Toarcian Event Research Articles

A Bajocian event of global coral reef growth: Record from northern Switzerland

Following the extinction event at the Triassic – Jurassic boundary and during the Pliensbachian – Toarcian events, coral reef builders were severely affected, and the number of reef domains during the early Middle Jurassic was at an all-time low. However, new and updated data seem to show that corals were more widespread than previously thought, particularly during the Bajocian (170–168 Ma), but biostratigraphic markers are lacking to constrain the precise timing of reef evolution of this interval. A recently discovered 40 m-thick carbonate unit in northern Switzerland, on the northern Tethys margin, documents this rapid growth. Two cores from this unit display a reef with a high number of specimen but a low coral diversity, dated as Bajocian, which likely grew at a considerable water depth (upper limit of the mesophotic zone or 20–30 m). Additionally, it developed on top of clay mineral-rich accumulation characterised by low relief. Twelve different coral genera were identified, with a strong predominance of Periseris, Isastrea, Thamnasteria and Dendraraea. In the absence of other biostratigraphic markers, palynomorphs were used to provide accurate dating, which shows that palynology is a powerful tool for dating reef deposits. The results show reef growth during the early Bajocian, in the Sauzei and Humphriesianum ammonite zones, based on the palynological data, yielding similar ages to other Bajocian reef localities in the Eastern Paris Basin in France and Luxembourg dated simultaneously. These results indicate that the Middle Jurassic and more specifically the last two ammonite zones of the early Bajocian were periods suitable for coral reef builders, and that global regrowth of coral reefs occurred more rapidly than expected. A new Bajocian reef distribution is presented, showing no reefs at latitudes below 15°. This distribution pattern is probably due to the fact that water temperature was too elevated (> 25 °C) towards the equator for corals, which prevented reef growth. This would tend to favour a scenario in which the global climate, in particular temperature, is a determining factor for coral reef growth.

Latest Triassic and Early Jurassic Spiriferinida (Brachiopoda) of Zealandia(New Zealand and New Caledonia).

The Late Triassic spiriferinides of Zealandia include the endemic or Austral genera Rastelligera, Psioidea and Psioidiella, and the cosmopolitan Zugmayerella. Mentzelia kawhiana is revised and placed in Callospiriferina. It appears in the late Rhaetian and is found in the Téremba Terrane in New Caledonia and throughout the Murihiku Terrane in New Zealand. The spiriferinides were severely affected at the Triassic-Jurassic boundary in Zealandia as elsewhere, but a moderately diverse fauna developed in the Early Jurassic. This has strong links to South America, and affinities with southern Europe and North Africa. In this study, a total of six species of Jurassic spiriferinides are recognised. Callospiriferina ongley is present in middle and late Hettangian and Sinemurian faunas. It is succeeded in the Pliensbachian and early Toarcian by Callospiriferina radiata. Two species of Spiriferina are recognised. S. sophiaealbae n. sp. first appears in the Middle Hettangian and is present in the Southland and Kawhia Synclines until the early Toarcian. S. arakiwa n. sp. is found mainly on the southwest limb of the Southland Syncline in the Pliensbachian and early Toarcian. The non-costate European genus Cisnerospira is represented in Zealandia by the small C. antipoda n. sp. This species ranges from Hettangian to early Toarcian. Two specimens of a spiriferinide with a costate sulcus are tentatively identified as Dispiriferina sp. cf. D. chilensis. The highest stratigraphic level at which Zealandian spiriferinides have been found is that of the Dactylioceras band at Kawhia, which is correlated with the Crassum Subzone of the Bifrons Zone (highest Early Toarcian) and includes three species of spiriferinide. This suggests that the Zealandian spiriferinides survived the Toarcian Event, only to meet their demise slightly later.

Stratification and productivity in the Western Tethys (NW Algeria) during early Toarcian

