Microbialites and global environmental change ... - Wiley Online Library

Geobiology (2012), 10, 25–47

DOI: 10.1111/j.1472-4669.2011.00302.x

Microbialites and global environmental change across the Permian–Triassic boundary: a synthesis S. KERSHAW,1 S. CRASQUIN,2 Y. LI,3 P.-Y. COLLIN,4 M.-B. FOREL,2 X. MU,3 A. BAUD,5 Y. WANG,6 S. XIE,6 F. MAURER7 AND L. GUO8 1

Institute for the Environment, Brunel University, Uxbridge, Middlesex, UK CNRS-UMR 7207, CR2P ‘Centre de Recherche sur la Paleóbiodiversite´ et les Paleóenvironnements’, Universite´ Pierre et Marie Curie–Paris 6, Paris Cedex, France 3 State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Nanjing, China 4 UMR 5561 Biogeósciences, Universite´ de Bourgogne, Baˆt. Sciences Gabriel, Dijon, France 5 BGC, Lausanne, Switzerland 6 Key Laboratory of Biogeology and Environmental Geology of Ministry of Education, Faculty of Earth Science, China University of Geosciences, Wuhan, China 7 Maersk Oil Qatar, PO Box 22050, Doha, State of Qatar 8 CASP, Department of Earth Sciences, University of Cambridge, Cambridge, UK 2

ABSTRACT Permian–Triassic boundary microbialites (PTBMs) are thin (0.05–15 m) carbonates formed after the endPermian 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 Renalcisgroup 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. Received 23 April 2011; accepted 27 September 2011 Corresponding author: S. Kershaw. Tel.: +44 1895 266094; fax: +44 1895 269761; e-mail: stephen.kershaw@ brunel.ac.uk

INTRODUCTION Microbialites grew in abundance following the end-Permian mass extinction (Flu¨gel, 2002; Erwin, 2006), mostly as layers and small build-ups on diversely fossiliferous latest Permian reef- and shallow-marine reef-associated carbonate facies, especially abundant in low-latitude Tethys Ocean (Fig. 1A).

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Subtidal microbial carbonates do not occur abundantly in open shelf environments after the Early Ordovician Period (Flu ¨ gel, 2004; p. 378), and their sudden reappearance after the end-Permian extinction is generally considered anomalous (Baud et al., 2007). Early Triassic microbialites formed in four stratigraphic levels following the mass extinction (Pruss et al., 2006), the first of which is the subject of this paper, formed

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24 Fig. 1 Geographic and stratigraphic distribution of Permian–Triassic boundary microbialites (PTBMs), and other relevant sites. PTBMs occur mostly in Tethys Ocean, in low-latitude settings. (A) Palaeogeographic map, based on the reconstruction by Golonka (2002). GH = Gujo-Hachiman, Japan; Gr = East Greenland; H = Hungary; Ir = Central and NW Iran; It = Italy; Ma = Madagascar; N ⁄ U = Nevada ⁄ Utah; AP = Arabian Plate; S = Slovenia; SC = south China block; TKP = Tibet, Kashmir and Pakistan; T = South Turkey; Ub = Ubara, Japan; Vi = Vietnam, part of south China block. (B) Stratigraphy of PTBMs in relation to Meishan lithological log, conodont stratigraphy and carbon isotopes, based on Xie et al. (2007, Fig. 2). PTBMs in all sites (with the possible exception of Nhi Toa in Vietnam) occur in a limited distribution and are: (i) not yet confirmed younger than the top of the Hindeodus parvus zone and (ii) occur in the lower of the two troughs in the carbon isotope curve, discussed in the text. In the log, white symbol is micritic clay-rich limestone; red bands are volcanic ashes. P = Permian; T = Triassic; large arrow is the Permian–Triassic boundary; box marked ‘M’ shows the stratigraphic range of PTBMs. Episodes I and II refer to blooms of cyanobacteria and are described in the text.

directly after the extinction in a very short stratigraphic interval, with complex variation of structures. The stratigraphic distribution of these microbialites crosses the Permian–Triassic boundary (PTB), detailed later, so they are referred to here as PTB microbialites (PTBMs). Interpretation of PTBM formation falls into two general areas: (i) some authors favour the concepts of disaster biotas (Schubert & Bottjer, 1992) and anachronistic facies (Sepkoski et al., 1991) because of anomalous sedimentary rocks, including PTBMs, associated with the end-Permian extinction event and its aftermath (Erwin, 2006; Pruss et al., 2006; see also data and discussions in Baud et al., 2007); (ii) other authors take a different approach and emphasise the importance of ocean carbonate supersaturation to stimulate microbialite calcification (Riding, 2005).

