Complexities in the use of bomb-curve radiocarbon to determine time since death of human skeletal remains.
Atmospheric testing of nuclear weapons during the 1950s and early 1960s doubled the level of radiocarbon ([sup.14]C) in the atmosphere ([sup.14]C[O.sub.2]). From its peak in 1963, the level of [sup.14]C[O.sub.2] has decreased exponentially, with a mean life of about 16 years, not because of radioactive decay, but because of mixing with large marine and terrestrial carbon reservoirs. Because radiocarbon is incorporated into all living things, the bomb pulse is an isotopic chronometer of the past half-century. The absence of bomb radiocarbon in skeletonized human remains generally indicates a date of death before 1950. Comparison of the radiocarbon values with the post-1950 bomb curve may also help elucidate when in the post-1950 era the individual was still alive. Such interpretation, however, must consider the age at death of the individual and the type of tissue sampled.
Estimation of time since death of skeletonized human remains represents an important yet difficult issue in forensic anthropology. Research and casework have demonstrated that many variables potentially affect postmortem alteration of both soft and hard tissues. It usually is difficult to determine from morphological information alone if skeletonized remains represent a postmortem interval of several or many years. This is unfortunate because it is advantageous to the investigation and identification processes to determine if the remains are ancient and thus not of medicolegal interest or if they are more recent and perhaps can be linked to a missing person.
Traditionally, radiocarbon dating has been considered an archeological tool rather than a forensic one. With a half-life of 5,730 years, the radioactive decay of carbon-14 ([sup.14]C) is minimal within the time periods of interest in forensic cases and applicable for samples more than 300 years of age. Radiocarbon analyses of bone traditionally use extracted collagen as the carbon source. Most of these analyses are of samples from archeological sites, often thousands of years old. Collagen is the most common protein in the body and the largest source of carbon in bone. Procedures to extract and purify collagen are well-developed. The mineral part of bone has not traditionally been used in radiocarbon dating because of the low carbon content and potential for mineral exchange of carbonate during long periods of burial.
Radiocarbon levels can be used to estimate time since death in modern forensic cases, using the modern bomb curve (Figure 1). Atmospheric testing of thermonuclear devices between 1950 and 1963 significantly increased the level of [sup.14]C in the atmosphere and food chain. The testing events nearly doubled levels of [sup.14]C in terrestrial organisms. In 1963, Great Britain, the United States, and the Soviet Union agreed to stop atmospheric testing, although France and China continued until 1980 (Nydal et al. 1979; Natural Resources Defense Council 2002). Despite this variation in testing cessation, laboratory research reveals that artificial radiocarbon in the stratosphere within the Northern Hemisphere reached maximum concentration in 1963 and subsequently has declined steadily (Berger 1979; Levin and Kromer 2004). Hua and Barbetti (2004) split the earth into four separate zones to account for the mixing of [sup.14]C[O.sub.2] from just a few anthropogenic sources to essentially uniform global distribution by the late 1960s. Through the food chain and related factors, these temporal fluctuations of artificially high levels of radiocarbon have been captured in organic material worldwide and thus offer an opportunity to elucidate time-since-death issues in recent forensic samples.
[FIGURE 1 OMITTED]
The adage "You are what you eat" holds true. The isotopic content of new plant growth reflects the atmospheric radiocarbon concentration. Herbivore values lag those of the atmosphere slightly because their primary carbon source is on the order of months old. Values of omnivores and carnivores lag the atmosphere further because their carbon source is another step removed. Within organisms, tissues turn over at different rates, so [sup.14]C levels vary among tissues. The date of formation of a tissue or specific biomolecule can be estimated from the bomb curve by considering these lags in incorporation and relating the [sup.14]C concentration (y-axis) with the date (x-axis). Of course, individual human diets vary considerably in the relative proportion of plant and animal foods. Using an annual average of the carbon intake over a growing season can account for much food-chain lag while also producing a simpler curve (Figure 2). Caution must be exercised when dating an elevated sample because the pulse is double-valued. Placing a sample on the ascending or descending side of the pulse often can be accomplished if additional tissue (e.g., hair, different bone, tooth) or other information is available. Routine radiocarbon analyses using accelerator mass spectrometry can be performed on samples containing as little as 200 [micro]g of carbon.
