This is where you will submit your responses to the graded questions in this activity.

Goals

The goals of the exercise are:

1. Learn about the age of Saharan groundwater.
2. Learn about the geography of the Saharan region.

Background

Both ¹⁴C and ¹⁴N exist in the earth's atmosphere and, hence, in rain water. Spontaneous decay of radioactive ¹⁴C to form ¹⁴N happens slowly but consistently within natural systems. You worked out the mechanics of the process in your problem set (¹⁴C gains a proton and loses a neutron), and can now apply that knowledge to the technique of dating. When rain water is below ground -- in an aquifer -- it is isolated from mixing with modern rain and the modern atmosphere. As a result, the decay process generates ¹⁴N at the expense of ¹⁴C, and neither ¹⁴N nor ¹⁴C can be added to the system from new rainfall.

The half-life of ¹⁴C is 5730 years. That means that one half of the atoms of ¹⁴C will decay to ¹⁴N every 5730 years. It means that three-fourths of the atoms of ¹⁴C will decay to ¹⁴N every 11460 years (half in 5730 years, and half of the remaining half in the subsequent 5730 years). This relationship is true no matter how much ¹⁴C was present at the time the water was isolated from atmospheric interaction.

Assuming one can measure the amount of radioactive ¹⁴C present in a water sample AND the rate at which that ¹⁴C is currently decaying, we can calculate the age of the sample. The special equation is:

t = -8270 ln (A/Ao)

where,

t = the age of the sample (i.e., the answer you want to get)

A = disintegration rate of ¹⁴C measured for your sample

Ao = disintegration rate of ¹⁴C in the atmosphere at the time the water was isolated. Before thermonuclear bombs were exploded in the air this rate was 10 disintegrations per minute per gram (dpm/g). The value of 10 is therefore appropriate for samples that are assumed to be more than about 50 years old. For Saharan groundwater this value is safe to use.

ln = natural log.

So, divide A by Ao and take the natural log before multiplying by -8270.

Suggestions for tackling the assignment:

Please download the activity sheet (MS Excel document). Be sure this file is saved on your computer and has not opened up in ANGEL. In this sheet you will find data on the measured ¹⁴C activity for Saharan water samples. The data are fictitious, but are consistent with what is known about the distribution of water within the region.

1. For all locations in the MS Excel file, calculate the apparent ¹⁴C age of the water and uncertainty. Report your answers in the data table. For each answer, be sure to include the uncertainty of the analysis. All laboratory analyses are subject to some error (particularly if the quantities to be measured are small), and your answers should reflect this uncertainty. After you calculate a date, add on the uncertainty that falls within the ranges shown below. Make sure that the value you use makes sense.

   For samples with dates of		The uncertainty is:

   100-12,000 years before present (ybp)	± 80-100 years

   12,000-20,000 (ybp)			± 100-400 years

   20,000-35,000 (ybp)			± 400-500 years

   a. Need help inserting the formula into Excel? See the instructional video for assistance.
   
   b. If you need help calculating the uncertainty values, please view the instructional video. Answers should be typed in the MS Excel file in the format: 24,625 ± 440 years.
2. Place your Excel file with completed calculations in the dropbox titled "The Age of Saharan Groundwater" in the Lesson 7 (NOT the "Dropboxes" folder). Keep a copy of the Excel file handy, as you will need your calculated numbers to complete your analyses. Once you place your Excel file in the dropbox, the "Online Tutorial" will unlock.
   
   NOTE: The Excel file is due in the dropbox on Friday, while the rest of the asssignment is due on Sunday!
3. Use the "The Age of Saharan Groundwater” Online Resources" link in ANGEL to help you locate the hydrographic basins in the Sahara. There are also links to additional maps and location information for each region.
4. Complete the The Age of Saharan Groundwater” Online Tutorial by entering your calculated water age values and initial interpretations and correlations between geography, location, and age. Note that this tutorial is not graded but is there to help you prepare your responses for the actual graded assignment questions listed below.
5. Once the "Online Tutorial" has been submitted, the form will be unlocked for "Radiocarbon Dating and the Age of Saharan Groundwater" Submission Form.
6. When you have finished formulating your responses to the assignment questions below, copy and paste your responses into the boxes in the"Radiocarbon Dating and the Age of Saharan Groundwater" Submission Form in the Lesson 07 folder of ANGEL.

Assignment Questions:

1. Did you upload the MS Excel file with the calculated age of water sample and uncertainty values by the Friday deadline?
• Dont' forget the uploaded Excel file needs ages AND uncertainty values!
2. Write a brief description of the spatial trends in the data, if you feel there are any.
3. Write a brief description of the temporal trends in the data, if you feel there are any.
4. When was the most recent period of groundwater recharge, and how long did it last?
5. What other periods of recharge can you identify?
6. References you used to complete this assignment (in APA format). *NOTE: This includes maps you used from online sources

Deadlines

All responses must be uploaded in ANGEL no later than 11:00 PM on the due date (see Calendar tab in ANGEL). Note that is 11:00 PM by ANGEL's clock, not by the clock on your computer! The submission form will disappear at 11:00 PM, and this assignment will not be accepted late under any circumstances. I have no problems giving students a zero for an assignment if they do not take the responsibility to submit by the deadline, so don't even think about trying to submit this late! REMINDER: The Excel file needs to be placed in the dropbox by 11PM Friday, while the assignment questions need to be completed by 11PM Sunday.

Resources to help get you started

The "Online resources" and "Online tutorial" are both designed to help you formulate your responses to the assignment. As this assignment requires you to visualize the age distribution in each basin and across the Sahara, it may be helpful to print off maps and your calculated ages in the Excel file for easy access. Keep in mind that this assignment is created as steps, where the Excel file must be in the dropbox before the online tutorial can be completed, then the graded response form becomes active.