Palynology

Palynomorphs are broadly defined as organic-walled microfossils between 5 and 500 micrometres in size. They are extracted from sedimentary rocks and sediment cores both physically, by ultrasonic treatment and wet sieving, and chemically, by chemical digestion to remove the non-organic fraction. Palynomorphs may be composed of organic material such as chitin, pseudochitin and sporopollenin. Palynomorphs that have a taxonomy description 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 mouthparts), 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.

A palynofacies is the complete assemblage of organic matter and palynomorphs in a fossil deposit. The term was introduced by the French geologist André Combaz in 1964. Palynofacies studies are often linked to investigations of the organic geochemistry of sedimentary rocks. The study of the palynofacies of a sedimentary depositional environment can be used to learn about the depositional palaeoenvironments of sedimentary rocks in exploration geology, often in conjunction with palynological analysis and vitrinite reflectance.[4][5][6]

Palynofacies can be used in two ways:

• Organic palynofacies considers all the acid insoluble particulate organic matter (POM), including kerogen and palynomorphs in sediments and palynological preparations of sedimentary rocks. The sieved or unsieved preparations may be examined using strew mounts on microscope slides that may be examined using a transmitted light biological microscope or ultraviolet (UV) fluorescence microscope. The abundance, composition and preservation of the various components, together with the thermal alteration of the organic matter is considered.
• Palynomorph palynofacies considers the abundance, composition and diversity of palynomorphs in a sieved palynological preparation of sediments or palynological preparation of sedimentary rocks. The ratio of marine fossil phytoplankton (acritarchs and dinoflagellate cysts), together with chitinozoans, to terrestrial palynomorphs (pollen and spores) can be used to derive a terrestrial input index in marine sediments.

The earliest reported observations of pollen under a microscope are likely to have been in the 1640s by the English botanist Nehemiah Grew,[7] who described pollen and the stamen, and concluded that pollen is required for sexual reproduction in flowering plants.

By the late 1870s, as optical microscopes improved and the principles of stratigraphy were worked out, Robert Kidston and P. Reinsch were able to examine the presence of fossil spores in the Devonian and Carboniferous coal seams and make comparisons between the living spores and the ancient fossil spores.[8] Early investigators include Christian Gottfried Ehrenberg (radiolarians, diatoms and dinoflagellate cysts), Gideon Mantell (desmids) and Henry Hopley White (dinoflagellate cysts).

Quantitative analysis of pollen began with Lennart von Post's published work.[9] Although he published in the Swedish language, his methodology gained a wide audience through his lectures. In particular, his Kristiania lecture of 1916 was important in gaining a wider audience.[10] Because the early investigations were published in the Nordic languages (Scandinavian languages), the field of pollen analysis was confined to those countries.[11] The isolation ended with the German publication of Gunnar Erdtman's 1921 thesis. The methodology of pollen analysis became widespread throughout Europe and North America and revolutionized Quaternary vegetation and climate change research.[10][12]

Earlier pollen researchers include Früh (1885),[13] who enumerated many common tree pollen types, and a considerable number of spores and herb pollen grains. There is a study of pollen samples taken from sediments of Swedish lakes by Trybom (1888);[14] pine and 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.[15] Lagerheim (in Witte 1905) and C. A.Weber (in H. A. Weber 1918) appear to be among the first to undertake 'percentage frequency' calculations.

The term palynology was introduced by Hyde and Williams in 1944, following correspondence with the Swedish geologist Ernst 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).[16]

Pollen analysis in North America stemmed from Phyllis Draper, an MS student under Sears at the University of Oklahoma. During her time as a student, she developed the first pollen diagram from a sample that depicted the percentage of several species at different depths at Curtis Bog. This was the introduction of pollen analysis in North America;[17] pollen diagrams today still often remain in the same format with depth on the y-axis and abundances of species on the x-axis.

Pollen analysis advanced rapidly in this period due to advances in optics and computers. Much of the science was revised by Johannes Iversen and Knut Fægri in their textbook on the subject.[18]

Chemical digestion follows a number of steps.[19] Initially the only chemical treatment used by researchers was treatment with Potassium hydroxide (KOH) to remove humic substances; defloculation was accomplished through surface treatment or ultra-sonic treatment, although sonification may cause the pollen exine to rupture.[11] In 1924, the use of hydrofluoric acid (HF) to digest silicate minerals was introduced by Assarson and Granlund, greatly reducing the amount of time required to scan slides for palynomorphs.[20] 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.[21] 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.[22]

Another treatment includes kerosene flotation for chitinous materials.

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.[23]

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,[24] traditional patterns of land use[25] or long term changes in regional climate[26]