[FIGURE 2 OMITTED]
The radiocarbon bomb pulse is most widely used in studies of ocean circulation (Key 2001) and the carbon cycle (Trumbore and Torn in press). In humans, the bomb pulse has been used to date pathogenic structures and to determine the birth dates of cells through isotopic analysis of DNA. Stenhouse (1979) used bomb-curve radiocarbon analysis to examine the formation time of coronary atherosclerotic lesions and gallstones, documenting that gallstones in one patient had formed at least 20 years before surgical removal. The progressive growth of gallstones has been examined through analysis of their layered morphology (Druffel and Mok 1983). Senile plaques and neurofibrillary tangles in brains with Alzheimer's disease have been dated to examine the progression of the disease (Lovell et al. 2002). Because DNA is synthesized only at the time of cell division, [sup.14]C analysis of purified DNA can place the birth dates of cells on the bomb curve. Spalding et al. (2005) used this approach to show that postnatal neurogenesis does not occur in the cortex of the adult human brain.
Bomb-Curve Radiocarbon Concentrations in Human Tissues
In 1959, Broecker et al. noted that bomb-curve radiocarbon concentrations in human tissues lag those of the atmosphere, relating to dietary issues and tissue turnover. They documented a lag time of about 1.1 years for blood and 1.8 years for lung tissue. Libby et al. (1964) further noted that concentrations in human brain tissue can lag atmospheric levels by only a few months, but levels in the collagen within cartilage were delayed considerably. Autopsy studies of the cartilage collagen of 70-year-old adults showed little increase in artificial radiocarbon despite the subjects' having lived through the entire rise of the bomb pulse from 1955 to 1964 (Libby et al. 1964). Harkness and Walton (1969) noted the important role that individual diet can play in establishing radiocarbon levels in humans. Nydal et al. (1971) found good agreement in radiocarbon levels among samples of human blood and hair.
Data from Individuals with Known Birth and Death Dates
In 1972, Harkness and Walton reported radiocarbon levels in various tissues of a 37-year-old female who died of coronary artery disease in 1969. Highest concentrations of [sup.14]C were found in the brain, followed by the muscles, ovaries, kidneys, liver, fat, uterus, bone marrow, bone collagen, and bone mineral. Bone collagen and mineral contained substantially lower amounts of radiocarbon than the other tissues, apparently reflecting their lower rate of tissue turnover.
Additional comparative data were added in 1977 from Stenhouse and Baxter's analyses of different tissues originating from a 72-year-old male with a birth date of 1899 and a death date of 1971. Soft-tissue levels were all much higher than that of bone collagen.
Wild et al. (2000) published additional data on tissues from four individuals with ages ranging from newborn to 85 years. As with the previous studies, they found [sup.14]C concentrations consistent with atmospheric levels in tissues with rapid turnover, such as lipids or hair, whereas bone collagen exhibited much slower turnover.
Shin et al. (2004) examined radiocarbon levels within six human forensic and surgical bone samples from individuals ranging in age from 19 to 84 years. They estimated cortical bone turnover at 2 to 8 percent per year and examined variability of radiocarbon within "light" and "heavy" density fractions, which they interpreted to represent recent and longer-term bone formations, respectively. For individuals less than 50 years of age, the relative values of the two types of bone correlated with placement on the bomb curve. This important study suggests that carbon turnover and recycling vary with bone tissue type and are manifested in radiocarbon concentrations.
Taylor et al. (1989), Ubelaker (2001), Ubelaker and Houck (2002), and Wild et al. (2000) all have called attention to the potential applications of bomb-curve radiocarbon interpretations in determining time since death in modern medicolegal cases. Basically, if human tissues are analyzed and the elevated bomb-curve levels of radiocarbon are not detected, those tissues date to time periods prior to nuclear testing (about 1950). Similarly, elevated levels suggest that the tissues postdate 1950, and the magnitude of the levels provides information regarding placement on the bomb curve. Such analysis has proven useful in applications to forensic cases involving skeletonized human remains in determining ancient (pre-1950) or modern (post-1950) status (Taylor et al. 1989; Ubelaker 2001; Ubelaker and Houck 2002).
