Pollen analysis

The Swedish geologist Lennart von Post (1884-1951; Figure 2.9) presented in 1916 the technique of pollen analysis at the 16th Scandinavian meeting of natural scientists in Christiania (now Oslo) and demonstrated its potential as a technique for relative dating and for reconstructing past vegetation and past climate. Although the original idea of pollen analysis can be attributed to the Swedish botanist Gustaf Lagerheim (1860-1926), it was von Post who had the vision of

Figure 2.9 Lennart von Post (1884-1951), a Swedish geologist who developed the technique of Quaternary pollen analysis as a tool for dating and for reconstructing past climate. Picture from (http://en.wikipedia.org/wiki/ Lennart_von_Post (accessed 30 October 2006).)

using pollen analysis as a dating technique and as a paleoclimatic tool (Fries 1950; Lundqvist 1965; Manten 1967a,b). In 1916 von Post proposed that in contrast to megafossils and macrofossils in peat, pollen could give a continuous record of vege-tational and hence climatic change. He demonstrated strong pollen-stratigraphic similarities within southern and central Sweden and strong pollen-stratigraphic differences between the two regions (Manten 1967b; von Post 1967). He later proposed (von Post 1946) the concept of regional parallelism, namely that the same overriding climate change will be reflected in different ways by different taxa in different geographic and climatic regions. Based on the concept of regional parallelism, pollen analysis was used prior to about 1960 as a means of "determining geologic time" (von Post 1946) by assuming that different pollen zones (e.g. Jessen's zones I-IX, Godwin's zones I-VIII) in an area were synchronous. Pollen analysis provided the major means of dating botanic, zoologic, and archaeolo-gic material in relation to the regional pollen zonation. Attempts were naturally made to correlate the pollen zonation schemes with the Blytt-Sernander climate periods (Iversen 1973; F^gri 1981; Mangerud 1982). Pollen stratigraphy soon became the only approach used to detect past climate changes and to identify the Blytt-Sernander periods. These periods almost became synonyms for pollen zones.

In a series of publications based on pollen-stratigraphic investigations in southern and central Sweden, von Post (e.g. 1920, 1924, 1933) emphasized climate as the principal cause of the inferred vegetation changes and interpreted the pollen record in a straightforward way and proposed that summer temperatures in the mid-Holocene were about 2°C higher than present-day. Even by 1933, von Post emphasized the need for a synthesis of paleoclimatic records and for improved chronologies. He wrote (von Post 1933, p. 58 - translated from Swedish)

"At some future date, as and when the time proves ripe, the variety of time-scales for the history of climate development might be fashioned into a unified whole. The system so finalized, naturally enough, should not only comprise a succession of periods. It ought to be based, firmly and squarely, on a time-scale of years, centuries, and millennia."

In the 1920s-1940s, many investigators made pollen-analytic studies in Europe, North America, South America, New Zealand, and China. Although these studies are, by modern standards, rather rudimentary, they lay the foundation for von Post's (1946) major synthesis on "The Prospect for Pollen Analysis in the Study of Earth's Climatic History", along the ideas he suggested in 1933. This global synthesis was the lecture von Post delivered when he was awarded the Vega Medal from the Swedish Anthropological and Geographic Society in 1944.

In this far-ranging and forward-looking paper, von Post emphasized that pollen analysis is "the most complete and most realistic register of climatic fluctuations throughout the past which we now have at our disposal". He proposed, based on pollen diagrams from Europe, New Zealand, Tierra del Fuego, North America, Hawaii, and China, that there is a consistent three-fold division of Holocene pollen stratigraphies. The first he interpreted as a period of increasing warmth, the second as a period of culminating warmth, and the third as a period of decreasing warmth. He characterized the dominant elements of the middle period as mediocratic, and of the earlier and later periods as protocratic and terminocratic, respectively. Von Post attempted to show that similar patterns of changing warmth had affected not only many parts of Europe, but many widely spaced regions on Earth. In this synthesis, von Post asked "now is it conceivable that anything other than climatic change could have brought about this general and, within certain regions, fundamental transformation of the forest distribution of our part of the world?" Besides discussing these broad-scale climatic trends, von Post also considered "minor paleoclimatic fluctuations" and suggested that the reader must be "wondering about these 'ripples'". He presented a Holocene pollen diagram from a bog near Stockholm and examined fluctuations in tree pollen concentrations (pollen per gram of peat) and the Betula (birch) pollen percentage curve. Using an early form of smoothing and spectral decomposition, von Post identified two long-term trends and three cycles with period lengths of 1700, 800-900, and 200-400 years within the birch pollen curve. The chronology was very crude and was based on peat-stratigraphic changes and the assumed ages of Granlund's peat recurrence surfaces.

