Peat stratigraphy megafossils and macrofossils

The impressive occurrence of large fossil trunks and stumps (megafossils) of pine trees preserved in peat bogs in north-west Europe (Figure 2.1a and b) naturally attracted attention from naturalists as early as the late 18th century (e.g. Tait, 1794), and raised problems for many scientists who assumed that the environment did not change greatly. For example, in Scotland, Maxwell (1915) suggested that "one of the greatest enigmas of natural science is presented in the remains of pine forest buried under a dismal treeless expanse on the Moor of Rannoch, and on the Highland hills up to and beyond 2000 feet altitude" (Figure 2.1a).

The first scientific study of peat and pine stumps was probably by Heinrich Dau (1790-1831) in Denmark. Dau (1829) recognized and described several different types of peat bog, the occurrence of pine trunks in peat, and the stratigraphic differences in peat color and peat type (fresh, pale unhumified peat and dark, humified peat - see Figure 2.1c). Dau interpreted the occurrence of pine megafossils as reflecting a phase in his hypothetical forest history of Denmark. Sadly, Dau died two years after the publication of his monograph and he was not able to test his forest-history hypothesis. The occurrence of buried pine trees in Danish peat bogs attracted so much public attention in the 1830s that the Danish Academy of Sciences offered a prize for "solving the problem" about how did pine trees once grow on Danish bogs and what caused the extinction of pine as a native tree in Denmark (Iversen 1973). The Danish zoologist and geologist Japetus Steenstrup (1813-1897; Figure 2.2) won the prize and he proposed (1841) that there had been four periods in Danish forest history - the aspen, pine, oak, and alder periods. Steenstrup emphasized the importance of plant and animal remains preserved in peat bogs as the best available means of investigating past environmental changes, including climate. He tentatively suggested that during the Danish post-glacial there had been changes in moisture and possibly temperature, thereby explaining the observed changes in peat stratigraphy and the occurrence of tree remains in peats. Japetus Steenstrup can thus be regarded as one of the fathers of Holocene paleoecology and climate research.

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Figure 2.1 (a) Fossil pine stumps on Rannoch Moor, western Scotland. These stumps are about 4000 years old (Birks 1975). (b) Fossil pine stump at Cooran Lane, Galloway, south-west Scotland. This stump is about 6000 years old (Birks 1975). (c) The "Grenzhorizont" or

Peat Stratigraphy

major recurrence surface at Chat Moss, Lancashire, northwest England. There is a conspicuous change from dark, humified peat to pale, less decomposed peat. This transition is radiocarbon-dated to about 2600 years ago. (Photographs: John Birks.)

Figure 2.1 (a) Fossil pine stumps on Rannoch Moor, western Scotland. These stumps are about 4000 years old (Birks 1975). (b) Fossil pine stump at Cooran Lane, Galloway, south-west Scotland. This stump is about 6000 years old (Birks 1975). (c) The "Grenzhorizont" or major recurrence surface at Chat Moss, Lancashire, northwest England. There is a conspicuous change from dark, humified peat to pale, less decomposed peat. This transition is radiocarbon-dated to about 2600 years ago. (Photographs: John Birks.)

Christian Vaupell (1821-1862; Figure 2.3) continued investigations on pine megafossils in Denmark. In addition to providing an ecologic explanation for the causes of long-term forest succession, he examined the width of the tree-rings of the buried pines. He concluded (1857) that the pines had not grown in cold conditions, as Steenstrup had proposed in his forest history, but that the pines had grown under warm but dry conditions. Vaupell suggested therefore that there had been changes not only in moisture (to explain the changes in peat stratigraphy) but also in temperature (to explain the changes in tree composition and growth) during the Holocene. The Swedish paleobotanist A.G. Nathorst (1850-1921; Figure 2.4) investigated plant macrofossils in the clays underlying the peat in southern Sweden (Nathorst, 1870) and subsequently elsewhere in Europe (Nathorst, 1892). He was the first to find remains of arctic plants such as Dryas octopetala, Salix polaris, and S. reticulata, plants that today are confined to high

Japetus Steenstrup

Figure 2.2 Japetus Steenstrup (1813-1897), a Danish scientist who pioneered the study of plant and animal remains preserved in peat as a means of reconstructing past environmental change and who provided one of the first explanations for how pine trees could once grow on Danish peat bogs. (With permission from the Royal Library, Denmark.)

