Paleopedology

Last updated

Paleopedology (palaeopedology in the United Kingdom) is the discipline that studies soils of past geological eras, from quite recent (Quaternary) to the earliest periods of the Earth's history. Paleopedology can be seen either as a branch of soil science (pedology) or of paleontology, since the methods it uses are in many ways a well-defined combination of the two disciplines.

Contents

History

Paleopedology's earliest developments arose from observations in Scotland circa 1795 whereby it was found that some soils in cliffs appeared to be remains of a former exposed land surface. During the 19th century there were many other finds of former soils throughout Europe and North America. However, most of this was only found in the search for animal and/or plant fossils, and it was not until soil science first developed that buried soils of past geological ages were considered of any value.

It was only when the first relationships between soils and climate were observed in the Eurasian Steppe that there was any interest in applying the finds of former soils to past ecosystems. This occurred because, by the 1920s, some soils in Russia had been found by K.D. Glinka that did not fit with present climates and were seen as relics of warmer climates in the past.

In 1892 Eugene W. Hilgard had related soil and climate in the United States in the same manner, and by the 1950s analysis of Quaternary stratigraphy to monitor recent environmental changes in the northern hemisphere had become firmly established. These developments have allowed soil fossils to be classified according to USDA soil taxonomy quite easily with all recent soils. Interest in earlier soil fossils was much slower to grow but has steadily developed since the 1960s owing to the development of such techniques as X-ray diffraction which permit their classification. This has allowed many developments in paleoecology and paleogeography to take place because soil chemistry can provide a good deal of evidence as to how life moved onto land during the Paleozoic era.

Finding soil fossils and their structure

Remains of former soils can either be found under deposited sediment in unglaciated areas or in extremely steep cliffs where the old soil can be seen below the younger present-day soil. In cases where volcanoes have been active, some soil fossils occur under the volcanic ash. If there is continued deposition of sediment, a sequence of soil fossils will form, especially after the retreat of glaciers during the Holocene. Soil fossils can also exist where a younger soil has been eroded (for instance by wind), as in the Badlands of South Dakota. (One must exclude areas where present-day soils are relics of former wetter climates, as with Australia and southern Africa. The soils of these regions are proper paleosols .)

Soil fossils, whether buried or exposed, suffer from alteration. This occurs largely because almost all past soils have lost their former vegetative covering, and the organic matter they once supported has been used up by plants since the soil was buried. However, if remains of plants can be found, the nature of the soil fossil can be made a great deal clearer than if no flora can be found because roots can nowadays be identified with respect to the plant group from which they come. Patterns of root traces including their shape and size, is good evidence for the vegetation type the former soil supported. Bluish colours in the soil tend to indicate the plants have mobilized nutrients within the soil.

The horizons of fossil soils typically are sharply defined only in the top layers, unless some of the parent material has not been obliterated by soil formation. The kinds of horizons in fossil soils are generally the same as those found in present-day soils, allowing easy classification in modern taxonomy of all but the oldest soils.

Analysis

Vertisol paleosol Watervol Onder. VertisolpaleosolWatervalOnder.jpg
Vertisol paleosol Watervol Onder.
Mollisol in Dayville Oregon. MollisolDayvilleOregon.jpg
Mollisol in Dayville Oregon.

Chemical analysis of soil fossils generally focuses on their lime content, which determines both their pH and how reactive they will be to dilute acids. Chemical analysis is also useful, usually through solvent extraction to determine key minerals. This analysis can be of some use in determining the structure of a soil fossil, but today X-ray diffraction is preferred because it permits the crystal structure of the former soil to be determined.

With the aid of X-ray diffraction, paleosols can now be classified into one of the 12 orders of soil taxonomy (Oxisols, Ultisols, Alfisols, Mollisols, Spodosols, Aridisols, Entisols, Inceptisols, Gelisols, Histosols, Vertisols and Andisols). Many Precambrian soils, however, when examined do not fit the characteristics for any of these soil orders and have been placed in an order called green clays. The green colour is due to the presence of certain unoxidised minerals found in the primitive Earth because O2 was not present. There are also some forest soils of more recent times that cannot clearly be classified as alfisols or as spodosols because, despite their sandy horizons, they are not nearly acidic enough to have the typical features of a spodosol.

