Erica Chambers

Journal of Undergraduate Research
Volume 5, Issue 6 - March 2004

PIXE and Paleodiet: Reconstructing Subsistence of Florida’s Middle Archaic Using a New Method of Trace Element Analysis

Erica Chambers

ABSTRACT

Trace elements are naturally occurring elements found in organic and inorganic materials. Archaeologists are interested in trace elements because it has been demonstrated that elemental concentrations within bone may reflect elemental concentrations of diet. This research studies trace element concentrations of human and non-human bone collected from Nona’s Site, a Middle Archaic locality (7,000-5,000 years BP) in Sarasota County, Florida. Sample analysis focuses on the use of Particle-Induced X-ray Emission (PIXE), a recently introduced ion beam analytical method. Comparison of the data obtained from PIXE with those derived from ICP-MS, a more traditional method of trace element analysis, indicates that PIXE is a valid method to determine trace element concentrations in archaeological bone remains. However, the data generated suggest a diagenetic signal rather than a dietary one, based on the elevated concentrations of strontium and calcium in the Nona’s Site bone samples. This is likely the result of inundation in an aqueous environment.

INTRODUCTION

The Trace element analysis of bone has demonstrated that the elemental composition of skeletal remains correlates with subsistence patterns of the sampled population (Radosevich 1993). This research examines the elemental composition of prehistoric bone material utilizing Particle-Induced X-ray Emission (PIXE), a relatively new spectroscopic technique, to determine if PIXE is an effective tool for paleodiet reconstruction. Sample analysis is restricted to elements demonstrated to correlate with diet: strontium (Sr), calcium (Ca), barium (Ba), magnesium (Mg), manganese (Mn), iron (Fe), zinc (Zn), and copper (Cu). To date, PIXE has been minimally used to infer paleodiet in prehistoric remains. Examination of remains from sites dating to the Florida Middle Archaic (i.e. Nona’s Site) provides benchmark data which can be used to redefine the ecology and settlement patterns of this time.

EXPERIMENTAL APPROACH

Particle-induced X-ray Emission (PIXE) is an ion beam analytical method introduced in the 1970s (Jankuhn 1997; Sandford 1993). Bombardment at high energy levels changes the sample’s atomic structure by displacing electrons from their current energy level, prompting other electrons to move in to replace them. This electron shift from one energy level to another releases a photon in the X-ray region of the electromagnetic spectrum. The emitted X-rays are collected by an Si(Li) detector, converted into electrical impulses, sent to a multi-channel analyzer, and displayed in a graph as the number of X-rays per energy. The area under each X-ray peak correlates to the element’s concentration within the sample. Quantitative analysis was conducted using RobWin analytical software and are reported in parts per million, ppm (Figure 1). The 1.7 MV tandem accelerator used for this PIXE investigation is housed in the University of Florida’s Department of Physics.

Figure 1. Spectrum created from PIXE analysis. Peaks on graph represent X-rays per energy level. Area under each peak is then interpreted as an elemental concentration.

Sample Selection

Samples for this study are from Nona’s Site in Sarasota County, Florida and date to the Middle Archaic period, ca. 7000-5000 years BP (Figures 2a, 2b). These remains, discovered in the 1980s, were found on the southeastern edge of a year-round pond (Luer 2002). The site is important as it is situated in close proximity to both coastal habitats of the Gulf and to Florida’s upland interior. This diversity of exploitable resources creates a wide-range of potential subsistence patterns available to the prehistoric inhabitants (DeLeon 1998).

Figure 2a. Nona's Site (from Luer 2002). Map of Sarasota County, Florida, location of Nona's Site. Demonstrates the close proximity to the coast and Florida's upland interior.

Figure 2b. Nona's Site (from Luer 2002). Plan view of excavations at Nona's Site. Illustrates layout of topography and various points of excavation.

PIXE analyses of ground bone include a total of ten samples, as listed in Table 1. Human samples were chosen based on bones with good cortex, mainly long bone fragments. Non-human samples were analyzed as an independent assessment of diagenesis, or alteration of bone chemistry. Sample preparation procedures for PIXE analysis included those used in previous PIXE experiments involving ground material and required minimal modifications (Kravchenko et al. 2003).

