Sylvie Crasquin

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.

Permian climatic and paleogeographic changes in Northern Gondwana: the Khuff Formation of Interior Oman

Abstract Detailed stratigraphic, paleontologic, and petrographic data from the Middle Permian Khuff Formation exposed in the Haushi–Huqf area of Interior Oman provide new insight into the Permian climatic evolution of the northern Gondwana margin, and on the still debated timing of Neotethys opening between Gondwana and the Cimmerian blocks. The Khuff Formation is interpreted to record a major transgression of Neotethyan waters in Wordian times (Middle Permian), at a stage of full oceanization and tectonic quiescence, when thermal subsidence caused final drowning of rift shoulders and deposition of marine carbonates onto vast portions of stable Arabia. The petrographic composition of Middle Permian sandstones indicates a post-rift stage, and documents a long-term increase in mineralogic stability ascribed to a shift toward warm–humid climatic conditions, coupled with reduced relief and longer transit times of detritus from source to basin. Raising temperatures and northward latitudinal drift towards lower tropic latitudes throughout the Permian are fully documented by rich transitional marine faunas and available paleomagnetic evidence.

Ostracods (Crustacea) and water oxygenation in the earliest Triassic of South China: implications for oceanic events at the end-Permian mass extinction

Ostracods (Crustacea) are benthic inhabitants well known for their consistent qualities as paleoenvironment markers. In particular, they are reliable indicators of water oxygenation level: filter feeders are more common in poor oxygen conditions, contrasting with deposit feeders, which are abundant in well-oxygenated settings. In the Permian/Triassic (P/Tr) boundary transition in the Great Bank of Guizhou, ostracod species are dominated by deposit feeders, showing well-oxygenated conditions from the latest Permian, through the extinction level into the earliest Triassic. These results are consistent with ostracod faunas from northwest Guangxi Province. However, these two examples are in contrast with coeval ostracods from Sichuan, which show lower-than-normal oxygen levels in the earliest Triassic. The Great Bank of Guizhou forms an isolated platform in the large Nanpanjiang Basin on the south side of the South China Block; northwest Guangxi is nearby, in a marginal setting: both faced the Panthalassa Ocean through the P/Tr boundary times according to several published paleogeographic reconstructions. In contrast, P/Tr boundary transition rocks in Sichuan Province, located ∼600 km north of the Great Bank of Guizhou, lie on the Tethyan side of the South China Block. Both the Great Bank of Guizhou and the Sichuan sites have earliest Triassic microbialites, but these are profoundly different in structure and composition. The difference between the two areas may reflect contrasts in the nature of circulating ocean waters, with reduced levels of oxygenation in the Tethys (Sichuan), associated with modelled slow circulation, in contrast to better circulated Panthalassa ocean waters (Great Bank of Guizhou and northwest Guangxi). This also may be an argument to show that low oxygenated, or even anoxic, waters were not the only reason for the P/Tr boundary crisis.

Microbialites as disaster forms in anachronistic facies following the end-Permian mass extinction: a discussion

‘Anachronistic facies’ and ‘disaster forms’ are interpretive terms applied from the early 1990s to sedimentary deposits and biotas in the aftermath of mass extinctions; both terms have been used especially for the deposits formed directly after the end-Permian mass extinction. Microbial carbonates (disaster forms) are abundant in the earliest Triassic and often considered as a return to environmental conditions typical of Neoproterozoic to Cambro-Ordovician times. However, this view does not take into account: (i) the growing evidence that microbialites are stimulated by bicarbonate-supersaturated waters irrespective of mass extinction; (ii) the potential oceanic and climatic effects of the Siberian Traps volcanics; and (iii) the unique global plate-tectonic setting of Pangaea at that time. The configuration of land masses led to near-isolation of Tethys from Panthalassa, with modelled slow circulation and accumulation of anoxic deep water in Tethys. Evidence of catastrophic overturn of the Tethys Ocean reflects instability, possibly driven by climate changes, which released anoxic bicarbonate-rich waters to the surface. Items (ii) and (iii) are features of the Permian–Triassic boundary transition and are not parallels of earlier episodes of Earth history. Taking the argument wider, not all mass extinctions are followed by widespread anachronistic facies and disaster biotas. Therefore, it may be argued that application of anachronism and disaster biota concepts is an oversimplification of mass extinction processes in general, and the Permian–Triassic boundary extinction in particular. Continued use of these terms generates a narrowed view of processes and hinders development of comprehensive interpretations of changes of facies and biotas in mass extinction research.

