An actualistic perspective into Archean worlds - (cyano-)bacterially induced sedimentary structures in the siliciclastic Nhlazatse Section, 2.9 Ga Pongola Supergroup, South Africa

Extensive microbial mats colonize sandy tidal flats that form along the coasts of today's Earth. The microbenthos (mainly cyanobacteria) respond to the prevailing physical sediment dynamics by biostabilization, baffling and trapping, as well as binding. This biotic-physical interaction gives rise to characteristic microbially induced sedimentary structures (MISS) that differ greatly from both purely physical structures and from stromatolites. Actualistic studies of the MISS on modern tidal flats have been shown to be the key for understanding equivalent fossil structures that occur in tidal and shelf sandstones of all Earth ages. However, until now the fossil record of Archean MISS has been poor, and relatively few specimens have been found. This paper describes a study location that displays a unique assemblage with a multitude of exceptionally preserved MISS in the 2.9-Ga-old Pongola Supergroup, South Africa. The 'Nhlazatse Section' includes structures such as 'erosional remnants and pockets', 'multidirected ripple marks', 'polygonal oscillation cracks', and 'gas domes'. Optical and geochemical analyses support the biogenicity of microscopic textures such as filamentous laminae or 'orientated grains'. Textures resembling filaments are lined by iron oxide and hydroxides, as well as clay minerals. They contain organic matter, whose isotope composition is consistent with carbon of biological origin. The ancient tidal flats of the Nhlazatse Section record four microbial mat facies that occur in modern tidal settings as well. We distinguish endobenthic and epibenthic microbial mats, including planar, tufted, and spongy subtypes. Each microbial mat facies is characterized by a distinct set of MISS, and relates to a typical tidal zone. The microbial mat structures are preserved in situ, and are consistent with similar features constructed today by benthic cyanobacteria. However, other mat-constructing microorganisms also could have formed the structures in the Archean tidal flats.     

Recent evaporite deposition associated with microbial mats,Al-Kharrar supratidal–intertidal sabkha,Rabigh area,Red Sea coastal plain of Saudi Arabia

The supratidal–intertidal sabkha of the Al-Kharrar area, Red Sea coast, Saudi Arabia, contains the evaporite minerals gypsum, anhydrite, and halite. Microbial mats flourish adjacent to the sabkha evaporites in tidal flats and pools of the Al Kharrar lagoon. Desiccation and decay of some microbial mats in tidal flat areas have led to precipitation of gypsum and halite there. The evaporite minerals have been precipitated through displacive, inclusive, and replacive growth within mud, sand, gravelly sand, and bioclastic sediment of the sabkha. Gypsum occurs as lenticular and tabular crystals whereas anhydrite occurs as nodular (individual, mosaic, and enterolithic) and pseudomorphs of lenticular gypsum crystals that grew displacively and replacively near the surface of the sabkha. Halite exists as a diagenetic cement within the sabkha sediment, or as primary rafts and skeletal crystals in desiccated tidal pools with salinity over 220‰. Microbial mats are growing on the surface of the upper tidal flat areas and in pools at a salinity range of 80–110‰, and they lead to biostabilization of the sediment. They have induced a range of sedimentary surface structures (MISS) including gas domes, reticulate patterns, tufts, pinnacles, wrinkles, and microbial shrinkage cracks. The occurrence, abundance, and association of evaporite minerals and MISS are controlled by local environmental factors such topography of the sabkha, emergence or submergence of tidal areas, surface area of the evaporite basin, contribution of meteoric water from floods from the adjoining Red Sea Mountains, and water salinity. These factors promote the growth of the microbial mats in the winter months, and deposition of evaporite minerals during summer months. Field and petrographic data indicate that the main recharge to the sabkha area is from tidal flow and water seepage from the Al-Kharrar lagoon. The results of this study indicate that within a small sabkha area of Al-Kharrar (3?×?17 km), a large variation in evaporite mineral types and morphologies grade into and are associated with MISS due to local environmental parameters. The interpretation of this association of evaporite minerals and MISS provides useful data for understanding the mechanisms responsible for precipitation of evaporite minerals and formation of MISS.     

