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This period (359-299mya) is called Carboniferous because it means 'containing or producing carbon'. It is particularly famous for the last part of the period when most of the world's coal was formed, but here we are looking at how early soil developed.
Most people associate the Carboniferous period as when coal was formed. Here we are more interested in how soil structures were evolving at that time, so may find clues that coal hunters missed.
In the graphic you can see 'developed soil'. "The 'theatre where water earth and air could interact'" became a reality. This was due to the increasing ability to build soil structures, probably still predominantly aerobically. Here is where the overall structure of the soil developed. More.
Soil becoming deeper and more debris about, While this age is famous for coal, what was going on in the rest of the land. There were many small soil animals like springtails and 'lower' oribatids, and early worms, but there is no evidence of main decomposing process - humification.
Orogens
Orogens
The end-Devonian mass extinction, or Hangenberg Crisis, 359 mya is now included among the first-order mass extinctions. The following Carboniferous Period was marked by tectonic activity including the uplift of mountain ranges and the creation of sedimentary basins. These geological processes, called orogens, led to the weathering of bedrocks, where early soils could have formed. Other weathering led to the transport of sediments, including mud, into low-lying areas.
The supercontinent, Gondwana, covered much of the Southern Hemisphere, while the continent of Siberia occupied the Northern Hemisphere, and an equatorial continent, Laurussia, was drifting towards Gondwana. to become one supercontinent called Pangea (watch), completed by the end of this period
Volcanic
Volcanic
The most abundant elements found in volcanic magma are silicon and oxygen, so most ash contain a lot of silica. Low energy eruptions of basalt produce a dark coloured ash containing ~45–55% silica, often in iron (Fe) and magnesium (Mg). More explosive eruptions produce ash nearly 70% silica, while other types of ash are between these figures.
The principal gases released during volcanic activity are water, carbon dioxide, sulphur dioxide, hydrogen, hydrogen sulphide and carbon monoxide. These sulphur and halogen gases and metals are removed from the atmosphere by processes of chemical reaction, dry and wet deposition, and by adsorption onto the surface of volcanic ash. A range of chloride and fluoride compounds are readily mobilised from fresh volcanic ash, due to rapid acid dissolution of ash particles within eruption plumes.
Volcanic ash is a mixture of rock, mineral, and glass particles expelled from a volcano during a volcanic eruption. The particles are very small—less than 2 millimetres in diameter. Due to their tiny size and low density, these particles can travel long distances, carried by wind. They tend to be pitted and full of holes, which gives them a low density. Pitted and full of holes – it is ideal as base for aggregates. Along with water vapor and other hot gases, volcanic ash is part of the dark ash column that rises above a volcano when it erupts.
We know there would be a lot of volcanic ash about – not only because a lot on Devonian seas, but also in the air all over the rocks.
After the great extinction, there appeared to be a gap in our knowledge. It was named after the person who proposed the name.
Orogens
Orogens
Orogens is where there are a lot of collisions of tectonic plates. This produced volcanic rock, some of which produces volcanic ash, with more to come and would be over a period of tens of millions of years. The Variscan or Hercynian orogeny was caused by the continental collision between Euramerica and Gondwana tectonic plates to form the supercontinent of Pangaea. The Variscon Orogony went on for 100 mill years between 350-250mya loads of time to supply soils with the 'essential' elements for vascular plant growth, added to the sedimanets deposited from older Caledonian orogony mountains. Orogenies tend to produce huge volumes of sediment rather than soils which form on weathering continental crust. Yet the volcanic ash is crucial in supplying the chemical elements required to grow vascular plants, as they require a wider range of elements (different ones for roots, stems and leaves) - found only together in volcanic dust. It is one of the 5 great periods of effusive volcanism.
