Stromatolite Formation Research Articles

Authigenic carbonate and native sulfur formation in Messinian (upper Miocene) marine sediments: Sedimentological, petrographical and geochemical constraints

Carbonate concretions accompanied by elemental sulfur were found in an early Messinian (Late Miocene) marine succession of NW Italy. The rocks were studied with an integrated approach including sedimentological, petrographical, stable isotope (carbon, oxygen, and multiple sulfur isotopes), and lipid biomarker analyses. Unlike other examples from Messinian strata of the Mediterranean area, the studied carbonate and sulfur concretions did not derive from the diagenetic replacement of sulfate minerals. Three lithofacies were distinguished: a) laminated lithofacies representing aphotic carbonate stromatolites enclosing fossils of filamentous sulfide-oxidizing bacteria; b) brecciated lithofacies deriving from the brecciation of carbonate stromatolites by mud injections; c) sulfur-bearing lithofacies deriving from the precipitation of thin laminae of elemental sulfur at or close to the sediment-water interface. The carbon and oxygen stable isotope composition of authigenic carbonate minerals and lipid biomarkers indicate that the initial formation of the laminated lithofacies was favored by organoclastic sulfate reduction in the shallow subsurface close to the sediment-water interface, producing sulfide that sustained dense microbial mats of sulfide-oxidizing bacteria at the seafloor. Calcification of the mats and consequent formation of stromatolites were possibly favored by nitrate-driven sulfide oxidation at the seafloor. The subsequent brecciation of the stromatolites was apparently the consequence of sulfate-driven anaerobic oxidation of methane (SD-AOM) in an underlying sulfate-methane transition zone (SMTZ). Focused fluid flow from below, possibly resulting from destabilization of gas hydrates, was not only responsible for the brecciation of the stromatolites, but also for the delivery of bicarbonate ions and the consequent precipitation of additional, 13C-depleted calcite (δ13C values as low as −52‰). Along with bicarbonate, also hydrogen sulfide was produced by SD-AOM at the SMTZ and was transported upwards. The oxidation of hydrogen sulfide at or close to the seafloor promoted the formation of elemental sulfur characterized by δ34S values and Δ33S values close to coeval seawater sulfate.

Water–Rock Interaction Processes in Groundwater and Flows in a Maar Lake in Central Mexico

Tropical maar lakes are distinct ecosystems with unique ecological features. To comprehend, manage, and conserve these lakes, it is essential to understand their water sources, particularly groundwater, and the hydrogeochemical processes shaping their water chemistry. This research focuses on the maar lake Alchichica in central Mexico, known for harboring 18 new and endemic species and a ring of stromatolites. With groundwater discharge as the primary source, concerns arise over anthropic extraction impacts on water levels and stromatolite survival. Sampling six wells and one piezometer revealed major ion (Ca2+, Mg2+, K+, Na+, Cl−, HCO3−, SO42−) and trace element (Fe, Al3+, SiO2) concentrations. Geochemical evolution was explored through diagrams, geological sections, and inverse geochemical models using the PHREEQC code. Findings indicate groundwater evolving along controlled flow paths, and influencing chemical composition through water–rock interactions. The lake’s unique conditions, resulting from the mixing of two flows, enable stromatolite formation. Water level reduction appears unrelated to evaporation at the sampled sites, suggesting a need for a broader study in a larger area. Analyzing the maar lake’s hydrochemistry provides valuable insights into unique characteristics supporting high endemism in this ecosystem. This research enhances our understanding of groundwater’s geochemical processes and hydrogeochemical evolution in maar lakes, with potential applications worldwide.

Строматолиты в верхневизейских отложениях разреза «Пача-Ласта» (р. Илыч, Северный Урал)

The object of the study is the carbonate section «Pacha-Lasta» (Ilych river, Northern Urals). The work is based on the study of the composition, structure of rocks and species diversity of the foraminifera contained in them. The rocks are represented by various limestones with numerous remains of foraminifera, brachiopods, corals, echinoderms, frequent calcispheres and rare stromatolites. Based on the presence in the complex of certain foraminifera genera Biseriella, Globivalvulina, various Eostaffella (including E. ikensis tenebrosa) as well as the species Endothyranopsis sphaerica and Asteroarchaediscus baschkiricus, the Venusian age of the deposits is reliably established. Stromatolite formations, which are not characteristic of the same-age deposits of the area, were established at two intervals of the studied section. They are composed of micritized colonies of cyanobacteria, among which Ortonella and Girvanella are identified. These formations belong to the type of skeletal stromatolites, which in the Silurian and Late Devonian had a certain paleogeographic position and fixed the margin of the carbonate platform. For the Ilych River basin, a consistent decrease in relative sea level is assumed after the Early Visean transgression with a change in the conditions of the open sublittoral (Tula and Aleksinsky horizons) to the conditions of insular shallow water (Mikhailovsky and Venevsky horizons). In the latter, stromatolites can sometines occur, which are not characteristic of the Upper Visean deposits of the area.

