Pollen Analysis
Pollen Analysis
Pollen analysis, or palynology, is the study of fossil pollen (and to a lesser degree, plant spores) preserved in lake sediments, bog peat, or other matrices. Usually, the goal of palynology is to reconstruct the probable character of local plant communities in the historical past, as inferred from the abundance of plant species in dated potions of the pollen record. Palynology is a very important tool for interpreting historical plant communities, and the speed and character of their response to changes in environmental conditions, especially climate change. Pollen analysis is also useful in archaeological and ecological reconstructions of the probable habitats of ancient humans and wild animals, and in determining what they might have eaten. Pollen analysis is also sometimes useful in exploration for resources of fossil fuels.
Pollen and spores
Pollen is a fine powdery substance, consisting of microscopic grains containing the male gametophyte of gymnosperms (conifers and their relatives) and angiosperms (monocotyledonous and dicotyledonous flowering plants). Pollen is designed for long-distance dispersal from the parent plant, so that fertilization can occur among individuals, in preference to self-fertilization. (However, many species of plants are indeed self-fertile, some of them exclusively so.) Plant spores are another type of reproductive grain intended for dissemination. Plant spores are capable of developing as a new individual, either directly or after fusion with another germinated spore. Among the vascular plants, these types of spores are produced by ferns, horsetails, and club-mosses. However, spores with somewhat simpler functions are also produced by mosses, liverworts, algae, fungi, and other less complex organisms.
Pollen of many plants can be microscopically identified to genus and often to species on the basis of the size, shape, and surface texturing of the grain. In general, spores can only be identified to higher taxonomic orders, such as family or order. This makes pollen, more so than spores, especially useful in typical palynological studies. The integrity of the outer cell wall of both pollen and spores is well maintained under conditions with little physical disturbance and poor in oxygen, and this is why these grains are so well preserved in lake sediment, bog peat, and even the drier deposits of archaeological sites. Fossil pollen has even been collected, and identified, from the teeth and viscera of extinct animals, such as mammoths found frozen in arctic permafrost.
Plant species are not represented in the record of fossil pollen of lake sediments and bog peat in a manner that directly reflects their abundance in the nearby vegetation. For example, plants that are pollinated by insects are rarely detected in the pollen record, because their relatively small production of pollen is not distributed into the environment in a diffuse manner. In contrast, wind-pollinated species are well represented, because these plants emit large quantities of pollen and disseminate it in a broadcast fashion. However, even among wind-pollinated plants, certain species are particularly copious producers of pollen, and these are disproportionately represented in the fossil record, as is the case of herbaceous species of ragweed (for example, Ambrosia artemesiifolia). Among temperate species of trees, pines are notably copious producers of pollen, and it is not unusual to find a distinct, pollen-containing, yellow froth along the edges of lakes and ponds in many areas during the pollen season of pines. Because of the large differences in pollen production among plant species, interpretation of the likely character of local vegetation based on observations of fossil pollen records requires an understanding of pollen production rates by the various species, as well as annual variations in this characteristic.
Dating palynological samples
Palynologists must understand the temporal context of their samples, which means that they must be dated. A number of methods are available to palynologists for dating their samples of mud or peat. Most commonly used in typical palynological studies is a method known as radiocarbon dating, which takes advantage of the fact that once an organism dies and is removed from the direct influence of the atmosphere, it no longer absorbs additional carbon-14, a rare, radioactive isotope of this element. Therefore, the amount of carbon-14 decreases progressively as a sample of dead biomass ages, and this fact can be used to estimated the age of organic samples on the basis of the remaining quantity of carbon-14, and its ratio to stable carbon-12. The rate of radioactive decay of carbon-14 is determined by its half-life, which is about 5,700 years. Radiological dating using carbon-14 is useful for samples aged between about 150,000 and 40,000-50000 years. Younger samples can sometimes be dated on the basis of their content of lead-210, and older samples using other elemental isotopes having longer half-lives.
