Triassic Mass Extinction Research Articles

Ca and Mg isotope constraints on the origin of Earth’s deepest [formula omitted]C excursion

Understanding the extreme carbon isotope excursions found in carbonate rocks of the Ediacaran Period (635–541Ma), where δ13C of marine carbonates (δ13Ccarb) reach their minimum (-12‰) for Earth history, is one of the most vexing problems in Precambrian geology. Known colloquially as the ‘Shuram’ excursion, the event has been interpreted by many as a product of a profoundly different Ediacaran carbon cycle. More recently, diagenetic processes have been invoked, with the very negative δ13C values of Ediacaran carbonates explained via meteoric alteration, late-stage burial diagenesis or growth of authigenic carbonates in the sediment column, thus challenging models which rely upon a dramatically changing redox state of the Ediacaran oceans. Here we present 257 δ44/40Ca and 131 δ26Mg measurements, along with [Mg], [Mn] and [Sr] data, from carbonates of the Ediacaran-aged Wonoka Formation (Fm.) of South Australia to bring new isotope systems to bear on understanding the ‘Shuram’ excursion. Data from four measured sections spanning the basin reveal stratigraphically coherent trends, with variability of ∼1.5‰ in δ26Mg and ∼1.2‰ in δ44/40Ca. This Ca isotope variability dwarfs the 0.2–0.3‰ change seen coeval with the Permian–Triassic mass extinction, the largest recorded in the rock record, and is on par with putative changes in the δ44/40Ca value of seawater seen over the Phanerozoic Eon. Changes in both isotopic systems are too large to explain with changes in the isotopic composition of Ca and Mg in global seawater given modern budgets and residence times, and thus must be products of alternative processes. Relationships between δ44/40Ca and [Sr] and δ26Mg and [Mg] are consistent with mineralogical control (e.g., aragonite vs. calcite, limestone vs. dolostone) on calcium and magnesium isotope variability. The most pristine samples in the Wonoka dataset, preserving Sr concentrations (in the 1000s of ppm range) and δ44/40Ca values inherited from an originally aragonitic polymorph, have δ13Ccarb of -8‰ to -7‰, thereby providing strong geochemical evidence that extremely negative δ13Ccarb values are primary products of the Ediacaran surface environment.

Amelioration of marine environments at the Smithian–Spathian boundary, Early Triassic

Abstract. The protracted recovery of marine ecosystems following the Permian–Triassic mass extinction may have been caused, in part, by episodic environmental and climatic crises during the Early Triassic, among which the Smithian–Spathian boundary (SSB) event is conspicuous. Here, we investigate the SSB event in the Shitouzhai section, Guizhou Province, South China, using a combination of carbonate carbon (δ13Ccarb) and carbonate-associated sulfate sulfur isotopes (δ34SCAS), rare earth elements, and elemental paleoredox and paleoproductivity proxies. The SSB at Shitouzhai is characterized by a +4‰ shift in δ13Ccarb and a −10 to −15‰ shift in δ34SCAS, recording negative covariation that diverges from the positive δ13Ccarb−δ34SCAS covariation that characterizes most of the Early Triassic. This pattern is inferred to reflect an increase in organic carbon burial (e.g., due to elevated marine productivity) concurrently with the oxidation of isotopically light H2S, as the result of enhanced vertical advection of nutrient- and sulfide-rich deep waters to the ocean-surface layer. Enhanced upwelling was likely a response to climatic cooling and the reinvigoration of global-ocean overturning circulation at the SSB. Coeval decreases in chemical weathering intensity and detrital sediment flux at Shitouzhai are also consistent with climatic cooling. A decline in marine biodiversity was probably associated with the late Smithian thermal maximum (LSTM) rather than with the SSB per se. The SSB thus marked the termination of the extreme hothouse conditions of the Griesbachian–Smithian substages of the Early Triassic and is significant as a record of accompanying climatic, environmental, and biotic changes. The ultimate cause of the SSB event is uncertain but may have been related to a reduction in intrusive magmatic activity in the Siberian Traps large igneous province.