Microbialites on the northern shelf of Lake Van, eastern Türkiye#: Morphology, texture, stable isotope geochemistry and age

ABSTRACTLake Van, the world's largest alkaline lake, hosts some of the largest microbialite towers worldwide, which are considered as modern analogues of ancient stromatolites. This study investigates the links between microbialite evolution, geology, climate and hydrology, and the role of biotic and abiotic processes in microbialite growth and morphology. For these objectives, the northern shelf of Lake Van was surveyed by sub‐bottom seismic profiling and diving, and two 9 m and 15 m high microbialite chimneys were sampled at 25 m water depth. Samples were analysed for stable oxygen and carbon isotopes, X‐ray diffractometry, scanning electron microscopy and U/Th age dating. Lake Van microbialites precipitate wherever focused Ca‐rich groundwater flows onto the lake floor to mix with alkaline lake water. Variable columnar, conical and branching morphologies of the microbialites indicate various processes of formation by groundwater channelling within the chimneys. Collectively, our data suggest that the microbialite chimneys have formed within the last millennium, most likely starting during the warm and humid Medieval Climate Anomaly (ca ad 800–1300), when lake level rose approximately to the present level due to enhanced Inputs of riverine Ca‐rich freshwater and groundwater. Our new scanning electron microscopy observations indicate that the internal structure of the microbialites below the outer cyanobacteria‐covered crust is constructed by calcified filaments, globular aggregates and nanocrystals of algal, cyanobacterial and heterobacterial origins and inorganically precipitated prismatic calcite crystals. These textural features, together with dive observations, clearly demonstrate the important role of inorganic carbonate precipitation at sites of groundwater discharge, followed by cyanobacteria and algal mucilage deposition and microbially meditated calcification in the photic zone in the rapid growth of the microbialite chimneys. Considering the close similarities of some textures with those of ancient stromatolites and meteorites, the results of this study provide new insights into the environmental conditions associated with stromatolite formation and extra‐terrestrial life evolution.

FRESHWATER STROMATOLITES FROM AN EARLY PERMIAN WETLAND (MANEBACH, THURINGIAN-FOREST BASIN, GERMANY): STRUCTURE, DEVELOPMENT, AND PALEOENVIRONMENTAL CONTEXT

Abstract Fossil stromatolites enclosing structurally preserved land plant remains have rarely been documented and studied in detail. Permineralized woody Tylodendron sp. conifer axes (slender stems, branches) from a lacustrine sedimentary sequence in the lower Permian fossil Lagerstätte of Manebach (Thuringian-Forest Basin, central Germany) are frequently surrounded by stromatolites that consist of successive, usually asymmetrical microbial layers. The stromatolites show various growth forms ranging from laminar to palisadic. They developed in stagnant water from microbial overgrowth dominated by slender, unbranched sessile cyanobacterial filaments aligned vertically into tufts or turf-like stands. Interspersed among the filaments were other filamentous and coccoid microorganisms. Preservation of the Tylodendron axes can be exquisite and sometimes even includes extraxylary tissues containing remains of fungi, suggesting that stromatolite formation began soon after the axes had entered the water and were perhaps even conducive to their preservation. Structurally similar fossil microbialitic structures from elsewhere likewise demonstrate that they were effective in preserving plant morphology. The Manebach stromatolites and the plant remains they contain contribute to a more accurate understanding of the complex biological processes in late Paleozoic lake ecosystems.

