Early Toarcian Anoxic Event Research Articles

Temperature-related body size change of marine benthic macroinvertebrates across the Early Toarcian Anoxic Event

The Toarcian Oceanic Anoxic Event (TOAE, Early Jurassic, ~182 Ma ago) was characterised by severe environmental perturbations which led to habitat degradation and extinction of marine species. Warming-induced anoxia is usually identified as main driver, but because marine life was also affected in oxygenated environments the role of raised temperature and its effects on marine life need to be addressed. Body size is a fundamental characteristic of organisms and is expected to decrease as a response to heat stress. We present quantitative size data of bivalves and brachiopods across the TOAE from oxygenated habitats in the Iberian Basin, integrated with geochemical proxy data (δ13C and δ18O), to investigate the relationship between changes in temperature and body size. We find a strong negative correlation between the mean shell size of bivalve communities and isotope-derived temperature estimates, suggesting heat stress as a main cause of body size reduction. While within-species size changes were minor, we identify changes in the abundance of differently sized species as the dominant mechanism of reduced community shell size during the TOAE. Brachiopods experienced a wholesale turnover across the early warming phase and were replaced by a virtually monotypic assemblage of a smaller-sized, opportunistic species.

Taphonomy and evolution of Lower Jurassic lithiotid bivalve accumulations in the Apennine Carbonate Platform (southern Italy)

Lower Jurassic Tethyan and Panthalassan marine shallow-water successions are characterized by aberrant lithiotid bivalves belonging to the Lithiotis Fauna. Their widespread occurrence, often in rock-forming abundance, represents a global biofacies, mostly restricted to the Pliensbachian–early Toarcian. Despite their wide occurrence and their prominent role as carbonate producers in shallow-water platforms, the biogeographic and stratigraphic distribution of this group of bivalves and their evolutionary history are obscure, mostly because they commonly have not been identified at the generic or specific level. In particular, their evolution and demise in relation to important global palaeoenvironmental perturbations, such as the Pliensbachian-Toarcian boundary event and the early Toarcian oceanic anoxic event are not yet known in detail.In the Apennine Carbonate Platform of southern Italy, the Lithiotis Member, in the upper part of the Lower Jurassic Palaeodasycladus Limestones Formation, is characterized by the abundant occurrence of lithiotid bivalves. They disappear abruptly in the lowermost beds of the overlying Oolitic-oncolitic Limestones Formation, at the onset of the early Toarcian Oceanic Anoxic Event. More than 60 lithiotid bivalve concentrations occur in a nearly 120m-thick succession spectacularly exposed on freshly cut walls in a quarry west of Mercato San Severino (Salerno). Field observations on the taxonomic composition and fabric of the shell beds (packing, maximum shell size, degree of shell articulation and fragmentation) allowed to distinguish four taphofacies (A–D). Taphofacies A records the appearance and spreading of the lithiotids, with accumulations characterized mainly by small-sized and loosely packed shells. Taphofacies B records the acme of lithiotid bivalves, with densely packed accumulations of large shells. These two taphofacies yield prevailing articulated individuals, commonly in life position. Taphofacies C records a decrease of the shell packing and frequency of articulated shells. However, it is not clear whether this represents the beginning of a prolonged crisis or just the local response to less favourable environmental conditions around a sequence boundary. Taphofacies D consists of three shell beds, one in the uppermost part of the Lithiotis Member and two within the lowermost part of the Oolitic-oncolitic Limestones Formation, in the stratigraphic interval characterized by the negative carbon isotope excursion of the early Toarcian OAE. The bivalve shells of these two beds consist exclusively of disarticulated and fragmented shells, possibly reworked from underlying levels. The demise of the lithiotids carbonate factory in the Apennine Carbonate Platform and the extinction of the largest aberrant bivalves of the Lithiotis Fauna at the onset of the early Toarcian anoxic event were probably due to the physiological stress imposed by ocean acidification and increased nutrient input.

