Gisela Gerdes

Microbially Induced Sedimentary Structures: A New Category within the Classification of Primary Sedimentary Structures

ABSTRACT Cyanobacterial films and mats syndepositonally influence erosion, deposition, and deformation of sediments. The biomass levels surface morphologies, and microbial mats stabilize depositional surfaces and shelter the sediment against erosion or degassing. Growing microbial mats dredge grains from their substrate upwards, whereas cyanobacterial filaments that are oriented perpendicular to the mat surface reach into the supernatant water and baffle, trap, and bind suspended particles. These and similar biotic-physical interactions are reflected in syndepositional formation of microbially induced sedimentary structures. We distinguish structures on bedding planes (leveled bedding surfaces, wrinkle structures, microbial mat chips, erosional remnants and pockets, multidirectional ripple marks, and mat curls) and internal bedding structures (sponge pore fabrics, gas domes, fenestrae structures, sinoidal laminae, oriented grains, benthic ooids, biolaminites, mat-layer-bound grain sizes). We propose to place this group of microbially mediated structures as a fifth category (bedding modified by microbial mats and biofilms) in Pettijohn and Potters (1964) existing classification of primary sedimentary structures.

Benthic cyanobacteria and their influence on the sedimentary dynamics of peritidal depositional systems (siliciclastic, evaporitic salty, and evaporitic carbonatic)

Abstract Peritidal sedimentary systems are widely colonized by benthic cyanobacteria that form biofilms and microbial mats. The bacterial communities interfere with physical and chemical sedimentary dynamics, which is documented by the formation of Microbially Induced Sedimentary Structures (MISS). The structures form a new fifth category in the existing Classification of Primary Sedimentary Structures. Siliciclastic depositional systems are dominated by physical dynamics. By biostabilization, cyanobacteria shelter their substrata against erosion during periods of intensive hydraulic reworking, or they permit flexible deformation of sandy sediments. During low hydrodynamic disturbance, the bacteria enhance deposition of sediments by baffling, trapping, and binding. Such biotic-physical interference is recorded by MISS such as erosional remnants and pockets or planar stromatolites. Chemical depositional systems include (i) evaporitic salty environments characterized by evaporation and dissolution and (ii) evaporitic carbonatic environments that include evaporation, dissolution, and in situ lithification of organic matter. Here, cyanobacterial mats experience periodical desiccation or evaporation of crystals and mat-related structures such as petees, and polygonal patterns of cracks are formed. Cyanobacteria and heterotrophic bacteria provide a chemical microenvironment that supports in situ lithification of organic matter. In thin-sections, carbonate precipitates as ooids or lines of decaying filaments are visible. MISS occur in modern and ancient depositional systems. They record (i) biological abilities of benthic cyanobacteria to cope with sedimentary dynamics and (ii) paleoclimate and paleoenvironmental conditions during Earth history. Similar structures are also expected in extraterrestrial (paleo)environments.

Contribution of microbial mats to sedimentary surface structures

SummaryThis paper summarizes studies of sedimentary surface structures in which microbial mats play a role. Intertidal/supratidal transitions of tidal flats of the North Sea coast, and shallow hypersaline water bodies of salterns (Bretagne, Canary and Balearic Islands), and Gavish Sabkha (Sinai) reveal a multitude of sedimentary surface structures which can be grouped and primary biologically controlled structures. Physically controlled surface structures include shrinkage cracks, erosion marks, deformation structures caused by water friction, gas pressure and mineral encrustation. Shrinkage cracks in microbial mats reveal the following features: (i) horizontally arranged cauliflower pattern that differs from the usually orthogonally regular crack morphology in clay, (ii) rounded edges and pillow-like thickening along the crack edges, caused by the growth of mats into the cracks. Criteria of erosion are pocket-like depressions and ripple marks on the thus exposed non-stabilized sand, and residual stacks of microbial mats. Deformation structures are due to water friction causing flotation of loosely attached microbial mats which fold and tear. Gas migration from deeper layers causes domal upheaval, protuberance structures, folds and “fairy rings”. Protuberance structures are caused by the rupture of gas domes and rapid escape of the enclosed gas. The sudden drop of pressure forces sediment to well up from below through the gas channels and to fill the internal hollow spaces of the domes. “Fairy rings” are horizontal ringshaped structures. Their center is the exit point of gas bubbles which escape from the substrate into the shallow water. The bubbles generate concentric waves which cause displacement of fine muddy sediments at the sediment-water interface Such gradual displacement guides mat-constructing microbes to grow concentrically. The “fairy rings” are crowned by pinnacle structures of bacterial and diatom origin. Pinnacles, “fairy rings” and pillow-like coatings of crack margins are biogenic structures which have to be genetically separated from purely physically controlled structures.