Microbial deposits in the aftermath of the end‐Permian mass extinction: A diverging case from the Mineral Mountains (Utah, USA)

AbstractThe 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.

High influx of carbon in walls of agglutinated foraminifers during the Permian–Triassic transition in global oceans

The Permian–Triassic mass extinction is postulated to be related to the rapid volcanism that produced the Siberian flood basalt (Traps). Unrelated volcanic eruptions producing several episodes of ash falls synchronous with the Siberian Traps are found in South China and Australia. Such regional eruptions could have caused wildfires, burning of coal deposits, and the dispersion of coal fly ash. These eruptions introduced a major influx of carbon into the atmosphere and oceans that can be recognized in the wall structure of foraminiferal tests present in survival populations in the boundary interval strata. Analysis of free specimens of foraminifers recovered from residues of conodont samples taken at a Permian–Triassic boundary section at Lung Cam in northern Vietnam has revealed the presence of a significant amount of elemental carbon, along with oxygen and silica, in their test wall structure, but an absence of calcium carbonate. These foraminifers, identified as Rectocornuspira kalhori, Cornuspira mahajeri, and Earlandia spp. and whose tests previously were considered to be calcareous, are confirmed to be agglutinated, and are now referred to as Ammodiscus kalhori and Hyperammina deformis. Measurement of the 207Pb/204Pb ratios in pyrite clusters attached to the foraminiferal tests confirmed that these tests inherited the Pb in their outer layer from carbon-contaminated seawater. We conclude that the source of the carbon could have been either global coal fly ash or forest fire-dispersed carbon, or a combination of both, that was dispersed into the Palaeo-Tethys Ocean immediately after the end-Permian extinction event.

Conodonts versus Triassic Climatic and Eustatic Changes

From astronomical, sedimentological, paleo-geographical and geochemical points of view, there is a general agreement that the Triassic period was exceptionally warm. The 51 MA long Triassic period started during the Siberian Traps and ended at the height of the Central Atlantic Magmatic Province activity. It is comprised between End Permian and End Triassic mass extinctions. Conodonts survived the former but got extinct by the end of the latter. Three major waves characterise the evolution of Triassic conodonts:I.The 5 MA long Scythian recovery by intensive speciation (23 S/MA);II.The 10 MA long Dinarian of intensive radiation (8.5 S/MA);III.The 16 MA long Julian - Lacian survival (2S/MA) and 20 MA of Alaunian – Rhaetian decline until extinction.The major causes of stress affecting the evolution of Triassic conodonts are environmental, such as marine regressions, anoxic episodes, and thermal or trophic (nutritional) stress. These stressful episodes caused dwarfing and extinction of larger forms (r-selection). Smaller forms with a short life span and rapid proliferation are the result of heterochrony such as proteromorphic retrogradation (Atavism).The disappearance of the conodonts was not caused by a single mass extinction event, but the result of summing up stress factors from hostile environments over a long time.

Systematics of putative euparkeriids (Diapsida: Archosauriformes) from the Triassic of China.

The South African species Euparkeria capensis is of great importance for understanding the early radiation of archosauromorphs (including archosaurs) following the Permo–Triassic mass extinction, as most phylogenetic analyses place it as the sister taxon to crown group Archosauria within the clade Archosauriformes. Although a number of species from Lower–Middle Triassic deposits worldwide have been referred to the putative clade Euparkeriidae, the monophyly of Euparkeriidae is controversial and has yet to be demonstrated by quantitative phylogenetic analysis. Three Chinese taxa have been recently suggested to be euparkeriids: Halazhaisuchus qiaoensis, ‘Turfanosuchus shageduensis’, and Wangisuchus tzeyii, all three of which were collected from the Middle Triassic Ermaying Formation of northern China. Here, we reassess the taxonomy and systematics of these taxa. We regard Wangisuchus tzeyii as a nomen dubium, because the holotype is undiagnostic and there is no convincing evidence that the previously referred additional specimens represent the same taxon as the holotype. We also regard ‘Turfanosuchus shageduensis’ as a nomen dubium as we are unable to identify any diagnostic features. We refer the holotype to Archosauriformes, and more tentatively to Euparkeriidae. Halazhaisuchus qiaoensis and the holotype of ‘Turfanosuchus shageduensis’ are resolved as sister taxa in a phylogenetic analysis, and are in turn the sister taxon to Euparkeria capensis, forming a monophyletic Euparkeriidae that is the sister to Archosauria+Phytosauria. This is the first quantitative phylogenetic analysis to recover a non-monospecific, monophyletic Euparkeriidae, but euparkeriid monophyly is only weakly supported and will require additional examination. Given their similar sizes, stratigraphic positions and phylogenetic placement, the holotype of ‘Turfanosuchus shageduensis’ may represent a second individual of Halazhaisuchus qiaoensis, but no apomorphies or unique character combination can be identified to unambiguously unite the two. Our results have important implications for understanding the species richness and palaeobiogeographical distribution of early archosauriforms.

