Course:EOSC311/2020/Studying ancient ecosystems with paleosols

Paleosols are fossil soils that have been preserved through lithification, and are becoming increasingly recognized by the geoscience community as an effective tool for reconstructing paleoenvironments. The study of paleosols is called "paleopedology". Research in paleopedology has traditionally been carried out more qualitatively, mostly by comparison to modern soil analogues, but recent development in quantitative modeling has allowed for new proxies for reconstructing terrestrial paleoenvironments. Since soil formation (pedogenesis) involves local interactions of multiple different environmental factors, paleosols can provide an increasingly accurate picture of past ecosystems.

Statement of connection and why you chose it

Global Resource Systems allows students within the Faculty of Land and Food Systems to create a course schedule that caters to the study of a specific resource and region. The resource that I have chosen to specialize in pertains to soil science, more specifically, soil ecology. From a young age I have been interested in paleontology, and studying paleosols allows me to unite my interests in soil science, ecology and paleontology together.

The Doctrine of Uniformity and Modern Soils

Paleopedologists apply the Doctrine of Uniformity when studying paleosols to infer the origins of their character. The Doctrine of Uniformity posits that the geologic forces of the present created paleoenvironments by the same mechanisms observed today. In the words of the first proponent, James Hutton, "The present is the key to the past." Since soils are the interface between rocks, air, water, and life, understanding how these forces create soil in the present allows for the ease of application to paleosols.

Soil Formation and Pedogenesis

Soils are defined as dynamic natural bodies that result from climatic and biotic activities acting on parent material along a certain relief over time. Contained within this definition are the five factors that soil scientists agree are involved with pedogenesis (soil formation). These factors control the rates and types of weathering that foster pedogenesis, and by extension foster the unique environments supported by soils. Paleopedologists study paleosols to understand the influence of these factors on the landscape, and thereby paint a picture of past environments.

The five soil forming factors in this diagram interact to create soil. The factors were devised in the 19th century by V.V. Dukochaev.

Weathering

As stated previously, topography, climate and biota act upon parent material over time to create soil, but they specifically do so by controlling the rate and type of weathering experienced by the soil. Weathering is defined as the destruction of exposed parent material (rocks and sediments) through physical disintegration and chemical decomposition. Types of weathering are denoted by the form it takes, and involve interactions between the five factors. Weathering is not a linear process, but an exponential one, meaning that weathering leads to more weathering.

Physical Weathering

Soil are primarily composed of fine-sized particles that result from the disintegration of parent materials. Three primary forms of physical weathering act upon rocks to transform them into soil parent material:

Exfoliation involves exposed rock expanding and breaking. The expansion can be brought upon by temperature fluxes, such as repeated fires, or due to pressure change from the removal of outer rock layers.

Wedging involves cracks in rocks are made larger by either the freeze-thaw cycles of ice (deemed "ice wedging"), or the evaporation and precipitation of soluble salts ("Salt wedging").

Erosion is the removal and transportation of weathered products. It is the most observable and temporally influential of the physical weathering processes, as it accelerates the rate of physical weathering. It takes many forms, with many different names, such as aeolian (wind), colluvial (gravity), alluvial (running water), and glacial. Deposition is the inverse process, where material carried by erosion is left in place, and allows for other pedogenic processes to create soil.

Chemical Weathering

Chemical weathering results in the chemical decomposition of parent material into ions or smaller chemically distinct minerals. This process is largely controlled by climate, which determines the precipitation, atmospheric composition and temperature of a region. Chemical weathering is responsible for transforming primary minerals into secondary minerals, most notably clays. There are four essential processes that are responsible for observable chemical weathering in soils, and usually encapsulate two reactions that are reversible with each other

Dissolution/Precipitation describes either the breakdown of a mineral into its component ions (the former) or the reformation of the mineral from its component ion (the latter). This requires the presence (or absence) or liquid water and is catalyzed by temperature. Not all parent materials dissolve evenly, which leads to another chemical weathering process known as hydrolysis. Hydrolysis is distinguished from dissolution in that dissolution creates two free ions in the soil solution, whereas hydrolysis creates secondary minerals. For example, the hydrolysis of primary aluminosilicate minerals results in the formation of clay minerals, which are more chemical stable and important for soil fertility and cation retention. Clay mineralogy is important to paleopedological analysis as it serves as a tell-tale indicator of the rate of hydrolysis in paleoenvironments.