Importance

Paleopedology is an important scientific discipline for the understanding of the ecology and evolution of ancient ecosystems, both on Earth and the emerging field of exoplanet research, or astropedology. In geochemistry, a knowledge of the structure of former soils is also valuable to understand the composition of paleo continents.

Models

The different definitions applied to soils are indicative of the different approaches taken to them. Where farmers and engineers concentrate on certain soil properties, soil scientists have a different view. [1] Essentially, these differing views of the definition of soil are different theoretical bases for their study. [2] Soils can be thought of as open systems in that they represent a boundary between the Earth and the atmosphere where materials are transported and are changed. There are four basic types of flux: additions, subtractions, transfers, and transformations. [3] [4] Examples of addition can include mineral grains and leaf litter, while subtractions can include surficial erosion of minerals and of organic matter. Transfers include the movement of a material within a soil profile, and transformations are the change of composition and form of the materials within a soil.

Soils can also be considered to be energy transformers in that they are physical structures of material that are modified by naturally occurring processes. The Sun constitutes the primary energy source for soils and significantly outweighs any heat generated by radioactive decay flowing up from deep within the Earth's crust. The deposition of sediment, or the addition of groundwater or rain, can also be considered an energy gain because new minerals and water can alter preexisting materials within the soil. These processes, coupled with the amount of energy available to fuel them, are what create a soil profile.

Another way to view soils is that they are environmental products that are molded over a period of time from the materials available to them. The large amount of influences that effect the formation of soils can be simplified to five main factors: climate, organisms, topographic relief, parent material, and time. [5] [6] These five factors can be easily remembered using the acronym "CLORPT". These categories are useful for mentally considering that aspects that occurred during the formation of a soil or paleosol. More importantly however, CLORPT allows for a theoretical framework when creating natural experiments for the study of soil formation. [2]

Climate

When soil science was first founded, climate was considered one of the most important factors regarding the formation of soil. For instance, temperate regions have widespread acidic sand spodosols, and in tropical regions red clayey oxisols are common. The tendency to use climatic data for the classification of soils has been challenged by efforts to base the classification of soil on observable features within the soils. However, paleoclimates cannot be interpreted from paleosols identified using paleoclimatic data. The identification of paleosols using climatic data is changing. For example, aridisols have been redefined [7] as soils that possess a calcic horizon of less than 1 meter in depth.

Soil climate is also a special kind of microclimate. It refers to the moisture, temperature, and other climatic indicators that are found within the pores of soil. For example, in well-drained soils, the soil climate is a somewhat subdued version of the regional climate. In waterlogged soils, soil climate is not related to regional climate because the temperature and oxygenation of waterlogged soils is more dependent on local groundwater paths and rates than on atmospheric conditions. Estimates of other types of soil climate are now beginning to find their way into the classification of soils, the models for soil formation, and into the study of soil biology.

The classification of climate from paleosols can be related using climatically sensitive features of soils that are sensitive to particular climatic variables, but even the best of these features lack precision. This is because soils are not as sensitive as meteorological instruments for recording climatic conditions. However, in a fairly broad category, climate can be interpreted from the sensitive features found in soils. One of the most large-scale influences regarding the classification of climate was created in 1918, then modified over two decades by the German meteorologist Vladimir Köppen. [8] He proposed there are five main climate groups (Köppen climate classification), each corresponding to the main types of terrestrial vegetation. Each climate type is designated by letters, with upper-case letters referring to the main climate groups and lower-case letters referring to subsidiary climatic features. [2]

Organisms

Bee nest ichnofossils from Wyoming. Beenestwyoming.jpg
Bee nest ichnofossils from Wyoming.

Large plants are only part of the organisms that play a role in soil formation. For example, fungi are closely associated with the roots of many vascular plants by making available nutrients like nitrogen and phosphorus in a way that their host plants can utilize, and play an important role in returning organic matter to the soil by decomposing leaf litter. The list of organisms that interact with and affect soil is extensive, and it is these interactions that allow for the presence of paleosols to be inferred.

