1 Published in Global and Planetary Change 117 (2014), 64–78 DOI: 10.1016/j.gloplacha.2014.03.008 which should be used for any reference to this work The middle Toarcian cold snap: trigger of mass extinction and carbonate factory demise Krencker F.N. 1 *, Bodin S. 1 , Hoffmann R. 1 , Suan G. 2 , Mattioli E. 2 , Kabiri L. 3 , Föllmi K.B. 4 , Immenhauser A. 1 1 Ruhr-Universität Bochum, Institut für Geologie, Mineralogie und Geophysik, D-44870 Bochum, Germany 2 UMR CNRS 5276 LGLTPE, Université Lyon 1, ENS Lyon, Campus de la Doua, Bâtiment Géode, F-69622 Villeurbanne Cedex, France 3 Faculty of Science and Techniques Errachidia, University of Moulay Ismaïl, BP 509, 52000 Boutalamine- Errachidia, Morocco 4 Institut des Sciences de la Terre, Quartier UNIL-Mouline, Bâtiment Géopolis, 1015 Lausanne, Switzerland * Corresponding author: [email protected] ; [email protected]Abstract The Pliensbachian and Toarcian (Early Jurassic) ages are characterised by several, relatively short-lived carbon cycle perturbations, climate change and faunal turnover. The cause(s) of biotic and abiotic disturbances remain unclear but most probably involved increased magmatic activity in the Karoo-Ferrar large igneous province. The Toarcian oceanic anoxic event (T-OAE) might represent the most extreme of these events, and as such, is becoming increasingly well documented worldwide. So far, other critical time intervals of the Pliensbachian – Toarcian have received considerably less attention. Here, the effects of the Middle Toarcian Variabilis event on the neritic-epeiric realm are explored making use of three well-exposed and extended stratigraphic sections in the Central High Atlas, Morocco. The carbon and oxygen isotopic composition of 112 bulk micrite samples were analysed and placed against 39 data points from carefully screened brachiopod valves in order to differentiate between palaeo-environmental and diagenetic patterns. Additionally, the phosphorus concentrations of 109 micrite samples were determined to evaluate the P-cycling. In all studied sections, an upper middle Toarcian major change from carbonate- to clastics-dominated sedimentation is recorded, pointing to a first-order carbonate production crisis. Our results reveal that these major sedimentological patterns coincide with an increase of oxygen-isotope ratios as well as a decrease of phosphorous accumulation rates. This suggests that the late middle Toarcian carbonate ramp crisis was related to a transient cooling event, potentially triggered by pulsed massive SO 4 exhalation events in the context of the Karoo large igneous province. Short-term cooling was likely amplified by the drawdown of atmospheric CO 2 levels related to the coeval decline of neritic carbonate precipitation and the warm water mass circulation disruption between the Tethys and the continental shelf. The data shown here provide the first evidence for coupled changes in carbon cycling, continental weathering and neritic systems in the aftermath of the T-OAE. Keywords Early Jurassic, carbonates; carbon cycle; geochemistry; climatic cooling; platform demise
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Published in Global and Planetary Change 117 (2014), 64–78DOI: 10.1016/j.gloplacha.2014.03.008
which should be used for any reference to this work
The middle Toarcian cold snap: trigger of mass extinction and carbonate factory demise
1 Ruhr-Universität Bochum, Institut für Geologie, Mineralogie und Geophysik, D-44870 Bochum, Germany2 UMR CNRS 5276 LGLTPE, Université Lyon 1, ENS Lyon, Campus de la Doua, Bâtiment Géode, F-69622 Villeurbanne Cedex, France3 Faculty of Science and Techniques Errachidia, University of Moulay Ismaïl, BP 509, 52000 Boutalamine-Errachidia, Morocco4 Institut des Sciences de la Terre, Quartier UNIL-Mouline, Bâtiment Géopolis, 1015 Lausanne, Switzerland
The Pliensbachian and Toarcian (Early Jurassic) ages are characterised by several, relatively short-lived carbon cycle perturbations, climate change and faunal turnover. The cause(s) of biotic and abiotic disturbances remain unclear but most probably involved increased magmatic activity in the Karoo-Ferrar large igneous province. The Toarcian oceanic anoxic event (T-OAE) might represent the most extreme of these events, and as such, is becoming increasingly well documented worldwide. So far, other critical time intervals of the Pliensbachian – Toarcian have received considerably less attention. Here, the effects of the Middle Toarcian Variabilis event on the neritic-epeiric realm are explored making use of three well-exposed and extended stratigraphic sections in the Central High Atlas, Morocco. The carbon and oxygen isotopic composition of 112 bulk micrite samples were analysed and placed against 39 data points from carefully screened brachiopod valves in order to differentiate between palaeo-environmental and diagenetic patterns. Additionally, the phosphorus concentrations of 109 micrite samples were determined to evaluate the P-cycling. In all studied sections, an upper middle Toarcian major change from carbonate- to clastics-dominated sedimentation is recorded, pointing to a first-order carbonate production crisis. Our results reveal that these major sedimentological patterns coincide with an increase of oxygen-isotope ratios as well as a decrease of phosphorous accumulation rates. This suggests that the late middle Toarcian carbonate ramp crisis was related to a transient cooling event, potentially triggered by pulsed massive SO4 exhalation events in the context of the Karoo large igneous province. Short-term cooling was likely amplified by the drawdown of atmospheric CO2 levels related to the coeval decline of neritic carbonate precipitation and the warm water mass circulation disruption between the Tethys and the continental shelf. The data shown here provide the first evidence for coupled changes in carbon cycling, continental weathering and neritic systems in the aftermath of the T-OAE.
Keywords
Early Jurassic, carbonates; carbon cycle; geochemistry; climatic cooling; platform demise
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1. Introduction
The Toarcian stage (Early Jurassic, 182.7 to 174.1 Ma, Gradstein et al., 2012) was punctuated by several major marine invertebrate extinction events (Little and Benton, 1995; Harries and Little, 1999; Cecca and Macchioni, 2004; Mattioli and Pittet, 2004; Mattioli et al., 2008; 2009; Suan et al., 2010, 2011). Previous work suggested that cephalopods in the Euro-boreal and Mediterranean realm, considered to be amongst the most environmentally sensitive marine organisms (Wang and Bush, 2008), were affected during four major Toarcian turnover events. Specifically, these took place (1) at the Pliensbachian-Toarcian boundary; (2) at the onset of the early Serpentinum ammonite zone; (3) around the Bifrons-Variabilis boundary and (4) during the Dispansum chronozone (Dommergues et al., 2009; Dera et al., 2010). Recently, these four extinction events were also recognised in northern panthalassic ammonites and foraminifera ecological patterns (Caruthers et al., 2013).
From a chemostratigraphical point of view, the Toarcian is characterised by several carbon isotopes excursions (CIEs) recorded in bulk carbonate, bulk organic matter, brachiopod valves and fossil wood samples. The most intensively studied CIE is undoubtedly that characterising the Toarcian ocean anoxic event (T-OAE; Jenkyns, 1985, 1988; Harries and Little, 1999; McArthur et al., 2000; Jenkyns et al., 2001; Röhl et al., 2001; Bailey et al., 2003; van de Schootbrugge et al., 2005; Mailliot et al., 2006; Cohen et al., 2007; Hesselbo et al., 2007; Gómez et al., 2008; Mattioli et al., 2008; Sabatino et al., 2009). Chronologically, the T-OAE is the second Toarcian event. Indeed, this event is preceded by a pronounced negative CIE spanning the Pliensbachian-Toarcian boundary (Hesselbo et al., 2007; Littler et al., 2010; Bodin et al., 2010), hereafter referred to as the Pliensbachian-Toarcian boundary event (PTo-E). Interestingly, the PTo-E and the T-OAE broadly coincide with the Pliensbachian-Toarcian boundary and the early Serpentinum cephalopod turnover events, respectively. Records of these two events share important similarities including the association with organic matter-rich stratigraphic intervals (Jenkyns, 1988; Jenkyns et al., 2002; Cohen et al., 2007; Wignall and Bond, 2008) and carbonate productivity shutdown (Dromart et al., 1996; Blomeier and Reijmer, 1999; Wilmsen and Neuweiler, 2008; Bodin et al., 2010; Trecalli et al., 2012).
Both of these events are also associated with significant warming of surficial water masses, as reflected by a marked decrease in oxygen-isotope ratios of brachiopod valves (Suan et al., 2008), belemnite rostra (McArthur et al., 2000; Bailey et al., 2003; Gomez et al., 2008) and fish tooth apatite (Dera et al., 2009b). Coeval increase of belemnite strontium-isotope ratios (86Sr/87Sr; McArthur et al., 2000), changes in clay mineral spectra (Dera et al., 2009a; Hermoso & Pellenard, in press) and increased phosphorus contents in bulk rock samples (Bodin et al., 2010) point to a concomitant increase in continental weathering and nutrient fluxes to coastal seas (Bodin et al., 2010; Jenkyns, 2010).
The above similarities of the PTo-E and T-OAE records caused previous workers (Suan et al., 2008; Littler et al., 2010) to suggest that similar processes triggered both events. Models invoked to explain concomitant atmospheric and oceanic change in carbon cycling during the PTo-E and the T-OAE all imply massive and relatively sudden input of an isotopically light carbon component in the ocean-atmosphere reservoirs. Hypotheses brought forward include gas hydrate release (Hesselbo et al., 2000) and the thermal metamorphism of carbon-rich sediments in the Karoo-Ferrar large igneous province (McElwain et al., 2005; Svensen et al., 2007), sustained injection of light carbon from a volcanogenic source (Suan et al., 2008), or a combination of these.
To date, only few studies have investigated the Bifrons-Variabilis (O’Dogherty et al., 2000; Bodin et al., 2010; Guex et al., 2012; Sandoval et al., 2012; Caruthers et al., 2013) and Dispansum events (Dera et al., 2010), both of which took place in the
Fig. 1: Simplified structural map of Morocco indicating position of studied outcrops (modified after Lachkar et al., 2009). Atlas Mountain range is shaded grey.
punctuated by phases of enhanced tectonic activity close to the Pliensbachian-Toarcian boundary and during the Middle Bajocian-Late Bathonian intervals. The basin subsequently experienced a passive infilling phase ending with a Late Cretaceous orogenic phase (Frizon de Lamotte et al., 2008).
The Palaeozoic meta-sedimentary rocks forming the crystalline basement were affected by ENE normal faulting slightly oblique to the regional EW trend of the mountain chain. This structuration, partly inherited from the Hercynian orogeny, formed an asymmetric basin composed of several sub-basins (Warme, 1988; El Arabi, 2007). Basin geometry controlled lateral facies changes from continental
aftermath of the T-OAE. Despite their arguably global extent and significance (Caruthers et al., 2013), geochemical data covering the corresponding stratigraphic intervals are still scarce. Consequently the significance of these two events is mainly deduced from marine faunal turnovers. Nonetheless, recent carbon- and oxygen isotope analyses of belemnite rostra suggested that, in the case of the Bifrons-Variabilis event, a CIE and a rapid cooling event (Gómez et al., 2008; Dera et al., 2011a) have been recorded. Moreover, regressive pulses have been suggested as the main mechanisms driving shallow marine extinction events during the Bifrons-Variabilis and Dispansum intervals (Sandoval et al., 2001; 2002; Dera et al., 2010). Nevertheless, given the low resolution of the few available geochemical and sedimentological records, theses mechanisms remains speculative and alternative causes remain feasible. Surprisingly, little attention has been paid to the impact of these perturbations on neritic carbonate ecosystems, which arguably react sensitively to changes in biotic and abiotic parameters (Hallock, 1986; Mutti and Hallock, 2003; Halfar et al., 2006; Föllmi et al., 2006; 2007) and hence represent detailed, albeit complex, archives of past environmental changes.