Profound environmental changes punctuated the Early Jurassic period, as recorded by marked carbon and oxygen isotope anomalies and major biotic crises. The response of low-latitude regions of Northern Gondwana to such intense changes is not documented as well as that of other Tethys areas. We present new calcareous nannofossil assemblages from three sections located in NW Algeria, in the Sahara and Tlemcen Basins, respectively. New stable carbon and nitrogen isotope data are provided from the Tlemcen Basin to reconstruct local environmental conditions in the wider context of a Toarcian greenhouse climate. We first established a solid chemo- and biostratigraphic framework by integrating stable carbon isotope data and calcareous nannofossil events. Calcareous nannofossil assemblages show common trends in the two basins, such as the occurrence in high proportions of the deep-dweller species Mitrolithus jansae , likely indicating stratification of the water column with a deep nutricline. This taxon dominated the assemblage during the negative carbon isotope excursion (CIE) interval, often used to delineate the base of the Toarcian anoxic event (T-OAE). Such a nannofossil record is unique in the Western Tethys domain, as M. jansae is known to drastically decrease in abundance during the T-OAE until its disappearance in the aftermath of the event. The NW Algeria nannofossil record indicates prolonged thermal stratification of water-masses, finally triggering hyper-oligotrophy and low productivity in shallow waters during the Toarcian CIE. Such peculiar conditions are likely related to the combined effects of a warm and arid climate dominating along the northern Gondwana margin and the presence of a strong clockwise gyre over the epicontinental shelf, which brought warm equatorial waters from the Tethys Ocean to the NW Algeria shelf. • The NW Algeria nannofossil record suggests prolonged thermal stratification of water-masses. • Hyper-oligotrophy and low productivity in shallow-waters occurred during the Toarcian Event. • Such conditions are likely related to the effects of a warm and arid climate of the Northern Gondwana.

Sedimentary organic matter and early Toarcian environmental changes in the Lusitanian Basin (Portugal)

The objective of this study is to investigate the relationship between early Toarcian climatic events and the composition of kerogen assemblages in the Lusitanian Basin (Portugal). In particular, we aim to understand how the Pliensbachian–Toarcian Event (Pl–Toa Event) and Toarcian Oceanic Anoxic Event (TOAE) affected the continental areas of the western Iberian margin and how possible variations in continental sources of organic matter (OM) were expressed in marginal-marine and hemipelagic depositional environments during the early Toarcian.We present here a characterisation [total organic carbon (TOC), total sulphur (TS), δ13C in kerogen concentrate (δ13CKerogen), and palynofacies] of kerogen assemblages from several uppermost Pliensbachian (emaciatum ammonite Zone)–lower Toarcian sections in the Lusitanian Basin (western Iberian margin), including the Peniche section, which contains the Toarcian GSSP. In all sections, TOC concentration is low (average 0.4 wt.%), with the highest values reaching up to 2.1 wt.% in a discrete level located approximately 10.5 m above the base of the levisoni Zone at Peniche. The TOAE negative carbon isotope excursion is observed in kerogen concentrate at the base of the levisoni Zone throughout the basin. Palynofacies analysis demonstrates that the kerogen assemblages are mostly of terrestrial affinity, with the dominance of the Phytoclast Group and terrestrial palynomorphs, and with punctual increases in amorphous organic matter, freshwater (e.g. Botryococcus sp.) and marine microplankton (dinoflagellate cysts, acritarchs, and prasinophyte algae) in specific stratigraphic intervals. A change in palynofacies assemblages associated with the TOAE is observed around the base of levisoni Zone. Although with slight differences between sections, the TOAE interval records an increase in non-opaque, translucent, phytoclasts (NOP) and cuticle fragments, and is also associated with an increased contribution of terrestrial palynomorphs (increase in sporomorphs and Classopollis in tetrads and agglomerates) and decrease in marine palynomorphs. The increases in NOP and terrestrial palynomorphs support the postulated enhancement of the hydrological cycle and increased export of terrestrial OM into marine environments during the early Toarcian, especially during the TOAE, in the western Iberian margin. Understanding the impact of the early Toarcian climatic events on land-based ecosystems may provide important insights into current climate change.

Late Pliensbachian–Early Toarcian palaeoenvironmental dynamics and the Pliensbachian–Toarcian Event in the Middle Atlas Basin (Morocco)