Skeletal material from larger Eusauropterygia (Reptilia: Eosauropterygia) with nothosaurian and cymatosaurian affinities from the Lower Muschelkalk of Winterswijk, The Netherlands

Eusauropterygian elements from the Lower Muschelkalk (Vossenveld Formation) of Winterswijk, The Netherlands, comprising three mandibles and a selection of postcranial material, are described. The new mandibles correspond to crania larger than eusauropterygian cranial material previously known from this locality. The postcranial material represents a selection of isolated and associated material of comparably large size. Two of the mandibles conform to the morphology commonly recognized in Nothosaurus from the Lower Muschelkalk, but the third mandible accommodates five fangs per ramus in the symphyseal domain and exhibits a symphyseal ratio of 1.04, a condition not yet described for Eusauropterygia from the Vossenveld Formation. Body length for the animals represented by the finds was estimated through dimensional comparison with MB.R.27, a nearly complete skeleton from the lower Middle Muschelkalk locality of Rudersdorf (Germany) assigned to Nothosaurus marchicus. Body length estimates for the individuals represented by the largest mandible and by a selection of associated postcranial material exceed 1,500 mm; approximately 50 % larger than the eusauropterygian taxa from Winterswijk described so far. Furthermore, certain morphological characters identified in one mandible and in the postcranial elements suggest a cymatosaurian affinity over a nothosaurian affinity. The present study indicates eusauropterygian diversity during the Lower Muschelkalk was greater than traditionally believed. This challenges the concept of a slow paced, gradual biotic response to the origination and expansion of the Muschelkalk Sea, which commenced only 5 million years after the Permian–Triassic mass extinction event.

Strontium isotope stratigraphy of the Gabbs Formation (Nevada): implications for global Norian–Rhaetian correlations and faunal turnover

The Luning and Gabbs formations in west-central Nevada, USA represent a Late Triassic shallow marine sedimentary succession with global significance (the Gabbs Formation was a candidate for the basal Jurassic GSSP). Typically, the Norian–Rhaetian stage boundary is placed at the contact between the formations, and the Rhaetian–Hettangian boundary (the Triassic–Jurassic boundary) is within the Muller Canyon Member of the Gabbs Formation. However, the use of different biostratigraphical index-species schemes in Norian–Rhaetian successions between Tethys and Panthalassa, the two largest ocean basins at the time, makes precise correlation problematic. Here, we compare 87Sr/86Sr measurements of well-preserved carbonate shell material from Nevada to the well-known and biostratigraphically constrained 87Sr/86Sr record from Tethys, where a negative excursion in 87Sr/86Sr is noted across the Norian–Rhaetian boundary. Our new 87Sr/86Sr data from the Luning and Gabbs formations reveal a comparable trend, with a sharp drop in 87Sr/86Sr within the Nun Mine Member of the Gabbs Formation, suggesting the position of the Norian–Rhaetian boundary is higher in the succession, and not between the Luning and Gabbs formations as previously defined. Relating the stage boundary using global isotopic signals is a useful tool for biostratigraphical correlation of successions between Tethys and Panthalassa, and for estimating the rate of faunal turnover at the Norian–Rhaetian stage boundary in comparison with the succeeding Late Triassic mass extinction. If correct, this biostratigraphical–chemostratigraphical correlation suggests that the current index groups for the Panthalassic stage boundary should be changed.