Oxidation/Reduction involves the loss (in the former) or gain (in the latter) of an electron in an element, which requires another element present to do the inverse. This process usually involves the presence (or absence) of oxygen as the receiving atom (hence the name). In soils that experience frequent waterlogged conditions, the absence of vaporous oxygen creates reducing conditions, leading to the reduction of ferric iron (Fe³⁺) to ferrous iron (Fe²⁺). This process creates greenish or grey mottles (areas characterized by colors different from the main soil color), which can be described as gleyed soils. Oxidation/reduction holds tantamount importance for paleopedology, as it imbues paleosol rhizoliths with distinct colors which provide clues pertaining to the conditions of their formation.

Hydration/Dehydration involves the incorporation of water molecules into the mineral structure. A classic example is the creation of gypsum from anhydrite and vice versa. Hydration/Dehydration results in changed volume occupied by a mineral, which may accelerate physical weathering processes such as exfoliation.

Biological Weathering

Biological weathering incorporates elements from both physical and chemical weathering, but the driving force is biotic in origin. Much of biological weathering that occurs in soils takes place in an area known as the rhizosphere. This area of soil (usually extends 2mm out from the root surface) is most significantly influenced by plant roots. The rhizosphere contains the main sites of soil ecological interactions and which result in many physio-chemical processes unique to soil organisms..

Humification describes the process of organic residues deposited in the soil being reduced to more inert organic substances collectively dubbed humus. This reducing reaction is carried out by all soil organisms to some degree, with the notable exceptions of plant roots. Depending on the C/N ratio of the organic residues, free nutrients (most commonly nitrates and phosphorous compounds) will be released into the soil solution (mineralization) or stored within the organisms (immobilization). Additionally, soil humus acts like clay minerals in its ability to hold exchangeable cations in the soil solution.

Chelation is the binding of metal cations to an organic compound. Soil microbes often release chelating compounds that lock up metal ions (most notably iron) in a complex organic compound within the rhizosphere, resulting in the rapid depletion of cations within the surrounding soil. This "zone of depletion" can create interesting rhizolithic structures within paleosols.

Bioturbation describes the process of soil-particle mixing and disintegration by soil fauna, such as arthropods (insects) and annelids (earthworms). Burrowing is the most common form of bioturbation, and fosters increased porosity within the soil organic horizon.

Pedogenesis

All the aforementioned weathering processes, when compounded over time, result in the creation of soil with unique properties. The transformation of soil from sediment (deemed regolith) is called pedogenesis. Four basic pedogenic processes encapsulate the collective results of these weathering processes:

Table 1. The four pedogenic processes as described by Brady and Weil 2001

Process	Description
Transformations	physical or chemical modifications within the soil
Translocations	movements of products within the soil, either laterally or vertically
Additions	inputs into the soil from outside sources
Losses	leaching of materials into the groundwater or erosion of the soil material

Soil Physical Properties

All forms of weathering create the unique architectural features that comprised a soil and imbue it with its unique physical properties. These physical properties allows soil to influence the factors of their formation, especially as it pertains to the above ground biota. Both soil scientists and paleopedologists recognize the same soil physical properties, although the purposes for observing these properties between the groups may vary. It is important to note that inferring the physical properties of paleosols is difficult (but not impossible, as is touched upon later) due to the physical alterations to the soil post-burial. Outlined below are the physical properties both groups recognize.

Soil Texture

Pedologists analyze three fine-sized particles within the soil: sand, silt, and clay. These are the foundational particles of soil and are responsible for imparting many physical characteristics. Although estimations may vary, the particles are differentiated by their diameter as follows:

Table 2. Primary soil particle sizes as adapted from Earle 2016

Particle	Size range (largest to smallest)
Sand	2mm - 63μm
Silt	63μm - 2μm
Clay	<2μm

Any particle greater than 2mm is deemed a 'coarse fragment'. The relative proportion of these soil particles allow pedologists and paleopedologists to describe the soils and paleosols in question. Relative proportions of the fine-sized particles are compared to the lexicon of soil textural classes: the soil texture triangle. The dominant particle size within the soil controls many physical properties of the soil, including density and porosity.

