Development of albian microbialites and microbialite reefs at marginal platform areas of the Vasco-Cantabrian Basin (Soba reef area,Cantabria, N. Spain)

The Middle Albian sequence from the western marginal area of the Vasco-Cantabrian Basin contains calcified microbialites in different marine depositional environments, individually well defined by microstructure, lamina characteristics and mode of formation. Microbialites may form the primary framework of reefs, which occur as composite stacks in mid to lower slope environments or as isolated bodies in small intraplatform basins. In most areas microbialite reef growth was initiated below the photic zone. Stratiform intercalations of microbialites and composite microbialite/foraminifer oncoids are restricted to well bedded carbonate platform deposits (Urgonian). Three basis types of microbialites are recognized:

Microbioerosion in Tahitian reefs: a record of environmental change during the last deglacial sea-level rise (IODP 310)

The main motivation for Integrated Ocean Drilling Program Expedition 310 to the Tahitian Archipelago was the assumption that the last deglacial sea‐level rise is precisely recorded in the coral reefs of this far‐field site. The Tahitian deglacial succession typically consists of coral framework subsequently encrusted by coralline algae and microbialites. The high abundance of microbialites is uncommon for shallow‐water coral reefs, and the environmental conditions favouring their development are still poorly understood. Microbioerosion patterns in the three principal framework components (corals, coralline algae, microbialites) are studied with respect to relative light availability during coral growth and subsequent encrustation, in order to constrain the palaeobathymetry and the relative timing of the encrustation. Unexpectedly for a tropical, light‐flooded setting, ichnotaxa typical for the deep‐euphotic to dysphotic zone dominate. The key ichnotaxa for the shallow euphotic zone are scarce in the analysed sample set, and are restricted to the base of the deglacial succession, thus reflecting the deglacial sea‐level rise. At the base of the deglacial reef succession, the ichnocoenoses present in the corals indicate shallower bathymetries than those in the encrusting microbialites. This is in agreement with radiocarbon data that indicate a time gap of more than 600 years between coral death and microbialite formation. At the top of the deglacial reef succession, in contrast, the microbioerosion patterns in the three framework components indicate a uniform palaeobathymetry, and radiocarbon ages imply that encrustation took place shortly after coral demise. An enigma arises from the fact that the ichnocoenoses imply photic conditions that appear very deep for zooxanthellate coral growth. During the deglacial sea‐level rise increased nutrients and fluvial influx may have led to (seasonal?) eutrophication, condensing the photic zonation. This would have exerted stress on the coral ecosystem and played a significant role in initiating microbialite development.     

Genesis of microbialites as contemporaneous framework components of deglacial coral reefs, Tahiti (IODP 310)

Deglacial reefs from Tahiti (IODP 310) feature a co-occurrence of zooxanthellate corals with microbialites that compose up to 80 vol% of the reef framework. The notion that microbialites tend to form in more nutrient-rich environments has previously led to the concept that such encrustations are considerably younger than the coral framework, and that they have formed in deeper storeys of the reef edifice, or that they represent severe disturbances of the reef ecosystem. As indicated by their repetitive interbedding with coralline red algae, the microbialites of this reef succession of Tahiti, however, formed immediately after coral growth under photic conditions. Clearly, the deglacial reef microbialites present in the IODP 310 cores did not follow disturbances such as drowning or suffocation by terrestrial material, and are not “disaster forms”. Given that the corals and the microbialites developed in close spatial proximity, highly elevated nutrient levels caused by fluvial or groundwater transport from the volcanic hinterland are an unlikely cause for the exceptionally voluminous development of microbialites. That voluminous deglacial reef microbialites generally are restricted to volcanic islands, however, implies that moderately, and possibly episodically elevated nutrient levels favored this type of microbialite formation.     

