Boundary Microbialites Research Articles

Benthic Pleurocapsales (Cyanobacteria) Blooms Catalyzing Carbonate Precipitation and Dolomitization Following the End‐Permian Mass Extinction

AbstractThe widespread development of Permian/Triassic boundary microbialites is a key biosedimentary response to the mass extinction event. The microbial communities that precipitated these structures remain, however, poorly resolved. Here, we report abundant columnar microfossils from five Permian‐Triassic successions in South China. Four phases of the microfossil life cycle are recorded: baeocytes, juvenile column, mature column, and baeocyte‐splitting, which are interpreted as Pleurocapsales (cyanobacterial clade that reproduces by multiple fission). The great thickness (up to 13 m) of these microbial deposits and Raman spectrum analysis indicate a strong calcification capacity of the Pleurocapsales. Pronounced peaks of Mg/(Mg + Ca) ratios from microbialite and distinct Mg‐enrichment in microfossils indicate that Pleurocapsales blooms may also have facilitated dolomitization of the microbialites. Common occurrence, strong biomineralization capacity, baffling framework, and the close association with carbonate minerals suggest that Pleurocapsales were key constructors of post‐extinction microbialites.

Calcilobes wangshenghaii n. gen., n. sp., microbial constructor of Permian\u2013Triassic boundary microbialites of South China, and its place in microbialite classification

Permian–Triassic boundary microbialites (PTBMs) that formed directly after the end-Permian extinction in the South China Block are dominated by one structure, a lobate-form calcium carbonate construction that created extensive very thin (ca. 2–20 m thick) framework biostromes in shallow marine environments, effectively occupying the ecological position of the prior pre-extinction Permian reefs and/or associated carbonates. In the field, vertical sections show the microbialite is dendrolite (branched) and thrombolite (clotted), but because thrombolite may include branched portions, its structure is overall best classed as thrombolite. In the field and in polished blocks, the microbial material appears as dark carbonate embedded in lighter-coloured micritic sediment, where details cannot be seen at that scale. In thin section, in contrast to the largely unaltered micritic matrix, the microbial constructor is preferentially partly to completely recrystallised, but commonly passes gradationally over distances of a few mm to better-preserved areas comprising 0.1–0.2 mm diameter uneven blobs of fine-grained calcium carbonate (micrite to microsparite). The lobate architecture comprises branches, layers and clusters of blobs ca. 1–20 mm in size, and includes constructed cavities with geopetal sediments, cements and some deposited small shelly fossils. Individual blobs in the matrix may be fortuitous tangential cross sections through margins of accumulated masses, but if separate, may represent building blocks of the masses. The lobate structure is recognised here as a unique microbial taxon and named Calcilobes wangshenghaii n. gen., n. sp. Calcilobes reflects its calcium carbonate composition and lobate form, wangshenghaii for the Chinese geologist (Shenghai Wang) who first detailed this facies in 1994. The structure is interpreted as organically built, and may have begun as separate blobs on the sea floor sediment (that was also composed of micrite but is interpreted as mostly inorganic), by microbial agglutination of micrite. Because of its interpreted original micritic–microsparitic nature, classification as either a calcimicrobe (calcified microbial fossil) or a sedimentary microbial structure is problematic, so C. wangshenghaii has uncertain affinity and nature. Calcilobes superficially resembles Renalcis and Tarthinia, which both form small clusters in shallow marine limestones and have similar problems of classification. Nevertheless, Calcilobes framework architecture contrasts both the open branched geometry of Renalcis, and the small tighter masses of Tarthinia, yet it is more similar to Tarthinia than to Renalcis, and may be a modification of Tarthinia, noting that Tarthinia is known from only the Cambrian. Calcilobes thus joins Renalcis, Tarthinia and also Epiphyton (dendritic form) and others, as problematic microbial structures. Calcilobes has not been recognised elsewhere in the geological record and may be unique to the post-end-Permian extinction facies. C. wangshenghaii occurs almost exclusively in the South China Block, which lay on the eastern margin of Tethys Ocean during Permian–Triassic boundary times; reasons for its absence in western Tethys, except for comparable fabrics in one site in Iran and another in Turkey, are unknown.