Gastropods from upper Pliensbachian–Toarcian (Lower Jurassic) sediments of Causses Basin, southern France and their recovery after the early Toarcian anoxic event

AbstractA gastropod fauna has been studied from upper Pliensbachian – upper Toarcian deposits of two sections of the Causses Basin (southern France) in order to investigate the mode of recovery after the early Toarcian anoxic event. The fauna consists of 15 species, one of which is new (Bathrotomaria kronzwilmesorumsp. nov.). Their stratigraphical distribution shows two peaks of diversity – in the Bifrons Zone (Bifrons Subzone) and in the Aalensis Zone (Mactra Subzone) – which reflect brief times during which the oxygen content and bottom consistency favoured the settlement of a relatively diversified fauna. In the Variabilis–Pseudoradiosa zones, gastropods are only represented by two species. This probably indicates more severe and unstable environmental conditions, only allowing the survival of gastropod taxa with wide adaptive capacities. The very low species diversity and the discontinuous and slow faunal recovery were probably determined by physiographic factors. The Causses area was a small basin confined by exposed lands and open towards the central part of western Tethys. Gastropods described here occur exclusively in the Toarcian – early Aalenian communities of the European epicontinental seas, whereas species from the central region of western Tethys are absent. Geographic isolation and marginal location of the Causses Basin restricted faunal exchange with the western European epicontinental seas, preventing fast recovery after the anoxic event. Gastropods of the central region of the western Tethys were probably unable to settle and colonize that area due to the strongly different environment.

Early Jurassic anoxia triggered the evolution of the oldest holoplanktonic gastropod Coelodiscus minutus by means of heterochrony

The tiny gastropod Coelodiscus minutus is superabundant in concretions of the Early Jurassic Posidonia Shale of South Germany which were formed under anoxic or extremely dysoxic conditions. Previous suggestions that C. minutus was a holoplanktonic organism are corroborated based on new evidence from exceptionally well-preserved specimens. The measurements of shell thickness show that the shell of Coelodiscus is very thin (mean 11 µm). In contrast to previous suggestions, the shell of Coelodiscus was not formed in three ontogenetic phases (embryonic, larval and adult shell) but in two phases comprising an embryonic and a secondary shell, the latter forming during an extended larval phase. Hostile conditions on the sea floor, absence or extreme scarcity of epibenthic animals as well as the small size also argue against a benthic life style of this gastropod. Coelodiscus minutus is the oldest known holoplanktonic gastropod. We speculate that Coelodiscus evolved during the Early Jurassic from a benthic precursor, which had a planktotrophic larval development. Probably under the influence of increasing frequency of dysoxic episodes along with hostile benthic conditions, the larval phase was extended neotenously and eventually, a holoplanktonic species evolved. During the Early Toarcian anoxic event, C. minutus was highly abundant in the plankton and dead shells rained down to the anoxic or dysoxic sea bottom. These thin and fragile shells formed an ooze similar to the pteropod ooze in the modern deep sea. The shells were preserved due to the absence or low level of deposit feeding and bioturbation as well as the formation of early diagenetic concretions.

Pleurotomaria DeFrance, 1826 (Gastropoda, Mollusca) from the Lower Bajocian (Middle Jurassic) sediments of Luxembourg, with considerations on its systematics, evolution and palaeobiogeographical history

Abstract: Pleurotomaria species from lower Bajocian (Middle Jurassic) sediments of south‐western Luxembourg housed in the National Natural History Museum of Luxembourg are described. Seven species are recognized, one of which is new, Pleurotomaria faberi sp. nov. A more detailed definition of the diagnostic characters of the genus is proposed and the morphological continuity between Talantodiscus and Pleurotomaria is demonstrated, suggesting that the former cannot be considered as a distinct taxon. The palaeoecology, evolution and palaeobiogeographical history of Pleurotomaria are outlined. Pleurotomaria presumably first appeared in late Middle Triassic of New Zealand where it underwent a relative diversification up to the Hettangian (Early Jurassic). From early Hettangian, most of its evolutionary history took place in Europe and western Tethys. In the European epicontinental seas, Pleurotomaria experienced two important radiations. The first occurred in the Early Jurassic, with a peak in the late Pliensbachian, and was marked by an expansion of the distribution to the central part of western Tethys. After a collapse in species diversity, probably related to the early Toarcian anoxic event, a second radiation occurred. This culminated in the early Bajocian and was mainly confined in a region encompassing southern England, Paris Basin and southern Germany. Low‐spired species, formerly attributed to Talantodiscus, probably originated independently and iteratively during the history of Pleurotomaria. The facies and associated benthic faunas suggest that Pleurotomaria probably lived on shallow soft bottoms composed of mixed calcareous–siliciclastic sediments. The two main Early Jurassic and early Middle Jurassic radiations of the genus took place in these environments. Records of the genus in Jurassic carbonate platform deposits are very few and concern mainly post‐Bajocian species.