Reefs, Deltas & Swamps
Reefs, Deltas & Swamps
In the Early Carboniferous, equatorial warm seas were depositing coral reefs (that became limestone) across much of what is now Europe and North America. The Middle Carboniferous marked a change when massive continental delta systems transgressed across those coral seas. Sediments were derived from weathering mountains upstream of the deltas - often made from quartz, feldspar and mica from the parent granites in the mountains - from Caledonian orogeny fold mountains. They covered these pro-deltas with muds (furthest offshore where deposition is slowest), that became sand deltas - deposits of sediments (particles of sand, gravel, and silt) at the mouths of rivers that flow into the ocean. It is during the Late Carboniferous that the river deltas were covered the with dense forests of giant tree ferns, horsetails and club mosses which encouraged swamps. These swamps - from what is now eastern USA to Ukraine - were periodically flooded by the sea, due to changes in higher latitude glaciation and interglacial melts controlling sea levels at this time (cyclothems). These seas would kill off anything in the swamps - and any soil developing nearby. The plant remains were buried, many metres deep, and preserved as the crust subsided beneath the mass volume of the deltas. Much of this later became coal.
Peak District
Peak District
Early Carboniferous coral seas now limestone White Peak
Mid-Carboniferous (pro-delta muds) Edale Shale Millstone Grit
(Sand Deltas)
Dark Peak,
Late Carboniferous Coal Measures
Palaeobotany
Palaeobotany
‘Introduction to Paleobotany, How Fossil Plants are Formed Thomas N. Taylor. Michael Krings, in Paleobotany (Second Edition), 2009
The development of paleoecology from an emerging field investigating past environments to a highly relevant applied science has developed over last decades [CH6] to help explain resource management questions in diverse habitats. Paleoecology, the study of past environments, is a rapidly changing field that involves the integration and synthesis of both botanical and geological information. In recent years there has been a concerted effort by many paleobotanists to understand the paleo-environment of fossil land plants more completely. A while back, Bateman and Scott (1990) examined the early Carboniferous plant-bearing deposits at Oxroad Bay, Scotland and found at eight levels in five successive facies (the character of a rock expressed by its formation, composition, and fossil content). These facies show the increasing influence of nearby volcanoes in the ocean-marginal setting. Details of the depositional environments through time at this site make it possible to understand plant adaptations to a rapidly changing, lowland environment, and to better understand both the biological and evolutionary importance of the floras.
Developing Soil
Developing Soil
This Carboniferous period played a significant role in early soil formation through the incorporation of these substances with much more organic matter and minerals now available in building aggregates. These created both resilient structures, and reduced movement providing stable spaces for life to live. The vastly increased surface area underground provided innumerable opportunities for interaction between land and water.
O Horizon Development:
O Horizon Development:
Thick layers of organic matter began to accumulate, leading to more pronounced O horizons particularly in swampy areas where waterlogged conditions had slow decomposition.
This period was when soil structure became and was called 'developed soil' in the RS paper. The challenge is: how did that 'developed soil' develop? Next to 'wetland'?
These early soils would have been formed from deltaic sediments, which would be different from most weathering profile soils, as they from top down rather than weathered bedrock up. There would also be a lot of lateral as well as vertical movement in this new dynamic.
There was the evolution of early vascular plants and the development of extensive forests of ferns, lycopsids (club mosses), and horsetails.
Witht he plant growth there was also more extensive root growth, creating rhizospheres for many other organisms to flourish. This RS article is primarily about the fungi round the roots, mycorrhiza, which we mapped out evolved nearly 400mya. The fungi produced glomalin, which springtails transform by glomalisation into GRSPs It is these substances that help stick the clay particles and bits of organic matter together. Were there other soil processes involved - like humification - producing humic substances, which we know play an important role in aggregation today?
Early soil building
Early soil building
Mountain ranges formed, volcanoes erupted and layers of sedimentary rocks turned upside down. This would have bought soil constituents together perhaps from different continents. New architectures arose that could resist these powerful new movements.
This period shows ‘rapid’ growth in early soil building of various sorts, depending on whether near sediments spreading, cracks opening up and lots of volcanic ash, along with swamps. That went on for tens of millions of years, when structures were now being thrown in with broken roots, rocks, silts, clays, volcanic ash along with that nutritious mud into crevices and cracks opening up all over. These would be the conditions to produce deeper soil, where more creatures can live.