Geochemical and palynofacies analysis of 38 samples from the Middle Atlas Basin (Aït Moussa and Issouka sections, Morocco) allowed to investigate sea-level changes, climate, and tectonics and impact on depositional environments in the westernmost Tethyan Gondwana-margin during the Late Pliensbachian–Early Toarcian and the Pliensbachian–Toarcian Event (Pl–Toa Event).The studied sections from the Middle Atlas Basin have low total organic carbon contents. Overall, kerogen assemblages are dominated by the Phytoclast Group, thus showing a strong terrestrial affinity and some degree of proximity to the source. Deposition occurred dominantly in oxic and proximal environments.Upper Pliensbachian kerogen assemblages from Aït Moussa are dominated by terrestrial particles and agree with the overall regressive character of the sedimentary succession. Uppermost Pliensbachian–lowermost Toarcian kerogen assemblages from Aït Moussa show an increase in marine particles. Considering the overall sedimentological context of the Middle Atlas Basin, these kerogen assemblages are interpreted to reflect local variation in accommodation space (driven by different sedimentation rates or tectonic compartmentalization) or ecological conditions (such as nutrient availability, temperature, turbidity, etc). In the Early Toarcian, above a major regional discontinuity and at the beginning of the Pl–Toa Event, increases in Botryococcus sp., AOM, and terrestrial palynomorphs suggest episodes of coastal erosion associated with transgression, likely driven by a combination of eustatic/tectonic changes and warmer and more humid climates leading to an increase in continental weathering and fluvial runoff. On the other hand, increases in sporomorphs and pollen grains occurring in tetrads and agglomerates indicate regressive episodes associated with increased sedimentation rates driven by enhanced continental weathering and fluvial runoff.This study shows the strong relationship between sea-level and the combined response of litho-, hydro-, and biosphere during the Early Toarcian, with implications to understand organic productivity and organic matter accumulation and preservation in the Tethys Ocean during the Pl–Toa Event.

Sedimentary organic matter and δ13CKerogen variation on the southern Iberian palaeomargin (Betic Cordillera, SE Spain) during the latest Pliensbachian–Early Toarcian

The Early Toarcian Oceanic Anoxic Event (T-OAE) is characterised by major palaeoenvironmental and palaeoceanographical changes (ocean anoxia, global warming, ocean acidification, among other), and a severe perturbation of the global carbon cycle. Although widespread oceanic anoxia was a significant control on the occurrence of organic-rich facies during the T-OAE in several locations of the northern Tethyan margin, the effects of anoxia, oceanic productivity, and dilution are poorly understood in other worldwide locations, such as the southern Tethyan and Iberian margins.In this study, we report new geochemical [total organic carbon (TOC), total sulphur (TS), total carbonate content (CaCO3), δ13C from kerogen isolates (δ13CKerogen)], palynofacies and thermal maturation data from the Upper Pliensbachian–Lower Toarcian successions in La Cerradura and Fuente Vidriera, External Subbetic domain, Betic Cordillera (SE Spain). The obtained results indicate that these successions have low TOC contents (~ 0.3 wt.%), with TOC reaching up to 0.46 wt.% around the Polymorphum–Serpentinum ammonite zone boundary in La Cerradura. δ13CKerogen negative carbon isotope excursions (CIE) are observed at the base of the Polymorphum Zone, the Pliensbachian–Toarcian Event (Pl-Toa), and base of the Serpentinum Zone (T-OAE). Overall, the low maturity kerogen assemblages are dominated by terrestrial OM and with a minor marine component. Small increments in terrestrial OM, non-opaque phytoclasts, and palynomorphs are observed just before and during the T-OAE in the studied area.During the Early Toarcian, in the External Subbetic domain, accumulation and preservation of OM was low and with a significant terrestrial contribution, similarly to other locations around the southern margins of Iberia and Tethys. The dominance of the opaque phytoclast subgroup, the occurrence of Classopollis, and the relatively more positive δ13C of terrestrial kerogen suggest that OM was mostly sourced from an area characterised by a semi-arid climate (i.e. with pronounced arid periods) during most of the Polymorphum Zone. The T-OAE negative CIE is accompanied by an increase in terrestrial OM, with a slight increase in the non-opaque phytoclast subgroup and terrestrial palynomorph subgroup. The increase of non-opaque phytoclast and terrestrial palynomorphs are in line with the posited enhancements of the hydrological cycle and export of terrestrial OM into marine environments during the T-OAE. This study suggests a link between 13C fractionation during photosynthesis in C3 plants and climate during the Early Toarcian. This link suggests coupling of the water and terrestrial carbon cycles before and during the T-OAE.

Substantial vegetation response to Early Jurassic global warming with impacts on oceanic anoxia