Diversity patterns of Early Jurassic brachiopod assemblages from the westernmost Tethys (Eastern Subbetic)

A detailed faunal succession based upon 6400 brachiopods from the Early Jurassic in the Eastern External Subbetic palaeogeographical domain (Betic Range, Spain) is characterized and analyzed. This study allows recognition of five assemblages with original biostratigraphical data in sediments where biostratigraphical markers were very scarce. It also enables a sound diversity dynamics pattern to be established among the brachiopod assemblages recorded. This pattern shows progressive diversification from the Sinemurian to the Late Pliensbachian, although with significant episodes. During the Sinemurian–Pliensbachian transition, the first initial pulses of the disintegration of the South Iberian palaeomargin played an important role in the brachiopod assemblages, as a notable faunal renewal event is detected (Assemblage 1 to Assemblage 2). Maximum biodiversity and abundance were reached in the Pliensbachian–Toarcian transition, due to the partial contemporaneity of Assemblages 2 and 3 and the settlement of taxa from nearby palaeogeographical basins. A decrease related to the Early Toarcian Anoxic Event (ETOAE) global effect is suggested, after which a slight recovery began, with colonization by opportunistic taxa (Soaresirhynchia bouchardi) but pre-ETOAE diversity and abundance were subsequently never again reached.A palaeogeographical analysis has been carried out to compare taxonomic diversity dynamics in the Eastern Subbetic with other representative Tethyan basins. This study also enables palaeobiogeographical affinities to be established for the Eastern Subbetic domain, showing Mediterranean palaeobioprovince influences until the Late Pliensbachian–Early Toarcian, a period when replacement occurred that shows faunal unification. Free connection between the Sub-boreal and Mediterranean provinces may be inferred from the Late Pliensbachian–Early Toarcian onwards.

Ecostratigraphy of benthic foraminifera for interpreting Arctic record of Early Toarcian biotic crisis (Northern Siberia, Russia)

The present contribution focusses on ecostratigraphic fluctuations of foraminiferal assemblages occurring during the benthic biotic crisis corresponding to the Early Toarcian anoxic event. We aim at understanding the response of benthic biota to the environmental changes that occurred during the crisis in a high latitude setting and compare with previous data from total organic carbon (TOC) and δ13C from previous works. The Kelimyar River section exposes a Lower Jurassic marine succession made up of siltstones, sandy siltstones (uppermost Pliensbachian) and black shales (Lower Toarcian) deposited close to the Early Jurassic North Pole in a relatively deep continental shelf. The Pliensbachian sediments are represented by a foraminiferal assemblage dominated by epifauna, mainly Trochammina, but showing a progressive diminution in diversity. This suggests an unfavourable microhabitat for infaunal forms, probably involving salinity fluctuations. Although the opportunist Trochammina tolerated salinity fluctuations, the decreasing proportions of aragonitic and calcitic hyaline foraminifera indicate the progressive deterioration of the benthic environment. A context of relative sea-level fall in the Arctic palaeobasin during the margaritatus Zone and the beginning of the viligaensis ammonite Zone related to an abrupt climatic cooling would explain these environmental changes (including salinity fluctuations) that lead to decreased diversity and even the disappearance of calcitic and aragonitic forms. The debut of the falciferum Zone (Lower Toarcian) is characterised by laminated black shales, high total organic carbon, a negative carbon isotopic excursion, and an abrupt decrease in the foraminiferal abundance and diversity. These features indicate oxygen-restricted conditions in the sea-bottom. Only opportunist Trochammina shows a rapid increase in abundance after the debut of the black shales, congruent with the relation of this genus with high TOC and a poorly oxygenated sediment water interface. Trochammina was able to colonise the sea-bottom after the development of hypoxic conditions in the Kelimyar River section. These adverse conditions occurred just after the negative carbon isotopic excursion and maximum values of TOC and correlate with the climatic warming that occurred in the Arctic palaeobasins referred to by previous authors. Detailed analysis of the sea-bottom recolonisation points to a foraminiferal replacement in which distinct foraminiferal morphogroups reach their optimum values. Finally, despite certain differences, the benthic foraminiferal assemblages from Kelimyar River section are comparable with those from the North African palaeomargin in the Ratnek El Kahla section (Algeria), thus illustrating an equivalent response of benthic foraminiferal assemblages to the Early Toarcian biotic crisis. In both cases the composition of assemblages, diversity and abundance of foraminifera were sensitive to restricted oxygen conditions, and opportunist species did colonize the sea bottom after this biotic crisis.

