During the Fall of 2017, I conducted a research internship at the Forensic Anthropology Research Facility (FARF), a component of the Forensic Anthropology Center at Texas State (FACTS). My research consisted of analyzing the effects of burial age on disturbances seen on the ground surface. The results of this research will help bioarchaeologists determine the relative age of a burial where documentation is not present.
Soil chemistry analysis and electrical resistivity was used to observe anomalies of the surface soil. The burials observed were positioned side by side with approximately one meter between each burial with control, or empty burials, alternating between filled burials. A total of 33 soil samples, three samples per burial, were processed using the X-ray fluorescence (XRF) handheld. Soil samples were collected from the center of the burial, one meter south from the center, and one meter west from the center of the burial. This methodology was used with the intention of mapping elements horizontally and vertically. The soil samples were finely grained using a mortar and pestle and transferred to sample cups for analysis. XRF is a non-invasive technique used to determine the elemental composition of materials, and allows on-site analysis of samples that might not otherwise be transported to laboratories (Pollard et al., 2007). Unfortunately, XRF is only able to detect elements from fluorine to uranium within the periodic table. Different filters may be used to shift the spectrum and focus on minor or major elements. Electrical resistivity was also used to observe anomalies of the surface soil. Buried remains affect the distribution of moisture within the soil, which affects the amount of electrical resistance (Clark, 2000). A tape of 30 meters was placed horizontally along the burials to roughly run across the center of each burial. Electrodes were positioned using a dipole-dipole array with a distance of 0.5 meters between each electrode. The dipole-dipole configuration allows unambiguous definition of shallow features (Clark, 2000), which is appropriate for the shallow burials used in this research. A total of 11 electrodes were used using this array with the first four probes acting as the trail current, lead current, trail potential, lead potential, and the remaining seven probes continuing the direction of advance. A current is passed through the current electrodes and a potential gradient is sampled between potential electrodes (Clark, 2000). The resistance is measured in ohms, Ω. The soil properties between the electrodes will influence resistance measured between the electrodes, so the further away the potential voltage electrodes are from the current electrodes, the greater depth of soil properties will affect the reading. I will be using data retrieved from the first potential electrodes, since only the shallow soil properties will influence the reading. The values are then inserted to a software, which generates a diagram of what the sub surface looks like. Any anomalies present over the burials will then be observed.
Before processing the soil samples, safety hazards and handling procedures of the XRF handheld had to be reviewed. Since the XRF handheld is an x-ray generating device, guidelines for keeping radiation exposure “as low as reasonably achievable” (ALARA) were strictly followed. The first precaution taken was using the instrument in a table top setting, which also allowed the samples to be processed in the laboratory. Other guidelines consisted of immediately shutting down the device if any risk of exposure was present by removing the key, keeping hands away from the spectrum, and being aware of hazard lights. The instrument also has safety features that reduce the risk of radiation exposure. X-rays will only be generated when the spectrum is against a hard surface; this feature prevents the handheld from being waived in the air like a gun while x-rays are generated. Although the device may be turned on, x-rays will only be generated for the designated amount of time the handler chooses. Before processing any samples, the instrument was also calibrated to ensure the PC trigger and hazard lights were functioning correctly. After the completion of the x-ray handler training, I was assigned a radiation badge which measures any amount of radiation exposure, and will flash red lights if medical attention is advised.
The soil chemistry analysis didn’t reveal any new elements, but rather changes in element concentrations around each burial. A distribution of iron was found throughout the field with spikes in concentration correlating with higher resistance values and filled burials (Figure 1). Similar changed in iron have been associated with burials (Perrone et al., 2014). Similar results were present with phosphate and manganese, showing a drop in concentration values over burials, albeit associated with a spike in resistance (Figures 2 and 3). In many cases, there was greater elemental activity present around the older burials (Figure 4).
Figure 1: Comparison of resistance values versus iron concentrations using GMP software. Burial positions are shown by solid rectangles, controls are indicated by outline blank rectangles.
Figure 2: Comparison of resistance values versus phosphate concentrations using GMP software. Burial positions are shown by solid rectangles, controls are indicated by outline blank rectangles.
Figure 3: Comparison of resistance values versus manganese concentrations using GMP software. Burial positions are shown by solid rectangles, controls are indicated by outline blank rectangles.
Figure 4: Comparison of resistance values versus silicon concentrations using GMP software. Burial positions are shown by solid rectangles, controls are indicated by outline blank rectangles.
The soil data also revealed a relations between iron and zirconium. Figure 5 shows zirconium and rubidium as exact mirror images of each other. This is due to the elements acting as enantiomers (Atkins and Paula, 2002).
Figure 5: Comparison of resistance values versus zirconium and rubidium concentrations using GMP software. Burial positions are shown by solid rectangles, controls are indicated by outline blank rectangles.
Anomalies revealed by electrical resistivity and soil chemistry revealed significant differences around burials, but no significant difference between burial age. The two older burials, buried in 2015, were also two double burials with no control burials alternating in between the two. For this reason, it is difficult to eliminate one variable over the other to attest that one was primarily responsible for the greater activity. It is also important to remember that correlation does not always equal causation. Several correlations were found in this research, however the topic requires more research to determine absolute causal factors. Further research is also recommended to include soil samples below the surface, and possibly areas underneath the burial. Gathering more soil data would also allow for a horizontal and vertical map of elemental concentrations around the burials.
This internship has taught me various skills such as research preparation, organization, handling x-ray generating equipment, and visually presenting data with different software. I was able to construct my own methodology with the guidance of faculty, catalog soil samples, handle x-ray generating equipment, and learn the steps of cleaning and processing data. This experience has definitely better prepared me for my future endeavors in graduate school, and continuing research. I recommend any future internship seekers to participate in an internship through FACTS!
Atkins, P., & Paula, J. (2002). Physical chemistry. 7th ed. New York (NY): Oxford University Press.
Clark, A. (2000). Seeing beneath the soil: Prospecting methods in archaeology. Antiquaries Journal. New York (NY).
Perrone, A., Finlayson, J.E., Bartelink, E.J., & Dalton, K.D. (2014). Application for portable x-ray fluorescence (xrf) for sorting commingled human remains. In A. Bradley & J. Byrd, Commingled human remains: methods in recovery, analysis, and identification (pp.145-165). San Diego (CA): Academic Press.
Pollard, M., Batt, C., Stern, B., Young, S.M.M. (2007). Analytical chemistry in archaeology. Cambridge (UK): Cambridge University Press.