Embedded Files
Marine
Marine
The clear seas, with creatures that have evolved over a couple of hundred million years, would be bathing these turbulent movements. Presumably some of their members moved from the sea into the mud. silt and sand engulfing them. Throughout the Carboniferous, there were significant fluctuations in sea levels. At times, the sea levels were relatively high, leading to the deposition of marine sediments. During periods of lower sea levels, coastal areas were exposed, and terrestrial sediments, including mud, could accumulate.
Deltas
Deltas
The clear tropical sea that dominated the early Carboniferous became increasingly choked with mud, silt and sand brought in by large rivers draining upland areas to the north. When a river empties into the sea, the sediment may build a delta.
Deltas usually form on tectonically subsiding continental crust where successive deltaic environments get buried. In areas where there is no subsidence, or the surface is rising relative to sea level, estuaries tend to develop.
Deltas are complex environments and are shaped by the interaction of waves, tides and river activity. Over time, sediment is deposited at the coastline and the delta is built up above sea level into a system of channels with intervening swampy areas, similar to the Mississippi delta.
These massive forces moving mud, silt and sand around will slow down in places, where they could mix with plants, debris and rhizospheres with fungi, bacteria and creatures, to create early soils.
Here's a simplified explanation of how several sorts of soil could have developed during the Carboniferous period, with most of the elements in abundance across Northern Hemisphere. The 3+ models are recognisable today, although now they have mixed together more.
Palaeosol evidence
Palaeosol evidence
The nearest to a description of the terrestrial landscape, in Early Carboniferous times, shows that “The key to understanding the palaeoenvironments where they lived is a detailed analysis of the sedimentary architecture of this formation, one of the thickest and most completely documented examples of a coastal floodplain and marginal marine succession from this important transitional time anywhere in the world. Palaeosols are abundant, providing a unique insight into the early Carboniferous habitats and climate. More than 200 separate palaeosols are described from three sections through the formation. The palaeosols range in thickness from 0.02 to 1.85 m and are diverse: most are Entisols and Inceptisols (63%), (the least developed soil types) indicating relatively brief periods of soil development. Gleyed Inseptisols and Vertisols are less common (37%). Vertisols are the thickest palaeosols (up to 185 cm) in the Ballagan Formation and have common vertic (al?) cracks. Roots are abundant through all the palaeosols, from shallow mats and thin hair-like traces to sporadic thicker root traces typical of arborescent lycopods. Geochemical, isotope and clay mineralogical analyses of the palaeosols indicate a range in soil alkalinity and amount of water logging. (Kearsey et al 2018)
"Geochemical, isotope and clay mineralogical analyses of the palaeosols indicate a range in soil alkalinity and amount of water logging. Estimates of mean annual rainfall from palaeosol compositions are 1000 –1500 mm per year. The high mean annual rainfall and variable soil alkalinities contrast markedly with dry periods that developed deep penetrating cracks and evaporite deposits. It is concluded that during the early Carboniferous, this region experienced a sharply contrasting seasonal climate and that the floodplain hosted a mosaic of closely juxtaposed but distinct habitats" (Kearsey et al 2018)
Weathering of Rocks
Weathering of Rocks
There must have been a lot of weathering to produce the vast amounts of mud around in the Northern Hemisphere in these times.
Weathering is the process by which rocks and minerals are broken down into smaller fragments or altered in composition due to exposure to various environmental factors. This fundamental geological process occurs continuously over time and plays a crucial role in shaping Earth's surface features.
Mechanical weathering, also known as physical weathering, involves the physical breakdown of rocks into smaller pieces without changing their chemical composition. One common mechanism of mechanical weathering is frost wedging, where water seeps into cracks in rocks, freezes, and expands, causing the rock to fracture. Another process, known as abrasion, occurs when rocks are worn down by the physical action of wind, water, or ice carrying sediment particles.
Chemical weathering, on the other hand, involves the alteration of rocks through chemical reactions that change their mineral composition. One of the most common forms of chemical weathering is hydration, where minerals absorb water molecules and undergo expansion, leading to the breakdown of rock structure. Another important process is oxidation, where minerals react with oxygen in the atmosphere to form new compounds, such as rust on iron-bearing minerals.
Biological weathering is the process by which living organisms, such as plants and microorganisms, contribute to the breakdown of rocks. Plant roots can penetrate cracks in rocks, causing them to expand and fracture, while microbial activity can produce acids that dissolve minerals and weaken rock structures. Lichens can also weather rocks.