Modeling evidences for global warming, Arctic seawater freshening, and sluggish oceanic circulation during the Early Toarcian anoxic event

The paleoecological disturbances recorded during the Early Toarcian warming event (183 Myr ago), including marine anoxia, sea level rise, seawater acidification, carbonate production crisis, and species extinctions, are often regarded as past examples of Earth's possible responses to the rapid emergence of super greenhouse conditions. However, physical mechanisms explaining both the global and local expressions of paleoenvironmental events are still highly debated. Here we analyze the paleoclimatic and paleoceanographic consequences of increases in atmosphericpCO2levels at a multiscale resolution using a fully coupled ocean–atmosphere model (FOAM). We show that, in association with stronger high‐latitude precipitation rates and enhanced continental runoff, the demise of polar sea ice due to the global warming event involved a regional freshening of Arctic surface seawaters. These disturbances lead to progressive slowdowns of the global oceanic circulation accountable for widespread ocean stratification and bottom anoxia processes in deep oceanic settings and epicontinental basins. In agreement with very negative oxygen isotope values measured on fossil shells from the NW Tethys, our simulations also show that recurrent discharges of brackish and nutrient‐rich Arctic surface waters through the Viking Corridor could have led to both vertical and geographical gradients in salinity and seawaterδ18O in the NW Tethyan seas. Locally contrasted conditions in water mass density and rises in productivity rates due to strong nutrient supplies could partly explain the regional severity of the anoxic event in the restricted Euro‐boreal domains, as it has been previously suggested and modeled regionally.

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.

Dramatic decrease of pelagic carbonate production by nannoplankton across the Early Toarcian anoxic event (T-OAE)

In this account we present estimates of nannofossil fluxes in four sections and one borehole all belonging to the Early Jurassic western Tethys. This study aims to map the distribution of pelagic carbonate production across the Early Toarcian anoxic event (T-OAE), and to understand which environmental parameters did control such production. Our results indicate important changes in carbonate production by nannoplankton occurring within the western Tethys and its variations through time. Nannofossil fluxes (specimens per m 2 per year) are extremely low during the T-OAE in all the studied settings. Higher fluxes are encountered in the westernmost part of the Tethys Ocean before the T-OAE, whilst pelagic carbonate production shifted towards the northern margin of the Tethys after the recovery from anoxic conditions. The dramatic decrease in nannoplankton production during the T-OAE has been interpreted in previous works as a biocalcification crisis related to high pCO 2 in the atmosphere/hydrosphere system. Although a high pCO 2 may have lowered the carbonate saturation state of Early Jurassic oceans and finally hampered biocalcification, we speculate that the most important effects of CO 2 increase were indirect, and affected pelagic producers via changes on climate and sea-level. Namely, it seems that precipitation/evaporation budgets and continental runoff that controlled nutrient levels and salinity in surface oceanic waters were important factors for pelagic biocalcifiers.