Not only can particular organisms be interpreted from paleosols, but also ancient ecosystems. The soil interaction of plants is different from community to community. They each have distinct patterns of root traces, soil structure, and overall profile form. Identifying these features is useful for providing an overall assessment of the influence past organisms had on any particular paleosol. However, qualifying these general effects of organism activity can be difficult because the level of their expression is as related to their nature as it is to the amount of time available for soil formation. Even when fossils that are found in paleosols are understood, much more can be learned regarding their preservation, ecology, and evolution by studying the paleosols they inhabited.

Fossil stumps in a paleosol. Fossilstumpsinpaleosol.jpg
Fossil stumps in a paleosol.

A fossilized footprint, burrow, or coprolite (fossil feces), are examples of trace fossils (ichnofossils). These trace fossils do not represent any physical part of an organism, but rather are evidence of an organism's activity within its environment. Whereas a bone, leaf, or stem might provide enough information to positively identify a particular species, trace fossils rarely allow for such a precise identification. However, unlike fossilized body parts which can be affected by many variables, trace fossils are not often transported away and are usually found in the place where the organism lived. This advantage makes trace fossils in paleosols especially important because they allow for interpretation of the animal's behavior in its natural environment. A great example of this is the simple shallow fossilized burrows of solitary bees that make their homes in soil.

Just as fossilized footprints, burrows, and coprolites represent trace fossils or organisms, paleosols can be considered trace fossils of an ancient ecosystem. Much like the small percentage of species that are fossilized, very few species within an ecosystem leave any discernible trace in paleosols. However, their more general effects within a paleosol may be preserved. A good example of this is root traces. Analyzing the pattern of root traces, the sequence of soil horizons, and other features can help identify the type of vegetation that was present during the formation of the soil.

General features such as stature and spacing determine what botanists call a "plant formation." Distinct from a community or association, plant formation is not defined by any particular species. Examples of plant formation include forests, woodlands, and grasslands. Because it may not be possible to determine whether a particular plant was an oak, eucalyptus, or other species, plant formations in paleosols make it possible to identify an ancient woodland ecosystem from an ancient grassland ecosystem. [2]

Topographic Relief

The nature of soils will vary with topography, which can be understood by comparing the thin rocky soils of mountain tops to the thick fertile soils of grass-covered lowlands. Even in a featureless lowland, the nature of a soil will vary greatly depending on whether or not it is well drained; although the drainage of soil is not completely independent because vegetation, microclimate, and the age of the land surfaces will vary within a given landscape. However, in smaller areas, the limiting factors may be so extensive that a variation in soils across a landscape will constitute a true topographical sequence, and the features within these soils can yield reliable topographic functions.

Bold landscapes like alpine ridges and peaks can be resolved based on distinct slope-related processes. For example, steep alpine slopes have sparse vegetation with soils that are eroded by snow melt, agitated by frost heave, and impacted by rock fall. These processes create thin, shallowly rooted, lightly weathered and rocky soils that are indicative of a mountain slope environment. The size and degree of these processes do not allow for strict analysis as topographic functions because of the extensive variation in climate, vegetation, parent materials, and land surface age at different elevations on a mountainside. [2]

Parent Material

The rock or sediment associated with a soil's development is referred to as its parent material, which is the starting point for the process of soil formation. During early formation, soils are not so different from their parent materials. With time however, soils will contain less features of their original parent material. In order to make an accurate assessment of the amount of soil formation that has occurred, the parent material must be known to establish a base line, or starting point in the soil's formation.

Igneous parent material. Parentmaterialigneous.tif
Igneous parent material.

In most instances, parent material is independent of soil formation. The formation of igneous rocks and metamorphic rocks occurs in locations and by processes below the surface of the Earth. These rocks are often the parent material for soils and are sometimes derived from soils, but the degree of sedimentary sorting and distribution varies so widely that these are also considered to be independent of soils.

Sedimentary parent material. Parentmaterialsedimentary.tif
Sedimentary parent material.