Characteristics of Early Neoproterozoic Stromatolites from Southern Liaoning, North China: Insights into the Formation of Stromatolites

Stromatolites, among the earliest fossils in Earth’s history, are widely distributed on the margins of the North China Precambrian carbonate platform. The formation processes of stromatolites reveal the biomineralization and evolution of early life in the Precambrian. The well-preserved stromatolitic dolostones recorded in the Ganjingzi Formation are developed around Yuanjiagou village, in southern Liaoning Province. The morphology of the Ganjingzi stromatolites manifests in stratiform, columnar, and domal forms. A tripartite lamina structure including light laminae and two types of dark laminae is observed in thin sections. The origins of dark laminae were related to microbial metabolism, while the light laminae were the result of the recrystallization of synsedimentary marine cement. Hardground substrate and carbonate fragments were suitable for microbes to colonize, suggesting that microbes can adapt to various current energy settings. A comparison of the growth environment, morphology, and laminae features between the Ganjingzi stromatolites and modern carbonate stromatolites from Hamelin Pool and Lagoa Vermelha suggest that the Ganjingzi stromatolites may have been formed in a restricted tidal-flat setting with high salinity and evaporation. The role of microbes that form modern stromatolites in inducing precipitation of carbonate or binding sediments, might contribute to the formation of the Ganjingzi stromatolites. The formation process of the Ganjingzi stromatolites indicates that the microbial communities, favorable substrate, and synsedimentary marine cement were the key factors in promoting the development of the Neoproterozoic stromatolites on the northeastern margin of the North China Craton.

Chemotrophy-based phosphatic microstromatolites from the Mississippian at Drewer, Rhenish Massif, Germany

Abstract The Drewer quarry located in the Rhenish Massif is a well-studied outcrop that comprises Upper Devonian (Famennian) to Lower Carboniferous (Viséan) strata. Within the Drewer deposits two black shale intervals have been described that are linked to two global oceanic anoxic events, the Hangenberg Event and the Lower Alum Shale Event. The black shales associated with the Middle Tournaisian Lower Alum Shale Event contain abundant phosphatic concretions, which were investigated using thin section petrography, powder X-ray diffraction, Fourier-transform infrared spectrometry and scanning electron microscopy. The concretions formed during several growth phases under anoxic and at least episodically sulphidic conditions within the sediment and served as a substrate for subsurface microbial mats that formed phosphatic microstromatolites. The microstromatolites occur either as partially branched columns of up to 600 µm in length attached to the phosphatic concretions or as smaller, bulbous aggregates surrounding the concretions. Element mapping identified the presence of pyrite and other metal sulphides within the phosphatic microstromatolites. The carbon and oxygen stable isotopic composition of phosphate-associated carbonate within the phosphatic microstromatolites suggests that the mat-forming microorganisms were probably anaerobic, chemotrophic microbial communities dwelling in the anoxic environment during the Lower Alum Shale Event. Such interpretation agrees with the deeper-water depositional setting of the Lower Alum Black Shale and its high content of organic matter, suggesting that chemotrophic microbial mats are potent agents of phosphogenesis in general, and of the formation of phosphatic stromatolites in particular.

No biological effect on magnesium isotope fractionation during stromatolite growth

The growth and morphology of stromatolites have been previously linked to diverse microbial activities. However, the role of microbial metabolisms on carbonate formation during stromatolite growth remains controversial. Magnesium isotopes have been proposed to serve as a tracer of microbial carbonate formation, implying a potential biological isotope fractionation. To further elucidate whether Mg isotope fractionation is modified during microbial carbonate formation, this study reports Mg isotope compositions of Holocene stromatolites and pore waters from Lagoa Salgada, a coastal ephemeral lake in Brazil. The stromatolitic carbonates are composed of high-Mg calcites characterized by extremely positive inorganic δ13C values, up to +20‰, and variable δ26Mg values, ranging from −2.98‰ to −0.68‰. Multiple pieces of evidence consistently demonstrate changes in microbial metabolism resulting from ecosystem fluid chemistry evolution. However, the direction of Mg isotope fractionation associated with carbonate precipitation remained invariable despite the changes in microbial activity. Mineralogical features indicate that the stromatolitic carbonates formed via 2-D nucleation. An isotopic mass balance calculation based on the observed variations in δ26Mg values and Mg concentrations of associated pore waters argues for an Mg isotopic equilibrium fractionation factor of −2.60‰ associating with the carbonate formation at Lagoa Salgada, well-matching experimental values for abiotic calcite precipitation. Thus, the observed δ26Mg variability of stromatolitic carbonates is primarily controlled by the changes of physicochemical conditions in the ambient fluid. We infer that during the formation of stromatolites, a consortium of different microbial communities produces extracellular polymeric substances, which serve as a substrate for carbonate nucleation from ambient fluid without significantly affecting the fractionation of Mg-isotopes. Our findings shed light on the effects of microbial processes on carbonate formation during stromatolite growth and improve the current understanding of the Mg isotope record in microbial carbonates.