Late Pliensbachian–Early Toarcian (Early Jurassic) environmental changes in an epicontinental basin of NW Europe (Causses area, central France): A micropaleontological and geochemical approach

We present an integrated work based on calcareous nannofossil and benthic foraminiferal assemblages, and geochemical analyses of two Upper Pliensbachian–Lower Toarcian sections located in the central-South France. The studied sections, Tournadous and Saint-Paul-des-Fonts, represent the proximal and the distal part, respectively, of the Jurassic Causses Basin, one of the small, partly enclosed basins belonging to the epicontinental shelf of the NW Tethys. At the transition from Late Pliensbachian to Early Toarcian, the Causses Basin recorded an emersion in response to the global sea-level fall. Our data indicate severe environmental conditions of marine waters, including salinity decrease and anoxia development, occurring in the Early Toarcian. The acme of this deterioration coincides with the Early Toarcian Anoxic Event (T-OAE) but, due to the restricted nature of the basin, anoxia persisted until the end of the Early Toarcian, mainly in the deeper parts of the basin. The micronutrients and organic organic-matter fluxes were probably high during the entire studied time interval, as shown by nannofossil and foraminiferal assemblages. However, nannoplankton production drastically decreased during the T-OAE, as demonstrated by very low nannofossil fluxes, and only taxa tolerant to low-saline surface waters could thrive. At the same time, benthic foraminifers temporarily disappeared in response to sea-bottom anoxia. Our study demonstrates that environmental changes related to the T-OAE are well-recorded even in small, partly enclosed basins of NW Europe, like the Causses Basin. Within this area, the effects of global changes, like sea sea-level and temperature fluctuations, are modulated by local conditions mainly controlled by the morphology of the basin.

Stable isotope records (δ18O and δ13C) of Lower-Middle Jurassic belemnites from the Western Balkan mountains (Bulgaria): Palaeoenvironmental application

Stable O and C isotope data of 110 Upper Pliensbachian-Lower Bajocian belemnites have been obtained and used to attempt a reconstruction of palaeotemperature and its variation in two epicontinental depositional environments from the Western Balkan mountains (Bulgaria). The samples were collected from 3 sections with high-resolution ammonite subdivision. Initially taphonomic, cathodoluminescence and geochemical analyses were used for evidence of diagenetic alteration. Non-luminescent parts of the belemnite rostra have been sampled for isotope analyses and 76 samples, having d 18 O � 0.5‰ (PDB), Fe 950 ppm and Sr/Mn ratio > 80 were used for palaeotemperature interpretations. The O and C isotope data generally exhibit little stratigraphical variability with minor fluctuations, however, there are prominent positive C isotope excursions and coeval negative O isotope shifts detected in the Lower Toarcian Tenuicostatum, Falciferum and Bifrons Zones. The O isotope data, interpreted in terms of palaeotemperature, revealed relatively high seawater temperatures during the Toarcian, Aalenian and Early Bajocian, with detectable temperature rises during the Early Toarcian (maximum value of 29.6 C). Both C isotope maxima and O isotope minima are used as evidence of the Early Toarcian anoxic event reported from many localities of the same age and in similar facies in Western Europe. In the study the latter is recognized as 3 episodes, which are closely related with the seawater temperature maxima. This isotope record pattern is considered as a consequence of a global Tethyan transgression during the Early Toarcian. 2008 Published by Elsevier Ltd.

Nannofacies analysis as a tool to reconstruct paleoenvironmental changes during the Early Toarcian anoxic event