Very few parent materials associated with soils are entirely uniform in their composition or structure. Frequently, there is some degree of irregularity including foliation, veining, jointing, or layering that in some cases helps with soil formation and in other cases hampers it. For example, some sedimentary layering promotes the formation of soil such as a silty cover on bedrock, or a sandy cover on a clayey alluvium layer. In both of these cases, a friable surface material has been established by nonpedogenic instances. Other instances of sedimentary surface cementation, or fine interbedded sequences of clay and sand, could be considered to be not conducive to the formation of a soil. Nonuniform parent materials may be difficult to find in soils and paleosols, although deviations from normally found minerals could lend clues to the original parent material. If grains of primary materials are not found in the parent material, it can be inferred that later additions occurred. For example, quartz is not found in basaltic phonolite, and olivine is not found in granite.

The role of parent material is best understood from studies of soils that formed under similar conditions on different parent materials or lithosequences (differing soil profile characteristics because of differing parent materials). This provides a starting point for understanding what role the parent material played during the formation of the soil. The generalized relationships obtained from these studies can be used to determine what effects the parent material had on the paleosol during its formation. The difficulty lies with the fact that the parent material no longer exists, and therefore its nature can only be estimated using nearby materials.

These estimates are typically based on four critical assumptions that should be recognized as assumptions and thus assessed cautiously when evaluating soils and paleosols. These four simplifying assumptions allow for a detailed analysis of the changes that occur during the formation of a soil and the burial of a soil. [2]

  1. The first assumption is that the parent material is fresh. This means that the parent material assumed to be a proxy for the original parent material must be both chemically and physically similar to that original material. For example, saprolite cannot be considered to be an accurate representation of a parent material derived from a forested soil on granite, but it could be representative of a cultivated soil formed after a clear-cut and erosion of a forested soil.
  2. The second assumption is that the parent material had a uniform composition within the soil profile. If the properties of the material found below the profile are to be considered representative of the parent material of the entire profile, this must be true. However, this is difficult considering that few rocks or sediments are uniform enough to be considered an accurate representation of the original parent material. For example, it is extremely difficult to detect a thin layer of windblown dust on top of granite within a thick clayey soil.
  3. The third assumption is that at least one of the constituents of the parent material is unaltered by weathering and is still present. The main problem with this is that no constituents are fully immune to the broad weathering processes that exist in nature. [9] For example, quartz is a fairly stable mineral in soils with pH>9, while alumina (Al2O3) is stable in between pH 4.5 and 8 (mostly in clay). Trace elements that are usually stable in soils over a wider range of environmental conditions include lead (Pb) and zirconium (Zr) but are not always sufficiently present to be useful.
  4. The fourth assumption is that volume change is proportional to thickness and density. This states that the loss of soil volume and the degree of compaction during burial are related to their density or thickness change. Although common sense suggests that volume and density are three dimensional and thickness is one dimensional, observations on various materials, including fossil plants of known shape [10] show that while under conditions of static vertical load, soils and fossils are maintained by pressure at the side.

See also

Related Research Articles

<span class="mw-page-title-main">Ecosystem</span> Community of living organisms together with the nonliving components of their environment

An ecosystem is a system that environments and their organisms form through their interaction. The biotic and abiotic components are linked together through nutrient cycles and energy flows.

<span class="mw-page-title-main">Fossil</span> Preserved remains or traces of organisms from a past geological age

A fossil is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in amber, hair, petrified wood and DNA remnants. The totality of fossils is known as the fossil record.

<span class="mw-page-title-main">Sedimentary rock</span> Rock formed by the deposition and cementation of particles

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

<span class="mw-page-title-main">Pedology</span> Study of soils in their natural environment

Pedology is a discipline within soil science which focuses on understanding and characterizing soil formation, evolution, and the theoretical frameworks for modeling soil bodies, often in the context of the natural environment. Pedology is often seen as one of two main branches of soil inquiry, the other being edaphology which is traditionally more agronomically oriented and focuses on how soil properties influence plant communities. In studying the fundamental phenomenology of soils, e.g. soil formation, pedologists pay particular attention to observing soil morphology and the geographic distributions of soils, and the placement of soil bodies into larger temporal and spatial contexts. In so doing, pedologists develop systems of soil classification, soil maps, and theories for characterizing temporal and spatial interrelations among soils. There are a few noteworthy sub-disciplines of pedology; namely pedometrics and soil geomorphology. Pedometrics focuses on the development of techniques for quantitative characterization of soils, especially for the purposes of mapping soil properties whereas soil geomorphology studies the interrelationships between geomorphic processes and soil formation.

