Archaeology: What's Hot Now: Timing is Everything

Archaeology: What's Hot Now These articles that had the largest increase in popularity over the last week // via fulltextrssfeed.com

Timing is Everything Dec 10th 2011, 11:01 Archaeological Dating Table of Contents | Part 2: Chronological Markers and Dendrochronology | Part 3: The Radiocarbon Revolution | Part 4: New Fangled Methods It is certainly no exaggeration to call the invention of radiocarbon dating a revolution. It finally provided the first common chronometric scale which could be applied across the world. Invented in the latter years of the 1940s by Willard Libby and his students and colleagues James R. Arnold and Ernest C. Anderson, radiocarbon dating was an outgrowth of the Manhattan Project, and was developed at the University of Chicago Metallurgical Laboratory. Although I am hardly a chemist or a physicist, and so will leave the detailed explanations to those who are better at it than I (for example, Anne Marie Helmenstine's page in About Chemistry), essentially radiocarbon dating uses the amount of carbon 14 available in living creatures as a measuring stick. All living things maintain a content of carbon 14 in equilibrium with that available in the atmosphere, right up to the moment of death. When an organism dies, the amount of C14 available within it begins to decay at a half life rate of 5730 years; i.e., it takes 5730 years for 1/2 of the C14 available in the organism to decay. Comparing the amount of C14 in a dead organism to available levels in the atmosphere, produces an estimate of when that organism died. So, for example, if a tree was used as a support for a structure, the date that tree stopped living (i.e., when it was cut down) can be used to date the building's construction date. The organisms which can be used in radiocarbon dating include charcoal, wood, marine shell, human or animal bone, antler, peat; in fact, most of what contains carbon during its life cycle can be used, assuming it's preserved in the archaeological record. The farthest back C14 can be used is about 10 half lives, or 57,000 years; the most recent, relatively reliable dates end at the Industrial Revolution, when humankind busied itself messing up the natural quantities of carbon in the atmosphere. Further limitations, such as the prevalence of modern environmental contamination, require that several dates (called a suite) be taken on different associated samples to permit a range of estimated dates. Calibration In the 50 or so years since Libby and his associates created the radiocarbon dating technique, refinements and calibrations have both improved the technique and revealed its weaknesses. Calibration of the dates may be completed by looking through tree ring data for a ring exhibiting the same amount of C14 as in a particular sample--thus providing a known date for the sample. Such investigations have identified wiggles in the data curve, such as at the end of the Archaic period in the United States, when atmospheric C14 fluctuated, adding further complexity to calibration. One of the first modifications to C14 dating came about in the first decade after the Libby-Arnold-Anderson work at Chicago. One limitation of the original C14 dating method is that it measures the current radioactive emissions; Accelerator Mass Spectrometry dating counts the atoms themselves, allowing for sample sizes up to 1000 times smaller than conventional C14 samples. While neither the first nor the last absolute dating methodology, C14 dating practices were clearly the most revolutionary, and some say helped to usher in a new scientific period to the field of archaeology.

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