Some palynological studies have investigated sediment collected from an unusual type of lake, called meromictic, in which there is a permanent stratification of the water caused by a steep density gradient associated with a rapid changes in temperature or salt-concentration. This circumstance prevents surface waters from mixing with deeper waters, which eventually become anoxic because the biological demand for oxygen exceeds its ability to diffuse into deeper waters. Because there is insufficient oxygen, animals cannot live in the sediment of meromictic lakes. Consequently, the seasonal stratigraphy of material deposition is not disturbed by benthic creatures, and meromictic lakes often have well-defined, annual sediment layers, called varves. These can be dated in carefully collected, frozen cores by directly counting backwards from the surface. Sometimes, a few radio-carbon dates are also measured in varved cores, to confirm the chronology, or to compensate for a poor collection of the youngest, surface layers. Although meromictic lakes are unusual and rare, palynologists seek them out enthusiastically, because of the great advantages that the varved cores have for dating and interpretation.
Sometimes, palynologists work in cooperation with archaeologists. In such cases, it may be possible to date sample locations through their physical association with cultural artifacts that have been dated by archaeologists, perhaps based on their known dates of occurrence elsewhere.
Sometimes it is not necessary to accurately know the absolute date of a sample—it may be enough to understand the relative age, that is, whether one sample is younger or older than another. Often, relative aging can be done on the basis of stratigraphic location, meaning that within any core of lake sediment or peat, older samples always occur deeper than younger samples.
Pollen analysis
Palynologists typically collect cores of sediment or peat, date layers occurring at various depths, and extract, identify, and enumerate samples of the fossil pollen grains that are contained in the layers. From the dated assemblages of fossil pollen, palynologists develop inferences about the nature of the forests and other plant communities that may have occurred in the local environment of the sampled lake or bog. These interpretations must be made carefully, because as noted above species do not occur in the pollen record in a fashion that directly reflects their abundance in the mature vegetation.
Most palynological investigations attempt to reconstruct the broad characteristics of the local vegetation at various times in the past. In the northern hemisphere, many palynological studies have been made of post-glacial changes in vegetation in places that now have a temperate climate. These vegetation changes have occurred since the continental-scale glaciers melted back, a process that began in some places 12,000-14,000 years ago. Although the particular, inferred dynamics of vegetation change vary among sites and regions, a commonly observed pattern is that
KEY TERMS
Half-life— The time it takes for one-half of an initial quantity of material to be transformed, for example, by radioactive decay of carbon–14.
Pollen analysis (palynology)— the inferred reconstruction of historical occurrences of local vegetation, as interpreted from the record of fossil pollen preserved in dated sediments of lakes or peat of bogs.
the pollen record of samples representing recently deglaciated times contains species that are now typical of northern tundra, while the pollen of somewhat younger samples suggests a boreal forest of spruces, fir, and birch. The pollen assemblage of younger samples is generally dominated by species such as oaks, maples, basswood, chestnut, hickory, and other species of trees that presently have a relatively southern distribution.
However, within the post-glacial palynological record there are clear indications of occasional climatic reversals-for example, periods of distinct cooling that interrupt otherwise warm intervals. The most recent of these coolings was the so-called “Little Ice Age” that occurred between the fourteenth and nineteenth centuries. However, palynology has detected much more severe climatic deteriorations, such as the Younger Dryas event that began about 11,000 years ago, and that caused the re-development of glaciers in many areas, and extensively reversed the broader patterns of post-glacial vegetation development.
Other interesting inferences from the palynological record have involved apparent declines of particular species of trees, occurring for reasons that are not known. For example, palynological records from various places in eastern North America have exhibited a large decline in the abundance of pollen of eastern hemlock (Tsuga canadensis), occurring over an approximately 50-year period about 4,800 years ago. It is unlikely that the hemlock decline was caused by climate change, because other tree species with similar ecological requirements did not decrease in abundance, and in fact, appear to have increased in abundance to compensate for the decline of hemlock. The hemlock decline may have been caused by an outbreak of an insect that specifically attacks that tree, by a disease, or by some other, undiscovered factor. Palynology has also found evidence for a similar phenomenon in Europe about 5,000 years ago, when there was a widespread decline of elms (Ulmus spp.). This decline could have been caused by widespread clearing of the forest by Neolithic humans, or by an unknown disease or insect.
Resources
BOOKS
Faegri, K. and J. Iversen. Textbook of Pollen Analysis. Caldwell, NJ: Blackburn Press, 2000.
Pielou, E.C. After the Ice Age. Chicago: University of Chicago Press, 1991.
