Organic staining on bone from exposure to wood and other plant materials
Pollock, Corey Rae
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Determining the depositional environment and the postmortem alterations to a set of remains are aspects of forensic investigations that are necessary to explain the circumstances surrounding the death of the individual. Further research on the taphonomic agents that can impact skeletal material can aid in the differentiation between various postmortem alterations that impact a single set of remains. The present study focuses on organic staining as a method for reconstructing the deposited environment of the remains and the taphonomic agents in which they came into contact. Organic staining results largely from tannins leaching from plant materials, including wood and leaves, and therefore can be seen on bone deposited in wooden coffin environments or on terrestrial surfaces. The present study hypothesized that the degree of staining observed on skeletal elements would increase as the length of exposure to the organic matter increased and that different plant materials, and environments, would leave different patterns or colorations of staining. The skeletal elements consisted of 150 commercially available pig (Sus scrofa) femora that had the epiphyses removed and were completely defleshed without utilizing chemicals or boiling. The sample was divided into three groups with differing conditions and/or types of organic material introduced. Some were buried in a marshy environment within wooden boxes constructed of ten wood types commonly utilized in coffin construction throughout U.S. history: hickory (Carya sp.), walnut (Juglans sp.), cherry (Prunus sp.), soft maple (Acer sp.), mahogany (Swietenia sp.), yellow pine (Pinus sp.), poplar (Populus sp.), cedar (Cedrus sp.), oak (Quercus sp.), and spruce (Picea sp.). Additional femora were deposited in plastic containers lined with the same wood types as above and filled with tap water. Five control bones were deposited in a container with tap water and five additional bones were placed in a container with commercial tannic acid. The final group of femora was deposited on the ground surface surrounded by four types of dead vegetation: evergreen pine needles (Pinus strobus), northern red oak leaves (Quercus rubra), sugar maple leaves (Acer saccharum), and acorns (Quercus rubra) collected from the Boston area. The bones were removed once a month from their experimental environments and left overnight to dry. The level of staining that manifested on the osseous material was recorded qualitatively using the Munsell Soil Color Chart under a consistent indoor 40- watt daylight light bulb. The staining was recorded after two months upon initiation of the study and every following month until the study’s completion. After the color staining was recorded, the bones were returned to their experimental environments until the next interval of data collection. An additional sample of 15 bones, which were previously buried with direct soil contact, was also analyzed. These bones were either buried within the O, A, or C soil horizons for an interval of 1, 2, or 3 years prior to analysis. They were photographed and the staining was classified on one occasion after which the bones were permanently withdrawn and not returned to the experimental environment. In all of the experimental environments, staining was present after two months of exposure, and the color darkened across the bone surface with each episode of data collection. Both groups exposed to the wood types displayed staining across the entire bone surface with a few major colors on the bone shaft, while minor colors were only expressed along the margins or as small patches along the shaft. As the buried boxes began to break down, which is commonly observed in coffin burials, soil was able to infiltrate the boxes and contact the bones. This process resulted in multiple shades of brown to be present in the staining across bones in multiple wood types. The bones in the plastic containers with wood exhibited a larger variation in color staining likely due to a higher concentration of tannins restricted to a smaller area around the bones combined with a lack of water inflow. The staining ranged from red for bones with mahogany to brown for bones with cedar to even dark gray or black on bones with walnut and tannic acid, respectively. The bones in plant matter differed in that the organic staining was sporadic, often with large areas of very pale brown or yellowish brown coloration and with smaller patches of shades of darker brown. The staining present on the buried soil bones was intermediate to the other samples, in that it was diffuse across the shaft with a large range of colorations present. The results from the present study indicate that staining can manifest on bone within a relatively short time frame once skeletonization occurs and a variety of colorations or patterns of staining can manifest based on the plant material. The present research demonstrates the potential of organic staining to aid in estimations of the postmortem interval as well as an environmental reconstruction through species identification.