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ARCHAEOLOGICAL

Archaeoastronomy

BIOLOGICAL

Packrat Middens
Amphibians and Reptiles
Arthropods
Birds
Dung
Mammals
Pollen

CHRONOLOGICAL

Dendrochronology
Fire Scars
Radiocarbon Dating
Other Techniques

GEOGRAPHICAL

GIS
Remote Sensing

GEOLOGICAL

Stratigraphic Sediments
Geomorphology
Volcanism
Glaciers

HISTORICAL

Land Surveys
Written Histories
Repeat Photography
Stream Gaging

ToolsRadiocarbon Dating

Radiocarbon (also called 14C) dating was invented in the late 1940s by Walter Libby, who was awarded the 1960 Nobel Prize for his discovery. The technique is typically used on organic remains, such as baskets, leather, dung, plant remains, amino acids, etc. With the development of accelerator mass spectrometry (AMS), beginning in the early 1970s, radiocarbon dating has become an increasingly useful way to measure time for archaeologists as well as scientists in other disciplines

Although the chemistry and physics involved is complex, the basis for the technique is relatively simple. Common nitrogen in the atmosphere (14N) can be changed to radioactive carbon-14 (14C) when bombarded by cosmic rays. Both are taken up by plants and animals along with the nutrients they need to survive. When the plant or animal dies, this process ceases and the 14C present begins decreasing through radioactive decay. Since the rate of decay is a known constant, the ratio of 14C to stable carbon can be used to determine the age of the sample.

The original measuring technique requires about enough material to fill a 35 mm film canister. Using radiometry, which is similar to counting the ticks given off by a Geiger counter, the amount of beta particle (electron) decays per 1000 minutes is measured. The fewer decays the older the sample. The amount of material required often made the technique too destructive to be use on rare or valuable artifacts. AMS dating uses a particle accelerator to count the number of 14C atoms remaining, and can be successful with samples the size of the head of a pin. AMS has reinvigorated radiocarbon dating as a tool for archaeologists.

Both techniques have statistical errors associated with them, expressed as a "+/-" number following the date. The B.P. following the date means "before present." By long standing convention, "present" is defined as A.D. 1950.


Resources:

Geyh, M. A. and Schleicher, H. 1990. Absolute age determination : physical and chemical dating methods and their application. Springer-Verlag, Berlin; New York, 503 pp.

Leavitt, S. W. and Long, A. 1989. The atmospheric 13C record as derived from 56 pinyon trees at 14 sites in the southwestern United States. Radiocarbon 31: 469-474.

Mead, J. I., Agenbroad, L. D., Davis, O. K. and Martin, P. S. 1986. Dung of Mammuthus in the arid southwest, North America. Quaternary Research 25: 121-127.

Mead, J., I. and Agenbroad, L. D. 1992. Isotope dating of Pleistocene dung deposits from the Colorado Plateau. Radiocarbon 34: 1-19.

Smiley, F. E. 1998. Archaeological chronometry: Radiocarbon and tree--ring models and applications from Black Mesa, Arizona. Report 16. Southern Illinois University Center for Archaeological Investigations, Carbondale, IL, 309 pp.

Tuniz, C., J.R Bird, D. Fink, and G.F Herzog. 1998. Accelerator Mass Spectrometry: Ultrasensitive analysis for global science. CRC Press.

Webb, R. H. and Betancourt, J. L. 1990. The spatial and temporal distribution of radiocarbon ages from packrat middens. Pp. 85-103 In: Betancourt, J. L., Devender, T. R. V. and Martin, P. S., editors. Packrat middens: The last 40,000 years of biotic change. University of Arizona Press, Tucson.