The soil texture triangle, a guide to classifying soils based on fine-particle proportions.

Density and Porosity

The density and porosity of the soil are two closely related physical aspects of the soil that controls physical properties such as drainage and aeration. Density refers to the mass of soil solids per unit volume. Soil scientists describe refer to two different measurements of density when analyzing soils. Particle density(Dp) is the mass of soil solids per unit volume of soil solids, and bulk density refers to the mass of soil solids per unit volume of soil solids and pore spaces (which are occupied by water or air). Both of these densities are required to calculate the porosity (Φ) of the soil which contains the proportion of soil volume occupied by pore space. It can be summarized by the following equation:

$\phi =1-(D_{b}/D_{p})$

Soil Structure

Soil structure refers to the arrangement of soil particles of a given soil. Peds (also known as aggregates) are the fundamental unit for characterizing soil structure, and form along natural zones of weakness within the soil. The structure of soil peds reflects the various forces acting upon the soil and include processes such as freeze/thaw, shrink/swell, bioturbation, and rhizospheric interactions. Various naming systems have been applied to soil peds, and reflect the form they take upon excavation. Soil peds may be preserved within paleosols, and provide qualitative insights into the processes influencing the formation of these structures in deep time.

Soil Color

Pedologists and paleopedologists alike rely on the manifest color of the soil for interpreting the dominant chemical processes in soils. Soil color gives easily observable insights into organic matter content, water content and the presence of iron and manganese oxides. Both aforementioned groups of scientists employ the Munsell color system as a means to communicate standardized color observations about soil. The Munsell system employs chips that describe three different components of color: hue (color present), value (lightness or darkness), and chroma (intensity). Hue is denoted with a letter signifying the color, while both value and chroma are denoted with a number.

Soil Ped Structure Types: A) Prismatic B) Columnar C) Angular Blocky D) Subangular Blocky E) Platy F) Granular

Qualitative Features of Paleosols

Common Locations

Quarternary Paleosols: Soils take anywhere between thousands to millions of years to form, which varies based on the rate and dominant type of weathering in a given area.The Quaternary period of geologic time contains the last 2.6 million years, and analyzing the paleosols (which may overlap with what are considered in-situ modern soils) from this period provides insights into how soil formation occurs from a temporal aspect. The area around the terminus of glaciers provide a great example of sequences of soil formation; as the glacier recedes the till left in its wake lies as fresh parent material. Farther away from the terminus of the glacier lies till subjected to more extensive weathering, thereby transforming into more recognizable soil. Such sequences provide paleoecologists useful insights into how long soils take to form after major glaciation events, and the types of environs they produce.

Paleosols at Major Unconformities: Paleosols are often found at major unconformities, and appear as deep, clearly differentiated layers of rock enrich with laterite, bauxite, silcrete, or calcrete. Rocks of Precambrian age showcase paleosols at major unconformities, but the conditions of their formation are somewhat confounding. The aforementioned minerals often form in the deeper saprolite of the soil (the C horizon), rather than the surface solum (A and B horizons), meaning that paleosols found at major unconformities only contain the deeper horizons. Laterite, in particular, takes a long time to form in-situ, and is very resistant to erosion, so the preexisting solum was likely truncated, leaving only the saprolite. Thus, the information to be gathered from paleosols at major unconformities remains limited.

Sedimentary and Volcanic Sequences: When present in sedimentary and volcanic sequences, paleosols represent stable conditions in which pedogenesis is allowed to play out. Many rocks present in sedimentary and volcanic sequences represent ancient soils, including red beds, variegated beds, tonstein, ganister, and cornstone. Many of these rocks, namely ganister, contain fossil root traces, showcasing the presence of a vegetated top soil. Paleosols found in sedimentary and volcanic sequences provide the most detailed accounts of paleoenvironments, and belong to some of the most famous sedimentary formations in the world. Areas like Badlands National Park in the United States contain as many as eighty-seven different successive paleosolic sequences

Badlands National Park, South Dakota, USA. This national park contains hills that have sedimentary sequences interbedded with paleosols dating back to the Eocene and Oligocene epochs.