Although von Post (1946) does not openly criticize the Blytt-Sernander scheme (von Post was after all a strong supporter of Rutger Sernander in the Sernander-Andersson feud and they had published together in 1910), von Post appears discretely to have modified his general views towards Andersson's idea of a mid-Holocene warmth. Von Post recognized that there were fluctuations in climate that correspond to the Blytt and Sernander phases but these were small compared with the major long-term climatic changes proposed by Andersson. This view, obliquely presented by von Post (1946), represents the paradigm of

Holocene climate change that had developed by the 1940s, despite the widespread application of the Blytt-Sernander scheme and its terminology (see also Hafsten 1970).

In his 1946 paper, von Post widened his discussion of Holocene climate change to the global scale and discussed "registrations of climate by the ocean surface", "the post-glacial changes in the world's system of circulation", and "theories" of climate change. He identified areas on Earth where pollen-analytic studies should be made (e.g. western North America, north-east Australia, Japan, Chile, western Africa, southern Africa, India, and Malaysia) to determine Holocene changes in the Earth's circulation systems. Under theories of climate change, von Post wrote (p. 212):

"In what I have said I have hesitated in my speech, when dealing with the nature of climatic changes, in such a manner as to astonish my audience, and about the causes, which are, nevertheless, the kernel of the problem, I have kept silent. Both these things I have done intentionally. For it might be indisputable that there was a wave ofwarmth in Europe, which determined the general course of post-glacial development, very likely this has in fact been the case in North America and Tierra del Fuego, but in the South Island of New Zealand and in China it is only more copious precipitation that is clearly indicated. About the driving force at the back of the climatic fluctuations of higher and lower order, which we may define or surmise, we will stumble in deep ignorance, in spite of all speculations about it."

He warns about the potential dangers of uncritical comparisons between data and models, when

"attempts are made to fit a course of development into a chronological system which is borrowed from a range of outside phenomena. If the facts appear to agree approximately then the theory becomes positively dangerous, for it may tempt us to wishful thinking which obscures our vision of the empirical realities. This has happened to a lamentable extent in dealing with Quaternary climatic history. We must, of course, at first define objectively the course of events that are to be explained, and neither for the post-glacial climate development nor for the climatic revolutions of the older Quaternary phases have we got the basis for a sound interpretation."

Despite his global view of vegetational and climatic development, von Post (1946) showed remarkable caution in relating observations to the theories about climate change then current (e.g. variations in solar radiation). Von Post mentions Milutin Milankovitch (1879-1958) and his ideas of orbital forcing to explain glacials and interglacials, but suggested "once the secret of the cause of the postglacial waves of climate is exposed, then the riddle of the Quaternary ice ages will no doubt be solved." In fact, it was the solving of the riddle of the Quaternary ice ages first (Imbrie and Imbrie 1979) that contributed to our understanding of Holocene climate change, rather than the reverse order predicted by von Post (1946).

Although von Post (1946) never mentioned the discovery by Johs. Iversen, Knut F^gri, Franz Firbas, and others in the late 1930s and 1940s (Iversen 1973; F^gri

1981; Lang 1994) that pollen analysis can detect the early impact of prehistoric people on vegetation, von Post was very aware of the potential role of climate change on human societies and societal change. He wrote (pp. 215-217) that:

"It is axiomatic that such considerable revolutions (climatic change is meant here - my addition) in the very heart of natural conditions, which we find registered in pollen diagrams from different parts of the world, must have set their trace on mankind's destiny from the disappearance of the very latest land ice and right on till material culture reached stages in which people could, at any rate in some measure, become independent of natural conditions of life. But until this took place, both the changes in climate themselves and their geographic results must often have brought about weal or woe for considerable groups of mankind. Particularly at the limits of human cultivation, such as Greenland or Iceland, or the extensive plains ofAsia, Africa, and America, which hovered between, on the one side, the barrenness of a desert, and on the other, the more or less good pastures and possibilities of cultivation in the grass steppes and forest land, it is inconceivable that changes in the natural conditions should not have had sweeping results on both the distribution and standards of living of mankind. Certainly cultural, social, political, and folk-psychological factors have many times been the driving force both of migrations of peoples or the flowering or fading of former cultures. None the less the conclusion seems inescapable that changes in natural factors must be considered if we wish to reach a true understanding of these phenomena, indeed that these changes may have occasionally been fundamental. In dealing with the problems of cultural history the reconstruction of the history of nature should be considered a necessity."

