Palynology The Scientific Study of Pollen and Spores

How Does Palynology Inform Paleoenvironmental Reconstruction?

Chicory Pollen Grains
Chicory Pollen Grains. Getty Images / Ian Cuming

Palynology is the scientific study of pollen and spores, those virtually indestructible, microscopic, but easily identifiable plant parts found in archaeological sites and adjacent soils and water bodies. These tiny organic materials are most commonly used to identify past environmental climates (called paleoenvironmental reconstruction), and track changes in climate over a period of time ranging from seasons to millennia.

Modern palynological studies often include all micro-fossils composed of highly resistant organic material called sporopollenin, which is produced by flowering plants and other biogenic organisms. Some palynologists also combine the study with those of organisms that fall into the same size range, like diatoms and micro-foraminifera; but for the most part, palynology focuses on the powdery pollen that floats on the air during the blooming seasons of our world.

Science History

The word palynology comes from the Greek word "palunein" meaning to sprinkle or scatter, and the Latin "pollen" meaning flour or dust. Pollen grains are produced by seed plants (Spermatophytes); spores are produced by seedless plants, mosses, club mosses, and ferns. Spore sizes range from 5-150 microns; pollens range from under 10 to more than 200 microns.

Palynology as a science is a little over 100 years old, pioneered by the work of the Swedish geologist Lennart von Post, who in a conference in 1916 produced the first pollen diagrams from peat deposits to reconstruct the climate of western Europe after the glaciers had receded.

Pollen grains were first recognized only after Robert Hooke invented the compound microscope in the 17th century.

Why is Pollen a Measure of Climate?

Palynology allows scientists to reconstruct the history of vegetation through time and past climate conditions, because during the blooming seasons, pollen and spores from local and regional vegetation are blown through an environment and deposited over the landscape.

Pollen grains are created by plants in most ecological settings, in all latitudes from the poles to the equator. Different plants have different blooming seasons, so in many places, they are deposited during much of the year.

Pollens and spores are well preserved in watery environments and are readily identifiable at the family, genus, and in some cases species level, based on their size and shape. Pollen grains are smooth, shiny, reticulate, and striated; they are spherical, oblate, and prolate; they come in single grains but also in clumps of two, three, four, and more. They have an astonishing level of variety, and a number of keys to pollen shapes have been published in the past century that make fascinating reading.

The first occurrence of spores on our planet comes from sedimentary rock dated to the mid-Ordovician, between 460-470 million years ago; and seeded plants with pollen developed about 320-300 mya during the Carboniferous period.

How it Works

Pollen and spores are deposited everywhere across the environment during the year, but palynologists are most interested in when they end up in bodies of water--lakes, estuaries, bogs--because sedimentary sequences in marine environments are more continuous than those in the terrestrial setting.

In terrestrial environments, pollen and spore deposits are likely to be disturbed by animal and human life, but in lakes, they are trapped in thin stratified layers on the bottom, mostly undisturbed by plant and animal life.

Palynologists put sediment core tools into lake deposits, and then they observe, identify and count the pollen in the soil brought up in those cores using an optical microscope at between 400-1000x magnification. Researchers must identify at least 200-300 pollen grains per taxa to accurately determine the concentration and percentages of particular taxa of plant. After they have identified all the taxa of pollen that reach that limit, they plot the percentages of the different taxa on a pollen diagram, a visual representation of the percentages of plants in each layer of a given sediment core that was first used by von Post.

That diagram provides a picture of pollen input changes through time.

Issues

At Von Post's very first presentation of pollen diagrams, one of his colleagues asked how he knew for sure that some of the pollen wasn't created by distant forests, an issue that is being resolved today by a set of sophisticated models. Pollen grains produced at higher elevations are more prone to be carried by the wind longer distances than those of plants closer to the ground. As a result, scholars have come to recognize the potential of an overrepresentation of species such as pine trees, based on how efficient the plant is at getting its pollen distributed.

Since von Post's day, scholars have modeled how pollen disperses from the top of the forest canopy, deposits on a lake surface, and mixes there before final accumulation as sediment in the lake bottom. The assumptions are that pollen accumulating in a lake comes from trees on all sides, and that the wind blows from various directions during the long season of pollen production. However, nearby trees are much more strongly represented by pollen than trees farther away, to a known magnitude.

In addition, it turns out that different sized bodies of water result in different diagrams. Very large lakes are dominated by regional pollen, and larger lakes are useful for recording regional vegetation and climate. Smaller lakes, however, are dominated by local pollens--so if you have two or three small lakes in a region, they might have different pollen diagrams, because their micro-ecosystem is different from one another. Scholars can use studies from a large number of small lakes to give them insight into local variations. In addition, smaller lakes can be used to monitor local changes, such as an increase in ragweed pollen associated with Euro-American settlement, and the effects of runoff, erosion, weathering and soil development.

Archaeology and Palynology

Pollen is one of several types of plant residues which have been retrieved from archaeological sites, either clinging to the inside of pots, on the edges of stone tools or within archaeological features such as storage pits or living floors.

Pollen from an archaeological site is assumed to reflect what people ate or grew, or used to build their homes or feed their animals, in addition to local climate change. The combination of pollen from an archaeological site and a nearby lake provides depth and richness of the paleoenvironmental reconstruction. Researchers in both fields stand to gain by working together.

Sources

Two highly recommended sources on pollen research are Owen Davis's Palynology page at the University of Arizona, and that of the University College of London.