The extinction of the Metacopina (Ostracoda)

The extinction of the Suborder Metacopina in the early Toarcian (Early Jurassic) was a major event in the macro-evolutionary history of the Ostracoda. The disappearance of this long-ranging, essentially Palaeozoic, group coincides with a change in the composition of marine ostracod faunas from ones with residual Palaeozoic aspect in the pre-Toarcian (due to the presence of the Metacopina) to those dominated by the Cytheroidea, Cypridoidea and Platycopina, a pattern that continues to the present. The Metacopina had been present in many Palaeozoic assemblages although usually at low diversity and abundance but they were particularly successful in Triassic and Early Jurassic times, being diverse, abundant and with an apparently cosmopolitan distribution. Why did such a successful group that had survived four of the ’Big Five’ Phanerozoic extinction events (end Ordovician, Late Devonian, end-Permian, end-Triassic), succumb at this second order and apparently less significant early Toarcian event? The rapid decline and extinction of the Metacopina followed a major regressive phase in the Pliensbachian and broadly coincided with the onset of the early Toarcian Oceanic Anoxic Event (TOAE), which has been linked to global eustatic sea-level rise and is marked by a major, negative carbon isotope (δ13C ) excursion in the global carbon cycle. The nature and likely causes of the Metacopina extinction event (MEE) are reviewed in the context of new environmental data for the Toarcian, with particular reference to evidence from the Mochras Borehole (West Wales) ; the group’s survival of earlier biotic events is also discussed. The one distinct feature that sets the Toarcian extinction event apart from previous evolutionary crises is that the Metacopina never had to compete against relatively advanced ostracod, largely cytheroidean, taxa that were present during the Early Jurassic. We propose that this may have been a key factor in the demise of the Metacopina at this particular time.

Lower Triassic ostracods (Crustacea) from the Meishan section, Permian–Triassic boundary GSSP (Zhejiang Province, South China)

The Permian–Triassic boundary Global Stratotype Section and Point (GSSP) is located at the Meishan section, Zhejiang Province, South China. The Yinkeng Formation and base of the Helongshan Formation, of Early Triassic Griesbachian age, have been sampled for ostracod analysis for the first time. Twenty-three species were recognized. A new species is described and figured: Bairdia cahuzaci sp. nov. The stratigraphical distributions of all 23 species are given and diversity variations are discussed.

Biodiversity evolution through the Permian-Triassic boundary event: Ostracods from the Bükk Mountains, Hungary

One of the most complete Permian-Triassic boundary sections located in the Bükk Mountains (Hungary) was sampled for ostracod study. Seventy-six species are recognized, belonging to twenty genera. Fifteen new species are described and figured: Acratia? jeanvannieri Forel sp. nov., Acratia nagyvisnyoensis Forel sp. nov., Bairdia anisongae Forel sp. nov., Bairdia davehornei Forel sp. nov., Callicythere? balvanyseptentrioensis Forel sp. nov., Cytherellina? magyarorszagensis Forel sp. nov., Eumiraculum desmaresae Forel sp. nov., Hollinella fengqinglaii Crasquin sp. nov., Hungarella gerennavarensis Crasquin sp. nov., Langdaia bullabalvanyensis Crasquin sp. nov., Liuzhinia venninae Forel sp. nov., Liuzhinia bankutensis Forel sp. nov., Microcheilinella egerensis Forel sp. nov., Reviya praecurukensis Forel sp. nov., Shemonaella? olempskaella Forel sp. nov. One species is renamed: Bairdia baudini Crasquin nom. nov. Comparison of the Bálvány North section with the Meishan section (Zhejiang Province, South China), Global Boundary Stratotype Section and Point (GSSP) of the Permian-Triassic Boundary (PTB), reveals discrepancies linked to the environmental setting and particularly to bathymetry. The stratigraphical distribution of all the species is given and diversity variations are discussed. The Bálvány North section exhibits the lowest extinction rate of all PTB sections studied for ostracods analysis associated with a high level of endemism.