Oldhamia radiata: the trace of an undermat miner from the Cambrian Series 2 Xinji Formation,western Henan,North China

Oldhamia trace fossils represent a complex horizontal burrow system usually associated with microbial mats, occurring in the upper part of the Cambrian Stage 2 of the Terreneuvian Series, with a peak in abundance during Stage 3 and 4 of Series 2, and then rapidly disappearing in the Wuliuan Stage of the Miaolingian Series. We here report Oldhamia radiata from the Cambrian Series 2 Xinji Formation in western Henan of North China, associated with microbial mats, as evidenced by the presence of wrinkle structures. The distinctive patterns indicate the exploitation of microbial mats by the tracemakers of O. radiata, featuring a specific feeding strategy under the mats where possibly anoxic and sulfidic conditions prevailed during the early Cambrian.     

Occurrence of photosynthetic microbial mats in a Lower Cretaceous black shale (central Italy): a shallow-water deposit

Cretaceous oceanic anoxic events (OAEs) were periods of high organic carbon burial corresponding to intervals with excellent organic matter (OM) preservation. This work focuses on the Urbino level, i.e., OAE1b, which is thought to be of regional extent. A detailed microscopical study of OM shows a dominance of microbial activity, characterized by a typical arrangement of exopolymeric substances (EPS) related to microbial mats, bacterial bodies, and some photosynthetic microorganisms, as shown by thylakoids. The latter lived where they have been found, i.e., at the sea bottom, which indicates that OM results from the diagenesis of benthic photosynthetic microbial mats, an interpretation supported through the comparison with a recent analogue. The exceptional preservation of such organic structures in OM points to the joint role of the selective and sorptive preservation pathways. These data and interpretation strongly differ from previous observations in OAE1b equivalents. They suggest that the Urbino level might be an atypical OAE of regional/local extent which was formed within the photic zone.     

Undirected motility of filamentous cyanobacteria produces reticulate mats

The roles of biology in the morphogenesis of microbial mats and stromatolites remain enigmatic due to the vast array of physical and chemical influences on morphology. However, certain microbial behaviors produce complex morphological features that can be directly attributed to motility patterns. Specifically, laboratory experiments with a strain of the cyanobacteria Pseudanabaena demonstrate that distinctive morphologies arise from the undirected gliding and colliding of filaments. When filamentous cells collide, they align and clump, producing intersecting ridges surrounding areas with low cell density, i.e. reticulate structures. Cell motility is essential for the development of reticulates and associated structures: filaments organize into reticulates faster than cell division and growth, and conditions that inhibit motility also inhibit reticulate formation. Cell density of the inoculum affects the frequency of cell–cell collisions, and thus the time required for biofilm organization into reticulate structures. This also affects the specific geometry of the reticulates. These patterns are propagated into larger structures as cyanobacterial cell numbers increase and cells remain motile. Thus, cell motility is important for templating and maintaining the morphology of these microbial communities, demonstrating a direct link between a microbial behavior and a community morphology. Reticulate geometries have been identified in natural microbial mats as well as in the fossil record, and these structures can be attributed to the motility of filamentous bacteria.     

Facies control on lower Cambrian wrinkle structure development and paleoenvironmental distribution, southern Great Basin, United States

Assessing the role that physical processes play in restricting microbial mat distribution has been difficult due to the primary control of bioturbation in the modern ocean. To isolate and determine the physical controls on microbial mat distribution and preservation, a time in Earth’s history must be examined when bioturbation was not the primary control. This restricts the window of observation primarily to the Precambrian and Cambrian, which precede the development of typical Phanerozoic and modern levels of bioturbation. Lower Cambrian strata of the southern Great Basin, United States, record the widespread development of seafloor microbial mats in shallow shelf and nearshore settings. These microbial mats are recorded by wrinkle structures, which consist of millimeter-scale ridges and sinuous troughs that represent the former presence of a surface microbial mat. Wrinkle structures within these strata occur exclusively within heterolithic deposits of the offshore transition, i.e., between fair-weather wave base and storm wave base, and within heterolithic tidal-flat deposits. Wrinkle structures are not preserved in siltstone-dominated offshore deposits or amalgamated shoreface sandstones. The preservation of wrinkle structures within these environments is due to: (1) the development of microbial mats atop clean quartz-rich sands for growth and casting of the structures; and (2) the draping of the microbial mat by finer-grained sediment to inhibit erosion. The exclusion from offshore deposits may be due to a lack of sufficient sunlight, whereas the restriction from the shoreface is likely due to the amalgamation of proximal tempestites, resulting in the erosion of any incipient microbial mat development.     

