Tenuicostatum Zone Research Articles

Early Jurassic extrinsic solar system dynamics versus intrinsic Earth processes: Toarcian sedimentation and benthic life in deep-sea contourite drift facies, Cardigan Bay Basin, UK

The Cardigan Bay Basin (UK) may have functioned as a deep and narrow strait, and thereby influenced Early Jurassic oceanic circulation through the northern and southern Laurasian Seaway, and between Boreal and Peri-Tethys domains. Toarcian hemipelagic deposits of the basin in the Mochras borehole show strongly bioturbated contourite facies. Trace fossils are strongly dominated by Phycosiphon incertum (represented by four morphotypes), which was produced by opportunistic colonizers. Thalassinoides, Schaubcylindrichnus and Trichichnus are common (the latter is a deep-tier trace fossil produced by filamentous sulfide-oxidizing bacteria with a high tolerance for dysoxia), accompanied by less common Zoophycos, Planolites, Palaeophycus, Teichichnus, Rhizocorallium, Chondrites, and dwelling and resting structures, such as cf. Polykladichnus, Siphonichnus, Skolithos, Arenicolites, Monocraterion and Lockeia. Ichnological and lithological signals suggest repetitive fluctuations in benthic conditions attributed to a hierarchy of orbital cycles (precession and obliquity [4th order], short eccentricity [3rd order], long eccentricity [2nd order] and Earth–Mars secular resonance [1st order]). The Pliensbachian–Toarcian transition appears to be a significant palaeoceanographic turning point in the Cardigan Bay Basin, starting a CaCO3 decline, and with the most severe oxygen crisis of the Tenuicostatum Zone (here dysoxic but not anoxic) ending at the onset, in the early Serpentinum Zone (Exaratum Subzone), of the Toarcian negative carbon isotope excursion (To-CIE—linked with the Toarcian Oceanic Anoxic Event occurring in the lower part in the Serpentinum Zone). This trend contrasts with the prevalence of anoxia synchronous with the To-CIE in many other settings. Minor dysoxia returned to the Mochras setting in the latest Thouarsense to Dispansum zone interval. Extreme climate warming during the To-CIE may have enhanced and caused a reversal in the direction of deep marine circulation, improving oxygenation of the sea floor. Spectral analysis of binary data on ichnotaxa appearances gives high confidence in orbital signals and allows refined estimation of ammonite zones and the duration of the Toarcian (minimum ~ 9.4 Myr).

The Toarcian Posidonia Shale at Salem (North Alpine Foreland Basin; South Germany): hydrocarbon potential and paleogeography

AbstractThe Posidonia Shale in the basement of the North Alpine Foreland Basin of southwestern Germany represents an important archive for environmental changes during the Toarcian oceanic anoxic event and the associated carbon isotope excursion (T-CIE). It is also an important hydrocarbon source rock. In the Salem borehole, the Posidonia Shale is ~ 10 m thick. The lower 7.5 m (1763.5–1756.0 m) of the Posidonian Shale and the uppermost part of the underlying Amaltheenton Formation were cored and studied using a total of 62 samples. Rock–Eval, palynological, maceral, biomarker and carbon isotope data were collected to assess variations in environmental conditions and to quantify the source rock potential. In contrast to most other Toarcian sections in southwest Germany, TOC contents are high in sediments deposited during the T-CIE, but reach a peak in post-CIE sediments. Biomarker ratios suggest that this reflects strong oxygen-depletion during the T-CIE (elegantulum to lower elegans subzones), but also during the falciferum Subzone, which is also reflected by a prolonged dinoflagellate cyst blackout. While sediments of the tenuicostatum Zone to the elegans Subzone are thinner than in neighbouring sections (e.g., Dotternhausen), sediments of the falciferum Subzone are unusually thick, suggesting that increased subsidence might have contributed to anoxia. The T-CIE interval is very thin (0.75 m). δ13C values of n-alkanes show that the maximum negative isotope shift predates the strongest basin restriction during the T-CIE and that the carbon isotope shift is recorded earlier for aquatic than for terrigenous organisms. In Salem, the Posidonia Shale is thermally mature and highly oil-prone. The residual source petroleum potential is about 0.8 tHC/m2. Graphical Abstract

Taphonomic alteration processes in Bouleiceras (Jurassic Ammonitina) of the Iberian Range. Paleoenvironmental and paleobiogeographical implications

