Notes on Palaeopalynology | Palynology

The below mentioned article provides a note on palaeopalynology.

Palaeopalynology (palaeo-/paleo- is the prefix used in a combining form to denote relation with fossils) is one of the disciplines of palynology and concerned with the study of fossil pollen grains and spores.

By extension, the study also includes chitinozoans (animal remains), fungal spores, dinoflagellates, acritarchs and other organisms (except diatom) that are preserved in sedimentary rocks, composed of sporopollenin and chitin etc., resistant to acids and the size being 5 µ to 500 µ.

Palaeopalynology has become an applicative discipline of palynology due to the following features possessed by pollen grains and spores:

1. Pollen and spores survive better and longer than other biological materials due to the presence of tough exine.

2. The exine is mainly composed of sporopollenin that renders the pollen grains and spores resistant to decay. Grains survive well where microbial activity is depressed due to drought, low availability of oxygen and presence of toxic salts in soil.

3. The resistant exine is always ornamented/ sculptured. The ornamentation occurs in a ‘species-specific-pattern’.

4. Pollen grains always have definite type and number of aperture(s) through which pollen tube emerges.

5. The type of ornamentation and the number, position and character of aperture(s) are genetically stable. Variations in ornamentation of exine and properties of aperture(s) provide means of identifying an unknown sporomorph up to its respective family or genera or even species. Definite identification of a pollen grain and spore is the essential prerequisite to study any discipline of palynology.

6. The size of a pollen grain and spore is small and this facilitates an ease of aerial transport to long distances. This property helps to trace the place of migration of a plant.

7. Pollen analysis, in comparison to megafossil studies is more advantageous by virtue of the fact that a little quantity of sample unfolds the vegetation of that area from where the samples are collected. Numerical abundance of pollen and possibility for uniform scattering help in statistical calculation and enable to determine the vegetation of the area. Uniform scattering of pollen and spore makes their fossils available in continental sediments and marine rocks. This is in contrast to marine fossils that are available only with marine rocks.

8. Pollen and spores are always produced in very large numbers. This is due to unreliability of particular grain finding its target. To be effective pollen and spores are produced in far large numbers than would be needed. This (excess pollen and spore) forms the basis upon which palaeopalynology depends. Pollen grains from atmosphere settle to soils from where they are washed out by rain water into lakes, streams, seashores, mires etc.

Pollen grains lie there for very long periods of time. Humus collects and thus pollen grains get fossilized. But the characteristics of surface pattern and aperture are still retained, and are of great diagnostic value. These deposits become a storehouse of information from where ancient vegetation can be reconstructed.

In such deposits types of pollen and their abundance become means of statistical analysis that leads to reconstruct palaeoenvironment. Changes of types of pollen and spores in the different strata reveal the change of vegetation and accordingly the changing pattern of past climate can be interpreted. It is to note that in the study of palaeopalynology most of pollen found come from anemophilous plants. Entomophilous pollen grains are under-represented.

Palaeopalynology is of immense value in a wide range of scientific studies, a few of which are mentioned below:

i. To trace the history of vegetation:

Palaeopalynology is most effectively exploited in the investigation of vegetation history. It is fruitfully employed with regard to the pollen grains of Quaternary deposits, because most of pollen in these deposits is referable to extant genera and species. Pollen and their chronological abundance in sediments provide a means of tracing the history of vegetation.

On the basis of palynological data Bryant et al. (1985) reconstructed the vegetation changes that occurred in Texas during late Quaternary. The data indicate that the flora of United States, including Picea glauca, invaded Texas during late Quaternary. Coniferous forests were present in Texas during the full- glacial period but those were not much widespread.

During post-glacial and part of the late-glacial period grassland savannas were present in North Texas. Chronological abundance of pollen grains provides information regarding arrival, expansion and decline of major trees like pine, birch and oak etc. Pollen stratigraphic studies enabled to calculate the population doubling time for forest trees during their phase of exponential population growth.

This type of study was carried out in early Holocene of Eastern England. The exponential population growth ranges from 58-73 years for Pinus sylvestris and about 100 years of Tilia cordata. Apart from origin and expansion the decline of plant taxa can also be detected through pollen stratigraphic studies, e.g. the decline of Castanea dentata and Tsuga canadensis in Northeastern United States.

Pollen analysis of peat profiles collected from Calcutta Metro Railway Excavation Project reveals the existence of Heritiera, Exocaria, Phoenix paludosa and Barringtonia etc., that form the present-day mangrove vegetation of Sundarban.

Pollen contents of peat samples were investigated at the laboratory of Division of Palynology and Environmental Biology, Bose Institute, Kolkata. With the aid of pollen stratigraphic studies the past vegetation of an area can be reconstructed. This becomes a tool in interpreting palaeoclimate, palaeoecology etc. of the region under study.

ii. To study plant assemblage at a specific stratigraphic horizon:

Each stratum of a particular region has definite types of fossil pollen and spores. The abundance of grains plus the durability of grains due to the presence of sporopollenin ensure pollen analysis from each stratum. The grains are most often preserved in bogs and lakes. Samples collected from these sources are treated chemically to stain pollen and spores thus allowing individual grains to be identified and counted.

The distinctive number, position and aperture plus the surface ornamentation of an individual pollen grain and spore allow them to be used to identify the plants that produced them.

Light microscopic study of the grains allows developing pollen spectrum. A pollen spectrum indicates the types of plant present in a specific stratigraphic horizon. Thus it reflects the plant assemblage that existed at the time of pollen and spore production.

Generally sediments have distinct groups of pollen and spore. These sporomorphs are called index fossil for that layer. The terms guide fossil, type fossil, zonal fossil, key fossil and diagnostic fossil are used as a synonym for index fossil. Index fossils provide information about the sediment from where they are found.

