Using Radiocarbon Dating and Paleontological Extraction Techniques in the Analysis of a Human Skull in an Unusual Context (Forensic Science Communications, October 2002)
October 2002 - Volume 4 - Number 4
Using Radiocarbon Dating and Paleontological Extraction Techniques in the Analysis of a Human Skull in an Unusual Context
Douglas H. Ubelaker
Department of Anthropology
National Museum of Natural History
Max M. Houck
Forensic Science Initiative
West Virginia University
Morgantown, West Virginia
A disarticulated cranium and mandible partially encased in an extremely hard plastic material were discovered in a riverbed in Pennsylvania. Several traditional fossil-preparation methods were used to extract the fragile skull from the plastic. Anthropological analysis of the skull indicated it was a male of African ancestry with an age at death greater than 50 years. To clarify time since death, radiocarbon analysis was conducted. The results were compared using the modern bomb curve. The skull revealed pre-1950 levels of radiocarbon, and thus it was not of recent origin.
On August 1, 1999, a poorly preserved metal bucket was recovered from a river in Lancaster, Pennsylvania. Inside the metal bucket was a smaller, white plastic bucket containing a human skull partially embedded in a gray plastic material. The recovered objects were taken to the local medical examiner to have the skull removed from the plastic matrix and to be analyzed. The medical examiner found the plastic to be very difficult to remove, to the extent that the motor on a bone saw burned out in an attempt to extract the skull. The skull and plastic were then sent to the FBI Laboratory to determine the composition of the matrix and to attempt to extricate and analyze the human remains.
Radiographs revealed a skull with the cranium and mandible disarticulated within the plastic matrix (Figure 1). The disarticulation suggested that the cranium and mandible were most likely skeletonized at the time of their immersion into the liquid plastic. The plastic itself was potentially an important clue. Its chemical composition indicated that it was similar to the plastic used in kitchen countertops, not commonly sold to the public in the quantity that was used to encase the skull.
The removal of the matrix proved to be quite a challenge. The resources of the Smithsonian Institution's Vertebrate Paleontology Preparation Laboratory were drawn upon to remove the plastic atrix with a pneumatic scribe. Several sessions of intensive, meticulous work with the so-called air scribe, used to separate fossils from their geological matrices, were required to extract the delicate skull from the surrounding plastic (Figures 2 and 3).
|Figure 1 Radiograph of a Human Skull with the Cranium and Mandible Disarticulated in a Plastic Matrix Click to enlarge image||Figure 2 Specimen Skull with Part of the Plastic Matrix Removed Click to enlarge image||Figure 3 Specimen Skull with Most of the Plastic Matrix Removed Click to enlarge image|
Once separated from their plastic context, the cranium and mandible were examined for geological materials which, if present, might indicate if the remains had been buried prior to submersion in the plastic. No soil, however, was found on either item. The cranium and the mandible were then analyzed for anthropological information.
The bone was well preserved with no soft tissue or hair present. The posterior portion of the left ramus of the mandible and the bones of the right cheek area were missing. These areas did not show obvious signs of recent fracture, but it could not be determined if the damage was peri- or postmortem. However, the coloration of the broken margin of the right zygomatic suggested relatively recent fracture. Numerous teeth were missing both ante- and postmortem.
The general robusticity of the remains, especially the large supraorbital ridges, suggested male sex. Discriminant function analysis of cranial measurements (Ousley and Jantz 1996) also suggested male sex.
The extent of cranial suture closure, antemortem tooth loss, and age-related changes in tooth structure suggested a relatively old age at death. In particular, application of the Lamendin technique (Lamendin et al. 1992; Prince and Ubelaker 2002) for age estimation from characteristics of single rooted teeth to the right maxillary canine indicated an age at death greater than 50 years. This technique produced an age estimate of 60.2 years, plus or minus about eight years. All of the collectively available information for estimating age suggested the remains originated from a mature adult, probably greater than 50 years of age.
Evidence for African ancestry consisted of relatively wide interorbital distance, wide nasal aperture, prognathism, and other features. A discriminant function analysis using the FORDISC 2.0 system (Ousley and Jantz 1996) also strongly supported African ancestry. No measurements were used that had been compromised by disease, age, or taphonomic conditions.
Time since death was more problematic. The lack of soft tissue and hair and the desiccated nature of the remains were consistent with considerable antiquity, but the possibility that the remains were sufficiently recent to be of forensic interest could not be ruled out. To address this question, radiocarbon analysis was conducted.
Standard 14C dating is based on the work of Nobel Laureate Willard Frank Libby (1908-1980) (Libby 1946), which was inspired by the research of a cosmic-ray physicist, Serge A. Korff (Korff and Danforth 1939). Although Libby initially kept his findings secret because he thought the idea was too ludicrous to gain financial support, 14C dating has ultimately revolutionized archeology, geology, geophysics, and other branches of science (Taylor 2000).
The radiocarbon dating method recognizes that living plants and animals maintain amounts of the carbon isotope 14C, which are close to atmospheric levels. After death, the amount of 14C gradually declines, with a half-life of about 5,730 years. By measuring the amount of radiocarbon remaining in the material to be dated, the analyst can calculate the antiquity. Traditionally, radiocarbon dating has been applied to materials older than 300 years because in younger materials, minimal radiocarbon decline and other factors usually allow only a modern label to be applied.
One of the variables involved in the interpretation of the so-called modern radiocarbon readings is the bomb 14C, or artificial radiocarbon, released into the atmosphere during the testing of thermonuclear devices between 1950 and 1963. Such testing, which increased to a peak in the mid-1960s, then gradually declined, left terrestrial plants and animals that were alive at the time with artificially elevated levels of 14C. The presence of greater than 1950 levels of 14C can be plotted on the earlier and later slope to suggest alternative post-1950 dates. For these reasons, bomb curve radiocarbon information is not useful in archaeological or geological investigations but is very helpful in forensic analysis (Ubelaker 2001; Wild et al. 2000).
To clarify the antiquity of the materials from Pennsylvania, an approximately square bone sample (measuring about two inches) taken from the cranial vault was sent to a commercial radiocarbon dating laboratory for analysis.
The results of the radiocarbon testing indicated that 14C values were below the 1950 levels, thus the individual's death (and production of the bone collagen) preceded 1950. Although this analysis establishes the important fact that the death was not recent, the exact antiquity of the remains is still not clear. The analysis produced a conventional radiocarbon age of 230 years, plus or minus 40 years, with concentrations approximately 98 percent that of modern (1950) levels. Calibration of these results using 95 percent probability levels reveals four periods when the individual could have died (Figure 4), which begin at AD 1530 and end at AD 1950. Using the less precise 68 percent probability levels, the range is shortened to between AD 1650 and AD 1800. The C13/C12 ratio of -13.6 suggests that the diet contained substantial amounts of marine foods and/or C4 plants such as corn.
Although it is not possible from these data to determine the precise antiquity within the broad interpretative framework outlined above, it is clear that the individual died prior to 1950. This information provides useful perspective to the investigation of the case.
This case represents an example of how techniques employed in paleontology can be useful in extracting human remains from a challenging modern context and how radiocarbon dating, especially comparison with the modern bomb curve, can clarify time since death issues in forensic situations.
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