Tidal Flats

Tidal flats form in protected coastal areas when strong wave action is absent and tidal range is wide (e.g. Prothero and Schwab, 2004). In arid climate zones, where tidal flats grade into coastal sabkhas, they may form refugia for diverse epibenthic microbial communities which thrive from the subtidal zone through the intertidal zone, with semi-daily inundation, up to the supratidal zone which is flooded during spring high tides and storm surges only. In such environments, microbial mats are best developed in the intertidal and lower supratidal zones, where they tend to form continuous, strongly cohesive layers on the sediment surface. Upslope towards the sabkha, they fade and loose their microbial consortium complexity due to deteriorating conditions, mainly increasing rates of desiccation, salinity and evaporite mineral precipitation.

Tidal flats are very low-gradient and low in relief, and are typically underlain by fine-grained unconsolidated sediment. Influx of sediment is mainly from the sea in the intertidal zone, and from the hinterland in the supratidal zone and sabkha, however, with a wide range of source overlapping. Most favorable for the establishment and growth of microbial mats are fine-grained sand to silt which possess the best capillarity combined with fastest capillary movement of pore water (e.g., Correns, 1934) and thus can provide mats with optimal capillary water in periods of subaerial exposure (Gerdes and Krumbein, 1987).

Hydrologic conditions on tidal flats are determined by regular or periodic tidal inundations and a continuous downslope flow of groundwater which is recharged episodically by exceptional flooding events and rainfall (e.g., Sanford and Wood, 2001). Upward leakage of groundwater combined with 'evaporative pumping' (Hsu and Siegenthaler, 1969) provide the mats with moisture in the supratidal zone and sabkha. Tidal flats may form in both terrigenous clastic and biogenic-chemical (carbonate and evaporite) coastal depositional systems, whereby climate is an important major control. In temperate humid zones (e.g., southern North Sea; east coast of North America), tidal flats usually are silici-clastic without major precipitation of carbonate and evaporite minerals; in semiarid to arid, subtropical and tropical zones (e.g., Mediterranean coast of southern Tunisia; Persian Gulf), evaporitic mineral formation and carbonate precipitation are of great importance, the latter supporting early lithification of microbial mats (cf. stromatolites).

In this paper, we present examples of microbial mats and related features developed in supratidal zones of siliciclastic tidal flats in (1) the temperate humid zone: Amrum Island, southern North Sea; (2) the subtropical arid zone: Bhar Alouane and El Gourine, Mediterranean coast of southern Tunisia.

4.2. AMRUM ISLAND; SOUTHERN NORTH SEA, GERMANY 4.2.1. Locality and Mats

The western coast and beach (Kniepsand) near Norddorf village on Amrum Island (Fig. 7A) is a type locality of mat-forming microbial communities, arranged in distinctly colored layers consisting chiefly of cyanobacteria and sulfur bacteria. The multilayered microbial mats have been named "Farbstreifensandwatt" (versicolored sand flat) by Schulz (1936). Their occurrence is restricted to the (lower) supratidal zone between a coast-parallel berm and stabilised dunes in the hinterland. Parts of the area are periodically inundated during spring tides, whereas rainfall may episodically provide additional water to the whole area. An important and, over long periods, continuous source of moisture for the mats is from upward leakage of groundwater whose hydraulic head is situated in the hinterland, several meters above the supratidal zone.

In section, the mats typically exhibit three differently colored layers: (1) a greenish surface layer, 2-10 mm thick, consisting of cyanobacteria among which Microcoleus chthonoplastes, Oscillatoria sp. and Lyngbya sp. predominate (Hoffmann, 1942); (2) a pink to red middle layer, 2-5 mm thick, consisting of anoxygenic phototrophic sulfur bacteria (e.g., Thiopedia rosea) which coat sediment grains and fill pore spaces; (3) a black bottom layer of variable thickness,

Figure 7. Location and structures related to progressive mat desiccation and shrinkage. All photographs from Amrum Island. (A) Location of Amrum Island within the German Wadden sea area. (B) Initial sigmoidal and tri-radiate shrinkage cracks. Scale (coin) is 2 cm. (C) Polygonal network of shrinkage cracks with upturned margins. Scale bar is 20 cm. (D) Shrinkage cracks with upturned margins. Scale (coin) is 2 cm. (E) Shrinkage cracks with involute margins surrounding irregular to subcircular openings developed in a thin mat. (F) Polygons of detached mat after intense desiccation. Scale bar is 20 cm. (G) Crumpled mat polygon after complete desiccation. Scale bar is 20 cm. (H) Detached mat deformed by tractional forces, e.g., currents or wind.

Figure 7. Location and structures related to progressive mat desiccation and shrinkage. All photographs from Amrum Island. (A) Location of Amrum Island within the German Wadden sea area. (B) Initial sigmoidal and tri-radiate shrinkage cracks. Scale (coin) is 2 cm. (C) Polygonal network of shrinkage cracks with upturned margins. Scale bar is 20 cm. (D) Shrinkage cracks with upturned margins. Scale (coin) is 2 cm. (E) Shrinkage cracks with involute margins surrounding irregular to subcircular openings developed in a thin mat. (F) Polygons of detached mat after intense desiccation. Scale bar is 20 cm. (G) Crumpled mat polygon after complete desiccation. Scale bar is 20 cm. (H) Detached mat deformed by tractional forces, e.g., currents or wind.

enriched in iron sulfide and H2S resulting from the activity of sulfate-reducing bacteria. Similar mats also occur on several of the other islands fringing the Wadden sea area along the North Sea coast from Denmark to the Netherlands (Fig. 7A). Detailed descriptions of their distribution and composition, specifically in respect of Mellum Island, have been given, e.g., by Gerdes et al. (1985a, b, 2000), Stal and Krumbein (1985), and Noffke and Krumbein (1999).