Rapid global warming and oceanic oxygen deficiency during the Early Jurassic Toarcian Oceanic Anoxic Event at around 183 Ma is associated with a major turnover of marine biota linked to volcanic activity. The impact of the event on land-based ecosystems and the processes that led to oceanic anoxia remain poorly understood. Here we present analyses of spore–pollen assemblages from Pliensbachian–Toarcian rock samples that record marked changes on land during the Toarcian Oceanic Anoxic Event. Vegetation shifted from a high-diversity mixture of conifers, seed ferns, wet-adapted ferns and lycophytes to a low-diversity assemblage dominated by cheirolepid conifers, cycads and Cerebropollenites-producers, which were able to survive in warm, drought-like conditions. Despite the rapid recovery of floras after Toarcian global warming, the overall community composition remained notably different after the event. In shelf seas, eutrophication continued throughout the Toarcian event. This is reflected in the overwhelming dominance of algae, which contributed to reduced oxygen conditions and to a marked decline in dinoflagellates. The substantial initial vegetation response across the Pliensbachian/Toarcian boundary compared with the relatively minor marine response highlights that the impacts of the early stages of volcanogenic global warming were more severe for continental ecosystems than marine ecosystems. Global warming impacts during the Toarcian Oceanic Anoxic Event were initially more severe for terrestrial ecosystems than marine ecosystems, and included a loss of vegetation diversity, according to spore–pollen assemblage data from Pliensbachian–Toarcian rock samples.

Recovery of benthic communities following the Toarcian oceanic anoxic event in the Cleveland Basin, UK

During the Toarcian oceanic anoxic event (OAE) considerable environmental changes occurred that were associated with global warming, perturbations to the C-cycle and ocean deoxygenation which resulted in a mass extinction of marine fauna. Recovery of the biota after the event was protracted and has to date undergone limited study. However, understanding the patterns and processes of recovery are critical to anticipating ecosystem responses to the environmental changes predicted for the near future. Results showed that increases in benthic diversity, and the re-establishment of the Toarcian infauna was gradual and followed the changing redox conditions. Pioneering infauna, such as Dacryomya ovum that dominated the seafloor after the event in the Cleveland Basin, Yorkshire, UK, can modify the physico-chemical environment and thus facilitate ecological succession after disturbance. The length of D. ovum increased >8 mm throughout the bifrons Zone and these body-size changes were linked with total organic carbon (TOC) content suggesting a link to primary productivity, although only at intermediate levels of deoxygenation. Major changes in the phytoplankton, and so food supply, seem to have driven changes in bivalve body size, across trophic guilds, both during and after the event in Yorkshire, and on the mid to lower shelf in Spain and France, respectively. Primary productivity collapse seems then to have been a major driver of biotic change throughout the Toarcian event, as it was during the Permian–Triassic, Triassic/Jurassic and Cretaceous/Tertiary mass extinctions. Further investigation of both the palaeontological and geochemical changes that occurred within early successional Toarcian infaunal communities are required to more fully understand the pattern of recovery after the OAE.

EARLY JURASSIC TEREBRATULIDE BRACHIOPODS FROM ZEALANDIA

Terebratulides, a minor group in New Zealand and New Caledonian Triassic faunas, become second in prominence only to rhynchonellides in the Jurassic. In this study, a total of seven genera and twelve species are recognised and eight new species are described. Lobothyris simesi n. sp. is present throughout the Sinemurian to middle Toarcian. Loboidothyris fordycei n. sp. is common in the late Sinemurian to Pliensbachian of the Hokonui Hills, and a few specimens are recorded from scattered Pliensbachian-Toarcian localities elsewhere. Zeilleria spiculata MacFarlan and Campbell is found in a narrow zone along the Triassic-Jurassic boundary in the Hokonui Hills. Z. terezowae n. sp. is found throughout most of the Early Jurassic, while Z. recessa n. sp. and Z. sacciformis n. sp. have more restricted ranges. The Sinemurian Aulacothyris stevensi n. sp. is known from the Hokonui Hills, and the coast north of Marokopa. Two further forms of Aulacothyris are present. Linguithyris agerorum Mancenido was described from Port Waikato, and is here recorded from New Caledonia. Tegulithyris? plencnerae n. sp. is known from the Sinemurian to Toarcian of Kawhia, and Rugithyris hasibuani n. sp. from the Awakino area. Lobothyris, Aulacothyris and Zeilleria are cosmopolitan and widely distributed. Loboidothyris is also cosmopolitan, but has a Tethyan aspect. Tegulithyris and Rugithyris are Tethyan. Linguithyris is also known from southern Europe, North Africa and western Asia. The highest Early Jurassic brachiopod faunas occur at or just above the Dactylioceras band near Kawhia, which is correlated with the top of the early Toarcian and lies above the Toarcian event in Western Europe.