As noted by Geyh (2001) and the studies cited above, interpretation in forensic contexts must consider the type of tissue as well as patterns of growth and aging, diet, and even medical treatment of pathological conditions. For example, Thompson and Ballou's 1956 study of rats given doses of a tritium oxide marker suggested that collagen is relatively inert and is not replaced during the lifetime of the animal. Thus analysis of collagen in an elderly individual might reflect the formation time at an earlier age, rather than the age at death.
Four studies (Harkness and Walton 1972; Shin et al. 2004; Stenhouse and Baxter 1977; Wild et al. 2000) present radiocarbon values derived from the tissues of 12 individuals with known birth dates and known death dates or dates of tissue death (surgical samples). These data are summarized in Table 1. The "Lab Numbers" (if available) are those designated in the individual publications. The "Age at Tissue Death" represents the age of the individual, either at death or at the time the tissue was surgically removed. Similarly, the "Tissue Death Date" represents either the date of death of the individual or the date of surgical removal of the specimen. Carbon-14 levels are given in F[sup.14]C, in which 1 is the atmospheric [sup.14]C level prior to the Industrial Revolution (Reimer et al. 2004; Stuiver and Polach 1977). "Average Age at Formation" represents the possible ages of the individual when the level of [sup.14]C in the tissue examined corresponds to the appropriate value on the bomb curve (Table 2). The slow, continuous turnover of bone means that this average age at formation is accrued over several to many years, depending on the tissue sampled. Two possible values for age at formation are provided for NOC2, NOC11, NOCX24, and VERA-0024 because of their more recent tissue death dates. The radiocarbon values could correspond to either the earlier, upward side of the bomb curve or its later, downward side. Given the values presented in VERA-0027, the former, earlier dates of formation are much more likely.
[TABLE 1 OMITTED]
Careful examination of Table 1 reveals the powerful role of individual age in comparisons of age at death and radiocarbon levels. With the obvious exception of the newborn, all other examples present a significant lag time between age at tissue formation and age at death. Furthermore, this lag time increases with increasing age at death. For example, in the 19-year-old of FOR2, radiocarbon values of the cortical femur sample suggest an average tissue formation age of 15 years. In contrast, the radiocarbon level of the 85-year-old long-bone collagen sample of VERA-0027 suggests an average formation age of 54 years, which in turn implies minimal bone turnover at that site in the final 30 years of the life of that individual. However, the radiocarbon value for the bone collagen sample of the 72-year-old reported by Stenhouse and Baxter (1977) suggests a more rapid bone turnover, at least for that individual in that particular tissue. Surprisingly, the mineral fraction of bone from the 72-year-old exhibited faster turnover than the collagen.
Bone is continuously remodeled during adulthood. In a closely coupled process, osteoclasts resorb old bone, and osteoblasts form new bone. Total annual bone turnover is often estimated at 10 percent. No forensic investigation samples a whole skeleton, however, and remodeling varies with the type of bone. Approximately 25 percent of trabecular bone and 3 percent of cortical bone are believed to be resorbed annually in adults (Manolagas and Jilka 1995). The extent to which resorbed material is recycled by osteoblasts reduces the carbon turnover in bone. The onset of rapid bone loss (osteoporosis) can discontinue the incorporation of new carbon into bone because the inhibited osteoblasts merely recycle material. The onset of rapid bone loss varies with age and sex. It starts to occur in postmenopausal women (at approximately age 40-50) and 20 to 30 years later in men. Looking again at Table 1, it is entirely possible that an 85-year-old woman with osteoporosis ceased to add new carbon to her bone the last 30 years of her life. A 72-year-old man is much more likely to have continued to turn over bone carbon and perhaps show no signs of osteoporosis.
The information presented here suggests that radiocarbon dating, with special attention to the bomb-curve values, provides a powerful new tool in the important forensic problem of estimating time since death of skeletonized human remains. Artificially high values clearly indicate that the tissue, and thus the individual, was alive in the post-1950 era. Considerable potential exists to fine-tune the estimates of time since death in such analyses with a fuller understanding of the factors that influence the timing of bone turnover in particular tissues and different anatomical areas. Modern accelerator mass spectrometry procedures require minimal sampling, and Lanting et al. (2001) have reported successful dating of the mineral (carbonate) component even of cremated bones.