In this paper, we first present new sedimentological and geochemical data for Pliensbachian-Aalenian sections in the High Atlas Mountain range of Morocco. By this, we second provide evidence on changes in carbon cycling, carbonate productivity, sea-surface palaeo-temperatures and nutrient discharge in Tethyan neritic environments in the aftermath of the T-OAE. Here, particular attention is given to the response of neritic carbonate ramps to the Bifrons-Variabilis event. Third, in the context of these palaeo-environmental patterns, the possible causes of the Bifrons-Variabilis and the Dispansum event are contrasted and compared. The data shown here are relevant for those concerned with the complex palaeo-environmental patterns in the aftermath of the T-OAE.
2. Geotectonic setting
The study area is located in the central High Atlas rift basin of Morocco (Fig. 1). The pre-orogenic history of the central High Atlas rift basin commences near the Permian-Triassic boundary. This time is characterised by the opening of the northern Atlantic and the western Tethys. During the Early Jurassic, the central High Atlas rift basin was active and characterised by relatively slow subsidence rates,
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Fig. 2: Toarcian palaeogeographic map. A: Western Tethyan realm (modified after Bassoulet et al., 1993) indicating approximate position of study area. The dashed rectangle indicates position of figure 2B. B: Palaeogeographic map of Morocco and western Algeria showing major geological provinces and location of Ait Athmane and Jbel Akenzoud (this study) and the Amellago section (Bodin et al., 2010) near the southern edge of the Central High Atlas basin (modified from Du Dresnay, 1971 and Blomeier and Reijmer, 1999).
to deep-water settings (Fig. 2). The central High Atlas rift basin opened towards the Tethys but was possibly separated from the Atlantic basin by the West Moroccan Arch topographic high (Ghorbal et al., 2008; Saddiqi et al., 2009).
In Morocco, the Lower Jurassic carbonate successions are interrupted by two regionally significant events of clastic deposition, one taking place near the Pliensbachian-Toarcian transition and one at the middle-Upper Toarcian transition. The first clastic pulse has been interpreted as representing a shift from arid to humid climate (Wilmsen and Neuweiler, 2008; Bodin et al., 2010). The second clastic pulse, expressed as the transition from a carbonate to a mixed carbonate-siliciclastic ramp (middle to Late Toarcian), remains poorly understood.
3. Materials and methods
3.1. Correlative ammonite zonation and calibrated geological timescale
Different ammonite zones exist between NW Europe and the Mediterranean realm. For instance, the Variabilis and Dispansum zones refer to the NW Europe ammonite zone terminology. Their Mediterranean counterparts are Gradata and Speciosum zone, respectively (Dean et al., 1961; Schlatter, 1980; Braga et al., 1982; Howarth, 1992; Page, 2003). To address this problem, the correlative Pliensbachian Toarcian scheme from Caruthers et al. (2013) is applied here. Numerical ages for ammonite zones are from Gradstein et al. (2012).
3.2. Field approaches and facies analysis
Two stratigraphic sections were measured in outcrops situated in the central High Atlas mountain belt (Fig. 2). The first section is located next to the village of Ait Athmane, approximately 15 km northeast of Errachidia, north of the South Atlas main fault (GPS coordinates: N32° 04’ 1.3”; W4° 22’ 56.7”). The second section is located on the north-western flank of the Dades Valley between the Taria n’Dades Gorge and the Col de Msemrir (GPS coordinates: N31° 37’ 17.7”; W5° 53’ 33.8”). A total of 450 m of Lower and Middle Jurassic sedimentary rocks were logged and described bed by bed. The focus was on lateral as well as stratigraphic facies change, sedimentary features and textures, biota, trace fossils and diagenetic features.
3.3. Bulk micrite geochemical analyses
A total of 112 bulk micrite samples from the Upper Pliensbachian to the lowermost Aalenian interval were collected across the Ait Athmane section with an average sample spacing between 0.3 and 1 m. For each horizon, several milligrams of the micritic part of non-weathered mudstone and marl samples lacking allochems and diagenetic veins were selected, ground to powder and homogenised within an agate mortar. Geochemical analyses performed on all samples include carbon (δ13C) and oxygen (δ18O) isotope composition and phosphorus contents (Data Repository 1).
3.4. Brachiopod valve geochemistry
In order to test the validity of the bulk micrite data set and to generate benchmark values against which observed geochemical trends could be evaluated, a total of 39 selected brachiopod valves from 29 different stratigraphic horizons were analysed for their carbon and oxygen isotopic compositions (Data Repository 1). Host sediment attached to valves was first removed mechanically using a dental drill. Subsequently, valves were cut longitudinally with a 1 mm diamond saw, polished with 25.8 μm grit on sandpaper and examined visually under a binocular microscope as a first screening step. Samples lacking well organised and translucent fibres in the secondary (inner) shell layer were excluded. The primary layer and the uppermost part of secondary layer, recording disequilibrium isotopic ratios compared to their aquatic environment in modern species (Carpenter
Fig. 3: Cathodoluminescence photographs of terebratulid and rhynchonellid brachiopod valves AA59-1 (A) and AA 97.5 (C) and their corresponding thin sections photographs (B and D). Note orange luminescence of the punctuae in terebratulid specimen AA59-1 (p). Non-luminescent character (i) of well-preserved fibrous secondary layer (ii) from specimen AA 97.5.
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and Lohmann, 1995; Auclair et al., 2003; Parkinson et al., 2005; Cusack et al., 2012), were removed with a dental tool under a binocular microscope.
The shell surface was then cleaned with twice deionised water in an ultrasonic bath and studied under a Phenom G2 Pure desktop Scanning Electron Microscope in back-scatter mode (BSEM; at the University of Lyon) in an attempt to recognise signs of diagenetic alteration. BSEM observations were coupled with cathodoluminescence (CL) analyses using a HC1-LM cathodoluminescence microscope (hot cathode device) at the Ruhr University of Bochum (Fig. 3). The instrument is linked to a Kappa DX30C video camera system and with an EG&G triple grating spectrograph connected to a liquid-N2 cooled CCD-detector. Sampling for isotopic analyses was performed using a stainless steel needle and focussed on the fibrous calcite of the inner secondary layer within the middle part of the shell that appeared dark grey under the BSEM. Within the limitations of carbonate archive research, these screening tests are considered sufficient to extract reasonably well-preserved shell material. Following previous work (van Geldern et al., 2006; Giles, 2012; Price and Harwood, 2012), the brachiopod valve data are considered the most reliable carbonate archives used here.
3.5. Carbon and oxygen isotope analysis
Carbon- and oxygen-isotope analyses were performed on 112 bulk micrite samples and 39 selected brachiopod valves from the Toarcian and Pliensbachian intervals using a Gasbench II coupled to a Finnigan MAT 253 mass spectrometer at the Ruhr University of Bochum. Depending on the calcium carbonate content of individual samples, between 0.4 to 1.7 mg of powder were weighted in vials and dried for 48 hours in a 105°C preheated oven and subsequently cooled in a refrigerator for 1 hour. The air present in the vials was flushed using helium in order to avoid any contamination. Carbonates were sublimated by adding anhydrous phosphoric acid (104%) using an auto-sampler. The quality of the measurements was controlled by NBS19, NBS18 and RUB internal standards. Isotope data were corrected using CO1 and CO8 carbonate standards. For micrite samples the reproducibility (3<n<4) calculated with RUB internal standard was better than ±σ 0.08‰ for δ13Cmicrite and ±σ 0.15 for δ18Omicrite. For brachiopod samples the reproducibility (n=6) calculated with RUB internal standard was better than ±σ 0.05‰ for δ13Cbrachiopod and ±σ 0.19 for δ18Obrachiopod.
3.6. Phosphorus analysis
A total of 109 samples were selected for phosphorus content analysis at Lausanne University (Switzerland). For each sample, about 100 mg of powdered bulk sediment was mixed with 1 ml of MgNO3, oven-dried at 45°C for 2 h and then ashed in a furnace at 550°C during 2 h 30 minutes. The cooled residue was reacted with 10 ml of 1N HCl for at least 16 h under constant shaking. The solution was filtered with a 63 µm filter, diluted 10 times, and mixed with ammonium molybdate and potassium antimonyl tartrate, which, in an acid medium, reacts with orthophosphate to form phosphomolybdic acid (Eaton et al., 1995). This acid was reduced with ascorbic acid to form an intense blue colour, the intensity of which was determined with a Perkin Elmer UV/Vis Photospectrometer Lambda 10. The concentration of PO4 in ppm was determined by calibration with known standard solutions. Individual samples were measured with a precision better than 5%. Replicate analyses of samples have a precision better than 6% in average.
Because the sedimentation rate is very likely to fluctuate through time, the single concentration of an element is potentially affected by dilution or condensation phenomena. In order to overcome this analytical problem, we calculated phosphorus accumulation rate by multiplying the P concentrations, the sample density and the sedimentation rate in mg per cm2 per kyr. Given the comparable marly facies of the analysed samples, a constant density of 2.5 g/cm3 was assumed for the entire succession. The sedimentation rate was estimated using the biostratigraphy of Ait Athmane section (Fig. 4) and the duration of ammonite zones given in the Geologic Time Scale 2012 (Gradstein et al., 2012). Due to the lack of ammonites in the Pliensbachian sedimentary rocks in the Ait Athmane section, these calculations were only possible for the Toarcian interval.
4. Biostratigraphic age, facies description and depositional environment of measured sections
4.1. Ait Athmane section – Description and interpretation
The Ait Athmane section (Fig. 4) has a stratigraphic thickness of about 200 m and comprises two formations. The base of the section consists of about 70 m of Upper Pliensbachian carbonate-
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Fig. 4: Biostratigraphy, lithology and depositional environment of Ait Athmane section. Font colour refers to ammonite (black), nannofossils (green) and foraminifera (in blue). Red dots correspond to samples estimated age. For each samples temporal range are displayed with horizontal lines. When not specified into bracket, the biostratigraphic attribution is from this study. A and B: field evidence of condensation process at the boundary between the Aganane and Ait Athmane Formation.
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rich sedimentary rocks belonging to the Aganane Formation (Septfontaine, 1986). This unit is capped by a ferruginous surface colonised by numerous oysters (Fig. 4A and 4B). Age-diagnostic ammonites and foraminifera date this lower interval as latest Pliensbachian to earliest Toarcian in age (Wilmsen et al., 2002; Wilmsen and Neuweiler, 2008). This ferruginous surface is overlain by 130 m of sedimentary rocks belonging to the Ait Athmane Formation (Wilmsen et al., 2002). Ammonites and calcareous nannofossils from the Ait Athmane section (Fig. 4) place the base of this unit in the latest Levisoni ammonite subzone. This implies that the ferruginous oyster hardground represents a hiatal surface corresponding to the Polymorphum zone and early Levisoni zone, representing about 750 kyrs, in line with the results of carbon-isotope stratigraphy. The top of this section is dated as Late Toarcian to earliest Aalenian based on ammonites and calcareous nannofossils biostratigraphy (Fig. 4).