In general, PTBMs are replacements of complex trophic systems of Late Permian reefs and reef-associated crinoidal limestones by communities interpreted as biased towards primary productivity of probable cyanobacteria-dominated systems. This scenario was previously considered compatible with rapid large-scale environmental change caused by overturn of slowly circulating deep-ocean waters, resulting in upwelling of bicarbonate-rich, low-oxygen deep-ocean water across the shallow shelf (Kershaw et al., 1999, 2007). In that interpretation, the result was rapid super-saturation of surface waters and abundant calcium carbonate precipitation as PTBMs. However, recently published work and new data from Turkey, south China, Hungary, Oman and Iran reported in this paper force reassessment of this simple model. New data were collected from a study of ca. 150 new thin


Permian-Triassic boundary microbialites sections and polished samples, plus re-examination of a further ca. 150 thin sections from material used in earlier work (Guo & Riding, 1992) by one author of this paper (Li Guo). The evidence indicates that upwelling only partly explains PTBM growth; geographic and stratigraphic variations in PTBMs detailed below show that they were also controlled by local factors operating within the shallow-marine environments in which they lived. In the light of evidence from new data and recent literature, this paper aims to provide a balanced view of interpretations of the nature and growth of PTBMs and presents a global synthesis of their occurrence. The work addresses the variety of PTBM constructors, microbialite growth forms, associated sedimentary sequences and geographic and stratigraphic variation of microbialites within the context of global Earth surface environmental change across the Permian– Triassic boundary.

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STRATIGRAPHY OF MICROBIALITES ACROSS THE PERMIAN–TRIASSIC BOUNDARY The base of all recorded occurrences of PTBMs is estimated to be not lower than the equivalent of the top of Bed 24 (Fig. 1B), the main extinction horizon in the Global Stratotype Section and Point (GSSP) at Meishan, east China (the PTB is in Bed 27, marked by the First Appearance Datum (FAD) of Hindeodus parvus at base of Bed 27c, Jiang et al., 2007). H. parvus occurs within PTBMs in south China (e.g. Kershaw et al., 2002; Ezaki et al., 2003); Chen et al. (2009) recorded the H. eurypyge, H. praeparvus and M. ultima and H. parvus zones in PTBMs at the Dawen section in the Great Bank of Guizhou, and Yang et al. (2011) recorded the N. meishanensis, H. changxingensis and H. parvus zones in PTBMs in Hunan and Hubei provinces, therefore those PTBMs are latest Permian to earliest Triassic (Fig. 2). Thus, 12

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Fig. 2 Logs of representative sections of Permian–Triassic boundary microbialites (PTBMs) in south China (measured by several authors of this paper) showing variation of microbialite sequences, as well as the facies directly below and above the PTBM units. Inset map shows location of sections. Thrombolites are the most abundant PTBMs in south China. Carbon isotope curve (Mu et al., 2009) from Laolongdong shows sustained large positive values below the extinction event, discussed in the text. Although the Chongyang section is labelled as undifferentiated microbialite, both stromatolites and thrombolites are present, but high-resolution study is needed to clarify the sequence. Y = Yudongzi, L = Laolongdong, D = Dongwan, B = Baizhuyuan, C = Chongyang, D = Dajiang. Yudongzi is previously undescribed, and currently under study by some authors of this paper; approximately 50 new samples were used to verify the log and provide material for photographs in Fig. 3. Arrows indicate the lowest occurrence of Hindeodus parvus as follows: Baizhuyuan: 1.1 m above microbialite base (Kershaw et al., 2002); Dongwan: 0.5 m above microbialite base (Ezaki et al., 2003); Chongyang: 8.4 m above microbialite base (Yang et al., 2011); Dajiang: base of microbialite (Ezaki et al., 2008).