Sample Number	Description	Dry Weight	DIUF Added	Yttrium μL	Yppm
Table 1 Preparation Procedures for PIXE Analysis of Ground Bone Samples
PNS0101	FS9 - snake	0.3461	150	350	961
PNS0203	FS10 - alligator	0.2891	200	300	1038
PNS0310	FS10 - turtle	0.3073	150	300	975
PNS0452	FS19 - turtle ss	0.2012	150	200	994
PNS0541	FS18 - human	0.536	300	500	932
PNS0654	FS19 - human	0.2185	150	200	915
PNS0757	FS31 - human	0.3238	250	350	1018
PNS0862	FS47 - human	0.1363	150	100	733
PNS0933	FS14 - deer	0.2777	150	350	900
PNS1023	FS10 - human	0.3784	200	350	924

Bone ash analysis: Inductively Coupled Plasma – Mass Spectroscopy (ICP-MS) and PIXE

Six samples were also examined using a more standard method of trace element analysis known as ICP-MS to provide a comparative baseline with which to interpret the PIXE results (Sandford 1993). ICP-MS protocol requires ashing the bone samples prior to analysis. This process removes organic material and decreases the probability of sample contamination. Therefore, additional preparation included baking ground bone samples in a muffle furnace at 720ƒ C for 8 hours.

The six samples, including 5 human and 1 deer, were ashed and sent to The University of Wisconsin’s Laboratory for Archaeological Chemistry. Samples were diluted in nitric acid, and an aqueous solution of known concentration was analyzed. The six bone ash samples were also subjected to PIXE analysis. Results of ground bone PIXE analysis and bone ash PIXE analysis could then be compared to other published studies as well as data obtained from ICP-MS analysis.

RESULTS

The PIXE analysis of ground bone yielded highly variable results. Table 2 outlines Pate’s (1995) range of elemental concentrations in relation to diet. In summary, diets relying on marine resources tend to have high Sr and Mg, but low Ba concentrations. Terrestrial diets generally result in low concentrations of Sr and Mg, but high concentrations of Ba. Herbivorous diets are high in Sr, Ba, and Mg, but have low concentrations of Zn. Carnivorous diets reflect low levels of Sr, Ba, and Mg, but have high concentrations of Zn. Accordingly, omnivorous diets have intermediate concentrations for Sr, Ba, Mg, and Zn.

Diet	Sr	Ba	Mg	Zn
Table 2 (From Pate, 1995) Chart of Bone Elemental Concentrations in Relation to Diet
Herbivorous	High	High	High	Low
Omnivorous	Intermediate	Intermediate	Intermediate	Intermediate
Carnivorous	Low	Low	Low	High
Marine	High	Low	High	N/A
Terrestrial	Low	High	Low	N/A

Elemental concentrations for the 5 human samples are shown in Figure 3 . Previous research has concluded that PIXE analysis is highly effective in determining concentrations of heavier elements, including Ca and Fe, but poorly reflects concentrations of lighter elements, such as Mg (Kravchenko et al. 2003).

Figure 3. Elemental concentration variability of PIXE ground human bone alanlysis for each of the studied elements.

The PIXE data was highly variable when compared to other published ranges utilizing trace element analysis. PIXE data interpretation relies on incorporation of quantitative analysis of raw data combined with appearance of each element on the spectrum graph. Ba and Mg quantitative levels matched those seen in other studies, however, since those levels are not apparent on the graph, data for these elements cannot be used for subsistence reconstruction. When compared to similar studies involving trace element analysis of bone, Zn and Mn concentrations seem comparable while highly elevated levels of Ca and Sr are apparent (Figure 4a). Sr and Ca are especially useful in paleodiet studies; however, the extremely high levels observed in Nona’s Site samples do not correspond with other published ranges and are off by over 100x. This strongly suggests diagenesis rather than a reflection of dietary influence in the samples analyzed.

Figure 4a. PIXE and ICP - MS results. Elemental concentrations resulting from analysis of ground bone using PIXE analysis in comparison to other published studies using trace element analysis.

PIXE analysis of bone ash shows similar results to the PIXE results of ground bone (Figure 4b). ICP-MS analysis of bone ash, when compared to published ranges (Figure 4c) show similar results for three of the four elements compared, including Ba, Mg, and Mn. Comparison of bone ash results from both PIXE and ICP-MS analysis (Figure 4d) demonstrates a wider range of results when using PIXE analysis. ICP-MS ranges for Ba, Mg, and Sr fall within the PIXE ranges, while Mn concentrations are somewhat high.

Figure 4b. Elemental concentrations resulting from analysis of bone ash using PIXE analysis in comparison to other published studies using trace element analysis.

Figure 4c. Elemental concentrations resulting from analysis of bone ash using ICP-MS analysis in comparison to other published studies using trace element analysis.

Figure 4d. Comparison of elemental concentrations from resulting analysis of bone ash using PIXE versus analysis of bone ash using ICP-MS.