Weathering occurs at various rates depending on factors such as rock type, climate, and environmental conditions. Rocks composed of softer minerals are typically more susceptible to weathering than those made of harder minerals. Additionally, factors such as temperature, moisture, and the presence of organic acids can accelerate the weathering process.
Overall, weathering is a fundamental geological process that continuously shapes Earth's surface features by breaking down rocks and minerals through mechanical, chemical, and biological mechanisms. This process plays a crucial role in soil formation, sediment transport, and the cycling of nutrients in Earth's ecosystems.
Weathering can occur from bottom-up chemical weathering and top-down biology. There is a variety of important processes that affect this weathering, including kinetics and thermodynamics and aspects like surface area of bedrock (e.g., fractured vs homogeneously solid, pH and Eh, etc.). Sediments, including mineral particles, would have been transported by water and wind and deposited in various places - as 'clasts' (below)
Four 'Early' Modern Soils
Four 'Early' Modern Soils
All that clay in the mud would have made for a massive mixing bed. Their particles and mineral structures, make clay stable both and reactive, making it a unique and versatile material that brings these unique properties to create early soils.
Today we tend to think of soil developing 'bottom up '('weathering' below) and 'top down'. However, these early soils also involve a lot of sideways involvement - from wind, sand, water and mud.
We are now looking to mud to see how particles stick together, as this would have had enormous influence on the movement of mud then.
Even without widespread humic substances which became much more extensive later, clay particles would have stuck to plant detritus. Clays help form larger aggregates through mechanisms like cation bridging, van der Waals forces, capillary action, microbial activity, and the physical interaction between clay and detritus. These processes contribute to the formation of soil structure and aggregation, supporting essential soil functions such as water retention and nutrient cycling. More on aggregation.....
Here are some key processes that would allow this aggregation to occur:
1. Electrostatic Forces (Cation Bridging)
1. Electrostatic Forces (Cation Bridging)
Clay particles often have negatively charged surfaces due to the presence of isomorphic substitution or broken bonds on the edges. When detritus (organic matter like plant debris) comes into contact with clay particles, cations (positively charged ions such as calcium, magnesium, iron, and aluminum) can act as a bridge between the negatively charged clay surfaces and the detritus.
Cation bridging can occur when these positively charged ions adhere to the negative surfaces of clay and detritus, facilitating aggregation by reducing the repulsion between the particles and binding them together.
2. Van der Waals Forces
2. Van der Waals Forces
Van der Waals forces are weak, short-range attractions between molecules or particles that can also play a role in binding clay to detritus. Even though these forces are relatively weak, they can still help hold particles together, particularly when clay particles and detritus are in close proximity.
3. Water Menisci and Capillary Forces
3. Water Menisci and Capillary Forces
In soils that have a mix of clay particles and organic detritus, capillary forces can help stick particles together. Water present in small pores between particles can form menisci, which creates a surface tension effect that pulls the particles together.
This process can help create aggregates, especially during cycles of drying and wetting. As the water evaporates, particles are drawn closer together, helping them form stable aggregates.
4. Polysaccharides and Other Organic Compounds
4. Polysaccharides and Other Organic Compounds
While humic substances appear later, other organic materials such as polysaccharides (from microbial or plant origins), proteins, and simple sugars can also contribute to particle adhesion. There would also be many Glomalin Related Soil Proteins about doing some glueing.
Polysaccharides, which are produced by bacteria, fungi, and plant roots, have sticky properties and can act as a natural glue that helps bind clay particles and detritus together, enhancing aggregate formation.
5. Microbial Activity
5. Microbial Activity
Soil microorganisms, such as bacteria and fungi, can produce extracellular polymeric substances (EPS) like proteins, polysaccharides, and lipids. These substances have adhesive properties and can help bind clay particles to organic matter.
Microbial decomposition of detritus can also lead to the release of smaller organic compounds, which can act as binding agents in the absence of humic substances.
6. Physical Enmeshment
6. Physical Enmeshment
Organic detritus, particularly fibrous plant material, can physically enmesh with clay particles, forming a network that stabilizes aggregates. This can occur as plant roots decompose, leaving behind fibrous residues that become interwoven with clay particles.
Root exudates from living plants can also help cement particles together by producing substances that promote particle adhesion.
