Palynology is the study and analysis of microscopic organic material, predominantly pollen and spores, but also a multitude of other organic particles with tough exterior surfaces that defy acid digestion, collectively known as Palynomorphs. Palynologists use the data on distribution and abundance of palynomorphs for a growing range of applications, from geology and archeology to paleoecology and forensic science, providing new insights and knowledge within each field. With the ability to resolve knowledge of ancient ecologies and climates, the application and importance of palynology to humankind grows coincidentally with our concerns about global climate change and environmental impacts.
Palynomorphs are derived from four of the five taxonomic kingdoms (Protista, Planta, Fungi and Animalia) where organisms have some part of their life cycle that produces a cell, tissue or organ with a type of wall that is highly resistant to organic decay or inorganic degradation.
Throughout time, vast quantities of palynomorphs have been released into the atmosphere, each carrying the unique design of its species. Transported by wind or water, this organic dust occurs on almost every surface in nature and can be ingested by animals, intentionally or not, when they eat, drink, and breathe. But the majority of palynomorphs rain down from the air to accumulate in lakes, bogs and oceans where they are incorporated into sediments and will readily fossilize. As the world’s vegetation changes, so, too, does the layered signature of pollen and spores in the geological record, preserving information of the plant assemblage at that point in time.
The predominant focus of palynology has been on spores and pollen grains. Ever since seed plants evolved in the Devonian period and spread across the planet, their reproductive structures evolved tough exteriors, the exine, to aid in dispersal strategies by resisting decay. The exine has walls consisting of chitin, a highly inert material related to cellulose, and sporopollenin, an enigmatic and nearly indestructible compound. This structural and chemical strength ensures that palynomorphs survive the rigors of transportation and sedimentation, making them ideal subjects for fossilization.
To study these microscopic objects, palynologists treat a sample of rock, soil, or other object with strong acids (such as hydrofluoric) to dissolve the constituents and leave an acid-insoluble residue. The tough exterior of palynomorphs protects them from digestion and thus concentrates them in the residue where they can be identified and counted.
However, there are conditions that are unfavorable for palynomorph preservation and subsequent analysis. Palynomorphs will oxidize when exposed to weathering, and they cannot survive in high-alkaline conditions. Because the carbonization of chitin and sporopollenin occurs at temperatures over 200°C, palynomorphs are destroyed in strata cooked by lava flows or igneous intrusions, and do not survive the processes of temperature and pressure into metamorphic or recrystallized rocks. Another challenge is that even in modern times it is difficult to match transported pollen and spores to the species of plant from which it originated, and even harder to match fossilized palynomorphs to a taxon that may not exist anymore.
History
The study of fossil plants dates back to the 6th century B.C. when fossil leaf compressions were noted by the Greek Xenophanes, but the identification and development of microscopic pollen as a scientific tool was to go unrecognized for some time, dependent upon prerequisite advances in magnification technology. Even after pollen was first observed microscopically around 1640, its scientific potential was not realized until the late 19th century when the German geologist Ehrenberg theorized that peat bogs and lake mud would contain pollen records. The commonly accepted date for the establishment of pollen analysis as a study is 1916, when Von Post published material on how to reconstruct ancient vegetation by counting the types of pollen it shed, layer by layer, in the growing peat bogs of Sweden. Then in 1944 Hyde and Williams coined the term palynology, from the Greek, meaning “I sprinkle,” cognate with the Latin “fine flour” or “dust.” Their definition became “the study of pollen and other spores and their dispersal, and applications thereof” but has since been revamped to include other organic microfossils in the range of 5 to 500 microns, not being made of calcium carbonate and resistant to acid digest, thus forming the collective palynomorphs.
Examples
The rapid growth and development of palynology in the latter part of the 20th century was in response to its most important economic application, the oil and gas industry. Palynological techniques offer relatively cheap and quick strata dates compared to other methods, providing data during the expensive drilling process and allowing optimization of fieldwork. Palynomorphs’ rapid evolutionary changes can help to provide a more accurate chronology of geological sequences and to correlate stratigraphy between exploration wells. In addition, the identification of indicative palynomorphs can lead to the discovery of hydrocarbon deposits in several ways. To produce useful hydrocarbons, organic-rich sediment needs to undergo a period of “cooking” or maturation. The duration and temperature of this process largely determines the nature of the product, whether it becomes oil, condensate, or gas. By examining and comparing the depth of color in spore exines, an estimate of sediment maturity and prospectivity can be made. It has also been found that palynomorphs can migrate through porous rocks along with the target petroleum products, so their very presence in certain strata can indicate that hydrocarbons are close by.
