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 .

Thanksgiving Staples: The Three Sisters

With the Thanksgiving Season upon us, how appropriate it would be to highlight the famous dietary trio that was most assuredly, in one form or another, on the table of the first Thanksgiving feast, the Three Sisters, corn, beans and squash. The term Three Sisters is a commonly used analogy for the practice of companion planting these crops, where each supports the other; through providing structure, moisture retention or nutrient exchange.

Both the nature and timing of the arrival of these cultigens into the Mid-Atlantic region continue to be intensively studied research topics in Archaeology, and the application of C-14 and AMS techniques have proved to be indispensable tools for dating these and a wide variety of other botanical remains.

As the body of data continues to accumulate, it is evident that each of the “Sisters” arrived in the region at different times in history, with squash (Cucurbita pepo) being the earliest at between 5000 and 2500 years before present (Hart and Sidell 1997). Next in the sequence, corn or maize (Zea maize) becomes common in archaeological settings post-dating roughly 1200 B.P., or about A.D. 700 (Klein 2003). Finally, the common bean (Phaseolus vulgaris) is the last to appear in the archaeological record, at approximately A.D. 1300 (Hart and Scarry 1999).

Native peoples in the area that would become Pennsylvania were raising all of the “Three Sisters” and enjoyed the nourishment they provided for hundreds of years prior to the settlement of Europeans in the New World. Today, we all are thankful for these staples of that first Thanksgiving feast and the many more that have followed.

Hart, John P. and C. Margaret Scarry (1999)
The Age of Common Beans (Phaseolus vulgaris) In the Northeastern United States
American Antiquity 64 (4) 653- 658

Hart, John P. and Nancy Asch Sidell (1997)
Additional Evidence for Early Curcurbit Use in the Northern Eastern Woodlands East of the Allegheny Front.
American Antiquity 62 (3): 523-537

Klein, Michael (2003)
Of Time and Three Rivers: Comments on Early and Middle Woodland Archaeology in Pennsylvania. In Foragers and Farmers of the Early and Middle Woodland Periods in Pennsylvania, edited by Paul A. Raber and Verna L. Cowin, pp. 117-129. Recent Research in Pennsylvania Archaeology, No.3, Pennsylvania Historical and Museum Commission, Harrisburg, PA.

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

Celebrating 60 years of using the carbon 14 dating method: Part II

Refinements in the method and improvements in procedures

In the past, C-14 dating required large samples of organic material. New processes, such as accelerator mass spectrometry (AMS) allow for extremely small samples to be accurately dated such as seeds or the remains of a burned meal adhering to the inside of a clay cooking pot. For example, the plus or minus factor for the above date would be reduced to 40 years or less.

A problem with C-14 dating is contamination of the samples – the addition of old or new carbon. A great deal of care must be used in collecting the samples. Further, charcoal is light in weight and can be moved around by wind and water. The same flood deposits that cover artifacts at a stratified site can bring in old charcoal eroded from a site upstream resulting in a C-14 date that does not accurately date the deposit. This type of contamination can be offset by getting many dates from a site. C-14 dates are relatively inexpensive (approximately $300 for standard dates and $600 for AMS dates) and presently it is a common practice, where the charcoal is available, to get over twenty dates from the same site. Archaeological analysis is a process of identifying patterns and C-14 dates are part of the patterning. If all of the artifacts and stratigraphy points to a date of 5000 B.P. and the C-14 date is 20,000 B.P., there is probably something wrong with the C-14 date.

Changes in the intensity of the sun, the burning of fossil fuels and the testing of nuclear weapons has had an effect on the accuracy of carbon 14 dating. This has resulted in two problems. First, dates less than 300 years old are not very dependable and other methods must be used to date artifacts from this period. Second, it turns out that the amount of C-14 in the atmosphere has changed over time. This was discovered through dendrochronology – tree ring dating. Using AMS to date individual trees rings, it has been discovered that C-14 years do not exactly correlate with tree ring dates which we are sure relate to calendar years. For example, C-14 dates of around 3500 B.P. are several hundred years too old. On the other hand, C-14 dates of 11,000 B.P. are almost 2000 years too young.

Formulas are being developed to convert radio carbon years into calendar years but the system still needs to be refined. In the meantime, archaeologists are using both systems - calendar years (cal yr B.P.) and radio carbon years (14C yr B.P.). This is probably going to be resolved in the next few years but in the mean time it’s confusing for both professional archaeologists and the general public. However, as one archaeologist, (David Hurst Thomas) has put it, “radiocarbon dating is the workhorse of archaeology”. It produces reasonably accurate dates, to within a few decades and it allows us to compare a variety of significant technological and cultural events.

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