Investigators applying this procedure should remain aware, however, that because of possible significant temporal differences between the time of bone formation and levels of atmospheric radiocarbon at the time of death, estimated ages or dates of individual tissues may lag actual age at death. This relationship appears to be influenced by diet, medical treatment, growth and remodeling patterns, age at death of the individual, types of tissue sampled, regional variation of [sup.14]C concentrations, and other factors. Forensic scientists incorporating radiocarbon analysis into their protocols should have at least a rudimentary understanding of the mechanisms involved and the complex factors that potentially impact interpretation.
This work was performed in part under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory, under contract W-7405-Eng-48.
The authors thank P. J. Reimer for providing the large atmospheric data sets in F[sup.14]C units.
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Douglas H. Ubelaker
Department of Anthropology
National Museum of Natural History
Bruce A. Buchholz
Center for Accelerator Mass Spectrometry
Lawrence Livermore National Laboratory
Table 2: Average Annual Tropospheric [sup.14]C[O.sub.2] in Mid-Latitudes of the Northern Hemisphere (1940-2003) Year [[DELTA].sup.14]C * pMC ([dagger]) F[sup.14]C 1940.5 -20.2 97.98 0.9849 1941.5 -19.4 98.06 0.9857 1942.5 -19.6 98.04 0.9855 1943.5 -22.5 97.75 0.9825 1944.5 -21.7 97.83 0.9834 1945.5 -22.1 97.79 0.9829 1946.5 -21.6 97.84 0.9835 1947.5 -21.1 97.89 0.9840 1948.5 -22.3 97.77 0.9827 1949.5 -24.6 97.54 0.9804 1950.5 -24.8 97.52 0.9802 1951.5 -24.8 97.52 0.9802 1952.5 -24.9 97.51 0.9801 1953.5 -23.9 97.61 0.9811 1954.5 -21.1 97.89 0.9840 1955.5 -8.2 99.18 0.9969 1956.5 26.5 102.65 1.0318 1957.5 73.0 107.30 1.0785 1958.5 140.2 114.02 1.1461 1959.5 228.0 122.80 1.2294 1960.5 212.3 121.23 1.2139 1961.5 221.6 122.16 1.2233 1962.5 358.5 135.85 1.3606 1963.5 718.3 171.83 1.7212 1964.5 835.7 183.57 1.8690 1965.5 756.3 175.63 1.7597 1966.5 691.9 169.19 1.6953 1967.5 623.6 162.36 1.6271 1968.5 564.5 156.45 1.5680 1969.5 545.4 154.54 1.5492 1970.5 529.1 152.91 1.5329 1971.5 499.4 149.94 1.5033 1972.5 465.6 146.56 1.4696 1973.5 418.6 141.86 1.4227 1974.5 400.8 140.08 1.4050 1975.5 369.8 136.98 1.3741 1976.5 352.5 135.25 1.3569 1977.5 333.9 133.39 1.3384 1978.5 325.8 132.58 1.3302 1979.5 295.8 129.58 1.3001 1980.5 264.5 126.45 1.2689 1981.5 256.7 125.67 1.2615 1982.5 238.3 123.83 1.2432 1983.5 224.2 122.42 1.2292 1984.5 209.3 120.93 1.2144 1985.5 201.3 120.13 1.2065 1986.5 191.1 119.11 1.1964 1987.5 182.6 118.26 1.1880 1988.5 173.4 117.34 1.1789 1989.5 163.5 116.35 1.1691 1990.5 152.5 115.25 1.1582 1991.5 142.9 114.29 1.1487 1992.5 136.4 113.64 1.1423 1993.5 128.4 112.84 1.1344 1994.5 122.1 112.21 1.1282 1995.5 115.5 111.55 1.1217 1996.5 109.9 110.99 1.1162 1997.5 104.3 110.43 1.1107 1998.5 100.3 110.03 1.1068 1999.5 96.3 109.63 1.1029 2000.5 88.6 108.86 1.0953 2001.5 81.6 108.16 1.0884 2002.5 77.8 107.78 1.0847 2003.5 71.5 107.15 1.0785 Source: Stuiver and Polach 1977 * [sup.14]C decay corrected ([dagger]) Percent modern carbon
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|Title Annotation:||Research and Technology|
|Author:||Ubelaker, Douglas H.; Buchholz, Bruce A.|
|Publication:||Forensic Science Communications|
|Date:||Jan 1, 2006|
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