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Peat development is closely tied to many of the classification schemes as, quite often, the environment that allowed the peat to start forming is the basis of the classification.

Here we will examine typical succession sequences, without going into details of typology, which was covered in section 3.2, above. As already mentioned, it is very rare for peat types to occur in isolation. The only exceptions are blanket mires, which tend to be thin and form on upland slopes where the net water input exceeds the runoff and loss through evapotranspiration. In these mires a relatively thin layer of terrestrial peat forms. Sometimes, a raised mire forms over a blanket mire, depending on the slope of the site.

3.3.1. Peat formation processes

In order for peat to start to form, one key condition must occur. The water input into the immediate environment needs to be larger than the output. This can result either from rainfall only, as in ombrogenous bogs, or from a mixture of rainfall and

throughflowing water, as in topogenous fens. Waterlogging slows down the decay of organic material, and as the oxygen level within the water is depleted by the limited decay process, decay of organic matter slows even further. The relationship is non-linear. If the oxygen levels are not replenished by diffusion (which is why fast flowing water systems tend not to form peat), the only decay processes left are anaerobic ones. This leads to very slow decay of organic matter, and once deposition rates outpace decay rates, peat will start to form from this accumulated matter.

Peat does not need open water to start to form; it can start to form in waterlogged soils as well. Key drivers in peat formation are the sphagnum mosses. This species of moss is ubiquitous in the ‘raised bog’ stage of peat development, and their dominance is key to bog formation (Koster & Favier 2005, 166) as they modify the ecosystem to make it more compatible for themselves and less so for other plants. The key to this is their ability to acidify the water around them through cation exchange, making the environment hostile to other plant species. Raised bogs are often therefore less species rich than fen systems, where sphagnum plays less of a role in the mire ecosystem.

Generally speaking, there are two main types of peat formation.

Peat growth may start in stagnant or slow flowing water, as organic sediments build up and gradually terrestrialize a lake or valley floor. This leads to a succession of peat types, starting with lacustrine, limnic peats or perhaps accumulations of dy, gyta, or lake marls, then telmatic peats as shallow water plants and swamp or reed bed vegetation moves in. Eventually, terrestrial peats will form. These might be woody fen peats at lower elevations and systems that are base rich or mineral rich, or at higher elevations or in more acidic, oligotrophic conditions, a raised bog might develop. It is also possible, given local conditions or shifts in climate, that a raised bog might succeed a fen mire and vice versa.

Alternatively, changes in climate or local topography might trigger a process of paludification, whereby a soil starts to receive more moisture than it sheds. Over time, telmatic and then terrestrial, or simply terrestrial peat may form as the vegetation changes in response to the shift in moisture regime (see Figure 3.2). This seems to have been the process by which much of the upland peat in great Britain was formed, due to climate changes in the Flandrian (Simmons 1996), and also partly due to human influences in the uplands in the Mesolithic and Neolithic, clearing trees and thus changing the hydrological conditions of the soils. The resulting mire types will largely depend on the climate and elevation the process occurs at. The inception stages and the deposits left behind will also be a function of this; in a forest you might get a horizon of preserved trees or tree stumps, in quite a woody peat layer, before the succession gives way to telmatic peats formed in the swamp and then terrestrial peats

as a raised bog forms. Over thin upland soils, with no forest present at the time of peat inception, there may be a pretty straightforward change to terrestrial peat in the form of a blanket mire, but with associated gleying of the underlying mineral soil due to the increased water throughputs that start the peat growing process.

Accumulation rates vary considerably. They can be as high as 5cm/ year in eutrophic lakes, but more typically 20-100cm per 1000 years (though values between 4 and 500 cm per 1000 years have been recorded (Koster & Favier 2005, 168). The

accumulation rate generally decreases with age, and also depends on mire types, for example in fens, primary productivity is higher, but so are decay rates. Accumulation is not just about the height of the bog; as material is added to the top of the system in the acrotelm, the catotelm is compressed (Clymo 1983). This causes serious

complications of interpretation for ecologists and archaeologists, because even if the accumulation rate is known or can be estimated, compression rates vary, so there is no simple correlation between depth and the passage of time since deposition. This can also compress and distort archaeological deposits, concatenating sequences and physically altering artefacts and structures.

3.3.2 Peat formation timescales

In the study region, all peat deposits have formed since the end of the last Devensian Ice Age and so belong in the Holocene. The Holocene has four climatic subdivisions (pollen zones V –IX) (Darvill 2002, QR4) which are more relevant to the formation of ombrotrophic peats (often started by climate shifts) than minerotrophic ones (that generally occur in association with rivers and coastal systems).

In the lowlands, peat formation commenced with the start of the Flandrian, largely by processes of terrestrialization in depressions in glacial till and in the newly forming valley systems of rivers. Changes in sea level and rainfall levels contributed, especially in coastal regions where hydrological systems were affected (i.e. slowed down or damned up) by eustatic and climatic sea level shifts. Extensive peat ecosystems formed in Northwest Europe (Koster & Favier 2005) by the time of the Neolithic, and continued to grow and evolve until heavy exploitation and drainage commenced during the Middle Ages.

In the uplands the system is more complex. Blanket bogs are rare in continental Europe but much more common in the UK and Ireland as they require a cool, humid oceanic climate. Upland raised bogs and blanket bogs cover a lot of the uplands of the UK, but this is not the ‘natural’ state of these uplands; in the early Flandrian, they were largely forested, to elevations that allowed tree growth. However, through a mixture of human activity (undisputed for the Neolithic, and tentatively identified in the Mesolithic in some areas (Simmons 1996; 2003) and shifts in climate during the second millennium BC towards wetter, cooler conditions and following the complex process shown in Figure 3.2, many of these areas became upland moors; extensive peatland environments. This process was ongoing throughout prehistory, and from the Iron Age onwards in the UK, seems to have kept human settlement activity away from these zones, at least until the Middle Ages (Van de Noort et al. 2002a).