Microbial deposits in the aftermath of the end‐Permian mass extinction: A diverging case from the Mineral Mountains (Utah, USA )
Emmanuelle VenninNicolas OlivierArnaud BrayardIvan BourChristophe ThomazoGilles EscarguelEmmanuel FaraKevin G. BylundJames F. JenksDaniel A. StephenRichard Hofmann
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Abstract The Lower Triassic Mineral Mountains area (Utah, USA ) preserves diversified Smithian and Spathian reefs and bioaccumulations that contain fenestral‐microbialites and various benthic and pelagic organisms. Ecological and environmental changes during the Early Triassic are commonly assumed to be associated with numerous perturbations (productivity changes, acidifica‐tion, redox changes, hypercapnia, eustatism and temperature changes) post‐dating the Permian–Triassic mass extinction. New data acquired in the Mineral Mountains sediments provide evidence to decipher the relationships between depositional environments and the growth and distribution of microbial structures. These data also help to understand better the controlling factors acting upon sedimentation and community turnovers through the Smithian–early Spathian. The studied section records a large‐scale depositional sequence during the Dienerian(?)–Spathian interval. During the transgression, depositional environments evolved from a coastal bay with continental deposits to intertidal fenestral–microbial limestones, shallow subtidal marine sponge–microbial reefs to deep subtidal mud‐dominated limestones. Storm‐induced deposits, microbialite–sponge reefs and shallow subtidal deposits indicate the regression. Three microbialite associations occur in ascending order: (i) a red beds microbialite association deposited in low‐energy hypersaline supratidal conditions where microbialites consist of microbial mats and poorly preserved microbially induced sedimentary structure; (ii) a Smithian microbialite association formed in moderate to high‐energy, tidal conditions where microbialites include stromatolites and associated carbonate grains (oncoids, ooids and peloids); and (iii) a Spathian microbialite association developed in low‐energy offshore conditions that is preserved as multiple decimetre thick isolated domes and coalescent domes. Data indicate that the morphologies of the three microbialite associations are controlled primarily by accommodation, hydrodynamics, bathymetry and grain supply. This study suggests that microbial constructions are controlled by changes between trapping and binding versus precipitation processes in variable hydrodynamic conditions. Due to the presence of numerous metazoans associated with microbialites throughout the Smithian increase in accommodation and Spathian decrease in accommodation, the commonly assumed anachronistic character of the Early Triassic microbialites and the traditional view of prolonged deleterious conditions during the Early Triassic time interval is questioned.Keywords:
Stromatolite
Microbial mat
Marine transgression
Early Triassic
Permian–Triassic extinction event
Ooid
The hothouse climate in the Early Triassic has been recognised for a decade. Yet it remains the most recently discovered hothouse and is poorly understood in many aspects. Initially triggered by the Siberian Traps in the latest Permian, the Early Triassic represents one of the most extreme and long-lasting greenhouses in the Phanerozoic. Although the outgassing of the Siberian Traps probably already decreased in the late Griesbachian, the Equatorial SSTs peaked at ~40 ℃ later in the late Smithian. The late Smithian thermal maximum coincided with resumed volcanic activities of a smaller scale. However, why lesser volcanism triggered Phanerozoic’s warmest hyperthermal is puzzling. The extreme warmth ameliorated in the latest Spathians, marking the termination of a ~5 Myr hothouse.Many key questions about the Early Triassic climate remain unanswered. These include how warm the poles were, how flat the latitudinal SST gradient was, and how climate interacted with the global ocean circulation. However, the most fundamental question is how to maintain such an extreme hothouse climate for such a long time.As most shelly fossils died out during the end-Permian mass extinction and the Early Triassic oceans were dominated by aragonite-shelled mollusks, reconstruction of Early Triassic seawater temperatures relies almost solely on oxygen isotope thermometer in conodont bioapatite. One of the key challenges is that Early Triassic conodonts are rare, small, and cannot be found everywhere due to the subduction of old ocean floors. These hinder the acquisition of proxy data in a broader palaeogeographic context. Future work combining proxy data with state-of-the-art Earth system modelling would be an ideal solution to better understand the hottest time in the Phanerozoic.
Early Triassic
Stromatolite
Permian–Triassic extinction event
Tethys Ocean
Polydiacetylenes
Conodont
Supercontinent
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Silurian calcareous algae, cyanobacteria, and microproblematica are abundantly preserved in the Alexander terrane of southeastern Alaska. They represent a diverse population of calcified microbes that contributed to the formation of a variety of shallow- and deep-water carbonate deposits. Five associations are recognized on the basis of recurring groups of microbial taxa. These include a Girvanella-Tuxekanella association that formed oncoids and thick encrustations on skeletal grains in shelf environments. A Renalcis association predominated in a stromatoporoid-coral reef that developed at the incipient shelf margin on a crinoid-solenoporid shoal (“ Solenopora ” association). Other organic buildups are characterized by a Ludlovia association, which constructed skeletal stromatolite reefs, and by an Epiphyton-Sphaerina association that contributed to the formation of a stromatolitic mud mound. A mixed microbial assemblage reflects transport and mixing of shallow-water microbial biotas that were deposited by turbidity currents, debris flows, and slumps in a slope environment.
