Serpentinum 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).

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.

Early Toarcian (Jurassic) brachiopods from the Balearic Islands (Spain) and their paleobiogeographic context

The record of brachiopods in the Lower Toarcian of the Balearic Islands is described after a reassessment of previous works and new samplings in the Tramuntana Range of Mallorca. The recognized species have been assimilated to the assemblages described in the Iberian Range in the Tenuicostatum and Serpentinum biozones. Moreover, a detailed comparison with other Western Tethys basins and the study of the dispersion of the brachiopod faunas in relation with the Early Toarcian Oceanic Anoxic Event, allow to refine the paleobiogeographic relationship of the Balearic brachiopods. They belong to the Euro-Boreal Province of brachiopods in the Tenuicostatum Biochron and correspond to the”Spanish Fauna” that emerged after the deep paleobiogeographic disruption that took place at the base of Serpentinum Biochron. The occurrence of Prionorhynchia msougari Rousselle in the Serpentinum Zone of Mallorca indicates that, within this last assemblage, there would be a closer connection between the Balearic region and the southern margin of the Tethys (Betic and North African basins) than with the northern margin (Eastern Iberian Platform System and Eastern Pyrenees). This paleobiogeographic conclusion is consistent with the supposed position of the Balearic area in the Early Toarcian, on the southeastern margin of Iberia and near the Alboran and Kabylian margins.

The last representatives of the Superfamily Wellerelloidea (Brachiopoda, Rhynchonellida) in the westernmost Tethys (Iberian paleomargins) prior to their demise in the early Toarcian Mass Extinction Event

AbstractThe last clade-level extinction episode affecting the Phylum Brachiopoda has been long-established in the Early Toarcian Mass Extinction Event (ETMEE) around the Pliensbachian-Toarcian transition, when several rhynchonellide groups became extinct and others underwent a notable renewal in the western Tethys. Among them, Wellerelloidea is a long-ranging superfamily severely affected by this environmental crisis, embodying the subfamily Cirpinae as the last wellerelloids worldwide, prior to their global extinction in the Pb-To transition. The profuse record of Lower Jurassic cirpines in the peri-Iberian paleomargins provides an opportunity to clarify the taxonomy of wellerelloid species in the pre-extinction interval. A new species (Cirpa lucentina) is erected and the revision of the Cirpinae taxa around the ETMEE is carried out. Morphometric analysis and the study of internal structures of the shells support separation of the genera Cirpa and Salgirella, adding new supplementary diagnostic criteria. The biogeographic distribution of this clade in the western Tethys and its evolutionary history in the Early Jurassic reveal a pervasive colonization pattern of both epicontinental and epioceanic habitats. The Mediterranean origin of the last representatives of this group is ascertained, but while diversification of Salgirella took place in epioceanic habitats, speciation of Cirpa is unrelated to biochorema boundaries, even colonizing epicontinental seas until their extinction prior to the hyperwarming event that occurred in the basal Serpentinum Zone. A rhynchonellide morphogroup epitomized by cirpines was resilient to this event in the epicontinental seas. This morphogroup is also recorded after the extinction interval by means of the genus Pseudogibbirhynchia, thus postulating potential pre- and post-extinction phyletic relationships.UUID: http://zoobank.org/100b06c0-a803-4f6f-b0fa-2d9e2d6678d2

Effects of the Pliensbachian–Toarcian Boundary Event on Carbonate Productivity of a Tethyan Platform and Slope

AbstractWe explore the effects of the Pliensbachian–Toarcian Boundary Event (P–ToBE) on tropical carbonate productivity in the interior to margin and slope of the Venetian Platform (Northern Italy). We document the P–ToBE for the first time in the shallow‐water platform margin, and we bio‐ and chemostratigraphically tie it to transgressive/regressive cycles. Following the latest Pliensbachian sea‐level drop and emersion, transgressive grainstones at the platform edge record the P–ToBE negative carbon isotope excursion (CIE) of 1–1.5‰, also found in marl/limestone couplets on the slope. Recovery of platform productivity was ephemeral, as the platform drowned right after the peak negative CIE and was covered by deep‐sea thin‐bedded micritic limestones. The end of the P–ToBE correlates with a regression and renewed recovery of carbonate productivity. The negative CIE of the subsequent Toarcian Oceanic Anoxic Event is recorded in open‐sea cherty limestones both at the marginal and interior platform. These limestones document an even wider transgression and the renewed partial drowning of the platform in the Serpentinus Zone. We investigate the causes of the carbon perturbation at the P–ToBE, using a simple carbon cycle model. The duration and magnitude of the CIE suggest a rapid release of methane in driving the CIE, perhaps related to the preceding sea‐level drop and associated cryosphere perturbation, or to thermogenic alteration of coals near the Karoo‐Ferrar Large Igneous Province (LIP). The extent of the warming and the magnitude of the P–ToBE CIE implies a contribution of volcanogenic carbon dioxide from the Karoo‐Ferrar LIP.