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the term ‘Earliest Triassic Microbialites’ (ETMs) used by Kershaw et al. (2007) is abandoned in favour of PTBMs. Although the boundary between latest Permian limestones and basal microbialites is a bed-parallel contact, erosion of the latest Permian sediments has been confirmed in: (i) the Taurides tectonic unit of southern Turkey, with oolite deposited on the erosion surface, followed by microbialite (Baud et al., 2005; Kershaw et al., 2010); and (ii) the Great Bank of Guizhou, south China, where PTBMs grew directly on eroded latest Permian grainstones that have subaerial cements proving exposure prior to microbialite growth (Collin et al., 2009). PTBMs therefore grew as sea level rose after the latest Permian lowstand. Biostratigraphic evidence is not yet sufficiently developed to fully constrain the stratigraphic range of PTBMs (Fig. 1B); because of zone definitions at Meishan, H. parvus fossils may be found above the H. parvus zone, so the youngest PTBMs may extend into the equivalents of Bed 28 or even 29 at Meishan, that is, the I. staeschei and I. Isarcica zones (Jiang et al., 2007). Changes in the inorganic carbon isotope curve are represented by a global negative excursion across the PTB (e.g. Mu et al., 2009; Richoz et al., 2010), and two small negative troughs have been recorded in many places, which approximately coincide with two periods of microbial blooming shown by biomarkers (Episodes I and II in Fig. 1B; Xie et al., 2007). In a global context, PTBMs occur in the first trough, and part of the intervening peak (Fig. 1B). Reasons for their absence from the second trough are discussed later.

PRINCIPAL FEATURES OF PTB MICROBIALITES Permian–Triassic boundary microbialites are carbonate-rich and replaced pre-existing Permian shallow-marine carbonate systems that are also rich in microbial structures and cements (Weidlich, 2002; Shen & Xu, 2005). PTBMs formed either directly on the latest Permian pre-extinction open shelf limestones with sharp contact (Kershaw et al., 2007 for south China) or on oolitic-bioclastic grainstones to wackestones that were deposited a short time after the extinction (Baud et al., 2005 for south Turkey). In most sites, stylolites obscure the detailed contacts between PTBMs and the underlying limestones. In some sites, oolitic-bioclastic grainstones also occur above PTBMs (Baud et al., 2005; Hips & Haas, 2006; Kershaw et al., 2010), but despite close stratigraphic positioning below and above PTBMs, oolitic-bioclastic grainstones are not found intermixed with PTBM fabrics. PTBMs are typified by sequences composed mostly of stromatolites and thrombolites and bioclastic wackestones. Note that some oolites are considered to be microbially formed (Flu¨gel, 2004) and were included by Baud et al. (2005) in post-extinction microbial facies in Turkey; however, because of potential confusion with inorganic ooids, and the presence of

grainstones ⁄ packstones composed only partially of ooids in post-extinction PTB facies, oolites are here excluded from the concept of microbialite for PTBMs (see Kershaw et al., 2010, for discussion). Riding (2000) grouped the geological record of microbialites into four morphological groups, essentially as a field classification: stromatolites (layered), thrombolites (clotted), dendrolites (branching calcimicrobes) and leiolites (structureless masses). There is overlap between thrombolites and dendrolites in some cases because of mixtures of components. PTBMs are mostly stromatolites and thrombolites (Pruss et al., 2006), with hybrid microbialites (mixtures of microbial and inorganic components, see Riding, 2008) in some sites. Dendrolites are uncommon (occur in south China, Kershaw et al., 2007) and leolites have not been described. In the three morphological groups of PTBMs, constructing elements may consist of one, or a mixture, of: (i) calcified skeletons of microbial organisms (calcimicrobes) (Flu¨gel, 2004), particularly the lobate structures of Renalcis-group fossils (Kershaw et al., 1999; Lehrmann, 1999); (ii) microbially mediated micrite, present as clotted, peloidal, layered and amorphous structures (Baud et al., 2005); and (iii) inorganic, and possibly microbially mediated, calcite cement (Baud et al., 2007; Kershaw et al., 2010). Uncommon hollow spherical objects occur, interpreted as coccoid cyanobacteria (e.g. Ezaki et al., 2003; Yang et al., 2011). Observations by authors of this paper show that PTBMs have a widespread association with micritic sediments, indicating that PTBMs grew in relatively low-energy conditions consistent with shallow shelf seas, possibly below the fairweather wave base. Therefore, PTBMs were facies-limited; they are not found in the shallowest high-energy facies. Also, no examples of brecciated microbialites have been described, and fragments are rare, consistent with growth in quieter waters. However, because of their dense microbial construction, including frame-building in some examples, PTBMs may be considered as microbial reefs, mostly with biostromal geometry. Some authors of this paper have observed wellconstrained examples of biostromal PTBMs in the Huaying Mountains of eastern Sichuan Province, China, where individual PTBMs have been traced for several hundred metres, with maximum of 2 m thickness. Micrite between layers, branches and patches of PTBMs usually contains abundant benthic shelly organisms, including ostracods, microgastropods, crinoids and bivalves (Richoz, 2006; Kershaw et al., 2007), other components (such as foraminifera) and macrophagous higher organisms (conodont elements). Shelly fossils are found occasionally encased within PTBMs, such as the foraminiferan Earlandia and microgastropods (Yang et al., 2011). Thus, there are elements of at least herbivorous secondary productivity, suggesting an oxygenated ecosystem was established very rapidly after the extinction, because these components may be found throughout the thickness of PTBMs. However,