Geospatial insights into the controls of microbialite formation at Laguna Negra,Argentina

Microbialites provide a record of the interaction of microorganisms with their environment constituting a record of microbial life and environments through geologic time. Our capacity to interpret this record is limited by an incomplete understanding of the microbial, geochemical, and physical processes that influence microbialite formation and morphogenesis. The modern system Laguna Negra in Catamarca Province, Argentina contains microbialites in a zone of carbonate precipitation associated with physico-chemical gradients and variable microbial community structure, making it an ideal location to study how these processes interact to drive microbialite formation. In this study, we investigated the geospatial relationships between carbonate morphology, geochemistry, and microbial community at the macro- (decimeter) to mega- (meter) scale by combining high-resolution imagery with field observations. We mapped the distribution of carbonate morphologies and allochtonously-derived volcaniclasts and correlated these with sedimentary matrices and geochemical parameters. Our work shows that the macroscale distribution of different carbonate morphologies spatially correlates with microbial mat distributions—a result consistent with previous microscale observations. Specifically, microbialitic carbonate morphologies more commonly occur associated with microbial mats while abiotically derived carbonate morphologies were less commonly associated with microbial mats. Spatial variability in the size and abundance of mineralized structures was also observed, however, the processes controlling this variability remains unclear and likely represent a combination of microbial, geochemical, and physical processes. Likewise, the processes controlling the spatial distribution of microbial mats at Laguna Negra are also unresolved. Our results suggest that in addition to the physical drivers observed in other modern environments, variability in the spatial distribution of microbialites and other carbonate morphologies at the macro- to megascale can be controlled by microbial processes. Overall, this study provides insight into the interpretation of microbialite occurrence and distributions in the geologic record and highlights the utility of geospatial statistics to probe the controls of microbialite formation in other environments.     

Evidence for seafloor microbial mats and associated metazoan lifestyles in Lower Cambrian phosphorites of Southwest China

The increase in the depth and intensity of bioturbation through the Proterozoic-­Phanerozoic transition changed the substrates on which marine benthos lived from being relatively firm with a sharp sediment-water interface to having a high water content and blurry sediment-water interface. Additionally, microbial mats, once dominant on normal marine Proterozoic seafloors, were relegated to stressed settings lacking intense metazoan activity. This change in substrates has been termed the 'agronomic revolution', and its impact on benthic metazoans has been termed the 'Cambrian substrate revolution'. The shallow marine phosphorites of the Lower Cambrian Meishucun Formation of southwest China contain evidence suggestive of the presence of seafloor microbial mats. This evidence includes abundant and distinctive red-colored bedding planes enriched in heavy iron minerals and mica, interpreted as resulting from mat-decay mineralization and mica trapping by microbial mats. The radular grazing trace fossil Radulichnus is also found in this formation, indicating a firm, microbial mat-bound substrate. These radular scratches are always preserved with circular impressions around 10 cm in diameter, possibly the fossils of soft-bodied organisms. The first relatively intense bioturbation in this region is found in this formation and is dominated by horizontal Thalassinoides burrows, which could represent undermat mining behavior. The evidence for the presence of microbial mats in the Lower Cambrian Meishucun Formation, and for metazoan lifestyles associated with such mat-bound seafloors, reveals that normal marine environments dominated by typical Proterozoic-style soft substrates still existed during the Cambrian substrate revolution.     

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

Summary Extensive 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.     

A microbial model for the Lower Devonian stromatactis mud mounds of the Montagne Noire (France)

Summary Lower Devonian mud mounds and stromatactis fabrics are exceptionally well exposed in quarry walls and industrially sawed blocks in the Montagne Noire in southern France. Interlayered red biomicrites and white to grey sparitic calcites form mounds up to 70 m high. The red biomicrites contain predominantly bryozoans, sponges and echinoderms. The sparitic layers show typical features of stromatactis fabrics, as outlined byBathurst (1982). We recognize two types of stromatactis fabrics: (1) Stromatactis type A: exentsive cavity systems filled by multiple cement generations, which are interpreted to be related to microbial mats, and (2) Stromatactis type B: smaller patches of blocky spar which are mainly diagenetic in origin, but show characteristic features of stromatactis. Type A is far more important in terms of rock volume. The cyclic interlayering of red biomicrites and sparitic layers is supposed to result from frequent changes in the composition of the mound biota. The bryozoan/sponge community was displaced by short term propagations of microbial mats during times of extremely low sedimentation. Sedimentation and thus the biotic community was probably determined by high frequency (6th order) sea level changes. Despite these changes, mound growth continued, because once established the ecological advantage over the surroundings was maintained by both communities alternating with each other. The microbial mats and the cavities they left after their decay were important for the stabilization of the mounds, the latter allowing for enormous quantities of dissolved carbonate to be transported and precipitated. We anticipate a close interrelation between mound formation and stromatactis formation, and we believe that it is not incidential that both, mud mounds and stromatactis, are mainly restricted to the same interval, namely the Paleozoic.     