The Aganane Formation consists of a stacking of several regressive sequences capped by subaerial exposure surfaces (Wilmsen and Neuweiler, 2008). These sequences are mainly composed of bio-oo-grainstones, bioclastic float- to rudstones, fenestral intra-peloid grain- to packstones, lagoonal marls and are capped by palaeo-soils. Bivalve (Lithiotis) bioherms are common features. The Formation has also yielded numerous dinosaur trackways (Jenny & Jossens 1982; Ishigaki 1988). The depositional setting of the Aganane Formation is probably best assigned to the inner, shallow neritic domain as based on facies arguments and the presence of numerous subaerial exposure surfaces.
The Ait Athmane Formation is built by limestone-marl alternations. The main lithologies include bioclastic wacke- to packstones that laterally grade into float- to rudstones. Allochems consist of abundant ammonites, brachiopods, diverse molluscs and crinoid osicles, typical of an open marine environment. Limestone beds are moderately affected by Thalassinoides-like and Rhizocorallium-like bioturbation. This association is commonly related to muddy firmgrounds located in the sub-littoral zone (Miller III, 2007). The presence of numerous flute casts and gullies indicate that the sharp contact between limestones and marls is likely erosive in origin.
Near the middle to Late Toarcian transition, marly intervals thicken significantly and over a short stratigraphic transition zone, leading to a significant
and abrupt increase of the marl-to-limestone ratio. This implies that the depositional environment of the Ait Athmane Formation is probably best assigned to a mid- to outer ramp setting.
4.2. Jbel Akenzoud section – Description and interpretation
The total stratigraphic thickness of the Jbel Akenzoud section (Fig. 5) is more than 250 m. From the base to the top, the section is composed of 70 section meters belonging to the upper portions of the Tafraout Formation (Milhi, 1992), followed by 182 section meters of the Azilal Formation (Jenny, 1988). The Tafraout and Azilal formations are separated by a major stratigraphic gap as revealed by the occurrence of the age assigning brachiopod Stroudithyris stephanoides in the lowermost part of the Azilal Formation (Ettaki et al., 2007). This hiatus corresponds to the erosion or non-deposition of the top part of the Gradata and the lower part of the Bonarelli ammonite zones (Fig. 5A). The upper portions of the Tafraout Formation were dated middle Toarcian in age as based on ammonite biostratigraphy (Ettaki et al., 2000), a notion that is supported by age-diagnostic brachiopods (Fig. 5). Numerous specimens of Stroudithyris infraoolithica (DESLONGCHAMPS) were collected in the uppermost part of the Tafraout Formation. This species is common in the Late Toarcian Thouarsense-Aalensis ammonite zones in NW Europe and northern Africa, where its earliest and latest occurrences correspond to the Variabilis (Illustris ammonite subzone; middle Toarcian) and the Opalinum zones (Early Aalenian), respectively (Alméras, 1996; Garcia Joral and Goy, 2004). Assuming that these stratigraphic range data are applicable to the sections under study, it is proposed that the top of the Tafraout section is latest middle Toarcian to Late Toarcian in age.
At Jbel Akenzoud, the uppermost interval of the Tafraout Formation corresponds to a carbonate-dominated facies that is best assigned to a shallow neritic depositional setting. Evidence for this includes sub- to intertidal facies as attested by the deposition of decametric ooid shoals displaying wave ripples and sedimentary features typical for tidal environments. Individual ooid shoals are intercalated by softer intervals of limestone-marl alternations. Limestones are formed mainly by bioclastic wack- to packstones and float- to rudstones. The faunal association is diverse and consists of bivalves, brachiopods, solitary corals and gastropods.
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Fig. 5: Jbel Akenzoud section: (A) Field image documenting demise of the ooidal carbonate factory at middle to Late Toarcian boundary (red line).
The carbonate-dominated facies of the upper portions of the Tafraout Formation is, via a stratigraphically thin transition interval, overlain by the mainly terrigenous Azilal Formation. The Azilal Formation consists mainly of claystones rich in continental plant debris and laminated microbial facies. Claystone intervals alternate with metric beds of dolo-mudstones, dolo-pel-packstones, dolo-oo-grainstones and two types of polymictic conglomerates here referred to as type A and type B as detailed below.
Conglomerate type A consists of sub-rounded to rounded clasts of dolo-oo-packstones, pelpackstones and dolo-mudstones. Conglomerate type B is composed of abundant angular to sub-angular extraclasts of metamorphic rocks and rounded to sub-rounded intraclasts of dolo-oo-grainstone and dolo-pel-packstones. For both conglomerate types, the maximum size of the observed clasts never exceeds 5 cm. Common sedimentary features related to conglomerate type A are wave ripples, cross bedding and Rhizocorallium-like ichnofossils. Conglomerate type B displays frequent cross-bedding features and imbricated pebbles. Both conglomerates correspond to channel fillings. Accordingly, conglomerates of type A are best assigned to tidal environments and conglomerates type B to alluvial plain. This interpretation is in line with the previous assignment of the Azilal Formation to a terrigenous continental context (Piqué, 1994).
5. Geochemical data
5.1. Bulk micrite carbon and oxygen isotope ratios
The carbon isotope profile of the Ait Athmane section is separated into four chemostratigraphic intervals each typified by its specific pattern (Fig. 6A). Interval 1 (Pliensbachian) records very variable δ13Cmicrite ratios oscillating between –2.2‰ and +3.8‰ with a mean value of +0.5‰. Interval 2 (upper Levisoni ammonite subzone - middle Falciferum ammonite subzone) is characterised by a sharp increase of δ13Cmicrite ratios increase from +1.3 to +2.5‰. Interval 3 (middle to Upper Toarcian) is characterised by an overall decreasing trend from +2.5 to –0.9‰ (Fig. 6A). Interval 4 records and increase of δ13Cmicrite ratios from –0.9‰ near the Toarcian-Aalenian transition to +0.8‰ in limestone beds near the top of the measured section.
The δ18Omicrite ratios oscillate around a mean value of –3.5‰ in interval 1 (Fig. 6B). In interval 2, the δ18Omicrite ratios increase sharply from –4.5 to –3‰. The δ18Omicrite values increase gradually from approximately –6 to –1‰ throughout intervals 3 to 4. Two important shifts toward lower δ18Omicrite values with amplitudes of 3 and 2‰ respectively are present in the lower and middle portions of interval 3.
5.2. Brachiopod valve carbon and oxygen isotope ratios
The carbon isotope ratios of brachiopods (δ13Cbrachiopod) range from 0‰ to +4.1‰ (Fig. 6A). The δ13Cbrachiopod values increase from 2.2‰ in the stratigraphically lowermost samples (Lower Toarcian) to +4.1‰ near the lower-middle Toarcian transition (interval 2). In interval 3, the δ13Cbrachiopod data record a long-term decreasing trend from +4.1‰ to 0‰ interrupted by a pronounced –1.5‰ rebound located within the middle portion of interval 3.
The minimum and maximum values recorded by δ18Obrachiopod are –4.1 and –1.6‰, respectively (Fig. 6B). The δ18Obrachiopod ratios depict a long term increasing trend from –4‰ at the base of interval 2 to –2‰ at the top of interval 4. Within the Gradata ammonite zone, this trend is interrupted by a twofold excursion toward positive values with amplitude of about +1.5‰ each. It can be noted that carbon and oxygen values from the same horizon are very similar in general but also independently of the brachiopod family (terebratulids or rynchonellids).
5.3. Phosphorus content
Phosphorus (Ptot) contents in measured sections can be divided into five main intervals based on their characteristics (Fig. 6C). The lowest values are recorded in the first part that corresponds stratigraphically to interval 1 as defined by carbon and oxygen isotope trends. Here the Ptot values increase from 50 to 150 ppm. Interval 2 is characterised by stable Ptot contents oscillating around a mean value of 280 ppm. The Ptot contents increase markedly across the middle Toarcian (interval 3a) and further upsection reach a plateau with mean value of 500 ppm in the upper Toarcian (Interval 3b).
The Toarcian-Aalenian boundary (Interval 4) is marked by a moderate decrease in Ptot, but its significance is questionable due to an only limited number of analyses available. The long-term Ptot trends are overprinted by two Ptot excursions in the lower portions of interval 3a and 3b. The excursion
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Fig. 6: Geochemical data from Ait Athmane section: Bulk rock records of carbon isotope ratios (A), oxygen isotope ratios (B), phosphorus content (C) and phosphorus accumulation rate (D). Brachiopod valve carbon isotope ratios (A), and oxygen isotope ratios (B): yellow squares represent data from terebratulids and red squares data from rhynchonellids. A significant oxygen isotope shift (>1‰) is recorded by the brachiopod data in the Gradata/Variabilis chronozone (red box). Please note that because of diagenetic processes the δ18Omicrite has been altered. Therefore, only δ18Obrachiopod from screened brachiopod valves can be used for seawater palaeotemperature reconstruction (see text for explanation) Note also the hiatal surface at the Pliensbachian/Toarcian boundary.
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toward higher Ptot values in interval 3a expands over approximately 20 meters and reaches an amplitude of 220 ppm. The excursion toward lower Ptot values in interval 3b expands over 10 meters and reaches an amplitude of 100 ppm.
The phosphorus accumulation rate in the Ait Athmane section can be divided into two segments as based on their characteristics (Fig. 6D). The first segment covers the lower and the middle Toarcian and corresponds to chemostratigraphic intervals 2 and 3a. The first segment is characterised by a generally decreasing trend from 2 to approximately 0.5 mg/cm2/kyr. This pattern is overprinted by a series of shifts reaching mean amplitudes of 0.5 mg/cm2/kyr. The second segment covers the Upper Toarcian and the lowermost Aalenian and corresponds to geochemical intervals 3b. Here, phosphorus accumulation rates fluctuate around 4 mg/cm2/kyr but in essence form a plateau. The transition zone between these two segments corresponds to an abrupt (10 m) phosphorus accumulation rate shift from 0.5 to 4 mg/cm2/kyr. This transition zone is located in the lower portion of geochemical Interval 3b. Within 10 meters, the phosphorus accumulation rate abruptly returns to values equivalent to those present at the top of Interval 3a (~0.5 mg/cm2/kyr).
6. Interpretation and Discussion
6.1. Degree of diagenetic alteration in the Ait Athmane section
Every assessment of isotope data from fossil carbonate archives requires a discussion of their potential diagenetic pathways including marine, meteoric and burial alteration (Brand et al., 2012). Particularly, the Upper Pliensbachian micritic limestones (Interval 1) of the Ait Athmane section record repeated subaerial exposure stages (Wilmsen & Neuweiler, 2008; Rachidi, 2012) that compromise the interpretation of the isotopic data from this interval. Indeed, the infiltration of 18O-depleted meteoric or marine-meteoric mixing zone pore fluids lowers the δ18Omicrite ratios of bulk carbonates significantly, whereas the presence of 13C-depleted soil-zone CO2 might decrease δ13Cmicrite values (Andrews, 1991; Christ et al., 2012). Bulk micrite δ18O values throughout the Toarcian intervals 2 to 4 display significant variability ranging from -6 to -1‰ (Fig. 6). Specifically, across the Gradata ammonite zone, several pronounced shifts to depleted values
are recorded. Both the irregular isotope pattern and the uncommonly depleted values are typical for an interval characterized by a more-than-average diagenetic alteration. The assumption of an altered bulk oxygen isotope signature is further corroborated by the simple application of a temperature equation to these values. During this comparably short interval (~2 Myr), bulk isotope values would suggest two successive heat waves, both with amplitudes of about 10°C each. On a longer-term period (~6 Myr), bulk oxygen isotope data suggest a 20°C cooling in seawater temperature (applying the Gradstein et al., 2012 time scale and the temperature equation of Anderson and Arthur, 1983). Both assumptions are neither geologically plausible nor supported by coeval sections elsewhere.