Permian-Triassic boundary microbialites oxygenation in PTBMs is a problematic issue, considered in the discussion. Precipitated calcium carbonate crystal fans, which might be expected to result from large-scale upwelling of bicarbonaterich deep waters as in the later Early Triassic (Woods et al., 1999), are rare and small in nearly all sites in facies after the mass extinction (e.g. Kershaw et al., 2007), except in parts of Iran and Oman (Baud et al., 2007). Environmental implications of the generally poor occurrence of precipitated fans are discussed later. The maximum depth of PTBMs appears to be deeper than the pre-extinction Permian reefs, although precise depth determinations are not possible. PTBMs have been described in carbonate ramps, in northern Hungary (Hips & Haas, 2006), in south Armenia (Baud et al., 1997) and in slope deposits of Wadi Maqam in Oman (Baud et al., 2001). However, PTBMs do not occur in the deep shelf settings of Shangsi (Sichuan Province) and Meishan (Zhejiang Province). Small structures suggested to be microbialites in Bed 27 at Meishan (Cao & Zheng, 2009) were re-examined in this study, revealing that they are micrite-filled burrows (C. Cao, pers. commun. with SK, 2010).

GEOGRAPHY OF PTB MICROBIALITES As stated earlier, PTBMs are concentrated in low-palaeolatitude shallow-water carbonate sequences on the small contiA

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nents within Tethys, uncommon in higher latitudes. PTBM structures vary geographically, and some general patterns can be recognised. The following inventory summarises the global occurrence of PTBMs.

Shallower shelf, low latitudes South China Microbialites in south China (Fig. 2) form units up to 10 m thick. They vary little across the region, but recrystallisation causes problems of classification. Stromatolites occur rarely (Fig. 3F), clotted forms are common (Figs 3–8), and calcimicrobes are abundant (Figs 4 and 5). Most of the microbialites are composed of branches and patches, commonly with lobate margins (Fig. 6). Thus, most of the microbialites are thrombolites or dendrolites. Because of recrystallisation, a full inventory of constructing elements cannot be achieved. Most microbialites are made of spar with micritic carbonate, showing widespread but mostly incomplete recrystallisation. The main evidence of constructing elements is the abundant preservation of small areas showing lobate structures of Renalcis and other Renalcis-group calcimicrobes (Fig. 4) enclosing small cavities (Fig. 4B), some of which contain peloids and possible cyanobacterial bodies (Fig. 4D). In the Nanpanjiang Basin, Renalcis-group fossils form frame-constructed layered architectures (see field photo-

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Fig. 3 Polished blocks showing examples of Permian–Triassic boundary microbialites (PTBMs) in the northern part of the south China block (SCB); all are vertical sections except for a horizontal section in (B). (A, B) Digitate thrombolite in Yudongzi. (C) Digitate thrombolite in Laolongdong. (D, E) variations of thrombolite framework in Yudongzi. (F) Stromatolites in Chongyang. (G) Digitate (upper) and equant (lower) thrombolites in Baizhuyuan. Further photographs from the northern SCB are published by Ezaki et al. (2003), Kershaw et al. (1999, 2002, 2007), Yang et al. (2011). Scale in all photos: large divisions are 10 mm.