ABSTRACT The genus Bouleiceras (Ammonitina, Bouleiceratinae) appears in the Iberian Range from the Tenuicostatum Zone to the Serpentinum Zone (Lower Toarcian, Jurassic). It displays a lower frequency in number of specimens (approximately 1%) in relation to other groups of ammonoids at the same levels within the studied sections. The studied species of this genus are B. nitescens Thévenin, B. elegans Arkell, B. arabicum Arkell, B. tumidum Arkell, B. marraticum Arkell, B. ibericum Martínez and B.? betetensis Martinez. They correspond to incomplete internal moulds and the record includes a scarce number of specimens per species. By identifying different types of taphonomic alterations within the specimens (biostratinomic and fossildiagenetic) it was possible to recognise the removal processes (resedimentation or reelaboration). Most of the specimens present different mechanisms of taphonomic alteration that denote prolonged processes of necroplanktonic drift and resedimentation or reelaboration such as colonisation, facet abrasion, distortion, disarticulation in internal moulds, etc. Despite the scarcity of material they can be classified as taphonic populations of type 3. Taphorecords point to a shallow marine environment with low sedimentation rates and episodic high-energy events. None of the data seems to correspond to areas linked to favourable environments for the ontogenic development of these species.

Do spores herald the Toarcian Oceanic Anoxic Event?

The environmental perturbations of the Toarcian Oceanic Anoxic Event (T-OAE, ~183 Ma) were associated with the widespread deposition of black shales, such as the Posidonia Shale Formation in southwest Germany. We recently published a detailed palynological analysis of the deposits of the Posidonia Shale Formation at Dormettingen, investigating pollen, spores, and marine plankton. In addition to the comprehensive palynological analysis, we point out the occurrence of possible teratological features (i.e. unseparated spore tetrads and dark-coloured sporomorphs) before the T-OAE. These features have been tentatively linked to environmental changes, such as increased UV-B irradiation or heavy metal pollution associated with the large-scale volcanism of the Karoo Ferrar province (Galasso et al., 2022). Our interpretation has drawn criticism (Riding et al., 2022). Here we present a more substantiated discussion of why spore tetrads and darkened spores might not be sedimentological artefacts but indicators of aggravating environmental conditions prior to the T-OAE. The combined occurrence of several teratomorphic features (i.e. spore tetrads, dark sporomorph walls, and even polyploidy) during the earliest Toarcian (Tenuicostatum Zone) is interpreted as a signal of environmental stress in land plants, although for each individual morphological variation a much easier explanation exists.

Biostratigraphy and sequence stratigraphy of the Toarcian Ludwigskanal section (Franconian Alb, Southern Germany)

Extensive construction work at the canal cutting of the Ludwigskanal near Dörlbach, Franconian Alb, provided the opportunity to re-investigate a scientific-historical and biostratigraphically important reference section of the South-German Toarcian. The 16 m thick section, described bed by bed with respect to lithology and macrofossils, starts within the Upper Pliensbachian Amaltheenton Formation, covers the Toarcian Posidonienschiefer and Jurensismergel Formation, and ends in basal parts of the Opalinuston Formation. Carbonate contents are high in the Posidonienschiefer and successively decline within the Jurensismergel to basal parts of the Opalinuston. The high carbonate contents in the Posidonienschiefer are associated with comparatively low organic carbon contents. However, organic carbon contents normalized to the silicate fraction are similarily high if compared to other regions in Germany. Only the persistence of high organic carbon levels into middle parts of the Upper Toarcian differs from those of most regions in central Europe.Ammonite biostratigraphy indicates a thickness of >9 m for the Upper Pliensbachian, 1.15–1.20 m for the Lower Toarcian, 5.04 m for the Upper Toarcian, and >0.5 m for the Lower Aalenian. Despite the low sediment thickness, all Toarcian ammonite zones and almost all subzones are present, except for major parts of the Tenuicostatum Zone and the Fallaciosum Subzone.On the basis of discontinuities, condensed beds, and correlations with neighbouring sections in Southern Germany, a sequence stratigraphic interpretation is proposed for the Toarcian of this region: (i) The Posidonienschiefer Formation corresponds to one 3rdorder T-R sequence, from the top of the Hawskerense Subzone to a fucoid bed at the top of the Variabilis Subzone, with a maximum flooding surface at the top of the Falciferum Zone. (ii) The Jurensismergel Formation exhibits two 3rdorder T-R sequences: The first ranges from the basis of the Illustris Subzone (i.e., the Intra-Variabilis-Discontinuity) to the top of the Thouarsense Zone, with a maximum flooding surface within the Thouarsense Zone. The “belemnite battlefield” reflects a transgressive “ravinement surface” within the first Jurensismergel Sequence, not a maximum regression surface at its basis. The second sequence extents from the erosive basis of the Dispansum Zone to the top of the Aalensis Subzone, with a maximum flooding surface at the Pseudoradiosa-Aalensis Zone boundary. Finally, the Opalinuston starts with a new sequence at the basis of the Torulosum Subzone. Transgressive system tracts of these 3rdorder T-R sequences are commonly phosphoritic, while some regressive system tracts show pyrite preservation of ammonites. The maximum regression surfaces at the basis of the Toarcian and within the Variabilis Zone reflect a significant submarine erosion and relief formation by seawater currents, while this effect is less pronounced at the basis of the Dispansum Zone and basis of the Torulosum Subzone (i.e., the boundary Jurensismergel-Opalinuston Formation).

