Dating Methods: Achieving the Correct Results

Carbon 14, Dendrochronology and Deep Time

This week we are visiting the topic of archaeological time and how two scientific dating methods have vastly enhanced our understanding of past human events.

 It was principally through the research of Willard F. Libby (1908-1980) (Anderson et al. 1947) that the method of radiocarbon dating, an accurate, absolute method of dating organic substances from archaeological, geological and environmental contexts, was developed. A key thresh-hold leading to the radiocarbon dating method was the discovery of the Carbon 14 atom’s half-life of 5,730 years*. By comparing the known decay rate of C14 with the amount of C14 remaining, Libby was able to calculate the age of the sample up to approximately 45,000 to 50,000 years. 

In 1948, following this breakthrough, Libby and a committee of anthropologists and geologists under the sponsorship of the American Anthropological Association tested different organic materials of known ages. For example, the funeral boat cedar wood belonging to the Egyptian pharaoh Sesostris, who died circa 3800 years B.P. (before the present). Libby’s radiocarbon age confirmed the boat’s known age at 3,800 +/-180 B.P. (Arnold and Libby 1949; Libby 1952; Poole 1961). Until Libby’s breakthrough discoveries in the radiocarbon dating process scientists could only estimate the age of archaeological/geological samples through relative means, such as stratigraphy and an artifact’s association with something else. For his pioneering research in the radiocarbon dating method Willard Libby was awarded the Nobel Prize in 1960.

Extracting wood charcoal for carbon dating

Since the discovery of radiocarbon dating its boundaries and accuracy has been enhanced by other scientific disciplines, especially dendrochronology or the study of tree ring patterns (Douglass 1919). Astronomer Andrew E. Douglass (1867-1962) is considered the father of dendrochronology. His scientific approach to correlating annular tree ring growth with solar (sun spot) activity subsequently led to other branches of multidisciplinary studies involving climate change, human use of, and adaptation to, the natural landscape, geological events, art and building history  and other related topics.  On point, is that Douglass developed a precise means by which he could bridge history to prehistory by working backward in time using overlapping tree ring patterns observed in the cross sections of wood beams. Much of his sampling took place at archaeological sites where roof beams were preserved in the arid environment of the prehistoric southwest.  Since Douglass’ pioneering, work much has been accomplished by dendrochronologists to advance the dating method which currently extends the chronology beyond 13,000 years (Ferguson 1970; Schulman 1956; Stuiver et al 1986).

Lab technician with sample specimen for dendrochronology dating 

Radiocarbon Dates + Tree Rings = Calibrated Accuracy

 Graph showing divergence between C14 and tree ring chronologies

                The multidisciplinary approach to dating archaeological samples using C14 and dendrology dating has its limitations and is not the panacea to knowing the exact age of something that has absorbed C14 during its entire life cycle. Fluctuations in the amount of C14, solar radiation, nuclear bomb radiation, volcanic activity etc., to name a few examples, can affect the true C14 age of a sample depending on its age and geographic location and the calibration program(s) used.  Radiocarbon laboratories globally have grappled with the problem by developing their own radiocarbon calibration programs and several of these are available online (i.e OxCal and CALPAL). Through diligence and refinement of these techniques, the applications for dating organic samples can only improve. In light of these improvements in dating methods, The State Museum of Pennsylvania reorganized its culture chronology chart for Pennsylvania.  Based on the recalibrated radiocarbon dates that are now available the following table provides our current culture chronology. Ages following AD.1000 are listed as BP (before present).

* Note: Calculated Libby half-life of C14 is 5568 years. Recalculated true half-life is 5730 years.

References:

Anderson, E.C., W. F. Libby et al.

1947       Natural Radiocarbon from Cosmic Radiation. Physical Review 72:931

Arnold, J.R., and  W.F. Libby

1949       Age Determination by Radiocarbon Content: Checks with Samples of Known Age. Science 110:678.

Douglass, A.E.

1919       Climatic Cycles and Tree Growth. Vol. 1 No. 289, Washington, D.C. Carnegie Institution of Washington.

Ferguson,C.W.

1970       Dendrochronology of Bristlecone Pine, Pinus arisata. Proceedings of the Twelfth Nobel Symposium, Upsala Sweden, August 11-15, 1969. Almquist and Wiksell, Stockholm.

Libby, W.F.

1952       Radiocarbon Dating. University of Chicago Press.

Poole, Lynn

1961       Carbon – 14 and other Science Methods that Date the Past. Whittlesey House. McGraw-Hill Book Company, Inc.


For more information, visit PAarchaeology.state.pa.us or the Hall of Anthropology and Archaeology at The State Museum of Pennsylvania .