Cenomanian/Turonian sponge microbialite deep-water hardground community (Liencres,Northern Spain)

Summary A benthic community of sessile metazoans dominated by coralline sponges (e.g.Acanthochaetetes andVaceletia) is found within a Cenomanian-Turonian deep water hardground succession cropping out at the coastal area of the Bay of Biscay near Santander. The characteristic K-strategic community exhibits a very close taxonomic relationship with modern communities from the Pacific realm, which allows for a comparison with Recent environmental conditions. The sponge community was associated with automicrites, microbialites, and thin mineralized limonitic biofilms. This biofacies is typically found in cryptic niches of reefal buildups (“telescoping”). The iron-rich biofilms had a strong electrochemical corrosive ability which explains the distinct submarine dissolution patterns. The hardground conditions are controlled, in part, by strong contour current regimes linked with extremely oligotrophic water masses. This system was established during the drowning of a distal carbonate ramp during the early Middle Cenomanian (A.rhotomagenese zone). In the uppermost portion of the hardground (Late Cenomaian, upperR. cushmani zone) the coralline sponge community was replaced by thick limonitic stromatolites with numerous encrusting foraminifera (Miniacina-type) and by colonies of the problematic iron bacteriumFrutexites. This event is accompanied by an increase of terrigenous influx and detrital glauconite, indicating a fundamental change in food web, and terminates the sponge dominated basal hardground interval. Thehardground was buried by hemipelagic sediments during the Middle Turonian (upperR. kallesi zone). Dedicated to the memory of Prof. Dr. JostWiedmann     

Metagenomic and stable isotopic analyses of modern freshwater microbialites in Cuatro Ciénegas, Mexico

Ancient biologically mediated sedimentary carbonate deposits, including stromatolites and other microbialites, provide insight into environmental conditions on early Earth. The primary limitation to interpreting these records is our lack of understanding regarding microbial processes and the preservation of geochemical signatures in contemporary microbialite systems. Using a combination of metagenomic sequencing and isotopic analyses, this study describes the identity, metabolic potential and chemical processes of microbial communities from living microbialites from Cuatro Ciénegas, Mexico. Metagenomic sequencing revealed a diverse, redox-dependent microbial community associated with the microbialites. The microbialite community is distinct from other marine and freshwater microbial communities, and demonstrates extensive environmental adaptation. The microbialite metagenomes contain a large number of genes involved in the production of exopolymeric substances and the formation of biofilms, creating a complex, spatially structured environment. In addition to the spatial complexity of the biofilm, microbial activity is tightly controlled by sensory and regulatory systems, which allow for coordination of autotrophic and heterotrophic processes. Isotopic measurements of the intracrystalline organic matter demonstrate the importance of heterotrophic respiration of photoautotrophic biomass in the precipitation of calcium carbonate. The genomic and stable isotopic data presented here significantly enhance our evolving knowledge of contemporary biomineralization processes, and are directly applicable to studies of ancient microbialites.     

Growth models of Bajocian coral-microbialite reefs of Chargey-lès-Port (eastern France): palaeoenvironmental interpretations

Very large amount of microbialites, up to 70% of the reef volume takes part in the edification of Lower Bajocian coral reefs in the Chargey-lès-Port quarry (Haute-Saône, France). Such high amounts of microbialites were unknown within bioconstructions of Middle Jurassic age. Along the 16 m-thick section, seven successive biohermal or biostromal units developed on a shallow platform. Bioconstructions display a first coral growth phase with either constratal or superstratal growth fabrics. Coral fauna is relatively poorly diversified and is dominated by massive forms (Isastrea, Thamnasteria, and Periseris) or branched phaceloid (Cladophyllia) and ramose (Dendraraea) colonies. Corals can be heavily encrusted by microbialites of diverse forms and fabrics (leiolitic, thrombolitic, and stromatolitic). According to the coral growth fabrics, microbialite crusts developed on top of or at the underside of coral colonies, forming a coral-microbialite elementary unit. Microbialites show a multiphase development: (i) directly at the coral surface, a first and mm-scale microbialite layer locally developed; (ii) a second, cm-scale microbialite layer (up to 8 cm thick) covered the entire coral reef framework and assumed the main building role; and (iii) a third, mm- to cm-scale, laminated microbialite layer may also be observed onlapping previous reef structures, before having been progressively buried under sediments. Contemporaneously to the coral growth phase, the first microbialite layer developed on dead portions of coral colonies. The transition between coral growth and microbialite development (i.e., second layer of microbialites) is interpreted as a result of a coral reef crisis, probably reflecting more nutrient-rich conditions. The passage to a stromatolitic (third) layer suggests a control of the accumulation rate. Composition and architecture of coral-microbialite reef units of Chargey-lès-Port highlight the relations between high-frequency fluctuating environmental factors (mainly accumulation rate and trophic conditions) and reef development.     

