Nora Noffke

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.

A new window into Early Archean life: Microbial mats in Earth's oldest siliciclastic tidal deposits (3.2 Ga Moodies Group, South Africa)

Newly discovered sedimentary structures produced by ancient microbial mats in Early Archean sandstones of the 3.2 Ga Moodies Group, South Africa, differ fundamentally in appearance and genesis from Early Archean stromatolites and bacterial cell fossils preserved in chert. Wrinkle structures, desiccation cracks, and roll-up structures record the previous existence of microbial mats that effectively stabilized sediment on the earliest known siliciclastic tidal flats. In thin-section, the sedimentary structures reveal carpet-like, laminated fabrics characteristic of microbial mats. Negative d 13 C isotope ratios of 220.1 to 221.5 6 0.2‰ are consistent with a biological origin for the carbon preserved in laminae. The biogenicity of the sedimentary structures in the Moodies Group is substantiated by comparative studies on identical mat-related features from similar tidal habitats throughout Earth history, including the present day. This study suggests that siliciclastic tidal-flat settings have been the habitat of thriving microbial ecosystems for at least 3.2 billion years. Independently of controversial silicified microfossils and stromatolites, the newly detected microbially induced sedimentary structures in sandstone support the presence of bacterial life in the Early Archean.

Sedimentary Controls on the Formation and Preservation of Microbial Mats in Siliciclastic Deposits: A Case Study from the Upper Neoproterozoic Nama Group, Namibia

Abstract Shallow-marine, siliciclastic depositional systems are dominated by physical sedimentary processes, with penecontemporaneous cementation playing only a minor role in sediment dynamics. For this reason, microbial mats rarely form stromatolites in siliciclastic environments; instead, mats are preserved as wrinkle structures on bedding surfaces. Microbial mat signatures should be widespread in siliciclastic rocks deposited before the Cambrian Period; however, siliciclastic shelf successions of the upper Neoproterozoic Nudaus Formation, Nama Group, Namibia, contain only sparsely distributed wrinkle structures. The facies distribution of observed structures reflects the superposition of a taphonomic window of mat preservation on the ecological window of mat development. Mat colonization is favored by clean, fine-grained, translucent quartz sands deposited at sites where hydrodynamic flow is sufficient to sweep mud from mat surfaces but insufficient to erode biostabilized laminae. During periods of reduced water agitation, microbial baffling, trapping, and binding entrain quartz grains into mat fabrics, increasing the thickness of the living mat layer. Mat preservation is facilitated by subsequent sedimentary events that bury the microbial structures without causing erosional destruction. Pressure originating from sediment loading forms molds and casts at bedding planes, inducing the formation of wrinkle structures. In storm-influenced shelf successions of the Nudaus Formation, wrinkle structures are restricted to quartz-rich fine sandstone beds, 2–20 cm thick, that alternate with thin interlayers of sandy mud- or siltstones. Such a lithological facies developed only sporadically on the Nudaus shelf, but is common in shallow-marine siliciclastic rocks of older Neoproterozoic age exposed in the Naukluft Nappe Complex. The observed relationship between sedimentary environment and microbial mat preservation can be observed in other Proterozoic and Phanerozoic siliciclastic rocks, as well as in modern environments. This facies dependence provides a paleoenvironmental and taphonomic framework within which investigations of secular change in mat abundance must be rooted. Understanding the physical sedimentary parameters that control the formation and preservation of microbial structures in siliciclastic regimes can facilitate exploration for biological signatures in early sedimentary rocks on Earth or other planets.

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.

Earth's earliest microbial mats in a siliciclastic marine environment (2.9 Ga Mozaan Group, South Africa)

This study provides evidence for the existence of filamentous (cyano-) bacteria forming sediment-stabilizing mats in shelf environments at 2.9 Ga-the oldest known occurrence of microbial mats in siliciclastic rocks. The Mesoarchean Mozaan Group, South Africa, features fine-grained quartzites of an ancient shallow-shelf environment. These sandstones contain wrinkle structures, which in thin section reveal filamentous textures forming carpet-like microbial mat fabrics. The textures resemble the trichomes of modern cyanobacteria, chloroflexi, or sulfur-oxidizing proteobacteria. Mineralogical, geochemical, and isotopic analyses acre consistent with a biological origin of the filament-like textures. Carbon filaments with biogenic isotopic signatures (δ 1 3 C = -24.2‰ ′ 0.5‰) are closely associated with hematite, goethite, and chert minerals, which may derive from the former presence of oxygen within the microbial mats. Detrital quartz, zircon, and rutile in the mats could indicate baffling, trapping, and binding of the bacterial communities.

Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in the ca. 3.48 Billion-Year-Old Dresser Formation, Pilbara, Western Australia

Microbially induced sedimentary structures (MISS) result from the response of microbial mats to physical sediment dynamics. MISS are cosmopolitan and found in many modern environments, including shelves, tidal flats, lagoons, riverine shores, lakes, interdune areas, and sabkhas. The structures record highly diverse communities of microbial mats and have been reported from numerous intervals in the geological record up to 3.2 billion years (Ga) old. This contribution describes a suite of MISS from some of the oldest well-preserved sedimentary rocks in the geological record, the early Archean (ca. 3.48 Ga) Dresser Formation, Western Australia. Outcrop mapping at the meter to millimeter scale defined five sub-environments characteristic of an ancient coastal sabkha. These sub-environments contain associations of distinct macroscopic and microscopic MISS. Macroscopic MISS include polygonal oscillation cracks and gas domes, erosional remnants and pockets, and mat chips. Microscopic MISS comprise tufts, sinoidal structures, and laminae fabrics; the microscopic laminae are composed of primary carbonaceous matter, pyrite, and hematite, plus trapped and bound grains. Identical suites of MISS occur in equivalent environmental settings through the entire subsequent history of Earth including the present time. This work extends the geological record of MISS by almost 300 million years. Complex mat-forming microbial communities likely existed almost 3.5 billion years ago.

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.

Multidirected ripple marks rising from biological and sedimentological processes in modern lower supratidal deposits (Mellum Island, southern North Sea)

The sandy lower supratidal zone, called “Westplate,” of Mellum Island (southern North Sea) is colonized by epipsammic cyanobacteria. The microbes form habitats of different stages of development adjacent to each other (biofilms: initial stages; mats: mature stages). Whereas biofilms do not contribute much to cohesion of sedimentary grains, mats significantly stabilize the supratidal surface, which leads to local conservation of a former physically shaped surface relief at microbially overgrown sites. In fall, the Westplate is covered by variously orientated ripple marks, termed “multidirected ripple marks.” Field measurements and investigations on the epipsammon revealed that ripple marks of similar orientations were covered by microbial assemblages of similar stages of development. The results permit the following interpretation. The sediments of the Westplate are reworked by high-water spring tide flood currents, the directions of which are frequently changed by strong winds. Because the water depth of the flood current is very shallow, some parts of the uneven sedimentary surface are more affected by hydrodynamic stress than others. At sites of calmer hydrodynamic conditions, predominantly filamentous cyanobacteria settle in time, forming mats. Constant physical reworking of slightly deeper parts of the supratidal surface, however, only permits the development of less-sediment –stabilizing biofilms composed mainly of coccoid cyanobacteria. After the supratidal area drains, the deeper parts are also colonized by mat-constructing cyanobacteria, and the ripple marks become consolidated by the organic layer. Repetition of these interactive processes of physical reworking and microbial colonization is documented by the patchy ripple structure of the supratidal surface.

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.

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.