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Fig. 4 (A–C) Partly preserved Renalcis-group calcimicrobes from Baizhuyuan, Huaying Mountains, Sichuan; these are common, but most South China Permian– Triassic boundary microbialites (PTBMs) are recrystallised. Calcimicrobes form an open framework infilled with a mixture of peloids, micrite and remaining space filled with sparite. (B) is an enlargement of (A) and shows the sharp contact between the Renalcis-group microbe and sediment infills, highlighting the rounded margins and hollow centres of the calcimicrobe. Such microbes have also been found in other sites nearby in the Huaying Mountains (east Sichuan and Chongqing SAR), in Yudongzi (northwest Sichuan) and in the Great Bank of Guizhou, see Fig. 5 and Lehrmann (1999). (D) microspherical objects from the microbialite in Chongyang, Hubei, interpreted as coccoid cyanobacteria.

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0.5 mm Fig. 5 Microbial framestone of the Great Bank of Guizhou, south China. (A) Vertical section of framestone from Langbai site, with primary cavities, growing on the latest Permian surface of eroded foraminiferal grainstone; an earlier erosion surface is marked by abrupt change from fine packstone to grainstone; see Collin et al. (2009) for details. (B, C) Vertical (upper) and horizontal (lower) polished slabs of microbial framestone from Dajiang site, showing variation in architecture. (D) Photomicrograph of framestone in a well-preserved sample from Dajiang, showing Renalcis-group structure, interpreted by Lehrmann (1999) as the principal constructor of the microbialite.


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0.5 mm Fig. 6 Recrystallised branches of digitate thrombolite from Jianshuigou site, Huaying Mountains, east Sichuan, China, showing the normal extensively altered appearance of the microbialites. (A, B) Sharp margins of altered microbiaite are lobate, B is an enlargement of A. (C–E) Micrite between the branches of another specimen contains a cluster of rounded sparitic objects with sharp margins, which closely resemble the margins of the adjacent thrombolite branches. This arrangement is interpreted as incipient growth of calcimicrobes in the thrombolite, such that the calcimicrobes began growth on the micritic substrate and form masses that become the digitate branches. The examples in these photographs may have been calcified biofilms, if the interpretation of Stephens & Sumner (2002) is correct (see text for discussion), but recrystallisation of the structure reduces clarity of interpretation. In this example, the process of growth of these structures is interpreted to have been arrested by sedimentation.

graphs in Lehrmann, 1999), details of which are shown in Fig. 5. South China PTBMs show vertical changes in architecture; for example, the northern south China thrombolites generally show tabular geometry in their lower parts, evolving into domal forms in their upper parts (see Fig. 8). These were interpreted by Ezaki et al. (2003) as a shallowing-up sequence, consistent with occurrence of oolites above the domed top of microbialite at Chongyang site (Fig. 2), for example. At the Dongwan site, an irregular surface near the microbialite top (Fig. 7) is likely to represent dissolution under either subaerial or submarine conditions. Payne et al. (2007) invoked submarine dissolution associated with ocean acidification during the extinction to explain an irregular contact between the base of the microbialite and the underlying latest Permian limestones in several sites in Tethys Ocean.


However, Collin et al. (2009) demonstrated subaerial erosion for that surface in Guizhou Province, south China. The new discovery at Dongwan reported here may be the first evidence of submarine dissolution associated with the PTBMs, but this surface is near the top of the microbialite, at a higher stratigraphic horizon than that described by Payne et al. (2007). Pyrite framboids have been found within PTBMs in the Laolongdong site, in Chongqing Province (Liao et al., 2010); they may represent low oxygen levels in the microbialite as it grew, the implications for which are discussed later. Vietnam The Nhi Tao site lies in the southern part of Nanpanjiang Basin of the SCB, but is classed separately from those described above because PTBMs lie above a thin oolite (Fig. 9A, see Algeo et al., 2007; Son et al., 2007) deposited