A microscopic sedimentary succession of graded sand and microbial mats in modern siliciclastic tidal flats

Microscopic studies of thin sections from modern siliciclastic tidal flat sediments in the southern North Sea demonstrate the significant role of microbial mats in the buildup of sedimentary sequences. This is documented by a unit only a few millimetres thick. It starts at the base with a fine- to medium-grained quartz sand often containing secondary pores (‘fenestrae type’) merging gradually into finer sediments. The lower siliciclastic part is superposed by an upper organically dominated layer built by microbial mats. Within the organic material, single quartz grains without any contact to each other are oriented with their long axes parallel to the bedding planes. Each siliciclastic part in the lower section of the unit indicates the initial deposition of coarser grains in a stronger flow regime followed by gradually decreasing flow velocities. Each microbial mat in the upper part essentially represents a period of low sedimentation rate. During its growth, grains still settle down onto the mat and become bound in the organic matrix. The orientation of these grains with their long axes parallel to the bedding plane points to an energetically suitable position to gravity achieved by the friction reduction of the soft organic matter. Repeated depositional events followed by low-rate deposition cause the buildup of various units. There is no visible reworking of the former surfaces, since the microbial mats prevent erosion during periods of increased flow. The buildup is characteristic of siliciclastic sediments repeatedly occupied, stabilized, and fixed by microbial films or mats.

Microbially induced sedimentary structures indicating climatological, Hydrological and depositional conditions within recent and pleistocene coastal facies zones (Southern Tunisia)

SummaryExtensive tidal areas of the Recent coast of southern Tunisia are overgrown by microbial mats. Different mat types of which each are dominated by distinct and well adapted cyanobacterial species develop. Ecological response of the mat-forming microorganisms to climatological hydrological and sedimentological factors produce characteristic sedimentary structures (=microbially induced sedimentary structures).A suecession of Pleistocene rocks crops out near the lagoon El Bibane, southern Tunisia. The stratigraphic section comprises structures that we regard as fossil equivalents to those microbially induced structures we observe in the Recent coastal area. Preservation of the structures is result of lithification of the microbial mats. This we conclude from fossil filaments of cyanobacteria visible within the rock matrix.The Recent microbially induced sedimentary structures indicate facies zones within the modern tidal environment. Comparison of the Recent structures with the fossil analogues recorded in the stratigraphic section aids to identify the same distinct facies zones within the Pleistocene coastal environment also.Erosion by water currents forms step-like cliffs, and the microbial mat is undermined and ripped off piece by piece. shallows within the supratidal area are overgrown by copious microbial mats comprising structures like biolaminites and—varvites, as well as polygons of cracks. The features originate from effects triggered by seasonal variations of climate. Tufts and reticulate pattern of bulges indicate supernatant water films covering the mat surfaces. Morphologically higher parts of the Recent tidal area are overgrown by single-layered mats forming petees, induced by microbial mat growth and evaporitive pumping.The study demonstrates that microbially induced sedimentary structures can be used to reconstruct small-scaled facies zones within coastal environments. The also include hints on paleoclimatological, hydrological and sedimentological conditions.