Novel diversity within Roseofilum (Desertifilaceae,Cyanobacteria) from marine benthic mats with description of four new species

Benthic cyanobacterial mats (BCMs) are natural phenomena in marine environments. Reports of BCMs occurring across coastal marine environments have increased, partly driven by nutrient loading and climate change; thus, there is a need to understand the diversity involved in the proliferations and potential toxicity of the BCMs. Furthermore, marine cyanobacterial mats are observed growing on and affecting the health of corals with one specific cyanobacterial genus, Roseofilum, dominating the microbial mats associated with black band disease (BBD), a destructive polymicrobial disease that affects corals. To explore the diversity of Roseofilum, cyanobacterial mats from various marine habitats were sampled, and individual isolates were identified based on morphology, 16S rRNA gene phylogenies, 16S–23S ITS rRNA region sequence dissimilarities, and phylogenomics. Four novel species of Roseofilum were isolated from benthic marine mats, three from the coasts of Florida, United States (R. capinflatum sp. nov., R. casamattae sp. nov., and R. acuticapitatum sp. nov.) and one from the coast of France (R. halophilum sp. nov.). Our analyses revealed that Roseofilum associated with coral BBD and those not associated with corals but rather from coastal benthic mats are systematically distinct based on both phylogenetic and phylogenomic analyses. Enzyme-linked immunosorbent assay (ELISA) and LC–MS data indicated that microcystin production was found in one of the four species.     

Microbial mats and physicochemistry in a saltern in the Bretagne (France) and in a laboratory scale saltern model

Abstract A saltern near La Baule (Bretagne, France) was remodeled in a programmable temperature and humidity controlled walk-in environmental chamber resembling the characteristics of the original saltern. The saltern showed different types of microbial mats predominantly composed of algae, oxy- and anoxyphotobacteria, and associated chemoorganotrophic bacteria, fungi and animals. Well-developed microbial mats were found up to a salinity of 10% during the three or four months in summer when salinity gradients and NaCl precipitation were established. The main phototrophic organisms were diatoms, the cyanobacteria Aphanothece, Microcoleus, Spirulina , and Oscillatoria , and Chromatiaceae. At higher salinity, Halobacterium sp., diatoms, and Dunaliella were dominant. Typical microbial mats and saltern-typical invertebrate, algal and bacterial species also developed in the saltern model, building up a stable community. The ionic composition of the brines and physicochemical parameters were similar to those determined for the original saltern. Different photosynthetic organisms, e.g. a filamentous purple bacterium and a hypersaline Chloroflexus -like organism, could be enriched within the microbial mats by changing the light regime.     

Evidence of oxygenic phototrophy in ancient phosphatic stromatolites from the Paleoproterozoic Vindhyan and Aravalli Supergroups,India