Creatures had many new spaces, in sands, ponds, mudflats and swamps, with plants developing more sophisticated structures with the aid of volcanic ash and warm wet conditions. If ever there were ecological niches waiting to be exploited this would be it.
Early Soil structures
Early Soil structures
From the mid carboniferous onward, early soil structures developed so soil became a distinct entity, a pivotal era in Earth's history. Many important soil processes were already around, including mineralisation and glomalisation. The rhizosphere produced under existing plants now had a new environment swirling round - mud. The existing rhizosphere helped the primary aggregation of organic matter with clay minerals, which would have been swirling around over miles of massive delta shores. Aggregates provide structure and spaces to hold water and enable life more deeply and widely.
If you follow Stephen Jay Gould, this period represents the fast evolution punctuating the slower equilibrium of soil evolution. He proposed that that “Punctuated equilibrium holds that the great majority of species originate in geological moments (punctuations) and persist in stasis.” (Gould 2007) ). Yet, this implies a linear evolution, whereas we find in this period that soil evolution may have 'radiated'. This was to provide a dynamic for later evolution. It also sets up the notion that there may not be a monophyly of soil evolution - that there may have been several 'origins' that came together later.
Primary Aggregation
Primary Aggregation
Here a form of ''aggregation', that I am calling 'primary' was probably becoming widespread along the sides of the streams and deltas. It involves GRSPs, produced by glomalisation from glomalin, providing the glue and flocculation. This involved mainly making microaggregates.
In the past in the West, we have come to see humic substances, from humification, as main glue of aggregation. Yet this probably did not happen for another 100+m years later - and hence I am calling that 'secondary' aggregation.
Was it possible that primary aggregation alone could explain what happened in this Carboniferous period? It looks like recent Chinese research can throw a light. More on aggregation
The humid climate of the period fostered plant growth. There would have been free-living bacteria like Azotobacter that would have been providing nutrients like nitrates and phosphates to enable this. Yet this would lead to the accumulation of thick layers of peat in the swampy environments. Pot worms (enchytraeids) are found in peat bogs today, where they are dominant compared with earthworms. This may indicate their role all those years ago. Later, the peat led to the formation of coal, again evidence of the decomposition process not working.
Aeration
Aeration
Just as the insects in this period were large, the higher atmospheric oxygen might well have accelerated early soil forming processes - both chemical weathering (oxidation of bedrock) and accelerating oxygen fuelled biology. It could have influenced soil processes and nutrient cycling, potentially affecting the types of organisms living in the soil and aiding their aeration.
"Oxygen made up 20 percent of the atmosphere—about today’s level—around 350 million years ago, and it rose to as much as 35 percent over the next 50 million years.
Hence the insects were big
Soil development
Soil development
Early soil developed in the Carboniferous period, alongside the lush swamp forests and diverse ecosystems that thrived at the interface between water and land. Understanding the condition of the soil during this time provides valuable insights - a lens - into the environmental factors that shaped life developing on land.
Early soil structure started to form in this period, as three key elements, which had existed separately previously, came together. The mud in the form of clay particles, along with massive amount of plant debris and the glues to bind them together as primary aggregates. Up till now plants had hung on to surfaces, now they could burrow. The soil structures would vary - from ones on the surface (MISS conceptions) to those deeper down, but there would have been a lot of mixing.
MISS conceptions
MISS conceptions
"In recent years, an increasing number of the sedimentary surface textures have been attributed to surficial (relating to the Earth's surface) microbial mats at the time of deposition, resulting in their classification as microbially induced sedimentary structures, or MISS." Davies et al 2016 in 'Resolving MISS conceptions and misconceptions'. Clearly MISS probably played its part along with primary aggregation, in developing soil structures, but not yet clear what the 'microbial processes' are.
