From Swamp to Coal: The Chemistry of Buried Forests

Why the forest didn't just rot: burial beats oxidation

Most dead plant matter is destroyed within a year. Aerobic microbes oxidize it back to inorganic carbon in the reaction that runs the whole biosphere in reverse: (CH₂O)ₙ + n O₂ → n CO₂ + n H₂O, where (CH₂O)ₙ is shorthand for carbohydrate biomass. This reaction releases about −470 kJ per mole of carbon and, given oxygen, is fast and thorough. Coal exists only where this reaction is starved.

In a peat swamp, standing water seals the sediment from air. Oxygen has a solubility of only ~9 mg/L in cool freshwater, and decomposition consumes it within millimeters of the surface, driving the pore water anoxic. Below that, decomposition can only proceed through progressively less energetic anaerobic pathways, in the order that thermodynamics dictates (each yields less energy per electron than the last):

• Denitrification: 5 CH₂O + 4 NO₃⁻ + 4 H⁺ → 5 CO₂ + 2 N₂ + 7 H₂O
• Sulfate reduction: 2 CH₂O + SO₄²⁻ → H₂S + 2 HCO₃⁻ (the source of pyrite, FeS₂, and sulfurous coals)
• Methanogenesis: CH₃COO⁻ + H⁺ → CH₄ + CO₂, and CO₂ + 4 H₂ → CH₄ + 2 H₂O

These pathways are slow and incomplete. Crucially, they cannot touch lignin — the rigid aromatic polymer (built from coniferyl and sinapyl alcohol units) that stiffens wood — because lignin degradation is an oxidative process requiring O₂ and fungal peroxidases. So in an anoxic swamp, lignin and its aromatic breakdown products accumulate, forming a black, humic gel: peat. Peat still holds ~90% water and only ~55–60% carbon on a dry, ash-free basis, but the die is cast — this carbon is now on the burial track, not the recycling track.

Diagenesis: humic acids, gel, and the birth of lignite

The first true chemical stage of coalification, diagenesis, happens in the top few hundred meters at temperatures below ~50 °C. It is driven by microbes and low-grade chemistry, not deep heat. Reactive plant biopolymers (cellulose, proteins, most carbohydrates) are consumed or hydrolyzed, while the resistant residue condenses.

The dominant products are humic acids and fulvic acids — dark, high-molecular-weight polymers riddled with carboxyl (–COOH) and phenolic hydroxyl (–OH) groups. These acidic groups matter: peat pore waters are typically pH 3.5–4.5, acidic enough to further suppress bacteria and "tan" the deposit into a stable geopolymer. As burial continues, these humic substances lose water and crosslink, forming the structureless brown gel that defines lignite.

The defining chemical move of this whole stage is decarboxylation — the loss of carboxyl groups as carbon dioxide:

• R–COOH → R–H + CO₂ (removes oxygen and lowers the O/C ratio sharply)

Because CO₂ takes away one carbon and two oxygens together, decarboxylation is an efficient way to raise the fraction of carbon in what remains. This is why, on a Van Krevelen diagram (a plot of H/C versus O/C atomic ratio), the coal maturation path first drops almost vertically down the O/C axis: oxygen is being stripped faster than hydrogen. Lignite emerges with O/C already fallen from ~0.5 to ~0.3, but its hydrogen content is still high.

Catagenesis: the deep cook that makes bituminous coal

Push the deposit down to 2–4 km and the geothermal gradient (typically 25–30 °C per km) takes over. Between roughly 50 °C and 150 °C, microbes die off and the chemistry becomes purely thermal — catagenesis. Now hydrogen starts leaving in earnest, and the maturation path on the Van Krevelen diagram bends and plunges toward the origin along the H/C axis.

The dominant reaction is demethanation — thermal cleavage of aliphatic (–CH₃, –CH₂–) side chains off the aromatic skeleton, releasing methane:

• R–CH₃ (bituminous) → R–H + CH₄ — strips hydrogen-rich fragments, so the residue becomes more aromatic and more carbon-rich
• Dehydration: 2 R–OH → R–O–R + H₂O, plus loss of ring hydroxyls as H₂O

This is the same thermal window that generates natural gas, and it is why coal seams are dangerous: the coalbed methane and "firedamp" that cause mine explosions are the CH₄ expelled here, much of it still adsorbed on the coal's internal micropore surface (a gram of coal can hold several square meters of internal surface). As aliphatic bridges are cleaved and aromatic rings condense and stack, the material develops the shiny, brittle, banded character of bituminous coal — the world's most abundant and most burned rank, and the feedstock that is baked into coke for steelmaking.