Boosted microbial productivity during the Toarcian Oceanic Anoxic Event in the Paris Basin, France: new evidence from organic geochemistry and petrographic analysis

Abstract The Toarcian Oceanic Anoxic Event (T-OAE) is marked by major palaeoenvironmental and palaeoceanographical changes on a global scale, associated with a severe disturbance of the global carbon cycle and organic-rich facies deposition. Here, a multiproxy approach (petrographic and geochemical techniques) was applied to the study of the organic content of the T-OAE of the Paris Basin, whose phytoplanktonic origin has been previously inferred by its geochemical signature. The top of the tenuicostatum Zone is characterized by palynomorphs and marine phytoplankton-derived amorphous organic matter (AOM), representing a proximal marine environment with emplacement of euxinic conditions at the top (total organic carbon/sulfur content and increase in AOM). At the base of the serpentinum Zone the proliferation of bacterial biomass begins, with phytoplankton playing a secondary role. This indicates the development of stagnant and restrictive conditions in a proximal environment, with water column stratification (neohop-13(18)-ene). The majority of the serpentinum Zone is dominated by bacterial biomass, suggesting a marine environment with bottom-water stagnation, possibly related to basin palaeogeomorphology and circulation patterns, with episodic euxinia. This therefore suggests that the T-OAE organic fraction is dominated by bacterial biomass, not phytoplankton, showing the importance of an integrated approach to the determination of the organic facies.

Hardground, gap and thin black shale: spatial heterogeneity of arrested carbonate sedimentation during the Jenkyns Event (T-OAE) in a Tethyan pelagic Basin (Gerecse Mts, Hungary)

Abstract The Jenkyns Event or Toarcian Oceanic Anoxic Event was an episode of severe environmental perturbations reflected in carbon isotope and other geochemical anomalies. Although well studied in the epicontinental basins in NW Europe, its effects are less understood in open marine environments. Here we present new geochemical (carbon isotope, CaCO 3 , [Mn]) and nannofossil biostratigraphic data from the Tölgyhát and Kisgerecse sections in the Gerecse Hills (Hungary). These sections record pelagic carbonate sedimentation near the margin of the Tethys Ocean. A negative carbon isotope excursion of c. 6‰ is observed in the Tölgyhát section, in a condensed clay and black shale layer where the CaCO 3 content drops in association with the Jenkyns Event. At Kisgerecse, bio- and chemostratigraphic data suggest a gap in the lower Toarcian. The presence of an uppermost Pliensbachian hardground, the absence of the lowermost Toarcian Tenuicostatum ammonite zone and the condensed record of the Jenkyns Event at Tölgyhát, together with a condensed Tenuicostatum Zone and the missing negative carbon isotope anomaly at Kisgerecse, imply arrested carbonate sedimentation. A calcification crisis and sea-level rise together led to a decrease in carbonate production and terrigenous input, suggesting that volcanogenic CO 2 -driven global warming may have been their common cause.

Stratigraphy, palaeoenvironment and mollusks (gastropods and bivalves) of the Toarcian succession of the Central Unit of the South Riffian Ridges (northern Morocco)