From index fossil the age of sediment and the environment in which the sediments were deposited can be determined. The relationship between sediments in distant locations can be established with the aid of index fossil. If the sediments from distant locations contain same index fossil, they are likely part of the same sediment and thus from the same time period.

Difference in the composition of index fossil in sediments from distant locations will indicate the sediments are distinct and not related to each other. With the aid of index fossil floral succession can be determined.

The followings are the characteristics of ideal index fossils:

(1) They must be abundant because it must be easy to find them in the sediments,

(2) They must be easy to identify,

(3) They must be short lived, and

(4) They are to be widely distributed.

Mention may made of the following pollen genera that are important index fossils in North Tethyan floral belt Mansoniaepollis, Reevesiapollis and Tahitiaoipollis.

iii. To correlate deposits and assigning dates:

Each stratum of a particular region during a particular geologic time has characteristic pollen assemblage. With the aid of pollen assemblage palynologists determine a relative chronology or a sequence of events that occurred during plant succession. The prominent changes in pollen proportions being correlated with a particular timing form the basis of using pollen analysis as dating tool.

But the absolute age of sediment cannot be determined using pollen assemblage alone. The absolute age is derived from absolute dating obtained by radiocarbon methods. A well-authenticated datum horizon can be of value on a particular region. A distinctive pollen horizon and sampling of adjacent horizon extend the records of the plant communities that occupied the site.

A number of distinctive pollen horizons over wide areas are of broader value. As for example the decline of elm at about 5000 years ago in North Europe and the decline of hemlock in Eastern North America were revealed by comparing a number of pollen horizons over wide areas. Now a days radiocarbon dating is favoured over pollen dating. Yet pollen analysis remains a major stratigraphic tool in older strata.

iv. To study climatic change:

Climate influences vegetation patterns. Determination of past vegetation types through pollen analysis is a useful tool for the study of past climates. Relative pollen frequencies are used exclusively in the reconstruction of past climate.

Pollen grains are produced in large numbers and most of them after dissemination settle to the ground throughout the year. In the following year another fresh layer of pollen grains deposits over the preexisting layer(s).

This process repeats year after year. Thus, the vegetation that provides pollen is recorded in each layer. Any change of vegetation is reflected to the change in pollen assemblage. Pollen analysis of each layer provides data to reconstruct vegetation of that layer.

The climate can be determined by studying the nature of vegetation. A comparison between pollen assemblages of two successive layers reveals the change of vegetation and accordingly the change of climate. Pollen analyses of soil, especially the soil of bogs and lakes provide data to interpret the change of climate pattern.

v. To study extinct genera:

Ex. Ephedripites and Vittatina. Wilson in 1959 reported pollen grains of the above two genera from the Middle Permian of Oklahoma. The grains are believed to be the ancestral forms of Ephedra and Wehvitschia. It is regarded that these grains are the most ancient records of Gnetales.

vi. To study evolution of plants and establish affinity:

It is regarded that monocolpate pollen grains are most primitive. Monocolpate pollen grains are found in extinct Bennettitales and extant Magnoliaceae of angiosperms. So there exists an affinity between Bennettitales and Magnoliaceae so far as pollen morphology is concerned.

Trichotomous aperture of bryophytic and pteridophytic spore is regarded as most primitive from which other morphotypes have originated. All known primitive vascular plants, both living and fossil, have trichotomous aperture.

vii. To study past distribution of flora:

Gnetales has-extremely scanty macrofossil record. But their pollen grains have been reported in fossil forms. Pollen grains attributed to Ephedra have been reported from early Tertiary horizons in Tasmania, South Australia etc. Pollen grains of Welwitschia have been found in Russia, United States and Australia. Pollen grains of Ephedra are polycolpate whereas Welwitschia has monocolpate pollen. Gnetum has no recognizable germinal opening.

viii. To study palaeoecology:

Pollen spectrum reflects the plant assemblage of a specific stratigraphic horizon. From the type of plants the past vegetation of an area can be reconstructed. This helps to interpret the palaeoecology of that area.

It has been proved that pollen grains are of immense value in determining changes in climate associated with changes in the extent of Pleistocene ice. Most plant genera of the Mesozoic and Paleozoic are extinct. In this case palaeoecology is determined on the basis of physical character of the deposit and correlative evidence from associated fossils.

ix. To determine coal-bearing strata:

Coal beds have characteristic abundance of certain pollen and spore genera on the basis of which they are identified (Fig. 5.5). The seams can be traced laterally by comparing the pollen assemblage of the adjacent sections. The pollen assemblage is expressed in the shape of tables and diagrams, and separate horizons that have definite pollen genera. Such horizons are compared with horizons of adjacent sections to correlate and extend the coal seam.

This correlation is successfully utilized in the Paleozoic, especially in the Pennsylvanian coal seams. The correlation tool has been proved to be of immense value in coal investigation of North America and Europe. In India Birbal Sahani Institute, Lucknow, has successfully employed such correlation tool in the investigations of coal basins of Assam, West Bengal, Bihar, Madhyapradesh and other states.

x. To define ancient shoreline:

The fossil fuel —oil accumulates in sediments that occur parallel to seashore. So in the exploration of oil, the defining of ancient shoreline bears special significance.

In this respect palaeopalynology has immense value. With the aid of palaeopalynology it is possible to determine strandlines, because pollen assemblages are limited near the shore. In modern marine sediments various oceanic plankton including dinoflagellates, diatoms and hystricosphaerids occur in association with pollen and spores (Fig. 5.5). By segregations the quantities of marine forms from pollen and spore the distance and direction of ancient shorelines can be determined. In seaward direction the quantity of marine forms increases with a corresponding decrease of pollen and spore.