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Fig. 7 Eroded top of a domal thrombolite, containing evidence of dissolution of the microbialite, from Dongwan site, Huaying Mountains, east Sichuan. (A) polished slab near the top of the microbialite showing eroded domal structure and overlain microgastropod grainstone-packstone; inset shows log and location of sample. Scale bar, large divisions are 10 mm. (B, C) Photomicrograph of terminal surface of dome, showing interpreted dissolution of microbial structure, with small amounts of precipitated CaCO3 crystals, and thin layer of opaque cement, prior to grainstone deposition. Sea-floor dissolution associated with end-Permian mass extinction was interpreted by Payne et al. (2007), but without clear evidence; photographs in this figure may provide that evidence, but at a later time than that discussed by Payne et al. (2007).

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Fig. 8 Reconstruction drawings of the interpreted growth appearance of calcimicrobial Permian–Triassic boundary microbialites (PTBMs) from the Huaying Mountains in Sichuan, China. (A, B) Digitate thrombolite, the branches probably protruded a few centimetres above the sea floor, detail in (B). (C, D) Equant thrombolite, detail in (D). (E) enlarged view of construction of Renalcis-group calcimicrobe, developing a small profile above the sea floor, with some cavities enclosed in the structure. (F) Interpretation of the appearance of the sea floor during the microbialite growth, showing digitate thrombolite mounds developed on top of tabular thrombolites as part of a biostrome. See text for discussion of water depth.



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Fig. 9 Logs of some Permian–Triassic boundary microbialites (PTBMs), together with carbon isotope curves, aligned along the base of the microbialite, for convenience, but no stratigraphic correlation is intended. See text for discussion. (A) Log of microbialite at Nhi Tao, Vietnam (redrawn from Algeo et al., 2007), in the southern part of Nanpanjiang basin, south China block. The age and nature of the microbialite are not fully established and further work is required, indicated by the question marks. In particular, the upper limit of microbialite is uncertain; however, the microbialite is underlain by oolite, as is also the case in Turkey (See Fig. 10). Chx = Changxing Formation; l-m = latidentatus-meishanensis zone; parv. = parvus zone; isar. = isarcica zone. (B) Stromatolite sequence from Aliguordaz, redrawn from Wang et al. (2007). Note that the position of the beginning of the negative swing of the carbon isotope curve does not coincide with the base of PTBM growth. (C) PTBM sequence in Hambast, Iran, redrawn from Richoz et al. (2010). Note the three layers of microbialites, composed of branching stromatolites interpreted to be formed in open shelf environments. (D) Composite log of PTB transition in Bu¨kk Mountains, northern Hungary, redrawn from Hips & Haas (2006). The microbialite unit is approximately 10 m thick. Note swing towards negative in the carbon isotope curve begins below the base of the microbialite, indicating the onset of microbialite growth, is not correlated to the oceanographic changes signified by the excursion.

after the extinction and during transgression, indicating the microbialite did not form in the shallowest water. However, no details of the nature or thickness of the deposit are available, and further work is required. Southern Turkey Permian–Triassic boundary microbialites (first described by Baud et al., 1997) occur in the Taurides tectonic unit of southern Turkey and are interlayered complexes of stromatolites, thrombolites and hybrid microbial-inorganic structures, up to a total 15 m thick (Figs 10 and 11, see Kershaw et al., 2010; for detailed illustrations). Stromatolites are complex and consist of at least two types of structures: (i) laminated micrite and (ii) layers of cloudy acicular crystals that may be microbially mediated precipitates. Thrombolites are composed of clotted micrites. Turkey PTBMs are much more variable than those in all other areas (compare the two sites in Fig. 10), but calcimicrobes are absent. Stratigraphy of the Taurides PTBMs is not fully resolved; H. praeparvus (latest Permian) was found in the oolites below the microbialite (Angiolini et al., 2007) and H. parvus in the microbialite (Richoz, 2006), but conodonts are rare, and more data are required. At the C ¸ u¨ru¨k Dag site (Fig. 10), Richoz (2006) reported the start of the negative excursion in carbon isotopes to occur below the base of the microbialite. The new locality at Oznur Tepe (currently under investigation by several authors of this paper) has stromatolites composed of clotted micrites (Fig. 12A,B), and thin thrombolites. However, much of the microbialite consists of oblique microbial structures (unique in PTBMs globally) interlayered with