In the Central Unit of the South Riffian Ridges, the Toarcian succession is represented by two thick terrigenous detrital formations, Sned and Lmachkha formations, each one is divided into two members. The lower member of the Sned Formation, often rich in belemnite rostra and ammonites of the Tenuicostatum Zone, consists of grey marlstones, with rare marly limestone intercalations. The upper member consists of silty marlstones, with frequent thin limestone beds and contains rare ammonites of the Serpentinus Zone. Its upper part is locally rich in terebratulids. The lower member of the Lmachkha Formation consists of a silty marlstone and silty limestone alternation, and is the richest in benthic macrofauna, composed of rhynchonellids, gastropods, common bivalves, and rare ammonites of the Bifrons Zone. Seven taxa of gastropods and six taxa of bivalves were recorded from this member and described for the first time from the study area. The upper member of the Lmachkha Formation consists of silty marlstone and silty limestone alternation, and contains rare bivalves and rare ammonites of late Toarcian age. The lithofacies and pelagic macrofauna of the lower member of the Sned Formation characterise a relative deep marine environment unfavourable to benthic macrofauna. The second member indicates a gradual decrease in sea-level allowing from its upper part the recurrence of benthic macrofauna, especially terebratulids. The lithofacies and macrofauna of the lower member of the Lmachkha Formation indicate a shallow terrigenous shelf environment. The second member, containing coral reefs in the western and southern peripheries of the Central Unit, characterises a still shallow marine environment. This significant but gradual decrease in sea-level of the Central Unit from the latest early Toarcian to late Toarcian is likely related to the acceleration of sea-floor spreading in the Central Atlantic Ocean.

Bouleiceras (Hildoceratidae, Ammonitina) from the lower Toarcian (Jurassic) of the Iberian Range (Spain): Taxonomy, stratigraphic distribution and insights on its dispersal

Bouleiceras is a very rare genus among the rich assemblages of ammonoids from the lower Toarcian of the Iberian Range. So far, only two dozen specimens have been recorded in numerous field campaigns carried out since 1965 by different authors. The interest of this taxon lies in its peculiar paleogeographical distribution in comparison with most other ammonoids of the same age. A review of these specimens is carried out, including those obtained in previous works and others recently collected in selected localities. Based mainly on the differences in the shape of the ventral section and the suture line, seven species have been identified; two of which are new: Bouleiceras ibericum nov. sp. and Bouleiceras? betetensis nov. sp. All the reviewed specimens are recorded in the Semicelatum Subzone of the Tenuicostatum Zone and the Elegantulum Subzone of the Serpentinum Zone from the Central Sector and the Levantine Sector of the Iberian Range. The global distribution of the genus is summarized from the known data, and its possible dispersal routes are analyzed, as well as the factors that could have conditioned them.

Kerogen assemblages and δ13CKerogen of the uppermost Pliensbachian–lower Toarcian succession of the Asturian Basin (northern Spain)

The Toarcian Oceanic Anoxic Event (TOAE) is characterised by a perturbation of the global carbon cycle and the widespread deposition of organic matter (OM)-rich strata/sediments in the northern areas of the West Tethys Shelf, in several northern European epicontinental basins. Contrariwise, in the southern areas of the West Tethys Shelf such as western and southern Iberian, and northern Gondwana margins, OM-rich strata/sediments are spatially and temporally restricted. To better understand the relationship between OM production, depositional conditions, and palaeoceanographic controls on the occurrence of OM-rich facies in the northern Iberian margin, we collected and analysed 18 samples from the Rodiles section (Asturian Basin) for geochemical (total organic carbon-TOC, total sulfur-TS, δ13CKerogen, and δ13CWood) and palynofacies analysis.The studied upper Pliensbachian–lower Toarcian succession has TOC values reaching up to 2.9 wt%. The relative dominance of non-opaque phytoclasts and the δ13CKerogen values (average of ~ −24‰) in the lowermost Toarcian (Tenuicostatum Zone) agrees with previous interpretations indicating that this area was located in a dominantly winter-wet climate zone. A change in the kerogen assemblages is observed at the Tenuicostatum/Serpentinum zones boundary (onset of TOAE), related to an increase in amorphous OM (AOM). A slight increase in non-opaque phytoclasts and terrestrial palynomorphs, and a negative δ13CKerogen excursion (reaching −31.5‰) is recorded in the same interval. These observations suggest a increase in continental weathering and fluvial runoff associated with more humid conditions, likely resulting in increased superficial productivity and deposition and preservation of OM under, at least, suboxic conditions. Above the TOAE interval, the δ13CKerogen record (values around −24‰) and the drastic drop in TOC and AOM content indicate that bottom water oxygenation improved after the TOAE.Our results demonstrate a link between early Toarcian climate dynamics and the deposition of OM-rich strata/sediments.

