Late Pliensbachian Research Articles

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.

Lower and Middle Jurassic Ammonoids of the Shemshak Group in Alborz, Iran and Their Palaeobiogeographical and Biostratigraphical Importance

The Shemshak Group at Shahmirzad (northern Iran) is characterized by the most frequent and extensive marine intercalations and contains the most abundant and diverse ammonite faunas hitherto known from the Lower and lower Middle Jurassic strata of the Alborz Range. So far, 62 ammonite taxa have been recorded from this area, including 25 taxa from earlier studies. The taxa belong to the families Cymbitidae, Echioceratidae, Amaltheidae, Dactylioceratidae, Hildoceratidae, Graphoceratidae, Hammatoceratidae, Erycitidae, and Stephanoceratidae with the new species Paradumortieria elmii and Pleydellia (P. ?) ruttneri. The fauna represents the Late Sinemurian, Late Pliensbachian, Toarcian, Aalenian, and Early Bajocian. Palaeobiogeographically, it is closely related to the Northwest European (Subboreal) Province, and exhibits only minor relations with the Mediterranean (Tethyan) Province.

Middle and Late Jurassic roots of brachyuran crabs: Palaeoenvironmental distribution during their early evolution

Knowledge of early, Mesozoic crabs is still rudimentary, mainly due to the poor preservational potential of crustacean decapods. Fossil evidence suggests that the family Prosopidae is ancestral to all other brachyurans, including Podotremata, with a very close phylogenetic relationship to Homolodromiidae, Dromiidae, Homolidae, Latreillidae, Dynomenidae, Xanthidae, Cyclodorippoidea and Calappoidea. Prosopidae is an extinct family, consisting mostly of Mesozoic species, almost exclusively known by their carapaces. This family appeared in the late Early Jurassic (Late Pliensbachian) and disappeared at the end of the Danian. The Early Jurassic (Pliensbachian) oldest crab species, Eocarcinus praecursor Withers, is transitional in many of its observable traits between the macruran Glypheoidea (Middle Triassic Pseudopemphix) and the early brachyuran prosopids, especially the earliest known species, Eoprosopon klugi Förster (Late Pliensbachian). Middle Jurassic prosopids lived in shallow-sea, soft bottom environments. The oldest known prosopid species lived on a silty sea floor, as did the first known crab (late Early Pliensbachian), and their presumed ancestors (Pemphicidae) were probably also shallow water organisms. Middle Jurassic prosopids lived both in shallow warm waters within organic buildups/shelly accumulations, and on silty sea-floors during the Bajocian/Bathonian, as illustrated by a new example from central Poland. Prosopids had an evolutionary climax during the Late Jurassic and were widely distributed in sponge-microbial (Oxfordian) buildups and coral reef (Kimmeridgian–Tithonian) environments of Europe ( Pithonoton, Coelopus, Longodromites, Prosopon, Nodoprosopon, Lecythocaris, Glaessneropsis). When biohermal and reef facies retreated at the end of the Jurassic, favourable conditions for prosopid crabs diminished and Cretaceous prosopids are rare and spatially dispersed. Their closely related descendants, the homolodromiids, preferentially inhabited soft muddy bottoms in deeper, colder waters, as is well documented by Cenozoic and Recent occurrences.

10.1007/s11506-008-1005-z

Stages in evolution of the Early Jurassic to Aalenian foraminifers and ostracodes are established based on the analyzed diversity dynamics of respective microfauna associations. Evolution of foraminifers is divided in two, the Hettangian-initial early Toarcian and the late early Toarcian-Aalenian stages, while the identical first stage in evolution of ostracodes has been followed by the late early Toarcian-Callovian stage. During the Jurassic, periodic migrations of foraminiferal and ostracod genera and species, which were interrelated with large transgressions and climatic changes, took place in the initial late and mid-late Pliensbachian, initial early Toarcian, and the late Toarcian-early Aalenian. Being isolated to the maximum extent in the second half of the late Aalenian, the Arctic basin lost connections with seas of northwestern Europe. The Early Jurassic to Aalenian biogeography of the Arctic basin is established based on the results of cluster analysis (group average link method, Jaccard coefficient, presence or absence of foraminifers and ostracodes genera) with due account for preceding and subsequent formation history of microbenthos structure in biochores. The distinguished biochores are ranked as realms and provinces of foraminifers and ostracodes. As is established, contours of the realms and provinces populated by different groups of microbenthos did not coincide and changed with time. Ecotones between the realms (e.g., the North Sea province) changed their localitization to be a part of the Arctic or Boreal Atlantic realms in different epochs. The Early-Middle Jurassic sedimentary successions of the Arctic basins reveal several levels of sharp taxonomic changes in composition of microbenthos under influence of the first-order abiotic factors.