Figure 2.2 Japetus Steenstrup (1813-1897), a Danish scientist who pioneered the study of plant and animal remains preserved in peat as a means of reconstructing past environmental change and who provided one of the first explanations for how pine trees could once grow on Danish peat bogs. (With permission from the Royal Library, Denmark.)

latitudes and/or high altitudes. With these discoveries Nathorst transformed peat-stratigraphic and macrofossil research and early ideas about climate change (Holmboe 1921). Temperature change immediately became recognized as the major factor influencing the history of plants and animals since the last Ice Age as a result of Nathorst's macrofossil studies. The final acceptance of the Ice Age theory

Wright The Quaternary Ice Age

Figure 2.4 Alfred Gabriel Nathorst (1850-1921), a Swedish paleobotanist who studied plant macrofossils in late-glacial and Holocene deposits in Scandinavia and elsewhere in Europe. His discovery of the remains of arctic-alpine plants in the clays below Holocene peats highlighted the large changes in temperature that occurred at the onset of the Holocene. (Photographer: Thor G. Halle, 1916. The Swedish Museum of Natural History.)

Figure 2.3 Christian Vaupell (1821-1862), a Danish scientist who pioneered the use of tree-rings in fossil pine stumps as a means of inferring past conditions of temperature and moisture. (Picture from Iversen 1973. Copyright: Geological Survey of Denmark and Greenland.)

Figure 2.4 Alfred Gabriel Nathorst (1850-1921), a Swedish paleobotanist who studied plant macrofossils in late-glacial and Holocene deposits in Scandinavia and elsewhere in Europe. His discovery of the remains of arctic-alpine plants in the clays below Holocene peats highlighted the large changes in temperature that occurred at the onset of the Holocene. (Photographer: Thor G. Halle, 1916. The Swedish Museum of Natural History.)

by geologists at the same time also contributed greatly to climate change occupying a key role in Quaternary research (Iversen 1973).

The idea that there had also been changes in temperature during the Holocene was investigated in detail in Sweden in the early 20th century. Sweden is ideal for such studies as there are marked south-north gradients in summer warmth today and hence well-defined northern geographic limits of many thermophilous plants and animals. Gunnar Andersson (1865-1928) discovered plant macrofossils (seeds, fruits, nuts, leaves, etc.) preserved in peat bogs. Finds of fossil nuts of Corylus avellana (hazel) well north of the present range of hazel suggested that the climate had once been warmer than today (Figure 2.5). The present-day northern limit of hazel coincides closely with the mean July temperature isotherm of 12°C today, whereas fossil nuts occurred as far north as the present-day July isotherm of

Figure 2.3 Christian Vaupell (1821-1862), a Danish scientist who pioneered the use of tree-rings in fossil pine stumps as a means of inferring past conditions of temperature and moisture. (Picture from Iversen 1973. Copyright: Geological Survey of Denmark and Greenland.)

Blytt Sernander Vegetation Succession

Figure 2.5 The present-day distribution (red cross-hatching) of Corylus avellana (hazel) in Denmark and Sweden in relation to the mean July temperature isotherm of 12°C and the distribution of fossil hazel nuts (red dots) of early- or mid-Holocene age. (From Wright 1936. Reproduced with permission of Palgrave Macmillan.)

Figure 2.5 The present-day distribution (red cross-hatching) of Corylus avellana (hazel) in Denmark and Sweden in relation to the mean July temperature isotherm of 12°C and the distribution of fossil hazel nuts (red dots) of early- or mid-Holocene age. (From Wright 1936. Reproduced with permission of Palgrave Macmillan.)

9.5°C, suggesting a change in July temperature of 2-2.5°C. On the basis of these and related investigations, Andersson (1902, 1909) emphasized changes in summer temperature and presented the idea of one long early- to mid-Holocene period with a "higher-than-today" temperature comparable with the modern

Figure 2.6 Axel Blytt (1843-1898), a Norwegian botanist who proposed a series of alternating wet and dry periods based on peat-stratigraphic changes and used these periods to explain the history of the Norwegian flora. (Photograph by Carl St0rmer with permission from the Norwegian Museum of Science and Technology.)

concept of the Holocene thermal maximum. Support for this idea came from discoveries of macrofossils of Cladium mariscus (saw-sedge), Carexpseudocyperus (cyperus sedge), Trapa natans (water-chestnut), Emys orbicularis (European pond tortoise), and other warmth-demanding plants and animals well north of their present northern limits in Europe.