Conversely, carefully selected inner shell layer low-Mg calcite brachiopod subsamples from the Ait Athmane section may, pending limited metabolic effects (Auclair et al., 2003; Parkinson et al., 2005; Pérez-Huerta et al., 2011; Cusack et al., 2012), represent reasonable carbonate archives allowing for tentative estimates of seawater palaeo-temperatures and the δ13C of the dissolved inorganic carbon (Takayanagi et al., 2013). Moreover, brachiopod valve isotope data serve as benchmarks against which bulk micrite data can be tested. A first observation is that both oxygen and carbon brachiopod isotope data in Morocco are commonly enriched in the heavy isotope relative to micrite. A second observation is that both brachiopod and bulk micrite data record comparable geochemical trends, although the micrite oxygen isotope data record much larger changes compared to those revealed by brachiopod data, with opposed trends in some intervals (Fig 6). Similar observations have been described by Suan et al. (2008) from the Peniche section in Portugal. There, carbon isotope data from brachiopod and bulk micrite show directly comparable trends, albeit with the brachiopod data being about 1‰ heavier than micrite from the same horizons. This may imply that the δ13Cmicrite data from Ait Athmane, albeit altered and depleted, still record the first order geochemical patterns of their depositional environment. Overall, the importance of a contrast-comparison between bulk and selected component specific geochemical data has been documented in numerous studies from many basins and time intervals worldwide (e.g., Pennsylvanian: Immenhauser et al., 2002; Toarcian: Gómez et al., 2008; Valanginian-Hauterivian: van de Schootbrugge et al., 2000; Early Aptian: Huck et al., 2010; Turonian: Wendler et al., 2013).
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The second possible interpretation, assuming concomitant diagenetic resetting of both, brachiopod and micrite data seems less likely. Indeed, screened brachiopod subsamples lack discernible evidence for diagenetic alteration (Fig. 3). In addition, a complete, homogenous diagenetic resetting should result in identical isotopic trends in both sample types. This notion is at odds with the fact that the offset between brachiopod and micrite is almost constant for δ13C (1‰ in average) while it varies substantially for δ18O (between 1 to 1.5‰; Fig. 6). Again, this element pattern is typical as diagenetic carbon is derived from the host rock whilst the source of diagenetic oxygen is the reactive fluid itself. This pattern becomes particularly evident in the diagenetically reactive bulk micrites whilst it is subdued in the considerably more stable brachiopod low-Mg calcite.
Clearly, assigning this offset to diagenetic alteration of micrites alone would represent an oversimplification. This is because micritic carbonates represent mineralogical averages of their depositional environment including calcitic and aragonitic, biogenic and abiogenic components. The differential fractionation of aragonite and calcite in seawater (Rubinson and Clayton, 1969; Tarutani et al., 1969; Grossman, 2012) implies, at deposition, bulk values that deviate from biogenic calcite. Moreover, the differential uptake of different carbonate species in inorganic marine environments compared to the complex biomineralisation processes in brachiopods (Parkinson et al., 2005; Pérez-Huerta et al., 2011; Cusack et al., 2012) represent significant obstacles.
Acknowledging all of these problems, the Ait Athmane carbon isotope chemostratigraphic pattern shares important similarities with that of bulk sections of Euroboreal and Panthalassic settings, a notion that is supported by independent biostratigraphic evidence. This implies that first order trends in carbon isotope chemostratigraphy are preserved despite differential diagenetic overprinting of isotope values at deposition (see Wendler, 2013, for a review).
Summing up, first, it is tentatively proposed that the geochemical data set shown here reflects fluctuations in Toarcian seawater DIC composition. Second, as typical for these lithologies, the bulk rock oxygen isotope record is subject to spatially and temporally variable diagenetic overprint. Judging from the data obtained here and previous works, the low-Mg calcite valves of screened brachiopods escaped diagenetic resetting and are used as qualitative archives of patterns in seawater palaeo-temperatures.
6.2. Chemostratigraphic and biostratigraphic correlations between Mediterranean and Euroboreal basins
With the exception of Interval 1 recording significant meteoric diagenetic alteration, a tentative regional and global correlation between Morocco (this study), Portugal (Peniche section; Hesselbo et al., 2007) and south of France (Grand Causses basin; Harazim et al., 2012) is here proposed (Figs 7). Arguments for intersection correlation come from carbon-isotope chemostratigraphy integrated with biostratigraphic data.
The carbon isotope record from the Peniche section (Hesselbo et al., 2007) represents one of the records with the best temporal resolution currently available for the Early Toarcian. From the Polymorphum to the Levisoni chronozones, the δ13Cmicrite data form a generally increasing trend that is recorded across the Tethyan realm (Sandoval et al., 2012). This pattern is interrupted by two negative CIEs corresponding to the Pliensbachian-Toarcian boundary event and the T-OAE (Fig. 7). The δ13Cmicrite data from Ait Athmane cover the upper portions of this long-term increasing pattern while they do not record, in agreement with biostratigraphic data (Fig. 4), the two Early Toarcian CIEs (Fig. 7). Possible explanations for this hiatal interval include non-deposition, significant condensation and/or erosion. Field evidence, including a regionally significant iron-impregnated marker bed (Wilmsen et al., 2002) at the top of the Pliensbachian is in favour of condensation and non-deposition. The hiatal interval is tentatively assigned to a second order regressive phase close to the Pliensbachian-Toarcian boundary (Guex et al., 2001; Suan et al., 2008). Following the Levisoni chronozone, the δ13Cmicrite chemostratigraphic trend in Tethyan and Panthalassic sections shares many important similarities. These include a long term decreasing pattern (Interval 3) followed by an increase in carbon isotope ratios during the latest Toarcian Aalensis ammonite zone (Interval 4). This pattern is found in many different sedimentary basins (Sandoval et al., 2012). Moreover an abrupt shift towards lower δ13C values has been described from the middle/Late Toarcian boundary.
Examples include sections in Morocco (Bodin et al., 2010), southern France (Harazim et al., 2012) and Peru (Guex et al., 2012). The carbon isotope ratios from the Ait Athmane section show a 1‰ excursion toward negative values at the middle/Late Toarcian boundary. Nonetheless, it is difficult to differentiate
Fig. 7: Correlation of δ13Cmicrite data from Ait Athmane (Morocco, this study) and composite δ13Cmicrite record from Hesselbo et al. (2007) and Harazim et al. (2012).
this excursion from small-scale variations in other portions of the section (Fig. 7). Therefore, at present a conclusive statement regarding a possible CIE at the
middle-Late Toarcian boundary is not supported by the new carbon isotope record from the Ait Athmane section.
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6.3. Palaeo-temperature reconstruction
A series of studies showed that many of the presently living articulate Terebratulina and Rhynchonellida build at least portions of their secondary fibrous valve in oxygen isotopic equilibrium with ambient seawater (Carpenter and Lohmann, 1995; Curry and Fallick, 2002; Auclair et al., 2003; Brand et al., 2003; Parkinson et al., 2005; Takayanagi et al., 2013). Therefore, the δ18O record of well-preserved secondary fibrous valves of brachiopods is commonly used for palaeo-temperature or salinity estimates. The authors acknowledge the problem of disentangling the seawater temperature and the seawater salinity component in fossil δ18O data and accept that recent articulate brachiopods might possibly not be good analogues for Jurassic ones. On the other hand, many groups of brachiopods seem rather conservative in their overall biomineralisation patterns and first order estimates of changes in seawater temperature seem feasible from the data set presented here. Brachiopods that passed screening have been used applying the seawater temperature equation of Anderson and Arthur (1983):
T(°C) = 16.0 - 4.14(δc-δw) + 0.13(δc-δw)²
where δc = δ18O PDB composition of the sample, and δw = δ18O SMOW of ambient seawater. Arguments for this come from the fossil biota and the open marine setting of sections investigated. A δw of –1‰ is commonly used (Rosales et al., 2004; Gómez et al., 2008; Suan et al., 2008) for the Toarcian assuming that this stage represents a largely ice free world. For the sake of comparability with previous work, the δw of –1‰ was applied here.
The Peniche (Suan et al., 2010) and the Ait Athmane (Fig. 8) sections are the closest sites in terms of their palaeo-geographic location, allowing for a comparison of brachiopod geochemical data shown here with published ones, respectively. Both datasets and resulting relative palaeo-temperature trends are displayed in Figure 8. Two major conclusions can be drawn. First, δ13Cbrachiopod data agree in their absolute values and secular patterns. Second, trends in δ18Obrachiopod share the overall temporal pattern but are offset by 1‰. Specifically, the Ait Athmane data are depleted relative to the Peniche data. This offset might reflect differential seawater salinities or temperatures. Based on palinspastic, physiographic and sedimentological information, the Ait Athmane section was probably situated in a more proximal setting, relative to the Peniche section, and at lower latitudes (Peniche ≈ 23° lat.; Ait Athmane ≈ 18° lat.).
All of these considerations agree with the data and their relative offset respectively.
From the viewpoint of the longer-term patterns resulting from this data set, a cooling by approximately 4°C through the middle to Late Toarcian in the Mediterranean realm is inferred. Moreover, a more short-lived cooling event with amplitudes in the order of 6°C was observed for the Variabilis/Gradata interval. Similar patterns were reported from belemnite data (McArthur et al., 2000; Gómez et al., 2008), an observation that might point to an at least provincial (global?) significance of this pattern. Similar to the brachiopod archive, the δ18Obelemnite ratios from England are depleted relative to the data from Spain (Fig. 8). Similarly to the brachiopod data, this shift is most likely due to salinity differences between these two localities and complicated by differential live modes or biomineralization patterns.
Additional evidence that δ18Obrachiopod ratios are primarily driven by sea-water temperature comes from alternative evidence for cooler conditions during the late Middle Toarcian. Indeed, the decrease of Mg/Ca ratios of belemnites from the Bifrons to Variabilis zone in the UK may represent independent evidence for cooling across this interval (McArthur et al., 2000; Bailey et al., 2003). In addition, the Variabilis event is also related to renewed provincialism amongst cephalopods that can be attributed to a sea-level fall and enhanced latitudinal thermal contrasts (Sandoval et al., 2001; Dera et al., 2011b).
6.4. Causes of long and short-term cooling
Evidence for a long-term and possibly for a short-term cooling event have been recognised in NW Tethyan belemnite and brachiopod archives. Possible mechanisms that may or may not explain this global cooling event are discussed below. For the sake of clarity, long- and short-term period processes are separated and discussed in two subsequent chapters. In the terminology used here, the term long-term refers to time intervals ≥2 Myr, whilst short-term refers to duration of ≤2 Myr.
6.4.1. Causes of long-term cooling
The T-OAE took place during one of the warmest periods of the Jurassic (Dera et al., 2011a). Several authors have postulated that this warming was triggered by the massive injection of greenhouse gases from the Karoo-Ferrar LIP accompanied by the destabilisation of seafloor methane hydrates
Fig. 8: Correlation of composite δ13Cbrachiopod and δ18Obrachiopod records from Morocco (Ait Athmane, this study) and Portugal (Peniche and Tomar, Suan et al., 2010) (A) and δ18Obelemnite record from Spain (La Almunia-Ricla, Gómez et al., 2008) (B). Note that δ18Obrachiopod values from Ait Athmane are shifted by +1 permil relative to those from Peniche (Suan et al., 2010).
