A neotaphonomic experiment in pollen oxidation and its implications for archaeopalynology

Abstract

Oxidation is considered to be a major taphonomic process involved in differential pollen preservation. An experimental protocol has been developed in the laboratory to test the impact of oxidation on pollen concentrations. Seven modern pollen taxa were selected among common trees, shrubs and herbaceous plants from the European flora, with regard to their typical pollen morphology ( Pinus, Alnus, deciduous Quercus, Cichorioideae, Caryophyllaceae, Ericaceae and Poaceae). An aqueous solution of potassium permanganate (KMnO 4) was used to oxidise the pollen grains. The impact of oxidation was measured as a function of increasing oxidation intensity and increasing exposure time. For each taxon, the pollen concentration (grains µl − 1) was measured following each oxidation stage. The results show that all pollen taxa are affected by oxidation. Both low oxidant concentrations and short oxidation exposure times led to a fast decrease in pollen concentrations. The thickness of the exine and a specific morphology appear to be the deciding factors controlling the ability to withstand oxidation. A simulation of the impact of oxidation was performed on two model pollen spectra simulating a forested and an open environment. Significant differences were recorded between the initial and final pollen rates. The AP/NAP ratio is modified to a greater extent in the spectrum associated with the open environment. However, over-representation of a specific taxon in a pollen spectrum cannot be attributed to the impact of oxidation alone. For instance, when Cichorioideae are recorded with high frequencies in pollen analyses carried out in an archaeological context, oxidation should not be regarded as the only taphonomic process responsible for this over-representation. The differential conservation of pollen is a complex process in which oxidation represents but one of a variety of interacting taphonomic factors (such as pollen transport, bacterial activity, temperature, pH, hygrometry).