DISCUSSION AND CONCLUSION

The variable results obtained from PIXE analysis, specifically the extremely high concentrations of Sr and Ca, suggest fluctuations due to factors other than diet. Sample selection and preparation as well as diagenetic variables have possibly affected the elemental concentrations. Samples were selected from various locations within Nona’s Site under the pretense that mean elemental concentrations would be more accurate when considering fluctuations due to diagenic influences. In trying to obtain an overall balance of diagenic variation, sample selection procedures limited the likelihood that bone specimens were from the same individual, thus increasing the variability within the study.

Postmortem changes can alter the elemental composition of bone, creating difficulty when interpreting subsistence patterns. Soft tissue decomposition, such as autolysis and putrefaction, combined with groundwater interaction alter chemical dietary signatures of human bone by forming acid by-products (Sandford 1993; Pate 1995). Further, elements are leached away in soils, thus altering the chemical signature that reflects diet (Pate 1995). The advantage of ashing bone is that organic material is removed, decreasing the possibility of sample contamination.

The results in this study demonstrate that PIXE data is comparable to ICP-MS analysis. Thus, PIXE, a fast and accurate method to analyze multiple elements simultaneously, can be included in the list of more traditional techniques, such as ICP-MS and AAS, which are currently used for trace element analysis. Sample preparation is relatively quick and straightforward. However, studies of human bone using PIXE require analysis of 3-5 targets to provide enough data to ensure accurate statistical range, therefore increasing the time and energy spent obtaining results. Further, PIXE analysis is considerably more specialized and expensive than other methods listed above.

Diagenesis greatly impacted the data obtained from the human samples, especially evident in the elevated levels of strontium and calcium. In addition, inconclusive Ba and Mg data create difficulties for subsistence reconstruction. However, it is important to note that the levels of Sr, Mn, and Zn all reflect a reliance on marine habitats. PIXE analysis of human bone ash, when compared to data obtained from ICP-MS analysis of human bone ash, show a wider range of data that is less precise in determining mean elemental concentrations. However, since nearly all of the ICP-MS data are within the range of the PIXE data, it is safe to assume that PIXE is providing accurate results. If, in future studies, diagenetic effects can be accounted for, the use of PIXE as an analytical method of trace element analysis will provide another helpful tool to contribute to the reconstruction of prehistoric subsistence.

REFERENCES

DeLeon, V. B. (1998). Stable Isotope Analysis and Paleodiet at the Bay West Site, Collier County, Florida. Unpublished MA Thesis, Anthropology Department, University of Florida.
Edward, J. B. and Robert A. Benfer (1993). The effects of diagenesis on the Paloma skeletal material. In (M. K. Sandford, ed.) Investigations of Ancient Human Tissue: Chemical Analyses in Anthropology. New York: Gordon and Breach, pp. 183-268.
Jankuhn, St., T. Butz, R. H. Flagmeyer, T. Reinert, J. Vogt (1997). Ion beam analysis of ancient human bone. In (J.L. Duggan and I.L. Morgan, eds.) Application of Accelerators in Research and Industry. New York: AIP Press, pp. 575-577.
Kravchenko, I.I., R.F. Kelly, F.E. Dunnam, H.A. Van Rinsvelt (2003). PIXE Study of Lacustrine Sediments Including a Sediment Core From Lake Maicuru, Para, Brazil. In (J.L. Duggan, I.L. Morgan, eds.) Application of Accelerators in Research and Industry: 17th Int’l Conference. American Institute of Physics.
Luer, George M. (2002). Three Middle Archaic Sites in North Port. In (George M. Luer, ed.) Archaeology of Upper Charlotte Harbor, Florida. Tallahassee: Florida Anthropological Society, Publication No. 15, pp. 3-33.
Pate, F. D. (1995). Bone chemistry and paleodiet. Journal of Archaeological Method and Theory. 1:161-209.
Radosevich, S. C. (1993). The six deadly sins of trace element analysis: A case of wishful thinking in science. In (M. K. Sandford, ed.) Investigations of Ancient Human Tissue: Chemical Analyses in Anthropology. New York: Gordon and Breach, pp. 269-332.
Sandford, M. K. (1993). Understanding the biogenic-diagenetic continuum: Interpreting elemental concentrations of archaeological bone. In (M. K. Sandford, ed.) Investigations of Ancient Human Tissue: Chemical Analyses in Anthropology. New York: Gordon and Breach, pp. 3-57.
Scharf, W. H. (1988). Particle Accelerators—Applications in Technology and Research. New York: John Wiley and Sons.

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