Introduction

Stromatolites, which possess laminated structures (Riding 2000), are one of the most intriguing benthic microbial deposits. They provide a unique window to investigate the history of photosynthesis, evolution of the early atmosphere and microbe–environment interactions through geological time (Awramik 1992, 2006; Kah and Riding 2007; Kershaw et al. 2007, 2009, 2012; Noffke and Awramik 2013). Early Triassic stromatolites have been reported from around the world (Schubert and Bottjer 1992; Sano and Nakashima 1997; Richoz et al. 2005; Hips and Haas 2006; Pruss et al. 2006; Farabegoli et al. 2007; Kershaw et al. 2011; Chen et al. 2012, 2014; Mata and Bottjer 2012) and are major components of post-extinction microbialites. Resurgence of microbial communities occurred throughout the entire Early Triassic recovery interval, during that time there were at least four major events of high microbialite abundance (Baud et al. 2007). Although three of these stromatolite events have been documented in detail. But there are few examples of Early Triassic stromatolites from the second event. In this study, a stromatolitic deposit from the second event is described. The deposit provides important evidence on the extent of devastated oceanographic conditions that prevailed in the global ocean during the Early Triassic (Bottjer et al. 2008; Chen et al. 2010; Algeo et al. 2011; Chen and Benton 2012).

Microbialites deposits (stromatolites, thrombolites and other forms) formation is often interpreted as the outcome of combined reduction in metazoan diversity and rising seawater saturation state in respect to CaCO3 (Riding and Liang 2005; Kershaw et al. 2007, 2012; Woods et al. 2007). However, recent studies show that the biogenic mechanisms were involved in microbialite formation. Microbes play an important role in accretion, lamination and early lithification of modern marine stromatolites forming in open marine environments of normal seawater salinity as on the margins of Exuma Sound, Bahamas (Reid et al. 2000). During periods of rapid sedimentation, filamentous cyanobacteria dominate over the stromatolite surfaces, whereas the climax communities of endolithic coccoidal bacteria develop during prolonged hiatal periods (Reid et al. 2000). This viewpoint opens a brand new window insight into the possible forming mechanism of microbialite deposits (Ezaki et al. 2003, 2008; Hips and Haas 2006; Kershaw et al. 2011, 2012; Yang et al. 2011; Chen et al. 2014; Luo et al. 2014).

The aims of this study are: (1) to describe the microbial structure and composition of the Xingyi stromatolites; (2) to elucidate the accretion and formation processes of the stromatolites; and (3) to discuss the possible palaeoenvironmental implications.

Geological background

The stromatolites described in this study were collected from the Bayou Section of the Xingyi area, about 12 km southwest of Xingyi city, southwestern Guizhou Province, southwest China (Fig. 1). The Xingyi area is situated in the northwestern part of the Nanpanjiang Basin, which is located on the eastern part of the Khandian platform. The Xingyi area is separated from the Nanpanjiang Basin by a shoal complex (Feng et al. 1994). In the Bayou Section, upper Permian mudstone and skeletal limestones are unconformably overlain by the Feixianguan Formation, which are in turn overlain by the Jialingjiang Formation.

Fig. 1
figure 1

Early Triassic palaeogeographic map of south China showing the palaeogeographic setting of the Xingyi area (red pentagon) during that time [base map was modified from Feng et al. (1994)]

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The Feixianguan Formation is subdivided into two Members. Member I is dominated by pewter-coloured siliciclastic sediments, representing deposition in a subtidal to intertidal environment. Laminated clastic deposits were widely developed during the earliest part of the Early Triassic, immediately after the Permian–Triassic boundary mass extinction in the upper Yangtze area, representing sluggish oceanic circulation.