Assessing the reliability of modern marine stromatolites as archives for the uranium isotope paleoredox proxy

Uranium isotopes (238U/235U) are commonly utilised in marine carbonates to reconstruct the redox evolution of ancient oceans and the atmosphere. However, questions remain concerning U isotope fractionation during carbonate precipitation and/or early sedimentary diagenesis. Moreover, the stromatolitic nature of many Precambrian limestones and dolostones brings further questions about U isotope fractionation during stromatolite formation due to microbial activity. Thus, it is unclear whether stromatolites are reliable archives for the U isotope composition of seawater. Here we measure the 238U/235U ratio of modern stromatolites from the hypersaline Shark Bay, Western Australia, a key study area that represents the largest modern example of stromatolite growth globally. A component-specific approach was utilised to analyse the uppermost crusts of modern stromatolites, older material from the deeper stromatolite laminae and the substrate upon which the stromatolites are forming. Our interpretations of δ238U data are aided by δ234U, δ18O, δ13C and trace element measurements, including some mineralogy and total organic carbon determinations for selected samples.Modern aragonitic crusts of stromatolites from the subtidal and intertidal zones around Shark Bay exhibited a narrow range of δ238U (−0.30 to −0.33‰), corresponding to a small offset of ca. +0.07‰ from seawater. This range of δ238U values overlaps with that of other primary carbonate precipitates in shallow marine environments such as corals but with a slightly higher mean δ238U. We investigated if this offset may be associated with the coprecipitation of U with aragonite from hypersaline seawater in Shark Bay, which has a higher proportion of dissolved U present as Ca2UO2(CO3)3 compared to open seawater. However, this measured offset is much smaller than that predicted using a speciation-dependent isotope fractionation model. Sub-recent material in the deeper stromatolite laminae exhibited higher and more variable δ238U (up to −0.15‰) compared to the modern aragonitic crusts of stromatolites. The strong inverse correlation between δ238U and the total organic carbon content (r = −0.83) and positive correlation with Mn concentrations (r = 0.91) suggest that 238U was preferentially reduced in the deeper laminae in association with organic carbon remineralisation under reducing conditions. The average δ238U offset of sub-recent stromatolite laminae from modern seawater was +0.13 ± 0.11‰ (1 s.d.), which is slightly lower than, but within uncertainty of, the offset measured for bulk carbonates from the Bahamas (+0.27 ± 0.14‰; 1 s.d.). These results demonstrate that stromatolites are valuable archives of global seawater δ238U, even in hypersaline restricted basins such as Shark Bay, but an offset to account for syndepositional diagenesis is likely required to accurately reconstruct seawater δ238U using ancient stromatolites.

Stromatolite formation by Anaerolineae-dominated microbial communities in hot spring travertine in North Sumatra, Indonesia

The travertine deposits could provide a prospective geomicrobiological snapshot of the early Earth considering its similarity with ancient stromatolites. Herein, we investigated the travertine carbonate deposits in a calcareous and sulfidic hot spring at Sipoholon, Northern Sumatra, Indonesia, with an emphasis on the formation of a laminated texture similar to one observed in the stromatolites. The travertine deposits were associated with microbial mats of different colors that change from yellow, pink, and green, lowering water temperature and H2S concentration along the water passages. A phylotype analysis of the Prokaryotic 16S rRNA gene revealed that chemolithoautotrophic sulfur and hydrogen oxidizing bacteria were abundant in the yellow sediment around vent sites. At the same time, photoheterotrophs, a linage of Anaerolineae, dominated the pink to green sediment at mid- to downstream. Laminated textures are developed in pink and green-colored sediments among these categories. Monitoring water chemical parameters and sediment texture for an Anaerolineae-dominated sample collected from a midstream site revealed that daily lamination was formed due to the daytime biofilm formation under the stable chemical composition of water conditions except for dissolved oxygen concentration. Daily laminations were also formed in other samples by microbial communities with lower Anaerolineae abundance. Further, the results showed that if mineral precipitation and microbial growth rates are appropriately balanced, cyanobacteria were not essential for the lamina development. This indicating that such a geochemical system could occur in early-time stromatolites even before the evolution of cyanobacteria. The microbial-mineral precipitation systems identified in this study provide significant insights into the formation of stromatolites, which can in turn shed light on the history of early Earth.

