The forensic utility of reworked geological materials in soil

Abstract

Geological materials such as rock fragments, microfossils and mineral grains are continuously being entrained (i.e. reworked) into soil during natural weathering processes. Distinctive reworked rock types in soil, and specific components of them such as palynomorphs (organic microfossils), can prove extremely useful in forensic investigations, i.e. to connect (match) people to places. If the outcrop area of a unique rock is small and well mapped, it potentially has substantial evidentiary value in soil forensic studies. Furthermore, clay minerals, geochemical data and minerals may support the presence of a suspect at a crime scene. Modern pollen and spores extracted from soil samples in forensic investigations can be invaluable in linking suspects to crime scenes. This is because the majority of localities, especially those with natural vegetation, have characteristic (often unique) floral character. Reworked (i.e. largely pre-Quaternary) palynomorphs and other microscopic fossils may co-occur with the in situ (indigenous) pollen and spores. If these reworked forms have relatively short geological ranges, they can indicate the age of the bedrock, thereby further helping to place a person at a location. However, stratigraphically recycled palynomorphs in the soil can be somewhat rare and sporadic, and many rock units are entirely or virtually devoid of palynomorphs. Furthermore, glacial sediments such as till can provide highly mixed reworked palynomorph associations due to their typically heterogenous nature. These diverse assemblages are frequently highly distinctive hence can potentially provide very powerful forensic evidence. The potential of geological materials derived from bedrock in soil for forensic investigations is absolutely clear. Hence, the use of reworked microfossils, minerals and rock should be considered in any major crime where the evidence includes soil.