Fossil microbiotas are rare in the early rock record, limiting the type of ecological information extractable from ancient microbialites. In the absence of body fossils, emphasis may instead be given to microbially derived features, such as microbialite growth patterns, microbial mat morphologies, and the presence of fossilized gas bubbles in lithified mats. The metabolic affinity of micro‐organisms associated with phosphatization may reveal important clues to the nature and accretion of apatite‐rich microbialites. Stromatolites from the 1.6 Ga Chitrakoot Formation (Semri Group, Vindhyan Supergroup) in central India contain abundant fossilized bubbles interspersed within fine‐grained in situ‐precipitated apatite mats with average δ¹³C_org indicative of carbon fixation by the Calvin cycle. In addition, the mats hold a synsedimentary fossil biota characteristic of cyanobacterial and rhodophyte morphotypes. Phosphatic oncoid cone‐like stromatolites from the Paleoproterozoic Aravalli Supergroup (Jhamarkotra Formation) comprise abundant mineralized bubbles enmeshed within tufted filamentous mat fabrics. Construction of these tufts is considered to be the result of filamentous bacteria gliding within microbial mats, and as fossilized bubbles within pristine mat laminae can be used as a proxy for oxygenic phototrophy, this provides a strong indication for cyanobacterial activity in the Aravalli mounds. We suggest that the activity of oxygenic phototrophs may have been significant for the formation of apatite in both Vindhyan and Aravalli stromatolites, mainly by concentrating phosphate and creating steep diurnal redox gradients within mat pore spaces, promoting apatite precipitation. The presence in the Indian stromatolites of alternating apatite‐carbonate lamina may result from local variations in pH and oxygen levels caused by photosynthesis–respiration in the mats. Altogether, this study presents new insights into the ecology of ancient phosphatic stromatolites and warrants further exploration into the role of oxygen‐producing biotas in the formation of Paleoproterozoic shallow‐basin phosphorites.     

Single-cell microliter-droplet screening system (MISS Cell): An integrated platform for automated high-throughput microbial monoclonal cultivation and picking

Solid plates have been used for microbial monoclonal isolation, cultivation, and colony picking since 1881. However, the process is labor- and resource-intensive for high-throughput requirements. Currently, several instruments have been integrated for automated and high-throughput picking, but complicated and expensive. To address these issues, we report a novel integrated platform, the single-cell microliter-droplet screening system (MISS Cell), for automated, high-throughput microbial monoclonal colony cultivation and picking. We verified the monoclonality of droplet cultures in the MISS Cell and characterized culture performance. Compared with solid plates, the MISS Cell generated a larger number of monoclonal colonies with higher initial growth rates using fewer resources. Finally, we established a workflow for automated high-throughput screening of Corynebacterium glutamicum using the MISS Cell and identified high glutamate-producing strains. The MISS Cell can serve as a universal platform to efficiently produce monoclonal colonies in high-throughput applications, overcoming the limitations of solid plates to promote rapid development in biotechnology.     

Diverse communities of Bacteria and Archaea flourished in Palaeoarchaean (3.5–3.3 Ga) microbial mats

Limited taxonomic classification is possible for Archaean microbial mats and this is a fundamental limitation in constraining early ecosystems. Applying Fourier transform infrared spectroscopy (FTIR), a powerful tool for identifying vibrational motions attributable to specific functional groups, we characterized fossilized biopolymers in 3.5–3.3 Ga microbial mats from the Barberton greenstone belt (South Africa). Microbial mats from four Palaeoarchaean horizons exhibit significant differences in taxonomically informative aliphatic contents, despite high aromaticity. This reflects precursor biological heterogeneity since all horizons show equally exceptional preservation and underwent similar grades of metamorphism. Low methylene to end-methyl (CH₂/CH₃) absorbance ratios in mats from the 3.472 Ga Middle Marker horizon signify short, highly branched n-alkanes interpreted as isoprenoid chains forming archaeal membranes. Mats from the 3.45 Ga Hooggenoeg Chert H5c, 3.334 Ga Footbridge Chert, and 3.33 Ga Josefsdal Chert exhibit higher CH₂/CH₃ ratios suggesting mostly longer, unbranched fatty acids from bacterial lipid precursors. Absorbance ratios of end-methyl to methylene (CH₃/CH₂) in Hooggenoeg, Josefsdal and Footbridge mats yield a range of values (0.20–0.80) suggesting mixed bacterial and archaeal architect communities based on comparison with modern examples. Higher (0.78–1.25) CH₃/CH₂ ratios in the Middle Marker mats identify Archaea. This exceptional preservation reflects early, rapid silicification preventing the alteration of biogeochemical signals inherited from biomass. Since silicification commenced during the lifetime of the microbial mat, FTIR signals estimate the affinities of the architect community and may be used in the reconstruction of Archaean ecosystems. Together, these results show that Bacteria and Archaea flourished together in Earth's earliest ecosystems.     