Lake level controls the recurrence of giant stromatolite facies

AbstractStromatolites have often served as a diagnostic carbonate facies for deep‐time palaeoclimatic and geobiological studies because they may form under favourable environmental conditions for microbially mediated carbonate production. ‘Giant’ (<5 m) stromatolites occur in the Laney Member of the Eocene Green River Formation in the Vermillion Creek section of the Sand Wash Basin (north‐west Colorado, USA). Giant stromatolite growth was hypothesized to have been promoted by both availability of large substrates as nucleation sites and physicochemical factors, including increased calcium carbonate mineral saturation states due to the generally warmer Eocene climate and a dynamic period in Lake Gosiute’s hydrological balance. Depositional horizons of giant stromatolites were observed at two distinct stratigraphic levels, which demonstrated that the formation of the giant morphotype was not a unique occurrence, and provided an opportunity to examine both onset and cessation of stromatolite development. Coincident increases in carbonate clumped isotope‐derived temperatures, the carbon and oxygen isotopic compositions of lake water (δ18Ow), carbonate mineral saturation states (Ω) estimated by ooid size, and salinity estimated by ostracod assemblages demonstrated that formation of giant stromatolites was facilitated by lake level drawdown. Decreased lake levels: (i) promoted carbonate precipitation; and (ii) positioned benthic microbial communities within the photic zone. Field and petrographic analyses revealed that the giants preserved micritic laminations, ‘trapped and bound’ grains, and aragonite fans, interpreted as reflecting contributions from both microbially mediated and abiotic carbonate mineral precipitation. Field and microscopic sedimentological and stable isotope data indicated that giant stromatolite growth ceased as a result of subaerial exposure of mounds during lake level lows. Although microbial mediation of carbonate chemistry was seemingly important for initiation of stromatolite growth, this work demonstrated that stromatolite macromorphology was dominantly controlled by availability of large substrates, lake level and resultant solute chemistry, i.e. increased Ω.

Cyanobacterial spheroids and other biosignatures from microdigitate stromatolites of Mesoproterozoic Wumishan Formation in Jixian, North China

Stromatolites have been widely reported from the Archean and Paleoproterozoic successions worldwide, and they could represent one of oldest life forms on Earth. Of these, a group of small stromatolites occur as microdigitate low-relief columns, and are also conspicuous in the field. However, biogenicity of these microdigitate stromatolites (MDSs) has long been disputed due to to the abundance of radial-fibrous texture and a lack of convincing microfossils. New examples of MDS are documented from the Mesoproterozoic Wumishan Formation of the Jixian area, North China. Vertically oriented fibrous fabrics are conspicuous and penetrate laminae as well as microscopic spheroids, which point to an abiotic genesis for this specific fabric. Stromatolite laminae contain abundant spheroids, typically 15–30 μm in diameter, with single or double outlines and they occur as solitary coccoid-like microfossils or in small aggregated colonies. Spheroids show strong fluorescence under both green and purple exciting lights, consistent with their composition of organic matter. Spheroids are abundant in the Wumishan stromatolites and they are categorized into two types. The first kind comprises micrite nuclei surrounded by sparitic sheaths, without nano-particle coatings. A smooth to grainy spheroidal surface defines the first kind of spheroids that also has a distinct rounded opening, which is often broken probably due to diagenesis and silicification. The second kind of spheroids is usually covered with nano-particles and lacks circular opening on surface. These spheroids possess large nuclei of single sparitic calcite coated with thin sparry sheaths. Overall, the Wumishan spheroids resemble coccoidal microorganisms reported from other Archean-Paleoproterozoic strata worldwide, but they are also better preserved. The rounded opening on spheroid surface is interpreted as division point of unicells during reproduction of the life cycle of bacteria akin to Myxococcoides grandis. Clump-like micro-particle aggregates in nuclei could represent daughter cells released from the parental envelope, similar to the reproduction process and life cycle suggested for similar spheroidal microfossils from other similar Precambrian occurrences. The Wumishan spheroids therefore may represent fossilized prokaryotes that could have contributed to construct the MDS. Moreover, filamentous microfossils are occasionally present in the coloumns of stromatolites, and they resemble filamentous cyanobacteria, but may not be major constructors of MDS due to their rarity in the buildups. Three types of nano-particles are also conspicuous: (1) putative organic relics, such as fragmented filaments and mucuslike biofilms (purported EPS), (2) organominerals, including nanoglobules, polyhedrons, and their aggregates, and (3) dumbbell-shaped nano-particle aggregates. All of these nano-particles are interpreted to be likely biogenic in origin, and many of them were found from the radial-fibrous fabrics of carbonate precipitates in the MDS, implying that some heterotrophic bacteria may have afficliated the precipitation of radial fibers in deep-time radial-fibrous carbonate precipitates. Therefore, abundant and diverse biosignatures (spheroids, tubular filaments, and nano-particles) are identified in the Wumishan MDSs, and we conclude that diverse filamentous and coccoidal micro-organismscontributed to the formation of the Wumishan stromatolites.