7. Fungal Hyphae
7. Fungal Hyphae
Fungal hyphae, both mycorrhizal and saprophytic can physically bind particles together by growing through the soil matrix, acting as a net that holds clay and organic detritus in place. The filaments of fungal hyphae can effectively link particles, contributing to the formation and stabilization of aggregates.
8. Drying and Shrinkage
8. Drying and Shrinkage
Upon drying, clay particles can shrink and change their structural orientation. As the water content decreases, the clay's surface may draw closer to organic matter and other particles, encouraging aggregation. This process is particularly common in clays with swelling properties, like montmorillonite.
We are going to try and work out what these early soils could have looked like: Each involves movements and mixes of sand, mud, water and organic matter.
SAND Sigillaria in sand and gravel. Gave rise to Entisols
POND Horsetails Gave rise to Inceptisols
MUD Paralycopodites and Lepididendron in mud with clastic covering. Gave rise to Vertisols
PEAT Lepidiophios and peat. Gave rise to Histosol
1. Sand
1. Sand
Sand can be derived from bedrock or through the clastic despositional processes - where weathered rock, now in various sized pieces, moves to somewhere else. Anybody who has done any building, we know that sand and gravel add strength to a cement mix, but on their own move around easily and are not stable until we add some sort of cement and water. In this period the most likely 'cement' would have been glomalin related glues.There would also be a little - but it seems not much - organic debris in the mix.
Sand and gravel (Sand =1/16mm-2mm, gravel >2mm- 64mm) rock particles can add stability to muddy soil by improving its drainage, increasing load-bearing capacity, and reducing its propensity to become excessively compacted.
Sigillaria was found mainly in sand and gravel levees growing below ferns.
Entisols of large river valleys and associated shore deposits provide cropland and habitat for millions of people worldwide. The central concept is soils developed in unconsolidated parent material with usually no genetic horizons except an A horizon. They are very diverse, and if a soil doesnt fit the other 11 soil orders, they are often allocated to Entisols.
"The palaeosols formed quickly before sediment buried them. This is characteristic of the more active areas of the floodplain of many fluvial systems today, for example on river sand bars which have been stable long enough for plants to become established. Based on the shallow rooting depths and absence of subsurface pedogenic horizons, this pedotype is most likely to be an Entisol" (Kearsley 2016)
Levee
Levee
We saw in previous times, the ferns had superficial rhizome systems with roots that are generally thin and wiry in texture and grow along the stem. They absorb water and nutrients and help secure the fern to its substrate. Also we saw how they and their associated fungi and creatures, can create a rhizosphere capable of hanging on to rocks.
Under the ferns tall growth cover, we see that the Sigillaria have special structures 'stigmaria' - rhizomes with 'rootlets' that leave a distinct scars. I need to look further into how similar they are to today's roots in terms of function. But they look as if they can absorb water and nutrients, but whether they have fungi, I need to check.
Together you can see how they could stabilise the heavy sand and gravel from moving round, to produce more stable levees. There was a light clastic layer to protect the stabilisation below.
The pores between the particles would allow previously shallow-living creatures live deeper down. In particular springtails could move, but now lost their jumping organ, as the need to follow water declined, and their eyes as they would be in continual dark. Onychiuridae - fungal feeders, now extending fungal networks deeper.
Fern roots and their fungi and springtails produce glomalin glues to stick particles together or to the roots. Sigallaria have strong stigmaria (probably weaker rootlets than can go between particles), to hold sand and gravel as if a mesh bag.
= Sandy Soils
= Sandy Soils
Sand & gravel held together by roots and the glomalin glues produces well-drained early soils
How sand mixes with mud
How sand mixes with mud
1 Improved Drainage:
Muddy soil typically has a high clay content, which retains water and leads to poor drainage.
Sand and gravel are both coarse materials that have excellent drainage properties. They allow water to pass through quickly, preventing waterlogging and reducing the likelihood of the soil becoming excessively muddy.
2 Increased Load-Bearing Capacity:
Muddy soil is generally weak and has a low load-bearing capacity. It can easily become unstable and compacted under the weight of structures or heavy machinery.
Mixing sand and gravel into the muddy soil can increase its load-bearing capacity by adding strength and cohesion to the soil matrix. This makes it more stable and capable of supporting heavier loads without excessive settlement or deformation.
3 Reduced Compaction:
Muddy soil tends to compact easily when subjected to heavy loads or traffic, leading to a loss of stability and increased sinking.
