Martin David Kamen (August 27, 1913—August 31, 2002) and Sam Ruben (November 5, 1913—September 28, 1943) discovered how to synthesize the isotope Carbon-14 in February 27, 1940. Ruben was studying the chemical reactions in photosynthesis and wanted a way to track carbon through the process. He worked with Kamen to synthesize Carbon-14 by hitting graphite (which is pure carbon) with radiation. They also synthesized other isotopes of carbon, including Carbon-11. Carbon-11 wasn’t as useful to trace because it had a very short half-life.

Sam Ruben, at Berkeley

Martin Kamen, at Berkeley

Carbon-14 Carbon is a molecule that normally has 6 neutrons and 6 protons, giving it the atomic number 12. Sometimes, radiation and other forces cause a molecule to change the number of neutrons in its nucleus. There are three naturally occurring isotopes (varieties) of carbon. Carbon-14 is the least common in nature and it is unstable—it will decay over time into nitrogen.

Carbon Dating Willard Libby originally posited the use of radiocarbon as a dating method. He and his assistant, Ernie Anderson, then created the process by which to measure the changes in Carbon-14 concentration to determine the age of an artifact. In 1960, Libby was awarded a Nobel Prize in Chemistry for this.

Carbon-14 is useful in dating objects for two reasons. First, Carbon-14 decays at a steady rate. The amount of Carbon-14 in a sample (present when the organism dies) will decrease by half every 5730 years. Second, dead tissues do not absorb Carbon-14, though living tissues do.

Samples for radiocarbon dating

 Artifacts from living things: Trees (paper), cotton, bones, tissue, ivory, food remains  Newer artifacts: >55,000 years old. Modern humans began migrating into Europe between 60-40 thousand years ago.  Comparing samples from before 1950 to after: Because of nuclear bomb testing, extra carbon-14 was created. These levels are useful in checking for modern forgeries. Neuroscientists are also using this carbon to check for the onset of Alzheimer’s.

Calibrating the Carbon Clock

Any tool used to measure has to be calibrated, whether it’s a scale or a clock. There are three main ways scientists calibrate the Carbon Clock. Known Artifacts Scientists can use the age of some known archaeological samples. For example, Mount Vesuvius erupted in 79 CE, destroying and preserving the village of Pompeii. Therefore, artifacts from this site have a known date of death and are very helpful in calibration.

Bread from Pompeii, British Museum Tree Rings (Dendrochronology) Scientists can use the reliable pattern in which trees form new rings to determine the age of the tree.

Lake Sediment Core samples taken from the lake bed of Lake Suigetsu in Tokyo has provided a record of over 650 carbon dates that can be used to help calibrate the carbon clock.

A sediment core layer, used in refining carbon dating

What about weird results?

Hundreds of thousands of artifacts have been dated using radiocarbon. Occasionally, the results of a carbon test are—well, weird. Contaminated Samples

In the early 1960s, campfire carbon remains from a Clovis site near Lewisville, Texas, gave a date of "greater than 37,000 years.” However, other samples in that area which had previously been dated were much younger. In 1978-80, a drought lowered lake levels so the site could be re-examined. Further excavations of hearths revealed that in addition to mesquite wood, the Indians were burning lignite, a soft form of coal that outcrops in the area. The lignite is much older than the date of the campfire, giving the unexpected old date. Irregular Carbon Absorption

Accuracy depends on the sample obtaining its carbon from the atmosphere, either directly, as with plants, or indirectly as with animals. The shells of living mollusks were once dated as being 2,300 years old! However, tests showed they were absorbing ancient carbon from the local limestone (calcium carbonate) in their habitat. Therefore, archeologists tend to avoid using shells for dating purposes.

References

 Ewen Callaway (October 2012). Carbon dating gets a reset. Scientific American.  Kate Wong (June 2001). The dating game. Scientific American.  Matthew Hedman. (2007). The Age of Everything.  Doug MacDougall. (2008). Nature’s Clocks  Caforio, L., Fedi, M. E., Mandò, P. A., Minarelli, F., Peccenini, E., Pellicori, V., ... & Taccetti, F. (2014). Discovering forgeries of modern art by the 14C Bomb Peak. The European Physical Journal Plus, 129(1), 1-5.  Smithsonian National Museum of Natural History. What does it mean to be human?: Dating. Retrieved from http://humanorigins.si.edu/evidence/dating  Hajdas, I.(2009). Applications of Radiocarbon Dating Method. Radiocarbon, 51 (1). 79-90

Forgeries Detected with Carbon Dating

Bomb testing from 1950-1960 drastically changed the amount of radiocarbon in the atmosphere, and therefore the radiocarbon in living organisms (because radiocarbon is not absorbed after death, it doesn’t affect existing samples). The 1955 “Bomb Peak” is helpful in dating newer items. A painting allegedly create by Fernard Leger was the first questioned piece of art to be dated using this technique. Radiocarbon dating proved that the cotton plant from which the canvas was made was cut no earlier than

1959—four years after the Contraste de forms of the Peggy Guggenheim Collection artist’s death. Persian Princess Mummy: Found in 2000; radiocarbon dating of the coffin showed it to be about 250 years old. The artifact was actually the mummified remains of a woman who had been murdered in 1996.

The Shroud of Turin: In 1988, the Shroud of Turin was radiocarbon tested with the permission of the Vatican. The fibers of the Shroud were from 1260-1390 CE. The first time the Shroud was ever mentioned was in the 1350s.

Limitations of Carbon Dating

 Sample Size: Sample must be large enough to test  Sample Source: Variations in carbon levels due to diet, atmosphere, or other effects can give erroneous results  Sample Age: Samples must be relatively new—less than 55 thousand years  Nonliving Samples: Samples from rock or other sources that have never been living cannot be tested using radiocarbon dating

Radiocarbon dating is one method of using isotopes and other changes in the physical properties of molecules for establishing the age of a sample. In almost all cases, samples are tested using more than one method to help insure accuracy.