Ostracods (Crustacea) of the Early-Middle Permian from Central Thailand (Indochina block). Part I. Order Palaeocopida

ABSTRACT Early to Middle Permian ostracods of the Indochina block were reported from Central Thailand. Twelve Permian localities spreading in the Loei, Phetchabun, and Nakhon Sawan-Lopburi areas were investigated and limestone samples were collected. Out of 135 samples collected, 82 yielded ostracods. In this paper, 39 species of 20 genera belonging to Order Palaeocopida, families Aparchitidae, Kloedenellidae, Knoxitidae, Paraparchitidae, Kirkbyidae, Amphissitidae, Youngiellidae, Hollinellidae, and Coelonellidae are identified; species of Order Podocopida will be described in a second paper. Eight species are newly described: Langdaia meesooki Chitnarin n. sp., Sargentina chantarameei Chitnarin n. sp., Geffenina posterodorsospina Chitnarin n. sp., Geffenina mariebeatriceae Chitnarin n. sp., Samarella sonei Chitnarin n. sp., Samarella viscusforma Chitnarin n. sp., Microcoelonella takliensis Chitnarin n. sp., and Microcoelonella takfaensis Chitnarin n. sp. This paper presents the first intensive systematics treatment of Permian ostracods of the Indochina block. Detailed analysis of the fauna recovered Palaeocopida and Podocopida, facilitates and will contribute to a palaeoenvironmental and palaeobiogeographical reconstruction.

COMMENT TO LEHRMANN ET AL. NEW SECTIONS AND OBSERVATIONS FROM THE NANPANJIANG BASIN, SOUTH CHINA

In the study of Earth-surface environmental processes during the events associated with the Permian–Triassic boundary, a key issue is the nature of the latest Permian pre-extinction surface in shallow marine limestones in numerous sites, principally within the Tethyan realm. Sediments below this surface pre-date the extinction event, so that the limestones comprising these latest Permian facies contain diverse fossil remains of organisms that lived just before the extinction. At all reported sites, this surface is disconformably overlain by post-extinction sediments, which contain microbialites in many places, particularly in Tethys. The nature of the youngest pre-extinction surface remains controversial, originating by either physical erosion or dissolution. Furthermore, if the surface was created by dissolution, this could reflect ocean acidification or, alternatively, subaerial dissolution. These arguments were discussed by Collin et al. (2009) and Kershaw et al. (2012a). In an attempt to solve the problem of the origin of the youngest pre-extinction surface, Lehrmann et al. (2015) provided a comprehensive treatment of the associated facies in the Nanpanjiang Basin in southern China, which although is of considerable value, contains some aspects we consider require further attention. Our comment primarily addresses their views regarding the environment of formation of calcium carbonate grain-coating cements in the boundary deposits. We also consider some other aspects of their paper, all presented under several subheadings on specific points listed below. Thus, in this comment, we aim to clarify some of their reported observations and interpretations of the boundary deposits. In preparing this comment, we reviewed thin sections used by Collin et al. (2009) and present further photographs showing the fabrics in better detail. Figure 1 shows outcrop views of a key site in the Great Bank of Guizhou (for location see Lehrmann et al. 2015, fig. 1). Figure 1C is a polished block showing …

Biodiversity across the Guadalupian-Lopingian Boundary: first results on the ostracod (Crustacea) fauna, Chaotian section (Sichuan Province, South China)

ABSTRACT The Middle Permian-Late Permian boundary (Guadalupian-Lopingian boundary, GLB) interval is characterised by important faunal assemblage changes. This extinction-turnover episode is considered by some authors to be the first step of the end-Permian biodiversity drop. The forty-five meters thickness of sediment encompassing the GLB in Chaotian section (Sichuan Province, South China) was sampled and processed for ostracod study. This study presents the first analysis of ostracod faunas in the GLB interval. A total of 154 species belonging to 29 genera are identified. Three species are described as new: Bairdia chaotianensis Zazzali, n. sp., Microcheilinella wujiapingensis Zazzali, n. sp., Microcheilinella pagodaensis Zazzali, n. sp. All the ostracods discovered in the section belong to shallow marine taxa. So these results are not consistent with previous interpretations (lagoonal environment or deep water setting) based on other evidences. Abundance and diversity present a rapid and noticeable decline in the Early Capitanian. Recovery is then recorded about three meters above the GLB. At specific level, a 93% extinction rate and a 96% turnover rate are recorded at the GLB. Moreover, Palaeocopida, straight dorsal border ostracods known to progressively disappear from the Late Permian to the basal Middle-Triassic, are here less abundant and diversified after the GLB. This could reflect the first step of their disappearance at the end of the Palaeozoic.