The addition of sand and gravel can help prevent excessive compaction by providing a granular structure that resists compression. This helps maintain the soil's porosity and reduces the risk of sinking and instability.
4 Enhanced Shear Strength:
The presence of sand and gravel particles in the soil matrix can increase its shear strength. Shear strength is the soil's ability to resist lateral deformation or sliding.
By improving shear strength, sand and gravel can help prevent soil erosion, slope failures, and other stability issues in muddy terrain.
5 Erosion Control:
Muddy soil is susceptible to erosion, especially in areas with heavy rainfall or water flow.
Sand and gravel can serve as erosion control measures by stabilizing the soil surface and reducing the risk of erosion. They act as a protective layer that helps retain the soil in place.
2. Pond
2. Pond
Could early soil also develop below or near the ponds. Think how quickly ponds and lakes become marsh, where certain trees - like willow and alder, and eventually oak woodland become established. There is mud and sand, a strong combination, with water on top. Could the horsetails have an impact. Anybody who enjoys their garden knows what a pest these horsetails can be forever popping back up. That is because they have deep roots, regulate nutrients (March et al 2000) and are notorious for the ability to regrow from any bit of its rhizome.
This must have had an effect on the shallow lakes system and provide all the ingredients for early soils - nutrients, mud & sand with plant debris. What creatures would be there at the bottom of the lake?
This environment may have given rise to evolution of Poduromorph springtails - like 'snow flees' ..
"The presence of a gleyed surface horizon and a cambic horizon below are characteristic of Inceptisols. This suggests that they formed further away from the active river system than the Entisols; episodic flooding deposited new sediment on top of the palaeosols thus preventing further development"
The most common palaeosol Inceptisol ...represents a relatively short period of soil development, long enough for clay-rich sub-horizons to develop, but not long enough to develop any other pedogenic structures; they are thought to take 100s to 1000s of years to form" (Kearsley 2016)
Carboniferous Sandstone (above) overlaying Mudstone (below), Swale, Yorkshire
3. Mud
3. Mud
Much mud is millions of clay particles moving about in water. We see how mudflats can develop into clay soils at this Carboniferous period In general, deltaic sediments muds are usually deposited during ‘over bank’ flood events when the deltaic river channels burst their banks, or breeched their levees - typically the gravel settles out first, then the sand, then the silt, then the mud. This dries out and gets vegetated, until it gets drowned in the next flood event, giving characteristic ‘graded bedding’.
When it comes to mud becoming soil, there would have to be some stability to start the process. Have you ever walked across or near estuarine mud banks and seen how in between the distributary channels the mudbanks become vegetated, initially with plants which can survive saline inundation on spring tides, and in many cases, especially if the land is rising (or sea level falling), eventually the mudbanks become clay soil.
Vertisols have a layer at least 25 cm thick that has slickensides or wedge-shaped peds with tilted long axes and at least 30 percent clay in all horizons to a depth of 50 cm, that cracks that opens and closes periodically.
Mud & Clear alternation
Mud & Clear alternation
The Upper Carboniferous ...are characterized by very distinctive sedimentary units—the whole set of strata is neither purely “muddy water sedimentary products, clastic rocks, nor entirely “clear water environment products ”, carbonates rocks, (include mudstone) but a mixed of clastic-carbonate succession formed by frequent “muddy and clear alternation”. According to the regular changes of rock composition, lithological ratio, and thickness of fine-grained sediments, the mixed system formed different cyclic units of several meters, ten meters to hundreds of meters thick, showing a multi-level rhythmic deposition pattern. The multilevel alternation of clastic and carbonate rocks indicates frequent changes in environmental factors such as water properties, material supply and climate changes during deposition" (Wei et al 2021)
Floodplains & Mud
Floodplains & Mud
The colonisation of the floodplains in the Bideford area during the Carboniferous (called the 'Culm') was controlled by the rate of deposition of muds and crevasse sands, the water input, and the pool of plant material. The calamites and accessory ferns (palaeosol Type I) were probably the best adapted to this unstable habitat, with its frequent flooding and sediment input. Calamite roots show analogies to modern rhizomes that are able to grow new shoots from nodes and buried axes after rapid sediment accumulation" The most common palaeosol was 'Type II' "Medium grey, mostly muddy, siltstone units of 15 to 20 cm thickness with a moderate to locally low rooting intensity of M. gracilis, P. capillacea and scattered small S. ficoides with appendages. The sedimentary structures are bioturbated around the Stigmaria only" (Hoffman 1992).