Upper Toarcian (Lower Jurassic) marine gastropods from the Cleveland Basin, England: systematics, palaeobiogeography and contribution to biotic recovery from the early Toarcian extinction event

AbstractHere we describe a new upper Toarcian (Lower Jurassic) marine gastropod fauna from rocks of the Cleveland Basin exposed on the North Yorkshire coast of England. The fossil assemblage consists of 16 species, of which three are new:Katosira?bicarinatasp. nov.,Turritelloidea stephenisp. nov. andStriactaenonina eleganssp. nov. Four species are described in open nomenclature asTricarilda? sp.,Jurildasp.,Cylindrobullinasp. andCossmanninasp. The other species have previously been described:Coelodiscus minutus(SchüblerinZieten),Procerithium quadrilineatum(Römer),Pseudokatosira undulata(Benzinvon Zieten),Palaeorissoinaaff.acuminata(Gründel),Pietteia unicarinata(Hudleston),Globulariacf.canina(Hudleston),Striactaeoninacf.richterorumSchulbert & Nützel,Striactaenoninaaff.tenuistriata(Hudleston) andSulcoactaeon sedgvici(Phillips). Most of these species are the earliest records of their respective genera and show palaeobiogeographical connections with contemporary gastropod associations from other regions of Europe and South America. The taxonomic composition of the upper Toarcian Cleveland Basin gastropod assemblage differs substantially from the faunas of the upper Pliensbachian and lower ToarcianTenuicostatumZone, showing the strong effect of the early Toarcian mass extinction event on the marine gastropod communities in the basin. Only a few gastropod species are shared between the late Toarcian faunas and the much more diverse Aalenian gastropod faunas in the Cleveland Basin, suggesting that there was a facies control on gastropod occurrences at that time. This is also a potential explanation for the taxonomic differences between the late Toarcian gastropod faunas in the Cleveland Basin and those in France, and northern and southern Germany.

Foraminiferal assemblages, extinctions and appearances associated with the Early Toarcian Oceanic Anoxic Event in the Llanbedr (Mochras Farm) Borehole, Cardigan Bay Basin, United Kingdom

The uppermost Pliensbachian-Lower Toarcian sediments recovered from Llanbedr (Mochras Farm) Borehole (North Wales, UK) were deposited in the Cardigan Bay Basin and mainly consist of alternating marls and marly limestones. The current study analyses the foraminiferal assemblages from the cored interval (143 m) that has been assigned to the Spinatum Zone (Upper Pliensbachian) to the lower part of the Bifrons Zone (Middle Toarcian). The foraminiferal assemblages are dominated by the suborders Lagenina and Robertinina. Four main ecostratigraphic intervals have been identified, based on changes in the foraminiferal assemblages that occurred before, during and after the Toarcian Oceanic Anoxic Event (T-OAE). Ecostratigraphic interval I (upper part of the Spinatum Zone) is characterized by a diverse assemblage with specialist, opportunist and intermediate forms. This assemblage is interpreted to represent a phase that pre-dates the biotic crisis. The ecostratigraphic interval II (Tenuicostatum Zone) shows an abrupt increase in opportunist forms such as the genus Reinholdella, and a facultative opportunist such as Paralingulina tenera, which was tolerant to oxygen depleted conditions. This second interval represents the initial biotic crisis phase, in response to decreasing oxygenation. The ecostratigraphic interval III is considered the peak of the biotic crisis, coincident with the negative carbon isotopic excursion (at the base of the Serpentinum Zone) and characterized by the dominance of the opportunist Reinholdella (87–97% of the assemblage) and low diversity. The ecostratigraphic interval IV (upper part of Serpentinum Zone and lower part of Bifrons Zone) is characterized by an increase of diversity and an abrupt decrease in Reinholdella. This assemblage is interpreted to reflect the increased availability of oxygen from deep-infaunal to epifaunal microhabitats and the return to normal conditions at the sea bottom. Ecostratigraphic fluctuations in the Mochras foraminiferal assemblages across the T-OAE event are comparable with those from other parts of the UK, the Lusitanian Basin (Portugal), the Umbria Marche Basin (Italy) and the Atlassic Basin (North Africa). In addition, a foraminiferal turnover is evident across the Pliensbachian-Toarcian boundary and the T-OAE, and reflects the effects of the Early Jurassic biotic crisis on benthic foraminifera. Controlling factors of the biotic crisis are discussed, regarding that anoxic conditions do not necessarily have a lethal effect on all foraminiferal species and other oceanographic and environmental factors need consideration (fragmentation of epicontinental platforms, sea-level changes, crisis of carbonate productivity and temperature changes).