The black shales of lower Toarcian (Schistes Carton in France, Jet Rock in England, Posidonia Shale in Germany) are sediments formed by fine alternations of millimetric to sub/millimetric clear and dark laminae. A study of polished core surfaces (Dotternhausen core, SW Germany) was carried out in SEM in order to characterize the micro‐fabrics of different laminae and their content in nannofossils. Two main intervals were recognized where fabrics and nannofossil content are significantly different. In the clay‐rich interval, characterized by high wt.%TOC and a large negative carbon isotope excursion (both in carbonates and organic matter) the dominant micro‐fabrics are well foliated or lumpy, giving evidence of the absence or reduction of benthic life. Nannofossils are scarce and only concentrated in discrete laminae separated by large, abiotic intervals. Assemblages are indicative of a deep nutricline, but sporadic nutrient inputs occurred, as demonstrated by fluctuation in abundance of specimens of the Biscutaceae. Nutrients were probably delivered to the German basin via riverine influx. Different sized framboids of pyrite were recognized in this interval, the smallest ones (∼ 2 μm) being probably precipitated in an anoxic and sulphidic water column. The clay‐rich interval was likely deposited during low sea level in a restricted basin. These conditions were followed by a sea‐level rise and better water‐mass circulation within the German basin. Sedimentation records an increase in carbonate deposition, organic matter content decreased, and carbon isotopes came back to pre‐excursion values. Micro‐fabrics became less markedly laminated, pyrite framboids nearly disappeared, and rich and diversified nannofossil assemblages are recorded. Sea‐water conditions were probably highly fluctuating, as indicated by alternation of intervals dominated in turn by Schizosphaerella spp., flourishing under generally oligotrophic conditions, where nutrients were temporarily available in surface waters due to storms, and by Crepidolithus spp., a deep‐dweller. The results of this work fit well with a silled‐basin model.

New, biostratigraphically significant ammonites from the Jurassic Fernie Formation, southern Canadian Rocky Mountains

Jurassic ammonites described here for the first time come from numerous localities in the Fernie Formation in the Foothills and Front Ranges of the Rocky Mountains of north- and south-eastern British Columbia and central- and south-western Alberta, and provide useful new age data for several members of the formation. The occurrence of Orthodactylites sp. in the uppermost beds of the Red Deer Member indicates this unit extends upwards into the basal Toarcian, which will be helpful in the search for carbon-isotope excursions associated with the early Toarcian anoxic event. The presence of Hettangian and lower Pliensbachian strata in outcrop of the Gordondale Member is confirmed. Diversity of the ammonite fauna in the Grey Beds is enlarged by the presence of ? Kepplerites – ? Cadomites and Choffatia spp. These faunas also provide biostratigraphic constraints for carbon- and oxygen-isotope trends and U–Pb ages recently recorded from the Highwood Member and Grey Beds exposed in Bighorn Creek and its eastern tributary, and in Fording River. The first occurrence of Zemistephanus richardsoni from cratonic rocks, recorded here, suggests close proximity of the allochthonous terranes Wrangellia and Stikinia, the only other areas from which this species is known, to the northwest North American craton by early Bajocian time. One new species, Cardioceras (Subvertebriceras) ferniensis, is described.

Biometric analysis of Pliensbachian-Toarcian (Lower Jurassic) coccoliths of the family Biscutaceae: intra- and interspecific variability versus palaeoenvironmental influence

Biometric analyses were applied to nine Lower Jurassic species of the genera Biscutum and Similiscutum in order to test whether these species can be typified by their morphological characters, and to evaluate the influence of the palaeoenvironment on coccolith morphology. This study was carried out on samples coming from different localities of the western Tethys, situated at different palaeolatitudes. The studied areas are: offshore Morocco (DSDP 547B), western Sicily, central Italy, and SW Germany. The analysed samples are of Pliensbachian and Toarcian ages. Length and width of the coccolith, and of its central area, were measured on digitally captured images of 1489 specimens, using a semi-automated method. Biometric analyses do not allow differentiation between all the considered species, with the exception of Biscutum grande, Biscutum dubium, and Biscutum intermedium that have peculiar morphological characters. Some other species form a cluster and cannot be significantly differentiated based on the measured parameters. These are: Similiscutum cruciulus, Similiscutum orbiculus and Similiscutum avitum, which likely represent different ecophenotypes of the same species. The biometric parameters analysed in this study (coccolith size, ellipticity, and central area dimensions) may be subject to palaeoenvironmental control. An increase in coccolith size may be related to high surface-water temperatures and/or to high nutrient concentration. An influence of critical conditions during the Early Toarcian anoxic event on coccolith size is speculated in the present work.

