Palynology

Palynology is the study of fossil pollen (and sometimes plant spores) extracted from lake sediment, peat bog, or other matrices. The most common goal of palynological research is to reconstruct the probable character of historical plant communities, inferred from the abundance of species in dated portions of the fossil pollen record. Pollen analysis is an extremely useful tool for understanding the character of ancient vegetation and its response to changes in environmental conditions, particularly in climate. Pollen analysis also has an economically important modern industrial use in the exploration for resources of fossil fuels. Palynology is also used to help reconstruct the probable habitats and foods of ancient humans and of wild animals.

Pollen consists of microscopic grains containing the male gametophyte of coniferous (cone-bearing) and angiosperm plants. Pollen of most species of plants undergoes a long-distance dispersal from the parent plant, so that fertilization can occur among individuals (instead of self-fertilization). A plant spore is a kind of reproductive grain capable of developing as a new individual, either directly or after fusion with another germinated spore, such as the kind produced by ferns, horsetails, and club-mosses. Spores with simpler functions are produced by mosses, liverworts, algae, fungi, and other less complex organisms.

The pollen of many plants can be classified by genus, and sometimes by species, on the basis of such characteristics as size, shape, and surface texture. In contrast, most spores can only be classified by higher taxonomic levels, such as family or order. Both pollen and spores are well preserved in lake sediment, peat bog, and many archaeological sites. Fossil pollen has even been identified from the bodies of extinct animals, such as mammoths discovered frozen in arctic permafrost (permanently frozen subsoil).

Plant species in the pollen record of lake sediment and peat are not represented in the same relative abundance they are in the nearby vegetation. Wind-pollinated plant species are most abundant, because these plants release huge amounts of pollen into the environment. For example, many species of pines, which are wind pollinated, are so prolific that during their flowering season a yellow froth of pollen may occur along the edges of lakes and ponds. Insect-pollinated plant species are more rare. The great differences in pollen production among plant species must be taken into account when interpreting the likely character of local vegetation on the basis of the fossil-pollen record.

Palynologists need to understand the historical context of their samples. The common method used to determine the age of samples of mud and peat is radiocarbon dating. This technique is based on the fact that after an organism dies, it no longer absorbs carbon-14 from the atmosphere. Because carbon-14 is a rare, radioactive isotope of carbon (that is, it "decays" into simpler isotopes or elements), its amount in dead biomass decreases progressively with time. This change can be used to estimate the age of organic material by calculating the ratio of carbon-14 to stable, non-radioactive carbon-12. Radiocarbon dating is effective for samples aged between 150 and 40-50 thousand years. Younger samples may be dated on the basis of their content of lead-210, and older samples using elemental isotopes with longer half-lives.

A typical palynological study might involve the collection of one or more cores of sediment or peat from a site. The layers occurring at various depths would be dated, and samples of the pollen grains contained in the layers would be extracted, classified, and enumerated. From the dated fossil pollen of various species or genera, the palynologist would develop inferences about the nature of the forests and other plant communities that may have occurred in the local environment at the time.

In the northern hemisphere, many palynological studies have been made of changes in vegetation occurring since the continental-scale glaciers melted back, beginning about 12-15 thousand years ago. A commonly observed pattern from the pollen record is that the oldest samples, representing recently deglaciated times, indicate plant species that are now typical of northern tundra, while somewhat younger samples suggest a boreal forest of spruces, fir, and birch. The pollen assemblage of younger, more recent samples is generally dominated by temperate trees such as oaks, maples, basswood, chestnut, hickory, and other species that now have a relatively southern distribution. There may also, however, be indications in the pollen record of occasional climatic reversals, such as periods of cooling that interrupt longer, warm intervals. The most recent of these cool periods was the "Little Ice Age" that occurred between about 1550 and 1850. However, palynology has also detected more severe climatic deteriorations in the past, such as the "Younger Dryas" event that began about 11,000 years ago, causing a re-development of glaciers in many areas and temporarily reversing post-glacial vegetation development.