Soil formation, also known as pedogenesis, is the process of soil genesis as regulated by the effects of place, environment, and history. Biogeochemical processes act to both create and destroy order (anisotropy) within soils. These alterations lead to the development of layers, termed soil horizons, distinguished by differences in color, structure, texture, and chemistry. These features occur in patterns of soil type distribution, forming in response to differences in soil forming factors.

<span class="mw-page-title-main">Ecological succession</span> Process of change in the species structure of an ecological community over time

Ecological succession is the process of change in the species that make up an ecological community over time.

<span class="mw-page-title-main">Petrified wood</span> Fossilized remains of plants

Petrified wood, also known as petrified tree, is the name given to a special type of fossilized wood, the fossilized remains of terrestrial vegetation. Petrifaction is the result of a tree or tree-like plants having been replaced by stone via a mineralization process that often includes permineralization and replacement. The organic materials making up cell walls have been replicated with minerals. In some instances, the original structure of the stem tissue may be partially retained. Unlike other plant fossils, which are typically impressions or compressions, petrified wood is a three-dimensional representation of the original organic material.

<span class="mw-page-title-main">Oxisol</span> Soil type known for occurring in tropical rain forests

Oxisols are a soil order in USDA soil taxonomy, best known for their occurrence in tropical rain forest within 25 degrees north and south of the Equator. In the World Reference Base for Soil Resources (WRB), they belong mainly to the ferralsols, but some are plinthosols or nitisols. Some oxisols have been previously classified as laterite soils.

<span class="mw-page-title-main">Florissant Formation</span> National monument in the United States

The Florissant Formation is a sedimentary geologic formation outcropping around Florissant, Teller County, Colorado. The formation is noted for the abundant and exceptionally preserved insect and plant fossils that are found in the mudstones and shales. Based on argon radiometric dating, the formation is Eocene in age and has been interpreted as a lake environment. The fossils have been preserved because of the interaction of the volcanic ash from the nearby Thirtynine Mile volcanic field with diatoms in the lake, causing a diatom bloom. As the diatoms fell to the bottom of the lake, any plants or animals that had recently died were preserved by the diatom falls. Fine layers of clays and muds interspersed with layers of ash form "paper shales" holding beautifully-preserved fossils. The Florissant Fossil Beds National Monument is a national monument established to preserve and study the geology and history of the area.

USDA soil taxonomy (ST) developed by the United States Department of Agriculture and the National Cooperative Soil Survey provides an elaborate classification of soil types according to several parameters and in several levels: Order, Suborder, Great Group, Subgroup, Family, and Series. The classification was originally developed by Guy Donald Smith, former director of the U.S. Department of Agriculture's soil survey investigations.

<span class="mw-page-title-main">Alfisol</span> Soil type

Alfisols are a soil order in USDA soil taxonomy. Alfisols form in semi-arid to humid areas, typically under a hardwood forest cover. They have a clay-enriched subsoil and relatively high native fertility. "Alf" refers to aluminium (Al) and iron (Fe). Because of their productivity and abundance, Alfisols represent one of the more important soil orders for food and fiber production. They are widely used both in agriculture and forestry, and are generally easier to keep fertile than other humid-climate soils, though those in Australia and Africa are still very deficient in nitrogen and available phosphorus. Those in monsoonal tropical regions, however, have a tendency to acidify when heavily cultivated, especially when nitrogenous fertilizers are used.

<span class="mw-page-title-main">Paleosol</span> Soil buried under sediment or not representative of current environmental conditions

In geoscience, paleosol is an ancient soil that formed in the past. The definition of the term in geology and paleontology is slightly different from its use in soil science.

The paleopedological record is, essentially, the fossil record of soils. The paleopedological record consists chiefly of paleosols buried by flood sediments, or preserved at geological unconformities, especially plateau escarpments or sides of river valleys. Other fossil soils occur in areas where volcanic activity has covered the ancient soils.