Muddle
Muddle
Mud, short for muddle, and is loam, silt or clay mixed with water. We experience a lot in this country after rain or near water sources. Mud deposits hardened from this period over geological time to form sedimentary rock called mudstone. When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds. There was a lot of mud around then. The particles must have came from the weathering of rocks (above) in the mountains being washed to the sea.
Mud doesn't move all the time, all mudrocks (shales, claystones, mudstones, mudrocks, and many types of siltstone and sansdstone) were originally muddy detrital sediments. Lacustrine muds, flood plain and estuarine muds along with marine muds are often preserved in the stratigraphic record. Most sedimentary rocks are mudrocks, there are far fewer sandstones and limestones.
But when it does..mountains of mud would move, in a slimey sort of way, with a force hard to stop - witness 'mudslides'. How could this be held back?
What is Mud?
What is Mud?
1. Wetness: Mud is characterized by its high moisture content - except when it is baked in the sun! . It is essentially a wet mixture of solid particles suspended in water. The water content can vary, resulting in mud that ranges from thick and gooey to more liquid and slurry-like.
2. Particle Composition: Mud can contain a range of fine particles, including clay, silt, sand, and organic matter. The proportions of these particles affect the texture and properties of the mud.
3. Stickiness: Mud is often sticky due to the presence of clay particles, which have adhesive properties [thixotropy and rheology are functions of grain size distribution, chemistry and surface electrical properties]. This stickiness can make it challenging to walk or drive through, as objects can become stuck in it.
4. Formation: Mud forms in various natural settings, such as riverbanks, wetlands, estuaries, and during heavy rainfall. It can also be created intentionally in various industrial processes or construction activities. Geologically muds constitute most continental shelf sediments, ultimately many are derived from soils washing into rivers and eventually the sea.
5. Uses: Mud has been used by humans for various purposes throughout history. It has applications in construction (e.g., making adobe bricks), agriculture (e.g., as a component of soil), pottery and ceramics (for shaping and firing clay), and spa treatments (mud baths).
6. Environmental Importance: Mud plays a vital role in ecosystems, providing habitat and nourishment for many aquatic organisms. It also helps filter water by trapping and removing impurities.
7. Challenges: While mud has its uses, it can also pose challenges, such as causing vehicles to get stuck in muddy terrain or contributing to erosion when it washes into water bodies. Managing mud and its impacts is essential in various industries and environmental contexts.
Overall, mud is a common and versatile substance found in nature and used by humans for various purposes, but its properties can vary widely depending on its composition and the conditions in which it forms.
The thickness of clastic substrates in the Carboniferous period could range from relatively thin layers to several meters or even tens of meters in thickness, depending on the local geological and environmental conditions. It would seem they were deeper over the muds rather than the levees.
Clastic Cover
Clastic Cover
'Clastic' means particles derived from elsewhere. They were not new, previous versions having been found up to a hundred million years previously, each with different markings left in the geological record.
Clastic substrates, such as sandstone, siltstone, and shale, are sedimentary rocks composed of 'clasts' or fragments. The fragments could be pre-existing rocks, mineral grains, sand and silt and organic matter that have been eroded, transported, and deposited by various geological processes.
Clastic cover is composed primarily of very fine mineral particles, typically smaller than 0.002 millimetres in diameter, which is much smaller than the particles found in sand or silt. This result is high density and little air space.
This clastic substrate at this period would have had to have been strong enough in places to hold mud moving about. Presumably this was due to a mixture of the clasts with some glomalin glues
"The rock record contains a rich variety of sedimentary surface textures on siliciclastic sandstone, siltstone and mudstone bedding planes. In recent years, an increasing number of these textures have been attributed to surficial microbial mats at the time of deposition, resulting in their classification as microbially induced sedimentary structures, or MISS" (Davies 2016) ...
"An increasing recognition amongst geologists of the physical role that micro-organisms can play in siliciclastic sedimentary environments. Observations ... have clearly demonstrated that microbial mats, biofilms and aggregates can be directly and indirectly responsible for sculpting a wide variety of sedimentary textures on bedding planes@
Were these factors at work then?