From field observations and logged sections, Member II of the Feixianguan Formation consists of two units (Fig. 2). Unit 1 (23.4–103 m) contains thick bedded siltstone and fine sandstone. The siltstone is partly fossiliferous— mainly bivalve fossils. The original physical sedimentary structures of the rock are tidal bedding and cross-bedding. The fine grained sediments represent intertidal to subtidal facies. Unit 1 in the lower part of Member II is similar to Member I, and consists mainly of siltstone and sandstone deposited in an intertidal to subtidal settings. Unit 2 (103–369.2 m) is composed of thin-bedded muddy limestone and alternations of oolitic limestone and mudstone (Fig. 2A), indicating deposition by episodic storms. The oolitic deposits (Fig. 2B) consist of dark to light grey, medium to thin bedded, granular and micritic limestone. The muddy limestone has a stromatolitic texture and contains the stromatolitic deposit (Fig. 2B) that is the focus of this study. Large amounts of bioclastic debris are present in the stromatolites, mainly bivalves and foraminifera. The stromatolites are covered by calcirudite (Fig. 2C). The calcirudite is rounded with good resistance, well sorted and cross-bedded, and represent storm-flow deposits. Thus, both lithofacies and palaeoecology suggest that Unit 2 overall represents an open, shallow platform to proximal ramp environment (Shi et al. 2010). Integration of the information on sedimentary facies and palaeoecology indicates that the Feixianguan Formation in the Xingyi area records a deepening-upward succession.

Fig. 2
figure 2

Lithology and palaeoenvironmental analysis of Member II of the Feixianguan formation at the logged section. Note the described stromatolite occurs at the middle part of unit 2(black arrow B)

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Conodont biostratigraphy was used to determine the age of the stromatolites. Unfortunately, conodonts were not recovered from near the stromatolites. Near the stromatolite-bearing horizons, however, the bivalves Claraia sp., Unionites sp., Claraia longyanensis Chen and Claraia cf. aurita (Hauer) were found at the top of Member I of the Feixianguan formation. These taxa indicate an early Dienerian age (Wu 1985; Yang et al. 1986; Tong et al. 2005). The conodonts Lonchodina nevadensis Mülleri, Neohindeodella triassica Mülleri, and Lonchodina cf. mülleri Tatge were recovered from the top of Member II of the Feixianguan Formation and the bottom of Member I of the Jialingjiang Formation. Moreover, the conodonts Pachycladina and Parachirognathus were found at the top of Member I of the Jialingjiang Formation. These taxa indicate that the top of Member II of the Feixianguan Formation is of early Olenekian age (Tong et al. 2005; Ji et al. 2011; Yan et al. 2013). The stromatolite-bearing horizon is present in the upper part of Member II (Fig. 2). The bivalves Bakevellia sp. was detected 55 m stratigraphically above the stromatolites. To summarize, the stromatolites may be of middle or late Dienerian age.

Methods

Polished slabs and petrographic thin sections were prepared to examine the internal fabrics and diagenetic features of the stromatolites. Fresh samples and polished chips were made separately for microanalysis under the scanning electron microscope (SEM). Freshly broken samples were cleaned with deionized water, immersed in 0.5 % hydrochloric acid for 3–5 h, and rinsed again with deionized water and ethyl alcohol. Samples for SEM analysis were polished with 200-mesh diamond dust prior to chemical etching and cleaning. Samples were gold-coated using a Hitachi-SU4800 FEGSEM. Elemental mapping of energy-dispersive X-ray spectrometry (EDS) analysis was performed with a Shimadz EPMA-1600 at the Southwest China Supervision and Inspection Center of Mineral Resources, Ministry of Land and Resources at the Chengdu Center of the China Geological Survey. Fluorescent imaging analysis was performed to assess the distribution of residual organic matter using a Zeiss Microscope at the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation at the Chengdu University of Technology. Recent studies have demonstrated that different colours of fluorescent light represent different levels of organic matter. A rock containing abundant organic matter will fluoresce strongly in both green and purple excitation light (Luo et al. 2014). In contrast, if a rock has little or no organic matter, the rock will weakly or not fluoresce (Cuif et al. 1990; Reitner and Neuweiler 1995; Russo et al. 1997, 2000; Mastandrea et al. 2006). In this study, the terminology and methods for documenting stromatolite microfabrics follow Shapiro (2000), who described microbial fabrics at three different scales.

Results

Mega-, macro- and mesostructures of Xingyi stromatolites

The substrate of the stromatolites is an oolitic limestone with a granular texture. The limestone is 20–30 cm thick and contains bioclasts. The matrix is dominated by superficial ooliths about 1 mm in size. The ooliths become larger gradually upward. Gastropod shell fragments 1–2 cm in size are mainly recrystallized as calcite.