Ichnofossils

Rhizoconcretions: mineralized remnants of ancient plant roots

Ichnofossils ("trace fossils") are preserved remnants of the presence of past organisms, and include objects such as burrows, fecal pellets and root traces. Their presence displays definite proof of past life in a paleoenvironment, and provides detailed clues about the composition of ancient ecosystems.

Rhizoliths

Rhizoliths are fossilized root traces, and may manifest as original cellular material, mineral replacements or mineral impregnations. Seeing that soil function as a medium for plant growth, the presence of rhizoliths within a sediment proves with near certainty that the rock layer represents a paleosol, however this only limits the identification of paleosols to when vascular plants first appeared in the Silurian epoch.

A Rhizohalo contained within a mudrock found in Nova Scotia

Root traces often appear in the form of altered minerals created through the biochemical interactions between the soil minerals, hydraulic regime, and the rhizosphere. Thus, analyzing the form and mineralogy of rhizoliths provide valuable information pertaining to the mineral-rhizospheric interactions and paleohydrology. The terminology describing rhizoliths evolves constantly, but most agree on the paleoenvironmental significance of certain structures. All rhizoliths include some kind of mold and cast, where the mold is the void left by a decayed root and the cast is the sediment that infills the mold. Rhizoconcretions (or rhizocretions) are minerals that form as nodules around a living or dead root. Calcerous rhizoconcretions form in desert environments or sandy beaches, where abundant calcium carbonate undergoes repeated cycles of dissolution and precipitation around plant roots, forming concretionary casts. Chelation also creates rhizoconcretions. Ferruginous rhizoconcretions appear as reddish-yellow molds around plant roots, and are created by the chelation of iron from the surrounding soil.

The most distinguishing mineralogical formation endemic to rhizoliths are the rhizohaloes. A rhizohalo is a mottle within a paleosol that develops in response to depletion of iron and manganese in the rhizosphere surrounding the root, often resulting from the change in soil moisture. They are referred to as 'drab haloes' by some due to the color they produce within the rhizolith. The color indicates the drainage of soil-water and the level at which the water table changes, which causes episodes of gleization. For example, rhizohaloes featuring an iron depleted interior and yellow-brown goethite exterior result from poorly drained, waterlogged soils, in which still water creates anaerobic, reducing conditions. Rhizohaloes almost always represent the last crop of plants in a unevenly, often poorly drained environment due to the fact that microbial decomposition of roots within well-drained soils is too rapid. Either rhizoconcretions and rhizohaloes must be present for clear delineation of fossil root traces, as root molds and casts cannot be discerned from the paleosolic matrix.

The density, pattern and depth of root traces indicate the type of ecosystem supported by the ancient soil. For instance, shallow and laterally spreading roots denote the presence of poorly-drained, swampy environments, while deep-rooting traces indicate well-drained conditions found in more coniferous forests. The thickness of the roots can be indicative of the types of plants occupying the ancient biome. Fine, deep rooting bunches are indicative of an tall grass prairie, while tubers and tussocks signal a short-grass prairie.

The large laterally spreading roots of Stigmaria ficoides indicate the presence of a swampy lowland environment, in which roots remain near the surface to obtain oxygen.

Soil fauna body fossils and ichnofossils

Often found alongside rhizoliths are the burrows, fecal pellets, and even preserved exoskeletons of soil fauna. Like root traces, the presence of soil fauna fossil have implications for soil paleohydrology. Termites will not found colonies near the water table, thus the presence of a termite colony in a paleosol may indicate a well-drained paleosol with a deep water table. In addition to paleohydrology, the direction and density of burrows within a paleosol is indicative of coarse fragments and cemented soil horizons. Although hard to see, an abundance of soil fauna fecal pellets and exoskeletons near the surface of the soil hints at high rates of humification within the organic horizon of the soil. Accounting for the overall level of faunal activity is difficult since occupancy rates of burrows are largely unknown.

The lighter colored mottles within the mudshale are the remnants of fossil burrows. They differ in appear from rhizoliths largely in their consistency of shape and irregular branching.

