Palynology - a microscopic view
into the past.
(Underlined words are defined in the Glossary)
Palynology is the study of non-mineralized 'organic' microfossils such as pollens and spores. All microfossils are of course, of organic origin, but 'organic' here refers to composition, not to origin.
In palynology techniques the crushed rock is digested in concentrated hydrofluoric acid, perhaps with some use of concentrated nitric acid, to eliminate all of the mineral content of the sediment. The residue consists of microfossils and nannofossils e.g. spores, pollens and other remains of higher plants, dinoflagellates, as well as amorphous organic matter (kerogen).
This drastic processing of the original rock requires a well equipped laboratory with an efficient fume-cupboard for safe working. The residues are carefully rinsed with pure water, dried and mounted as a strew in Canada Balsam under a glass cover-slip. (See safety footnote).
To illustrate how palynology can provide an insight into the plants, climate and habitats existing in prehistoric times, samples from the Dorset coast sediments, UK are presented. This work extends previous work on the Mesozoic rock samples from the same area, using cellulose lacquer peels.
Further to the 'peel' work, it was decided in the early 1990's to seek the help of Dr David J. Batten of the Institute of Earth Studies, University of Wales. Dr Batten, who was very experienced in palynology techniques, had offered very kindly to examine rock samples provided by the present writer, by preparing mounted slides of strews of the organic residues. I would examine these in detail with the Nikon microscope, both visually and photographically. Figures (1) to (8) below illustrate some of the results obtained.
Calcareous nannofossils (coccoliths) occur in vast numbers in the various Kimmeridge Clay sediments found on the Dorset coast, and these are completely dissolved in the concentrated acids used; but transparent moulds of these can be seen in the kerogen. Examination of Figures (1) to (8), and comparisons with corresponding cellulose lacquer peel images, indicate that the peels like 'thin sections', give a more complete picture of the microscopic character of these sediments; however palynological techniques can lead to a more detailed study of individual species.
One type of residue shown in the images below are 'bordered pits'. These are a specific remnant of woody plants. Like pollen and spores these can often be identified to species to show the type of plants living in prehistoric times. As these are lignified (i.e. 'woody') they resist chemical treatments in palynology and are found in the residues. Bordered pits are actually the non-perforated pores in the end walls of the long narrow cells making up plant xylem (the vessels that conduct water).
A distinct magnified Mesozoic conifer wood fragment showing a definite bordered pit structure with 15 pits in a single row. Diameters of the bordered pits are about l6.3 mm. Phase contrast, objective 40X. Probably Protocupressinoxylon purbeckensis (Francis 1983).
A magnified Araucariod type Mesozoic conifer wood fragment, showing its distinct bordered pit structure. The picture shows some processed kerogen with coccolith moulds. Diameters of the bordered pits are about 15 mm. Phase contrast, objective 40X.
Highly magnified Araucariod type bordered pit structure. Diameters of the bordered pits are about 17 to 18 mm. Phase contrast, objective 100X.
A highly magnified partly exposed dinoflagellate cyst with fusain. Diameter of the dinoflagellate cyst is about 100 mm. Brightfield illumination, objective 100X.
Coccolith moulds in kerogen treated with concentrated acid. Diameters of coccolith moulds range from ca. 4.0 to 6.0 mm. Brightfield illumination, objective 100X.
An isolated dinoflagellate cyst showing some faint surface coccolith moulds. Diameter of the dinoflagellate cyst is ca. 65 mm. Brightfield illumination, objective 100X.
A cluster of Classopolis conifer pollen grains, seen at high magnification. Average diameter of the pollen grains estimated as about 37 mm. Brightfield illumination, objective 100X.
Comparisons between the above palynological studies and the results from cellulose lacquer rock peels, confirm the mixed origins for microfossils, nannofossils and plant fragments, found in the Kimmeridge Clay and Lower Purbeck beds. These origins point to both marine and terrestial sources. For example the presence of Mesozoic conifer wood fragments mixed with numerous coccolith nannofossils as well as Mesozoic conifer pollen and dinoflagellate cysts confirm the mixed nature of the kerogen.