Applications of fossil taxonomy in palaeoenvironmental reconstruction: a case study of ostracod identification and diversity in Permian–Triassic boundary microbialites

Applications of fossil taxonomy in palaeoenvironmental reconstruction: a case study of ostracod identification and diversity in Permian–Triassic boundary microbialites

Different accretion and diagenetic patterns within the fabrics of the Permian\u2013Triassic boundary microbialites on the Leye isolated carbonate platform, South China Block

A comprehensive study on the accretion and diagenesis of the Permian–Triassic boundary microbialites is conducive to a better understanding of the ecological community after the end-Permian mass extinction. Here we studied the special microbialite sequences at the Tianba section of Leye isolated carbonate platform, South China Block. The microbialites are shown as small columnar stromatolites, stromatolitic thrombolites, spotted thrombolites, and domical digitate thrombolites in an ascending order. Thin section analyses, coupled with cathodoluminescence photos and oxygen isotopic data, reveal that all types of microbially-mediated laminae/clots are partly recrystallized. Layers of the Polybessurus-like fossils commonly occur in the recrystallized fabrics of stromatolitic laminae. However, the Polybessurus-like fossils are rare in quantity and generally fragmentary and structureless in stromatolitic clots and spotted clots. Such taphonomic features are likely interpreted as the early decomposition by heterotrophic bacteria in an oxygen-depleted microenvironment caused by rapid accumulations of organic matter in the calm water. More enrichments of 13C in the laminae of stromatolite and in the clots of stromatolitic thrombolite and spotted thrombolite than in adjacent interstitial matrixes signify the photosynthesis-dominated isotopic fractionation during the growth of microbial communities. Rare calcimicrobial structures but many calcite crystal fans were found in the 13C-depleted digitate clots. These phenomena indicate that seawater on the carbonate platform was 12C-enriched and supersaturated, accelerating carbonate precipitations and decompositions of organic matter within the microbial community. Different preservations of the Polybessurus-like fossil revealed the complicated microbially-dominated sedimentation and post-depositional diagenesis in the abnormal seawater after the catastrophe.

Polybessurus-like fossils as key contributors to Permian–Triassic boundary microbialites in South China

Massive occurrences of microbialites near the Permian–Triassic boundary have been interpreted as being due to benthic microbial proliferation in unusual marine conditions after the end-Permian mass extinction. Here, we present large populations of Polybessurus-like microfossils in the microbialites of the South China Craton to reveal their morphological structures, growth patterns, and ecological implications. These distinctive microfossils are composed of layered stalks and spherical endpoints. The spheroids, 20–40 μm in diameter, comprising external thin micritic walls and internal sparry parts, are interpreted as coccoid cyanobacteria. The stalks consist of multiple “stacked-cup” micritic envelopes consisting of micron-sized magnesian calcite/dolomite grains and are catalogued as calcified remains of coccoid cyanobacterial extracellular polymeric substances (EPS). Early microbially mediated calcification of EPS increased the preservation potential of microfossils. The microfossils stand upright as coccoid unicells supported by the highly unidirectional secretion of EPS, revealing their benthic behaviours. The successively secreted EPS lift the cells away from the substrate, expanding their ecological niches to gain more sunlight for photosynthesis in the barren marine floor. The distinctive Polybessurus-like microfossils can be viewed both in laminae of stromatolites and in spotted/digitate clots of thrombolites, suggesting their critical roles in microbe-dominated reefs in the aftermath of the end-Permian mass extinction. The morphological features and growth patterns of the Polybessurus-like fossils resemble their Proterozoic counterpart—Polybessurus bipartitus Fairchild ex Green et al. (1987). The widespread Polybessurus-like fossils in the Permian–Triassic boundary microbialites of the South China Craton suggest pervasive retrogressive ecosystems on carbonate platforms after the largest extinction event during the Earth's history.