A Mediterranean Holocene restricted coastal lagoon under arid climate: Case of the sedimentary record of Sabkha Boujmel (SE Tunisia)

The Holocene sedimentary record of Sabkha Boujmel (SE Tunisia) is expressed by a shallowing-upward carbonate lagoon-tidal flat cycle (2.3 m thick) unconformably overlying continental silt-sandy sediment, Late Würmian in age. The sedimentary package of this cycle starts with transgressive marginal shallow marine (intertidal to subtidal) bioclastic sands grading upwards to black mudstone, rich in organic matter (T.O.C. up to 1.3%) deposited within a lagoon protected from the sea by Upper Pleistocene lithified sand spits.The uppermost part of the cycle is represented by oobioclastic carbonate sands covered with dead biodegraded microbial mats and/or reddish sands of aeolian origin deposited in intertidal to supratidal environments. The facies arrangement, particularly the spatial distribution of the ancient and the more recent microbial mats, records the progressive infilling of the lagoon as well as the progradation of the shoreline during the last 2000 years. The organic-rich facies which provide an age varying between 4130 and 6800 yr B.P. were deposited when the Boujmel lagoon started to be progressively separated from the Mediterranean Sea.The main factors controlling the facies and the thickness variation are the local topographic sea-floor irregularities most likely controlled by the inheritance morphology resulting from an important fluviatile digging that occurred during the last glacial maximum, the relative sea-level fluctuations, the hydro-isostatic rebound and the climate.     

Microbial mats associated with bryozoans (Coorong Lagoon, South Australia)

Summary Bryostromatolites are laminated carbonate rocks composed of bryozoan zoarial laminae. The laminated texture is frequently caused by patterns of bryozoan self overgrowth as a regular defensive tactic against microbial fouling. In the Coorong Lagoon (South Australia), another type of bryostromatolite is present where the laminated growth of the weakly calcifying bryozoan speciesConopeum aciculata is postmortally stabilized by cyanobacterial mats at the surface, and fungal mats settling in the zooecial cavities. A tough extracellular slime network produced by benthic cyanobacteria is a trap for sediment particles, provides a method of adhesion to the bryozoan substrate, and produces a biological lamination by the vertical stratification of dead bryozoan skeletons. These slimes are also important for the preservation of cell structures and for their fossilization. Seasonal fluctuations in salinity and water level are the most important regional control factors, causing a phase displacement in the growth optima of microbial mats and bryozoans, thereby resulting in a rigid bryostromatolitic fabric.     

Methane-derived carbonate build-ups and associated microbial communities at cold seeps on the lower Crimean shelf (Black Sea)

In the euxinic waters of the NW’ Black Sea shelf, tower-like carbonate build-ups up to several metres in height grow at sites of cold methane seepage. These structures are part of an unique microbial ecosystem that shows a considerable biodiversity and a remarkable degree of organization. The accretion of the build-ups is promoted by the growth of centimetre-sized, methane-filled spheres constructed by calcifying microbial mats. Progressive mineralization of these spheres involves the early precipitation of strongly luminescent high-Mg-calcite rich in iron sulphides, and closely interfingered aragonite phases that finally create the stable (mega-) thrombolithic fabric of the towers. Within the microbial mats, microorganisms occur in distinctive spatial arrangements. Major players among the microbial consortia are the archaea groups ANME-1 and ANME-2, Crenarchaeota, and sulphate-reducing bacteria (SRB) of the Desulfosarcina/Desulfobacterium group. The intracellular precipitation of iron sulphides (greigite) by some of these bacteria, growing in close association with ANME-2, suggests iron cycling as an additional biogeochemical pathway involved in the anaerobic oxidation of methane (AOM).     

A seaward prograding siliciclastic sequence from upper tidal flats to salt marsh facies (Southern North Sea)

Summary The uppermost part of the seaward prograding sequence in quartz sandy areas of the southern North Sea coast starts at the base with tidal flat layers, often strongly perturbated by burrowing endobenthic animals. This section is overlain by microbial mats intercalated by tempestites, and in part bioturbated by shaped burrows. The top consists of salt marsh facies strongly penetrated by roots. The seaward prograding sequence is characterized by numerous vertical changes in small-scale facies. The facies development and changes are caused by both organisms (halophytes, mat building microbes, benthic animals) and primary physical processes (erosion, transport and sedimentation, or sedimentation alone). The small-scale facies includes (i) bioturbation structures, (ii) biogenic growth structures caused by microbial mats, (iii) subaerial and subaqueous tempestites. Typical is the sudden breakdown of the biocoenosis by fast sedimentation due to tempestites or wind-blown sand sheets. Typical is also the renewal of the former biocoenosis in the new top section. Such sequences may be preserved by wind-blown sand sheets or by washover fans. The sequence is thus an important and unique tool for the identification of fossilized sequences of the intertidal-supratidal transition. It is also a useful mark for estimating the mean sea level and the level of the lower supratidal horizon within vertical sections. Finally the sequence indicates progress in island development governed by the interaction of biological components with physical processes.