The evolution and expansion of vast Mesozoic conifer forests along the separating coasts of Europe and North America during the Jurassic Period, with periodic changes in sea levels, marine incursions and retreats, would help to explain the mixed origin of microfossils, nannofossils and plant fragments. These microscopic objects reported from the cellulose lacquer peels, and confirmed here by palynological techniques, point to the mixed coastal swampy conifer forest and marine environments of the Kimmeridge Clay and lagoon environments of the Lower Purbeck.
The dramatic emergence of the dinosaur "ruling reptiles" seems to have been related to these ancient Mesozoic conifer forests. The extremely large herbivore reptiles are believed to have been "tree browsers" (cf. modern elephants) with specially adapted large body structure, movement, teeth and digestive systems. Likewise tree attacking insects, related to modern "cockroaches", were dominant in Jurassic times. Nectar consuming insects evolved in the late Cretaceous Period because of the development of flowering plants. The dominant status of birds and mammals would only arise after the mass extinction of the dinosaurs and related reptiles, at the close of the Cretaceous Period, about 65 million years B.P. By this time the Atlantic Ocean had developed substantially into a large area of open sea, which continued to spread during the Tertiary Period and into recent geological times, by virtue of tectonic plate movement, i.e. ocean floor spreading on either side of the central Atlantic Ridge.
Very special thanks are due to Dr David Batten of the Institute of Earth Studies, the University of Wales, for his help in preparing the above palynological microscope slides. All the above images shown were taken from these slides.
Images taken using a Nikon SKt microscope.
Objectives: 10X - N.A. 0.25; 40X - N.A. 0.65; 100X oil immersion - NA 1.25.
Slides 1, 2, 4, 5, 7, 8
From the Jurassic, Lower Purbeck, Cypris Freestones, Micrite with wood fragments. Map Reference SY.972.785. Swanworth Quarry, West of Swanage, Dorset coast.
From the Jurassic, Kimmeridge Clay, Maple Ledge Shales. Map Reference SY.908.790. East of Gaulter Gap, Kimmeridge Bay, Dorset coast.
From the Jurassic, Kimmeridge Clay, Freshwater Steps Stone Band. Map Reference SY.943.772. Freshwater Steps, West of Hounstout Cliff, Dorset coast.
Araucariod : a member of the family of conifers Araucariaceae (or genus Araucaria). The monkey puzzle tree is a present day member.
Calcareous nannofossils : Nannofossils largely composed of calcium carbonate. Many forms belong to the Coccolithophyceae.
Classopollis : an extinct genus of conifer of the Mesozoic era with distinct pollen grains.
Coccoliths : minute calcium carbonate platelets secreted by coccolithophores which bear them as surface plates.
Coccolithophore : a unicellular planktonic organism of uncertain type (protozoan or algae?). Currently assigned the phylum Haptomonada.
Dinoflagellate : a microscopic single-celled organism having two dissimilar flagellae. A major component of marine phytoplankton.
Fusain : carbonaceous material derived from decaying vegetation or wood.
Kerogen : A solid complex organic material which yields petroleum type hydrocarbons under heat and pressure.
Microfossil : a fossil or fossil fragment that can only be seen with a microscope.
Nannofossil : fossils of minute planktonic organisms, especially calcareous unicellular algae.
Period: Jurassic - ca. 213 - 144 million years ago; Cretaceous - ca. 144 - 65 million years ago.
Editor's note: Some of the quality of the author's original 35mm slides is lost in the scanned and compressed web images. Comments to the author are welcomed, who can be contacted directly at the address below or comments can be passed on via the Micscape Editor, see magazine index for contact details.
Keith Abineri. 42 West Borough, Wimborne, Dorset BH21 1NQ, UK. Tel. 01202 885547
Safety footnote: as the author stresses, palynology uses very hazardous materials; the techniques should only be carried out by very experienced workers in the correct laboratory facilities and using appropriate protective equipment.
Prepared for the Web by David Walker.
Published in the November 1999 edition of Micscape Magazine.
Article at http://www.microscopy-uk.net/mag/artnov99/kamast6.html
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