This is a theme that is explored in depth by Oldfield (2005, this volume).

Von Post (1946) foresaw the need for major international collaboration in establishing the patterns of Holocene climate change by writing a "plan of work of the extent here sketched can scarcely be carried out without organized international collaboration. At the head of this must stand a scientist with experience, foresight, mental acuity and activity, and he must be young, for the task is a life's work."

Von Post's Vega lecture was published 60 years ago and his original paper on pollen analysis was published 90 years ago. It is little wonder that the late Ed Deevey (1967) wrote "von Post's simple idea that a series of changes in pollen proportions in accumulating peat was a four-dimensional look at vegetation, must rank with the double-helix as one of the most productive suggestions of modern times."

Major advances in pollen-analytic methodology and interpretation were made in the 1940s-1960s by the Danish botanist Johs. Iversen (1904-1971; Figure 2.10a), the Norwegian botanist Knut F^gri (1909-2001; Figure 2.10b), and others (see, for example, Lang's (1994) review). In terms of Holocene climate research, one of the most important developments was Iversen's (1944) use of Viscum album (mistletoe), Hedera helix (ivy), and Ilex aquifolium (holly) as "indicator species" of summer and winter temperatures. Iversen studied in detail the occurrence, growth, flowering, and fruit production of these three shrubs at or near their

Figure 2.10 (a) Johs. Iversen (1904-1971), a Danish botanist (left) and (b) Knut Fsgri (1909-2001), a Norwegian botanist. Together they made major advances in pollen-analytic methodology and interpretation. (The picture of Iversen is from Iversen (1973; copyright: Geological Survey of Denmark and Greenland). The picture of Fsgri was taken by Jan Berge on the occasion of Fsgri's 90th birthday.)

northern and/or eastern limits in Denmark in relation to the mean temperatures of the warmest and coldest months. He delimited "thermal limits" for these species. All three have distinctive pollen. From fossil pollen occurrences in Holocene sediments in southern Scandinavia, Iversen inferred that mid-Holocene summer temperatures may have been 2-2.5°C warmer than today and that winter temperatures may have been 0.5-1.5°C warmer than today, thereby building on Samuelsson's (1916) idea that both summer and winter temperatures may have changed in the Holocene. Iversen's (1944) brilliant analysis (see also F^gri 1950 and Hintikka 1963) of the climatic sensitivity of Ilex has been elegantly confirmed by Walther etal. (2005), who show shifts in its northern margin in the past 50 years in response to recent climate changes. These shifts are entirely predictable, given Iversen's thermal limit and recent climate changes.

In terms of direct relevance to Holocene climate research, the other major development in pollen analysis in the 1940s-1960s was the demonstration of the impact of prehistoric people on vegetation through forest clearance and agriculture. This demonstration was a result of improved pollen identifications (e.g. cereals, agricultural weeds), fine resolution analyses, identification and counting of charcoal particles, and research collaboration between pollen analysts and archaeologists (e.g. Iversen 1973). Although the interpretation of the observed pollen-stratigraphic changes in terms of human activity is not in doubt, recent concerns about the changing extent of forest and cleared land during the Holocene and the role of humans in Holocene climate change (Oldfield 2005, this volume) have stimulated research into the relationship between landscape cover and pollen deposition (e.g. Sugita et al. 1999). In general, pollen-analytic data underestimate the extent of cleared land because of the huge differences in pollen representation between forest trees and cleared-land herbs and shrubs (Sugita et al. 1999).

The paradigm of Holocene climate change based on pollen analysis and earlier macrofossil and megafossil analysis at about 1950 was that proposed by von Post (1946), namely long-term regional-scale or even global changes in temperature and precipitation at broad millennial scales and more local-scale shifts in moisture and possibly in temperature at centennial scales. Changes in both summer and winter temperature had occurred and there may have been periods of rapid climate change in the late Holocene. There was also evidence for the effects of prehistoric people on vegetation, and interest naturally centered on whether the extent of human activity and resulting land-use change were related to climate change. At that time there were no means of testing the implicit assumption that pollenstratigraphic changes and hence inferred climate changes were synchronous over large areas. The next major stage in Holocene research dawned with the development of radiocarbon dating and absolute chronologies.

0 0

Responses

  • juhana
    How does pollen analysis show climate change?
    8 years ago
  • cornelia labingi
    How can pollen analysis help us reconstruct past climates?
    8 years ago
  • Franco
    Why is reconstructing the holocene climate interesting pollen analysis?
    8 years ago
  • Amalio
    What has pollen done to the climate in the past?
    8 years ago

Post a comment