Palynology

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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 "study of dust" (from Greek παλύνω - palunō, "strew, sprinkle"[2] and -logy) or "particles that are strewn". A classic palynologist analyses particulate samples collected from the air, water, or from deposits including sediments of any age. The condition and identification of those particles, organic and inorganic, give the palynologist clues to the life, the environment, and energetic conditions that produced them.

The term is sometimes narrowly used to refer to a subset of the discipline, which is defined as "the study of microscopic objects of macromolecular organic composition (i.e. compounds of carbon, hydrogen, nitrogen and oxygen), not capable of dissolution in hydrochloric or hydrofluoric acids."[3] It is the science that studies contemporary and fossil palynomorphs, including pollen, spores, orbicules, dinocysts, 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 siliceous 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[4] who described pollen, the stamen, and correctly predicted that pollen is required for sexual reproduction in flowering plants. As microscopes began to improve, further studies included work by Robert Kidston and P. Reinsch who examined the presence of spores in coal and compared them to modern spores.[5] The early pioneers also included Christian Gottfried Ehrenberg (radiolarians, diatoms and dinoflagellate cysts), Gideon Mantell (desmids) and Henry Hopley White (dinoflagellate cysts).

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.[6] Pollen analysis was initially confined to Nordic countries because many early publications were in Nordic languages.[7] 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.[6]

Modern pollen analysis dates back to Lennart von Post when he presented a paper on fossil pollen grains in Swedish bogs to the 1916 Scandinavian Scientist Conference in Oslo.[8] The paper was repeated in the same year in Stockholm but was not fully published until 1918. Antecedents of this work can be traced to the writings of scientists such as Früh (1885),[9] who enumerated most of the common tree pollen types, together with a considerable number of spores and herb pollen grains. In a study of bottom samples from Swedish lakes by Trybom (1888),[10] Pinus (Pine) and Picea (Spruce) pollen was found in such profusion that he considered them to be serviceable as "index fossils". Georg F.L. Sarauw studied fossil pollen of middle Pleistocene age (Cromerian) from the harbour of Copenhagen.[11] Lagerheim (in Witte 1905) and C.A.Weber (in H.A.Weber 1918) appear to be among the first to undertake 'percentage frequency' calculations.

Pollen analysis was refined and developed by Johannes Iversen and Knut Fægri in their now classical textbook on the subject.[12]

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).[13]

Methods of study

Palynomorphs

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.

A palynomorph (from Greek, meaning "strewn or sprinkled forms") is a particle of a size between five and 500 micrometres, found in rock deposits (sedimentary rocks) and composed of organic material such as chitin, pseudochitin and sporopollenin. Described palynomorphs are sometimes referred to as palynotaxa.

Palynomorphs form a geological record of importance in determining the type of prehistoric life that existed at the time the sedimentary formation was laid down. As a result, these microfossils give important clues to the prevailing climatic conditions of the time. Their paleontological utility derives from an abundance numbering in millions of cells per gram in organic marine deposits, even when such deposits are generally not fossiliferous. Palynomorphs, however, generally have been destroyed in metamorphic or recrystallized rocks.

Typically, palynomorphs are dinoflagellate cysts, acritarchs, spores, pollen, fungi, scolecodonts (scleroprotein teeth, jaws and associated features of polychaete annelid worms), arthropod organs (such as insect-mouth parts), chitinozoans and microforams. Palynomorph microscopic structures that are abundant in most sediments are resistant to routine pollen extraction including strong acids and bases, and acetolysis, or density separation.

Chemical preparation

Chemical digestion follows a number of steps.[14] 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.[7] 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.[15] 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.[16] In acetolysis the specimen is treated with acetic anhydride and sulfuric acid, dissolving cellulistic materials and thus providing better visibility for palynomorphs.

Some steps of the chemical treatments require special care for safety reasons, in particular the use of HF which diffuses very fast through the skin and, causes severe chemical burns, and can be fatal.[17]

Another treatment includes kerosene flotation for chitinous materials.