(Hesselbo et al., 2000; Jenkyns, 2010). According to this model, the resulting enhanced greenhouse effect contributed to an accelerated hydrological cycle, leading to substantial increase of continent-derived nutrient supply into the world oceans (Jenkyns, 2010). Ocean fertilisation favoured organic matter deposition and triggered CO2 drawdown from the atmosphere. This process acts as a negative feedback on global temperature and constitutes one possible explanation for the long-term cooling observed in the aftermath of the T-OAE. Supporting evidence for this mechanism comes from the long-term rise in Lower Toarcian δ13C ratios recorded by belemnites, brachiopods and bulk micrites in northwestern Tethys (e.g., Fig. 8; Bailey et al., 2003; Hesselbo et al., 2007; Harazim et al., 2012). This feature is probably best explained by enhanced burial of 12C-enriched organic matter (Jenkyns, 2010).
Nevertheless, the enhanced organic matter deposition during the T-OAE has been questioned by several authors (Gómez et al., 2008; Gröcke et al., 2011; Macchioni, 2002; Wignall et al., 2005; McArthur et al., 2008), pointing out that several sections lack coeval organic matter rich deposits. An alternative/complementary model, summarised best under the label “basalt weathering hypothesis”, is proposed here.
Recent studies have shown that basalt weathering represents nearly 30% of the present-day total atmospheric CO2 sink (Gaillardet et al., 1999) and might have been even more significant during LIP activity intervals (Dessert et al., 2001). The global significance of basalt weathering depends on the total basaltic rock surface area in warm and humid regions of the world (Dessert et al., 2003) where weathering is most efficient.
During the Early Jurassic, the Karoo Ferrar basalts were located in the southern part of Gondwana. Early Toarcian palaeo-climate modelling (Dera and Donnadieu, 2012) suggested that during the Toarcian warming event, this region was affected by enhanced runoff and net moisture. Therefore, water supply might have been sufficient to yield an efficient weathering of the Karoo Ferrar basalts and the same pattern accounts for example for the central Atlantic magmatic province (CAMP) basalts. As a consequence, the enhanced weathering of the Karoo Ferrar and the CAMP basalts in the aftermath of the T-OAE would have resulted in decreasing CO2 level in the atmosphere and therefore contributed to the long-term fall in mean palaeo-temperatures.
6.4.2. Causes of Variabilis/Gradata short-term cooling
Three model approaches are brought forward in order to explain the postulated upper Variabilis/Gradata cold snap. These are best labelled (i) SO2, (ii) platform demise and (iii) ocean circulation model. We emphasise that each of these models on its own is perhaps insufficient to explain the data observed.
H2SO4 is a climate-relevant compound released and produced during volcanic activities (Self, 2005; Self and Blake, 2008; Bryan and Ferrari, 2013). H2SO4 disturbs solar radiations that eventually will produce an atmospheric cooling (Rampino and Self, 1992). Several parameters seem to enhance H2SO4-related cooling. These include the amount of SO2 released during volcanic eruptions and the altitude reached by the eruption column. For this reason, silicic volcanism, due to its explosive nature, is more prone to induce atmospheric cooling than basalt-type volcanism. According to Jourdan et al. (2008), a volcanic silicic-pulse from the Karoo LIP is recorded close to the Bifrons-Variabilis chronozone transition. The high latitude location of the Karoo LIP (between 45° and 75°) could have also eased the H2SO4 dissemination. This is because the boundary between the troposphere and the stratosphere is lower at higher latitudes, and SO2 remains longer in the stratosphere than in the troposphere (Self, 2005). The Karoo LIP could have injected enough SO2 to produce, or at least to trigger, the Variabilis cold snap, although the ability of LIP’s to produce and sustain a cooling over time intervals of 1 Myr remains debated. Therefore a complementary model is needed.
From a short term perspective, carbonate precipitation results in an increase of atmospheric pCO2 (Zeebe and Wolf-Gladrow, 2001). Thus, the demise of numerous carbonate ecosystems during the Bifrons-Variabilis interval (Cope et al., 1980) deprived the atmosphere of a substantial CO2 source. This process has been evoked to explain other brief glacial episodes during the Mesozoic. A prominent example for the relation between platform demise and pCO2 is detailed in a sophisticated modelling approach documented in Donnadieu et al. (2011). Specifically, these authors propose that the rapid cooling documented at the Middle-Late Jurassic transition has been triggered by a drastic decrease of carbonate productivity of low latitudes carbonate platforms. Standard simulations reproduce a 4°C cooling that is in agreement with the 4 to 6°C cooling recorded by belemnite and bivalve δ18O from this
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time interval. Hence, the demise of neritic carbonate production recorded in the Variabilis/Gradata Zone may well have accentuated and sustained the initial cooling caused by the Karoo LIP silicic eruption.
Finally, palaeo-geographic changes might also have contributed to the recorded cooling. During the middle to Late Toarcian, Morocco was located in the European Epicontinental Sea (EES). The EES was connected to the Tethyan Ocean through narrow seaways (Vörös, 1977; Meister and Stampfli, 2000). Connections to the Tethys were restricted due to physical barriers such as shallow carbonate platform or emerged land belts (Alméras and Elmi, 1987; Arias, 2007). Overall, this setting is very sensitive to changes in relative sea level. Previous work suggested a thermohaline circulation between the EES and the Tethys (Kutzbach et al., 1990; Arias, 2007; Dera and Donnadieu, 2012). This circulation was possibly an efficient source of warm and salty water from the Tethys to the EES, which in turn might have acted as a positive feedback on palaeo-temperatures. The relative sea-level fall in the Variabilis/Gradata interval might have disrupted the thermohaline circulation, eventually causing additional cooling. Summing up, the underlying reasons for the Variabilis/Gradata cooling event are clearly complex and many factors were most likely interlinked.
6.5. Impact of the Variabilis/Gradata cold snap
The Bifrons-Variabilis biological crisis, first reported from the Tethyan realm (Alméras et al., 1994; O’Dogherty et al., 2000; Sandoval et al., 2001; Dera et al., 2010) was most recently also found in the Panthalassic realm (Caruthers et al., 2013), underlining its global significance. A commonly found assumption is that the Bifrons-Variabilis event is linked to a regressive sea-level pattern that might have contributed extinction patterns by reducing the size of the epicontinental environment (Sandoval et al., 2001; 2002; Dera et al., 2010). Data shown here agree with this concept. It appears likely that faunas adapted to warm conditions during the T-OAE warming episode would have suffered from decreasing seawater temperature. Moreover, decreasing the seawater temperature could potentially also disrupt the ooid production, as observed at the transition between the Tafraout and the Azilal formations in the High Atlas Basin. Indeed, by decreasing seawater temperature, more CO2 will dissolve into the water column, acidifying seawater and hence potentially inhibiting ooid production
(Loreau and Purser, 1973; Lees, 1975; Lucas et al., 1976; Simone, 1981; Loreau, 1982; Tucker et al., 1990).
6.6. The Dispansum/Speciosum event: possible link with enhanced trophic levels
A biologic crisis is documented during the Late Toarcian (Dispansum/Speciosum ammonite chronozone; Caruthers et al., 2013; Dera et al., 2010). In Morocco, this event is recorded in the interval 3b of the Ait Athmane section (Fig. 6), corresponding to high phosphorus accumulation rate. As riverine fluxes play a key role in the input of phosphorus into the ocean (Compton et al., 2000), the data presented here might hint at the reinstallation of humid conditions during the Late Toarcian interval following a more arid middle Toarcian. Evidence for more humid condition on a global scale during the Late Toarcian comes from palynofacies data from Argentina (Zavattieri et al., 2008), from clay mineralogy spectra from Quercy Basin (France, Lezin et al., 2007) and from the common occurrence of siliciclastics-dominated facies in Upper Toarcian sedimentary rocks (Cope et al., 1980; Hesselbo, 1995; Dromart et al., 1996).
This latest observation is made in Morocco as well. Indeed, field evidence reveals the abrupt Late Toarcian disappearance of the characteristic middle Toarcian ooidal shoal facies in the shallow marine setting. This trend comes along with the appearance of polymictic conglomerates and other terrigenous deposits in proximal settings (e.g., Azilal Fm; Fig. 5). In outer ramp setting, this, results in the increase of the claystone/limestone ratio, accentuated by the reduced shedding of carbonate oozes from proximal areas.
Enhanced nutrient input and increased sea-water turbidity are known to severely inhibit the neritic carbonate productivity (Wood 1993; Lokier et al., 2009). Moreover the addition of large amount of freshwater in the High Atlas Basin may also have lowered the sea-water salinity contributing to the inhibition of the ooidal carbonate factory (Loreau and Purser, 1973; Lees, 1975; Lucas et al., 1976; Simone, 1981; Loreau, 1982; Tucker et al., 1990) and limiting their reinstallation after the Variabilis/Gradata cooling event. This hypothesis has previously been evoked to explain the demise of carbonate production at the Aalenian/Bajocian (outcrops in Morocco) and at the Bathonian/Callovian (outcrops in France) transitions (Brigaud et al., 2009; Pierre et al., 2010).
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If the carbonate factory recovery was inhibited due to humid conditions and enhanced influx of nutrient-rich waters into coastal settings, then this hypothesis fails, however, to explain the Dispansum/Speciosum extinction event. Indeed, the establishment of more humid conditions takes place at the onset of the Late Toarcian, thus clearly predating the Dispansum/Speciosum event. Obviously, more work is therefore needed in order to constrain the causes and mechanisms causing and controlling the Late Toarcian biological crises.
7. Conclusions
An episode of carbonate ooidal-ramp demise coeval with a second order mass extinction event is reported from the upper middle Toarcian (Variabilis/Gradata ammonite zone) in Morocco and beyond. This event occurred during a transient seawater cooling, arguably triggered by massive and sustained SO2 exhalation from Karoo LIP during its silicic phase. Cooling of seawater temperatures were possibly enhanced and sustained by the demise of the carbonate factory following the model described in Donnadieu et al. (2011). The concomitant allopatric speciation of ammonites documented during this interval suggest that the warm water mass circulation between the Tethys and the continental shelf was disrupted, conducting fact that contributed to further cooling
Brachiopod oxygen isotope data suggest that warm seawater temperatures re-established during the early Late Toarcian. Nevertheless, a coeval re-establishment of neritic carbonate production is lacking. In the view of the authors, this is due to an increase of riverine influx and their siliciclastic sedimentary and dissolve ionic load in the coastal marine domain. Moreover, it is here suggested that enhanced weathering is related to the onset of more humid condition during the Late Toarcian.
7. Acknowledgments
This research was financed by the Deutsche Forschungsgemeinschaft (DFG, project n° BO 3655/1-1). L. Henkel, M. Hönig and T. Kothe are thanked for their help during field expeditions and for laboratory work at Bochum. Analytical work in the isotope laboratories at Bochum were supported by A. Niedermayr. L. Bonvallet and B. Brahimsamba are acknowledged for technical support in the laboratory at Lausanne.
8. References
Alméras, Y., 1996. Brachiopodes toarciens et aaléniens de la Téthys nord-occidentale. Centre des sciences de la terre, Université Claude-Bernard-Lyon I.
