Applying Carbon-14 Dating to Recent Human Remains by Philip Bulman with Danielle McLeod-Henning
Measuring carbon-14 levels in human tissue could help forensic scientists determine age and year of death in cases involving unidentified human remains.
rchaeologists have long used have been significantly perturbed carbon-14 dating (also known by mid-20th-century episodes of Aas radiocarbon dating) to above-ground nuclear weapons estimate the age of certain objects. testing. Before the nuclear age, the Traditional radiocarbon dating is amount of radiocarbon in the envi- applied to organic remains between ronment varied little in the span of 500 and 50,000 years old and a century. In contrast, from 1955 to exploits the fact that trace amounts 1963, atmospheric radiocarbon levels of radioactive carbon are found in almost doubled. Since then they the natural environment. Now, new have been dropping back toward applications for the technique are natural levels. Over the past six emerging in forensics, thanks to decades, the amount of radiocarbon research funded by NIJ and other in people or their remains depends organizations. In recent years, foren- heavily on when they were born or, sic scientists have started to apply more precisely, when their tissues carbon-14 dating to cases in which were formed. law enforcement agencies hope to find out the age of a skeleton or Forensic anthropologists at The other unidentified human remains. University of Arizona took advantage of this fact in a recent study funded The new method is based on the by NIJ. The researchers wanted fact that over the past 60 years, to find out if they could identify a environmental levels of radiocarbon person’s year of birth or year of
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What Is Carbon Dating?
tandard carbon-14 testing, Sas used by archaeologists, Figure 1: Northern Hemisphere Atmospheric Radiocarbon Levels is based on the natural process 1940 to 2003 A.D. of radioactive carbon formation 1.9 that results from cosmic ray bombardment of nitrogen in the 1.8 earth’s upper atmosphere. The radioactive carbon is taken from 1.7 the atmosphere and incorporated 1.6 into plant tissues by plant photo- synthesis. It is then incorporated 1.5 into all living organisms by means of the food chain. After an organ- 1.4 ism dies, its level of carbon-14 1.3
gradually declines at a predictable (Fraction Modern) pace, with a half-life of about 5,730 1.2 years. Archaeologists precisely measure levels of the isotope in Atmospheric radiocarbon content 1.1 organic remains. Knowing the half- 1.0 life, they back calculate how much time must have passed since 0.9 the remains had levels identical 1920 1940 1960 1980 2000 2020 to living organisms. Radiocarbon Calendar year measurement can date organic The radiocarbon levels are expressed in terms of Fraction Modern (FM), where remains up to about 50,000 years 1.0 FM represents the level of atmospheric radiocarbon in 1950 A.D. without old. Objects younger than 500 anthropogenic influence. Archaeological material 5,730 years or one half-life years old are rarely radiocarbon old contains 0.5 FM radiocarbon by definition; two half-lives, 0.25 FM, etc. dated. Natural and anthropogenic The scale is used for expressing recent radiocarbon measurements because fluctuations in environmental radio- 1950 A.D. was selected as the “zero year” by convention. Using FM avoids the paradox of expressing radiocarbon dates from the past 60 years in terms carbon levels mean that organisms of negative years. The curve is a composite of data sets from Stuiver et al.1 and living in different centuries within Levin and Kromer,2 and it has been annually smoothed. the past 500 years can have identical radiocarbon contents. the biosphere. The radiocarbon tissue’s formation, one calculates the Forensic scientists use carbon-14 created by the thermonuclear explo- year of birth or death of the organism measurement in a subtly differ- sions is identical to naturally created from which the tissue was derived. ent manner. A large increase in radiocarbon, and its route into plant atmospheric carbon-14 occurred and animal tissues is the same. It when the United States and has resulted in artificially high levels several other countries tested Notes of carbon-14 in plants and animals nuclear weapons aboveground 1. Stuiver, Minze, Paula J. Reimer, Edouard living in the past 60 years. during the 1950s and 1960s (see Bard, J. Warren Beck, G.S. Burr, Konrad A. Hughen, Bernd Kromer, Gerry McCormac, Figure 1). The Nuclear Test-Ban The forensic use of carbon-14 Johannes Van Der Plicht, and Marco Treaty of 1963 effectively ended measurement does not rely on the Spurk, “INTCAL98 Radiocarbon Age the era of atmospheric testing for slow decay process. Instead, tissue Calibration, 24,000-0 cal BP,” Radiocarbon most nuclear powers. Since then, carbon-14 levels are directly com- 40 (1998): 1041-1083. 2. Levin, Ingeborg, and Bernd Kromer, “The atmospheric carbon-14 levels have pared to levels in a rapidly changing 14 Tropospheric CO2 Level in Mid-latitudes been declining as the radiocarbon atmosphere. Using biological knowl- of the Northern Hemisphere (1959-2003),” is soaked up by the oceans and edge of the timing of a particular Radiocarbon 46 (2004): 1261-1272.
