The thermal evolution of sporopollenin

Abstract

Micro Fourier-Transform Infrared (FT–IR) spectroscopy in combination with transmitted and reflected light microspectrophotometry relates the chemical and physical properties of sporopollenin during thermal maturation; the physical properties measured being colour, as the chromaticity coordinates a*, b* and L*, (luminance) and the reflectance (Rsp) of the sporinite wall layers in the polished section. During maturation, sporopollenin exhibits a wide range of colours before there are any significant changes in Rsp. The immature phase is characterised by subtle colour changes through a series of progressively darkening yellows. This coincides with a reduction in the relative proportion of >CO groups and an increase in the relative proportion of aliphatic –CH2 and –CH3 groups. During the mature phase, functional groups within spores and pollen are thermally cracked to generate hydrocarbons. Their colours change rapidly through orange and brown and the FT–IR data indicate the loss of a considerable portion of the aliphatic groups and increases in the CC content associated with aromatic rings. Significant structural reorganisation during the spore ‘oil-window’ results in the formation of isolated aromatic rings. A further increase in maturity yields little change in colour but a rapid increase in reflectivity. This is caused by the formation of multi-ring aromatic units from isolated aromatic units. The size of these polyaromatic units increases with rank. Investigation of artificially matured samples of Lycopodium clavatum spores indicates considerable chemical differences in >CO, CC and aromatic skeletal structure, in comparison to fossil palynomorphs, although they progress through a similar series of colours. Only the behaviour of the aliphatic CH2, CH3 groups, in artificially heated samples replicates that seen in samples matured naturally, under geological conditions.