Microbial mats that repeatedly are subaerially exposed, as in the supratidal zone of Amrum Island, tend to develop strongly cohesive, 'felty' layers in the upper photic zone, and a 'leathery' surface. The internal structure of the 'felty' layers is best described as a "condensed fibrillar meshwork" consisting of more or less parallel, "horizontally stretched ensheated filament bundles" (Gerdes et al., 2000, 285) which in neighbouring laminae develop perpendicular orientations, leading to a kind of 'plywood texture' (Fenchel and Kühl, 2000). Bacterial layers below are much less cohesive due to the very uncommon presence of filamentous species. A further component in microbial mats is various EPS which are secreted by coccoid cyanobacteria in great volume, surround the bacterial cells and sediment grains, and partly may exist in an aggregated gel state of high cohesiveness (Decho, 1994; Stal, 2000). Such EPS frequently form a 'leathery' surface film which combines high mechanical resistance with very low permeability, inhibiting the escape of water and even gas rising up from below. Finally, a sedimentary component is introduced into the mat by 'trapping and binding' of detrital grains on the mucilaginous mat surface, and subsequent overgrowth and incorporation of grains into the mat.

4.2.2. Extreme Conditions

Although in the supratidal zone of Amrum Island mats have been more or less continuously observed since the middle of the nineteenth century (e.g., Oerstedt, 1842), they are repeatedly exposed to extreme conditions in two contrasting situations: (1) in hot summers with prevailing eastern winds over longer periods, when spring tides do not reach the supratidal zone and groundwater supply ceases due to lacking recharge by rainfall; (2) in the winter season (between November and April) when strong western winds and storms may raise the water level up to several meters above normal. In both these cases, the mats may be widely destroyed by desiccation or erosion, respectively, but nevertheless recur in the following spring to early summer, thus demonstrating their ability to survive even under extreme conditions.

4.2.3. Structures Related to Subaerial Exposure and Desiccation

Solar irradiation and related evaporation gradually lead to desiccation of the subaerially exposed mat. Thereby, EPS containing >95% water by weight (Sutherland, 1977) act as a buffer for some time, though themselves shrinking in volume with progressive loss of water. At some stage, the contractional force exerted by the shrinkage process overcomes the physical strength of the material and cracking occurs. In the early stages of desiccation, shrinkage and cracking usually are restricted to the upper, green layer of filamentous cyanobacteria, which during the process gradually becomes detached from the layers below. This happens at the latest when groundwater flow has ceased and capillary water is no longer available.

Shrinkage cracks: Shrinkage cracking may either start at the tip of a small, dome-like elevation and develop a triple junction from which characteristic 'tri-radiate cracks' propagate along the mat surface; or on the crest of a small ridge and then leads to more linear and sigmoidally 'curved cracks' with tapering ends (Fig. 7B). Since the entire mat surface is under contractional stress during desiccation, cracking likely will occur at numerous places simultaneously. This and crack propagation with progressive shrinkage will eventually lead to networks of cracks, typically arranged in polygonal patterns (Fig. 7C).

Upturned and curled crack margins: A further typical feature of shrinkage cracks are their 'upturned margins' (Fig. 7D). Upturning of crack margins results from differential contraction of the shrinking mat layer and EPS film on top. At places where the upper green mat layer is only thinly developed, e.g., towards the outer margins of the mat-covered area, differential contraction may lead to involute or 'curled margins' with more than a full revolution of the mat layer (Fig. 7E). This behaviour is usually combined with a tendency to create irregular to circular openings in the mat, exposing deeper layers of the system.

Fatal desiccation: Once a polygonal network of shrinkage cracks has developed all over the mat, and the upper, green mat layer has been detached from the layers below, cracks increasingly become wider due to further shrinkage of the polygons (Fig. 7F). Finally, the isolated polygons themselves undergo intense shrinkage into irregularly folded and crumpled fragments (Fig. 7G). In these, the completely desiccated mat has remained as a rigid skin, c. 1 mm thick, which can survive transport by wind over considerable distances. Crumpled mat fragments, or pieces thereof, may thus be found embedded in terrestrial sediments far away in the hinterland (see also "meteor paper", Ehrenberg, 1839; Krumbein et al., 2003).

4.2.4. Structures Related to Mat Erosion

Once established on the sediment surface and not seriously injured by desiccation, mats are quite resistant against 'normal' wave action during spring high tides. Shrinkage cracks, however, provide points of attack for erosion, whereas strongly desiccated and detached mats will instantaneously float and become deformed (Fig. 7H). If a mat is locally eroded by current or wave action, 'erosion pockets' may form and develop a rippled surface on the exposed non-cohesive sediment. Vice-versa, if a mat is widely eroded, parts of it may still remain as 'erosion remnants' (see Gerdes et al., 1993; 2000 and documentation therein). According to Gerdes et al. (1993), 'erosion pockets' preferentially develop in the upper intertidal zone where the mat is juvenile and thin, whereas 'erosion remnants' occur towards the supratidal zone where the mat is thicker and tough.

Fragments of eroded mats may survive for some time and be transported by currents and waves to finally be deposited along a trash line or be included in new sedimentary deposits. Such 'mat chips' which may be described as 'organically bound mineral aggregates' (Gerdes et al., 2000) are characterized by high cohe-siveness and may attain various shapes ranging from very irregular to well rounded (see 'sand clasts' below).

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