Pierre-Yves Collin

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.

Erosional truncation of uppermost Permian shallow-marine carbonates and implications for Permian - Triassic boundary events: Comment.

[Payne et al. (2007)][1] suggest that there is widespread evidence for a carbonate dissolution surface at the level of the end-Permian mass extinction, and that this crisis was at least partly due to ocean acidification that preferentially affected heavily calcified marine invertebrates. These ideas

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.

Taphonomic sequences—A new tool for sequence stratigraphy

The Middle-Upper Jurassic boundary in western Europe is characterized by extensive condensed sections containing authigenic minerals together with abundant and varied fossils, both reworked and not reworked. We have analyzed ammonite shells and chronologically ordered taphonomic events in a taphonomic sequence extending from the time the dead organism settled on the seafloor to the time it became permanently incorporated in the sediment. Three types of taphonomic sequence are recognized; they are characteristic of (1) sedimentation in depositional environments having little and only occasional hydrodynamic activity (coinciding with periods of increasing accommodation), (2) sedimentation during maximum regional reductions in accommodation corresponding to third-order sequence boundaries at the scale of western Europe, or (3) sedimentation involving a sudden lithological change and an onset of a large-scale transgressive trend. In condensed sections, where little sediment is preserved, applying this classification is a useful new tool for sequence stratigraphy.

Ostracods (Crustacea) through the Permian-Triassic boundary in South China: the Meishan stratotype (Zhejiang Province)

The Global Stratotype Section and Point (GSSP) of the Permian–Triassic boundary, located in Meishan, Zhejiang Province, South China, was sampled bed-by-bed for ostracods. Ninety-eight species belonging to forty-three genera are recognized. Twenty-five new species are described and figured: Bairdia bassoni Crasquin sp. nov., B. broutini Crasquin sp. nov., B. deweveri Crasquin sp. nov., B. fangnianqiaoi Crasquin sp. nov., B. gaelleae Crasquin sp. nov., B. limatusformis Forel sp. nov., B. paussi Crasquin sp. nov., B. pierrevalentini Crasquin sp. nov., B. wushunbaoi Crasquin sp. nov., Baschkirina ballei Crasquin sp. nov., B. huzhouensis Forel sp. nov., Basslerella annesophieae Crasquin sp. nov., Hollinella martensiformis Crasquin sp. nov., Kempfina taihuensis Forel sp. nov., Knightina hongfui Crasquin sp. nov., Liuzhinia praeantalyaensis Forel sp. nov., Microcheilinella rectodorsata Forel sp. nov., M. shicheni Crasquin sp. nov., M.? multinodosa Forel sp. nov., Orthobairdia lemairei Crasquin sp. nov., Paraparchites chenshii Crasquin sp. nov., Praezabythocypris? pulchraformis Forel sp. nov., Samarella meishanella Forel sp. nov., S. victori Crasquin sp. nov., and Silenites? zhejiangensis Forel sp. nov. One new genus name is proposed: Kempfina Crasquin nom. nov. The stratigraphic distribution of all the species is given. Ostracod biodiversity variations are analyzed through the section. The main drop is recorded in bed 22, occurring a little earlier in ostracods than in foraminifers, corals and brachiopods. The decline in ostracod diversity is more progressive than in the other groups. Variations in ostracod biodiversity during the Changhsingian are related to fluctuations in palaeoenvironmental conditions.

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.

Geochemistry of post-extinction microbialites as a powerful tool to assess the oxygenation of shallow marine water in the immediate aftermath of the end-Permian mass extinction

Rapid and profound changes in earth surface environments and biota across the Permian–Triassic boundary are well known and relate to the end-Permian mass extinction event. This major crisis is demonstrated by abrupt facies change and the development of microbialite carbonates on the shallow marine shelves around Palaeo-Tethys and western Panthalassa. Microbialites have been described from a range of sites in end-Permian and basal Triassic marine sedimentary rocks, immediately following the end-Permian mass extinction. Here, we present geochemical data primarily focused on microbialites. Our geochemical analysis shows that U, V, Mo and REE (Ce anomaly) may be used as robust redox proxies so that the microbialites record the chemistry of the ancient ambient sea water. Among the three trace metals reputed to be reliable redox proxies, one (V) is correlated here with terrigenous supply, the other two elements (U and Mo) do not show any significant authigenic enrichment, thereby indicating that oxic conditions prevailed during the growth of microbialites. REE profiles show a prominent negative Ce anomaly, also showing that the shallow marine waters were oxic. Our geochemical data are consistent with the presence of some benthic organisms (ostracods, scattered microgastropods, microbrachiopods and foraminifers) in shallow marine waters that survived the mass extinction event.