Bill Freedman
Pollen Analysis
Pollen analysis
Pollen analysis, or palynology , is the study of fossil pollen (and to a lesser degree, plant spores) preserved in lake sediments, bog peat, or other matrices. Usually, the goal of palynology is to reconstruct the probable character of local plant communities in the historical past, as inferred from the abundance of plant species in dated potions of the pollen record. Palynology is a very important tool for interpreting historical plant communities, and the speed and character of their response to changes in environmental conditions, especially climate change. Pollen analysis is also useful in archaeological and ecological reconstructions of the probable habitats of ancient humans and wild animals, and in determining what they might have eaten. Pollen analysis is also sometimes useful in exploration for resources of fossil fuels .
Pollen and spores
Pollen is a fine powdery substance, consisting of microscopic grains containing the male gametophyte of gymnosperms (conifers and their relatives) and angiosperms (monocotyledonous and dicotyledonous flowering plants). Pollen is designed for long-distance dispersal from the parent plant, so that fertilization can occur among individuals, in preference to self-fertilization. (However, many species of plants are indeed self-fertile, some of them exclusively so.) Plant spores are another type of reproductive grain intended for dissemination. Plant spores are capable of developing as a new individual , either directly or after fusion with another germinated spore . Among the vascular plants, these types of spores are produced by ferns , horsetails , and club-mosses. However, spores with somewhat simpler functions are also produced by mosses, liverworts, algae , fungi , and other less complex organisms.
Pollen of many plants can be microscopically identified to genus and often to species on the basis of the size, shape, and surface texturing of the grain. In general, spores can only be identified to higher taxonomic orders, such as family or order. This makes pollen, more so than spores, especially useful in typical palynological studies. The integrity of the outer cell wall of both pollen and spores is well maintained under conditions with little physical disturbance and poor in oxygen , and this is why these grains are so well preserved in lake sediment, bog peat, and even the drier deposits of archaeological sites. Fossil pollen has even been collected, and identified, from the teeth and viscera of extinct animals, such as mammoths found frozen in arctic permafrost .
Plant species are not represented in the record of fossil pollen of lake sediments and bog peat in a manner that directly reflects their abundance in the nearby vegetation. For example, plants that are pollinated by insects are rarely detected in the pollen record, because their relatively small production of pollen is not distributed into the environment in a diffuse manner. In contrast, wind-pollinated species are well represented, because these plants emit large quantities of pollen and disseminate it in a broadcast fashion. However, even among wind-pollinated plants, certain species are particularly copious producers of pollen, and these are disproportionately represented in the fossil record, as is the case of herbaceous species of ragweed (for example, Ambrosia artemesiifolia). Among temperate species of trees, pines are notably copious producers of pollen, and it is not unusual to find a distinct, pollen-containing, yellow froth along the edges of lakes and ponds in many areas during the pollen season of pines. Because of the large differences in pollen production among plant species, interpretation of the likely character of local vegetation based on observations of fossil pollen records requires an understanding of pollen production rates by the various species, as well as annual variations in this characteristic.
Dating palynological samples
Palynologists must understand the temporal context of their samples, which means that they must be dated. A number of methods are available to palynologists for dating their samples of mud or peat. Most commonly used in typical palynological studies is a method known as radiocarbon dating, which takes advantage of the fact that once an organism dies and is removed from the direct influence of the atmosphere, it no longer absorbs additional carbon-14, a rare, radioactive isotope of this element. Therefore, the amount of carbon-14 decreases progressively as a sample of dead biomass ages, and this fact can be used to estimated the age of organic samples on the basis of the remaining quantity of carbon-14, and its ratio to stable carbon-12. The rate of radioactive decay of carbon-14 is determined by its half-life , which is about 5,700 years. Radiological dating using carbon-14 is useful for samples aged between about 150,000 and 40,000-50000 years. Younger samples can sometimes be dated on the basis of their content of lead-210, and older samples using other elemental isotopes having longer half-lives.