How mudplains become clay soil
How mudplains become clay soil
A mud plain becomes clay soil through a series of geological and environmental processes that involve sedimentation, compaction, and chemical changes over time. Here’s a step-by-step explanation of how this transformation occurs:
1. Sedimentation
1. Sedimentation
Mud plains are typically formed in areas where fine sediments, including silt and clay particles, are deposited by slow-moving water bodies like rivers, lakes, and estuaries. These sediments settle out of the water and accumulate in layers.
2. Compaction
2. Compaction
Over time, the layers of mud are buried under additional layers of sediment. The weight of these overlying sediments exerts pressure on the lower layers, causing them to compact. This compaction reduces the pore spaces between the particles, expelling water and increasing the density of the sediment.
3. Dehydration
3. Dehydration
As compaction progresses, the water content in the mud decreases further. This dehydration process is crucial in transforming the mud into more solid and cohesive soil.
4. Chemical Changes
4. Chemical Changes
Several chemical processes contribute to the transformation of mud into clay soil:
Ion Exchange: Clay minerals can exchange cations (positively charged ions) with the surrounding environment. This can lead to changes in the clay mineral structure, making it more stable and cohesive.
Diagenesis: This is a process where the chemical composition of the sediment changes over time. It includes the reorganization of mineral structures and the formation of new minerals. In mud plains, the dominant fine particles (like silt and very fine sand) gradually alter to form clay minerals such as kaolinite, illite, and montmorillonite.
5. Biological Activity
5. Biological Activity
Organic matter from decaying plants and animals can mix with the sediment, contributing to the formation of humus. The presence of organic material helps in the formation of soil structure and improves its fertility.
6. Weathering
6. Weathering
Weathering processes, both chemical and physical, break down the mineral particles further. Chemical weathering involves reactions with water and atmospheric gases, leading to the breakdown of minerals and formation of clay minerals. Physical weathering involves the breaking apart of particles due to temperature fluctuations, freeze-thaw cycles, and other mechanical forces.
7. Soil Formation Processes
7. Soil Formation Processes
As these processes continue, the sediment in the mud plain gradually transitions into clay soil. This soil is characterized by:
High Plasticity: Clay particles are very small and have a plate-like structure, giving the soil a high plasticity when wet.
Low Permeability: The tiny size and flat shape of clay particles mean that water moves very slowly through clay soil.
Nutrient Retention: Clay soils can hold onto nutrients better than sandy soils due to their high cation exchange capacity.
In summary, a mud plain becomes clay soil through a combination of sedimentation, compaction, dehydration, chemical alterations, biological activity, weathering, and soil formation processes. These changes occur over long periods and result in the fine-grained, dense, and nutrient-rich soil we recognize as clay.
4. Peat
4. Peat
On the floor of the swamps was a think layer of peat accumulated from dead plant debris that hadnt decomposed. There may have been a little humification (anoxic decomposition) to provide nutrients but nowhere near enough to recycle properly. Nutrients and energy lay at the bottom of the swamp, enough to provide millions of tons of coal later. The peat overlaid the mud, now stable. The mud would have provided support for the trees to grow, but there seems to be little mixing of the two main medium.
Of the three 'early soils' models, this is most like todays in terms of fertility and 'bottom up' meets 'top down', but without the great mixing like our present day soils. There would have been small worms - the enchytraeids, if their habitat today is anything to go by. They dominate peat habitats. But they were not strong enough to make the mixing of peat and mud. That was to wait a hundred million years.
It has been estimated that at their peak the swamps were responsible for the sequestration of 13–47 × 109 t of carbon per annum (Cleal and Thomas, 2004)
Peat built up over years, and got compressed (5-1) volume later to make coal
Lack of well-defined soil horizons: The continual deposition of organic-rich sediments and the dynamic nature of the swampy environments may have prevented the development of well-defined soil horizons typical of modern soils. Instead, the soils may have exhibited a more homogeneous and stratified structure with varying degrees of organic enrichment.
Histosols contain at least 20-30 percent organic matter by weight and are more than 40 cm thick. Bulk densities are quite low, often less than 0.3 g cm3. They are often referred to as peats and mucks and have physical properties that restrict their use for engineering purposes, because of low weight-bearing capacity. They are either saturated with water for at least 30 days per year unless drained and have a minimum thickness of 40-60 cm or constitute at least 2/3 of the total soil thickness to a root-restrictive layer
Swamp
Swamp
The later Carboniferous Period is often associated with the development of vast swampy environments. These swamps grew behind the massive deltaic environment and provided ideal conditions for the preservation of organic material, including plant debris. The stagnant water in these swamps also limited the decomposition of organic matter, allowing it to accumulate.