The capping facies of the stromatolites is calcirudite (Fig. 2C) that has a thickness of more than 50 cm. Calcirudite with irregular appearance 2–3 mm in size, and are poorly sorted. Both the oolitic limestone and the calcirudite contain abundant, irregularly shaped grains with few fossil fragments. In contrast, the stromatolites are dominated by light-coloured and dark laminae with few grains.

Stromatolites with columnar structures formed by alternations of light-coloured and dark laminae are distinct from the surrounding rocks (Fig. 2B). The columnar structures are 8–25 cm high and 3–6 cm long. In cross-section, the stromatolites are discrete hemispheroids in shape (Fig. 3A, B). These hemispheroids are stacked vertically to form the columnar structures. In polished slabs, the concentric laminae are 1–2 mm thick (Fig. 3B).

Fig. 3
figure 3

Polished surfaces and field photos show stromatolite of Member II of the Xingyi formation. A Field photos showing note columns preserved as rounded reliefs (white curve). B Polished surfaces showing spheroidal structures

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Stromatolite microstructures

Microscopic observation showed that the stromatolites have a columnar structure, characterized by undulating laminations with few grains (Fig. 4A, B). The grains, which do not have oolitic envelopes and are small in size, are different from the grains in inter-stromatolites (Fig. 10B, D).

Fig. 4
figure 4

Photomicrographs show the Xingyi stromatolite. A–C Showing the well-preserved filamentous tube colony comprising individual upright filaments in life position (white arrows), A-b Showing grain packstone caps the laminated stromatolite. D Showing prostrate filament and fenestrae in dark laminae

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Stromatolitic laminae can be divided into two categories, dark and light, on the basis of colour. The boundaries of these laminae are not complete. Dark grey laminated layers, which have an average thickness of 400 µm, are mainly composed of dark grey micritic calcite and abundant filamentous structures (Fig. 4A, B). There are two types of filamentous structure: upright and prostrate. The upright structures, which are usually perpendicular to the dark laminae and grow straight upward, possess three main features: they have microsparitic cores and micritic walls, appear as unbranched tubes in outline, and do not possess nodes. The diameters of the upright filamentous tubes range from 5 to 15 µm with the mean value of 8 µm, based on the measurements of 43 individuals.

The prostrate filamentous structures are strongly curved and are, in general, distributed nearly parallel to the thin laminae (Fig. 4D). Individual filamentous structures are composed of microsparite in the core, with walls defined by micrite (Fig. 4D). The diameters of the prostrate filamentous structures are similar to those of the upright tubes. In contrast, the light-coloured laminae with poor lateral continuity, which are usually composed of microsparite, are 2–8 mm thick (Fig. 4D). Under the fluorescent microscope, the dark laminae showed strong fluorescence under blue excitation light (Fig. 5A, B). Moreover, the microsparite areas within dark laminae that lack well-preserved filaments (Fig. 5A, B) also showed strong fluorescence. In contrast, the light-coloured laminae have very weak fluorescence (Fig. 5C, D); however, the filamentous tubes fluoresce the same colour as the micritic cement (Fig. 5D). Strong fluorescence indicates the presence of abundant organic matter in these microfabrics (Cuif et al. 1990; Reitner and Neuweiler 1995; Russo et al. 1997, 2000; Mastandrea et al. 2006).

Fig. 5
figure 5

Filament-rich and clotted fabric areas in dark laminated layers of stromatolite in transmitted light and fluorescent light. A The prostrate filamentous tube in the dark laminae. B The same area as that in view A show the dark laminae showed a strong fluorescence under blue exciting light. C–D Another view of clotted fabrics in the bright laminae of stromatolite. Filamentous tube (white arrows). D Coarsely sparry calcite cementation was poorly responded to fluorescent light. Filamentous tube (white arrows) was similar to micrites cementation

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Filamentous microfossils

Under SEM, well-preserved filamentous microfossils are common in the dark laminae. This is a distinct difference from the substrate of the stromatolites, which contains only microcrystalline calcite. The filamentous microfossils are characterized by being flexuous and unsegmented and having unbranched tubular outlines (Fig. 4A–C). Each individual is delimited by micritic walls, appear as short columns in the plane (Fig. 4A) and show a few pores in cross-section (Fig. 4D). The diameter of the filamentous microfossils ranges from 5 to 18 µm and the wall thickness ranges from 1 to 3 µm, based on measurements of 19 individuals. In contrast, the filamentous structures are 20–150 µm long, with the mean value of 35 µm. Irregularly shaped polyhedral (Fig. 6B) and tabular calcite (Fig. 6A, C) are associated with thin tubes (Fig. 6B).

