An effective method for the observation and documentation of highly mature palynomorphs using reflected light microscopy

Abstract

Palynomorphs undergo progressive darkening and loss of translucency as temperature increases over geological time periods. The use of oxidising agents that lighten highly mature palynomorphs enables their observation and documentation using transmitted light microscopes, but in certain cases, oxidation destroys part of or the entire organic residue. A new method was tested using palynological residues derived from Devonian and Carboniferous metasediments that allows the microscopic observation and documentation of highly mature palynomorphs without loss of specimens. In its simplest form, it uses non-oxidized, air-dried 20-μm-sieved palynological residues mounted on acrylic slides without the use of cover slips. Observation and documentation were performed routinely using reflected light petrographic microscopes. The resulting photomicrographs are scanning electron micrograph (SEM)-like in the sense that the observed objects are opaque. Depth-of-field issues can be easily overcome by editing the partially focussed images with z-stack image software. The method was found to be useful up to estimated peak rock temperatures of 290°C and over a wide range of tectonic deformation degrees. The application of this method may allow the biostratigraphic and paleaeoenvironmental study of metasediments in areas where this kind of information was assumed to be unattainable.

Keywords:

• palynomorphs
• metasediments
• reflect light microscopy
• vitrinite reflectance

Acknowledgements

Graham Booth is acknowledged for a critical review of the work prior to submission. Philippe Steemans and John Marshall are acknowledged for their reviews.

Additional information

Funding

This work was supported by the Fundação para a Ciência e Tecnologia (Portugal) under PhD Grant SFRH/BD/23787/2005.