The organic matter accumulated as peat, as it was not washed away across the floodplains. It is the most productive substrate to have developed on Earth to this point - witness its success in growing plants.
It was here that the great biomass was laid down, that became crushed and coalified later.
Trees in swamps
Trees in swamps
We tend to think that the trees planted themselves in swamps. But what came first - the tree or the swamp? Trees will affect the movement of water, and their presence at the head of the floodplains would slow water movements - leading to swamp conditions.
Lepididendron could live in both mud flats and swamps. For example: "The density of these remains within this peat mass, possibly penetrated by Psaronius (tree fern) outer roots" (DiMichelle & Bateman 2020)
This would have led to a build-up of vegetation in the swamps "liana-like ferns and seedferns were climbing up along the Psaronius trunks. Stems have been found with up to eight climbing axes of one plant. Often the rootlets of these epiphytes are found among the rootlets of Psaronius. They are strikingly different in structure from the Psaronius roots."
"The possibility exists that parts of the stigmarian system were involved in CO2 acquisition from substrates; some may have been photosynthetic. These functions are considered in the context of the light sharing and diffuse photosynthesis evident in the pole architecture. The combination of such possibilities is related, in part, to the xeromorphic characteristics of the arborescent habit, raising the question about a modified kind of C3 photosynthesis such as CAM (Crassulacean Acid Metabolism). Stigmarian lycopsids dominated tropical Westphalian coal swamps...Species appear to exhibit different levels of tolerance to disturbances and range from colonizers to site occupiers. Paralycopodites, with prolific, free sporing, bisporangiate cones, was most abundant in frequently disturbed, partially exposed, peat- to mineral-rich habitats (ecotonal). Monosporangiate Lepidophloios and Lepidodendron were associated typically with deeper, standing-water habitats...Plants were characteristically much taller than other trees, yet did not shade out lower vegetation. They also were a major stabilizing influence on substrates with their extensive, baffling and anchoring systems in the high disturbance and abiotically stressed environments of peat swamps. The environmental circumstances of the first major coal age appear to have selected against long-lived or slow-growing trees in most coal swamps..Sigillaria was less closely associated with peat swamps, as a sporadic occupant associated with major disturbances, such as flood/dry down cycles. " (Phillips & DiMichelle 1992). Most trees then lived for opnl;y 10-15 years
No fungi
Because conditions were anoxic, there would be no fungi, either saprophytic breaking down debris nor mycorrhizal fungi.Hence no glomalin nor glomalin glues, and no micro-aggregation. There would have been some humification (but clearly not much in the way of decomposition) and hence some humic substances, but again not enough to produce a lot of aggregation. So there were no soil structures at this point
But what creatures?
Coal
Coal
As plant matter (and other organic matter) from these dense, swampy forests accumulated and lay decaying in the bottom of these swamps, the material became buried and subsequently (but when???) compacted. From this partially decomposed organic matter, anaerobic conditions from a lack of oxygen formed peat.
Peat resembles a spongy mass of brown, fibrous material, and the standing water in the Carboniferous swamps created excellent conditions to prevent the oxidation or fungal and usual bacterial destruction of the material. If you were to become buried in peat soon after your demise, then as new peat is formed on top, humic acid (there would have been a little humification, but clearly not enough to break peat down to humus in Carboniferous times) is released and your body may be preserved, in much the same way as pickling.
Peat becomes lignite
Peat becomes lignite
Once peat is buried between layers of sediments, biological degradation halts and changes begin to occur that are a result of increased temperature and pressure from the burial.
When this peat was buried at relatively shallow depths, continued heat and pressure compressed it between layers of sediment into lignite: a soft, brownish-black coal-like material with a high moisture content. It will also often contain wood and plant remains in a fine-grained groundmass. Lignite is also known as brown coal, and there are two types: xyloid lignite (fossilised wood) and compact lignite. The gemstone jet is formed from lignite.
At the end pf this period, we have 4 recognisable soil types - Histosols from the peaty swamps, entisols from the sandy beaches, inceptisols from ponds, and vertisols from the mudplains. They have yet to develop 'horizons' which distinguish many soils today. Nor were there earthworms or insect larvae. How will these early soils interact in the future and how will they help form other more developed soils? We will find out over the next 300 million years..
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