Irradiance Regulation of Photosynthesis and Respiration in Modern Marine Microbialites Built by Benthic Cyanobacteria in a Tropical Lagoon (New Caledonia)

Microbialites are organosedimentary deposits that have built up as a result of the growth and binding of detrital sediment by a benthic microbial community. This study focuses on microbialites built by monospecific populations of cyanobacteria in the south-west lagoon of New Caledonia, where they have been observed down to 20–25 m depth. The aim was to study their photosynthetic and respiratory responses to various light intensities. The Phormidium sp. TK1 microbialite was collected at 19 m depth and the P. crosbyanum (Tilden) microbialite was collected at 0.5 and 13 m depth. Phormidium sp. TK1 showed all the characteristic features of a low-light adapted species. The initial slope of the Photosynthesis versus Irradiance curve for this microbialite was close to the maximum quantum yield indicating an efficient light absorption and utilization at low light. The photosynthesis maximum was located 0.2–0.4 mm below the surface and did not shift with changing light intensity. Respiration rates were low and not enhanced by light; photoinhibition was observed at higher light intensities. In Phormidium crosbyanum (Tilden) microbialites, the photosynthesis maximum shifted downward to lower depths with increasing light, probably as a result of phototactic migration of cyanobacterial filaments, and light-enhanced respiration was observed at light intensities above light saturation. The photosynthetic para- meters measured in P. crosbyanum indicate that P. crosbyanum is capable of photo-acclimation at high light intensities. The gross productivity of the different microbialites was comparable to values measured in cyanobacterial stromatolites observed in other shallow environments. However, the microbialites studied here were characterized by a lower respiration / production ratio which indicates a higher growth efficiency.     

Palaeoecology Of A Late Devonian Back Reef: Canning Basin, Western Australia

Back‐reef ecologies within the celebrated mixed carbonate‐siliciclastic Late Devonian (late Frasnian) Pillara Limestone of Windjana Gorge, in the Canning Basin, Western Australia, are re‐interpreted as being dominated by microbial communities. Proposed microbialites are expressed as weakly‐laminated, fenestral micrite, that show unsupported primary voids, peloidal textures, disseminated bioclastic debris, and traces of microfilaments. These grew as either extensive free‐standing mounds or columns, often intergrown with encrusting metazoans, or thick post‐mortem encrustations upon skeletal benthos. In some cases, microbial encrustations are inferred to have developed in protected cavities formed by progressive burial of the reef. The calcimicrobe Shuguria also shows a preferentially cryptic habit, encrusting either primary cavities formed by skeletal benthos, microbialite, or the ceilings of mm‐sized fenestrae within microbialite. A further calcimicrobe, Rothpletzella, formed columns up to 0.3 m high in areas enriched by very coarse siliciclastic sediment. Stromatoporoid sponges with a diverse range of morphologies also formed in situ growth fabrics. Monospecific thickets of closely‐aggregating dendroid stromatoporoid sponges (Stachyodes costulata), and platy‐laminar forms (?Hermatostroma spp.) were common, as were remarkably large stromatoporoids (Actinostroma spp.) that grew as isolated individuals up to 5 m in diameter. Such sponges showed impressive powers of regeneration from partial mortality, and individual clones may have been capable of substantial longevities of up to 500 years. Actinostroma spp. showed highly complex growth forms including platy‐multicolumnar (A. windjanicum), and a hitherto undescribed inferred whorl‐forming foliaceous morphology (Actinostroma sp.) reminiscent of the modern photosymbiotic coral Acropora palmata. These complex morphologies formed abundant primary cavities, previously thought to be only rarely developed in association with stromatoporoids.key words : Late Devonian, Canning Basin, reefs, palaeoecology, microbialite.     