Paleomagnetism of trap intrusions, East Siberia: Implications to flood basalt emplacement and the Permo–Triassic crisis of biosphere

Despite decades of concentrated studies, large igneous provinces (LIPs) are still among the most enigmatic phenomena of the Earth evolution. Among the myriad questions facing researchers, the duration of eruptive magmatic activity and its distribution through time are probably the most mysterious ones. Although the accuracy of modern dating techniques offers relatively high resolution, it is impossible to discriminate whether magmatic activity was more or less uniform or concentrated in few powerful outbursts. Consequently, many “trap-related” questions, from their origin to possible impact on the biosphere, still cannot be answered. Some of the above questions can be addressed with paleomagnetic data as have been effectively done in the case of the end-Cretaceous Deccan traps. The Siberian traps are one of the largest LIPs in the Phanerozoic and are thought, by some, to be the triggering event for the end-Permian mass extinction. We conducted a paleomagnetic study of trap intrusions along two long profiles in the eastern part of the province and found that most site-mean directions are grouped several times tighter than is common for the data on thick lava series and dyke swarms; similar over-grouping is found in other parts of the Siberian trap province too. We argue that this phenomenon is the most likely related to nearly simultaneous emplacement of trap intrusions over areas of several ten to few hundred kilometers in dimension for few millenniums or even faster. Moreover, such brief events account for all, or nearly all, magmatism in each area; how such events correlate over the entire Siberian LIP, however, remains an open question. Still, our data together with paleomagnetic results on thick lava series in Siberia strongly indicate that magmatic activity consisted of a number of brief pulses. In comparison to the other LIPs, the Siberian one is underlain by the thickest succession of carbonates, evaporites and coal-bearing clastics that are capable of producing huge amount of green-house and poisonous gases. We hypothesize that these very fast injections of magma into thick underlying sediments and the unique geological setting of the Siberian LIP are the major reasons for the Permo–Triassic mass extinction being the strongest in Earth history.

ORGANISM-ENVIRONMENT INTERACTIONS DURING THE PERMIAN-TRIASSIC MASS EXTINCTION AND ITS AFTERMATH

ORGANISM-ENVIRONMENT INTERACTIONS DURING THE PERMIAN-TRIASSIC MASS EXTINCTION AND ITS AFTERMATH

Restructuring of Terrestrial Environments in Southern Pangea Following the Permian–Triassic Mass Extinction

Restructuring of Terrestrial Environments in Southern Pangea Following the Permian–Triassic Mass Extinction

Origin of volcanic ash beds across the Permian–Triassic boundary, Daxiakou, South China: Petrology and U–Pb age, trace elements and Hf-isotope composition of zircon

Abundant volcanic ash layers, largely altered to clay, characterize the Permian–Triassic boundary beds of the Daxiakou section, South China. Volcanism may have had an important role in the Permian–Triassic mass extinction, so it is important to understand the origin of these ash beds. The lithology of ash bed 260, as constrained by modes of crystal fragments (66.7% plagioclase, 7.2% alkali feldspar, and 26.1% quartz), suggests that the ash represents dacitic volcanism. The U–Pb ages, trace elements and Hf-isotope compositions of zircons from ten ash beds have been analyzed using LA-ICPMS and LA-MC-ICPMS. The zircons can be divided into two groups: magmatic zircons (~97%, 227–279Ma) and inherited zircons (~3%, 678–2424Ma). Magmatic zircons have Y, Hf, Th and U contents and Nb/Ta ratios typical of zircons from silicic calc-alkaline volcanism. εHf(t) values of magmatic zircons vary from −10.9 to +5.3, implying magmatic mixing between juvenile crustal material (probably Neoproterozoic in age) and ancient crustal material (probably Archean). Beds 252–259-b have relatively depleted Hf-isotope compositions with εHf(t) values of −10.9–+5.3 and averages of −5.0–−2.5, while other beds have εHf(t) values of −10.8–+4.4 (mostly −10.8–−3.5) and averages of −8.9–−6.8, implying more input of juvenile crustal material in the volcanism of Beds 252–259-b. Integration of the Hf-isotope and trace-element compositions of magmatic zircons suggests that the volcanism producing the ashes took place along the convergent continent margins during the formation of the Pangea supercontinent. This intense silicic volcanism may have played an important role in causing the Permian–Triassic mass extinction in South China.