Sinuous stromatolites of the Chandi Formation, Chattisgarh Basin, India: their origin and implications for Mesoproterozoic seawater

AbstractRemnants of some of the planet’s most ancient life forms, stromatolites in the late Mesoproterozoic sea of the Chattisgarh Basin, India, preserve a conspicuous sinuous pattern. They occur as successive biostromes, 10–30 cm thick, separated by 2–5-cm-thick marly layers and discrete bioherms up to several metres thick and 20 m across. Stromatolite columns in the Chandi Formation are 5–10 cm high, sinuous, inclined and straight, with both branched and non-branched types. These stromatolites are composed of calcite micrite and show well defined light and dark laminae with evidence of erosion between lamina sets. The column sinuosity probably originated as a response to changes in direction and strength of currents. Successive flat beds of stromatolite (biostromes), separated by marl/clay horizons, impart a rhythmic pattern to the succession. The Chandi sinuous stromatolite columns resemble those occurring in China, North America and Siberia, of a comparable age, suggesting that similar marine conditions of stromatolite formation might have been operating in the late Mesoproterozoic seas worldwide. However, the petrographic and sedimentological analyses of these stromatolites indicate their development throughin situproduction of carbonate with some trapping and binding of detrital sediment. As a result of the presence of terrigenous material within the stromatolites, whole-rock geochemical analyses for trace elements and rare earth elements cannot be used for interpretation of seawater chemistry and the redox conditions at the time.

Comparison of thresholding methods for shoreline extraction from Sentinel-2 and Landsat-8 imagery: Extreme Lake Salda, track of Mars on Earth

Lake Salda's extreme environment is geologically similar to Jezero Crater paleolake on Mars due to the formation of stromatolites and its extremely alkaline and cold water. It is critical to accurately determine the shoreline in the littoral zone where stromatolite formation occurs, in alkaline clean lakes like Salda, which contain traces of life on Mars, and in monitoring the change that occurs with climate and anthropogenic effect. The performance of global automatic thresholding algorithms on shoreline determination from NDWI and mNDWI water indices is compared in this study using Sentinel-2 and Landsat-8 images atmospherically corrected by different algorithms. Satellite images data acquired on August 12, 2020 for Sentinel-2 and August 11, 2020 for Landsat-8 on Lake Salda were used to determining the shoreline. The shoreline data measured in situ concurrently with the Sentinel-2 satellite acquisition was used as reference data. In the accuracy analysis, ground control points created inside and outside the lake at a distance of 1 pixel and 0.5 pixel to the reference shoreline for each satellite image were used. The performance of the optimal threshold values determined by each thresholding algorithm in the water index images was assessed using Kappa coefficient, Overall Accuracy (OA), %OA of Inside (%OAinside) and %OA of Outside (%OAoutside) statistics metrics. The optimal threshold values vary depending on the image and the atmospheric correction algorithm applied to the image. The NDWI index produces more accurate results in both Sentinel-2 and Landsat-8 satellite images. While atmospheric correction algorithms do not affect the results in Landsat-8 images, the Sen2Cor algorithm outperforms iCOR in Sentinel-2 images. For thresholding algorithms to be used in different water index and satellite images, Intermode, Isodata, IJ_Isodata, Minimum and Otsu algorithms in Landsat-8_LaSRC_NDWI and Landsat-8_iCOR_NDWI images, and Intermode, Minimum and Huang algorithms in Sentinel-2_Sen2Cor_NDWI images produce the best results. Because the Minimum algorithm causes significant gaps in the lake surface, the Huang and Intermode algorithms should be used for Sentinel-2_Sen2Cor_NDWI images. The 0 (zero) threshold value in the water indices images has a high accuracy only in the NDWI water indices generated from the Landsat-8 image.

