What Is a Bog? Waterlogged Ground That Preserves the Past

Wetlands are classified by their water source, and the distinction matters because it determines chemistry and vegetation. A bog is ombrotrophic, meaning it receives water only from rainfall and has no contact with groundwater or streams, so it gets almost no dissolved minerals and is consequently acidic and nutrient poor, typically with a pH between three and four and a half. A fen, by contrast, is fed by groundwater, is much richer in minerals, is closer to neutral and supports quite different plants. Marshes and swamps are wetter still and dominated by grasses or trees respectively. Bogs are built largely by sphagnum mosses, which do more than grow there: sphagnum actively acidifies its surroundings by exchanging hydrogen ions for nutrients, holds many times its own weight in water, and grows upward while its lower parts die and compress, so the moss engineers the very conditions that suit it and excludes competitors. A raised bog eventually forms a dome standing above the surrounding land, fed entirely by rain.

Decomposition is mostly the work of microbes, and four conditions in a bog suppress them at once: waterlogging, which blocks oxygen since water holds far less than air and diffusion through it is slow; acidity, which most decomposer bacteria tolerate poorly; low nutrients, particularly nitrogen, which limits microbial growth; and low temperatures in the northern bogs where most peat occurs. Sphagnum adds a fifth by releasing polysaccharides and phenolic compounds that inhibit microbial activity directly. The result is that organic matter accumulates faster than it breaks down, and peat builds at something like half a millimetre to a millimetre a year, so a bog several metres deep represents many thousands of years of continuous accumulation. That is why peat bogs hold an estimated third or more of all soil carbon worldwide while covering only about three percent of land area, and why draining or burning them releases carbon that took millennia to accumulate.

Because the layers accumulate in order and the contents survive, a peat core is a chronological record readable by several methods at once:

• •Pollen grains, which have extremely durable outer walls and are identifiable to genus, so each layer records what was growing in the region and reveals forest clearance, the arrival of agriculture and climate shifts
• •Plant macrofossils, seeds, leaves and mosses, which record conditions on the bog surface itself and therefore wetness
• •Testate amoebae, single-celled organisms with preserved shells whose species composition tracks the water table closely
• •Charcoal, recording fire history, and volcanic ash layers, which give precisely dated tie points across separated sites
• •Atmospheric metal deposition, since lead from Roman smelting and from twentieth-century petrol is recorded in the layers, making bogs a record of industrial pollution
• •Radiocarbon dating throughout, giving the whole sequence a timescale

The same conditions preserve human remains in a way that is unlike anything else. In an acidic bog, calcium dissolves out of bone while sphagnum's polysaccharides bind to skin proteins and tan them, so bodies emerge with skin, hair, nails, stomach contents and facial expressions intact while the skeleton has softened or vanished. Several hundred are known from northern Europe, with famous examples including Tollund Man and Grauballe Man in Denmark and Lindow Man in England. Many date from the Iron Age, many died violently by strangulation, throat cutting or blows, several show multiple causes of death applied at once, and the combination has generated a long debate over whether they were executed criminals, sacrificial victims or something else, with no resolution that covers all cases. The preservation is good enough that stomach contents reveal the final meal, isotopes in hair reveal where the person had been living in their last months, and facial reconstruction is possible. The bodies are also fragile once exposed, needing immediate conservation, and most were found by peat cutters rather than archaeologists, so context was frequently destroyed.

Bogs have been drained for agriculture and forestry, cut for fuel and for horticultural compost, and burned for grouse moor management, and a drained bog stops storing carbon and starts releasing it, turning a sink into a source that can emit for decades. Peat extraction for gardening has been progressively restricted in several countries because the material is functionally non-renewable on human timescales. Restoration is possible and increasingly practised: blocking drainage ditches to raise the water table, removing invading trees that dry the surface further, and reintroducing sphagnum, after which a degraded bog can begin accumulating again, though rebuilding what was cut takes millennia. Bogs also reduce flood peaks by holding water, support specialised species found nowhere else including carnivorous sundews adapted to the nitrogen shortage, and their protection is now treated primarily as climate policy rather than only as nature conservation.

A bog is fed only by rainfall, which makes it acidic and nutrient poor, and sphagnum moss actively maintains those conditions. Waterlogging, acidity, low nutrients and cold together stop decomposition, so dead plants accumulate as peat at roughly a millimetre a year, storing a third or more of the world's soil carbon on three percent of its land. The ordered layers preserve pollen, charcoal, metals and radiocarbon dates, making a core a detailed environmental archive, and the same chemistry tans human remains into bog bodies.