ASWT Radiocarbon Update #2

ASWT-C14 Updates-Logo

In this post you will find an update on Emily McCuistion’s progress in her radiocarbon dating learning curve, and “First Dates,” a history of the invention of radiocarbon dating excerpted from Emily’s thesis draft.

Footprints  Carbon Footprints

Since the previous update I’ve had some great opportunities to further my knowledge of radiocarbon dating.  Here are the highlights:

In the Fall of 2017, I worked with Dr. Raymond Mauldin at the Center for Archaeological Research (CAR) at the University of Texas at San Antonio, to learn how samples are chemically pretreated, or cleaned, prior to being radiocarbon dated.

Blog-Bison-hair

Bison hair undergoing pretreatment at CAR.

This process generally consists of an acid-base-acid sequence to eliminate external contaminants such as humic substances found in soil. We treated one unusual sample—bison hair from a Lower Pecos rockshelter—which required a little research and experimentation to determine the best pretreatment method. Prior to treating the archaeological sample, we practiced on modern bison hair, donated to us by Thunder Heart Bison for that purpose.

During Spring break 2018 I had the rare opportunity of touring DirectAMS’s radiocarbon facilities in Seattle and Bothell, Washington. Over two days I witnessed the radiocarbon dating process, from pretreatment and graphitization through measurement by AMS. The staff at DirectAMS were incredibly generous in sharing their knowledge and time with me.

Direct AMS

At DirectAMS’s lab in Bothell, WA: (left to right) Director of Laboratory Operations Alyssa Tate, Emily McCuistion, and Brittany Hundman, Director of Archaeological Services.

Also during the Spring 2018 semester, I attended a Bayesian modeling workshop held in advance of the Society for American Archaeology (SAA) conference in Washington D.C.  The workshop was located in the bowels of the Smithsonian Institution! It was put on by Drs. Tony Krus and Derek Hamilton from the University of Glasgow. It was a great, and challenging, workshop, and I’d recommend it to anyone interested in using Bayesian statistics for radiocarbon analysis—this year the workshop is again coinciding with the SAAs. For the 2018 SAAs, I presented a poster on my preliminary thesis data and proposed work. I had a lot of interest and good conversations with people from around the world who have undertaken similar projects, who shared advice and articles with me—a fruitful trip to D.C. all around!

This past fall, Dr. Black and I were invited to participate in a Texas Archeological Society poster symposium on radiocarbon dating across Texas. Participants were asked to chart radiocarbon dates for their region (ours was the Lower Pecos Canyonlands) using summed probability distributions (SPDs). 000479For our poster we investigated patterns in an SPD of all dates from Lower Pecos rockshelter sites—open sites were excluded to avoid preservation biases encountered at open sites—and an SPD of directly-dated desert succulents (i.e., sotol, agaves, prickly pear, and yucca).

I would like to thank Texas State University and the Texas Archeological Society for financially supporting my thesis trips and research, the staff at DirectAMS, and many people in the Texas archaeology community who have helped me on this long learning curve. I’ve still got a distance to go.

If you’d like to get in touch, please email me at: erm63@txstate.edu. Thanks for reading!

 

First Dates: A review of the early history of 14C

The radiocarbon dating method was published a week before calendar pages turned to January 1950 (Arnold and Libby 1949). January 1, 1950 would, in time, become a significant placeholder on the Western time scale: day-zero Before Present (BP). The year 1950 was elected to divide radiocarbon time because global atmospheric carbon levels were, by then, drastically altered by human activities. Fossil fuel emissions decreased quantities of 14C (the Suess Effect) while atomic testing resulted in increases in the production of 14C (known as bomb carbon) (Taylor and Bar-Yosef 2014:23). It has also been proposed that “BP” stand for “Before Physics,” meaning before atomic testing, to avoid the confusion of “present” (Flint and Deevey 1962). The year A.D. 1950 represents a turning point in chronometrics and is an homage to Willard Libby and his colleagues’ accomplishment. Arguably, “BP” is also a symbol of an increasingly secular world, one in which scientific breakthroughs such as the atom bomb were rippling across the world.