The chemostratigraphy and environmental significance of the Marlstone and Junction Bed (Beacon Limestone, Toarcian, Lower Jurassic, Dorset, UK)

AbstractTwo fallen blocks of the Marlstone and stratigraphically overlying Junction Bed sampled on the beach below Doghouse Cliff in Dorset, UK (Wessex Basin) have been examined for carbon and oxygen isotopes of bulk carbonate as well as for strontium, carbon and oxygen isotopes and Mg:Ca ratios in the contained belemnites. The sequence, which contains most of the Toarcian zones and subzones within a metre or less of grey to yellow to pink, red and brown fossil-rich nodular limestone, is extremely condensed and lithologically similar to pelagic red limestones of the Tethyan Jurassic that are locally mineralized with Fe-Mn oxyhydroxides (e.g., Rosso Ammonitico). Strontium-isotope ratios of the contained belemnites are compatible with existing reference curves and both blocks show a rise to more radiogenic values post-dating the Pliensbachian–Toarcian boundary. The high degree of correlation between the relatively negative carbon and oxygen isotopes of the bulk carbonate is compatible with significant diagenetic overprint, and contrasts with higher carbon-isotope values in coeval condensed coccolith-rich limestones elsewhere. Evidence for the characteristic signature of the Toarcian Oceanic Anoxic Event, as represented by organic-rich sediment, is absent, possibly owing to a stratigraphic gap. Both blocks exhibit abrupt carbon-isotope shifts to lower values, one of which could represent the limbs of an incompletely recorded negative excursion associated with the Toarcian Oceanic Anoxic Event. That the Toarcian Oceanic Anoxic Event was also a significant hyperthermal is illustrated in both blocks by a drop in oxygen-isotope values and rise in Mg:Ca ratios of belemnites close to the base of the Junction Bed in the lowest part of theserpentinumzone.

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.

Microfossil assemblages and geochemistry for interpreting the incidence of the Jenkyns Event (early Toarcian) in the south-eastern Iberian Palaeomargin (External Subbetic, SE Spain)

Abstract. By studying the facies, geochemistry, and microfossil assemblages of the uppermost Pliensbachian and lower Toarcian of the Cueva del Agua section, I was able to appraise the impact of the Jenkyns Event in the eastern part of the South Iberian Palaeomargin (Western Tethys). Depleted oxygen conditions are envisaged for the Polymorphum–Serpentinum Zone boundary (lower Toarcian), represented by dark marls, precisely in a laminated interval. The decrease in the α diversity of foraminifera and ostracods, along with greater proportions of opportunists such as Lenticulina, Eoguttulina, and Cytherella just before the negative carbon isotopic excursion (CIE), would indicate a disturbance of the environmental conditions during the initial phase of the biotic crisis. The peak of the biotic crisis is characterized by an absence of trace fossils, increased organic matter content, an increased Mo∕Al ratio, and negative CIE and δ18O, as well as fewer specialist forms and more opportunists. This biotic crisis peak is related to oxygen-depleted conditions in the bottom waters and in the sediment pore water, while warming negatively affected microfauna – to the point of leaving a barren benthic horizon in the record. Recovery is evidenced by the occurrence of carbonate layers with hummocky cross-stratification and a decrease in organic matter content, the Mo∕Al ratio, and the trace fossil record. In terms of microfauna, the first phase of recovery shows highly abundant foraminifera, ostracods, and microgastropods, mainly opportunist forms. After the proliferation of opportunist forms, a second phase of recovery is marked by a progressive increase in α diversity.

Assessing anoxia, recovery and carbonate production setback in a hemipelagic Tethyan basin during the Toarcian Oceanic Anoxic Event (Western Carpathians)