Bulk-carbonate and belemnite carbon-isotope records across the Pliensbachian-Toarcian boundary on the northern margin of Gondwana (Issouka, Middle Atlas, Morocco)

The data presented here provide the first high-resolution investigation of carbon isotope and geochemical analyses derived from the Pliensbachian-Toarcian boundary, of Issouka, Middle Atlas, Morocco. The isotope data recorded in micrite reveal a stepwise negative carbon isotope excursion with values dropping to −1.8‰ within the Polymorphum Zone. This excursion coincides with major marine biological changes and extinctions and corresponds with European records, supporting the assertion that the excursion was global in extent. The Issouka section is relatively expanded compared to other well–studied sections in Europe. The excursion at the Pliensbachian–Toarcian boundary also shows several similarities with the negative Early Toarcian Event. In contrast, carbon isotope values derived from coeval belemnites show positive values. The belemnite δ13C data presented here suggest spatial heterogeneity in the carbon isotopic composition of dissolved inorganic carbon (DIC) in the Early Jurassic ocean. Overturning or upwelling of a stratified water mass is inconsistent with our data, as it requires the belemnites to have lived elsewhere and only later to have migrated into the Middle Atlas area where they became fossilized. The oxygen isotope values from belemnite calcite show no distinct trend across the Pliensbachian-Toarcian boundary event, indicative of either no significant change in temperatures or change in seawater δ18O. We suggest that the introduction of any light carbon (e.g. from a volcanogenic source) must have resulted in spatial variability in the δ13C of the dissolved inorganic carbon of seawater. Alternatively, a regional change in the source of the carbonate carrying the isotope signal, could lead to a negative shift in the δ13Cmicrite signature without any relation to variations in the global carbon isotope trend.

Osmium isotope evidence for two pulses of increased continental weathering linked to Early Jurassic volcanism and climate change

Abstract Large igneous provinces (LIPs) are proposed to have caused a number of episodes of abrupt environmental change by increasing atmospheric CO2 levels, which were subsequently alleviated by drawdown of CO2 via enhanced continental weathering and burial of organic matter. Here the sedimentary records of two such episodes of environmental change, the Toarcian oceanic anoxic event (T-OAE) and preceding Pliensbachian–Toarcian (Pl-To) event (both possibly linked to the Karoo-Ferrar LIP), are investigated using a new suite of geochemical proxies that have not been previously compared. Stratigraphic variations in osmium isotope (187Os/188Os) records are compared with those of mercury (Hg) and carbon isotopes (δ13C) in samples from the Mochras core, Llanbedr Farm, Cardigan Bay Basin, Wales. These sedimentary rocks are confirmed as recording an open-marine setting by analysis of molybdenum/uranium enrichment trends, indicating that the Os isotope record in these samples reflects the isotopic composition of the global ocean. The Os isotope data include the first results across the Pl-To boundary, when seawater 187Os/188Os increased from ∼0.40 to ∼0.53, in addition to new data that show elevated 187Os/188Os (from ∼0.42 to ∼0.68) during the T-OAE. Both increases in 187Os/188Os correlate with negative carbon isotope excursions and increased mercury concentrations, supporting an interplay between terrestrial volcanism, weathering, and climate that was instrumental in driving these distinct episodes of global environmental change. These observations also indicate that the environmental impact of the Karoo-Ferrar LIP was not limited solely to the T-OAE.

Demise of the last two spire-bearing brachiopod orders (Spiriferinida and Athyridida) at the Toarcian (Early Jurassic) extinction event

Brachiopods were severely hit by several mass extinctions which fundamentally shaped their long evolutionary history. After the devastating end-Permian extinction, the fate of the four surviving orders differed significantly during the Triassic and Jurassic. Two orders, the rhynchonellids and terebratulids are extant today, whereas spiriferinids and athyridids, which possess spiral brachidia, suffered heavy losses at the end of the Triassic and became extinct in the Early Jurassic Toarcian event. Although the doom of the spire-bearing orders has been thought to be related to physiological traits, extinction selectivity across the end-Triassic and Toarcian event has not been rigorously assessed previously, and the reasons for their demise at the later and lesser Toarcian event, rather than at the earlier and greater end-Triassic crisis remained unexplored. Using primarily the Paleobiology Database, we constructed diversity curves, estimated taxonomic rates, and assessed the temporal changes in geographic distribution of the two spire-bearing and two other orders in the Triassic-Jurassic interval. After shared trends and similar origination rates in the post-Permian recovery leading to a Late Triassic diversity maximum, the end-Triassic extinction was selective and preferentially eliminated the spire-bearers. In contrast to the rebound of rhynchonellids and terebratulids, spire-bearers failed to recover in the Early Jurassic, and their repeated selective extinction at the Toarcian event led to their final demise. The end-Triassic event also terminated the worldwide geographic distribution of spire-bearers, confining them to the Western Tethys, whereas the other groups were able to re-establish their cosmopolitan distribution. The morphologically diverse spire-bearers represent specialized adaptation, which further increased their extinction vulnerability compared to the other groups with conservative biconvex shell morphology. Another key difference is the physiological disadvantage of the fixed lophophore and passive feeding of spire-bearers, which became critical at times of increased environmental stress. The spire-bearing spiriferinids and athyridids were “dead clades walking” in the Early Jurassic and their disappearance in the Early Toarcian represents the last major, order-level extinction event for the brachiopods.