Evidence for major environmental perturbation prior to and during the Toarcian (Early Jurassic) oceanic anoxic event from the Lusitanian Basin, Portugal

The timing and causal relationships between the pronounced negative C isotope excursion and paleoenvironmental perturbations associated with the Toarcian oceanic anoxic event (Early Jurassic) remain unclear, particularly because biotic crises and carbonate production decrease appear to have been initiated earlier than the main C isotope anomaly. Here we present a new quantification of Late Pliensbachian–Early Toarcian calcareous nannofossils abundance and size from the Peniche reference section (Portugal) together with O and C isotope records of well‐preserved brachiopod shells from the same section. The brachiopod shell δ13C curve parallels that of bulk carbonate and records two pronounced negative isotopic excursions, close to the Pliensbachian‐Toarcian boundary (∼−2‰) and during the Toarcian oceanic anoxic event (∼−3.5‰). Our results indicate that both C isotope negative excursions were characteristic of benthic and shallow‐water environments, suggesting that these two carbon cycle perturbations affected all epioceanic reservoirs. Coeval shifts toward lower values of brachiopod oxygen isotope compositions and closely correlated northward migrations of Mediterranean ammonite fauna suggest that both events coincided with major rises in seawater temperatures, probably as a result of increased CO2 levels and enhanced greenhouse conditions. CO2‐induced changes in seawater chemistry likely affected the calcification potential of both neritic and pelagic systems, as evidenced by synchronous drops of platform‐derived carbonate accumulation and drastic reductions in size (>3 μm) of the main pelagic carbonate producer Schizosphaerella. We suggest that the Early Toarcian paleoenvironmental crisis occurred in two distinct episodes that were most likely related to two successive phases of intense volcanic degassing in the Karoo‐Ferrar province.

Sequence stratigraphy and inferred relative sea-level change from the onshore British Jurassic

Sequence stratigraphy — the subdivision of successions into fades packages bounded by surfaces representing major shifts in depositional environment — has provided a systematic methodology for inference of relative sea-level change from vertical facies successions. In the present review, sequence stratigraphic work on exposures of British Jurassic successions is summarized. Resultant inferred relative sea-level curves for Early Jurassic and early Mid Jurassic successions show some strong similarities at the scale of ammonite zones between widely separated basins, implying sea-level change of at least regional extent or, alternatively, regionally co-ordinated changes in sediment supply. Also well developed are coeval stage-level influxes of sandy sediments into widely separated marine basins during the Late Pliensbachian, Late Toarcian—Aalenian and Oxfordian. Similarly, widespread synchronous shifts to more offshore facies are evident in the Early Pliensbachian, Early Toarcian, Early Callovian and Early Kimmeridgian.

Pliensbachian radiolarians in Teltschengraben (Northern Calcareous Alps, Austria): a keystone in reconstructing the Early Jurassic evolution of the Tethys

In the "Hallstatt Mélange" in northwest Bad Mitterndorf, a slide of earliest late Pliensbachian marly radiolarites occurs in an upper-Middle to lower-Upper Jurassic succession. The succession consists of radiolarites, cherty limestones and marls, dated by radiolarians as upper-Middle Jurassic (Callovian) and different slides of Triassic to early Jurassic age derived from the Hallstatt facies zone. The facies and lithology (mainly cherty sediments) of the Pliensbachian slide are nearly identical to those of the succession, but it belongs to the Lower Jurassic Dürrnberg Formation of the outer-shelf area of the Northern Calcareous Alps (Hallstatt Zone), which was paleogeographically situated in the north-western rim of the Neotethys Ocean. The radiolarian fauna has low diversity but its good preservation allows an accurate age determination. The dating of the cherty slide as earliest late Pliensbachian is of great importance because: 1) it is the first record of sediments younger than Sinemurian in the Hallstatt Zone of the Northern Calcareous Alps; 2) it is the first record of Pliensbachian radiolarians in the European Alpine area; 3) it confirms that the northwestern passive margin of the Neotethys Ocean persisted in this region at least until the Pliensbachian. Our paleontological and stratigraphic data prove that the closure of the Neotethys Ocean in this region is younger than earliest late Pliensbachian, but older than Callovian. The complete radiolarian fauna is illustrated and described. Fourteen taxa are identified to species level, from which one new genus Paradroltus and two new species Paradroltus mitterndorfensis and Lantus sutnal are formally described.