Metagenesis: aromatic sheets and the road to anthracite

Above ~150–200 °C, in the deepest basins or where tectonic heat intrudes, coalification enters metagenesis. Almost all the remaining oxygen and hydrogen is gone; what's left is nearly pure carbon reorganizing itself. The aromatic rings — already the dominant motif — condense into ever-larger polycyclic aromatic sheets that begin to stack and align into parallel layers, approaching the structure of graphite.

This structural ordering is exactly what vitrinite reflectance measures. Vitrinite, the coal maceral derived from woody tissue, reflects more light as its aromatic layers grow flatter and more ordered. Reflectance Rₒ climbs from ~0.3% in lignite to over 2.5% in anthracite, making it geology's most reliable paleothermometer — a fossilized record of the maximum temperature the rock ever reached.

The end product, anthracite, is over 92% carbon, burns with almost no smoke or flame (little volatile matter is left to release), and has a hardness and metallic luster closer to a mineral than to wood. Take it a step further — to >300 °C and high pressure under regional metamorphism — and the carbon sheets lock into true crystalline order: graphite. Coal is, chemically, a snapshot of an unfinished journey from cellulose toward graphite, frozen at whatever temperature the burial stopped.

Reading the ratios: the Van Krevelen fingerprint

The single most useful way to see coalification chemistry is the Van Krevelen diagram, which plots the atomic H/C ratio (vertical) against the atomic O/C ratio (horizontal). Every stage of maturation moves the material along a characteristic track toward the origin (pure carbon):

• Diagenesis (peat → lignite): a steep drop in O/C from ~0.5 to ~0.3, with little H/C change — this is decarboxylation and dehydration stripping oxygen.
• Catagenesis (lignite → bituminous): the path bends left and down as H/C falls from ~0.9 to ~0.6 — this is demethanation stripping hydrogen as CH₄.
• Metagenesis (→ anthracite): both ratios collapse toward zero as the residue becomes aromatic carbon.

The starting point on this diagram also reveals the source material. Coals derived mostly from land-plant lignin (Type III kerogen) start low in H/C (~1.5) and are gas-prone. By contrast, algal and planktonic organic matter (Type I/II kerogen) starts high in H/C (~1.7) and is oil-prone — which is why lakes and marine basins make petroleum while swamps make coal. The same anoxic-burial chemistry, applied to different feedstocks, forks into the world's two great fossil-fuel families.

The Carboniferous anomaly and the carbon it locked away

Roughly 90% of the world's coal formed in two intervals: the Carboniferous–Permian (~360–250 Ma) and the Cretaceous–Paleogene. The Carboniferous burst was extraordinary. Tropical continents were covered in vast lowland swamps of giant lycophytes (like Lepidodendron, 30-meter "scale trees") whose bark was up to ~50% lignin and other decay-resistant compounds. One long-debated hypothesis holds that white-rot fungi — the only organisms that can fully break down lignin, using ligninolytic peroxidases — had not yet evolved the enzymatic toolkit to keep pace, so lignin piled up faster than anything could destroy it.

The consequence was planetary. Burying that much reduced carbon is chemically identical to running photosynthesis and not reversing it: for every carbon atom locked in a swamp, an O₂ molecule was left in the air. Organic-carbon burial drew atmospheric CO₂ down and pushed O₂ toward an estimated ~35% (versus 21% today), enabling insects that breathe by diffusion to grow to enormous sizes — Meganeura dragonflies with 70 cm wingspans. The coal we burn today is, quite literally, the buried remains of that oxygen surplus. Combusting it now — C + O₂ → CO₂, ~−394 kJ/mol — reverses in decades a carbon burial that took the Carboniferous tens of millions of years, which is the core reason coal is the most carbon-intensive fossil fuel per unit of energy (~95 kg CO₂ per gigajoule for anthracite).

The coalification series: from living plant to anthracite, tracked by composition and rank indicators.

Stage	Carbon (wt%, daf)	Approx. H/C atomic	Approx. O/C atomic	Vitrinite Rₒ (%)
Wood / plant tissue	~50	~1.5	~0.7	—
Peat	55–60	~1.4	~0.5	~0.2–0.3
Lignite (brown coal)	65–72	~0.9	~0.3	0.3–0.5
Sub-bituminous	72–78	~0.8	~0.2	0.5–0.6
Bituminous	78–92	~0.7	~0.1	0.6–2.0
Anthracite	92–98	~0.4	<0.05	2.0–>3.0