Soil Horizons

Paleopedologists use the same nomenclature as modern pedologists for describing the horizons of paleosols. The upper horizon of paleosols is sharply truncated, while the lower horizons gradually mesh with the parent material, creating a distinct geologic layer within a sedimentary sequence. Texture, structure (peds), mineralogy (primary and secondary minerals, crystals, nodules, mottles etc.), color and ichnofossils are often the features paleopedologists use to identify and describe paleosol horizons. Color and structure are the most discernible properties that can be observed in-situ, as removal and laboratory storage may alter the composition of these properties. The two aforementioned properties are clues to pedogenic processes responsible for creating the differentiated horizons, which can provide key paleoclimatic insights. Most of the field methods used to describe these aspects of soil horizons mimic methods used by modern pedologists. The texture triangle and the Munsell color system are good examples of modern systems adapted to paleosols.

However, recognizing the boundaries of horizons remains a complicated process, largely due to the changes brought upon by compaction and lithification. Oftentimes upper horizons may be all or partially truncated in the soil profile, meaning its content must be inferred base on the lower horizons.

Paleosol Classification

Paleopedologists have developed many different ways of classifying paleosols, and constant debate surrounds this very topic. For most of paleopedology's history, paleosols have been named based on the modern soil orders most alike in character, but this is complicated by the alterations of paleosols upon burial, and by the difference in environmental factors (namely vegetation types) between paleosols and the modern analogs. Two competing naming theories have been developed; Mack et al. ignores the modern classification system entirely and designates new names for paleosols, while Retallack modifies modern US soil taxonomy to name the paleosols.

However, many paleopedologists frequently categorize paleosols based on observable characteristics, namely degrees of pedogenesis/sedimentationand drainage. Furthermore, many paleosols represent repeated depositional events, creating many layers of paleosols that represent soil-forming after said events. A reliable classification system divides the paleosol profile into four types based on the rate of sedimentation versus pedogenesis:

Table 3. Classification of paleosol profile as according to Tabor & Myers 2015

Profile Type	Sedimentary accumulation
Simple	None
Cumulative	Slow, constant, small
Composite	Rapid, intermittent, medium
Compound	Rapid, intermittent, large

The rate of sedimentary accumulation is telling of the frequency of deposition events, which coupled with grain-size and pedogenetic development, provides a detailed reconstruction of disturbance events and rates of weathering within paleoenvironments.

Some paleopedologists group paleosols into "pedotypes", which are orders of paleosols sharing similar features. The approach has the strength of being able to compare non-genetic soils in character, but has been criticized for being an unreliable method for paleoenvironmental reconstruction, and denoting paleosols unintuitively. Many more approaches for classifying and naming paleosols abound, all of which come with strengths and weaknesses depending on the purpose of denomination.

Quantitative Paleosol Analysis

For most of paleopedology's history, analysis of paleosols has been limited to the qualitative features aforementioned. While these qualitative methods still contain a wealth of information, recent advancements in mineralogical techniques have allowed paleopedologists to gain more direct proxies for paleoenvironmental analysis. Some of these methods have been adapted from other forms of sedimentology, and still others are new and unique to paleosolic analysis. The breadth and depth of these new methods cannot be understated, and the following descriptions are by no means a comprehensive list, however, the methods that follow are the most widely applied. It should be noted that only certain methods are applicable depending on the paleosol type present, and may be confounded by pedogenic factors during formation and post-burial.

Clay mineralogy: Clay minerals within the soil make up the main soil colloidal surfaces, and hold the soil cations required by soil biota. Additionally, clay minerals are the end result of chemical weathering, so the type and amount of clay minerals present provides useful proxies for climate and temperature. Paleopedologists analyze the clay content of a paleosol firstly by finding the clay-size fraction (the proportion of clay-size particles from the total particles), and then apply X-ray diffraction (XRD) to the sample to obtain mineralogical composition. Since not all clay minerals may not possess the same cation exchange capacity, XRD provides a more precise estimation based on the type of clay mineral present.