Diversity dynamics of Early Jurassic ostracods of the Cordillera Ibérica (Spain) and the re-evaluation of the Pliensbachian–Toarcian mass extinction

The extinction and recovery of Ostracoda at the Pliensbachian–Toarcian (P–T) boundary are analyzed based on a database of taxonomically revised Pliensbachian to Toarcian transition ostracod assemblages. In contrast to earlier assertions, the results of this study indicate that ostracod extinction rates were significant in comparison with other marine invertebrates. An extinction rate of 54% has been calculated for upper Pliensbachian ostracod species occurring in more than one section. Diversification took place in the latest Pliensbachian (Spinatum Zone) and early Toarcian (Tenuicostatum Zone), whereas diversity decrease occurred in the middle early Toarcian (Strangewaysi Subzone, Serpentinus Zone). This notable diversity decline in the early Toarcian corresponds to a global mass extinction time, whose peak has been documented in the Tenuicostatum Zone. Meanwhile, the ostracod mass extinction occurred within the Serpentinus Zone and was followed by radiation and recovery in the succeeding Bifrons Zone. Similar diversity changes of ostracods are observed in other European areas, although in the Cordillera Ibérica, the demise began later. Many aspects of this event are still debated, and there is no common cause or single set of climatic or environmental changes common to this event. The supposed extinction-causing environmental changes resulting from anoxia episodes are unclear and are unlikely to have been of sufficient intensity or geographic extent to cause this global extinction. In this paper, the decrease in marine species diversity is explained by a new palaeoceanographic scenario, in which a rapid global cooling episode is regarded as the ultimate cause.

Stratigraphy and Palaeobiogeography of Mesozoic Benthic Foraminifera - Part 3

The Adriatic Carbonate Platform (AdCP), was a separate shallowmarine depositional system characterized by a lack of terrigenous input and was connected to Gondwana towards the South via Gavrovo–Tripolitza or Apulia. It existed for approximately 120 MY, from the Early Jurassic Pliensbachian/Toarcian) to the end of the Cretaceous, resulting in a 4000–6500 m thick succession of almost pure carbonates. However, this is part of a thicker (>8000 m) sequence of predominantly carbonate rocks which forms the Karst Dinarides, and was deposited during more than 270 MY – at least from the Carboniferous (Moscovian) to the Late Eocene. Among many different groups of fossil organisms, benthic foraminifera are especially abundant and well preserved, so they, along with calcareous algae (Dasycladales), are the most important fossils used for age determination and stratigraphic subdivision of shallow-marine carbonate deposits. Within the 257 determined taxa belonging to different foraminiferal families which lived through the Mesozoic, numerous different index fossils occur in assemblages indicating various ages: Early Triassic, Anisian, Carnian, Norian–Rhaetian, Late Sinemurian, Early and Late Pliensbachian (Carixian and Domerian), Early and Late Aalenian, Early and Late Bajocian, Early and Late Bathonian, Callovian, Early and Late Oxfordian, Kimmeridgian, Tithonian, Berriasian, Valanginian, Late Hauterivian, Late Barremian, Early and Late Aptian (Bedulian and Gargasian), Early and Late Albian, Early, Middle and Late Cenomanian, Turonian, Coniacian, Santonian, Early and Late Campanian and Early and Late Maastrichtian. A total of 64 biostratigraphic units – biozones of different categories, from subzone to superzone, were defined within the stratigraphic interval from the Carnian to the Late Maastrichtian. This enabled very detailed biostratigraphic subdivision of the carbonate deposits within the Karst Dinarides. This is one of the most precise sequences, not only in this area, but also among former shallow marine deposits of the entire Neotethyan realm in the present Mediterranean region. The palaeobiogeographic characteristics of biotopes and the composition of foraminiferal assemblages during the Mesozoic were controlled by the position of the study area within the Neotethyan bioprovinces. Until the Albian, this area represented part of the Southern Neotethyan bioprovince, while from the Cenomanian to its final disintegration at the end of the Cretaceous it belonged to a separate, Central Mediterranean Neotethyan bioprovince.

