Taphonomic alteration to juvenile porcine bone after exposure to a marine environment

Abstract

Research into the effects of different aquatic environments, particularly marine environments, on the taphonomic alteration of the skeleton, is an under-studied area of forensic research. Taphonomic alteration is environment-driven and, as such, geographically specific research is critical to develop a clear understanding of the range and degree of diagenetic processes to bone when exposed to a marine environment. Bone density is probably the most well studied intrinsic factor to bone survivability and differs greatly pre- and post-puberty, with bone thickness also fluctuating during growth. Because of these age-related differences in bone, it can be expected that juvenile remains would be modified at a different rate, and perhaps in a different way, to mature adult bone. This work aimed to explore the impact of depositional marine environment on the taphonomic alteration to juvenile porcine skeletal remains. In particular, this research focused on the impact of time and seasonal variation based on initial exposure (summer and winter). Partial remains of 80 mixed breed white domestic piglets (Sus scrofa domesticus) were exposed to two marine environments: submerged and intertidal. Remains were exposed for 6 to 24 weeks, in the Otago Harbour, Dunedin, New Zealand. The experiment involved deposition in summer (January) and winter (July). Long bones (74 femora and 68 tibiae) were recovered from the two environments and analysed using three different analytical techniques: morphological assessment, Fourier transform (FT)-Raman spectroscopy, and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS). Exposed bones were also compared to 14 non-exposed controls (7 femora and 7 tibiae) macerated in laboratory conditions. The presence of algae on newly recovered intertidal bones and possible iron sulphide (FeS2) and hematite (Fe2O3) discolouration on submerged bones were characteristics that differentiated the two depositional environments. Permanent staining of various colours persisted on bones once dried from all submerged and intertidal samples after at least 12 weeks exposure. Whereas the pattern of increased scavenging and general bioerosion over time was similar between samples from the submerged and intertidal zones and matched the patterns of algal growth during the summer and winter experiments. Using principal component analysis (PCA), sample-specific spectral indicators could be identified within the FT-Raman spectra of exposed bone samples. Intertidal samples had greater spectral variation than submerged and control samples and were generally defined by the presence of carotenoid peaks combined with amide degradation and a broadening of the phosphate peak, associated with increased carbonate substitution in the bioapatite. Quadratic discriminant function analysis (QDA) of an independent test set achieved 84% overall correct classification for environment, and 42% total correct classification for environment and length of exposure. Importantly, using FT-Raman spectroscopy, exposed and non-exposed bones were not misclassified as each other. Examination of the chemical composition of bone through SEM-EDS confirmed the presence of key marine sediment elements such as Al, Si, and Fe. The presence of these, or other, unique depositional elements may be of particular use in confirming the depositional environment if the sedimentology of the local marine environment is known. However, SEM-EDS offered little evidence for systematic depositional context-related diagenetic change over time. While many aspects of marine taphonomy remain unknown, this research has shown that there is a need for more environment- and age-specific research into taphonomic alteration. In addition, this research has demonstrated the potential of three different methods that could be further developed to improve early exposure interval estimates. FT-Raman spectroscopy combined with PCA has shown its ability as a tool for defining taphonomic alteration in bones exposed to a marine environment and warrants further research.