Multiproxy geochemical analysis of a Panthalassic margin record of the early Toarcian oceanic anoxic event (Toyora area, Japan)

The early Toarcian oceanic anoxic event (OAE) was a significant palaeoenvironmental perturbation that led to marked changes in ocean chemistry and climate, and which also had a long-lasting impact on marine ecosystems. The global significance of the event has been recognised from the widespread occurrence of a ~3–7‰ negative excursion in the carbon-isotope (δ13C) composition of marine organic and inorganic matter and terrestrial plant material. This feature of the event is indicative of a pronounced perturbation to the global carbon cycle; an inference further supported by widespread evidence for seawater deoxygenation and elevated rates of organic carbon burial. Nevertheless, the precise palaeoenvironmental impacts of this event from sections outside of the Boreal and Tethyan realms are uncertain. Here, we present the results of a multiproxy geochemical study of an expanded record of the early Toarcian event from the northwest Panthalassa Ocean margin exposed in southwest Japan (Toyora area, Yamaguchi prefecture). Our results indicate that in the studied succession, organic matter enrichment persisted through the early Toarcian event, but elemental redox proxies do not support persistent seawater anoxia. Analyses of terrigenously derived major and trace element abundances coupled with sedimentological observations reveal an increase in coarse-grained sediment close to the onset of a ~−4‰ excursion in δ13Corg, and coincident with an inferred increase in terrestrial organic matter flux. These observations are consistent with previously published evidence for a marked strengthening of hydrological cycling and increased runoff that occurred contemporaneously with abrupt warming at the onset of the carbon isotope event.

Microbes, mud and methane: cause and consequence of recurrent Early Jurassic anoxia following the end‐Triassic mass extinction

AbstractThe end‐Triassic mass extinction (c. 201.6 Ma) was one of the five largest mass‐extinction events in the history of animal life. It was also associated with a dramatic, long‐lasting change in sedimentation style along the margins of the Tethys Ocean, from generally organic‐matter‐poor sediments during the Triassic to generally organic‐matter‐rich black shales during the Jurassic. New core material from Germany provides biomarker evidence of persistent photic‐zone euxinia during the Hettangian, the onset of which is associated with a series of both negative and positive carbon isotope excursions. Combined inorganic and organic geochemical and micropalaeontological analyses reveal strong similarities between the Hettangian and the better‐known Toarcian anoxic event. These events appear to be the most clearly expressed events within a series of anoxic episodes that also include poorly studied black shale intervals during the Sinemurian and Pliensbachian. Both the Hettangian and Toarcian events are marked by important changes in phytoplankton assemblages from chromophyte‐ to chlorophyte‐dominated assemblages within the European Epicontinental Seaway. Phytoplankton changes occurred in association with the establishment of photic‐zone euxinia, driven by a general increase in salinity stratification and warming of surface waters. For both events, the causes of large negative carbon isotope excursions remain incompletely understood; evidence exists for both variation in the δ13C of atmospheric CO2 and variation in the sources of organic carbon. Regardless of the causes of δ13C variability, long‐term ocean anoxia during the Early Jurassic can be attributed to greenhouse warming and increased nutrient delivery to the oceans triggered by flood basalt volcanism.