Alméras, Y., Elmi, S., 1987. Evolution des peuplements des brachiopodes en fonction de l’ environnent dans le Lias ardéchois. Cahiers Scientifiques de l’Université Catholique de Lyon, Science, Lyon 1, 21–56.
Alméras, Y., Boullier, A., Laurin, B., 1994. La zonation du Jurassique Français par les Brachiopodes: Limites de résolution. Geobios 27, Supplement 2, 69–77.
Anderson, T.F., Arthur, M.A., 1983. Stable isotopes of oxygen and carbon and their application to sedimentologic and palaeoenvironmental problems, Tulsa, pp. 1–15, 57 Figs., 10 Tabs.
Andrews, J.E., 1991. Geochemical indicators of depositional and early diagenetic facies in Holocene carbonate muds, and their preservation potential during stabilisation. Chemical Geology 93, 267–289.
Arias, C., 2007. Pliensbachian–Toarcian ostracod biogeography in NW Europe: Evidence for water mass structure evolution. Palaeogeography, Palaeoclimatology, Palaeoecology 251, 398–421.
Auclair, A.-C., Joachimski, M.M., Lécuyer, C., 2003. Deciphering kinetic, metabolic and environmental controls on stable isotope fractionations between seawater and the shell of Terebratalia transversa (Brachiopoda). Chemical Geology 202, 59–78.
Bailey, T.R., Rosenthal, Y., McArthur, J.M., van de Schootbrugge, B., Thirlwall, M.F., 2003. Palaeoceanographic changes of the Late Pliensbachian-Early Toarcian interval: a possible link to the genesis of an Oceanic Anoxic Event. Earth and Planetary Science Letters 212, 307–320.
Blomeier, D.G., Reijmer, J.G., 1999. Drowning of a Lower Jurassic carbonate platform: Jbel Bou Dahar, High Atlas, Morocco. Facies 41, 81–110.
Bodin, S., Mattioli, E., Fröhlich, S., Marshall, J.D., Boutib, L., Lahsini, S., Redfern, J., 2010. Toarcian carbon isotope shifts and nutrient changes from the Northern margin of Gondwana (High Atlas, Morocco, Jurassic): Palaeoenvironmental implications. Palaeogeography, Palaeoclimatology, Palaeoecology 297, 377–390.
Braga, J.C., Comas-Rengifo, M.J., Goy, A., Rivas, P., 1982. Comparaciones faunisticas y correlaciones en el Pliensbachiense de la Zona Subbética y Cordillera Ibérica. Bolletin de la Reale Sociedad Española de Historia Naturale, Seccion Geologica 80, 133–152.
Brand, U., Logan, A., Hiller, N., Richardson, J., 2003. Geochemistry of modern brachiopods: applications and implications for oceanography and palaeoceanography. Chemical Geology 198, 305–334.
Brand, U., Jiang, G., Azmy, K., Bishop, J., Montañez, I.P., 2012. Diagenetic evaluation of a Pennsylvanian carbonate succession (Bird Spring Formation, Arrow Canyon, Nevada, U.S.A.) — 1: Brachiopod and whole rock comparison. Chemical Geology 308–309, 26–39.
Brigaud, B., Durlet, C., Deconinck, J.F., Vincent, B., Puceat, E., Thierry, J., Trouiller, A., 2009. Facies and climate/environmental changes recorded on a carbonate ramp: A sedimentological and geochemical approach on Middle Jurassic carbonates (Paris Basin, France). Sedimentary Geology 222, 181–206.
19
Bryan, S.E., Ferrari, L., 2013. Large igneous provinces and silicic large igneous provinces: Progress in our understanding over the last 25 years. Geological Society of America Bulletin 125, 1053–1078.
Carpenter, S.J., Lohmann, K.C., 1995. δ18O and δ13C values of modern brachiopod valves. Geochimica et Cosmochimica Acta 59, 3749–3764.
Caruthers, A.H., Smith, P.L., Gröcke, D.R., 2013. The Pliensbachian–Toarcian (Early Jurassic) extinction, a global multi-phased event. Palaeogeography, Palaeoclimatology, Palaeoecology.
Cecca, F., Macchioni, F., 2004. The two Early Toarcian (Early Jurassic) extinction events in ammonoids. Lethaia 37, 35–56.
Christ, N., Immenhauser, A., Amour, F., Mutti, M., Preston, R., Whitaker, F.F., Peterhänsel, A., Egenhoff, S.O., Dunn, P.A., Agar, S.M., 2012. Triassic Latemar cycle tops — Subaerial exposure of platform carbonates under tropical arid climate. Sedimentary Geology 265–266, 1–29.
Cohen, A.S., Coe, A.L., Kemp, D.B., 2007. The late Palaeocene-Early Eocene and Toarcian (Early Jurassic) carbon isotope excursions: a comparison of their time scales, associated environmental changes, causes and consequences. J Geol Soc London 164, 1093–1108.
Compton, J., Mallinson, D., Glenn, C.R., Filippelli, G.M., Föllmi, K.B., Shields, G., Zanin, Y., 2000. Variations in the global phosphorus cycle, in: Glenn, C.R., Prévôt-Lucas, L., Lucas, J. (Eds.), Marine authigenesis; from global to microbial. SEPM Special Publication n° 66, pp. 21–33.
Cope, J.C.W., Getty, T.A., Howarth, M.K., Morton, N., Torrens, H.S., 1980. A Correlation of Jurassic Rocks in the British Isles. Part One: Introduction and Lower Jurassic. Spec. Report Geol. SOC. Lond. 14, 73 pp.
Curry, G.B., Fallick, A.E., 2002. Use of stable oxygen isotope determinations from brachiopod valves in palaeoenvironmental reconstruction. Palaeogeography, Palaeoclimatology, Palaeoecology 182, 133–143.
Cusack, M., Huerta, A.P., Eimf, 2012. Brachiopods recording seawater temperature—A matter of class or maturation? Chemical Geology 334, 139–143.
Dean, W.T., Donovan, D.T., Howarth, M.K., 1961. The Liassic Ammonite zones and subzones of northwest European province; Bulletin of the British Museum (Natural History). Geology 4, 435–505.
Dera, G., Donnadieu, Y., 2012. Modeling evidences for global warming, Arctic seawater freshening, and sluggish oceanic circulation during the Early Toarcian anoxic event. Paleoceanography 27, PA2211.
Dera, G., Pellenard, P., Neige, P., Deconinck, J.-F., Pucéat, E., Dommergues, J.-L., 2009a. Distribution of clay minerals in Early Jurassic Peritethyan seas: Palaeoclimatic significance inferred from multiproxy comparisons. Palaeogeography, Palaeoclimatology, Palaeoecology 271, 39–51.
Dera, G., Pucéat, E., Pellenard, P., Neige, P., Delsate, D., Joachimski, M.M., Reisberg, L., Martinez, M., 2009b. Water mass exchange and variations in seawater temperature in the NW Tethys during the Early Jurassic: Evidence from neodymium and oxygen isotopes of fish teeth and belemnites. Earth and Planetary Science Letters 286, 198–207.
Dera, G., Neige, P., Dommergues, J.L., Fara, E., Laffont, R., Pellenard, P., 2010. High-resolution dynamics of Early
Jurassic marine extinctions: the case of Pliensbachian-Toarcian ammonites (Cephalopoda). J Geol Soc London 167, 21–33.
Dera, G., Brigaud, B., Monna, F., Laffont, R., Puceat, E., Deconinck, J.F., Pellenard, P., Joachimski, M.M., Durlet, C., 2011a. Climatic ups and downs in a disturbed Jurassic world. Geology 39, 215–218.
Dera, G., Neige, P., Dommergues, J.-L., Brayard, A., 2011b. Ammonite paleobiogeography during the Pliensbachian–Toarcian crisis (Early Jurassic) reflecting paleoclimate, eustasy, and extinctions. Global and Planetary Change 78, 92–105.
Dessert, C., Dupre, B., Francois, L.M., Schott, J., Gaillardet, J., Chakrapani, G., Bajpai, S., 2001. Erosion of Deccan Traps determined by river geochemistry: impact on the global climate and the Sr-87/Sr-86 ratio of seawater. Earth and Planetary Science Letters 188, 459–474.
Dessert, C., Dupré, B., Gaillardet, J., François, L.M., Allègre, C.J., 2003. Basalt weathering laws and the impact of basalt weathering on the global carbon cycle. Chemical Geology 202, 257–273.
Dommergues, J.L., Fara, E., Meister, C., 2009. Ammonite diversity and its palaeobiogeographical structure during the early Pliensbachian (Jurassic) in the western Tethys and adjacent areas. Palaeogeography Palaeoclimatology Palaeoecology 280, 64–77.
Donnadieu, Y., Dromart, G., Goddéris, Y., Pucéat, E., Brigaud, B., Dera, G., Dumas, C., Olivier, N., 2011. A mechanism for brief glacial episodes in the Mesozoic greenhouse. Paleoceanography 26, PA3212.
Dromart, G., Allemand, P., Garcia, J.P., Robin, C., 1996. Cyclic fluctuation of carbonate production through the Jurassic along a Burgundy-Ardeche cross-section, eastern France. Bulletin de la Société Géologique de France 167, 423–433.
Du Dresnay, R., 1971. Extension et developpement des phenomenes recifaux jurassique dans le domaine atlasique marocain, particulierement au Lias moyen. Bulletin de la Société Géologique de France 13, 46–56.
Eaton, A.D., Clesceri, L.S., Greenberg, A.E., 1995. Standard methods for the examination of water and wastewater. American Public Health Association Publications, New-York.
El Arabi, E.H., 2007. La série permienne et triasique du rift haut-atlasique: nouvelles datations; evolution tectono-sédimentaire, Faculté des sciences Ain Chok. Université de Casablanca Hassan II, Casablanca, p. 225p.
Ettaki, M., Chellai, E.H., Milhi, A., Sadki, D., Boudchiche, L., 2000. Le passage Lias moyen-Lias superieur dans la region de Todrha-Dades : evenements bio-sedimentaires et geodynamiques (Haut Atlas central, Maroc) - The Middle Lias-Upper Lias crossing in the Todrha-Dades (central High Atlas, Morocco): geodynamic and bio-sedimentary events. Comptes Rendus de l’Academie des Sciences - Series IIA - Earth and Planetary Science 331, 667–674.
Ettaki, M., Ibouh, H., Chellaï, E.H., 2007. Tectono-sedimentary events during Lias-Dogger at the southern margin of the Central High-Atlas, Morocco. Estudios Geológicos 63, 103–125.
Föllmi, K.B., Godet, A., Bodin, S., Linder, P., 2006. Interactions between environmental change and shallow-water carbonate build-up along the northern Tethyan margin and their impact on the early Cretaceous carbon-isotope
20
record. Paleoceanography 21, PA4211, 16 PP.Föllmi, K.B., Bodin, S., Godet, A., Linder, P., Van de
Schootbrugge, B., 2007. Unlocking paleo-environmental information from Early Cretaceous shelf sediments in the Helvetic Alps: stratigraphy is the key! Swiss Journal of Geosciences 100, 349–369.
Frizon de Lamotte, D., Zizi, M., Missenard, Y., Hafid, M., El Azzouzi, M., Maury, R.C., Charrière, A., Taki, Z., Benammi, M., Michard, A., 2008. The Atlas system, in: Michard, A., Saddiqi, O., Chalouan, A., Frizon de Lamotte, D. (Eds.), Continental Evolution: The geology of Morocco. Lecture Notes in Earth Sciences. Springer-Verlag, Berlin, pp. 133–202.