The Depositional Record of Sandy, Versicolored Tidal Flats (Mellum Island, Southern North Sea)

ABSTRACT Cyanobacteria fix and bind sediments and generate versicolored, laminated patterns in quartz sand flats. They are developed in the lower supratidal zone (between mean high water and mean high water springs). The cyanobacteria mats protect the sediments from erosion and desiccation and provide food for marine and terrestrial invertebrates. Burrowing and grazing polychaetes, amphipods, and gastropods are present as well as a diverse meiofauna. The bioturbation structures are described. At some places the population density of the macrofauna averages 20,000 individuals per m2. Typical are smooth surfaces and erosional pockets with rippled sand. The deposits consist of laminated sand in which organic-rich mat generations are intercalated. The sediment is medium to fine-grained sand. The development of versicolored systems by interaction of migrating cyanobacteria and episodic low-rate sedimentation was studied in the laboratory. The depositional record reveals characteristics from both, the terrestrial and the marine surrounding. There are bioturbation structures from marine and terrestrial burrowing animals, hard skeletal parts from shelled marine organisms and plant roots of halophytes. Fenestrae produced by burrowing beetles are observed. The described mats characterize the type of quartz sandy potential stromatolites.

Salinity and Water Activity Related Zonation of Microbial Communities and Potential Stromatolites of the Gavish Sabkha

Laminated microbial mats are found in a variety of extreme environments, including hypersaline ecosystems, thermal hot springs, deserts, and even permanently ice-covered lakes in the Antarctic (Bauld 1981 a, b, Borowitzka 1981, Brock 1978, Castenholz 1973, Y. Cohen et al. 1977 b, c, Horodyski 1977 a, Javor 1979, Krumbein et al. 1977, Krumbein and Lange-Giele 1979, Parker et al. 1981, Wright and Burton 1981).

Microbial Modification of Sedimentary Surface Structures

In coastal salterns, physical forces cause the upfolding of surface layers. Gelsticky, microfibrous substrates produced by microbes interfere with these processes. Results are folds and buckles, which differ from abiogenic tepee structures. The modified tepees are termed petees. Basing upon the over-thrusting mechanism involved, petees are classified into three types: (1) alpha-petees are buckles or folds which initially derive from subsurficial gas pressure, wind or water friction. In alpha-petees, mineral crystallization is either non-existent or initial, crests remain closed and rounded. (2) Beta-petees are advanced stages of alpha-petees with ruptured crests. Mineral crystallization takes place in the whole field of preformed folds, buckles and interspaces. Alpha- and beta-petees can be arranged irregulary or as parallel folds. (3) Gamma-petees are folds around polygons, formed in coherent biogenic surface layers. Their crests are sometimes rounded, but more often ruptured. Gamma-petees are visually closest to tepees and evolve from the same process which is lateral expansion of surface crusts by crystallization pressure.

Evaporite Microbial Sediments

Signatures of microbial life in shallow evaporite systems are discussed using examples from modern coastal hypersaline settings. Organisms contributing to microbial sediments are assigned to moderate halophiles (e.g. cyanobacteria, other phototrophic bacteria, diatoms, non-phototrophic eubacteria) and extremely halophilic taxa (e.g. green algae and halobacteria). Primary production creates the organic base upon which biogeochemical cycles are based that produce a variety of authigenic minerals found in deposits of hypersaline settings. Characteristic microbial sediments include stromatolitic laminae, biolaminoid facies and sedimentary augen structures. Communities dominated by stenotopic major taxa often contribute with less unambiguous laminated structures, e.g. flocculent organics, to the sedimentary record. Based on the criteria of brine depth and salinity, a biofacies classification of marine-derived microbial sediments is proposed.

Stromatolites, oncolites and oolites biogenically formed in situ

Observations on ancient and potential (recent) stromatolites reveal that ooids and oncoids, generally described as forming in agitated water, in fact have developed within microbial mats under quiescent conditions. Coated grains grow around various nuclei including biogenic gas bubbles within lensshaped cavities. Mats in which ooids and oncoids occur as independent or compound bodies had been formed by photosynthetic and/or chemoorganotrophic bacteria or fungi. In their formational environment, the microorganisms building the coated grains will protrude and fuse with the microbial community of the mat. Oolitic beds devoid of organic remains suggest destruction of mats, separation of hard grains, and their subsequent deposition elsewhere. The fresh and outermost microbial coatings of grains may be removed during transportation. The Minette iron ore (Lorraine) is an example of in situ formation of coated grains and stromatolites. The importance of microbial mats in the formation of Europes largest iron ore deposit is demonstrated.