While there was some aerobic decomposition of plant debris by saprophytic fungi, partly dependent on the water table, there was little evidence of widespread decomposition by de-lignification or universal humification. This means most of the soil glue would have came from glomalin in the mycorrhiza and broken down by springtails to GRSP glues. There were no earthworms or insect larvae around, as they come much later when there is a lot more soil to turn over..
When the trees died in carboniferous times, "the bacteria, fungi, and other microbes that today would have chewed the dead wood into smaller and smaller bits were missing, they were not yet present. Trees would fall and not decompose back ”
(Ward and Kirschvink 2015)
Rhizosphere as a holobiont
Rhizosphere as a holobiont
We saw in earlier times that the rhizosphere was a holobiont of multiple species working together The roots with their fungi and creatures had a mud or sand medium to grow and expand into. As it did so it would have formed a mesh of filaments that would trap some mud and sand particles where they would stick to the roots with the aid of the GRSPs, produced by glomalisation via springtails and their like. GRSPs would be be encouraging microaggregates that would help the mud particles flocculate and thus reduce its movement. The roots, with mud sticking to them, had new boundaries to explore and further stabilise the mud.
This enabled the roots to grow more and provide more physical support for the plants. These larger plants with their vascular systems could also move water higher up vertically and so the plants could grow taller. There would have been a myriad variations of how this happened, depending on the sorts of roots, the mud and sand movements, and the creatures moving in and out. The springtails would provide their gluey poo, and the enchytraeid worms provide tunnels through the mud. They were not as strong as present-day earthworms, so not adept at burrowing more solid soil, but vary able to make tunnels in mud, providing ducts and airways for other to explore.
The soil of the Carboniferous period was not the soil we know today. These early soil relied on aggregation due primality to GRSPs, there was virtually no de-lignifcation and much less humification that makes many - larger - aggregates now. The decomposition then was not like it is today in many parts of the world. While good at building soil to support plants, the existing soil processes - largely aerobic - were not yet so good at decomposing dead debris left by these plants. There would have been some aerobic decomposition due to saprophytic fungi, and lower oribatids, but the process we know as 'humification' as the major process contributing to recycling processes, was not universal then. There would have been patches occurring in the environment, but not in the guts of billions of creatures running round.
Late Carboniferous
Late Carboniferous
In these swamps, dead plant material accumulated. Rapid production and, especially, rapid burial were key features in preserving the plant material, and with little anaerobic fermentation, this resulted in the accumulation of peat, which transformed into lignite - and later, coal under pressure and heat. There were also, organic rich, early soils called histosols. These organic-rich soils were often acidic due to the partial decomposition of plant material, and they supported the growth of specific types of vegetation adapted to such conditions.
The land surface now provided pervious habitat for a variety of arthropods, mainly springtails and early insects, like cockroaches. There was now also a substrate for creatures like mites, along with potworms and roundworms to explore.
The movement between aquatic to and terrestrial ecosystems during this period set the stage for the development of more complex terrestrial communities, including vertebrates, later. Early winged insects such as dragonflies and mayflies were totally on or in water. Insects had taken to the air, but not from land. The relationship of these early water borne flyers to later insect forms, is - you guessed it - subject to debate.
The Smithsonian say:“If you visited what is now North America 300 million years ago, near the close of the Carboniferous period, you would have been greeted by a very unfamiliar scene. The landscape was dominated by vast swamps filled with huge lycopods (relatives of club mosses that grew to the size of trees). Newly evolved characteristic of plants that drove oxygen levels to as high as 35 percent of the atmosphere during the Late Carboniferous. Lush equatorial forests produced a considerable amount of oxygen as a by-product of photosynthesis, but that alone wasn’t enough to drive atmospheric oxygen to such high levels. Bacteria use oxygen as they break down carbon-rich material, but lignin prevented this process until bacteria evolved the ability to decompose the compound. This biological quirk caused oxygen levels to soar.”
At the end of the Carboniferous period, when conditions drier, the coastal plain with its coal swamps became a central part of a giant continent called Pangaea. So surrounding soils would have been present at the centre of the new world in the, next,
Permian Period
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