Phytoplankton evidence for the timing and correlation of palaeoceanographical changes during the early Toarcian oceanic anoxic event (Early Jurassic)

The causes and duration of the early Toarcian anoxic event are controversial. Integration of data from calcareous and organic phytoplankton provides a biochronological framework that allows precise correlations across the western Tethys. In particular, the Carinolithus superbus nannofossil Zone can be used to correlate the levels enriched in organic matter and the related δ 13 C negative excursion. Although a variable duration is given in the literature for this negative excursion, it is likely that it lasted between 520 and 650 ka. Increased atmospheric p CO 2 , related to excess volcanic emissions (magmatic activity in the Karoo and Ferrar Provinces), had an impact on climate and ocean chemistry, and marked the inception of a biotic crisis affecting many organisms. The beginning of the crisis within shallow carbonate platforms, documented at southern latitudes, predates the levels enriched in organic matter. Dinoflagellate cysts experienced a decrease in abundance in the C. superbus Zone, until they temporarily disappeared. The nannoplankton crisis was twofold: a decrease in size and low calcified specimens are observed in addition to a drastic decrease in absolute abundance. The increased atmospheric p CO 2 , as a result of the magmatic activity and temporarily amplified by transient methane release, could have been the trigger for the biocalcification crisis, which first affected the probably more reactive neritic system, and eventually the nannoplankton community.

Spatial and temporal distribution of calcareous nannofossils along a proximal–distal transect in the Lower Jurassic of the Umbria–Marche Basin (central Italy)

Three Upper Pliensbachian/Lower Toarcian successions of the Umbria–Marche Basin were analysed for sedimentology and calcareous nannofossil content across a 120-km-long transect from proximal to distal areas with respect to the shallow Latium–Abruzzi Platform. The uppermost part of the Corniola Unit (Uppermost Pliensbachian) and the Marne di Monte Serrone Formation (p.p., Lower Toarcian; Polymorphum to Levisoni Ammonite Zones) that contains organic matter-rich levels were studied in the three localities. The analysis of the sedimentary structures in the studied successions indicates that the basin was a shallow-water system, above storm-wave base, during the Late Pliensbachian/Early Toarcian. The stratigraphic evolution of the sedimentary structures recorded in the studied localities shows that the Upper Pliensbachian corresponded to a regressive sedimentary interval, whereas a complete transgressive–regressive relative sea-level cycle occurred during the Lower Toarcian Polymorphum Zone. A further transgression occurred in the lowermost Levisoni Zone (Lower Toarcian). Changes from carbonate-dominated to marl-dominated sediments form decimetre- to metre-scale lithological units, that can be correlated from succession to succession, and may have resulted from short-term sea-level and/or palaeoenvironmental variations. The mean duration of these lithological units suggests that they formed in tune with the first orbital eccentricity cycle (100 ka). The analysis of calcareous nannofossil quantities (abundance per mm 2 of the smear slide, percentage, mean abundance) shows both spatial and temporal changes in the community structure. Schizosphaerella spp. and Crepidolithus crassus were more abundant in proximity to the shallow Latium–Abruzzi Platform. These two taxa were likely affiliated to oligotrophic conditions in marine surface waters, explaining why they were generally more abundant in carbonate-rich intervals than in marls. Mitrolithus jansae is interpreted as being a deep-dwelling species, which was more abundant during rising sea level in the earliest Toarcian. Species of the genera Lotharingius, Biscutum and Calyculus were more abundant when higher trophic conditions occurred, and were thus more common during periods of marl deposition. The stratigraphic distribution of calcareous nannoplankton and its interpretation in terms of palaeoecological affinities allow three main phases in the environmental evolution during the Early Toarcian anoxic event to be recognised. The first phase corresponds to a long-term transgression, and high-nannoplankton abundance in distal areas (mainly Lotharingius, Biscutum and Calyculus species) suggests high productivity that may have resulted from upwelling. The beginning of accumulation and preservation of organic matter in the sediments occurred during this phase. The scarcity of nannoplankton during the second phase may indicate oligotrophic conditions in surface waters, possibly due to water stratification during the highest sea-level stand. A high abundance of Schizosphaerella spp. and C. crassus is recorded in proximal areas, probably resulting from sporadic nutrient input from continental or shallow-platform areas. The maximum of organic matter preservation is recorded during this phase. During the third phase, relative sea-level fall and efficient water mixing by frequent storm events led to enhanced nannoplankton productivity in proximal and distal areas and to a decrease in organic matter accumulation in the Umbria–Marche Basin.