Assessing Early Triassic paleoceanographic conditions via unusual sedimentary fabrics and features

Analysis of the Lower Triassic sedimentary rock record has revealed a variety of unusual sedimentary fabrics and features, many of which were more common in the Proterozoic than the Phanerozoic. These so-called “anachronistic facies” developed across Lower Triassic carbonate platforms from shallow subtidal environments to deep, basinal environments and include features associated with the enhanced precipitation of calcium carbonate (e.g., synsedimentary seafloor cements), microbialites, and fabrics that formed due to a reduction in the intensity and depth of bioturbation (e.g., vermicular limestones). Three interacting factors controlled the development of anachronistic facies during the Early Triassic including: 1) environmental conditions (i.e., the unusual chemistry of Early Triassic oceans); 2) biotic factors (e.g., bioturbation and grazing pressures); and, 3) actualistic sedimentologic processes (e.g., waves and currents, sedimentation). Because these three factors typically acted in concert with each other, it is important to consider the role of each when using anachronistic facies to reconstruct Early Triassic paleoceanographic conditions. Overall, anachronistic facies are indicative of enhanced calcium carbonate precipitation during the Early Triassic, but do not necessarily indicate the former presence of anoxic or suboxic conditions where they were deposited. The influence of seawater chemistry on the deposition of anachronistic facies was most pronounced in deeper water environments, where anomalous paleoceanographic conditions heightened calcium carbonate precipitation and caused a decrease in bioturbation and grazing pressures via the upwelling of anoxic, alkaline waters. The degree of environmental influence lessened shorewards, where biotic factors and actualistic processes were more important, and slight modification of seawater by microbial activity, shallow burial, or CO2-degassing was necessary for calcium carbonate precipitation to occur. Anachronistic facies are a useful tool for reconstructing paleoceanographic conditions and examining shifts in the distribution of environmental stress during the recovery from the Permian–Triassic mass extinction, but only when the biologic, sedimentologic, and geochemical factors that led to their growth are carefully considered.

Elevated pCO2 leading to Late Triassic extinction, persistent photic zone euxinia, and rising sea levels

The Late Triassic mass extinction event is the most severe global warming–related crisis to have affected important extant marine groups such as scleractinian corals, and offers potential insights into climate change scenarios. Here we present evidence from Chlorobi-derived biomarkers of episodic and persistent photic zone euxinia. From biomarkers and stable carbon isotopes, we present evidence of rapid mixing of atmospheric and oceanic carbon reservoirs. Global versus regional trends are resolved in kerogen organic matter type, carbonate δ13C, and bulk and pyrite δ34S. This suite of data demonstrates for the first time a comprehensive organic and stable isotope geochemical reconstruction of events leading up to the Late Triassic extinction event and its aftermath. The cascade of events prior to, during, and after the extinction is remarkably similar to those reported for the Late Permian extinction, the largest extinction event of the Phanerozoic. We predict that similar conditions will have occurred during all past episodes of rapid global warming and biotic crisis that are associated with similar rises in p CO2.

Ammonoid recovery after the Permian–Triassic mass extinction: a re-exploration of morphological and phylogenetic diversity patterns