Fig. 6
figure 6

SEM images of filamentous structures detected in dark laminae of the Xingyi stromatolites. A Thin tubular filament was cementated by calcite. B Close-up of boxed area in view A show filamentous microfossils and polyhedron. C Filamentous tube associated with tabular calcite. D Few amount of pores in cross-section

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Smaller bacilli

Under SEM, it can be seen that smaller bacilli are mostly aggregated locally to form irregular rod-like aggregates (Fig. 7A, B) and reticular rounded pits (Fig. 7D). These structures are commonly present in the light-coloured laminae of stromatolites. The diameter of the irregular and meshed rod-like aggregates (Fig. 7A) ranges from 2 to 8 µm, with the mean value of 5 µm based on measurements of nine individuals. Reticular rounded pits (Fig. 7C, D) were also commonly observed in the light-coloured laminated layers of stromatolites. The reticular rounded pits (Fig. 7D) are circular in outline and 0.8–1.5 µm in diameter with an average diameter of 1.2 µm, based on measurement of 12 individuals. Both these pits and irregular mesh consortia are formed of smaller bacilli, but are interwoven in different ways. The smaller bacilli (Fig. 7B, D) are 0.1–0.2 µm in diameter with the mean value of 0.15 µm, based on measurements of 19 individuals. The smaller bacilli are 0.2–1.5 µm in length with the mean value of 0.35 µm, based on measurements of 31 individuals. The smaller bacilli split continuously, as shown by cells that are Y-shaped in outline (Fig. 7B, D).

Fig. 7
figure 7

SEM images of Smaller bacilli detected in dark laminae of the Xingyi stromatolites. A Irregular rod-like aggregates. B Close-up of boxed area in view A showing irregular rod-like aggregates. C Reticular rounded pits. D Close-up of boxed area in view C showing reticular rounded pits and smaller bacillus is characterized by cells with “Y” in outline

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Coccoid-like spheroids and oval algae

Small spheroidal structures are common in the dark-coloured laminae of the stromatolites. Some spheroids are globular in outline, 10–12 µm in diameter, and are formed of clusters of many minute rhombic calcite crystals (Fig. 8A). Under SEM, well-preserved oval algae (Fig. 8B–D) are commonly present in the dark laminated layers of stromatolites. These algae are markedly different from the spheroidal structures. The oval algae are cystic (Fig. 8C, D) in outline and 20–60 µm in size, with the mean value of 35 µm based on measurements of 22 individuals. These algae always have verrucae that are 0.2–1 µm in size (Fig. 8C). Some have a smooth surface with no folds or little folding, and sometimes bear a small protuberance (Fig. 8D).

Fig. 8
figure 8

SEM images of coccoid-like spheroids and oval algae are detected in dark laminae of the Xingyi stromatolites. A Spheroid balls are globular in outline and lustered by many minute rhombic limestone crystals. B The oval algae always have verruca. C Close-up of boxed area in view B verruca. D The oval algae have smooth surface with no fold or little fold, but have a small protuberance

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Extracellular polymeric substance and associated polyhedrons

Another feature typifying the cyanobacteria is the common presence of extracellular polymeric substance (EPS) and associated polyhedrons (Po) (Fig. 9A–D). These EPS form fibrous structures, which cover or around the organic matter (Fig. 9A, C). Po cover or around the EPS (Fig. 9A, D).

Fig. 9
figure 9

SEM images of extracellular polymeric substance (EPS), associated polyhedrons (Po) and filameatous bacteria relies (Fi) A–D Extracellular polymeric substance (EPS), associated polyhedrons (Po) and filamentous bacteria relies (Fi) in dark lamina. B Close-up of boxed area in view A. D Close-up of boxed area in view C

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Framboidal pyrite

Pyrite framboids are also common in the stromatolite microstructures. The framboids coexist with sparry calcite within stromatolitic laminae, have strawberry-shaped, globose outlines (Fig. 10A–D) and are formed of smaller euhedral pyrite crystals. The majority of the smaller euhedral pyrite crystals (more than 70 %) have a spheroplastic appearance (Fig. 10B) and a few (less than 30 %) are tetrahedral in shape (Fig. 10D). The size of the framboids is 4–18 µm with the mean value of 10 µm, based on measurements of 29 specimens. The framboids commonly occur in clusters, and sometimes as loose groups.