Stratigraphy and Palaeobiogeography of Mesozoic Benthic Foraminifera - Part 2

The Adriatic Carbonate Platform (AdCP), was a separate shallowmarine depositional system characterized by a lack of terrigenous input and was connected to Gondwana towards the South via Gavrovo–Tripolitza or Apulia. It existed for approximately 120 MY, from the Early Jurassic Pliensbachian/Toarcian) to the end of the Cretaceous, resulting in a 4000–6500 m thick succession of almost pure carbonates. However, this is part of a thicker (>8000 m) sequence of predominantly carbonate rocks which forms the Karst Dinarides, and was deposited during more than 270 MY – at least from the Carboniferous (Moscovian) to the Late Eocene. Among many different groups of fossil organisms, benthic foraminifera are especially abundant and well preserved, so they, along with calcareous algae (Dasycladales), are the most important fossils used for age determination and stratigraphic subdivision of shallow-marine carbonate deposits. Within the 257 determined taxa belonging to different foraminiferal families which lived through the Mesozoic, numerous different index fossils occur in assemblages indicating various ages: Early Triassic, Anisian, Carnian, Norian–Rhaetian, Late Sinemurian, Early and Late Pliensbachian (Carixian and Domerian), Early and Late Aalenian, Early and Late Bajocian, Early and Late Bathonian, Callovian, Early and Late Oxfordian, Kimmeridgian, Tithonian, Berriasian, Valanginian, Late Hauterivian, Late Barremian, Early and Late Aptian (Bedulian and Gargasian), Early and Late Albian, Early, Middle and Late Cenomanian, Turonian, Coniacian, Santonian, Early and Late Campanian and Early and Late Maastrichtian. A total of 64 biostratigraphic units – biozones of different categories, from subzone to superzone, were defined within the stratigraphic interval from the Carnian to the Late Maastrichtian. This enabled very detailed biostratigraphic subdivision of the carbonate deposits within the Karst Dinarides. This is one of the most precise sequences, not only in this area, but also among former shallow marine deposits of the entire Neotethyan realm in the present Mediterranean region. The palaeobiogeographic characteristics of biotopes and the composition of foraminiferal assemblages during the Mesozoic were controlled by the position of the study area within the Neotethyan bioprovinces. Until the Albian, this area represented part of the Southern Neotethyan bioprovince, while from the Cenomanian to its final disintegration at the end of the Cretaceous it belonged to a separate, Central Mediterranean Neotethyan bioprovince.

Stratigraphy and Palaeobiogeography of Mesozoic Benthic Foraminifera - Part 1

The Adriatic Carbonate Platform (AdCP), was a separate shallowmarine depositional system characterized by a lack of terrigenous input and was connected to Gondwana towards the South via Gavrovo–Tripolitza or Apulia. It existed for approximately 120 MY, from the Early Jurassic Pliensbachian/Toarcian) to the end of the Cretaceous, resulting in a 4000–6500 m thick succession of almost pure carbonates. However, this is part of a thicker (>8000 m) sequence of predominantly carbonate rocks which forms the Karst Dinarides, and was deposited during more than 270 MY – at least from the Carboniferous (Moscovian) to the Late Eocene. Among many different groups of fossil organisms, benthic foraminifera are especially abundant and well preserved, so they, along with calcareous algae (Dasycladales), are the most important fossils used for age determination and stratigraphic subdivision of shallow-marine carbonate deposits. Within the 257 determined taxa belonging to different foraminiferal families which lived through the Mesozoic, numerous different index fossils occur in assemblages indicating various ages: Early Triassic, Anisian, Carnian, Norian–Rhaetian, Late Sinemurian, Early and Late Pliensbachian (Carixian and Domerian), Early and Late Aalenian, Early and Late Bajocian, Early and Late Bathonian, Callovian, Early and Late Oxfordian, Kimmeridgian, Tithonian, Berriasian, Valanginian, Late Hauterivian, Late Barremian, Early and Late Aptian (Bedulian and Gargasian), Early and Late Albian, Early, Middle and Late Cenomanian, Turonian, Coniacian, Santonian, Early and Late Campanian and Early and Late Maastrichtian. A total of 64 biostratigraphic units – biozones of different categories, from subzone to superzone, were defined within the stratigraphic interval from the Carnian to the Late Maastrichtian. This enabled very detailed biostratigraphic subdivision of the carbonate deposits within the Karst Dinarides. This is one of the most precise sequences, not only in this area, but also among former shallow marine deposits of the entire Neotethyan realm in the present Mediterranean region. The palaeobiogeographic characteristics of biotopes and the composition of foraminiferal assemblages during the Mesozoic were controlled by the position of the study area within the Neotethyan bioprovinces. Until the Albian, this area represented part of the Southern Neotethyan bioprovince, while from the Cenomanian to its final disintegration at the end of the Cretaceous it belonged to a separate, Central Mediterranean Neotethyan bioprovince.