Mixed carbonate-siliciclastic sedimentation in a mesoproterozoic storm-dominated ramp: Depositional processes and stromatolite development

Records of shallow-marine ramps with the mixing of carbonate and siliciclastic sediments are common throughout the geological time. All these records have pure carbonate and pure siliciclastic deposits as end members, occurring contemporaneously in distinct depositional regions along the ramp, and transitional hybrid facies between them. The two end member can mix in different scales and can alternate in time due to climatic changes and regressions and transgressions. This work presents a detailed reconstruction of a Mesoproterozoic storm-dominated mixed carbonate-siliciclastic ramp composed of hybrid sediments and whithout the presence of pure siliciclastic or carbonate deposits, a rare example in the geological record. Based on a high resolution logged section (in 1:20 scale) and qualitative thin sections, eleven lithofacies were identified and grouped into three lithofacies associations (offshore, offshore transition and shoreface), which are stacked vertically forming a transgressive–regressive cycle. This faciological distribution indicates a low relief ramp with wide microbial colonization from shallow to relatively deep waters (below storm-wave base level). In offshore low-energy distal areas, microbial mats spread laterally over large distances with little or no interference from currents, while in the offshore transition the morphology of the bioherms is shaped by currents induced by waves. In turn, the high wave energy in the shoreface inhibits the formation of stromatolites, restricting their occurrence to thin layers of microbial carpets or intraclastic lags. The mixing occurs in compositional scale and is relatively homogeneous along the whole logged interval, independent of the shifts in lithofacies or lithofacies associations. This compositional homogeneity is linked to the wide distribution and regularity in the input of siliciclastic sediments during the sedimentary accumulation. Strong currents induced by storms allow the transport and mixing of siliciclastic sediments with carbonate grains generated in the basin during fair-weather periods.

Builders, tenants, and squatters: the origins of genetic material in modern stromatolites.

Micro-organisms have long been implicated in the construction of stromatolites. Yet, establishing a microbial role in modern stromatolite growth via molecular analysis is not always straightforward because DNA in stromatolites can have multiple origins. For example, the genomic material could represent the microbes responsible for the construction of the stromatolite (i.e., "builders"), microbes that inhabited the structure after it was built (i.e., "tenants"), or microbes/organic matter that were passively incorporated after construction from the water column or later diagenetic fluids (i.e., "squatters"). Disentangling the role of micro-organisms in stromatolite construction, already difficult in modern systems, becomes more difficult as organic signatures degrade, and their context is obscured. To evaluate our ability to accurately decipher the role of micro-organisms in stromatolite formation in geologically recent settings, 16/18S SSU rRNA gene sequences were analyzed from three systems where the context of growth was well understood: (a) an actively growing stromatolite from a silicic hot spring in Yellowstone National Park, Wyoming, where the construction of the structure is controlled by cyanobacteria; (b) a mixed carbonate and silica precipitate from Little Hot Creek, a hot spring in the Long Valley Caldera of California that has both abiogenic and biogenic components to accretion; and (c) a near-modern lacustrine carbonate stromatolite from Walker Lake, Nevada that is likely abiogenic. In all cases, the largest percentage of recovered DNA sequences, especially when focused on the deeper portions of the structures, belonged to either the tenant or squatter communities, not the actual builders. Once removed from their environmental context, correct interpretation of biology's role in stromatolite morphogenesis was difficult. Because high-throughput genomic analysis may easily lead to incorrect assumptions even in these modern and near-modern structures, caution must be exercised when interpreting micro-organismal involvement in the construction of accretionary structures throughout the rock record.

Conserved bacterial genomes from two geographically isolated peritidal stromatolite formations shed light on potential functional guilds.

Stromatolites are complex microbial mats that form lithified layers. Fossilized stromatolites are the oldest evidence of cellular life on Earth, dating back over 3.4 billion years. Modern stromatolites are relatively rare but may provide clues about the function and evolution of their ancient counterparts. In this study, we focus on peritidal stromatolites occurring at Cape Recife and Schoenmakerskop on the southeastern South African coastline, the former being morphologically and structurally similar to fossilized phosphatic stromatolites formations. Using assembled shotgun metagenomic analysis, we obtained 183 genomic bins, of which the most dominant taxa were from the Cyanobacteria phylum. We identified functional gene sets in genomic bins conserved across two geographically isolated stromatolite formations, which included relatively high copy numbers of genes involved in the reduction of nitrates and phosphatic compounds. Additionally, we found little evidence of Archaeal species in these stromatolites, suggesting that they may not play an important role in peritidal stromatolite formations, as proposed for hypersaline formations.