Fig. 10
figure 10

SEM images of framboidal pyrite and authigenic quartz detected in dark laminae of the Xingyi stromatolites. A Framboidal pyrite plexus. B Close-up of boxed area in view A showing framboidal pyrite have the appearance of like spheroplast. C Framboidal pyrite plexus. D Close-up of boxed area in view C showing framboidal pyrite have the appearance of tetrahedron. E Authigenic Quartz crystals. F Close-up of boxed area in view E

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Authigenic quartz

Quartz crystals are also obvious in the stromatolite microstructures. The crystals coexist with sparry calcite within the stromatolitic laminae. The crystals have subidiomorphic outlines and pronounced crystal structures on the surfaces (Fig. 10E, F). The crystal size is 2–6 µm, with the mean value of 4 µm based on measurements of 27 crystals. The quartz (Fig. 10E, F) are scattered in the stromatolite laminae and not concentrated to form layers or horizons. They show no signs of abrasion, which are typical features of detrital quartz grains. As these crystals show no indications of transported, they are probably authigenic in origin.

Inter-stromatolite microstructures

The spaces between the stromatolites are dominated by grains (Fig. 11A–D) that fall into two size categories. The diameter of the smaller grains is 50–120 µm with the mean value of 80 µm, based on measurements of 49 individuals. Although fragmented relicts are observed, individual primary grains are difficult to distinguish because the smallest grains have been recrystallized and strongly calcitized (Fig. 11B). Other particles are oncoids composed of an irregular cortex and nuclei (Fig. 11B, C). Grapestone is characterized by multiple nuclei and common envelopes (Fig. 11C). The diameter of the oncoids is 200–600 µm with the mean value of 400 µm, based on measurements of 52 grains. Some oncoids are infilled by pyrite (Fig. 11D), which occur only within the cortex and do not appear outside the cortex. Small gastropods occur sporadically among the grains (Fig. 11D).

Fig. 11
figure 11

Polished surfaces and microscopic photos show the parts of interstromatolite. A Inter-stromatolite facies is dominated by oncoids, the geode of gypsum or salt (White arrow) scattered in the stromatolite. B Close-up of boxed area in view of A-a showing grain consisting an irregular micrite clots. C Close-up of boxed area in view of A-a showing grapestones and lumps. D Close-up of boxed area in view of A-b showing shell fragments of bivalves. oncoids is filled by pyrite (Black arrow)

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Discussion

Biogenic origin of stromatolites

A large numbers of evidences show that various microbial colonies are present in different stromatolite laminae. The metabolic processes of these microbes interact to promote the formation of stromatolites (Dupraz et al. 2004). Microorganisms can be divided into six typical functional groups on the basis of their roles in rock diagenetic processes (Dupraz and Visscher 2005; Visscher and Stolz 2005; Dupraz et al. 2009). These groups are as follows. (1) Oxygenic phototrophs (cyanobacteria) are major primary producers and use light energy to fix CO2. Filamentous cyanobacteria and spherical bacteria play an important role in the capture and cohesion of sedimentary grains with EPS) and biofilms. (2) Anoxygenic phototrophs such as purple and green sulfur bacteria use H2S for photosynthesis. (3) Aerobic heterotrophic bacteria live by respiring oxygen and consuming organic matter. (4) Fermenting bacteria use organic matter or sulfur compounds in their metabolism. (5) Anaerobic heterotrophs (a major type is sulfate-reducing bacteria; SRB) consume organic matter, including SO4 2−, to yield HS−. (6) Sulfide-oxidizing bacteria, which are inorganic chemical autotrophic bacteria, play an important role in fixing CO2 and reducing sulfur. Studies of modern stromatolites have shown that all microbial groups together establish a semi-closed system in the stromatolites, which maintains the circulation of effective elements and the highest metabolic rate (Visscher et al. 1998; Dupraz et al. 2004). However, not all of these six microbial groups are found at the same time, because microbial metabolism within the microbial mat varies strongly from place to place (Reid et al. 2000; Baumgartner et al. 2006; Dupraz et al. 2009; Heindel et al. 2012; Voorhies et al. 2012). Biosignatures from ancient lithified examples are important for understanding the genesis and palaeoenvironmental implications of stromatolites in the geological past (Noffke and Awramik 2013). In this study, three key microbial groups, which were involved in the stromatolite accretion processes, were detected in the Xingyi stromatolites.