Stromatolite-dominated microbialites at the Permian–Triassic boundary of the Xikou section on South Qinling Block, China

Permian–Triassic boundary microbialites (PTBMs) are organosedimentary carbonates formed immediately after the end-Permian mass extinction. All those reported PTBMs constrained by convincing conodont biozones are present stratigraphycally not higher than the Hindeodus parvus zone and most of them are dominated by thrombolites. This paper provides the first record of a brief, but spectacular development of stromatolite-dominated PTBMs within the basal Isarcicella isarcica conodont zone of the earliest Triassic from the Xikou section of South Qinling Block that was at the margin of the North China Block during the Permian–Triassic transition and was geographically separated from the major occurrence of post-extinction microbialites in the South China Block. This stromatolite cap overlies a 3.7-m-thick oolitic limestone and is composed of a lower 0.2-m-thick bed and an upper 0.5-m-thick bed, separated by a 0.2-m-thick greyish green siliciclastic mudstone. These two stromatolite beds mainly consist of columnar stromatolites with subordinate domal stromatolites. The intercolumn and interstitial spaces within the stromatolites are filled with oolitic grainstones. At the microscopic scale, laminoid structures in stromatolites comprise wavy, millimetric-domical and tangled laminae. The increased grain and fossil contents and/or bioturbation in the domical and tangled laminae indicate that the formation of these laminae is likely related to an increase in the populations and the disruptions by benthic metazoans, as well as an influx of sediment grains. The δ13Ccarb values fluctuate between 2‰ and 3‰ in the uppermost Permian strata; a distinct negative shift of 1.9‰ occurs at the topmost oolitic grainstone, just below the lower stromatolite bed, and the lowest value of −0.1‰ is located at the base of the upper stromatolite bed. The stratigraphic succession from stromatolites to thrombolites of the PTBMs may represent a transgressive succession and/or a transient ecosystem recovery immediately after the end-Permian mass extinction. The thrombolites-dominated PTBMs mainly developed in near-equator shallow marine geographic locations, and stromatolite-dominated PTBMs mainly developed at higher latitude settings, which probably indicates that a relatively lower diversity and abundance of marine benthic metazoans existed at higher latitudes after the end-Permian mass extinction.

Anachronistic facies and carbon isotopes during the end-Permian biocrisis: Evidence from the mid-Tethys (Kisejin, Iran)

Late Permian mass extinction (LPME) research has focused primarily on Tethyan sections because it is believed that these successions are more complete than those from other localities, and provide a more comprehensive record of the largest, most devastating extinction event in Earth history. The Kisejin section, a previously undocumented mid-Tethyan, Upper Permian-Lower Triassic succession located in the Central-Iran Plate. The Kisejin section contains a continuous Permian-Triassic sequence with only small breaks in sedimentation and was examined in order to determine sedimentologic, biostratigraphic, and carbon isotopic trends across the Permian-Triassic transition. Abnormal (anachronistic) carbonates developed in the study area following the LPME, and include microbialites, edgewise conglomerates, sparitic microspheres, and microbially-coated grains; microbialites occur as thrombolites, stromatolites, and as agglutinated forms. Renalcis-type calcimicrobes are documented for the first time from Permian-Triassic boundary microbialites (PTBMs) of Iran. Coated grains fall into two groups, and include pre-PTBM ooids, which are dense and cloudy, and possibly microbial in origin, and post-PTBM cortoids with destructive and constructive micrite envelopes. Latest Permian conodonts (H. praeparvus, M. ultima), coupled with carbon isotopic values, place the Permian-Triassic boundary within the lowermost thrombolite unit, about 2.1 m above the boundary between the Nessen Formation and the Elika Formation, and indicate that microbialite growth began during the latest Permian. Our study of this previously unknown section shows that, unlike other well-known PTB sections from Iran, the microbialite pattern is complicated, and is more similar to PTBM successions from Turkey. In addition, we note a similarity between the unusual facies of the Kisejin section and those of eastern Tethyan sections from China (i.e. sparry microspheres and Renalcis-type calcimicrobes); these unusual facies have not been previously reported from other well-known Iranian sections, including Julfa and Abadeh, which are thick and have been extensively studied.