Settlement down again patterns of a carbonated platform following up a sedimentary crisis: example of the middle-late Jurassic boundary in the southeastern part of the Paris Basin

Following a sedimentary crisis which begins in the late Lower Callovian and spans all the early Oxfordian, the settlement down patterns of a platform with carbonated sedimentation are analysed in a southeastern area of the Paris Basin (fig. 1). Ten lithostratigraphic units (reefal formations, associated bioclastic facies and marly distal lateral facies ; fig. 2) are defined (fig. 3). New ammonite and brachiopod faunas, collected in situ, allow to date accurately the sedimentary units with a precision matching an ammonite subzone of the standard bio-chronostratigraphic scale (fig. 4) of the middle-late Oxfordian (from the Parandieri Subzone, at the base of the Transversarium Zone, to the Planula Subzone, at the top of the Planula Zone). The sedimentologic analysis coupled with the study of the benthic and pelagic faunal communities allow to define twelve type-facies (tabl. I and II). Regrouped into three associations, these characterise depositional environments which occur in succession, following three platform models (fig. 5). As witnesses of the evolution of the accomodation/sedimentation ratio, the resulting time succession of sedimentary bodies shows a depositional dynamics organised into three sequence tracks (fig. 5 and 6) : - a retrogradation phase is characterised by a moderately deep and open platform, dominated by low energy and marly sedimentation, which ranges from the Middle Oxfordian (Plicatilis Zone and Transversarium Zone) to the lowermost late Oxfordian (Bifurcatus Zone) ; - during the late Oxfordian (Bimammatum Zone, from the Semimammatum Subzone to the Bimammatum Subzone) an aggradation phase corresponds to the installation of three successive shallow platforms with contrasted morphology. Indicating the re-initiation of carbonated production, these platforms are well limited and represent high energy shallows with reef buildings, which lateraly grade into dismantling bioclastic facies, then secondly and more laterally again into low energy and medium deep marly facies ; - the upper part of the late Oxfordian (Hauffianum Subzone, in the uppermost Bimammatum Zone, and Planula Zone) shows the wide extent of a low energy and morphologically very little contrasted distal platform. This one has a high potential of carbonated production characterised by bioclastic, oolitic and micritic facies which illustrate a progradation phase ; such a phase proceeds in the early Kimmeridgian. The collected data and the corresponding phenomenons pointed out on this area of the southeastern border of the Paris Basin are discussed and replaced in the general framework of the sedimentary, biologic, palaeogeographic and palaeoclimatic events henceforth recognised at the middle-late Jurassic boundary on the peri-Tethyan intracratonic domains of western Europe. The demise of the carbonate production seems to be correlated with a global cooling of both marine waters and atmosphere, which is considered as a limiting factor. During the Middle Oxfordian, the re-initiation of carbonate production with the developement of reef buildings should correspond to a large scale warming of the marine waters still observed elswhere on the Russian Platform, in the North Sea and in the Paris Basin. However, the geographic distribution and the chronologic succession of the facies and deduced palaeoenvironments is probably equally related to a synsedimentary tectonic activity which operates as a favorable factor at both a local and regional scale.

Séries condensées et indice de préservation d'unité biostratigraphique: exemple de l'ennoiement de la plate-forme nord-bourguignonne (France) au Callovo-Oxfordien

Abstract Several depositional environments associated with Callovian-Oxfordian condensed series of the southeast of the Paris Basin are defined. As sedimentation rates were very low, variations in palaeodepth, which were synchronous at regional scale, were taken to directly reflect fluctuations in accommodation potential. Within a major trend toward increased accommodation potential, five sharp upturns and one sharp downturn are detected. A biostratigraphic unit preservation index is denned to quantify and discuss the low preservation of sediments in these condensed series. This index is seen to vary with accommodation potential.

Coupures morphologiques et biochronologie chez les Kosmoceratinae de l'Est de la France (Callovien inférieur pp. à Callovien supérieur pp.)

Abstract For the first time in eastern France, a detailed succession of the uppermost Lower Callovian to Lower Upper Callovian Kosmoceratinae is presented, according to the fossils collected in situ at Blye (Jura, France). It is compared with the abundant data from Champagne and Bourgogne. These ammonites may allow more accurate correlations between the subboreal and subtethysian biostratigraphical frameworks. They record numerous morphological changes, sometimes different from those generally accepted. They allow the precise recognition of the limits of the biostratigraphical units, and the definition of successive faunal units available in the lowermost Upper Callovian.