In Norway, Axel Blytt (1843-1898; Figure 2.6), perhaps influenced by the work of Dau (1829), Steenstrup (1841), and Vaupell (1857), interpreted tree layers in peat bogs and changes from dark, humified to pale, fresh peat (Figure 2.1c) as evidence for alterations between dry (continental = Boreal) and wet (oceanic = Atlantic) periods (Blytt, 1876). He proposed an elaborate theory for the immigration of the Norwegian flora and its various floristic elements during these oceanic and continental periods. Blytt assumed that the floristic elements of the present-day flora of Norway (e.g. boreal, atlantic) had immigrated during successive climatic periods, with the arctic element first and the sub-atlantic element last (Mangerud et al. 1974). In light of the present-day distributions of these elements, Blytt proposed that the boreal and sub-boreal elements had immigrated during periods of continental climate, whereas the atlantic and sub-atlantic elements had immigrated during periods of oceanic climate. In this early work Blytt used the terms boreal and atlantic to refer to floristic elements and not to phases within the Holocene. He widened their use in 1893 after Sernander's early publications.

The Swedish botanist Rutger Sernander (1866-1944; Figure 2.7), who had strong interests in both climate history and plant geography (Fries 1950), combined the Swedish ideas of summer-temperature changes with Blytt's (1881) moisture changes to propose the famous Blytt-Sernander four periods of post-glacial time (Sernander, 1893, 1894, 1908, 1909, 1910 - see Table 2.1). Sernander (1889) initially used the concepts of Blytt's "atlantiska period" and "sub-boreala period"

Rutger Sernander

Figure 2.7 Rutger Sernander (1866-1944), a Swedish botanist and Quaternary geologist who expanded Blytt's ideas into the Blytt-Sernander scheme for Holocene climate history (see Table 2.1) that became the major paradigm for Holocene climate until about 1960. (From unknown Internet source that is no longer on line.)

Table 2.1 The Blytt-Sernander division of the Holocene as proposed by Sernander (1890, 1894). The Pre-Boreal was added by Fsgri (1940) for the earliest Holocene with a cool-sub-arctic climate

Period Inferred climate Approximate age (years BP)

Table 2.1 The Blytt-Sernander division of the Holocene as proposed by Sernander (1890, 1894). The Pre-Boreal was added by Fsgri (1940) for the earliest Holocene with a cool-sub-arctic climate

Period Inferred climate Approximate age (years BP)

Sub-Atlantic

Cool, wet

0-2500

Sub-Boreal

Warm, dry

2500-5000

Atlantic

Warmest, wet

5000-8000

Boreal

Warm, dry

8000-10 000

Pre-Boreal

Cool, sub-arctic

>10 000

to link Blytt's floristic terms atlantic, sub-boreal, etc. to Blytt's descriptions of peat stratigraphy in Norway and Blytt's associated climatic interpretations (Mangerud et al. 1974). Later Sernander (1890) used all Blytt's floristic terms for successive time periods, characterized by a different climate based on Blytt's peat investigations. Blytt (1893) then applied his own floristic terms to his peat-stratigraphic layers, thereby introducing the sub-atlantic peat-bed, the sub-boreal stump layer, the atlantic peat-bed, etc. (Mangerud et al. 1974).

The Blytt-Sernander paradigm of Holocene climate change, in the form that later became widespread, was probably fully established by Sernander (1894) in his doctoral thesis at the University of Uppsala (Mangerud 1982). In this remarkable monograph, Sernander first discussed the basis for dating and correlation of geologic sequences. He regarded the stratigraphic sequences based on plant macrofossils

Table 2.2

Sernander's (1894) correlation scheme for the Holocene (modified from

Mangerud 1982)

Sea-level

Peat bogs in southern

Gotland vegetation

Period

Correlations

phase

Scandinavia

history

(Steenstrup,

Nathorst, et al.)

Littorina

Spruce zone

Spruce

Sub-Atlantic

Littorina

Sub-Boreal

transgression

maximum

Oak zone

Oak

Atlantic

Ancylus

Pine zone

Pine Cladium

Boreal

Ancylus Lake

Carex

Sub-Arctic

transgression

pseudocyperus

maximum

Iris pseudacorus

Aspen zone

Pine Northern salix

Betula nana

Dryas

Polar Sea

Dryas zone

Dryas with Salix polaris

Arctic

and megafossils described by Steenstrup (1841) as the basis for Holocene climate change in Scandinavia. However, in contrast to other researchers at that time, Sernander doubted that vegetational changes in, for example, Denmark could be synchronous with changes in, for example, north-central Sweden because plant spread over several hundred kilometers must have required a long time. Sernander thus sought other methods to try to determine the age of these changes. In terms of climate history, Sernander accepted Blytt's observations and interpretations and wrote (Sernander, 1894, p. 71 - translated from Swedish)

"We introduce therefore to the peat bog-sequences in Gotland the terms of Blytt, from the top downwards: sub-atlantic, sub-boreal, atlantic, and boreal beds, and we will also use these names for the time periods corresponding to the beds."

