palynology

Pine Pollen under the microscope

A late Silurian sporangium bearing trilete spores. Such spores are the earliest evidence of life on land.^[1] Green: A spore tetrad. Blue: A spore bearing a trilete mark – the Y-shaped scar. The spores are about 30–35 μm across.

Palynology is the science that studies contemporary and fossil palynomorphs, including pollen, spores, orbicules, dinoflagellate cysts, acritarchs, chitinozoans and scolecodonts, together with particulate organic matter (POM) and kerogen found in sedimentary rocks and sediments. Palynology does not include diatoms, foraminiferans or other organisms with silicaceous or calcareous exoskeletons.

Palynology is an interdisciplinary science and is a branch of earth science (geology or geological science) and biological science (biology), particularly plant science (botany). Stratigraphical palynology is a branch of micropalaeontology and paleobotany which studies fossil palynomorphs from the Precambrian to the Holocene.

A history of palynology

Early history

The earliest reported observations of pollen under a microscope are likely to have been in the 1640s by the English botanist Nehemiah Grew^[2] who described pollen, the stamen and successfully predicted that pollen was required for successful reproduction in plants. As microscopes began to improve further studies included work by Robert Kidston and P. Reinsch examined the presence of spores in coal and compared them to modern spores^[3]. The early pioneers also included Christian Gottfried Ehrenberg (radiolarians and diatoms), Gideon Mantell (desmids) and Henry Hopley White (dinoflagellates).

Modern palynology

The earliest quantitative analysis of pollen was published by Lennart von Post who laid out the foundations of modern pollen analysis in his Kristiania lecture of 1916.^[4] Pollen analysis was initially confined to Nordic countries because many early publications were in Nordic languages.^[5] This isolation ended with the publication of Gunnar Erdtman's thesis of 1921 when pollen analysis became widespread throughout Europe and North America for use in studies of Quaternary vegetation and climate change^[4].

The term palynology was introduced by Hyde and Williams in 1944, following correspondence with the Swedish geologist Antevs, in the pages of the Pollen Analysis Circular (one of the first journals devoted to pollen analysis, produced by Paul Sears in North America). Hyde and Williams chose palynology on the basis of the Greek words paluno meaning 'to sprinkle' and pale meaning 'dust' (and thus similar to the Latin word pollen).^[6]

Methods of study

Palynomorphs are broadly defined as organic-walled microfossils between 5 and 500 micrometres in size. They are extracted from rocks and sediment cores both physically, by wet sieving, often after ultrasonic treatment, and chemically, by using chemical digestion to remove the non-organic fraction.

Chemical preparation

Chemical digestion follows a number of steps. Initially the only chemical treatment used by researchers was treatment with KOH to remove humic substances; defloculation was accomplished through surface treatment or ultra-sonic treatment, although sonification may cause the pollen exine to rupture.^[5] The use of hydrofluoric acid (HF) to digest silicate minerals was introduced by Assarson and Granlund in 1924, greatly reducing the amount of time required to scan slides for palynomorphs.^[7] Palynological studies using peats presented a particular challenge because of the presence of well preserved organic material including fine rootlets, moss leaflets and organic litter. This was the last major challenge in the chemical preparation of materials for palynological study. Acetolysis was developed by Gunnar Erdtman and his brother to remove these fine cellulose materials by dissolving them.^[8]. In acetolysis the material is treated with acetic anhydride and sulfuric acid, dissolving cellulistic materials and providing better visibility for palynomorphs.

Some steps of the chemical treatments require special care for safety reason, in particular the use of HF which diffuses very fast through the skin and could cause severe chemical burns.

Other treatment include kerosene flotation for chitinous materials.

Analysis

Once samples have been prepared chemically, samples are mounted on microscope slides using silicon oil, glycerol or glycerol-jelly and examined using light microscopy or scanning electron microscopy.

Researchers will often study either modern samples from a number of unique sites within a given area, or samples from a single site with a record through time, such as samples obtained from peat or lake sediments. More recent studies have used the modern analog technique in which paleo-samples are compared to modern samples for which the parent vegetation is known^[9]

When the slides are observed under a microscope the researcher will count the number of grains from each pollen taxon. This record is then used to produce a pollen diagram. This data can be used to detect anthropogenic effects such as logging^[10], traditional patterns of land use^[11] or long term changes in regional climate^[12]

Palynology can be applied to problems in many fields including geology, botany, paleontology, archaeology, pedology (soil study), and geography.

Applications

Palynology is used for a diverse range of applications, related to many scientific disciplines:

• Biostratigraphy and geochronology. Geologists use palynological studies in biostratigraphy to correlate strata and determine the relative age of a given bed, horizon, formation or stratigraphical sequence.
• Palaeoecology and climate change. Palynology can be used to reconstruct past vegetation (land plants) and marine and freshwater phytoplankton communities, and so infer past environmental (palaeoenvironmental) and palaeoclimatic conditions.
• Organic palynofacies studies, which examine the preservation of the particulate organic matter and palynomorphs provides information on the depositional environment of sediments and depositional palaeoenvironments of sedimentary rocks.
• Geothermal alteration studies examine the colour of palynomorphs extracted from rocks to give the thermal alteration and maturation of sedimentary sequences, which provides estimates of maximum palaeotemperatures.
• Limnology studies. Freshwater palynomorphs and animal and plant fragments, including the prasinophytes and desmids (green algae) can be used to study past lake levels and long term climate change.
• Taxonomy and evolutionary studies.
• Forensic palynology — the study of pollen and other palynomorphs for evidence at a crime scene.
• Allergy studies. Studies of the geographic distribution and seasonal production of pollen, can help sufferers of allergies such as hay fever.
• Melissopalynology — the study of pollen and spores found in honey.
• Archaeological Palynology examines human uses of plants in the past. This can help determine seasonality of site occupation, presence or absence of agricultural practices or products and plant-related activity areas within an archaeological context. Bonfire Shelter is one such example of this application.

Because the distribution of acritarchs, chitinozoans, dinoflagellate cysts, pollen and spores provides evidence of stratigraphical correlation through biostratigraphy and palaeoenvironmental reconstruction, one common and lucrative application of palynology is in oil and gas exploration.

Palynology also allows scientists to infer the climatic conditions from the vegetation present in an area thousands or millions of years ago. This is a fundamental part of research into climate change.

References

• Moore, P.D., et al. (1991), Pollen Analysis (Second Edition). Blackwell Scientific Publications. ISBN 0-632-02176-4
• Traverse, A. (1988), Paleopalynology. Unwin Hyman ISBN 0-04-561001-0
• Roberts, N. (1998), The Holocene an environmental history, Blackwell Publishing. ISBN 0-631-18638-7

See also

• Aperture (botany)

External links

• International Federation of Palynological Societies
• Palynology Laboratory, French Institute of Pondicherry, India
• The Palynology Unit, Kew Gardens, UK
• PalDat, palynological database hosted by the University of Vienna, Austria
• The Micropalaeontological Society
• The American Association of Stratigraphic Palynologists (AASP)
• Commission Internationale de Microflore du Paléozoique (CIMP), international commission for Palaeozoic palynology.
• CIMP Subcommission on Acritarchs
• CIMP Chitinozoan Subcommission
• Linnean Society Palynology Specialist Group (LSPSG)
• Canadian Association of Palynologists
• Pollen and Spore Identification Literature