Youngest early carboniferous (late Visean) shallow-water patch reefs in eastern Australia (Rockhampton Group, Queensland): Combining quantitative micro- and macro-scale data

Summary Although skeletal organisms have received most of the emphasis in studies on Phanerozoic roef history, the roles of non-skeletal (non-enzymatic) carbonates (e.g., synsedimentary cements, automicrite, microbialite, etc.) in reef framework construction are becoming increasingly better understood. One problem in understanding the role of non-enzymatic carbonates in reef construction has been the difficulty in recognizing them in reef facies. Whereas skeletal organisms commonly can be recognized and documented in the field, non-enzymatic carbonates may be recognizable only in thin section. This paper describes the application of a new sampling technique that allows the quantitative comparison of skeletal macrofauna and flora with associated non-enzymatic carbonates and other microfaunal/microfloral constituents. The technique involves the point counting of thin sections made from small diameter cores that are systematically recovered from grids and line transects that cover a reasonable area (m²) of reef facies. Small, shallow-water patch reefs are abundant in scattered oolitic intervals in the Lower Carboniferous strata of eastern Australia. The youngest known Carboniferous reefs in eastern Australia occur in uppermost Visean strata (limestone FC5) near the top of the Rockhampton Group, approximately 50 km west-northwest of Rockhampton, Queensland. The largest sampled reef was 15 m thick and 42 m in diameter, with synsedimentary relief above the sea floor of at least 2 m during the primary growth phase. The reef occurs within bioclasticoolitic grainstones representing a shallow shelf setting and consists of eight common framework microfacies: 1) coral boundstone; 2) bryozoan boundstone; 3) mixed crinoid-bryozoan boundstone; 4) tubular problematica boundstone; 5) sponge-automicrite boundstone; 6) encrusted thrombolite boundstone; 7) mixed automicrite boundstone; and 8) thrombolitic wackestone-packstone. Reef growth was initiated by automicrite-producing biofilms, sponges and a tubular problematic organism. Primary relief building was accomplished by automicrite-dominated frameworks and lithistid sponges, crinoids, and corals. Large cerioidAphrophyllum coral colonies had a heterogeneous distribution through the reef. The framework of the main relief-bearing portion of the reef consists on average of 44.4% automicrite and automicrite-bound detritus, excluding automicrite-bound sponge body fossils, and at most 19.6% skeletal organisms in growth position (minimum of 7.2%). If sponge body fossils are included as automicrite framework, because they are preserved only as a result of automicrite formation, the percentage of automicrite and bound sediment is 54.9%. A smaller sampled reef consisting of the same basic facies had 39.5% automicrite and automicrite-bound sediment in its fremework (50.2% including sponges) and, at most, 33.4% skeletal organisms in growth position (minimum of 22.7%). The greater volume of skeletal framework in the small reef reflects a greater proportion of large corals. Of framebuilding skeletal organisms, automicrite-preserved lithistid and other sponges and cerioid rugose corals provided the greatest volume. However, crinoid holdfasts were the most widespread skeletal framework components. The dominant framework facies are sponge-automicrite boundstone, encrusted thrombolite, boundstone, mixed automicrite boundstone, and coral boundstone. The reefs are similar in overall framework construction and ecological succession to slightly older Visean reefs in eastern Australia and to some of the late Visean reefs of northern England. Surprisingly, framework similarities also exist between the reefs and certain thrombolite-lithistid-coral reefs of the European Jurassic.     

Patch reef development in the florigemma-Bank Member (Oxfordian) from the Deister Mts (NW Germany): a type example for Late Jurassic coral thrombolite thickets

Small reefal bioconstructions that developed in lagoonal settings are widespread in a few horizons of the Late Jurassic (Oxfordian) succession of the Korallenoolith Formation, exposed southwest of Hannover, Northwest Germany. Especially the florigemma-Bank Member, “sandwiched” between oolite shoal deposits, exposes a high variety of build-ups, ranging from coral thrombolite patch reefs, to biostromes and to coral meadows. The reefs show a distribution with gradual facies variations along an outcrop belt that extends about 30 km from the Wesergebirge in the NW to the Osterwald Mts in the SE.The patch reefs from the Deister Mts locality at the “Speckhals” are developed as coral-chaetetid-solenoporid-microbialite reefs and represent a reef type that was hitherto unknown so far north of its Tethyan counterparts. They are mainly built up by coral thickets that are preserved in situ up to 1.5 m in height and a few metres in diameter. They contain up to 20 coral species of different morphotypes but are chiefly composed of phaceloid Stylosmilia corallina and Goniocora socialis subordinately. The tightly branched Stylosmilia colonies are stabilized by their anastomosing growth. The coral branches are coated with microbial crusts and micro-encrusters reinforcing the coral framework. Encrusters and other biota within the thicket show a typical community replacement sequence: Lithocodium aggregatum, Koskinobullina socialis and Iberopora bodeuri are pioneer organisms, whereas the occurrence of non-rigid sponges represents the terminal growth stage. The latter are preserved in situ and seem to be characteristic so far poorly known constituents of the Late Jurassic cryptobiont reef dweller community. The distance and overall arrangement of branches seems to be the crucial factor for the manifestation of a (cryptic) habitat promoting such community replacement sequences. Widely spaced branches often lack any encrusting and/or other reef dwelling organisms, whereas tightly branched corals, as is St. corallina, stimulate such biota. Hence, such reefs are well suited for research on coelobites and community sequences of encrusting and cavity dwelling organisms.     