Final Results and Recommendations of the Last 10 Years IGCP 572 and 630 Field Workshops in South Turkey, Oman, India (Kashmir) and Armenia

Being involved in the organization of IGCP 572 field workshops in Turkey (2009), in Oman (2010), as in the following IGCP 630 field workshops in Kashmir (India, 2014) and in Armenia (2017), I co-wrote for each of them detailed guidebooks and extended reports that can be found and downloaded from ResearchGate website. This paper summarizes the final results obtained by IGCP 572 and 630 members from these four important field workshop sites in the past 10 years, and also attempts to make recommendations for future studies based on these unique localities surrounding the Tethys Ocean. The first field workshop of IGCP 572 was organized in South Turkey in September, 2009, with the logistical support of Dr. Erdal Kosun from the Antalya University. Following the South Turkey field workshop, eight important papers have been published and concerned the well-preserved Permian-Triassic boundary (PTB) microbialites and their ecosystems (including ostracodes, brachiopods, and many of other clades). The 3rd IGCP 572 annual field workshop was held during February, 2010 in the Sultanate of Oman, with Dr. Michaela Bernecker and the deep support from the GuTech University in Muscat. More than 15 papers have derived from the P-Tr successions that IGCP 572 members have investigated during this workshop, including the first study on oceanic acidification based on Boron isotopes. New paleontological studies are still on progress, and concern the Smithian ammonoids, Lower Triassic shark teeth and dental remains as well as basal Triassic conodonts, crinoids and ammonoids. The 1st IGCP 630 annual field workshop was held during November, 2014 in Srinagar, Kashmir, and North India, with the helpful assistance of Prof. Ghulam Bhat from Jammu University. This meeting renewed studies on the classic Guryul Ravine and adjacent P-Tr boundary sections in that region, with new geochemical works on carbon isotopes and pyrite framboids as well as a detailed Induan conodont taxonomy and zonation. Other new paleontological works concern the palynology of the Guryul Ravine Section as some well-preserved organic eukariotic protist (thecamoebians) that cross without damage the PTB. Lilit Sahakyan, Aymon Baud, and Zhong-Qiang Chen organized the 5th IGCP 630 annual conference and field workshop in Armenia on October 8–14, 2017. Several PTB and Lower Triassic sections have been systematically sampled for biogeochemical and geobiological studies. These Armenian sections provide unique paleoenvironmental settings that have the longest record, from Griesbachian to Dienerian, of microbial proliferation following the P-Tr mass extinction in the world. The basal Triassic giant sponge-microbial build-ups are encouraging more comprehensive studies on metazoan-microbial reef developments after the greatest extinction event.

Environmental instability prior to end-Permian mass extinction reflected in biotic and facies changes on shallow carbonate platforms of the Nanpanjiang Basin (South China)

Shallow carbonate platforms exhibit major changes in faunal composition and facies types during the latest Permian and earliest Triassic. Although the microbialites that developed following the latest Permian mass extinction (LPME) have attracted wide attention, temporal variations in shallow-platform facies and faunas prior to the LPME have been less thoroughly studied. Here, we analyze diversity patterns and variation in skeletal composition in three Upper Permian sections from isolated carbonate platforms of the Nanpanjiang Basin. In addition to the well-known transition from fossil-rich Upper Permian limestones to fossil-poor Permian-Triassic boundary (PTB) microbialites, these sections exhibit several distinct changes that predate the LPME. First, foram faunas show a shift from non-fusulinid-dominated to fusulinid-dominated communities in the <1-m interval below the LPME horizon of each section, reflecting a shallowing trend over ~20–30 kyr preceding the mass extinction. Second, a “foram gap” and concurrent “detrital event” are observed below the LPME in all three sections, recording a rapid influx of detrital siliciclastics that predated the mass extinction by <60 kyr. These features reflect a degree of marine environmental instability prior to the end-Permian mass extinction. The sudden influx of siliciclastics may represent an early perturbation to terrestrial ecosystems linked to incipient Siberian Traps magmatism, resulting in secondary effects in marine environments through increased sediment yields.