Rostrum size differences between Toarcian belemnite battlefields

Abstract. Body size changes have been reported across crisis intervals. Belemnites – now considered extinct stem-decabrachians – have rarely been investigated for this purpose, and the few studies have resulted in ambiguous outcomes. Here we investigate two Toarcian belemnite accumulations in southern Germany from a morphometric point of view with the support of computed tomography data. The aim of this study is to test whether a difference in size can be observed between the rostra of the two studied samples, from individual lineage to community, and which proxy is more reliable. A significant decrease in median size from the Early Toarcian (Dactylioceras tenuicostatum Zone) to the Middle Toarcian (Haugia variabilis Zone) is recognized. This is observed at the community level of organization, considering the whole assemblage, but also within Passaloteuthis–Acrocoelites lineage, at the genus level. It is also demonstrated that diameter-based measurements or maximum preserved length are not reliable proxies for size, and therefore apical length or three-dimensional approximations, such as the geometric mean or the post-phragmocone volume, are more advisable. This is especially important when comparing specimens with markedly different rostrum shapes. Further studies are, however, still necessary to disentangle the mechanisms behind the reduction in rostrum size within the Toarcian and their putative environmental causes.

Isotope records (C-O-Sr) of late Pliensbachian-early Toarcian environmental perturbations in the westernmost Tethys (Majorca Island, Spain)

The late Pliensbachian–early Toarcian (Early Jurassic) was a time of major environmental changes that culminated with the Early Toarcian Oceanic Anoxic Event (T-OAE, ca. ~183 Ma). This period is marked by significant disturbances in the carbon cycle and rapid climatic changes. To improve the understanding of the expression of these events in westernmost Tethyan domains, this study provides new belemnite and bulk carbonate C and O stable isotope records of a ~5 Myr upper Pliensbachian to middle Toarcian marine succession of the Balearic Basin (Es Cosconar section, Majorca). Time resolution has been improved by combination of biostratigraphic (ammonoids and brachiopods) and geochronologic (87Sr/86Sr) methods. Seawater paleotemperatures derived from δ18O belemnite records reveal cooler paleotemperatures in the upper part of the Spinatum Zone. The uppermost Spinatum Zone is characterized by the onset of a warming event that crosses the Pliensbachian-Toarcian boundary, culminating with the warmer temperatures (up to ~10 °C of warming) for the Serpentinum Zone of the lower Toarcian. This warming event has been detected contemporaneously in many other European and Tethyan basins and is interpreted to represent generalized raised seawater temperatures linked to the T-OAE. Four significant δ13C events have been recorded in belemnite and bulk carbonate records. The first is a negative carbon isotope excursion (CIE) around the Pliensbachian-Toarcian boundary, which is best represented in the belemnite record. Soon after, the bulk‑carbonate record shows a positive shift in the lower Tenuicostatum Zone concomitant with a return to background values in the belemnite record, suggesting strong water stratification or decoupling probably related with export of neritic carbonate to the basin. The third is a negative CIE represented in bulk carbonate across the Tenuicostatum-Serpentinum zonal transition, which could be correlated with the negative excursion characterizing the onset of the T-OAE in other sections. The position of this excursion corresponds with a gap in the belemnite record. Finally, in the lower Toarcian, both the bulk carbonate and belemnite carbon isotope records show pronounced positive CIEs in the lower-middle part of the Serpentinum Zone. These CIEs testify the impact of the T-OAE in the Balearic basin.

New Saurorhynchus (Actinopterygii: Saurichthyidae) material from the Early Jurassic of Alberta, Canada

Saurichthyidae are a family of elongate, piscivorous actinopterygian fishes with a stratigraphic range extending from the late Permian to the Middle Jurassic. There are four recognized Early Jurassic species, all from Europe and all referred to the genus Saurorhynchus. Only a single non-European Jurassic occurrence has been reported, a partial Saurorhynchus skull from the Fernie Formation of western Alberta, Canada. Here, we evaluate the Saurorhynchus material from Canada, including a new skull from Ya Ha Tinda, Alberta, and a fragmentary mandible from Canyon Creek, Alberta. All diagnostic Canadian Saurorhynchus specimens are consistent with the European species Saurorhynchus acutus, on the basis of external narial morphology and reduced dermal ornamentation. Saurorhynchus acutus was present in the Toarcian of western North America from the tenuicostatum Zone to the late serpentinum – early bifrons Zone, similar to the stratigraphic range documented in Europe. The absence of divergence between eastern Panthalassan and western Tethyan saurichthyids is surprising, given that divergence has been well-documented in coeval invertebrates. Saurorhynchus may have had slower rates of morphological evolution than contemporaneous marine invertebrates; alternatively greater mobility, broader environmental tolerance, or both in these fish may have allowed ongoing gene flow between the two oceanic basins in the early Toarcian.