Non-rigid cryptic sponges in oyster patch reefs (Lower Kimmeridgian, Langenberg/Oker, Germany)

Summary Two patch reefs which predominately consist of the oysterNanogyra nana (Sowerby 1822) are exposed in Lower Kimmeridigian strata of the Langenberg hillrange, central Germany. Left oyster valves making up the frame-work of the reefs formed small abundant cavities that were inhabited by a unique sponge community. The excellent preservation of non-rigid sponges was related to early organomineralization within the decaying sponge tissue. As a process of sponge taphonomy, different types of microbially induced carbonates precipitated preserving spicule aggregates. Organomineralization within sponge soft tissues is especially favored with the Langenberg patch reefs due to the closed or semi-closed system conditions with the cavities. The δ¹³C values ofin situ formed microbialities reveal that carbonate precipitation was in equilibrium with Jurassic seawater. The carbon of the microbialites does not derive from the bacterial remineralization of organic matter, but is of a marine source. Likewise, organomineralization is probably related to bacterial EPS or decaying sponge tissues providing an organic matrix for initial carbonate precipitation. Biomarker analyses revealed, that the patch reef microbialites contain terminally branched fatty acids (iso-andanteiso-pentadecanoic acid) in significant concentrations. These fatty acids, like hopanoid hydrocarbons, are most likely of a bacterial source. This is in agreement with sulfate-reducing bacteria remineralizing the decaying sponges as further indicated by the occurrence of framboidal pyrite in sponge microbialites.     

Reconstruction of limnology and microbialite formation conditions from carbonate clumped isotope thermometry

Quantitative tools for deciphering the environment of microbialite formation are relatively limited. For example, the oxygen isotope carbonate‐water geothermometer requires assumptions about the isotopic composition of the water of formation. We explored the utility of using ‘clumped’ isotope thermometry as a tool to study the temperatures of microbialite formation. We studied microbialites recovered from water depths of 10–55 m in Pavilion Lake, and 10–25 m in Kelly Lake, spanning the thermocline in both lakes. We determined the temperature of carbonate growth and the ¹⁸O/¹⁶O ratio of the waters that microbialites grew in. Results were then compared to current limnological data from the lakes to reconstruct the history of microbialite formation. Modern microbialites collected at shallow depths (11.7 m) in both lakes yield clumped isotope‐based temperatures of formation that are within error of summer water temperatures, suggesting that clumped isotope analyses may be used to reconstruct past climates and to probe the environments in which microbialites formed. The deepest microbialites (21.7–55 m) were recovered from below the present‐day thermoclines in both lakes and yield radioisotope ages indicating they primarily formed earlier in the Holocene. During this time, pollen data and our reconstructed water ¹⁸O/¹⁶O ratios indicate a period of aridity, with lower lake levels. At present, there is a close association between both photosynthetic and heterotrophic communities, and carbonate precipitation/microbialite formation, with biosignatures of photosynthetic influences on carbonate detected in microbialites from the photic zone and above the thermocline (i.e., depths of generally <20 m). Given the deeper microbialites are receiving <1% of photosynthetically active radiation (PAR), it is likely these microbialites primarily formed when lower lake levels resulted in microbialites being located higher in the photic zone, in warm surface waters.     