Sponge-microbial build-ups from the lowermost Triassic Chanakhchi section in southern Armenia: Microfacies and stable carbon isotopes

The end-Permian mass extinction was the most severe biotic crisis in Earth's history. In its direct aftermath microbial communities colonized some of the space left vacant after the severe decline of skeletal metazoans. The Permian-Triassic boundary microbialites were peculiarly abundant on low-latitude shallow-marine carbonate shelves of central Tethyan continents. Armenia features particularly well preserved and diverse basal Triassic sponge-microbial build-ups (BTSMBs), which were not studied in detail to date. Here, the Chanakhchi section in southern Armenia is described petrographically and by means of stable isotope analyses. The Armenian BTSMBs formed in a distally open marine setting on a pelagic carbonate ramp in the course of two phases of microbial growth during the Induan (Lower Triassic). The BTSMBs are represented by predominantly thrombolitic but also dendrolitic and digitate stromatolite biostromes and mounds that vary in height between 5cm to 12m. The digitate stromatolites are associated with calcium carbonate crystal fans (CCFs). Microfacies analyses revealed that the BTSMBs exhibit a number of different growth forms and internal fabrics. The formation of CCFs was apparently not devoid of biological influence and took place above the sediment surface. The abundance of sponges in the BTSMBs reveals that ecologically complex metazoan-microbial reefs have been present already early after the end-Permian mass extinction. However, the formation of biostromes and mounds did not depend on sponges or other metazoans. BTSMBs that formed during the second microbial growth phase revealed similar δ13C-values like the surrounding sediment. In contrast, the δ13Cmicrobialite and δ13Csediment values from the BTSMBs and CCFs of the first growth phase show a difference of up to +2.3‰, suggesting a significant influence of photoautotrophy during microbially induced carbonate precipitation.

Permian–Triassic boundary microbialites at Zuodeng Section, Guangxi Province, South China: Geobiology and palaeoceanographic implications

A previously unknown microbialite bed in the Permian–Triassic (P–Tr) boundary beds of Zuodeng section, Tiandong County, Guangxi, South China comprises a thin (5cm maximum thickness) stromatolite in the lower part and the remaining 6m is thrombolite. The Zuodeng microbialite has a pronounced irregular contact between the latest Permian bioclastic limestone and microbialite, as in other sites in the region. The stromatolite comprises low-relief columnar and broad domal geometries, containing faint laminations. The thrombolite displays an irregular mixture of sparitic dark coloured altered microbial fabric and light coloured interstitial sediment in polished blocks. Abundant microproblematic calcimicrobe structures identified here as Gakhumella are preserved in dark coloured laminated areas of the stromatolite and sparitic areas in thrombolites (i.e. the calcimicrobial part, not the interstitial sediment) and are orientated perpendicular to stromatolitic laminae. Each Gakhumella individual has densely arranged segments, which form a column- to fan-shaped structure. Single segments are arch-shaped and form a thin chamber between segments. Gakhumella individuals in the stromatolite and thrombolite are slightly different from each other, but are readily distinguished from the Gakhumella- and Renalcis-like fossils reported from other P–Tr boundary microbialites in having a smaller size, unbranching columns and densely arranged, arch-shaped segments. Renalcids usually possess a larger body size and branching, lobate outlines. Filament sheath aggregates are also observed in the stromatolite and they are all orientated in one direction. Both Gakhumella and filament sheath aggregates may be photosynthetic algae, which may have played an important role in constructing the Zuodeng microbialites. Other calcimicrobes in the Zuodeng microbialite are spheroids, of which a total of five morphological types are recognized from both stromatolite and thrombolite: (1) sparry calcite spheroid without outer sheaths, (2) a large sparry calcite nucleus coated with a thin sparry calcite sheath, (3) a large nucleus of micrite framboid aggregates rimmed by a thin sparry calcite sheath (bacterial clump-like spheroids), (4) a large nucleus of micrite framboid aggregates coated with a thin micritic sheath, and (5) a small sparry nuclei rimmed by coarse-grained, radiated euhedral rays. The irregular contact beneath the Zuodeng microbialites is interpreted as a subaerial exposure surface due to regional regression in South China. The demise of the Zuodeng microbialites may have been due to rapid rise in sea-level because they grew in relatively shallow marine conditions and are overlain by muddy limestones containing pelagic conodonts. Also siliciclastic content increases above the microbialite, suggesting a possible climate-related increase in weathering as the transgression progressed.