Abstract The chemostratigraphic record of the Toarcian Oceanic Anoxic Event (T-OAE) is generally well documented in epicontinental basins; however, the duration of anoxia and recovery remains poorly constrained in the Tethys Ocean owing to limited stratigraphic resolution of oceanic sections with hiatuses. In this paper, we describe a relatively continuous hemipelagic record in the Central Western Carpathians of Slovakia (Skladana Skala section), which contains the Pliensbachian/Toarcian boundary, the T-OAE, and the subsequent recovery phase. In our section, the Pliensbachian Spinatum Zone is represented by a ~ 7 m thick alternation of spotted, spiculitic marly limestones and limestones of the Allgau Formation that contain moderately diverse trace fossils including abundant Zoophycos, Lamellaeichnus and Teichichnus. In the uppermost part of the Spinatum Zone, a ~ 0.8‰ decline in δ13Ccarb values marks the carbon isotope excursion (CIE) at the Pliensbachian/Toarcian boundary. This isotope anomaly is overlain by a ~ 3 m-thick interval of spotted marly limestones and marls, which contains a positive CIE that represents the base of the T-OAE. The T-OAE interval comprises a 60 cm-thick pyritic black shale interval that lacks bioturbation and shows a negative CIE (δ13Corg values drop by 4–5‰), coincident with an increase in TOC values (from 0.3% to 2–3%), and a drop in CaCO3 values (from 50 to 80% to 3–7%). The black shale is overlain by ~8 m thick marls with rare intercalations of marly limestones, which show ammonites of the Exaratum Subzone in the lowermost part (~1 m) succeeded by Serpentinum Zone beds. This unit exhibits a second positive CIE (δ13Corg increases by 2.5‰ and δ13Ccarb increases by 1.5‰), significantly more negative δ18O values relative to the pre-T-OAE interval, and a low diversity of trace fossils dominated by Chondrites and extremely thin Zoophycos. Sponge spicules re-appear in the uppermost 6.5 m-thick interval of the section formed by the alternation of spotted marls and limestones, with ammonites of the Falciferum Subzone in the lower part and of the Bifrons Zone in the uppermost parts; however, diversity of trace fossils remains low. Based on these findings, we argue that the 60 cm-thick pyritic black shale that represents the T-OAE is a strongly condensed succession because it includes the onset of the negative CIE and the subsequent rebound, and thereby encompasses the full duration of the anoxic event. Anoxia in basins open to the oceanic Tethys thus coincided not only with reduced carbonate production but also with reduced siliciclastic supply, generating very thin black shale deposits. In contrast to current-swept swells where black shales associated with hardgrounds are thin or missing owing to winnowing, carbonate starvation was induced by a calcification crisis coincident with the T-OAE and siliciclastic starvation was probably induced by rapid sea-level rise. Although bottom waters were not anoxic in the aftermath of T-OAE, bioturbation remained restricted during the Falciferum Subzone in basinal settings, indicating that environmental stress induced by limited ventilation and/or high bottom-water temperatures were persistent. The reduced diversity of trace fossils and the re-appearance of sponge spicules close to the lower/middle Toarcian boundary indicate that the ecological recovery in basins open to the oceanic Tethys was delayed, a pattern similar to the epicontinental seas that experienced anoxia.

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.

Orbital pacing and secular evolution of the Early Jurassic carbon cycle

Global perturbations to the Early Jurassic environment (∼201 to ∼174 Ma), notably during the Triassic-Jurassic transition and Toarcian Oceanic Anoxic Event, are well studied and largely associated with volcanogenic greenhouse gas emissions released by large igneous provinces. The long-term secular evolution, timing, and pacing of changes in the Early Jurassic carbon cycle that provide context for these events are thus far poorly understood due to a lack of continuous high-resolution δ13C data. Here we present a δ13CTOC record for the uppermost Rhaetian (Triassic) to Pliensbachian (Lower Jurassic), derived from a calcareous mudstone succession of the exceptionally expanded Llanbedr (Mochras Farm) borehole, Cardigan Bay Basin, Wales, United Kingdom. Combined with existing δ13CTOC data from the Toarcian, the compilation covers the entire Lower Jurassic. The dataset reproduces large-amplitude δ13CTOC excursions (>3‰) recognized elsewhere, at the Sinemurian-Pliensbachian transition and in the lower Toarcian serpentinum zone, as well as several previously identified medium-amplitude (∼0.5 to 2‰) shifts in the Hettangian to Pliensbachian interval. In addition, multiple hitherto undiscovered isotope shifts of comparable amplitude and stratigraphic extent are recorded, demonstrating that those similar features described earlier from stratigraphically more limited sections are nonunique in a long-term context. These shifts are identified as long-eccentricity (∼405-ky) orbital cycles. Orbital tuning of the δ13CTOC record provides the basis for an astrochronological duration estimate for the Pliensbachian and Sinemurian, giving implications for the duration of the Hettangian Stage. Overall the chemostratigraphy illustrates particular sensitivity of the marine carbon cycle to long-eccentricity orbital forcing.

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.

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).

