New technologies are enabling archaeologists to uncover exciting finds that can help us learn more about ancient civilizations. How can archaeologists use their knowledge of radioactive isotopes to unlock some secrets of the past?

National Geographic Society explorer-in-residence Robert Ballard and his team have discovered evidence that humans once lived in an area now covered by the Black Sea north of Turkey:

• The mixture of freshwater shells and saltwater shells in the area indicates that the present saltwater sea was once a freshwater lake.
  
  
• Indications of an ancient coastline under the Black Sea suggest that the land once extended miles farther than the present coastline.
  
  
• The discovery of a structure similar to those built during the Stone Age indicates that the submerged land was once inhabited by people.

Ballard has proposed a theory that the area was suddenly submerged by a great flood—perhaps the flood described in the Biblical story of Noah's Ark or in the Mesopotamian text, the Gilgamesh Epic. Ballard's team hopes to get permission from the Turkish government to take a sample of the wooden artifacts they have discovered. They could then perform a carbon dating test to determine if this human settlement indeed fits into the timeline of Noah and the Biblical flood.

• Students can read about the expedition's find in the National Geographic press release, "Black Sea Expedition Finds of Human Habitation ."
  
  
• They can listen to Ballard's description of the current expedition in the Riverdeep Today article, "21st Century Explorers: Robert Ballard."
  
  
• They can follow Ballard's expedition at the National Geographic site, Ballard & the Black Sea.

Teaching the Problem

Students must synthesize a number of scientific principles in order to understand radioactive dating:

1. Isotopes are atoms of an element that have the same atomic number (i.e., the same number of protons), but a different number of neutrons—and, therefore, different atomic masses. Carbon has several common isotopes, including carbon-14, which is a radioisotope. Radioisotopes have unstable nuclei that lose energy by emitting radiation during the process of radioactive decay.
   
   During photosynthesis, plants absorb carbon dioxide from Earth's atmosphere. A small percentage of atmospheric CO ₂ contains carbon-14. Therefore, as a result of photosynthesis, this radioisotope is present in all plants and in living organisms that eat plants anywhere along the food chain.
   
   (The following SimLibrary activities require Logal Express. Get a free trial subscription.)
   
   
   • Students can view carbon in the periodic table in the Chemistry Gateways activity, Periodic Table: An Introduction.
     
     
   • Students can learn how plants absorb carbon dioxide during photosynthesis in the Biology Gateways activity, Fueling Photosynthesis with Carbon Dioxide and Water.
2. Every radioisotope has a characteristic rate of decay, called its half-life. The half-life is the time required for one-half of the atoms of the isotope to decay into a different element. After one half-life period, one half of the original number of radioactive atoms remains. After two half-life periods, only one quarter of the atoms remain.
   
   While an organism is alive, the radioisotope carbon-14 is present within it in a fixed ratio. The decay begins when a living organism dies and carbon-14 is no longer absorbed. In the case of carbon-14, the half-life is 5,730 years, and it decays into nitrogen-14. By measuring the radioactivity of a once-living artifact, scientists can determine how much carbon-14 is present, and therefore they can determine when the organism lived.
   
   (The following Destination MATH activity is currently available on CD. An online version will be available soon.)
   
   
   • Students can use exponents and their reciprocals to calculate half-life. They learn how to work with powers in the Destination MATH activity, Algebra I, Course II: Working with Powers.
3. Scientists must extract the radioisotopes from the artifacts in order to measure their weight. They ionize the sample and pass the ions through an instrument called a mass spectrometer. In the spectrometer, the ions pass between the poles of a magnet, which deflects them into a curved path. For ions with the same charge, the angle of deflection depends on mass, so that the ions are automatically sorted by isotope, according to their masses.
   
   
   • Students can explore the paths of isotopes as they are sorted in a simulated mass spectrometer in the Physics Explorer activity, The Mass Spectrometer.
     
     

Analyzing the Problem

Ask students the following questions:

• Why does Ballard need a sample of wood in order to date the underwater structure? Why can't he date a stone tool? Can you predict whether or not marine life samples are efficient for carbon-14 dating? Why or why not?
  
  
• Why is carbon-14 dating only effective for artifacts that range between 200 to 50,000 years of age? What amount of carbon-14 would be present in a fossil that is 1 million years old? (Give the answer as an exponent.)

1. Students can read about Ballard's current and past expeditions at the Mystic Aquarium's Institute for Exploration. Ballard is also founder of the JASON Project, a year-round scientific expedition designed to engage students in science and technology.
   
   
2. Students can get an interactive lesson in carbon-dating at Virtual Dating.
   
   
3. The Minnesota State EMuseum explores the different dating methods used by archaeologists in the exhibit, Dating Techniques.
   
   
4. Underwater archaeology presents an interesting set of challenges for archaeologists, but some recent finds have been spectacular. Students can explore different developments in the field at the following sites:
   
   
   • What is Underwater Archaeology?
     
     
   • Combined Caesarea Expeditions (Caesarea, Israel): Includes learning modules and lesson plans
     
     
   • Herakleion and Canopus (off the coast of Alexandria, Egypt)
     
5. Students can read the Mesopotamian flood narrative from the Gilgamesh Epic (ca. 2700 B.C.). The flood is described in Tablet 11.