Molecular weathering ratios: Rate of weathering directly relates to the rate of pedogenesis, and certain effects of weathering can be discerned in paleosols from the relative amounts of minerals left within the rock:

Table 4. Common molecular and trace element ratios used to measure weathering adapted from Sheldon and Tabor 2009

Ratio	Formula	Pedogenic process
ΣBases/Al	ΣBases/Al	Hydrolysis
Base Loss	Base/Ti	Leaching
Clayeyness	Al/Si	Hydrolysis
Gleization	Fe2+/Fe3+	Oxidation
Provenance	Ti/Al	Acidification
Salinization	K+Na/Al	Salinization
Leaching	Ba/Sr	Leaching/Hydrolysis
Parent Material	La/Ce, Sm/Nd, U/Th	Acidification

Many ratios overlap in the weathering effects they measure. A common element incorporated into many of these ratios is aluminum as it has unique soil chemical properties that give useful insights into the pedogenic processes, such as hydrolysis. For instance, aluminum rapidly replaces basic cations in the soil solution as parent materials undergo hydrolysis, and replaces silicon ions in silicate clays via isomorphic substitution. A high alumina to basic oxides ratio indicates frequent weathering, and provides proxy for mean annual precipitation.

Isotopic geochemistry: Analyzing the stable isotopic geochemistry provides qualitative insights into the mean annual temperature, mean annual precipitation, and paleosol organic matter respiration rates. The most common stable isotopes used are those of O¹⁶/O¹⁸, H¹/H², and C¹⁶/C¹⁸. Methods involving isotopic geochemistry involve numerous requirements pertaining to the amount and distribution of isotopes within the environment, thus the analysis of these forms is quite complicated and situationally variant.

Nonetheless, isotopic analysis can provide a wealth of paleoenvironmental insights. Due to its larger atomic mass, δ¹⁸O more commonly precipitates out as calcite during evapotranspiration (ET), indicating that calcite-enriched with δ¹⁸O found in paleosols implies paleoclimates that experience high ET. Since ET is controlled by temperature and relative humidity, comparing the δ¹⁸O values of various paleosols yields compelling data pertaining to paleotemperature across latitudes and time.

Soil respiration responds to changes in organic residue inputs, and the rates of respiration indicate soil biotic acitivity. Soil respiration is often difficult to measure in modern soils, and adding the complication of burial and lithification in paleosols has made measuring biological activity of paleosols difficult. However, paleopedologists often used the carbon isotopic composition of the mineral goethite within paleosols to approximate soil respiration. Soil carbon dioxide is generally isotopically lighter than atmospheric carbon dioxide, suggesting a dearth of isotopically heavy carbon should be apparent within the goethite near the paleosol surface.

Windows into the Past: Example paleoenvironments

As paleopedological analysis gains increasing traction in the paleontological community, paleosols have helped to paint more accurate, and surprising, pictures of past paleolandscapes. Their application has shed light on previously unknown stages in the history of life on Earth, and has challenged the paleontological community's understanding of the history of life.

The First Forests

In the twenty-fifth edition of the magazine Palaios, Mintz et al. published an account of the paleoforests of the Devonian. The team of researchers analyzed paleosols in the Catskill mountains in New York and were able to describe a diverse array of forest ecosystems, ranging from periodically reduced, poorly drained swamps, to well-drained coniferous like forests. These were the first established forests, and their presence is concurrent with decreases in atmospheric carbon dioxide and subsequent Paleozoic glaciations

Early Life on Land - A Precambrian debacle

Since plants did not conquer the land until at lest the Silurian epoch, identifying paleosols from the before this time is made difficult by the absence of rhizoliths. Despite this difficulty, the acclaimed paleopedologist Gregory Retallack has proposed an interesting, albeit controversial theory, about life in the Precambrian. He suggests that terrestrial life has existed in a form similar to modern biocrusts as early as the Mesoarchean, 3.2 billion years ago. He cites the presence of paleosolic structures that require the presence of microbial life, microfossils found within deposits of terrestrial origin, and biostructures that are non-aquatic. Some of the structures he describes bear a remarkable resemblance to modern biocrustal fungi and lichen, which would make fungi some of the oldest eukaryotes on Earth. Despite skepticism from the paleontological community, Retallack continues to advocate for his position.

Conclusion

Paleopedology provides a very multidisciplinary approach to studying paleoenvironments of any time in Earth's history. As more advancements in sedimentology and fossil analysis arise, paleopedology will only continue to grow in the scope of information it can provide about ancient ecosystems.