Pollen Deterioration in a Tropical Surface Soil and its Impact on Forensic Palynology

Abstract

Although pollen evidence can remain at a crime scene for many years, some pollen species are more susceptible to decay than others. Mature pollen grains of 9 pollen species were placed in nylon gauze bags and buried in 3 types of tropical soil, leaf mold, swamp clay, and sandy soil, in June 2007. Degradation rates and effects of some environmental factors on pollen deterioration were then determined. Observations were made after 2, 4, and 6 months of burial, and the pollen classified into 3 types: identifiable, non-identifiable to species, and absent in the case of complete destruction. Results showed that pollen from different plant species had different rates of deterioration but in all cases pollen deteriorated more slowly in sandy soil. Soil Eh and pH, and infection by fungi seemed to have little influence on pollen preservation. However, pollen preservation of some pollen species was affected by time. It is apparent that complex destructive factors play different roles in different depositional settings. This allowed each area to yield a unique pollen composition. From a forensic point of view, this specific composition becomes part of the evidence that may link a suspect to the precise crime scene.

Pollen degradation in mangrove sediments: A short-term experiment

“…In the present study, in most cases, more than half of the palynomorph percentages and concentrations were still observed even after 6 months of burial. This result differs from a previous experimental study in other tropical environments where the pollen disappeared within 2 months of deposition (Phuphumirat et al, 2009). The waterlogging and inundation in mangroves produce a dysaerobic environment, thereby retarding the oxidative degradation of the pollen.…”

“…Although rapid palynomorph destruction occurs in tropical surface sediments due to high rates of energy flow speeding the decomposition of sedimentary organic matter (Montagnini and Jordan, 2005;Phuphumirat et al, 2009), spore and pollen deterioration seem to proceed more slowly in mangroves. In the present study, in most cases, more than half of the palynomorph percentages and concentrations were still observed even after 6 months of burial.…”

“…Previous studies showed that better pollen preservation was found in saline sediments as pollen grains encapsuled in salt crystals are more resistant to mechanical damage (Campbell and Campbell, 1994;Phuphumirat et al, 2009). It is also possible that the salt will cause dehydration, which will initiate pollen shrinkage and also prevent further fermentation and decomposition.…”

“…However, such palynological data should be used with caution since the tropical zone provides the impetus for rapid pollen destruction (Montagnini and Jordan, 2005). In a tropical surface soil, more than half of the buried pollen was degraded after only 2 months of burial (Phuphumirat et al, 2009). Thus, pollen preservation is an important taphonomic factor that must be taken into account when interpreting the palynological record.…”

“…In such a reducing environment (low Eh value) where only a small amount of oxygen is present and hydrogen sulphide and carbon dioxide are produced by microbial organisms as by-products of their respirations, leading to a decrease in soil pH, complete oxidation of pollen is restricted (Tschudy, 1969;Bryant et al, 1994). As pointed out by many investigators, sediment with low Eh and pH values appears to be ideal for pollen preservation (Dimbleby, 1957;Tschudy, 1969;Bryant et al, 1994;Phuphumirat et al, 2009). In addition, high salinity of sediment also provides a suitable habitat for pollen to be preserved (Campbell and Campbell, 1994).…”