The roots of radiocarbon science predate Libby’s 1949 accomplishment. Many others’ work laid the foundation upon which radiocarbon dating was born. Just 15 years before, it was not known that 14C existed at all. Physicist Franz Kurie was the first to publish suspicions that 14C may be artificially created (Kurie 1934), based on anomalous particle behavior (recoil tracks) seen when 14N was bombarded with “fast neutrons” in a particle accelerator; if the recoil tracks were from a proton being ejected, and not from an alpha-particle, 14N must transform into 14C. Imagery of the recoil tracks led Kurie to posit that it was a proton being ejected, though additional work was needed to confirm this possibility (Kamen 1963:235). The next year, two parties independently reported that the same particle behavior could be created with “slow neutrons,” though it was still uncertain whether the particle was a proton.

libby_willard_2

“Wild Bill” Libby. Photo Credit: Columbia University.

In 1936, further support for Kurie’s supposition came from a study by Burcham and Goldhaber, which showed that the particle emission produced in this interaction was almost certainly a proton. Also in 1936, physical chemist Martin Kamen completed a doctoral dissertation for which he examined 730 recoil tracks; his observations were the same as those made by Kurie (Kamen 1963:236). In 1937, Kamen and Kurie began working together at the Berkley Radiation Laboratory with the aim of investigating neutron-nuclear interactions. At this point the existence of 14C was sufficiently proved, at least in a laboratory setting, though little was known about the isotope. It was believed that 14C was an unstable, radioactive, isotope, and that the half-life was short—mere hours or days, or at most, months. However, this was yet to be confirmed.

The late 1930s were a time of burgeoning research into the use of isotopes as biological tracers. It was hoped that a radioactive-isotope of one of the abundant biological elements—Hydrogen, Oxygen, Carbon, or Nitrogen—would be found to have a long-enough half-life to be used for biological tracer studies (Kamen 1963:239). Thus, research into 14C during this time was focused on its possible utility in such applications. Technological advances in cyclotrons made by Ernest Orlando Lawrence, and internal-target preparation advances by Kamen, set the stage for the future of radioactive-isotope research.

Kamen

Interview with Dr. Martin D. Kamen (left). Photo credit: (an2_a30_11_11), University Communications. Public Relations Materials. RSS 6020. Special Collections & Archives, UC San Diego

Finally, in 1940, Kamen and Samuel Ruben, a student of Willard Libby, found that 14C had a much longer half-life than previously believed (Kamen 1963:241); however, Kamen and Ruben believed the half-life of 14C was 25,000 years (AIP 1979b)!  The inaccuracy of their half-life calculation aside, Kamen and Ruben are credited for “discovering” 14C (AIP 1979b), at least as a tool for biological and chemical research.

Not only was the 14C created in labs artificial, so were the neutrons that produced 14C through bombardment of 14N. In the 1930s it was unknown whether either neutrons or 14C occurred naturally. In the late 1930s, cosmic-ray physicist Serge A. Korff at the Bartol Research Foundation was trying to detect neutrons in natural radiation by sending Geiger counters to various levels of the atmosphere with balloons (Schuur et al. 2016:26). Eventually Korff and Danforth (1939) found increasing neutron intensity with elevation. They suggested that this was the result of cosmic radiation interacting with the atmosphere. It followed that if neutrons could be identified in the atmosphere, 14C must also be present. This study was, according to Libby, the catalyst for his radiocarbon dating work (AIP 1979b).

Willard “Wild Bill” Libby [1908-1980] graduated from University of California, Berkley with his undergraduate degree in 1931. He triple-majored in chemistry, math, and physics, and built the first Geiger counter in the United States for his senior project (AIP 1979a). In 1933, Libby was awarded his doctoral degree from Berkeley (Schuur et al. 2016:23). After receiving his PhD., Libby continued at Berkley as faculty; he is regarded as Berkley’s first nuclear chemist (Marlowe 1999:10).