micrite and fine sparite (Fig. 12C,D) and therefore form hybrid microbialites. Iran Stromatolites and thrombolites (Fig. 9B,C) occur in Zagros and Elbourz areas (Baud et al., 1997) in a tectonic complex including terranes originating from the northern Gondwana margin and from small continents (Cimmeria) within Tethys Ocean. Tethys Ocean sites are the most important, described below. In Zagros, Richoz et al. (2010) described only stromatolites (with unusual branched form) in three horizons of several sites including Hambast (Fig. 9C) and Shareeza; these formed shortly after the extinction horizon, and Richoz et al. (2010) interpreted them as being in open shelf waters. At Hambast, and in two other Zagros sites (Kuh e Surmeh and Dena; see Insalaco et al., 2006), the carbon isotope excursion began below the microbialite. Also, in Hambast, abundant crystal fans are present, but grew on stromatolites (Fig. 13A,B), therefore slightly postdate them. At Aliguordaz in Zagros, Wang et al. (2007) reported stromatolites in which the carbon isotope curve began its negative excursion within the microbialite (Fig. 9B). In the Elikah Valley in Elbourz, thrombolites and stromatolites occur (Altiner et al., 1980; Iranian-Japanese Research Group, 1981; Gaetani et al., 2009), see Fig. 13C,D. Eastern Arabian area PTBMs here are on the southern part of the Arabian Plate, attached to the Tethyan margin of Gondwana in PTB times,

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S. KERSHAW e t al. 13

δ CCARB

Çürük Dag

0

1

2

(‰) 3

4

15

14

Shelly micrite with intraclasts

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12

Thrombolite heads

Tabular bedded thrombolite

Oznur Tepe

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9

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8

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7

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6

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Mini-stromatolite domes Thin beds of micrite

Tabular thrombolite

Hybrid microbialite & interbedded carbonates

Domal stromatolite

Oolite-bioclastic grainstone

Late Permian wackestone with erosion surface

m 0

m 0

Basal microbialite (thrombolite) 13

δ CCARB 0

1

2

(‰) 3

4

Fig. 10 Logs of Permian–Triassic boundary microbialites (PTBMs) in the Antalya Nappe of southern Turkey. (A) Çu¨ru¨k Dag, described in detail by Baud et al. (2005) and Kershaw et al. (2010). (B) Oznur Tepe, previously undescribed, and currently under study by some authors of this paper; approximately 50 new samples were used to verify the log and provide material for photographs in Fig. 11. Note the differences between details of sequence between these two localities currently located several tens of kilometres apart in the Taurides nappes, but with unknown original separation. In both sites, the microbialite is composed of stromatolites and thrombolites, with some hybrid microbialites, but calcimicrobes are missing. White areas between thrombolite branches are micritic sediment; in Çu¨ru¨k Dag, mini-stromatolites occur as a 15-cm-thick layer at 1.1 m. In Oznur Tepe, the wavy lines between 2 and 2.5 m are stromatolitic layers. Thrombolite heads have rounded tops, defined by bold lines, and are overlain by more microbialite and ⁄ or deposited sediment.



A

35

C

B

E F

D

Fig. 11 Photographs of vertical sections of microbialite from Çu¨ru¨k Dag (A, B, D) and Oznur Tepe (C, E, F), southern Turkey. (A, E) Stromatolite; (B, C) Thrombolite. (D, F) Hybrid microbialite. All scales, large divisions are 10 mm (in E, black and white squares are 10 mm). Full description and interpretation of Permian–Triassic boundary microbialites (PTBMs) in Çu¨ru¨k Dag are given by Kershaw et al. (2010).

A

B

0.5 mm

1.0 mm

D

C

0.5 mm

0.5 mm

Fig. 12 Photomicrographs of Permian–Triassic boundary microbialites (PTBMs) from Oznur Tepe site, southern Turkey. (A, B) Stromatolites composed of clotted micrites, near the base of the microbialite. (C, D) Hybrid microbialites containing obliquely aligned clots in layers; some appear chambered but these are not identifiable as renalcid-group calcimicrobes and are not equivalent to the calcimicrobes described for south China in Figs 4 and 5. See Kershaw et al. (2010) for photomicrographs of PTBMs from Çu¨ru¨k Dag, illustrating contrast with Oznur Tepe.