Carbonate platform evidence of ocean acidification at the onset of the early Toarcian oceanic anoxic event

The early Toarcian oceanic anoxic event (Early Jurassic;∼183Myr ago) is associated with one of the largest negative carbon isotope excursion (CIE) in the whole Phanerozoic (3–7‰). Estimates of the magnitude and rate of CO2 injection in the ocean–atmosphere system are compatible with a scenario of ocean acidification. Many carbonate platforms drowned in the Pliensbachian, well before the early Toarcian event. In this paper we test the hypothesis of surface water ocean acidification by presenting data from a resilient carbonate platform: the Apennine Carbonate Platform of southern Italy.The studied sections document a dramatic shift of the carbonate factory from massive biocalcification to chemical precipitation. Lithiotis bivalves and calcareous algae (Palaeodasycladus mediterraneus), which were the most prolific carbonate producers of Pliensbachian carbonate platforms, disappear during the first phase of the early Toarcian CIE, before the most depleted values are reached. We discuss the local versus supraregional significance of this shift and propose a scenario involving abrupt decline of carbonate saturation, forced by CO2 release at the beginning of the early Toarcian CIE, followed by a calcification overshoot, driven by the recovery of ocean alkalinity. Attribution of the demise of carbonate platform hypercalcifiers to ocean acidification is supported by palaeophysiology and reinforced by experimental data on the detrimental effects of ocean acidification on recent shellfishes and calcareous algae.

Low marine sulfate concentrations and the isolation of the European epicontinental sea during the Early Jurassic

Research Article| January 01, 2011 Low marine sulfate concentrations and the isolation of the European epicontinental sea during the Early Jurassic Robert J. Newton; Robert J. Newton 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK Search for other works by this author on: GSW Google Scholar Eoghan P. Reeves; Eoghan P. Reeves * 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK *Current address: Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Search for other works by this author on: GSW Google Scholar Nefeli Kafousia; Nefeli Kafousia † 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK †Current address: Department of Geology and Geoenvironment, Section of Historical Geology and Paleontology, National University of Athens, Panepistimiopolis, 15784 Ilisia, Athens, Greece Search for other works by this author on: GSW Google Scholar Paul B. Wignall; Paul B. Wignall 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK Search for other works by this author on: GSW Google Scholar Simon H. Bottrell; Simon H. Bottrell 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK Search for other works by this author on: GSW Google Scholar Jin-Geng Sha Jin-Geng Sha 2LPS (Laboratory of Palaeobiology and Stratigraphy), Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, 39 East Beijing Road, Nanjing 210008, China Search for other works by this author on: GSW Google Scholar Author and Article Information Robert J. Newton 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK Eoghan P. Reeves * 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK Nefeli Kafousia † 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK Paul B. Wignall 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK Simon H. Bottrell 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK Jin-Geng Sha 2LPS (Laboratory of Palaeobiology and Stratigraphy), Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, 39 East Beijing Road, Nanjing 210008, China *Current address: Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA †Current address: Department of Geology and Geoenvironment, Section of Historical Geology and Paleontology, National University of Athens, Panepistimiopolis, 15784 Ilisia, Athens, Greece Publisher: Geological Society of America Received: 26 Apr 2010 Revision Received: 22 Jul 2010 Accepted: 25 Jul 2010 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2011 Geological Society of America Geology (2011) 39 (1): 7–10. https://doi.org/10.1130/G31326.1 Article history Received: 26 Apr 2010 Revision Received: 22 Jul 2010 Accepted: 25 Jul 2010 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Robert J. Newton, Eoghan P. Reeves, Nefeli Kafousia, Paul B. Wignall, Simon H. Bottrell, Jin-Geng Sha; Low marine sulfate concentrations and the isolation of the European epicontinental sea during the Early Jurassic. Geology 2011;; 39 (1): 7–10. doi: https://doi.org/10.1130/G31326.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Two records of seawater sulfate isotope composition from the Early Jurassic demonstrate that large isotopic gradients existed between the European epicontinental sea and the open Tethys Ocean. These differences can be explained by the modification of open-ocean sulfate isotopic compositions by water-mass isolation, sea-level rise, and the effects of changing regional weathering and pyrite burial fluxes, during a time of rapid environmental change. Both records contain large positive isotopic excursions. In the section from Europe (Yorkshire, UK), a 6‰ excursion begins in the early exaratum subzone of the Toarcian in the middle of the organic-rich shale representing a well-known oceanic anoxic event. An open Tethyan margin record from Tibet records a much larger 19‰ excursion, but the section is less well dated. Two age interpretations are possible: sparse biostratigraphic evidence places this excursion in the Aalenian, but we suggest that it may correlate with the positive excursion in Yorkshire. Hence these records may document both a Toarcian event and an Aalenian sulfur isotope event, or the early Toarcian anoxic event alone. Conservative estimates of the rate of isotopic change with time based on the Tibetan section suggest that Early Jurassic seawater sulfate concentrations were between 1 and 5 mM, much lower than previously thought. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Toarcian carbon isotope shifts and nutrient changes from the Northern margin of Gondwana (High Atlas, Morocco, Jurassic): Palaeoenvironmental implications