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Zusammenfassung Die Verlandungssequenz sandiger Küsten der Nordsee besteht aus einer Abfolge, die von Wattenschichten mit überwiegend ungestalteter Verwühlung im Liegenden über Mikrobenmatten, in die Tempestite mit vorwiegend vertikal angelegten G?ngen eingeschaltet sind, zu supralitoralen Salzwiesen mit starker Durchwurzelung führt. Sie wird durch zahlreiche, oft nur zentimeterm?chtige Fazieswechsel charakterisiert. An der Entwicklung und am Wechsel der Fazies sind Organismen (benthische Fauna, mattenbildende Mikroorganismen und Halophyten) sowie prim?r physikalische, subaquatische wie subaerische Transport-und Sedimentationsprozesse beteiligt. Bezeichnend für die Verlandungssequenz sind wiederholte Zusammenbrüche der lokalen Bioz?nosen durch eingeschaltete Tempestite mit nachfolgender Entwicklung der gleichen Bioz?nose bei hinreichender sediment?rer Ruhezeit. Die beschriebene Sequenz ist ein diagnostisches Mittel zur Identifizierung flachmariner Verlandungsabfolgen. Sie erlaubt, die H?he der mittleren Tidehochwasserlinie festzulegen. Solche Sequenzen k?nnen durch Windsanddecken oder sturmflutbedingte Durchbruchsf?cher überdeckt und erhalten werden.

     

Late Neogene planktonic foraminifera of the Cibao Valley (northern Dominican Republic): Biostratigraphy and paleoceanography

An assemblage of planktonic foraminifera is described from 125 samples taken from the Cercado, Gurabo, and Mao Formations in the Cibao Valley, northern Dominican Republic. The primary objectives of this study are to establish a biochronologic model for the late Neogene of the Dominican Republic and to examine sea surface conditions within the Cibao Basin during this interval. The Cercado Formation is loosely confined to Zones N17 and N18 (∼ 7.0–5.9 Ma). The Gurabo Formation spans Zones N18 and N19 (∼ 5.9–4.5 Ma). The Mao Formation is placed in Zone N19 (∼ 4.5–3.6 Ma). Changes in the relative abundances of indicator species are used to reconstruct sea surface conditions within the basin. Increasing relative abundances of Globigerinoides sacculifer and Globigerinoides ruber, in conjunction with a decreasing relative abundance of Globigerina bulloides, suggests the onset of increasing sea surface temperature and salinity in conjunction with diminishing primary productivity at ∼ 6.0 Ma. Abrupt increases in the relative abundances of G. sacculifer and G. ruber at ∼ 4.8 Ma suggest a major increase in sea surface temperature and salinity in the early Pliocene. The most likely mechanism for these changes is isolation of the Caribbean Ocean through progressive restriction of Pacific–Caribbean transfer via the Central American Seaway. Periods of high productivity associated with upwelling events are recorded in the upper Cercado Formation (∼ 6.1 Ma) and in the middle Mao Formation (∼ 4.2 Ma) by spikes in G. bulloides and Neogloboquadrina spp. respectively. The timing of major increases in sea surface salinity and temperature as well as decreasing productivity (∼ 4.8 Ma) and periods of upwelling (∼ 6.1and 4.2 Ma) in the Cibao Basin generally corroborate previously suggested Caribbean oceanographic changes related to the uplift of Panama. Changes in sea surface conditions depicted by paleobiogeographic distributions in the Cibao Basin suggest that shoaling along the Isthmus of Panama had implications in a shallow Caribbean basin as early as 6.0 Ma. Major paleobiologic changes between ∼ 4.8 and 4.2 Ma likely represent the period of final closure of the CAS and a nearly complete disconnection between Pacific and Caribbean water masses. This study illustrates the use of planktonic foraminifera in establishing some paleoceanographic conditions (salinity, temperature, productivity, and upwelling) within a shallow water basin, outlining the connection between regional and localized oceanographic changes.