Interaction of tectonics, eustasy, climate and carbonate production on the sedimentary evolution of an early/middle Jurassic extensional basin (Southern Provence Sub‐basin, SE France)

ABSTRACTThis paper describes the evolution of an extensional basin in regard to the nature and sequence stratigraphic arrangement of its carbonate deposits. The purpose of this study is to evaluate the respective effects of tectonism, eustasy, climate and oceanography on a carbonate sedimentary record. The case study is the early to mid‐Jurassic age carbonate succession of the Southern Provence Sub‐basin (SE France), located within the southern part of the extensional Western European Tethyan Margin. This work is based on sedimentologic, biostratigraphic (using ammonites and brachiopods) and sequence stratigraphic analysis of the carbonate facies of the Cherty Reddish Limestone Formation (late Sinemurian to earliest Bajocian). These strata were deposited in shoreface to lower offshore depositional environments. The succession of the various environments together with the recognition of key stratigraphic surfaces allow us to define four second‐order depositional sequences; of late Sinemurian to earliest Pliensbachian, early Pliensbachian to late Pliensbachian, earliest Toarcian to middle Aalenian and late Aalenian to early Bathonian ages. The architecture of the depositional sequences (thickness and facies variations within the systems tracts, wedge‐shaped geometries) reflects a strong tectonic control. The sub‐basin was structured by extensional faults (oriented approximately 070–090/250–270). Sea‐level variations, fluctuations in carbonate production and preservation, and environmental changes were also significant controlling factors of the carbonate deposition. The interplay of the tectonic control with the other factors resulted in five main phases in the sedimentary evolution of the sub‐basin: (1) dominant tectonic control during the initial rifting stage (late Sinemurian to early Pliensbachian); (2) increasing extensional tectonics (mid‐Pliensbachian); (3) global climato‐eustatic sea‐level fall (latest Pliensbachian) and global climato‐eustatic sea‐level rise plus hypoxia/anoxia (early Toarcian); (4) relative sea‐level fall linked to tectonic uplift related to the ‘Mid‐Cimmerian phase’ (mid‐Aalenian) and (5) oceanographic events (upwelling) and reduction in carbonate production (hypoxia/anoxia) plus tectonic downwarping (late Aalenian/earliest Bajocian).

Evidence for the longest stratigraphic range of a post-Triassic Ichthyosaur: a Leptonectes tenuirostris from the Pliensbachian (Lower Jurassic) of Switzerland

A three-dimensional well-preserved ichthyosaur skull and parts of the postcranial skeleton are attributed to the species Leptonectes tenuirostris (Conybeare, 1822). It was found vertically embedded in Pliensbachian deposits representing three successive biozones ( ibex to margaritatus Zone). The find is dated as early Late Pliensbachian ( margaritatus Zone) by a rich ammonite and ostracod fauna. It is the first record of the genus Leptonectes from Switzerland and from the Late Pliensbachian. It is so far the best preserved and most complete ichthyosaur from this time interval worldwide. With diagnostic specimens known from the Rhaetian (Late Keuper) up to the early Late Pliensbachian (Middle Liassic), L. tenuirostris (Conybeare, 1822) has the most extensive stratigraphic range documented for any post-Triassic ichthyosaur so far.

The Early Toarcian anoxia, a synchronous event in the Western Tethys? An approach by quantitative biochronology (Unitary Associations), applied on calcareous nannofossils