Palynology has been increasingly used in the field of archaeology and is now applied routinely in many investigations. Pollen and spore analysis can help track the cultivation and domestication of plants as well as the human impact on vegetation, and it can reveal exquisite details of prehistoric life when found associated with artifacts, burial sites and archeological features. In the celebrated discovery of “the ice man,” a 5300-year-old human corpse preserved by an Austrian Alp glacier, the colon contents contained 30 different types of pollen, two spore types, and 24 diatom species. Most of the pollen was ingested as swallowed food and showed a diet predominant in legumes and cereals including einkorn, an ancient wheat grain previously thought to be consumed only by livestock. Pollen grains trapped within the man’s clothing indicated the path and direction from whence he travelled, and even the season in which he met his demise.
Palynology in the service of paleontology is usually used to date strata in which fossils occur, but can also enhance our understanding of the environments and diets of prehistoric animals. The discovery of a naturally mummified brachylophosaurus with intact stomach contents allowed identification of pollen and spores forming not only the cretaceous flora, but also presumably the diet of this 77-million-year-old dinosaur.
Honey contains abundant pollen that palynologists use in the fields of economic science and archaeology. The plants which bees in the Nile valley visited several thousand years ago can be inferred from pollen in honey found in ancient Egyptian tombs. The pollen residues in old drinking horns would technically allow reconstruction of the Viking recipe for mead. Today, apiarists and the food industry use palynology to examine bees’ foraging habits and pollen load variations to assist the development of honey production. It has also been applied as a new tool in the protection of consumers from fraudulent labeling and food “forgery”: Palynological analysis of provincially unique foodstuffs can detect the presence of foreign pollen, indicating that the food has been mixed with cheaper substitutes or revealing other deceptive trade practices.
Another recent application of palynology is in the field of forensic science, where it has now emerged as a powerful tool to solve crimes. Particularly in outdoor crimes where palynomorphs can drift unseen on air currents, settling on clothes or adhering to certain objects, the analysis of pollen and spores can provide compelling scientific evidence for a jury. It is often possible to be very specific about when and where a person or object has been from the pollen types that occur together in a sample. In one example, a man went missing near Vienna; although the police held a suspect for murder, the body could not be located. Palynological analysis of soil adhering to the suspect’s shoes revealed a mixture of pine and alder pollen, but also traces of older, Oligocene-age pollen. This unique combination pointed to a specific location just south of Vienna where pine and alder trees grow on Oligocene strata. When confronted with this knowledge, the suspect quickly confessed and the missing man’s body was recovered.
Palynology is of great importance in describing changes in tree population with time, thereby allowing inference of past climate changes. The prospect of rapid, trace-gas-induced climate change over the next 100 to 150 years has raised scientific and public interest in the rates at which plant species, particularly tree species, can respond to these changes. Because palynomorphs are sensitive to any minor fluctuation in their surroundings, they are highly indicative of the environment in which they are deposited. Any changes brought about by climatic fluctuations or the activities of man are often recorded in bog or lake sediments as changes in the pollen assemblage. For example, in some Pleistocene-age, interglacial sediments, the palynological analysis reveals the major climatic changes reflected in the tree population. During the cool conditions just before the ice sheet retreated, palynomorphs of a pine and birch assemblage with lots of grasses demonstrated the pretemperate climate; then, as the ice receded during the interglacial period, warmth-loving oak, hazel, and alder trees appeared in the sedimentary record, followed by the anticipated return of pine, birch, and grass as the ice sheet re-advanced in the succeeding glacial period when temperatures dropped.
Although the primary practice of palynological sample collection and preparation has not evolved much since the relatively recent birth of palynology as a science, there has been explosive growth in applications to many fields. Further advances in palynology can only continue to provide a better picture of our planet’s history with which to approach our immediate and long-term future.
References:
- Glover, J. (2003). Geological journeys: From artifacts to zircon. The Geological Society of Australia.
- Jansonius, J. & McGregor, D. C. (Eds). (1996). Palynology: Principles and applications. American Association of Stratigraphic Palynologists Foundation.
- Moore, P. D., Webb, J. A., & Collinson, M. E. (1991). Pollen Analysis. Oxford: Blackwell Scientific Publications.
- Stewart, W. N. & Rothwell, G.W. (1993). Paleobotany and the evolution ofplants. Cambridge: Cambridge University Press.
- Thomas, B. (1981). The evolution of plants and flowers. Eurobook.
- Traverse, A. (1988). Paleopalynology. Boston: Unwin Hyman.