Palaeogeographic variation in the Permian–Triassic boundary microbialites: A discussion of microbial and ocean processes after the end-Permian mass extinction

Shallow marine carbonate sediments that formed after the end-Permian mass extinction are rich in a thin (maximum ca. 15 m) deposit of microbialites. Microbial communities that constructed the microbialites have geographic variability of composition, broadly divisible into two groups: 1) eastern Tethys sites are calcimicrobe-dominated (appearing as thrombolites in the field), with rare occurrence of sediment-constructed microbialites and uncommon cements either within microbial structure or as inorganic precipitates, 2) other Tethys sites are sediment-dominated structures forming stromatolites and thrombolites, composed of micrites and cements, with some inorganic precipitates. These other Tethys locations include western and central Tethys sites but their palaeogeographic positions depend on the accuracy of continental reconstructions, of which there are several opinions. In contrast to geographic variation of microbialites, the conodont Hindeodus parvus, which appeared after the extinction and defines the base of the Triassic, is widespread, indicating easy lateral migration throughout Tethys. Conodont animals were active nekton, although being small animals were presumably at least partly carried by water currents, implying active Tethyan surface water circulation after the extinction event. Post-extinction ammonoid taxa, presumed active swimmers, show poor evidence of a wide distribution in the Griesbachian beds immediately after the extinction, but are more cosmopolitan higher up, in the Dienerian strata in Tethys. Other shelly fossils also have poorly defined distributions after the extinction, but ostracods show some wider distribution suggesting migration was possible after the extinction. Therefore there is a contrast between the geographic differences of microbialites and some shelly fossils.Determining the cause of geographic variation of post-extinction microbialites is problematic and may include one or more of the following possibilities: 1) because calcifying microbial organisms that create calcimicrobes were benthic, they may have lacked planktonic stages that would have allowed migration, 2) eastern Tethyan seas were possibly more saturated with respect to calcium carbonates and microbes, so microbes there were possibly more able to calcify, 3) significant reduction of Tethyan ocean circulation, perhaps by large-scale upwelling disrupting ocean surface circulation, may have limited lateral migration of benthic microbial communities but did not prevent migration of other organisms, and 4) microbes may have been subject to local environmental controls, the mechanisms of which have not yet been recognized in the facies. The difficulty of distinguishing between these possibilities (and maybe others not identified) demonstrates that there is a lot still to learn about the post-extinction microbialites and their controls.

Permian\u2013Triassic boundary microbialites (PTBMs) in southwest China: implications for paleoenvironment reconstruction

Permian–Triassic boundary microbialites (PTBMs) are commonly interpreted to be a sedimentary response to upwelling of anoxic alkaline seawater and indicate a harsh marine environment in the Permian–Triassic transition. However, recent studies propose that PTBMs may instead be developed in an oxic environment, therefore necessitating the need to reassess the paleoenvironment of formation of PTBMs. This paper is an integrated study of the PTBM sequence at Yudongzi, northwest Sichuan Basin, which is one of the thickest units of PTBMs in south China. Analysis of conodont biostratigraphy, mega- to microscopic microbialite structures, stratigraphic variations in abundance and size of metazoan fossils, and total organic carbon (TOC) and total sulfur (TS) contents within the PTBM reveals the following results: (1) the microbialites occur mainly in the Hindeodus parvus Zone but may cross the Permian–Triassic boundary, and are comprised of, from bottom to top: lamellar thrombolites, dendritic thrombolites and lamellar-reticular thrombolites; (2) most metazoan fossils of the microbialite succession increase in abundance upsection, so does the sizes of bivalve and brachiopod fossils; (3) TOC and TS values of microbialites account respectively for 0.07 and 0.31 wt% on average, both of which are very low. The combination of increase in abundance and size of metazoan fossils upsection, together with the low TOC and TS contents, is evidence that the Yudongzi PTBMs developed in oxic seawater. We thus dispute the previous view, at least for the Chinese sequences, of low-oxygen seawater for microbialite growth, and question whether it is now appropriate to associate PTBMs with anoxic, harsh environments associated with the end-Permian extinction. Instead, we interpret those conditions as fully oxygenated.