Prokaryote populations of extant microbialites along a depth gradient in Pavilion Lake,British Columbia,Canada

Pavilion Lake in British Columbia, Canada, is home to modern‐day microbialites that are actively growing at multiple depths within the lake. While microbialite morphology changes with depth and previous isotopic investigations suggested a biological role in the formation of these carbonate structures, little is known about their microbial communities. Microbialite samples acquired through the Pavilion Lake Research Project (PLRP) were first investigated for phototrophic populations using Cyanobacteria‐specific primers and 16S rRNA gene cloning. These data were expounded on by high‐throughput tagged sequencing analyses of the general bacteria population. These molecular analyses show that the microbial communities of Pavilion Lake microbialites are diverse compared to non‐lithifying microbial mats also found in the lake. Phototrophs and heterotrophs were detected, including species from the recently described Chloroacidobacteria genus, a photoheterotroph that has not been previously observed in microbialite systems. Phototrophs were shown as the most influential contributors to community differences above and below 25 meters, and corresponding shifts in heterotrophic populations were observed at this interface as well. The isotopic composition of carbonate also mirrored this shift in community states. Comparisons to previous studies indicated this population shift may be a consequence of changes in lake chemistry at this depth. Microbial community composition did not correlate with changing microbialite morphology with depth, suggesting something other than community changes may be a key to observed variations in microbialite structure.     

Carbonate fabrics in the modern microbialites of Pavilion Lake: two suites of microfabrics that reflect variation in microbial community morphology,growth habit,and lithification

Modern microbialites in Pavilion Lake, BC, provide an analog for ancient non‐stromatolitic microbialites that formed from in situ mineralization. Because Pavilion microbialites are mineralizing under the influence of microbial communities, they provide insights into how biological processes influence microbialite microfabrics and mesostructures. Hemispherical nodules and micrite–microbial crusts are two mesostructures within Pavilion microbialites that are directly associated with photosynthetic communities. Both filamentous cyanobacteria in hemispherical nodules and branching filamentous green algae in micrite–microbial crusts were associated with calcite precipitation at microbialite surfaces and with characteristic microfabrics in the lithified microbialite. Hemispherical nodules formed at microbialite surfaces when calcite precipitated around filamentous cyanobacteria with a radial growth habit. The radial filament pattern was preserved within the microbialite to varying degrees. Some subsurface nodules contained well‐defined filaments, whereas others contained only dispersed organic inclusions. Variation in filament preservation is interpreted to reflect differences in timing and amount of carbonate precipitation relative to heterotrophic decay, with more defined filaments reflecting greater lithification prior to degradation than more diffuse filaments. Micrite–microbial crusts produce the second suite of microfabrics and form in association with filamentous green algae oriented perpendicular to the microbialite surface. Some crusts include calcified filaments, whereas others contained voids that reflect the filamentous community in shape, size, and distribution. Pavilion microbialites demonstrate that microfabric variation can reflect differences in lithification processes and microbial metabolisms as well as microbial community morphology and organization. Even when the morphology of individual filaments or cells is not well preserved, the microbial growth habit can be captured in mesoscale microbialite structures. These results suggest that when petrographic preservation is extremely good, ancient microbialite growth structures and microfabrics can be interpreted in the context of variation in community organization, community composition, and lithification history. Even in the absence of distinct microbial microfabrics, mesostructures can capture microbial community morphology.     

Microbialite response to an anthropogenic salinity gradient in Great Salt Lake,Utah

A railroad causeway across Great Salt Lake, Utah (GSL), has restricted water flow since its construction in 1959, resulting in a more saline North Arm (NA; 24%–31% salinity) and a less saline South Arm (SA; 11%–14% salinity). Here, we characterized microbial carbonates collected from the SA and the NA to evaluate the effect of increased salinity on community composition and abundance and to determine whether the communities present in the NA are still actively precipitating carbonate or if they are remnant features from prior to causeway construction. SSU rRNA gene abundances associated with the NA microbialite were three orders of magnitude lower than those associated with the SA microbialite, indicating that the latter community is more productive. SSU rRNA gene sequencing and functional gene microarray analyses indicated that SA and NA microbialite communities are distinct. In particular, abundant sequences affiliated with photoautotrophic taxa including cyanobacteria and diatoms that may drive carbonate precipitation and thus still actively form microbialites were identified in the SA microbialite; sequences affiliated with photoautotrophic taxa were in low abundance in the NA microbialite. SA and NA microbialites comprise smooth prismatic aragonite crystals. However, the SA microbialite also contained micritic aragonite, which can be formed as a result of biological activity. Collectively, these observations suggest that NA microbialites are likely to be remnant features from prior to causeway construction and indicate a strong decrease in the ability of NA microbialite communities to actively precipitate carbonate minerals. Moreover, the results suggest a role for cyanobacteria and diatoms in carbonate precipitation and microbialite formation in the SA of GSL.     