Oxygenic phototrophs in stromatolites

Upright and prostrate filamentous tubes are often present in the undulating laminae of the Xingyi stromatolites. Morphologically, these filamentous tubes are flexuous, unsegmented, unbranched tubes. Each individual has well-defined dark walls in longitudinal section (Fig. 4A–C, Fig. 5C, D, Fig. 6A–C) and are circular in transverse section (Fig. 6D). Each filamentous tube has a constant diameter. The external diameter of the filaments is 5–18 µm and the wall thickness is 1–3 µm. Considering these characteristics, it is concluded that the filamentous tubes are similar to the morphological group of tubiforms, which are non-fan forms of cyanobacteria (Flügel 2010; Martin 2010; Reitner and Neuweiler 1995; Riding 1991).

Spheroidal balls, which are formed of clusters of many minute rhombic calcite crystals, are similar to small ball-like objects reported from Late Triassic stromatolites (Mastandrea et al. 2006; Perri and Tucker 2007) and modern examples of mineralized capsules from Lake Vai Si’I, Tonga (Kaźmierczak and Altermann 2002) in both outline and size.

Oval algae with no plastic sheath and single cell have a mean diameter of 25 µm, and are often observed in Xingyi stromatolites. Oval algae are a type of cyanobacteria that have been reported from Late Triassic limestone (Liu 1982; Liu and Liu 1984).

Regardless of the classification of these different cyanobacteria, which coexisted in the dark laminae, the dark laminae usually show strong fluorescence under the fluorescent microscope (Fig. 5B–D), indicating that these layers are rich in organic matter, as in other stromatolites (Cuif et al. 1990; Reitner and Neuweiler 1995; Russo et al. 1997, 2000; Mastandrea et al. 2006; Luo et al. 2014). Modern research has shown that fossilized cyanobacteria may have been the primary producers (Kaźmierczak et al. 2011; Voorhies et al. 2012) that built the Xingyi stromatolites. In the Xingyi stromatolites, oxygenic phototrophs may have contributed to carbon fixation and oxygen production, which is favourable for growth of other microbial groups (Kaźmierczak et al. 2011; Voorhies et al. 2012). It has been documented that some filamentous cyanobacteria can thrive in low-O2 environments and carry out primary production by anoxygenic photosynthesis (Voorhies et al. 2012); however, the material in that study was not lithified. Thus, cyanobacterial fossils may indicate the presence of oxygenic phototrophs rather than anoxygenic phototrophs in the Xingyi stromatolites. This view has been widely adopted to explain the Permian–Triassic boundary microbialites and Lower Triassic microbialites with abundant filamentous cyanobacteria (Reid et al. 2000; Decho et al. 2005; Chen et al. 2012; Luo et al. 2014).

Sulfate-reducing bacteria in stromatolites

Many evidences show that sulfate reduction may also have been an important mechanism of carbonate precipitation in these fossilized structures (Visscher et al. 2000). Although bacterial sulfate reduction in the subsurface environment (Wright and Wacey 2005) has not been directly observed, there are three pieces of evidence indicating the presence of SRB in the Xingyi stromatolites. First, framboidal pyrite is a common type of pyrite in sedimentary rocks, and is composed of many tiny crystallites that have similar size and composition (Rickard 1970; Wilkin and Barnes 1996, 1997). Microcrystalline pyrite can form a variety of structures (Butler and Rickard 2000; Ohfuji and Rickard 2005), including spheroidal, pentagonal like spheroplasts, hexahedra and tetrahedra. Studies show that the formation of framboidal pyrite is related to the presence of SRB (Berner 1984; Coleman et al. 1993). The H2S generated by SRB play an important role in formation of framboidal pyrite (Wilkin and Barnes 1997; Grimes et al. 2001). Therefore, the presence of framboidal pyrite indicates that SRB were present in the Xingyi stromatolites.