Role and lifestyle of calcified cyanobacteria (Stanieria) in Permian–Triassic boundary microbialites

Calcified coccoid fossils in the Permian–Triassic boundary microbialites were widely considered to represent the remains of the organisms that formed the microbialites. However, this role has been recently attributed to the fossil casts of Microcystis cyanobacterial colonies, leaving the ecological role of the coccoid fossils, as well as their affinities, uncertain. Here we propose that these coccoid fossils bear morphological similarity and are related to the extant epiphytic cyanobacterium Stanieria, and that they lived in surface waters by attaching themselves to bloom-forming phytoplankton, in a hitch-hiking lifestyle. Calcification of the envelopes of these interpreted Stanieria cells produced the small thin-walled spherical calcareous coccoid fossils. Different from ordinary pseudoplanktons, Stanieria cells cannot attach themselves to sparse planktons, but only to densely packed phytoplanktons in bloom. The dependence of these calcified Stanieria fossils on this lifestyle makes them possible indicators of ancient planktonic blooms.

Modern Black Sea oceanography applied to the end-Permian extinction event

The modern Black Sea has a mixed upper layer in the top 150–200 m of the water column, below which the water is anoxic, separated from the mixed layer by a redox boundary. There is limited vertical movement of water. Pyrite framboids form in the water column of the anoxic zone, then have been traditionally interpreted to sink immediately and accumulate in the sediments of the Black Sea. Thus the occurrence of framboids in sediments in the rock record is widely interpreted to indicate poorly oxygenated to anoxic conditions in ancient environments. However, in the Permian–Triassic boundary (PTB) microbialites of South China, which formed in shallow marine conditions in contact with the atmosphere, the published occurrence of framboids is inconsistent with abundant gastropod and ostracod shells in the microbialite. Furthermore, in the modern Black Sea, (a) framboids may be suspended, attached to organic matter in the water column, thus not settle to the sea floor immediately after formation; and (b) the redox zone is an unstable complex area subject to rapid vertical water movement including occasional upwelling. The model presented here supposes that upwelling through the redox zone can lead to upward transport of suspended pyrite framboids into the mixed layer. Advective circulation could then draw suspended framboids onto the shelf to be deposited in oxygenated sediments. In the Permian–Triassic transition, if framboids were up-welled from below the redox boundary and mixed with oxygenated waters, sediment deposited in these conditions could provide a mixed signal for potentially misleading interpretations of low oxygen conditions. However, stratigraphic sampling resolution of post-extinction microbialites is currently insufficient to demonstrate possible separation of framboid-bearing layers from those where framboids are absent.Profound differences between microbialite constructors and sequences between the western and eastern Tethys demonstrate barriers to migration of microbial organisms. However, framboid occurrences in both areas indicate upwelling and emphasize vertical movement of water from the lower to upper ocean, yet the mixed layer advective motion may not have been as effective as in modern oceans. In the modern Black Sea, such advection is highly effective in water mixing, and provides an interesting contrast with the PTB times.

Early Middle Triassic stromatolites from the Luoping area, Yunnan Province, Southwest China: Geobiologic features and environmental implications

An early Middle Triassic stromatolite deposit is documented from the Guanling Formation of the Luoping area, Yunnan, SW China. The Luoping stromatolite shows five types of constructional microbial forms in various magnifications: 1) typical stratified columnar structures, up to 20cm high, with crinkled laminae. Dark coloured laminae, 1mm thick, are composed of upright filamentous tubes, averagely 29.4μm in diameter, showing a vertical growth fabric. 2) Laminoid fenestrae, 0.5–1mm wide, and 3) prostrate filaments, which are reflected by strong fluorescence in sharp contrast to dolomite cement in fluorescent images. 4) Rod-like aggregates, 4.6–18μm in diameter, composed of minute dolomite rhombs, are very common in stromatolitic laminae; they resemble present-day cyanobacterial trichome, and thus may represent putative fossilized cyanobacteria. 5) Moreover, small pits, coccoid-like spheroids, calcified biofilms, and fibrous structures are also common in stromatolitic laminae. The last two may represent calcified extracellular polymeric substances (EPS) that contributed to the development and lithification of stromatolites. Authigenic quartz grains are common and may also have involved biological processes in stromatolite formation. Of these microbial functional-groups driving accretion and lithification processes of stromatolite documented in literature, both lithified cyanobacteria/oxygenic phototrophs and possible sulfate-reducing bacteria (SRB), which induced microbial formation of dolomite and contributed to the accretion of the Luoping stromatolite, are suggestive of biogenic origin. The Luoping stromatolite differs from the Permian–Triassic boundary microbialites (PTBMs) in having abundant filamentous structures and growing in an oxic marine environment. Both sedimentary facies analysis and abundant fossilized cyanobacteria may indicate proliferation of oxygenic phototrophs in a normal, oxic habitat during the middle Anisian (early Middle Triassic), a period when hospitable environments, coupled with biotic diversification, prevailed in South China and set an agenda for the full recovery of marine ecosystems in middle–late Anisian. However, the post-extinction stromatolites and other anachronistic facies are not necessarily indicative of anoxic or oxic conditions, and their environmental settings are much more complex than previously thought.