It would be five years between reading Korff and Damforth’s article and Libby taking time to develop the radiocarbon method; in 1940, Libby obtained a Guggenheim Fellowship and took sabbatical from Berkley to conduct research at Princeton University. Soon thereafter, the United States’ entered World War II, and Libby went to work on the Manhattan Project at Columbia University to develop atomic bombs. In 1945, after the war, Libby began working at the University of Chicago, which was then becoming the leading institution in atomic sciences. It was there, at Chicago’s Department of Chemistry and Institute for Nuclear Studies, that Libby would develop radiocarbon dating. Thirty years later, when asked why he was the person to come up with the method and not someone else, Libby answered that the obstacle for others was the idea of global mixing: “Here I was talking about the ocean, I mean the entire ocean mass, the entire biosphere, the entire atmosphere, as though it were in my test tube…Once you get over that, the whole carbon dating thing falls into place” (AIP 1979b).

Libby’s early work with radiocarbon dating was conducted in total secrecy, for fear that funding would be withheld from him because of the outlandish-nature of this project (AIP 1979b).

Anderson

Ph.D. student Ernest C. Anderson (left) and Willard F. Libby (right). Photo credit: University of Chicago Photographic Archive, [apf1-03868], Special Collections Research Center, University of Chicago Library.

Without breaching his secrecy, Libby put a student and an assistant to researching 14C; graduate student Ernest Anderson was applied to the task of identifying the natural abundance of 14C, and James Arnold was tasked with isolating and measuring 14C. Anderson was able to complete his project by obtaining samples of modern wood from around the world, and thereby also solved the aforementioned obstacle of worldwide mixing.

The radiocarbon dating method, though conceptually straight-forward, faced several practical challenges. Libby still needed to determine if it was practicable given the costs of access to equipment, sample sizes, and time— it often took four days of round-the-clock counting to get the measurement for a single sample. In addition, Libby and his colleagues needed access to a detector that was sensitive enough to count 14C. Obtaining samples of historical materials to date was not easy, and required the assistance of archaeologists. Libby stated, “Those museum dogs were not going to give it [samples] to a bunch of physical chemists to burn up, no way” (AIP 1979b). Once samples were obtained, they required cleaning of contaminants, another step Libby cites as critical in the development of radiocarbon dating.

The shared history of radiocarbon dating and archaeology begins in 1947. At this point Libby is certain radiocarbon dating is feasible, but needs funding and access to equipment to test the method. Libby first discloses his plans for radiocarbon dating to those close to him in 1946, and in 1947 James Arnold’s father provides unsolicited Egyptian specimens to Libby, obtained from Ambrose Lansing at the Department of Egyptian Art at New York’s Metropolitan Museum of Art (Marlow 1999:11-12). The year 1947 also saw the informal creation of a University of Chicago seminar club to discuss the role of social science in the atomic age, spearheaded by Chicago researchers Harold Urey (a 1934 Nobel laureate in chemistry, and an ally of Libby’s), associate professor Harrison Brown, and anthropologist and dean of social sciences, Robert Redfield (Marlow 1999:13). That same year, radiocarbon dating was for the first time presented to an audience outside Chicago, at a Viking Fund Supper Conference. Though two-dozen anthropologists and archaeologists were in attendance, it was asked that the development of radiocarbon dating not yet be made public (Marlow 1999:19). Soon after the conference, the Viking Fund financially backed Libby’s radiocarbon dating project.

Though communication about the radiocarbon method was slow and fraught with misunderstandings, Libby’s project had well-connected advocates and plenty of interest, as well as controversy, among archaeologists. Debate swirled around who should oversee the integration of the new method into archaeology before the method was even shown to be practicable. Organizations proposed for this task included the Society for American Archaeology, the American Anthropological Association, the Committee for the Recovery of Archaeological Remains, the National Research Council, and the Viking Fund, among others (Marlow 1999). In great part the calls to delegate an organization came from fears that the radiocarbon method was going to be controlled by the University of Chicago or the Viking Fund, and that the tool would not be made available to all who sought to use it (Marlow 1999:22). There were other concerns as well, such as whether old-world archaeologists would have representation in discussions of radiocarbon dating. A historic meeting occurred in January 1948 at a Viking Fund Supper Conference with a presentation by Libby, which was well attended by archaeologists. Here, the dispute over who should represent archaeologists was settled—the American Anthropological Association was chosen as the representative body, “to collaborate with Libby’s group, coach its brethren to be scrupulous in fulfilling their reciprocal responsibilities, and mediate the inevitable disputes and misunderstandings that arose” (Marlow 1999:25).