Applying Carbon-14 Dating to Recent Human Remains | 9 NIJ JOURNAL / ISSUE NO. 269 ■ MARCH 2012
death using precise measurements The spike in atmospheric carbon-14 of carbon-14 levels in different post- Over the past six levels during the 1950s and early mortem tissues. They measured decades, the amount of 1960s makes this approach possible, carbon-14 levels in various tissues but it also means it will have a limited from 36 humans whose birth and radiocarbon in people period of utility because the amount death dates were known. of carbon-14 in the atmosphere is depends heavily on slowly returning to its natural level. To determine year of birth, the Barring any future nuclear detona- researchers focused on tooth when their tissues tions, this method should continue to enamel. Adult teeth are formed at be useful for year-of-birth determina- known intervals during childhood. were formed. tions for people born during the next The researchers found that if they 10 or 20 years. Everyone born after assumed tooth enamel radiocarbon that would be expected to have the content to be determined by the hair — had radiocarbon levels identi- same level of carbon-14 that pre- atmospheric level at the time the cal to the contemporary atmosphere. vailed before the nuclear testing era. tooth was formed, then they could Therefore, the radiocarbon level in deduce the year of birth. They found those tissues post-mortem would All the people whose tissues were that for teeth formed after 1965, indicate the year of death. The tested for the study were residents enamel radiocarbon content pre- researchers found that year-of-death of the United States. Atmospheric dicted year of birth within 1.5 years. determinations based on nails were dispersion tends to create uniform Radiocarbon levels in teeth formed accurate to within three years. levels of carbon-14 around the globe, before then contained less radiocar- and researchers believe that these bon than expected, so when applied The generally poor post-mortem would be reflected in human tissues to teeth formed during that period, preservation of soft tissues would regardless of location. However, the method was less precise. be a limiting factor to this approach. more testing is needed to confirm However, the researchers suggested that belief. To determine year of death, the that soft tissue radiocarbon content researchers used radiocarbon levels would be transferred to, and pre- in soft tissues. Unlike tooth enamel, served in, the pupal cases of insects About the authors: Philip Bulman is soft tissues are constantly being whose larvae feed on these tissues. a writer and editor at NIJ. Danielle made and remade during life. Thus, Such insects are simply another link McLeod-Henning is a program manager their radiocarbon levels mirror those in the food chain. Thus, pupal case and physical scientist at NIJ. in the changing environment. The radiocarbon content would serve as a researchers found that certain soft decay-resistant proxy for the tissues, NCJ 237722 tissues — notably blood, nails and yielding the year of death.
For more information: ■ Read the full report: Hodgins, Gregory W. L., “Measuring Atomic Bomb- Derived 14C Levels in Human Remains to Determine Year of Birth and/or Year of Death,” Final report to the National Institute of Justice, grant number 2005-IJ-CX-K013, August 2009, NCJ 227839, available at https://www.ncjrs. gov/pdffiles1/nij/grants/227839.pdf.
10 | Applying Carbon-14 Dating to Recent Human Remains