Lytoceratids (Cephalopoda, Ammonoidea) from the Lower Posidonienschiefer Formation (Tenuicostatum Zone, Early Jurassic) of Baden-Württemberg (south-western Germany)

Lytoceratids (Cephalopoda, Ammonoidea) from the Lower Posidonienschiefer Formation (Tenuicostatum Zone, Early Jurassic) of Baden-Württemberg (south-western Germany)

Brachiopod faunal exchange through an epioceanic-epicontinental transitional area from the Early Jurassic South Iberian platform system

The La Mola region (eastern External Betic Zone) can be regarded as one of the easternmost complete Jurassic successions of the Betic Cordillera in the Iberian Peninsula, but the paleogeographical setting of their outcrops remains widely discussed. Analysis of brachiopod assemblages from the Lower Jurassic improves the accuracy of previous paleogeographical data, enabling identification of a mainly epioceanic transitional area in which influences of epicontinental habitats are also detected. Assemblage 1, mainly with a Mediterranean affinity but also sharing several constituents with the African and Northeastern Iberian basins, typifies the Sinemurian–Pliensbachian transition. Assemblage 2, as a whole, shows a transitional character between epioceanic and epicontinental habitats; it is subdivided into two successive and interrelated sub-assemblages: Ass. 2a (Demonense–Tenuicostatum Zones) reveals a free connection with the epioceanic Subbetic area, whereas Ass. 2b (Uppermost Pliensbachian–Lower Toarcian) shows a closer relationship with epicontinental environments. Assemblage 3 (Uppermost Spinatum–basal Serpentinum Zones) is commonly recorded in the peri-Iberian epicontinental platform system integrated within the NW-European bioprovince, but it can also be regarded as a marginal assemblage that is widespread in the westernmost Tethyan margin prior to the Early Toarcian extinction event. Assemblage 2 constitutes a suitable index for assessing the paleobiogeographical affinity of the La Mola region, as Ass. 2a is progressively replaced by Ass. 2b, thus triggering the arrival of epicontinental taxa to the more intra-epioceanic Subbetic environments, inferring a possible connection through the La Mola transitional slope. Consequently, this region enabled a faunal mixing and exchange between both environments, and La Mola likely remained as an area that would facilitate migration and an effective dispersal seaway or, at least, did not constitute an ecological filter-barrier for brachiopods. Biostratigraphical data from brachiopods and ammonites are correlated for the first time in La Mola, refining and calibrating biochronostratigraphical gaps in the pre-Domerian deposits where biochronological markers are usually scarce, and around the Pliensbachian–Toarcian boundary, a crucial timespan in which the Early Toarcian extinction event took place.

LOWER TOARCIAN FORAMINIFERA OF THE SAN ANDRÉS SECTION (BASQUE-CANTABRIAN BASIN, SPAIN)

A study of 11 samples from the Pliensbachian-Toarcian transition in the San Andres Section yielded a total of 4756 specimens representing 50 taxa. The species are here described and the diversity and abundance trends of the foraminiferal assemblages are shown. Lagenina is the best-represented suborder. Lenticulina toarcense is the most abundant species and is present from the Spinatum Zone to the Bifrons Zone. Miliolina, Robertinina, Spirillinina and Textulariina are also recorded, but in smaller proportions. The succession of foraminiferal assemblages in the Upper Pliensbachian Lower Toarcian of the examined section shows a number of significant biostratigraphic events, including a step-wise extinction of typical long-ranging Lower Jurassic species such as Mesodentalina matutina, Ichthyolaria sulcata, Astacolus speciosus, Icizthyolaria intumescens, Prodentalina terquemi, Planularia pulchra, Planularia inaequistriata, Astacolus matutinus, Saracenella sublaevis and Marginulina spinata in the Tenuicostatum Zone. Stratigraphically short-ranging species, such as Eoguttulina palomerensis and Planularia obonensis are found in the Spinatum and Tenuicostatum zones. Finally, an important renewal and diversification event, including the first record of the genus Citharina and its species C. colliezi and C. clathrata, occurs in the Serpentinum Zone. Comparison of these assemblages with those identified in other European, North African and American basins reveals similar patterns.