For many archaeologists at the time, radiocarbon dating was intimidating. In part this was due to its association with the atom bomb; while radiocarbon dating was not directly related to the development of the bomb, it was developed by atomic scientists and in a social climate of fear and awe of the power of the atom (Marlow 1999:23). Additionally, most archaeologists lacked the necessary background to understand how radiocarbon dating worked, and thus were reluctant to adopt the technology. Radiocarbon dating was also viewed as a threat to established dating methods and chronologies. Some even postulated that it could render obsolete their job as an archaeologist, as all the questions could now be easily answered (Marlow 1999:22-23).

The first published radiocarbon-dated samples were run on wood with known or assumed dates (Arnold and Libby 1949). These samples consisted of two dendrochronological samples, a floor fragment from a Syrian palace, two ancient Egyptian wood fragments (from a coffin and a funerary boat), and two samples from Egyptian tombs which were assayed as one sample. The measured ages were found to be satisfactory in comparison to expected dates of the samples. The half-life used to calculate the ages was 5720 ± 47 years. The study established that the radiocarbon method was useful for up to 4600 year ago and expressed the author’s hope that future research could evaluate the accuracy of the method up to 20,000 years ago. This article was radiocarbon’s seminal unveiling to the wider scientific public.

Though the new technology was discomfiting to many archaeologists at the time of its development, by the end of the 1950s it was widely accepted in archaeology as well as in other fields of study (e.g., geology); by then twenty radiocarbon labs had been established around the world, and the journal Radiocarbon was in circulation to consolidate radiocarbon date lists from the labs and ensure sufficient information was being published (Taylor and Bar-Yosef 2014:288). Most of these early radiocarbon labs were established at universities or research institutions, though one commercial lab was opened in the United States as well. In 1960, Libby won the Nobel Prize in Chemistry for the radiocarbon dating method. Taylor and Bar-Yosef point out that archaeology has only been mentioned once in a Nobel award citation, and that was for Libby’s Nobel (2014:289).

References Cited

AIP
1979a       Interview of Willard Libby by Greg Marlowe on 1979 April 12, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA. Electronic document, www.aip.org/history-programs/niels-bohr-library/oral-histories/4743-1, accessed February 9, 2019.

1979b      Interview of Willard Libby by Greg Marlowe on 1979 April 16, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA. Electronic document, http://www.aip.org/history-programs/niels-bohr-library/oral-histories/4743-2, accessed February 9, 2019.

Arnold, J. R., and W. F. Libby
1949    Age Determinations by Radiocarbon Content: Checks with Samples of Known Age. Science 110(2869):678–680.

Flint, Richard Foster and Edward S. Deevey
1962    Editorial statement. Radiocarbon 4(1):i–ii.

Kamen, Martin D.
1963    The early history of carbon-14. Journal of Chemical Education 40(5):234.

Korff, S. A., and W. E. Danforth
1939    Neutron Measurements with Boron-Trifluoride Counters. Physical Review 55(10):980–980.

Kurie, Franz N.D.
1934    A new mode of disintegration induced by neutrons. Physical Review 45(12):904.

Marlowe, Greg
1999    Year One: Radiocarbon Dating and American Archaeology, 1947-1948. American Antiquity 64(01):9–32.

Schuur, Edward A.G., Ellen Druffel, and Susan E. Trumbore (editors)
2016    Radiocarbon and Climate Change. Springer International Publishing.

Taylor, R.E., and Ofer Bar-Yosef
2014    Radiocarbon Dating: An Archaeological Perspective. Left Coast Press, Inc. Walnut, California.

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