Analysis

Once samples have been prepared chemically, they are mounted on microscope slides using silicon oil, glycerol or glycerol-jelly and examined using light microscopy or mounted on a stub for 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[18]

When the slides are observed under a microscope, the researcher counts the number of grains of each pollen taxon. This record is next used to produce a pollen diagram. These data can be used to detect anthropogenic effects, such as logging,[19] traditional patterns of land use[20] or long term changes in regional climate[21]

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

See also

References

  1. ↑ Gray, J.; Chaloner, W. G.; Westoll, T. S. (1985). "The Microfossil Record of Early Land Plants: Advances in Understanding of Early Terrestrialization, 1970–1984". Philosophical Transactions of the Royal Society B 309 (1138): 167–195. Bibcode:1985RSPTB.309..167G. doi:10.1098/rstb.1985.0077. JSTOR 2396358. 
  2. ↑ παλύνω, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
  3. ↑ W.A.S. Sarjeant, 2002. 'As chimney-sweeps, come to dust': a history of palynology to 1970. pp. 273-327 In: Oldroyd, D. R. The earth inside and out: some major contributions to geology in the twentieth century. Geological Society (London) Special Publication no. 192.
  4. ↑ Bradbury, S (1967). The Evolution of the Microscope. New York: Pergamon Press. pp. 375 p. 
  5. ↑ Jansonius, J; D.C. McGregor (1996). "Introduction, Palynology: Principles and Applications". AASP Foundation 1: 1–10. 
  6. ↑ 6.0 6.1 Fægri, Knut; Johs. Iversen (1964). Textbook of Pollen Analysis. Oxford: Blackwell Scientific Publications. 
  7. ↑ 7.0 7.1 Faegri, Knut (1973). "In memoriam O. Gunnar E. Erdtman". Pollen et Spores 15: 5–12. 
  8. ↑ von Post, L (1918) "Skogsträdpollen i sydsvenska torvmosslagerföljder", Forhandlinger ved de Skandinaviske naturforskeres 16. møte i Kristiania 1916: p. 433
  9. ↑ Früh, J (1885) "Kritische Beiträge zur Kenntnis des Torfes", Jahrb.k.k.Geol.Reichsanstalt 35
  10. ↑ Trybom, F (1888) "Bottenprof fran svenska insjöar", Geol.Foren.Forhandl.10
  11. ↑ Sarauw, G. F. L. (1897). "Cromer-skovlaget i Frihavnen og trælevningerne i de ravførende sandlag ved København" [The Cromer Forest layer in the Free Harbour and Wood Remains in the Amber containing strata near Copenhagen]. Meddelelser fra Dansk Geologisk Forening / Bulletin of the Geological Society of Denmark (in Danish) 1 (4): 17–44. 
  12. ↑ Fægri, K. & Iversen, J. (1989) Textbook of pollen analysis. 4th ed. John Wiley & Sons, Chichester. 328 p.
  13. ↑ Hyde, H.A.; D.A. Williams (1944). "The Right Word". Pollen Analysis Circular 8: 6. 
  14. ↑ Bennett, K.D.; Willis, K.J. (2001). "Pollen". In Smol, John P.; Birks, H. John B.; Last, William M. Tracking Environmental Change Using Lake Sediments. Volume 3: Terrestrial, algal, and siliceous indicators. Dordrecht: Kluwer Academic Publishers. pp. 5–32. 
  15. ↑ Assarson, G. och E.; Granlund, E. (1924). "En metod for pollenanalys av minerogena jordarter". Geol. Foren. Stockh. Forh. 46: 76–82. doi:10.1080/11035892409444879. 
  16. ↑ Erdtman, O.G.E. "Uber die Verwendung von Essigsaureanhydrid bei Pollenuntersuchungen". Sven. Bot. Tidskr. 28: 354–358. 
  17. ↑ "Hydrofluoric acid fatality in Perth - hazard alert". 1995-03-06. Retrieved 2011-12-18. 
  18. ↑ Overpeck, J.T.; T. Webb, I.C. Prentice (1985). "Quantitative interpretation of fossil pollen spectra: Dissimilarity coefficients and the method of modern analogs". Quaternary Research 23: 87–108. Bibcode:1985QuRes..23...87O. doi:10.1016/0033-5894(85)90074-2. 
  19. ↑ Niklasson, Mats; Matts Lindbladh, Leif Björkman (2002). "A long-term record of Quercus decline, logging and fires in a southern Swedish Fagus-Picea forest". Journal of Vegetation Science 13 (6): 765–774. doi:10.1658/1100-9233(2002)013[0765:ALROQD]2.0.CO;2. 
  20. ↑ Hebda, R.J.; R.W. Mathewes (1984). "Holocene history of cedar and native cultures on the North American Pacific Coast". Science 225 (4663): 711–713. Bibcode:1984Sci...225..711H. doi:10.1126/science.225.4663.711. PMID 17810290. 
  21. ↑ Heusser, Calvin J.; L.E. Heusser, D.M. Peteet (1985). "Late-Quaternary climatic change on the American North Pacific coast". Nature 315 (6019): 485–487. Bibcode:1985Natur.315..485H. doi:10.1038/315485a0. 
  • 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

External links

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