An approximate chronology for these periods was proposed based on correlations with archaeology, sea-level changes (Littorina and Ancylus transgressions), and the Swedish varve chronology of Gerard de Geer (1858-1943) by Sernander (1894) (see Table 2.2). The change from the Sub-Boreal to the Sub-Atlantic was thought by Sernander to be an abrupt climate change, even a catastrophe, the Fimbulwinter of the Sagas (Iversen 1973).

The 1910 International Geologic Congress in Stockholm was, according to Knut F^gri (personal communication 1995), a major event in the dissemination and acceptance of the Blytt-Sernander classification scheme. Interestingly there was close co-operation at this time between Sernander and Lennart von Post (the founder of pollen analysis - see below) and they organized a major excursion to Swedish peat bogs (von Post and Sernander 1910). After 1910, the Blytt-Sernander scheme was widely used in Scandinavia for many decades without any major change in its meaning or interpretation (Mangerud et al. 1974; Mangerud 1982). F^gri (1940) introduced the term Pre-Boreal as a unit between the Boreal and the Younger Dryas of the late-glacial (Table 2.1).

A major conflict and acrimonious debate ensued for about 20 years between the "grand old men" (Lundqvist 1965) of Swedish post-glacial climate research, namely Gunnar Andersson and Rutger Sernander. A rapid polarization of ideas developed between the Blytt-Sernander scheme with its alternating dry and wet periods and rapid climate change and Andersson's more uniform, gradually rising temperature curve, a thermal maximum, and subsequent decrease. The debate was partly one of scientific personalities (Fries 1950; Danielsen et al. 2000) and partly one about research techniques (Iversen 1973). Andersson's approach was based exclusively on macrofossils whereas the Blytt-Sernander scheme was based on peat stratigraphy and megafossils. The conflict between the two Swedish schools was not satisfactorily resolved until another Swede, Lennart von Post (1946), proposed that post-glacial climate history involved both broad-scale temperature changes (Andersson) and finer-scale precipitation changes (Blytt-Sernander). This new paradigm did not develop until the development of pollen analysis, which will be discussed in the next section.

As part of the Andersson-Sernander debate, Samuelsson (1916) analyzed the northern limit of hazel in Sweden in considerable detail and showed that summer temperature was far from uniform along the limit today. He showed that a lower summer temperature could be compensated for by a longer growing season, and he modeled the climatic demands of hazel in terms of both summer temperature and the length of the growing season. He proposed that both summer and winter temperatures, and hence the length of the growing season, may have changed during the Holocene. This idea was followed up in detail by Iversen (1944) using pollen analysis and by Hintikka (1963) in plant geography.

During the early part of the 20th century, the Blytt-Sernander scheme became the dominant paradigm for Holocene climate history and many peat and megafossil stratigraphies in Scotland, central Europe, and the Alps were interpreted in terms of the Blytt and Sernander model (e.g. Samuelsson 1910; Gams and Nordhagen 1923). As peat stratigraphies were examined in more detail, several recurrence surfaces (changes from dark, humified to fresh, unhumified peat -Figure 2.1c) were identified by Granlund (1932), suggesting several shifts in moisture during the late Holocene. Granlund proposed at least two "Sub-Atlantic" climate phases during Sernander's Sub-Boreal period and after Sernander's Fimbulwinter the climate changed twice over to the Sub-Boreal type and then again to the Sub-Atlantic. Von Post (1946) summarized Granlund's (1932) modification of the Blytt-Sernander paradigm as follows:

"Granlund's work showed that the post-glacial warm period did not end abruptly with a climatic catastrophe about 500 bc, as supposed by Sernander; but that instead of this event we must suppose a gradually advancing climatic deterioration spread over at least 4000 years. This process was, however, not continuous. Just in the same way as, during autumn, summer gradually changes to winter during repeated alternations between days of summer warmth and wintry cold, so the climate falls and rises from the warm period to the present time, through the bronze age, iron age, and our so-called historic time, swaying between fimbul winters and phases which were echoes of the glorious climate of the stone age."