Acid Mine Drainage as Energizing Microbial Niches for the Formation of Iron Stromatolites: The Tintillo River in Southwest Spain.

The Iberian Pyrite Belt in southwest Spain hosts some of the largest and diverse extreme acidic environments with textural variation across rapidly changing biogeochemical gradients at multiple scales. After almost three decades of studies, mostly focused on molecular evolution and metagenomics, there is an increasing awareness of the multidisciplinary potential of these types of settings, especially for astrobiology. Since modern automatized exploration on extraterrestrial surfaces is essentially based on the morphological recognition of biosignatures, a macroscopic characterization of such sedimentary extreme environments and how they look is crucial to identify life properties, but it is a perspective that most molecular approaches frequently miss. Although acid mine drainage (AMD) systems are toxic and contaminated, they offer at the same time the bioengineering tools for natural remediation strategies. This work presents a biosedimentological characterization of the clastic iron stromatolites in the Tintillo river. They occur as laminated terraced iron formations that are the most distinctive sedimentary facies at the Tintillo river, which is polluted by AMD. Iron stromatolites originate from fluvial abiotic factors that interact with biological zonation. The authigenic precipitation of schwertmannite and jarosite results from microbial-mineral interactions between mineral and organic matrices. The Tintillo iron stromatolites are composed of bacterial filaments and diatoms as Nitzschia aurariae, Pinnularia aljustrelica, Stauroneis kriegeri, and Fragilaria sp. Furthermore, the active biosorption and bioleaching of sulfur are suggested by the black and white coloration of microbial filaments inside stromatolites. AMD systems are hazardous due to physical, chemical, and biological agents, but they also provide biogeochemical sources with which to infer past geochemical conditions on Earth and inform exploration efforts on extraterrestrial surfaces in the future.

Erosion‐initiated stromatolite and thrombolite formation in a present‐day coastal sabkha setting

AbstractLaminated microbial mats and microbialites have been documented from a variety of coastal marine environments. This study aims to provide the first detailed descriptions of intertidal pools, along with their hosted thrombolite and stromatolite structures, from Abu Dhabi, and to propose a model for their formation and evolution. It is proposed that the development of pools within the upper intertidal zone was initiated by localized erosion of the laminated microbial mats during high energy events. The removal of the protective mats permitted erosion of the underlying unconsolidated sediment to produce erosional scours that continued to develop to create the pools observed today. The margins of the newly‐created submerged environment were colonized by a cyanobacteria dominated microbial community. The precipitation of aragonite cement, associated with the cyanobacteria, stabilized the pool walls and cemented the microbial communities to form stromatolitic and thrombolitic fabrics. Syndepositional cementation was further enhanced by the precipitation of marine cements as a result of evaporation‐driven Ca2+ and Mg2+ supersaturation. Erosion behind and below the cemented pool wall eventually resulted in rim‐collapse and the formation of the observed pool margin parallel thrombolite bands. Successive generations of lithification and erosion increased the area of the pool with the earliest thrombolites eroding and becoming increasingly isolated. In summary, the resultant microbialites developed through the complex interplay of erosion, abiotic early lithification and microbially‐mediated processes, and represent a continuum between unlithified laminated microbial mats and domal microbialites. These features are most likely produced during a sea‐level scenario of stillstand or transgression and, as such, may be useful as a diagnostic tool to elucidate the onset of transgression. The newly proposed model for stromatolite formation has significant implications for the recognition and interpretation of similar structures observed in the fossil record.

Between a Rock and a Soft Place: The Role of Viruses in Lithification of Modern Microbial Mats

Stromatolites are geobiological systems formed by complex microbial communities, and fossilized stromatolites provide a record of some of the oldest life on Earth. Microbial mats are precursors of extant stromatolites; however, the mechanisms of transition from mat to stromatolite are controversial and are still not well understood. To fully recognize the profound impact that these ecosystems have had on the evolution of the biosphere requires an understanding of modern lithification mechanisms and how they relate to the geological record. We propose here viral mechanisms in carbonate precipitation, leading to stromatolite formation, whereby viruses directly or indirectly impact microbial metabolisms that govern the transition from microbial mat to stromatolite. Finding a tangible link between host-virus interactions and changes in biogeochemical processes will provide tools to interpret mineral biosignatures through geologic time, including those on Earth and beyond.