Palaeoecology of coralline sponge-coral meadows from the upper jurassic of Portugal

TheComophyllia polymorpha-Crispispongia cf.expansa association of the Kimmeridgian Alcobaça Formation occurs in a 5–10 m thick unit that can be followed for at least 10 km in the vicinity of Alcobaça (Estremadura). Corals, coralline sponges (mainly Calcarea), cryptalgal crusts and, to a lesser extent, crinoids are the dominant constituents of the autochthonous community relic which can be grouped in framebuilders, framework encrusters, frame binders, reef-dwellers, and reef destroyers. These organisms formed low meadows in a shallow, fully marine environment subject to low rates of sedimentation and moderate to low energy levels punctuated by rare high energy events. The abundance of coralline sponges in reefs and reef-like communities is uncommon in the Jurassic and appears to be restricted to very shallow water environments.     

Calcified epibionts as palaeoecological tools: examples from the Recent and Pleistocene reefs of Barbados

Calcified epibionts (crustose coralline algae, bryozoans, foraminiferans and serpulid worms) which colinize primary framebuilders of Recent Barbados reefs exhibit a well-defined zonation of species and morphological growth forms in response to environmental factors such as water turbulence and light. Exposed environments are characterized by thick crusts of coralline algae whereas cryptic environments are dominated by thin crusts of algae, bryozoans, foraminiferans and serpulid worms. A model, based on this zonation, was used to decipher the environments of growth and early burial of Pleistocene reefs. Lagoonal corals possess an assemblage of encrusters which document prolonged growth in a uniform environment. Reef crest corals support a mixed succession of shallow water encrusters which record a gradual decrease in light as substrates are smothered by accumulating debris. Sequences such as these represent growth under stable conditions. The model can also be used to interpret sequences formed by catastrophic events and fluctuations in sea level.     

Microbial mat controls on infaunal abundance and diversity in modern marine microbialites

Microbialites are the most abundant macrofossils of the Precambrian. Decline in microbialite abundance and diversity during the terminal Proterozoic and early Phanerozoic has historically been attributed to the concurrent radiation of complex metazoans. Similarly, the apparent resurgence of microbialites in the wake of Paleozoic and Mesozoic mass extinctions is frequently linked to drastic declines in metazoan diversity and abundance. However, it has become increasing clear that microbialites are relatively common in certain modern shallow, normal marine carbonate environments—foremost the Bahamas. For the first time, we present data, collected from the Exuma Cays, the Bahamas, systematically characterizing the relationship between framework‐building cyanobacteria, microbialite fabrics, and microbialite‐associated metazoan abundance and diversity. We document the coexistence of diverse microbialite and infaunal metazoan communities and demonstrate that the predominant control upon both microbialite fabric and metazoan community structure is microbial mat type. These findings necessitate that we rethink prevalent interpretations of microbialite–metazoan interactions and imply that microbialites are not passive recipients of metazoan‐mediated alteration. Additionally, this work provides support for the theory that certain Precambrian microbialites may have been havens of early complex metazoan life, rather than bereft of metazoans, as has been traditionally envisaged.     

Benthic-pelagic coupling on coral reefs: Feeding and growth of Caribbean sponges

Benthic-pelagic coupling and the role of bottom-up versus top-down processes are recognized as having a major impact on the community structure of intertidal and shallow subtidal marine communities. Bottom-up processes, however, are still viewed as principally affecting the outcome of top-down processes. Sponges on coral reefs are important members of the benthic community and provide a crucial coupling between water-column productivity and the benthos. Other than scleractinian corals, sponges dominate many of these habitats where water column productivity is composed of mostly autotrophic and heterotrophic picoplankton that sponges actively filter. While predation upon sponges by invertebrates, fish, and turtles occurs, the sponges Callyspongia vaginalis, Agelas conifera, and Aplysina fistularis from Florida, Belize, and the Bahamas, respectively, exhibit a consistent and significant pattern of greater biomass, rates of growth, and feeding, as does their food supply, with increasing depth. Sponges consume 65-93% of the available particulate food supply and, at all sites, sponges increase in size and growth rate as depth increases, suggesting that food supply and, therefore, bottom-up processes significantly influence the distribution and abundance of sponges in these habitats.     

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.