Gaillardet, J., Dupré, B., Louvat, P., Allègre, C.J., 1999. Global silicate weathering and CO2 consumption rates deduced from the chemistry of large rivers. Chemical Geology 159, 3–30.
Garcia Joral, F., Goy, A., 2004. Caracterización de Choffatirhynchia nov. gen. (Brachiopoda, Rhynchonellida) en el Toarciense (Jurásico) de la Cordillera Ibérica (España). Boletín de la Real Sociedad Española de Historia Natural. Sección geológica 99, 237–250.
Ghorbal, B., Bertotti, G., Foeken, J., Andriessen, P., 2008. Unexpected Jurassic to Neogene vertical movements in ‘stable’ parts of NW Africa revealed by low temperature geochronology. Terra Nova 20, 355–363.
Giles, P.S., 2012. Low-latitude Ordovician to Triassic brachiopod habitat temperatures (BHTs) determined from δ18O[brachiopod calcite]: A cold hard look at ice-house tropical oceans. Palaeogeography, Palaeoclimatology, Palaeoecology 317–318, 134–152.
Gómez, J.J., Goy, A., Canales, M.L., 2008. Seawater temperature and carbon isotope variations in belemnites linked to mass extinction during the Toarcian (Early Jurassic) in Central and Northern Spain. Comparison with other European sections. Palaeogeography, Palaeoclimatology, Palaeoecology 258, 28–58.
Gradstein, F.M., Ogg, J.G., Hilgen, F.J., 2012. On The Geologic Time Scale. Newsl Stratigr 45, 171–188.
Gröcke, D.R., Hori, R.S., Trabucho-Alexandre, J., Kemp, D.B., Schwark, L., 2011. An open ocean record of the Toarcian oceanic anoxic event. Solid Earth 2, 245–257.
Grossman, E.L., 2012. Applying Oxygen Isotope Paleothermometry in Deep Time. In L. C. Ivany and B. T. Huber (eds.), Paleontological Society Papers, v. 18. Paleontological Society, p. 39–67.
Guex, J., Morard, A., Bartolini, A., Morettini, E., 2001. Découverte d’une importante lacune stratigraphique à la limite Domérien-Toarcien: implication paléo-océanographiques. Bulletin de la Société Vaudoise des Sciences Naturelles 345, 277–284.
Guex, J., Bartolini, A., Spangenberg, J., Vicente, J.C., Schaltegger, U., 2012. Ammonoid multi-extinction crises during the Late Pliensbachian – Toarcian and carbon cycle instabilities. Solid Earth Discussions 4, 1205–1228.
Halfar, J., Godinez-Orta, L., Mutti, M., Valdez-Holguin, J.E., Borges, J.M., 2006. Carbonates calibrated against oceanographic parameters along a latitudinal transect in the Gulf of California, Mexico. Sedimentology 53, 297–320.
Hallock, P., and Schlager, W., 1986. Nutrient excess and the demise of coral reefs and carbonate platforms. Palaios
1, 389–398.Harazim, D., van de Schootbrugge, B., Sorichter, K., Fiebig,
J., Weug, A., Suan, G., Oschmann, W., 2012. Spatial variability of watermass conditions within the European Epicontinental Seaway during the Early Jurassic (Pliensbachian-Toarcian). Sedimentology 60, 359–390.
Harries, P.J., Little, C.T.S., 1999. The early Toarcian (Early Jurassic) and the Cenomanian–Turonian (Late Cretaceous) mass extinctions: similarities and contrasts. Palaeogeography, Palaeoclimatology, Palaeoecology 154, 39–66.
Hermoso, M., Pellenard, P., Continental weathering and climatic changes inferred from clay mineralogy and paired carbon isotopes across the early to middle Toarcian in the Paris Basin. Palaeogeography, Palaeoclimatology, Palaeoecology.
Hesselbo, S.P., and Jenkyns, H.C., 1995. A comparison of the Hettangian to Bajocian successions of Dorset and Yorkshire, in: Talyor, P.D. (Ed.), Field Geology of the British Jurassic. Geological Society, London, pp. 105–150.
Hesselbo, S.P., Grocke, D.R., Jenkyns, H.C., Bjerrum, C.J., Farrimond, P., Bell, H.S.M., Green, O.R., 2000. Massive dissociation of gas hydrate during a Jurassic oceanic anoxic event. Nature 406, 392–395.
Hesselbo, S.P., Jenkyns, H.C., Duarte, L.V., Oliveira, L.C.V., 2007. Carbon-isotope record of the Early Jurassic (Toarcian) Oceanic Anoxic Event from fossil wood and marine carbonate (Lusitanian Basin, Portugal). Earth and Planetary Science Letters 253, 455–470.
Howarth, M.K., 1992. The ammonite family Hildoceratidae in the Lower Jurassic of Britain. Palaeontographical Society Monograph 145, 1–200.
Huck, S., Rameil, N., Korbar, T., Heimhofer, U., Wieczorek, T.D., Immenhauser, A., 2010. Latitudinally different responses of Tethyan shoal-water carbonate systems to the Early Aptian oceanic anoxic event (OAE 1a). Sedimentology 57, 1585–1614.
Immenhauser, A., Kenter, J.A.M., Ganssen, G., Bahamonde, J.R., Van Vliet, A., Saher, M.H., 2002. Origin and Significance of Isotope Shifts in Pennsylvanian Carbonates (Asturias, NW Spain). Journal of Sedimentary Research 72, 82–94.
Ishigaki S. 1988. — Les empreintes de dinosaures du Jurassique inférieur du Haut Atlas central marocain. Notes du Service géologique du Maroc 44: 79–86.
Jenkyns, H.C., 1985. The Early Toarcian and Cenomanian-Turonian anoxic events in Europe: comparisons and contrasts. Geologische Rundschau 74, 505–518.
Jenkyns, H.C., 1988. The Early Toarcian (Jurassic) Anoxic Event: stratigraphy, sedimentary, and geochemical evidence. American Journal of Science 288, 101–151.
Jenkyns, H.C., Grocke, D.R., Hesselbo, S.P., 2001. Nitrogen isotope evidence for water mass denitrification during the early Toarcian (Jurassic) oceanic anoxic event. Paleoceanography 16, 593–603.
Jenkyns, H.C., Jones, C.E., Grocke, D.R., Hesselbo, S.P., Parkinson, D.N., 2002. Chemostratigraphy of the Jurassic System: applications, limitations and implications for palaeoceanography. J Geol Soc London 159, 351–378.
Jenny, J., 1988. Carte géologique du Maroc au 1/100.000, feuille Azilal (Haut Atlas Central). Notes et Mém. Serv.
21
géol. Maroc 339, 92.Jenny J. & Jossens J. A. 1982. — Découverte d’empreintes
de pas de dinosauriens dans le Jurassique inférieur (Pliensbachien) du Haut Atlas central (Maroc). Comptes Rendus de l’Académie des Sciences, Paris 294: 223–226.
Jourdan, F., Féraud, G., Bertrand, H., Watkeys, M.K., Renne, P.R., 2008. The 40Ar/39Ar ages of the sill complex of the Karoo large igneous province: Implications for the Pliensbachian-Toarcian climate change. Geochemistry, Geophysics, Geosystems 9, Q06009.
Kutzbach, J.E., Guetter, P.J., Washington, W.M., 1990. Simulated circulation of an idealized ocean for Pangaean time. Paleoceanography 5, 299–317.
Lachkar, N., Guiraud, M., El Harfi, A., Dommergues, J.-L., Dera, G., Durlet, C., 2009. Early Jurassic normal faulting in a carbonate extensional basin: characterization of tectonically driven platform drowning (High Atlas rift, Morocco). Journal of Geological Society of London 166, 413–430.
Lees, A., 1975. Possible influence of salinity and temperature on modern shelf carbonate sedimentation. 19, 159–198.
Lezin, C., Rey, J., Faure, P., Cubaynes, R., Pelissie, T., Durlet, C., Deconinck, J.-F., 2007. Tectono-biosedimentary recordings at the Lias-Dogger transition: example of the Quercy carbonate platform (Aquitaine Basin, France). Bulletin de la Société Géologique de France 178, 275–291.
Little, C.T.S., Benton, M.J., 1995. Early Jurassic Mass Extinction - a Global Long-Term Event. Geology 23, 495–498.
Littler, K., Hesselbo, S.P., Jenkyns, H.C., 2010. A carbon-isotope perturbation at the Pliensbachian-Toarcian boundary: evidence from the Lias Group, NE England. Geological Magazine 147, 181–192.
Loreau, J.-P., 1982. Sédiments aragonitiques et leur genese. 47, 300pp.
Loreau, J.P., Purser, B.H., 1973. Distribution and ultrastructure of Holocene ooids in the Persian Gulf. Springer, Berlin, pp. 279–328, 220 Figs.
Lucas, G., Cros, P., Lang, J., 1976. Les roches sedimentaires. 2: Etude microscopique des roches meubles et consolidees. Doin, Paris.
Macchioni, F., 2002. Myths and legends in the correlation between the Boreal and Tethyan Realms. Implications on the dating of the Early Toarcian mass extinctions and the Oceanic Anoxic Event. Geobios 35, Supplement 1, 150–164.
Mailliot, S., Mattioli, E., Guex, J., Pittet, B., 2006. The Early Toarcian anoxia, a synchronous event in the Western Tethys? An approach by quantitative biochronology (Unitary Associations), applied on calcareous nannofossils. Palaeogeography, Palaeoclimatology, Palaeoecology 240, 562–586.
Mattioli, E., Pittet, B., 2004. Spatial and temporal distribution of calcareous nannofossils along a proximal-distal transect in the Lower Jurassic of the Umbria-Marche Basin (central Italy). Palaeogeography Palaeoclimatology Palaeoecology 205, 295–316.
Mattioli, E., Pittet, B., Suan, G., Mailliot, S., 2008. Calcareous
nannoplankton changes across the early Toarcian oceanic anoxic event in the western Tethys. Paleoceanography 23, PA3208.
Mattioli, E., Pittet, B., Petitpierre, L., Mailliot, S., 2009. Dramatic decrease of pelagic carbonate production by nannoplankton across the Early Toarcian anoxic event (T-OAE). Global and Planetary Change 65, 134–145.
McArthur, J.M., Donovan, D.T., Thirlwall, M.F., Fouke, B.W., Mattey, D., 2000. Strontium isotope profile of the early Toarcian (Jurassic) oceanic anoxic event, the duration of ammonite biozones, and belemnite palaeotemperatures. Earth and Planetary Science Letters 179, 269–285.
McArthur, J.M., Algeo, T.J., van de Schootbrugge, B., Li, Q., Howarth, R.J., 2008. Basinal restriction, black shales, Re-Os dating, and the Early Toarcian (Jurassic) oceanic anoxic event. Paleoceanography 23, PA4217.
McElwain, J.C., Wade-Murphy, J., Hesselbo, S.P., 2005. Changes in carbon dioxide during an oceanic anoxic event linked to intrusion into Gondwana coals. Nature 435, 479–482.
Meister, C., Stampfli, G., 2000. Les ammonites du Lias moyen (Pliensbachien) de la Néotéthys et de ses confins; compositions fauniques, affinités paléogéographiques et biodiversité. Revue de Paléobiologie 19, 227–292.