Second, authigenic quartz grains are abundant in stromatolitic laminae. Although sources of silica are common, an acidic environment is necessary for formation of authigenic quartz crystals (Campbell et al. 2002; Peckmann and Thiel 2004; Joye et al. 2010), in which bacterial sulfate reduction and sulfide oxidation play an important role (Friedman and Shukla 1980). It should also be noted that the possibility that the euhedral quartz may have had a volcanic origin cannot be ruled out, because morphologically identical authigenic quartz produced by volcanic eruptions has been recorded in Permian–Triassic rocks in South China (Yin et al. 1992).

Third, in the Xingyi stromatolites, tabular calcite and calcite in the shapes of irregular polyhedra, which are composed of many nanoparticles, are found closely associated with EPS (Pacton et al. 2010). EPS (Fig. 9A, B) and SRB have a crucial role in calcium carbonate precipitation (Spadafora et al. 2010; Perri and Spadafora 2011; Perri et al. 2012). EPS can serve as a template to induce calcite formation directly from solution, and can be regarded as an alveolar organic network within which calcite precipitation can be initiated (Bontognali et al. 2010). In addition, SRB disintegrate EPS, consuming sulfate in the process; this raises the alkalinity of the microenvironment, which is favourable for carbonate precipitation (Dupraz et al. 2004, 2009).

EDS analysis of two different lithified cyanobacteria and calcite (Fig. 12A, B) suggests that the lithified cyanobacteria and EPS are rich in Mg and Ca with a small percentage of Si (Fig. 12B). This was probably caused by subsequent Ca incorporation within the previously formed Si–Mg phase that was encapsulated by the EPS matrix. Calcified EPS produced by microbial communities is important in the development and lithification of stromatolites (Dupraz et al. 2004, 2009; Spadafora et al. 2010; Glunk et al. 2011). SRB, supersaturated CaCO3 and high alkalinity of the microenvironment are the main factors required for stromatolite lithification (Glunk et al. 2011).

Fig. 12
figure 12

EDS analytical results of two different components of caclite grains in the Xingyi stromatolites. White cross corresponds to analysing point. Element Au indicates goad coating of samples. A Flamentous tubes that showed a composition of Ca and Mg caclite (indicated by EDS spectrum in A), with low percentage of element Si. B Caclite grains, which showed high percentage of element Ca incomposition (indicated by EDS spectrum in B)

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Possible anoxygenic phototrophs in stromatolites

Smaller aggregates of bacilli such as irregular rod-like aggregates (Fig. 7A, B) and reticular rounded pits (Fig. 7D) are common in the undulating laminae of the Xingyi stromatolites. Smaller bacilli split to form Y-shaped structures, which combine together to form three-dimensional reticular aggregates. This Y-shaped structure is similar to the microscopic structure of Pelodictyon clathratiforme (Pfennig 1968). These bacilli are smaller than P. clathratiforme, which is 0.7–1.2 µm in width and 1.5–2.5 µm in length; however, the aspect ratio of the Xingyi bacilli and P. clathratiforme is the same, having a value of 2. From only this evidence, it is not possible to infer that the smaller bacilli and P. clathratiforme are of the same bacterial type. However, the bacilli were involved in the formation of the stromatolites, as demonstrated by the capture or adhesion of microcrystalline particles in the bacterial aggregates (Fig. 7B, D).

There have been few detailed studies of geomicrobial taxonomy, in contrast to the large amount of work that has been conducted on geomicrobial ecology (Chen et al. 2014). The smaller bacilli cannot be definitely identified as P. clathratiforme, but there is also no evidence that they were not. P. clathratiforme is one of the Chlorobiaceae (Pfennig 1968), which are frequently found in shallow estuarine habitats such as closed bays and lagoons (Caumette 2011). This organism grows in exposed mud containing H2S, stagnant water and hot springs (Pfennig 1977, 1978) and carries out photosynthesis using H2S. These fossils that resemble P. clathratiforme probably represent anoxygenic phototrophs in the Xingyi stromatolites.