Shallow water redox conditions from the Permian–Triassic boundary microbialite: The rare earth element and iodine geochemistry of carbonates from Turkey and South China

Redox sensitive elements serve as useful proxies of the oxygenation state of ancient environments, but their interpretation may not always be straightforward. To evaluate the inherent complexities, rare earths and yttrium (REY), iodine, and major element concentrations are determined in two carbonate sections spanning the Permian–Triassic (P–Tr) transition in Demirtas, Turkey and Cili, South China. We use major oxides to identify non-seawater REY sources such as siliciclastics, Fe-oxides, phosphates and diagenetic fluids. Additionally, we employ Y/Ho ratio, La anomaly, and light rare earth element depletion to identify which samples preserve a seawater-like REYSN distribution. In contrast to past interpretations, we find that the P–Tr boundary microbialites in both sections contain REY signatures indicative of deposition in an oxic environment. These boundary microbialites have their base at the extinction horizon and are widespread within the Tethyan region. In the Cili section, the underlying Permian limestone also preserves a seawater-like REY signature with a negative Ce anomaly. This indicates that the water column was oxygenated both before and after the extinction event. The Permian limestone in the Demirtas section does not preserve a seawater-like REYSN distribution, so the absence of a Ce anomaly cannot be used to distinguish prevailing redox conditions during deposition. However, in these samples, we find the presence of a diverse Permian benthic community sufficient to identify deposition in an oxic environment. The geochemical evidence for a continuously oxic environment during the deposition of the boundary microbialite presented in this study strongly supports work done using ostracods as redox indicators within the boundary microbialite in South China. The microbialite has been proposed as a disaster facies, in part resulting from the exclusion of grazers. Although anoxia is one of the suggested mechanisms for the exclusion of grazers, we find that it is not supported by geochemical and biotic evidence.

Microbialites and global environmental change across the Permian–Triassic boundary: a synthesis

Permian-Triassic boundary microbialites (PTBMs) are thin (0.05-15 m) carbonates formed after the end-Permian mass extinction. They comprise Renalcis-group calcimicrobes, microbially mediated micrite, presumed inorganic micrite, calcite cement (some may be microbially influenced) and shelly faunas. PTBMs are abundant in low-latitude shallow-marine carbonate shelves in central Tethyan continents but are rare in higher latitudes, likely inhibited by clastic supply on Pangaea margins. PTBMs occupied broadly similar environments to Late Permian reefs in Tethys, but extended into deeper waters. Late Permian reefs are also rich in microbes (and cements), so post-extinction seawater carbonate saturation was likely similar to the Late Permian. However, PTBMs lack widespread abundant inorganic carbonate cement fans, so a previous interpretation that anoxic bicarbonate-rich water upwelled to rapidly increase carbonate saturation of shallow seawater, post-extinction, is problematic. Preliminary pyrite framboid evidence shows anoxia in PTBM facies, but interbedded shelly faunas indicate oxygenated water, perhaps there was short-term pulsing of normally saturated anoxic water from the oxygen-minimum zone to surface waters. In Tethys, PTBMs show geographic variations: (i) in south China, PTBMs are mostly thrombolites in open shelf settings, largely recrystallised, with remnant structure of Renalcis-group calcimicrobes; (ii) in south Turkey, in shallow waters, stromatolites and thrombolites, lacking calcimicrobes, are interbedded, likely depth-controlled; and (iii) in the Middle East, especially Iran, stromatolites and thrombolites (calcimicrobes uncommon) occur in different sites on open shelves, where controls are unclear. Thus, PTBMs were under more complex control than previously portrayed, with local facies control playing a significant role in their structure and composition.