During the Early Toarcian, major paleoenvironnemental and paleoceanographical changes occurred, leading to an oceanic anoxic event (OAE) and to a perturbation of the carbon isotope cycle. Although the standard biochronology of the Lower Jurassic is essentially based upon ammonites, in recent years biostratigraphy based on calcareous nannofossils and dinoflagellate cysts is increasingly used to date Jurassic rocks. However, the precise dating and correlation of the Early Toarcian OAE, and of the associated δ 13C anomaly in different settings of the western Tethys, are still partly problematic, and it is still unclear whether these events are synchronous or not. In order to allow more accurate correlations of the organic rich levels recorded in the Lower Toarcian OAE, this account proposes a new biozonation based on a quantitative biochronology approach, the Unitary Associations (UA), applied to calcareous nannofossils. This study represents the first attempt to apply the UA method to Jurassic nannofossils. The study incorporates eighteen sections distributed across western Tethys and ranging from the Pliensbachian to Aalenian, comprising 1220 samples and 72 calcareous nannofossil taxa. The BioGraph [Savary, J., Guex, J., 1999. Discrete biochronological scales and unitary associations: description of the Biograph Computer program. Mémoires de Géologie de Lausanne 34, 282pp] and UA-Graph (Copyright Hammer Ø., Guex and Savary, 2002) softwares provide a discrete biochronological framework based upon multi-taxa concurrent range zones in the different sections. The optimized dataset generates nine UAs using the co-occurrences of 56 taxa. These UAs are grouped into six Unitary Association Zones (UA-Z), which constitute a robust biostratigraphic synthesis of all the observed or deduced biostratigraphic relationships between the analysed taxa. The UA zonation proposed here is compared to “classic” calcareous nannofossil biozonations, which are commonly used for the southern and the northern sides of Tethys. The biostratigraphic resolution of the UA-Zones varies from one nannofossil subzone or part of it to several subzones, and can be related to the pattern of calcareous nannoplankton originations and extinctions during the studied time interval. The Late Pliensbachian – Early Toarcian interval (corresponding to the UA-Z II) represents a major step in the Jurassic nannoplankton radiation. The recognized UA-Zones are also compared to the carbon isotopic negative excursion and TOC maximum in five sections of central Italy, Germany and England, with the aim of providing a more reliable correlation tool for the Early Toarcian OAE, and of the associated isotopic anomaly, between the southern and northern part of western Tethys. The results of this work show that the TOC maximum and δ 13C negative excursion correspond to the upper part of the UA-Z II (i.e., UA 3) in the sections analysed. This suggests that the Early Toarcian OAE was a synchronous event within the western Tethys.

Early Jurassic climate change and the radiation of organic-walled phytoplankton in the Tethys Ocean

During the Early Jurassic, cyst-forming dinoflagellates began a long-term radiation that would portend ecological importance of these taxa in the pelagic plankton community throughout the rest of the Mesozoic era. The factors that contributed to the evolutionary success of dinofla- gellates are poorly understood. Here we examine the relationship between oceanographic and cli- matic conditions during the Hettangian-Toarcian interval in relation to the radiation of dinofla- gellates and other organic-walled phytoplankton taxa in the Tethys Ocean. Our analysis is based on two data sets. The first includes d 13Ccarb, d13Corg, total organic carbon (TOC), and quantitative palynological observations derived from the Mochras Core (Wales, U.K.), which spans the complete Early Jurassic. The second is a coupled Mg/Ca and d 18O record derived from analyses of belemnite calcite obtained from three sections in northern Spain, covering the upper Sinemurian to Toarcian. From these two data sets we reconstructed the influence of sea level, trophism, temperature, and salinity on dinoflagellate cyst abundance and diversity in northwest Europe. Our results suggest that organic-walled phytoplankton (acritarchs, prasinophytes, and dinoflagellates) diversity in- creased through the Early Jurassic. The radiation coincides with a long-term eustatic rise and over- all increase in the areal extent of continental shelves, a factor critical to cyst germination. On shorter timescales, we observed short bursts of dinoflagellate diversification during the late Sinemurian and late Pliensbachian. The former diversification is consistent with the opening of the Hispanic Corridor during the late Sinemurian, which apparently allowed the pioneer dinoflagellate, Liasi- dium variabile, to invade the Tethys from the Paleo-Pacific. A true radiation pulse during the late Pliensbachian, with predominantly cold-water taxa, occurred during sea level fall, suggesting that climate change was critical to setting the evolutionary tempo. Our belemnite d 18O and Mg/Ca data indicate that late Pliensbachian water masses cooled (DT 268C) and became more saline (DS 12 psu). Cooling episodes during generally warm and humid Early Jurassic climate conditions would have produced stronger winter monsoon northeast trade winds, resulting in hydrographic instability, increased vertical mixing, and ventilation of bottom waters. During the late Pliensba- chian, dinoflagellates replaced green algae, including prasinophytes and acritarchs, as primary pro- ducers. By producing benthic resting cysts, dinoflagellates may have been better adapted to oxi- dized ocean regimes. This hypothesis is supported by palynological data from the early Toarcian ocean anoxic event, which was marked by highly stratified anoxic bottom water overlain by low- salinity, warm surface waters. These conditions were advantageous to green algae, while cyst-pro- ducing dinoflagellates temporarily disappeared. Our results suggest that the rise in dinoflagellate diversity later in the Jurassic appears to correspond to deep water ventilation as a result of the opening of the Atlantic seaway, conditions that appear to have simultaneously led to a loss of pra- sinophyte dominance in the global oceans.