Oceanic response to Pliensbachian and Toarcian magmatic events: Implications from an organic-rich basinal succession in the NW Tethys

The Bächental bituminous marls (Bächentaler Bitumenmergel) belonging to the Sachrang Member of the Lower Jurassic Middle Allgäu Formation were investigated using a multidisciplinary approach to determine environmental controls on the formation of organic-rich deposits in a semi-restricted basin of the NW Tethys during the Early Jurassic. The marls are subdivided into three units on the basis of mineralogical composition, source-rock parameters, redox conditions, salinity variations, and diagenetic processes. Redox proxies (e.g., pristane/phytane ratio; aryl isoprenoids; bioturbation; ternary plot of iron, total organic carbon, and sulphur) indicate varying suboxic to euxinic conditions during deposition of the Bächental section. Redox variations were mainly controlled by sea-level fluctuations with the tectonically complex bathymetry of the Bächental basin determining watermass exchange with the Tethys Ocean. Accordingly, strongest anoxia and highest total organic carbon content (up to 13%) occur in the middle part of the profile (upper tenuicostatum and lower falciferum zones), coincident with an increase in surface-water productivity during a period of relative sea-level lowstand that induced salinity stratification in a stagnant basin setting. This level corresponds to the time interval of the lower Toarcian oceanic anoxic event (T-OAE). However, the absence of the widely observed lower Toarcian negative carbon isotope excursion in the study section questions its unrestricted use as a global chemostratigraphic marker. Stratigraphic correlation of the thermally immature Bächental bituminous marls with the Posidonia Shale of SW Germany on the basis of C27/C29 sterane ratio profiles and ammonite data suggests that deposition of organic matter-rich sediments in isolated basins in the Alpine realm commenced earlier (late Pliensbachian margaritatus Zone) than in regionally proximal epicontinental seas (early Toarcian tenuicostatum Zone). The late Pliensbachian onset of reducing conditions in the Bächental basin coincided with an influx of volcaniclastic detritus that was possibly connected to complex rifting processes of the Alpine Tethys and with a globally observed eruption-induced extinction event. The level of maximum organic matter accumulation in the Bächental basin corresponds to the main eruptive phase of the Karoo-Ferrar large igneous province (LIP), confirming its massive impact on global climate and oceanic conditions during the Early Jurassic. The Bächental marl succession is thus a record of the complex interaction of global (i.e., LIP) and local (e.g., redox and salinity variations, basin morphology) factors that caused reducing conditions and organic matter enrichment in the Bächental basin. These developments resulted in highly inhomogeneous environmental conditions in semi-restricted basins of the NW Tethyan domain during late Pliensbachian and early Toarcian time.

Chronology of the Early Toarcian environmental crisis in the Lorraine Sub-Basin (NE Paris Basin)

Early Toarcian (Jurassic; ∼183 Ma) sediments recorded profound environmental changes, including mass extinction, global warming, marine transgression as well as widespread bottom water anoxia and organic matter accumulation on the Western Tethyan shelf. Enhanced organic matter accumulation was accompanied by a positive carbon isotope excursion (CIE) in pelagic carbonate, which marks the Toarcian Oceanic Anoxic Event. These environmental changes were accompanied by a major perturbation of the global carbon cycle, expressed by negative CIE, interrupting the positive trend. The duration of the carbon cycle perturbation is still debated, with estimates for the negative CIE range from ∼200 to ∼600 kyr. Here we present ultra high-resolution (<1 kyr) measurements of magnetic susceptibility and sediment color from a marine section located in the Lorraine Sub-Basin (NE Paris Basin) documenting Milankovitch-controlled fluctuations in depositional conditions that occurred superimposed onto the overall sea level evolution. Differences in the wavelength of the sedimentary cycles indicate variable sediment accumulation rates that mainly resulted from rapid sea level fluctuations. The most pronounced sea level rise that took place within the uppermost tenuicostatum zone resulted in a strong condensation of the basal Schistes Carton formation. Strong condensation can explain the discrepancy between durations previously calculated for the CIE placed at this stratigraphic interval. Our data support durations of ∼900 kyr and ∼600 kyr for the positive and negative CIE, respectively. The cyclostratigraphy-based timescale further proposes a duration of >555 kyr for the tenuicostatum zone and 1310 kyr for the serpentinum zone. The durations of the elegantulum and falciferum subzones can be estimated to ∼790 kyr and ∼520 kyr, respectively. A change in the orbital response from eccentricity- to obliquity-forcing, evident from other locations, is well-expressed in the Lorraine Sub-Basin and occurred within the CIE interval. The strong impact of the obliquity component in post-event deposits hints to processes most effective at high latitudes, such as the waxing and waning of polar ice. Paleogeographic features of the Western Tethyan shelf supported the tele-connection of higher to lower latitude processes via water exchange through the Viking Corridor.