In other words, the accepted paradigm by the 1930s based on peat stratigraphy and megafossils was of alternating wet and dry phases but perhaps with increased variability and several short-lived climate shifts in the late Holocene and with a mid-Holocene warm period.

Independently of the Scandinavian botanists and peat stratigraphers, the paradigm of a Holocene climate optimum or thermal maximum had already been presented by Thomas F. Jamieson (1829-1913; Figure 2.8) in his study of the fossil molluscan fauna of mid-Holocene estuarine clays in central Scotland (Wright 1936). Jamieson found species that do not occur as far north as Scotland today. The Irish naturalist Robert Lloyd Praeger (1865-1963) reached similar conclusions in his studies of estuarine clays and raised beaches in Northern Ireland, as did W.C. Br0gger (1900/01) in the Oslo Fjord (Wright 1936). Mitchell (1976) discusses whether W.B. Wright's (1936) dedication in his second edition of The Quaternary Ice Ages to "R. Lloyd Praeger, Discoverer of the Climatic Optimum" is appropriate, given Jamieson's pioneer work in 1865 that Praeger cites frequently in his Irish studies of 1887 and 1892. Perhaps credit for the discovery of the climatic optimum should be given to T.F. Jamieson in 1865, and not to Praeger, Br0gger, or Andersson.

In retrospect, the advances made by Dau, Steenstrup, Vaupell, Andersson, Jamieson, Praeger, Blytt, Sernander, Br0gger, and Granlund in reconstructing

Heinrich Dau 1790 1831

Figure 2.8 Thomas F. Jamieson (1829-1913), a Scottish geologist who discovered the mid-Holocene climatic optimum or thermal maximum in his investigations of Mollusca preserved in estuarine clays in central Scotland. (Picture from www.fettes.com/(Artist anknown.))

Holocene climate change were very remarkable, given the very limited range of techniques then available. A flavor of the excitement of these early investigations based around peat stratigraphy and macrofossils can be obtained from Godwin (1978, 1981) and Mitchell's (1990) autobiographies. It was not until the development of pollen analysis that the different views of Andersson and Sernander could be satisfactorily reconciled and the Holocene climate history reconstructed for other parts of the world where peat stratigraphy is not always possible. I will return to modern detailed studies of peat stratigraphy later when I discuss paleolimno-logy, paleohydrology, and evidence for hydrologic changes during the Holocene.

The study of peat stratigraphy in Scandinavia was largely ignored after the development of pollen analysis as a tool for reconstructing Holocene climate change. It was not until the mid-1970s that peat stratigraphy was revived in Scandinavia using new techniques for estimating peat humification, detecting changes in bog moisture using testate amoebae, and establishing detailed chronologies by radiocarbon dating (e.g. Aaby and Tauber 1975; Aaby 1976). These new studies suggested consistent shifts from dry to moist conditions with a periodicity of260 years over the past 5500 years.

The study of plant macrofossils and megafossils in north-west Europe was similarly ignored for about 70 years. Hilary Birks (1975) re-examined buried pine stumps (Figure 2.1) in Scottish blanket bogs using pollen analysis, peat stratigraphy, plant macrofossils, and radiocarbon dating and showed that the temporal patterns of pine stumps were considerably more complex than predicted from the application of the Blytt-Sernander scheme to Scottish peats by Samuelsson (1910). In recent decades Kullman (e.g. 1998a,b 2000, 2002) has studied tree megafossils and macrofossils in several areas of the Swedish mountains and has made spectacular discoveries of Picea and Pinus remains in the early Holocene and the late-glacial, of thermophilous trees in the early Holocene, and of Larix sibirica in the Swedish Mountains in the early Holocene. Larix sibirica today is found 1000 km to the east in Russia. The finds of its cones and wood (Kullman 1998a) suggest major changes in its distribution since the early Holocene and its extinction from Fennoscandia. These studies and others (see Birks 2005; Birks et al. 2005) emphasize the importance of integrated studies involving detailed plant macrofossil and megafossil analyses, pollen analysis, lithostratigraphic investigations, and radiocarbon dating if the complex patterns of tree spreading and climate change in the early Holocene are to be unraveled.

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Responses

  • daisy
    Which historian discovered holocene?
    8 years ago
  • vito
    What can tree remains in peat bogs tell us about past climates?
    8 years ago

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