Milhi, A., 1992. Stratigraphie, Fazies und Paläogeographie des Jura am Südrand des zentralen Hohen Atlas (Marokko). Berliner Geowissenschaftliche Abhandlungen 144, 93.
Miller III, W., 2007. Trace Fossils, Concepts, Problems, Prospects, Elsevier Science ed.
Mutti, M., Hallock, P., 2003. Carbonate systems along nutrient and temperature gradients: some sedimentological and geochemical constraints. International Journal of Earth Sciences (Geol. Rundsch.) 92, 465–475.
O’Dogherty, L., Sandoval, J., Vera, J.A., 2000. Ammonite faunal turnover tracing sea-level changes during the Jurassic (Betic Cordillera, southern Spain). J Geol Soc London 157, 723–736.
Page, K.N., 2003. The Lower Jurassic of Europe: its subdivision and correlation. Geological Survey of Denmark and Greenland Bulletin 1, 23–59.
Parkinson, D., Curry, G.B., Cusack, M., Fallick, A.E., 2005. Shell structure, patterns and trends of oxygen and carbon stable isotopes in modern brachiopod valves. Chemical Geology 219, 193–235.
Pérez-Huerta, A., Cusack, M., Dalbeck, P., 2011. Crystallographic contribution to the vital effect in biogenic carbonates Mg/Ca thermometry. Earth Env Sci T R So 102, 35–41.
Pierre, A., Durlet, C., Razin, P., Chellaï, E.H., 2010. Spatial and temporal distribution of ooids along a Jurassic carbonate ramp: Amellago outcrop transect, High-Atlas, Morocco, in: van Buchem, F.S.P., Gerdes, K.D., Esteban, M. (Eds.), Mesozoic and Cenozoic carbonate systems of the Mediterranean and the Middle East: Stratigraphic and diagenetic reference models. Geological Society of London, Special Publication 329, pp. 65–88.
Piqué, A., 1994. Géologie du Maroc - Les domaines régionaux et leur évolution structurale. Pumag, 284 pp.
Price, G.D., Harwood, E., 2012. Isotopic analysis of belemnites and brachiopods from the Cretaceous (Albian) Hunstanton Red Chalk Formation (Hunstanton, Norfolk, UK). Proceedings of the Geologists’ Association 123, 479–485.
Rachidi, M., 2012. La diagénèse d’enfouissement et l’évolution
22
des fluides dans un contexte structural (Jurassique, Moyen et Haut Atlas, Maroc), Faculté des études supérieures et postdoctorales de l’Université Laval. Laval University, Laval, p. 398.
Rampino, M.R., Self, S., 1992. Volcanic winter and accelerated glaciation following the Toba super-eruption. Nature 359, 50–52.
Röhl, H.-J., Schmid-Röhl, A., Oschmann, W., Frimmel, A., Schwark, L., 2001. The Posidonia Shale (Lower Toarcian) of SW-Germany: an oxygen-depleted ecosystem controlled by sea level and palaeoclimate. Palaeogeography, Palaeoclimatology, Palaeoecology 165, 27–52.
Rosales, I., Quesada, S., Robles, S., 2004. Paleotemperature variations of Early Jurassic seawater recorded in geochemical trends of belemnites from the Basque-Cantabrian basin, northern Spain. Palaeogeography Palaeoclimatology Palaeoecology 203, 253–275.
Rubinson, M., Clayton, R.N., 1969. Carbon-13 fractionation between aragonite and calcite. Geochimica et Cosmochimica Acta 33, 997–1002.
Sabatino, N., Neri, R., Bellanca, A., Jenkyns, H.C., Baudin, F., Parisi, G., Masetti, D., 2009. Carbon-isotope records of the Early Jurassic (Toarcian) oceanic anoxic event from the Valdorbia (Umbria-Marche Apennines) and Monte Mangart (Julian Alps) sections: palaeoceanographic and stratigraphic implications. Sedimentology 56, 1307–1328.
Saddiqi, O., El Haimer, F.Z., Michard, A., Barbarand, J., Ruiz, G.M.H., Mansour, E.M., Leturmy, P., Frizon de Lamotte, D., 2009. Apatite fission-track analyses on basement granites from south-western Meseta, Morocco: Paleogeographic implications and interpretation of AFT age discrepancies. Tectonophysics 475, 29–37.
Sandoval, J., O’Dogherty, L., Guex, J., 2001. Evolutionary rates of Jurassic ammonites in relation to sea-level fluctuations. Palaios 16, 311–335.
Sandoval, J., O’Dogherty, L., Vera, J.A., Guex, J., 2002. Sea-level changes and ammonite faunal turnover during the Lias/Dogger transition in the western Tethys. Bulletin de la Société Géologique de France 173, 57–66.
Sandoval, J., Bill, M., Aguado, R., O’Dogherty, L., Rivas, P., Morard, A., Guex, J., 2012. The Toarcian in the Subbetic basin (southern Spain): Bio-events (ammonite and calcareous nannofossils) and carbon-isotope stratigraphy. Palaeogeography, Palaeoclimatology, Palaeoecology 342-343, 40–63.
Schlatter, R.V., 1980. Biostratigraphie und Ammonitenfauna des Unter-Pliensbachium im Typusgebiet (Pliensbach, Holzmaden und Nürtingen;Württemberg, SW.-Deutschland). Stuttgarter Beiträge zur Naturkunde, Serie B (Geologie und Paläontologie) 65 (251 pp.).
Self, S., Blake, S., 2008. Consequences of Explosive Supereruptions. Elements 4, 41–46.
Self, S., Thordarson, T., and Widdowson, M., 2005. Gas fluxes from flood basalt eruptions. Elements 1, 283–287.
Septfontaine, M., 1986. Milieux de dépôt et foraminifères (Lituolidés) de la plate-forme carbonatée du Lias moyen du Maroc. Revue de Micropaléontologie 28, 265–289.
Simone, L., 1981. Ooids: a review. 16, 319–355, 325 Figs.Suan, G., Mattioli, E., Pittet, B., Mailliot, S., Lécuyer, C.,
2008. Evidence for major environmental perturbation prior to and during the Toarcian (Early Jurassic) oceanic anoxic event from the Lusitanian Basin, Portugal.
B., Duarte, L.V., Philippe, M., Reggiani, L., Martineau, F., 2010. Secular environmental precursors to Early Toarcian (Jurassic) extreme climate changes. Earth and Planetary Science Letters 290, 448–458.
Suan, G., Nikitenko, B.L., Rogov, M.A., Baudin, F., Spangenberg, J.E., Knyazev, V.G., Glinskikh, L.A., Goryacheva, A.A., Adatte, T., Riding, J.B., Föllmi, K.B., Pittet, B., Mattioli, E., Lécuyer, C., 2011. Polar record of Early Jurassic massive carbon injection. Earth and Planetary Science Letters 312, 102–113.
Svensen, H., Planke, S., Chevallier, L., Malthe-Sorenssen, A., Corfu, F., Jamtveit, B., 2007. Hydrothermal venting of greenhouse gases triggering Early Jurassic global warming. Earth and Planetary Science Letters 256, 554–566.
Takayanagi, H., Asami, R., Abe, O., Miyajima, T., Kitagawa, H., Sasaki, K., Iryu, Y., 2013. Intraspecific variations in carbon-isotope and oxygen-isotope compositions of a brachiopod Basiliola lucida collected off Okinawa-jima, southwestern Japan. Geochimica et Cosmochimica Acta 115, 115–136.
Tarutani, T., Clayton, R.N., Mayeda, T.K., 1969. The effect of polymorphism and magnesium substitution on oxygen isotope fractionation between calcium carbonate and water. Geochimica et Cosmochimica Acta 33, 987–996.
Trecalli, A., Spangenberg, J., Adatte, T., Föllmi, K.B., Parente, M., 2012. Carbonate platform evidence of ocean acidification at the onset of the early Toarcian oceanic anoxic event. Earth and Planetary Science Letters 357-358, 214–225.
Tucker, M.E., Wright, V.P., Dickson, J.A.D., 1990. Carbonate sedimentology. Blackwell Scientific Publications ; Distributors, USA, Publishers’ Business Services, Oxford [England]; Boston; Brookline Village, Mass.
van de Schootbrugge, B., Föllmi, K.B., Bulot, L.G., Burns, S.J., 2000. Paleoceanographic changes during the early Cretaceous (Valanginian–Hauterivian): evidence from oxygen and carbon stable isotopes. Earth and Planetary Science Letters 181, 15–31.
van de Schootbrugge, B., McArthur, J.M., Bailey, T.R., Rosenthal, Y., Wright, J.D., and Miller, K.G., 2005. Toarcian oceanic anoxic event: An assessment of global causes using belemnite C isotope records. Paleoceanography 20, PA3008.
van Geldern, R., Joachimski, M.M., Day, J., Jansen, U., Alvarez, F., Yolkin, E.A., Ma, X.-P., 2006. Carbon, oxygen and strontium isotope records of Devonian brachiopod shell calcite. Palaeogeography, Palaeoclimatology, Palaeoecology 240, 47–67.
Vörös, A., 1977. Provinciality of Mediterranean Lower Jurassic Brachiopod Fauna - Causes and Plate-Tectonic Implications. Palaeogeography Palaeoclimatology Palaeoecology 21, 1–16.
Wang, S.C., Bush, A.M., 2008. Adjusting global extinction rates to account for taxonomic susceptibility. Paleobiology 34, 434–455.
Warme, J.E., 1988. Jurassic carbonate facies of the Central and Eastern High Atlas rift, Morocco. Lecture Notes in Earth Sciences 15, 169–199.
Wendler, I., 2013. A critical evaluation of carbon isotope
23
stratigraphy and biostratigraphic implications for Late Cretaceous global correlation. Earth-Science Reviews 126, 116–146.
Wendler, I., Huber, B.T., MacLeod, K.G., Wendler, J.E., 2013. Stable oxygen and carbon isotope systematics of exquisitely preserved Turonian foraminifera from Tanzania — Understanding isotopic signatures in fossils. Marine Micropaleontology 102, 1–33.
Wignall, P.B., Bond, D.P.G., 2008. The end-Triassic and Early Jurassic mass extinction records in the British Isles. P Geologist Assoc 119, 73–84.
Wignall, P.B., Newton, R.J., Little, C.T.S., 2005. The timing of paleoenvironmental change and cause-and-effect relationships during the early Jurassic mass extinction in Europe. American Journal of Science 305, 1014–1032.
Wilmsen, M., Neuweiler, F., 2008. Biosedimentology of the Early Jurassic post-extinction carbonate depositional
system, central High Atlas rift basin, Morocco. Sedimentology 55, 773–807.
Wilmsen, M., Blau, J., Meister, C., Mehdi, M., Neuweiler, F., 2002. Early Jurassic (Sinemurian to Toarcian) ammonites from the central High Atlas (Morocco) between Er-Rachidia and Rich. Revue de Paléobiologie, Genève 21, 149–175.
Wood, R., 1993. Nutrients, predation and the history of reef-building. Palaios 8, 526–543.
Zavattieri, A.M., Rosenfeld, U., Volkheimer, W., 2008. Palynofacies analysis and sedimentary environment of Early Jurassic coastal sediments at the southern border of the Neuquén Basin, Argentina. Journal of South American Earth Sciences 25, 227–245.
Zeebe, R.E., Wolf-Gladrow, D., 2001. CO2 in Seawater: Equilibrium, Kinetics, Isotopes. Elsevier Science.