<span class="mw-page-title-main">Brown earth</span> Soil type

Brown earth is a type of soil. Brown earths are mostly located between 35° and 55° north of the Equator. The largest expanses cover western and central Europe, large areas of western and trans-Uralian Russia, the east coast of America and eastern Asia. Here, areas of brown earth soil types are found particularly in Japan, Korea, China, eastern Australia and New Zealand. Brown earths cover 45% of the land in England and Wales. They are common in lowland areas on permeable parent material. The most common vegetation types are deciduous woodland and grassland. Due to the reasonable natural fertility of brown earths, large tracts of deciduous woodland have been cut down and the land is now used for farming. They are normally located in regions with a humid temperate climate. Rainfall totals are moderate, usually below 76 cm per year, and temperatures range from 4 °C in the winter to 18 °C in the summer. They are well-drained fertile soils with a pH of between 5.0 and 6.5.

<span class="mw-page-title-main">Paleolimnology</span> Scientific study of ancient lakes and streams

Paleolimnology is a scientific sub-discipline closely related to both limnology and paleoecology. Paleolimnological studies focus on reconstructing the past environments of inland waters using the geologic record, especially with regard to events such as climatic change, eutrophication, acidification, and internal ontogenic processes.

Rhizoliths are organosedimentary structures formed in soils or fossil soils (paleosols) by plant roots. They include root moulds, casts, and tubules, root petrifactions, and rhizocretions. Rhizoliths, and other distinctive modifications of carbonate soil texture by plant roots, are important for identifying paleosols in the post-Silurian geologic record. Rock units whose structure and fabric were established largely by the activity of plant roots are called rhizolites.

<span class="mw-page-title-main">Gregory Retallack</span> American paleontologist

Gregory John Retallack is an Australian paleontologist, geologist, and author who specializes in the study of fossil soils (paleopedology). His research has examined the fossil record of soils though major events in Earth history, extending back some 4.6 billion years. Among his publications he has written two standard paleopedology textbooks, said N. Jones in Nature Geoscience "Retallack has literally written the book on ancient soils."

A relict, in geology, is a structure or mineral from a parent rock that did not undergo metamorphic change when the surrounding rock did, or a rock that survived a destructive geologic process.

The term humus form is not the same as the term humus. Forest humus form describes the various arrangement of organic and mineral horizons at the top of soil profiles. It can be composed entirely of organic horizons, meaning an absence of the mineral horizon. Experts worldwide have developed different types of classifications over time, and humus forms are mainly categorized into mull, mor, and moder orders in the ecosystems of British Columbia. Mull humus form is distinguishable from the other two forms in formation, nutrient cycling, productivity, etc.

References

  1. Johnson, D. & Watson-Stegner, D. (1987). "Evolution model of pedogenesis". Soil Science. 143 (5): 349–366. Bibcode:1987SoilS.143..349J. doi:10.1097/00010694-198705000-00005. S2CID   140140410.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  2. 1 2 3 4 5 6 Retallack, Gregory J. (2001). Soils of the Past: An introduction to paleopedology (2nd ed.). Malden, MA: Blackwell Science. pp.  171–172, 180–182. ISBN   9780632053766.
  3. Simonson, R.W. (1978). A multiple-process model of soil genesis. Norwich: Geoabstracts. pp. 1–25.
  4. Anderson, D.W. (1988). "The effect of parent material and soil development on nutrient cycling in temperate ecosystems". Biogeochemistry. 5: 71–97. doi:10.1007/bf02180318. S2CID   95971825.
  5. Jenny, H.J. (1941). Factors in Soil Formation. New York: McGraw-Hill.
  6. Buol, S.W. (1997). Soil Genesis and Classification (4th ed.). Ames: Iowa State University Press.
  7. Soil Survey Staff (1998). Keys to Soil Taxonomy. Blacksburg, VA: Pocahontas Press.
  8. Trewartha, G.T. (1982). Earth's Problem Climates. Madison, WI: University of Wisconsin Press.
  9. Gardner, L.R. (1980). "Mobilization of Al and Ti during weathering - isovolumetric chemical evidence". Chemical Geology. 30 (1–2): 151–165. Bibcode:1980ChGeo..30..151G. doi:10.1016/0009-2541(80)90122-9.
  10. Walton, J. "On the factors which influence the external form of fossil plants; with description of some species of the Paleozoic equisetalean genus Annularia Sternberg". Philosophical Transactions of the Royal Society of London. Series B, 226: 219–237. doi: 10.1098/rstb.1936.0008 .