Upper Pliensbachian-Lower Toarcian methane cold seeps interpreted from geochemical and mineralogical characteristics of celestine concretions (South Iberian palaeo-margin)

Abundant sulfate mineralizations are recorded in the Upper Pliensbachian-Lower Toarcian marly-limestones and marls of La Cerradura section (Betic Cordillera, South Spain). These mineralizations are celestine concretions (6–41 cm in diameter), sulfated wood coal remains and yellow-powder patches composed mainly of natrojarosite, native sulfur and gypsum. These deposits are mostly recorded in the base of Serpentinum Zone (Lower Toarcian) in the interval with the highest TOC values (0.4 wt%) and a negative carbon isotopic excursion, correlative with the Toarcian Oceanic Anoxic Event. The 87Sr/86Sr and δ18O values of celestine are within the values of the Early Toarcian seawater but δ34S values are anomalously high. The isotopic composition of these sulfates is congruent with their precipitation from fluids that have been submitted to variable degrees of microbial sulfate-reduction combined with anaerobic oxidation of methane, as reported from recent examples. The record of thin laminated crusts composed of celestine with calcite as the nucleus of concretions containing fossil filaments (observed under scanning electron microscopy) is compatible with the activity of sulfate-reducing bacteria. Therefore, celestine concretions have been related to methane cold seeps or high methane concentrations in the sediment. The source of the methane, which moved within the pore-space of marine sediment via diffusion or as bubbles, was the microbial methanogenesis of the organic-rich marls. The very high δ34S values indicate that celestine crystals formed in a semi-closed system below the sediment-water interface, which favored the efficient microbial sulfate-reduction that preferentially removed the light 32S isotope from pore-water. The celestine crystals grew from mixing of this 34S-enriched sulfate pool with reducing and Sr-rich seep fluids advecting with the methane hydrates. The precipitation of celestine in the dark marls of the Serpentinum Zone occurred in some cases over wood coal remains. Fossil chimneys related to the cold seeps have not been recorded in the studied outcrop indicating that the amount of methane hydrates reaching the sea-bottom was not enough for building this kind of structures. The growth of each celestine concretions ended when the source of methane, the organic matter, was spent and the activity of methanogenic microbes and sulfate-reducing bacteria decreased. New celestine concretions developed when new methane bubbles were channeled upwards in different layers.

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.

Early Jurassic carbon and oxygen isotope records and seawater temperature variations: Insights from marine carbonate and belemnite rostra (Pieniny Klippen Belt, Carpathians)

New carbon and oxygen isotope records and discussion of the main variations in seawater temperature through the Sinemurian–Aalenian of the Pieniny Klippen Basin (northern Tethys Ocean) are presented herein. Comparison of the recorded changes in stable-isotope compositions of bulk carbonate and belemnite rostra from an open-marine environment with previously documented, predominantly restricted epicontinental data enables determination of major climatic events that were most likely of worldwide extent. A slight positive δ13C shift is recorded in the lowermost Upper Pliensbachian. A significant positive excursion in carbonate carbon isotope values is documented in the Lower Toarcian Serpentinum Zone. Furthermore, the δ13C values display a falling trend in the Lower–Upper Toarcian and relatively constant values in the Aalenian. Temperatures inferred from the δ18O values of well-preserved belemnite rostra suggest rather cool seawater conditions (10–13 °C) in the Pieniny Klippen Basin during the Late Sinemurian, warming by 4 °C in the Early Pliensbachian and then cooling by 8 °C in the Late Pliensbachian. The seawater temperature rose once more in the Early Toarcian and began to fall again during the Middle Toarcian. In the Middle–?Late Aalenian, seawater temperatures oscillated between 10 and 13 °C.

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.

Evolution of the last koninckinids (Athyridida, Koninckinidae), a precursor signal of the early Toarcian mass extinction event in the Western Tethys

Koninckinids are a suitable group to shed light on the biotic crisis suffered by brachiopod fauna in the Early Jurassic. Koninckinid fauna recorded in the late Pliensbachian–early Toarcian from the easternmost Subbetic basin is analyzed and identified as a precursor signal for one of the most conspicuous mass extinction events of the Phylum Brachiopoda, a multi-phased interval with episodes of changing environmental conditions, whose onset can be detected from the Elisa–Mirabile subzones up to the early Toarcian extinction boundary in the lowermost Serpentinum Zone (T-OAE). The koninckinid fauna had a previously well-established migration pattern from the intra-Tethyan to the NW-European basins but a first phase with a progressive warming episode in the Pliensbachian–Toarcian transition triggered a koninckinid fauna exodus from the eastern/central Tethys toward the westernmost Mediterranean margins. A second stage shows an adaptive response to more adverse conditions in the westernmost Tethyan margins and finally, an escape and extinction phase is detected in the Atlantic areas from the mid-Polymorphum Zone onwards up to their global extinction in the lowermost Serpentinum Zone. This migration pattern is independent of the paleogeographic bioprovinciality and is unrelated to a facies-controlled pattern. The anoxic/suboxic environmental conditions should only be considered as a minor factor of partial control since well-oxygenated habitats are noted in the intra-Tethyan basins and this factor is noticeable only in the second westward migratory stage (with dwarf taxa and oligotypical assemblages). The analysis of cold-seep proxies in the Subbetic deposits suggests a radiation that is independent of methane releases in the Subbetic basin.

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.