Some palynological studies have investigated sediment collected from an unusual type of lake, called meromictic, in which there is a permanent stratification of the water caused by a steep density gradient associated with a rapid changes in temperature or salt concentration. This circumstance prevents surface waters from mixing with deeper waters, which eventually become anoxic because the biological demand for oxygen exceeds its ability to diffuse into deeper waters. Because there is insufficient oxygen, animals cannot live in the sediment of meromictic lakes. Consequently, the seasonal stratigraphy of material deposition is not disturbed by benthic creatures, and meromictic lakes often have well-defined, annual sediment layers, called varves. These can be dated in carefully collected, frozen cores by directly counting backwards from the surface. Sometimes, a few radiocarbon dates are also measured in varved cores, to confirm the chronology, or to compensate for a poor collection of the youngest, surface layers. Although meromictic lakes are unusual and rare, palynologists seek them out enthusiastically, because of the great advantages that the varved cores have for dating and interpretation.
Sometimes, palynologists work in cooperation with archaeologists. In such cases, it may be possible to date sample locations through their physical association with cultural artifacts that have been dated by archaeologists, perhaps based on their known dates of occurrence elsewhere.
Sometimes it is not necessary to accurately know the absolute date of a sample—it may be enough to understand the relative age, that is, whether one sample is younger or older than another. Often, relative aging can be done on the basis of stratigraphic location, meaning that within any core of lake sediment or peat, older samples always occur deeper than younger samples.
Pollen analysis
Palynologists typically collect cores of sediment or peat, date layers occurring at various depths, and extract, identify, and enumerate samples of the fossil pollen grains that are contained in the layers. From the dated assemblages of fossil pollen, palynologists develop inferences about the nature of the forests and other plant communities that may have occurred in the local environment of the sampled lake or bog. These interpretations must be made carefully, because as noted above species do not occur in the pollen record in a fashion that directly reflects their abundance in the mature vegetation.
Most palynological investigations attempt to reconstruct the broad characteristics of the local vegetation at various times in the past. In the northern hemisphere, many palynological studies have been made of post-glacial changes in vegetation in places that now have a temperate climate. These vegetation changes have occurred since the continental-scale glaciers melted back, a process that began in some places 12,000-14,000 years ago. Although the particular, inferred dynamics of vegetation change vary among sites and regions, a commonly observed pattern is that the pollen record of samples representing recently deglaciated times contains species that are now typical of northern tundra , while the pollen of somewhat younger samples suggests a boreal forest of spruces, fir, and birch. The pollen assemblage of younger samples is generally dominated by species such as oaks , maples , basswood , chestnut , hickory, and other species of trees that presently have a relatively southern distribution.
However, within the post-glacial palynological record there are clear indications of occasional climatic reversalsfor example, periods of distinct cooling that interrupt otherwise warm intervals. The most recent of these coolings was the so-called "Little Ice Age" that occurred between the fourteenth and nineteenth centuries.. However, palynology has detected much more severe climatic deteriorations, such as the Younger Dryas event that began about 11,000 years ago, and that caused the re-development of glaciers in many areas, and extensively reversed the broader patterns of post-glacial vegetation development.
Other interesting inferences from the palynological record have involved apparent declines of particular species of trees, occurring for reasons that are not known. For example, palynological records from various places in eastern North America have exhibited a large decline in the abundance of pollen of eastern hemlock (Tsuga canadensis), occurring over an approximately 50-year period about 4,800 years ago. It is unlikely that the hemlock decline was caused by climate change, because other tree species with similar ecological requirements did not decrease in abundance, and in fact, appear to have increased in abundance to compensate for the decline of hemlock. The hemlock decline may have been caused by an outbreak of an insect that specifically attacks that tree, by a disease , or by some other, undiscovered factor. Palynology has also found evidence for a similar phenomenon in Europe about 5,000 years ago, when there was a widespread decline of elms (Ulmus spp.). This decline could have been caused by widespread clearing of the forest by Neolithic humans, or by an unknown disease or insect.
Resources
books
Faegri, K., and J. Iversen. Textbook of Pollen Analysis. New York: Hafner Press, 1975.
Pielou, E.C. After the Ice Age. Chicago: University of Chicago Press, 1991.
Bill Freedman
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Half-life
—The time it takes for one-half of an initial quantity of material to be transformed, for example, by radioactive decay of carbon–14.
- Pollen analysis (palynology)
—the inferred reconstruction of historical occurrences of local vegetation, as interpreted from the record of fossil pollen preserved in dated sediments of lakes or peat of bogs.