The explosive ammonoid rediversification after the Permian–Triassic mass extinction is now well understood in terms of taxonomic richness and biogeography. Using an updated dataset of Early Triassic ammonoids, we compare morphological disparity and taxonomic richness patterns at the regional and global scales. Disparity evolved similarly at both scales, suggesting a global influence of abiotic factors. Morphological diversification occurred early in the Smithian and a marked contraction of the morphospace took place during the end-Smithian extinction. We confirm that trends in disparity and richness were decoupled during the Griesbachian and Dienerian. Three macroevolutionary processes may be involved: (1) a nonselective extinction at the Permian–Triassic boundary; (2) a Dienerian constrained radiation with several homeomorphic genera; (3) a potential deterministic extinction during the end-Smithian crisis. We also demonstrate a superfamily imprint upon disparity for the Spathian when most superfamilies occupied a restricted part of the morphospace. Sphaerocones were the most affected by the Dienerian and end-Smithian extinction, but explanations remain elusive. On the one hand, this may be linked to widespread harsh conditions at those times. On the other hand, as the sphaerocones occurred episodically during the Early Triassic, this might be explained by a relaxing of ecological constraints or simply by convergent evolution. Supplementary materials: The database, including measurements of specimens illustrated in previously published plates and of unpublished specimens from Utah, South China and Spiti, as well as the number of genera present in each studied substage of the Early Triassic, are available at www.geolsoc.org.uk/SUP18571 .

Sulfur isotope profiles in the pelagic Panthalassic deep sea during the Permian–Triassic transition

Mesozoic accretionary complexes in Japan and New Zealand contain Panthalassic low latitude and southern mid-latitude deep-water sedimentary rock respectively. These sedimentary rocks record environmental changes in the pelagic Panthalassic Ocean during the transition associated with the severe Permian–Triassic mass extinction. This study presents sulfur isotope records of sulfide from continuous deep-sea Permian–Triassic boundary sections located in northeast Japan (the Akkamori section-2, the most continuous section among other previously reported deep-sea sections) and North Island of New Zealand (the Waiheke-1 section, providing the first sulfur isotopic record from a southern hemisphere deep-sea section). Both sections show sharp ~15‰ drops of the sulfur isotope ratio coupled with a negative shift of organic carbon isotope ratio. Similar decreases in sulfur isotope ratio of carbonate-associated sulfates by ~10‰ accompanied with a negative shift of inorganic carbon isotope ratio at the end-Permian mass extinction horizon have been reported in some shallow water Paleotethyan sections. These sulfur isotope changes suggest that a massive release of 32S-enriched sulfur from the H2S-rich water to the oxic surface-waters coincided with the end-Permian mass extinction.

Uppermost Permian reefs from the Wubaiti structure in eastern Sichuan, China: implications for exploration of natural gas reservoirs

Changhsingian (latest Permian) reefs are reported from the Wubaiti structure, northern Sichuan Basin of southwest China. Here, reefs grew along the platform margins and their tempo‐spatial distribution and growth stages were determined based on the integration of detailed seismic profiles, well logs and core data. The Wubaiti reefs experienced two distinct development stages. The first episode of reef development was characterized by the growth of coral and sponge patch reefs during early–middle Changhsingian time. The second episode, the main growth phase of the Wubaiti reefs, occurred in late Changhsingian time when both sponges and algae acted as major reef‐builders. These reef‐building organisms disappeared in the Permian–Triassic mass extinction. Reef growth coincided with a local transgression event, but ceased due to a fall in local sea‐level in the latest Changhsingian time. Dolostones of various parts of the Wubaiti reefs provide considerably high‐quality reservoirs for natural gas in the Sichuan Basin. Copyright © 2012 John Wiley & Sons, Ltd.

Microbial ooids and cortoids from the Lower Triassic (Spathian) Virgin Limestone, Nevada, USA: Evidence for an Early Triassic microbial bloom in shallow depositional environments