Trophic regimes of benthic foraminiferal assemblages in Lower Jurassic shallow water carbonates from northeastern Italy (Calcari Grigi, Trento Platform, Venetian Prealps)

Benthic foraminiferal distribution patterns in the Lower Jurassic Rotzo Member (Trento Platform, northeastern Italy) are analysed in terms of trophic gradients to provide a better understanding of the benthic communities during the time of the initial diversification of Lower Jurassic large foraminifera in the Southern Alps. The Rotzo Member deposited on the Trento Platform records a complex mosaic of distinct shallow water ecosystems. The changes in the foraminiferal assemblages of the Rotzo Member are considered to be primarily a reflection of gradients in trophic resources and oxygen levels. The comparative analysis of benthic foraminiferal assemblages (characteristic taxa, number of species, test size, reproduction strategy), the associated sediments (lithofacies) and the macrofossil and microfossil content allow three trophic regimes to be identified during the time of deposition of the Rotzo Member: (a) eutrophic, (b) mesotrophic, and (c) oligotrophic. (a) The eutrophic regime, marked by episodically dysaerobic bottom conditions and high organic matter supply, occurs stratigraphically below and within the Orbitopsella spp. assemblage (Sinemurian pro parte-Carixian). Gradual change in the benthic foraminiferal assemblage may have been caused by a combination of reduced nutrient supply and increased bottom-water circulation which led to a mesotrophic regime (b). The latter, which has been recognised in the basal and middle part of the Rotzo Member alternating with the eutrophic assemblage, is characterised by a foraminiferal assemblage dominated by Orbitopsella sp. A major foraminiferal faunal turnover across the Carixian/Domerian boundary, corresponding to the disappearance of Orbitopsella spp., indicates a significant environmental change. An oligotrophic regime (c) became established towards the upper part of the Rotzo Member. New taxa appeared and diversity of the foraminiferal faunas was higher than in the previous ecosystems. This part of the member, characterised by the maximum lateral and vertical development of large Lithiotis bivalve mounds, reflects low and more stable nutrient supply, which provided optimal conditions for the more specialised benthic communities. Evolution of these benthic foraminiferal faunas from the Late Pliensbachian onwards was obliterated by events that led to the drowning of the Trento Platform, which is marked by a regional unconformity.

The two Early Toarcian (Early Jurassic) extinction events in ammonoids

The Early Toarcian (Early Jurassic) biological crisis was one of the ‘minor’ mass extinctions. It is linked with an oceanic anoxic event. Fossil data from sections located in northwestern European (epicontinental platforms and basins) and Tethyan (distal, epioceanic) areas indicate that Late Pliensbachian–Early Toarcian ammonoids experienced two extinction events during the Early Toarcian. The older one is linked with disruption of the Tethyan–Boreal provinciality, whereas the younger event correlates with the onset of anoxia and corresponds with the Early Toarcian mass-extinction event. These two extinctions cannot be interpreted as episodes of a single, stepwise, event. Values of the net diversification, more than the number of extinctions, allow the two extinction events to be clearly recognized and distinguished. Values of regional net diversification for northwestern European and Tethyan faunas point to greater evolutionary dynamics in the epioceanic areas. The inclusion of Mediterranean faunas in the database proves that the ammonite turnover at the Early Toarcian mass-extinction event was more important than previously thought. Progenitor (evolute Neolioceratoides), survivor (Dactylioceras, Polyplectus pluricostatus) and Lazarus (Procliviceras) taxa have been recognized. Different selectivity patterns are shown for the two events. The first one, linked to the disruption of the Tethyan–Boreal provinciality, has mainly affected ammonites adapted to epicontinental platforms. In the mass-extinction event, no selectivity is recognized, because also Phylloceratina and Lytoceratina were deeply affected at species level, although their wide biogeographical distribution at clade level was a significant buffer against extinction. In contrast to Palaeozoic mass extinctions, ammonoid survivors and Lazarus taxa are characterized by complex sutures: Phylloceratina (long-ranging ammonoids) and Polyplectus (relatively long-ranging compared to other Ammonitina).

