The Plants

How Coal Forms

Peat

Subsidence

Coalification

The Negative Greenhouse

Capturing Carbon Fast Enough

Bibliography

Figure 2.1 The main coalfields in England and Wales in Westphalian times which formed in swamps north and south of the Wales-Brabant upland area. Exposed coalfields shown in black, concealed coalfields in grey, areas of non-deposition in brown; areas of marine deposition in fine stipple. For coalfields immediately north of the Wales-Brabant Massif: 1, Leicestershire – S. Derbysire; 2, Warwickshire; 3, S. Staffordshire; 4, Wyre Forest; 5, Coalbrookdale; 6, Shrewsbury; 7, Denbigh; 8, Flint. From Cleal et al. (2010).

The Plants

The Carboniferous lowland forest was dominated by five major plant groups, lycopsids (club mosses), sphenopsids (horsetails), filicopsids (ferns), pteridosperms (seed ferns – a slightly more evolved form of a fern) and cordaites (rather like a conifer). The first three reproduced with simple spores, while the last two used seeds. All five of these types produced large trees but also smaller bushes and shrubs (Figure 2.2). These are the plants that formed Carboniferous coal.

But what is interesting here is that swamp forests and dryland forests were a new feature of the land at this remote time in Earth’s past. Although plants were well established on some parts of the land, large areas were probably still un-colonized by plants. To understand why ancient forests like these were so different to modern forests we have to understand a little of how land plants evolved.

Fossil evidence for the earliest land plants is limited to spores from moss or hornwort-like plants 475 million years old, 175 million years before the coal swamp forests. At this time there was no sign of the mysterious plants that produced the spores, and there was no sign of them until 50 million years later! The plants, when they eventually do appear as fossils, show a simple vascular system in that they had tissues for conducting water, minerals and photosynthetic products through the plant. This also meant that they had a certain rigidity (from vascular pressure) and so could reach up for sunlight, though not very far. The earliest known of these plants Cooksonia (mostly from the northern hemisphere) and Baragwanathia (from Australia) were only very small, millimetres high at the most. By 400 million years ago primitive plants had created the first recognizable soils that harboured mites and scorpions. Strangely these plants didn’t have leaves; small leafless shrubs filled the landscape until the tree-like fern Archaeopteris appeared with the first leaves. By the start of the Carboniferous period 360 million years ago where all the action of this chapter takes place, plants and trees with leaves were common and the first seed-forming plants had appeared. For our story, the next most important event was the appearance of lignin.

Lignin (from the Latin word lignum meaning wood) is one of the most abundant organic polymers on Earth, exceeded only by cellulose. At the beginning of the Carboniferous it appears that a number of plant types were able to make lignin. Its main function was to strengthen wood (or xylem cells) in the stems of trees so that they could grow taller – at least taller than their competitors. Vascular pressure is not sufficient to allow trees to grow very tall. This ‘invention’ not only made trees taller and therefore more efficient photosynthesizers but also made them more difficult to digest.

Let me explain this. The lignin in trees – being a large complex molecule – is much more difficult to degrade than other plant tissues and is physically stronger too. When Carboniferous trees and plants with lignin died, their lignin tissues persisted longer than other tissues, remaining solid and un-rotted even in swamp waters. It may be that early primitive Carboniferous microbial scavengers, fungi and bacteria weren’t able to break the lignin down either. A lot of carbon in trees is in lignin, and so if the trees died and were preserved in peat quickly, most of the carbon was buried. A large coal-forming Westphalian tree might have contained 3200 kg of carbon, mostly in its lignin, and if this was buried then most of the carbon was too. If we think of this happening over a vast area of 2 million square kilometres that’s a lot of carbon. We could say then that we owe much of our coal to the invention of lignin.

Apart from the enormous amount of carbon burial going on, the multi-storey photosynthesis machine that was a Carboniferous forest was also producing an enormous amount of oxygen. Computer models indicate a rise of oxygen from 21% to 35% in the atmosphere due to the appearance of the Carboniferous forests. The high oxygen levels meant that wildfires were probably more common, being easily and often started by lightning strikes (Figure 2.3). The abundant oxygen also stimulated the metabolisms of insects and encouraged insects like dragon flies to grow very large (e.g. the Carboniferous fossil dragonfly Meganeura which had a wingspan of 60 cm).

What would the coal forests have looked like? Much of our detailed knowledge of the ecology of the swamp forests comes from the so-called coal balls. These unlikely objects are not coal at all but concretions of minerals that formed shortly after the burial of the plant material in the swamp (Figure 2.4). Two factors favour the lucky palaeontologist who finds a coal ball: one is that plant material (as well as insects and other flora and fauna) is often very well preserved inside; the other is that the material is not squashed as it would be in normal coal. The organic matter that accumulates to make coal is often compacted by a factor of 10 before it becomes coal. In a coal ball, the material is almost uncompressed, the hard minerals resisting most of the compaction. The first scientific description of coal balls was made in 1855 by Sir Joseph Dalton Hooker and Edward William Binney, who reported on examples in the coal seams of Yorkshire and Lancashire, England; but more recently much of the concentrated research on coal balls has...