Salt marshes, most now embanked, together with genetically related wetlands and high intertidal flats, make a major environmental contribution to the lowland coasts of Northwest Europe. They occur in many different contexts, but chiefly on open and barrier coasts and in estuaries and embayments, and range greatly in scale, from a modest total that measure hundreds of square kilometres in individual extent, to an enormous number each of an area no greater than tens to a few hundred hectares. These marshes and associated environments are under complex natural controls and experienced from the mid-Holocene onward human exploitation and, increasingly, interference. The main external controls are the sea-level, tidal and sediment-supply regimes. Intrinsic infuences are provided by the halophytic vegetation and sediment autocompaction. Upward sea-level movements and autocompaction combine to provide accomodation space within which marshes build upward. Field data and simulation modelling show that youthful mineralogenic marshes grow up rapidly and can mature within a few hundred years of inception. They consist of a vegetated platform dissected typically by extensive networks of blind-ended, branching tidal creeks and gullies. The how-resistant surface vegetation, shaping the combined wave-tide boundary layer on the platforms, both traps and binds tidally introduced mineral sediment, but also contributes an organic component of indigenous origin to the deposit. When sea-level becomes stable or falls, however, in response to century-millennial scale fluctuations, the organic sediment component becomes dominant and mineralogenic marshes are transformed into organogenic ones. Organogenic marshes normally display a considerable range of sub-environments which create much spatial variation in the peat facies which accumulate. At an advanced stage, domed raised bogs, rising significantly above the general landscape, may appear on the marshes. Because peat is such a porous and permeable sediment, and there is little or no tidal inundation, organogenic marshes in Northwest Europe typically lack surface channels for internal drainage. The stratigraphic sequences accumulated during the Holocene beneath coastal marshes and high tidal flats typically present an alternation on a vertical scale of decimetres to metres of silts (mineralogenic marshes, high intertidal mudflats) and peats (organogenic highest intertidal-supratidal marshes). Coastal barriers and some channels are represented by local accumulations of sand and/or gravel. The silts and peats form couplets which are generally considered to be related to fluctuations of sea-level about the general upward trend. Field investigations and modelling show that, in areas where marshes are mature, the upward change from an organogenic to a sequence of mineralogenic marshes (transgressive overlaps) is accompanied by the initiation and invasive development of a branching network of tidal creeks. These decay and infill during the reversal of the environmental sequence and the approach, expressed as a series of repressive overlaps, of the next set of peat-forming conditions. The operation of the continuous, progressive, irreversible and asymptotic process of sediment autocompaction exerts a major, secondary control on depositional regimes and marsh behaviour. A variety of local responses are consequently possible in an extensive marsh, even though the marsh may be everywhere in dynamic equilibrium with environmental factors. Autocompaction also strongly shapes the character of Holocene coastal sequences as now perceived, introducing significant stratigraphic distortions and displacements which, for the time being, limit the accuracy of sea-level curves and rates of sea-lever change based on dated intercalated peats. Prehistoric humans benefitted from the resources of coastal salt marshes, especially at the times when peat marshes begin to be transgressed. Feat domes not yet fully collapsed may have provided vantage points for seasonal ocupancy or even settlement from which the richer resource of the mineralogenic marshes developing on lower ground could be exploited. Increasing human interference on salt marshes over the last millennium, chiefly through wholesale land-claim and set-back, augmented by rising sea levels and continuing autocompaction, has led to a variety of poorly understood but generally deleterious effects in the lowland coastal zone. These include major changes to tidal and sedimentary regimes. (C) 2000 Elsevier Science Ltd. All rights reserved.