Quantitative Meso-/Cenozoic development of the eastern Central Atlantic continental shelf, western High Atlas, Morocco

The well exposed Late Paleozoic to Cenozoic succession in the western High Atlas (Morocco) documents the early rift to mature drift development of the eastern Central Atlantic continental shelf basin. Vertical sections, depositional geometries and unconformities have been used to reconstruct the basin architecture prior to Atlasian inversion. Two-dimensional reverse basin modeling has been performed to quantitatively analyze the development of the continental shelf between the latest Paleozoic to Early Cenozoic. Basin evolution stages include (i) early rift, Late Permian to Anisian; (ii) rift climax, Ladinian to Carnian; (iii) sag, Norian to Early Pliensbachian; (iv) early drift, Late Pliensbachian to Tithonian; (v) mature drift, Berriasian to Cenomanian; (vi) mature drift with initial Atlasian deformation, Turonian to Late Eocene; (vii) Atlasian deformation; Late Eocene to Early Miocene; (viii) Atlasian uplift and basin inversion, Early Miocene to Recent. The Late Permian to Late Cretaceous basin development comprises eight subsidence trends of 10–35 Myr duration. Trends were initiated by changes in thermo-tectonic subsidence, which in turn triggered positive and negative feedback processes between sediment flux, flexural and compaction-induced subsidence. Plate-tectonic reconfigurations in the Atlantic domain controlled the thermo-tectonic subsidence history and the basin development of the Agadir segment of the northwest African passive continental margin: (1) major shifts in the sea-floor spreading axis; (2) significant decreases in sea-floor spreading rates; (3) the stepwise migration of crustal separation and sea-floor spreading in and beyond the Central Atlantic; (4) African–Eurasian relative plate motions and convergence rates. During the Early Pliensbachian/Toarcian to Cenomanian the first three plate-tectonic reconfigurations triggered changes in ridge-push forces and modulated the extensional stress field of Central Atlantic plate drifting. Since the Turonian, African–Eurasian relative plate motions and convergence rates represented the dominant control on the thermo-tectonic subsidence history in the Agadir Basin. Major variations in sediment flux and total subsidence characterize the development of the northwest African passive continental margin. The explanation of typical stratigraphic sequences as caused predominantly by sea-level fluctuations, and rough assumptions on sediment input/production and subsidence, is not necessarily applicable to passive continental margins. The methodology applied in this study, including the newly developed tool of ‘Compositional Accommodation Analysis’, allows to develop more rigorous genetic models for the development of continental shelf basins.

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.

Foraminifera and ostracodes across the Pliensbachian-Toarcian boundary in the Arctice Realm (stratigraphy, palaeobiogeography and biofacies)

Abstract The analysis of samples from numerous Pliensbachian and Toarcian sections from the northern regions of Russia and northern Alaska, as well as published data, allow us to estimate the correlative significance of foraminifera and ostracodes, and to develop an Arctic zonal standard based on these microfauna. During the Late Pliensbachian-Early Toarcian depositional period in the Arctic Basin and northwestern seas of Western Europe, a succession of almost simultaneous biotic and abiotic events occurred: widespread development of black bituminous shale at the beginning of the Early Toarcian, and a microbiota crisis (mass extinction event) in the Northern Hemisphere. The Early Toarcian microbiotic crisis was very sharp in both Arctic and Western European palaeobasins. In the Arctic seas, the generic and family composition of ostracode communities was completely replaced by new taxa. The species composition of foraminiferal assemblages changed considerably, while the generic composition of the foraminifera is only characterized by partial changes. In the Western European seas, the Early Toarcian microbenthos crisis caused taxonomic changes generally at the species level among foraminifera and at the generic level among ostracodes. The comprehensive analysis of the biogeographical distribution of Late Pliensbachian and Early Toarcian foraminifera and ostracodes, as well as Jaccard cluster analysis, allow us to define several biogeographical units within the Arctic and Boreal-Atlantic realms. A pattern of ecological distribution of microbenthos in bathymetric zones in Siberian palaeobasins has been developed, providing the basis for palaeoenvironmental reconstructions.