Lower Triassic sedimentary rocks contain a variety of unusual facies and fabrics, with microbialites being a distinctive component of many carbonates deposited following the Permian–Triassic mass extinction. Coated grains are common in shallow water facies from the upper Lower Triassic (Spathian) Virgin Limestone (Moenkopi Formation) in southern Nevada, and were investigated in order to determine their origin. Petrographic analysis reveals that the majority of the coated grains found within the Virgin Limestone are micritic ooids with a concentric fabric, or with a homogenous fabric composed of dense, often cloudy micrite. In addition, asymmetric ooids, aggregate grains, and distorted ooids are also locally common in some oolitic units; low-Mg calcite ooids and bimineralic ooids composed of low-Mg calcite and dense, cloudy micrite are less commonly found, but are also documented from the Virgin Limestone. Cortoids (i.e., grains that are coated with constructive micrite envelopes) are a minor component of oolitic grainstones and packstones (typically 10–15% of the grains), although they may also comprise entire beds. The cortoids are coated with micrite similar to that which comprises the ooid cortices, and may be finely laminated or dense and cloudy in nature. The micrite ooids and constructive micrite envelopes are interpreted as microbial in origin based on the finely laminated or cloudy, dense nature of the micrite, as well as coatings that are uneven, or often of greater thickness on one side of elongate nuclei, such as bivalve shells or phylliod algae blades. The origin of the low-Mg calcite ooids and layers is less certain, but may also be microbial. The results of this study suggest that a microbial bloom occurred in shallow water environments, which was the result of 3 factors: (1) the unusual chemistry of Early Triassic oceans; (2) runoff of nutrient-rich waters, which enhanced microbialite growth; and, (3) wave agitation and warm waters that led to CO2 degassing and further supersaturation of shallow waters with respect to calcium carbonate.

Systematics and palaeoecology of Changhsingian (Late Permian) Ambocoeliidae brachiopods from South China and implications for the end-Permian mass extinction

Weihong He, G.R. Shi, Yang Zhang, Tinglu Yang, Fei Teng.& Shunbao Wu, December 2012. Systematics and palaeoecology of Changhsingian (Late Permian) Ambocoeliidae brachiopods from South China and implications for the end-Permian mass extinction. Alcheringa 36, 515–530. ISSN 0311-5518. Four Ambocoeliidae brachiopod species including one new species (Crurithyris tazawai sp. nov., Crurithyris sp., Paracrurithyris pygmaea and Attenuatella mengi) are described from the Changhsingian (Late Permian) deep-water facies of South China. Analysis of the morphology, palaeoecology and palaeogeographical and temporal distributions of these species revealed that the presence of a delthyrium and/or the micro-ornaments among three of the four species (Crurithyris tazawai sp. nov., Paracrurithyris pygmaea and Attenuatella mengi) favoured an epifaunal (epiphytic) lifestyle. Morphological differences suggest that Paracrurithyris pygmaea may have been more effective metabolically in forming the shell compared with Attenuatella mengi and Crurithyris tazawai. The temporal and palaeogeographical distribution of Attenuatella suggests that A. mengi inhabited cool or cold deep waters. Both Crurithyris tazawai and Attenuatella mengi disappeared earlier in the stratigraphic record than Paracrurithyis pygmaea during the Permian–Triassic mass extinction. These differences in timing of extinction, morphology and palaeogeographical distributions suggest that oxygen deficiency and trophic resource limitation (a consequence of the changing composition of marine phytoplankton in the seas) may have contributed to the end-Permian mass extinction.

Geochemical consequences of the Permian–Triassic mass extinction in a non‐marine succession, Sydney Basin, Australia

The identification of the Permian–Triassic boundary (PTB) and Permian–Triassic mass extinction event (PTME) in eastern Australia is based primarily on palynological and plant fossil evidence with limited δ13Corg data from the northern part of the Sydney Basin. Consequently, the PTB in the Sydney Basin has been placed at the top of the last Permian coal measures. Here we report carbon and nitrogen isotopic data, together with major element, trace element and mineralogical data, for a continuous non‐marine Permian–Triassic (P–Tr) section from the southern Sydney Basin. The PTME is identified by a negative shift in δ13Corg of ~3.8‰ occurring approximately 1m above the Bulli Coal seam. Lack of correlation between δ13Corg and TOC indicates that the isotopic signature has not been affected by contamination or diagenetic processes. The interrelationships between C, N and S data indicate severe environmental disruption and increased weathering at the time of the extinction event with die off of terrestrial vegetation and the injection of sulphuric acid into the water column. Positive excursions in Ni, Cr, and Co are coincident with the negative δ13Corg shift and are most consistent with a reduction in oxygen availability due to a major volcanic eruption.