Other important microbial components, such as aerobic heterotrophs and sulfide-oxidizing bacteria, were not identified in Xingyi stromatolites. Two major microbial functional groups, oxygenic phototrophs indicated by fossilized filamentous cyanobacteria and SRB represented by framboidal pyrite, are obvious in the dark laminae of the Xingyi stromatolites. These remains might represent the most active microbial communities in the stromatolite ecosystem, and may have contributed to the accretion of stromatolites. In addition, another possible microbial functional group, anoxygenic phototrophs represented by lithified structures resembling P. clathratiforme, may have been present in the Xingyi stromatolites. Even if this functional group was not present, the smaller bacilli were involved in stromatolite formation. To summarize, the Xingyi stromatolites are biogenic in origin.

Implications for Early Triassic stressed marine environments and microbial proliferation

Early Triassic microbialites, including stromatolites, thrombolites and other forms, are characteristic of P–Tr successions in shallow marine facies around the world (Kershaw et al. 2012). Although the debate on the genesis of P–Tr microbialites still continues (Kershaw et al. 2007, 2012; Mata and Bottjer 2012), microbes existed widely in various niches of the post-extinction oceans (Xie et al. 2005; Baud et al. 2007; Ezaki et al. 2012; Chen et al. 2014; Heindel et al. 2015; Luo et al. 2014). Sun et al. (2012) tested oxygen isotopes from Early Triassic conodonts, and estimated the temperature of sea water to have been up to 40 °C during the late Smithian. In contrast, the temperature of sea water during the late Permian was 25 °C (Joachimski et al. 2012; Sun et al. 2012). This warmer temperature would have been favourable for flourishing of cyanobacteria. In addition, significant quantities of nutrients (such as Fe, N, and P) are deposited in the sea after volcanic eruptions (Algeo and Twitchett 2010; Lindenthal et al. 2012; Ayris and Delmelle 2012), which would have also contributed to cyanobacterial blooms (Abraham et al. 2000; Duggen et al. 2010; Olgun et al. 2011). Moreover, after the Permian mass extinction, a lack of metazoan interference resulted in proliferation of cyanobacteria (Schubert and Bottjer 1992, 1995). The occurrence of Dienerian stromatolites in the northwest Nanpanjiang Basin confirms the proliferation of microbialites in the Dienerian, which was the second stage of four distinct growth episodes of enhanced microbialite growth during the Early Triassic (Baud et al. 2007). Microbial proliferation during the Early Triassic is thought to indicate an unhealthy trophic structure in the marine ecosystem and environmental devastation, which delayed biotic recovery following the P–Tr crisis (Chen and Benton 2012). The devastated, unstable marine ecosystem in the Dienerian is also indicated by the large shifts in carbon isotopic values in the Nanpanjiang Basin (Tong et al. 2002, 2007; Payne et al. 2004). Both the Yangtze platform and the Nanpanjiang Basin preserve large δ13C fluctuations (3–8 %) during the Griesbachian to late Smithian time interval (Huang et al. 2012, Song et al. 2012, Song et al. 2014). This carbon isotope positive excursion indicates burial of large amounts of organic matter during the Dienerian. The lack of Early Triassic land coal deposits indicates that terrestrial plants made very little contribution to the organic burial rate; instead, cyanobacteria were extremely important for the organic burial rate. Cyanobacteria have been confirmed to occupy an important position in the ocean carbon cycle (Kolber et al. 2001). As a result, the occurrence of the Xingyi stromatolites is consistent with the earlier view that microbes proliferated in time and space after the severe P–Tr mass extinction (Pruss et al. 2006; Baud et al. 2007).

Conclusions

  1. 1.

    The Xingyi stromatolites show typical stratified columnar structures formed of crinkled laminae. Dark-coloured laminae are composed of upright and prostrate filamentous structures. Upright filamentous structures show a straight-upwards growth fabric.

  2. 2.

    Two main microbial functioning groups, oxygenic phototrophs represented by lithified Cyanobacteria and probable SRB represented by framboidal pyrite, are distinct during the growth of stromatolites. Another possible microbial functional group, anoxygenic phototrophs represented by lithified remains similar to P. clathratiforme, may be present in the Xingyi stromatolites. The smaller bacilli that resemble P. clathratiforme were involved in the formation of stromatolites by capture or adhesion of microcrystalline particles. All of this evidence demonstrates that these Early Triassic stromatolites are biogenic.

  3. 3.

    The occurrence of the Xingyi stromatolites, corresponding to a second episode of microbial growth, reveals that post-extinction microbialites were widespread during the Dienerian.