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Microbial mat
Crinoid
Shoal
Ooid
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Permian–Triassic extinction event
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Ooid
Early Triassic
Nekton
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Most Phanerozoic oolites are marked by ooids with a diameter less than 2 mm. Observations on a Neoproterozoic oolite have resulted in a change of concept. The term "pisolite" that traditionally referred to oolites with a grain size of more than 2 mm, is now restricted to those coated carbonate grains formed by meteoritic freshwater diagenesis; oolites with a grain size of more than 2 mm are now defined as "giant". Particular unusual giant oolites within a set of oolitic-bank limestones with thicknesses of more than 40 m in the top part of the Lower Triassic (Induan) Daye (Ruiping) Formation at the Lichuan section in the western part of Hubei Province in South China, represent an important sedimentological phenomenon in both the specific geological period and the geological setting that is related to the end-Permian biological mass extinction. Like the giant oolites of the Neoproterozoic that represent deposits where oolites formed in a vast low-angle carbonate ramp at that special geological period, the Triassic Daye Formation at the study section are significant because they provide a comparative example to help understand the evolving carbonate world reflected by oolites, the origin of which is still uncertain, and they give insight into the sedimentation pattern of the desolate sea floor, which resulted from the mass extinction at the turn of the Permian into the Triassic.
Early Triassic
Permian–Triassic extinction event
Ooid
Tethys Ocean
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Abstract Lower Triassic limestones contain giant ooids (>2 mm) along with other precipitated carbonate textures more typical of Precambrian strata. These features appear to have resulted from changes in seawater chemistry associated with the end-Permian mass extinction, but quantifying the carbonate chemistry of Early Triassic seawater has remained challenging. To constrain seawater carbonate saturation state, dissolved inorganic carbon, alkalinity, and pH, we applied a physicochemical model of ooid formation constrained by new size data on Lower Triassic ooids from south China, finding that the Triassic giant ooids require a higher carbonate saturation state than typifies modern sites of ooid formation. Model calculations indicate that Early Triassic oceans were at least seven times supersaturated with respect to aragonite and calcite. When combined with independent constraints on atmospheric pCO2 and oceanic [Ca2+], these findings require that Early Triassic oceans had more than twice the modern levels of dissolved inorganic carbon and alkalinity and a pH near 7.6. Such conditions may have played a role in inhibiting the recovery of skeletal animals and algae during Early Triassic time.
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Alkalinity
Stromatolite
Early Triassic
Permian–Triassic extinction event
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Abstract Thrombolite and stromatolite habitats are becoming increasingly recognized as important refuges for invertebrates during Phanerozoic Oceanic Anoxic Events ( OAE s); it is posited that oxygenic photosynthesis by cyanobacteria in these microbialites provided a refuge from anoxic conditions (i.e., the “microbialite refuge” hypothesis). Here, we test this hypothesis by investigating the distribution of ~34, 500 benthic invertebrate fossils found in ~100 samples from a microbialite succession that developed following the latest Permian mass extinction event on the Great Bank of Guizhou (South China), representing microbial (stromatolites and thrombolites) and non‐microbial facies. The stromatolites were the least taxonomically diverse facies, and the thrombolites also recorded significantly lower diversities when compared to the non‐microbial facies. Based on the distribution and ornamentation of the bioclasts within the thrombolites and stromatolites, the bioclasts are inferred to have been transported and concentrated in the non‐microbial fabrics, that is, cavities around the microbial framework. Therefore, many of the identified metazoans from the post‐extinction microbialites are not observed to have been living within a microbial mat. Furthermore, the lifestyle of many of the taxa identified from the microbialites was not suited for, or even amenable to, life within a benthic microbial mat. The high diversity of oxygen‐dependent metazoans in the non‐microbial facies on the Great Bank of Guizhou, and inferences from geochemical records, suggests that the microbialites and benthic communities developed in oxygenated environments, which disproves that the microbes were the source of the oxygenation. Instead, we posit that microbialite successions represent a taphonomic window for exceptional preservation of the biota, similar to a Konzentrat‐Lagerstätte, which has allowed for diverse fossil assemblages to be preserved during intervals of poor preservation.
Stromatolite
Permian–Triassic extinction event
Microbial mat
Paleoecology
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Conodont
Early Triassic
Permian–Triassic extinction event
Ooid
Carbonate platform
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Microbialites (stromatolites and thrombolites) are mineralized mat structures formed via the complex interactions of diverse microbial-mat communities. At Highborne Cay, in the Bahamas, the carbonate component of these features is mostly comprised of ooids. These are small, spherical to ellipsoidal grains characterized by concentric layers of calcium carbonate and organic matter and these sand-sized particles are incorporated with the aid of extra-cellular polymeric substances (EPS), into the matrix of laminated stromatolites and clotted thrombolite mats. Here, we present a comparison of the bacterial diversity within oolitic sand samples and bacterial diversity previously reported in thrombolitic and stromatolitic mats of Highborne Cay based on analysis of clone libraries of small subunit ribosomal RNA gene fragments and lipid biomarkers. The 16S-rRNA data indicate that the overall bacterial diversity within ooids is comparable to that found within thrombolites and stromatolites of Highborne Cay, and this significant overlap in taxonomic groups suggests that ooid sands may be a source for much of the bacterial diversity found in the local microbialites. Cyanobacteria were the most diverse taxonomic group detected, followed by Alphaproteobacteria, Gammaproteobacteria, Planctomyces, Deltaproteobacteria, and several other groups also found in mat structures. The distributions of intact polar lipids, the fatty acids derived from them, and bacteriohopanepolyols provide broad general support for the bacterial diversity identified through analysis of nucleic acid clone libraries.
Microbial mat
Stromatolite
Alphaproteobacteria
Deltaproteobacteria
Ooid
Prokaryote
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