The Early Toarcian is marked by a global perturbation of the carbon cycle and major marine biological changes. These coincide with a general decrease in calcium carbonate production and an increase in organic carbon burial, and culminate in the so-called Toarcian Oceanic Anoxic Event. It is believed that the environmental crisis was triggered by the activity of the Karoo–Ferrar large igneous province. In order to further document the Early Toarcian palaeoenvironmental perturbations, carbon isotope, total organic matter, calcareous nannofossils and phosphorus content of the Amellago section in the High Atlas rift basin of Morocco were investigated. This section is extremely expanded compared to the well-studied European sections. Its position along the northern margin of the Gondwana continent is of critical importance because it enables an assessment of changes of river nutrient input into the western Tethyan realm. The carbon isotope curve shows two negative excursions of equal thickness and amplitude, at the Pliensbachian–Toarcian boundary and at the transition from the Polymorphum to the Levisoni Zone. This confirms the supra-regional nature of these shifts and highlights the possible condensation of the first “boundary” shift in European sections. Phosphorus content is used to trace palaeo-nutrient changes and shows that the two negative carbon isotope shifts are associated with increased nutrient levels, confirming that these episodes are related to enhanced continental weathering, probably due to elevated greenhouse gases in the atmosphere. In the High Atlas Basin, the increase in nutrient levels at the Pliensbachian–Toarcian boundary is moreover likely to be the main factor responsible for the coeval demise of the Saharan carbonate platform. A middle Toarcian event, centered on the boundary between the Bifrons and Gradata Zones, characterized by a positive carbon isotope excursion and nutrient level rise, is documented in the Amellago section.

Fluctuations in the body size of marine invertebrates through the Pliensbachian–Toarcian extinction event

The sizes of a selection of common fossil marine invertebrates, including belemnites, ammonites and bivalves, from the Cleveland basin of northern England were measured, in order to assess whether there were any statistically significant changes in body size through the Pliensbachian–Toarcian extinction event. Bed-by-bed fluctuations in body size of pre-extinction bivalves from the Upper Pliensbachian may be influenced by taphonomic effects, whereas other size changes recorded probably reflect real biological change. One taxon, the belemnite species Passaloteuthis bisulcata, records a significant size increase at the start of the extinction interval ( hawskerense through to tenuicostatum subzones). The taxa Dactylioceras and Pseudomytiloides dubius both survive the extinction interval and record their smallest sizes in the immediate aftermath of the event (the early survival interval): a phenomenon known as the Lilliput effect. This is the first documented account of the Lilliput effect in the aftermath of the Pliensbachian–Toarcian extinction event. Apart from the belemnite Acrocoelites subtenuis, all taxa present in the aftermath of the event increase in body size through the later survival interval and subsequent recovery interval. In the Early Toarcian, size decrease may have been caused by low oxygen conditions and, possibly, changes in water depth and food supply. Growth line data indicate that a bedding plane assemblage of Pseudomytiloides dubius from the semicelatum Subzone had slower rates of growth (more growth lines per millimetre) than a larger-sized assemblage from an adjacent horizon. Changes in variance also occur during the Pliensbachian–Toarcian event. Species show greatest variability in the survival interval, which may be the result of reduced interspecific competition experienced by those few taxa fortunate enough to survive the extinction crisis.

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.

The “Schistes carton” of Quercy (Tarn, France): a lithological signature of a methane hydrate dissociation event in the Early Toarcian. Implications for correlations between Boreal and Tethyan realms

Abstract The geochemical events (negative shifts in δ13C and δ18O, and Mn peak) associated with the “Schistes carton” of Quercy (SW France) do not appear to be due only to lithological variations and a differential diagenesis. They correspond to the Lower Toarcian global event and seem to be connected to a destabilization phase of gas hydrates. During its oxidation by seawater, the release of methane gas with a very low carbon isotopic ratio (− 60%) led to the production of CO2 and carbonates with a negative δ13C. The consumption of oxygen in the seawater resulted in conditions that were first dysoxic and then anoxic. The reduction of burrowing and bioturbation associated with this oxidation resulted in the laminated appearance of the “Schistes carton”. The decrease in the oxygen content led to a decrease in MnO2 microparticles present in the water column and sediments. This created a complementary source of Mn2+ that was incorporated into the carbonates produced during this event. Furthermore, an increase in the average seawater temperature (δ18O) appears associated with this event. Finally, the apparent diachronism of geochemical events in the Boreal and the Tethyan realms is discussed.