36

S. KERSHAW e t al.

A

B

D

C

Fig. 13 Illustrations of PTBMs from Iran, all in vertical section. (A, B) Branching stromatolites from Hambast (samples from Aymon Baud), with some calcite cement growth on the branches. (C, D) Polished blocks of thrombolites from the Elikah Valley (samples from Sylvie Crasquin), showing branching architecture in (C), and small stromatolite domes in (D). All samples illustrated in vertical section, right way up. Scale bars: large divisions are 10 mm.

A southwest Japan 20

Microbial unit

Dolomite

m

6

Changhsingian

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Mostly oolitic grainstone

1 m 0

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C Bulla, Italy

Thrombolite

Lower Mazzin Mbr

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Mostly grainstone & packstone

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U (ppm) 0

Werfen Fm

Oncolitic limestone

P/T transition

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8

Bellerophon Fm

Shelly limestones

Inaudn

B Wadi Shahha, United Arab Emirates

1 m 0

Tesero Oolite

Bulla Mbr

Mostly packstone & wackestone

Fig. 14 Three logs through Permian–Triassic boundary microbialites (PTBMs) with no isotope curve, aligned along the microbialite base for convenience, but no stratigraphic correlation is intended. See text for discussion. (A) Log of PTB transition in Takachiho, southern Kyushu, Japan. Several metres of microbially bound sediment lie on latest Permian dolomite. Redrawn from Sano & Nakashima (1997, Fig. 4). This example shows PTBMs formed in seamounts in Panthalassa Ocean, separated from the Tethyan PTBMs. (B) Log through the PTB, in Wadi Shahha, northern United Arab Emirates, redrawn from Maurer et al. (2009). Note the microbialite is only 5-20 cm thick, and occurs in a sequence of grainstones and packstones, some metres above the extinction horizon. Uranium data show significant shift associated with the extinction level and may indicate decreased oxygenation of the seawater. However, the microbialite unit lies some distance above the shift in uranium, so its relationship with changes in oxygenation is unclear, discussed in the text. (C) Log of PTB transition from Bulla section, northern Italy, showing a thin stromatolite overlying the Tesero Oolite. Redrawn from Groves et al. (2007). In general, PTBMs are poorly developed in the Italian sequences, for reasons that are unclear.



37

A

Fig. 15 Illustrations of PTBM from Wadi Shahha, Ras Al Khaimah, United Arab Emirates. (A) Vertical section of polished block showing tabular thrombolite architecture. Note that this sample is the entire thickness of the microbialite at this sampling site and is 6 cm thick, likely the thinnest PTBM globally. The sample does not show any vertical change in its development. Scale: large divisions are 10 mm. (B, C) Photomicrographs from sample in A, showing altered thrombolite, with possible calcimicrobial construction.

B

and separate from Iranian areas mentioned in the previous section (Figs 14 and 15). In eastern Arabia, PTBMs form a single bed, varying from 5 cm to no more than 20 cm thick, of thrombolite (Maurer et al., 2009; Fig. 14B); these are the thinnest PTBMs so far recorded. Figure 15 shows the entire thickness of the thrombolite in Wadi Shahha, and the partially recrystallised structure, the original fabric of which may include calcimicrobes. There is no progressive change through the thickness of this sample, suggesting that the processes which caused it were rapidly switched on and off, discussed later. As noted by Insalaco et al. (2006), and Maurer et al. (2009), PTBMs from the southern part of the Arabian Plate decrease in thickness from two metres in the west (Qatar) to less than 20 cm in the northern United Arab Emirates (Figs 14 and 15). Further east, in the Jebel Akhdar area of Oman, they are lacking completely (Richoz, 2006). Japan At Takachiho in southern Kyushu, clotted peloids of likely microbial origin and stromatolites 1–5 m thick (Sano & Nakashima, 1997) formed after the mass extinction (Fig. 14A). These deposits formed in shallow-marine conditions on the top of seamounts in Panthalassa Ocean, described in detail by Sano & Nakashima (1997). Italy Stromatolites are recorded at the Bulla section, Italian Dolomites, where a bed of stromatolites