Paleotemperature variations of Early Jurassic seawater recorded in geochemical trends of belemnites from the Basque–Cantabrian basin, northern Spain

Here we present new O-isotope, Mg/Ca and Sr/Ca data of Early Jurassic belemnites from the Basque–Cantabrian basin of northern Spain, a succession deposited in a periodically anoxic, restricted hemipelagic basin. Nearly 200 Pliensbachian–Lower Toarcian belemnites were collected and examined using optical, cathodoluminescence and chemical techniques to constrain the effects of potential diagenetic alteration. After a precise diagenetic screening of samples, 176 belemnites were selected to provide records of changes in seawater geochemistry and temperature. The records show a fairly good correlation of δ 18O with Mg/Ca ratios ( r=−0.65) and a weak correlation of δ 18O with Sr/Ca ratios ( r=0.34). The oxygen isotope data, if interpreted in terms of paleotemperature, have revealed a period of relatively uniform and warm seawater temperatures during the Early Pliensbachian, followed by a sharp temperature drop during the Late Pliensbachian–Earliest Toarcian. The onset of this cooler period occurred within the Protogrammoceras celebratum Subzone and the coolest seawater temperatures were reached at the end Pliensbachian Pleuroceras spinatum Zone. This was followed by a rapid seawater warming during the Harpoceras serpentinus Zone of the Early Toarcian. In addition, the data set provides new information regarding paleoceanographic conditions for anoxic events in a critical region of connection between the Boreal and Tethys realms. It is the background to relatively high δ 18O values that prevailed during the anoxic episodes. Concerning the impact of the Early Toarcian anoxic event in northern Spain, the detected δ 18O and Mg/Ca excursions reached their minimum and maximum, respectively, during the lower H. serpentinus Zone, while the anoxic black shale interval developed earlier, during the upper Dactylioceras tenuicostatum–lowermost H. serpentinus Zones.

Victims of the Early Toarcian anoxic event: the radiation and extinction of Jurassic Koninckinidae (Brachiopoda)

The significant mass extinction attributed to the Early Toarcian anoxic event had a severe impact on the phylum Brachiopoda. Beyond a serious decrease of species diversity, the extinction of the orders Spiriferidina and Athyridida is connected with this episode. The order Athyridida was represented by the family Koninckinidae in the Early Jurassic. The stratigraphical and geographical distribution of the three Early Jurassic koninckinid genera (Koninckella, Koninckodonta, Amphiclinodonta) shows a definite radiative pattern. The number of their nominal species increased from 2 to 17 from the Sinemurian to Early Toarcian; in the same time interval, their area increased from the Alpine region to the whole Mediterranean and the NW-European domains. This radiative evolution can be explained as the result of different factors: (1) morphological adaptation to muddy bottoms, (2) fundamental changes in the current pattern in the Tethys/Laurasian Seaway, and, possibly, (3) utilization of methane-based chemosynthesis as alternative food source. The radiation of koninckinids, leading from the cryptic habitats of the Tethyan rocky floors to the extensive muddy bottoms of the open European shelves, was abruptly terminated by the anoxic event in the Early Toarcian Falciferum Zone. The main causes of the extinction might be: (1) the excessive warming of Tethyan deep waters by thermohaline circulation, (2) the anoxic event, which was not survived by the spire-bearers, handicapped by their stiff, calcareous spiralia. Brachiopoda, Early Jurassic, Europe, extinction, Koninckinidae, radiation, Tethys.

Victims of the Early Toarcian anoxic event: the radiation and extinction of Jurassic Koninckinidae (Brachiopoda)

Victims of the Early Toarcian anoxic event: the radiation and extinction of Jurassic Koninckinidae (Brachiopoda)