Julien Plancq
email: julien.plancq@glasgow.ac.uk

Emanuela Mattioli
email: emanuela.mattioli@univ-lyon1.fr

Bernard Pittet
email: bernard.pittet@univ-lyon1.fr

François Baudin
email: francois.baudin@upmc.fr

Vitor Luis

Duarte
email: lduarte@dct.uc.pt

V Boussaha

Vincent Grossi
email: vincent.grossi@univ-lyon1.fr

L V Duarte

M Boussaha
email: myriam.boussaha@ign.ku.dk

A calcareous nannofossil and organic geochemical study of marine palaeoenvironmental changes across the Sinemurian

Keywords: productivity, sea-level, evolutionary events, Hispanic Corridor, coccoliths, lipid biomarkers

come   L'archive ouverte pluridisciplinaire

A C C E P T E D

M A N U S C R I P T

Introduction

The early Jurassic is punctuated by episodes of organic matter (OM) deposition and preservation, probably testifying to the sporadic development of dysoxic/anoxic conditions in the epicontinental basins of the western Tethys associated with peculiar tectonic and climatic conditions (e.g., [START_REF] Jenkyns | Chemostratigraphy of the Jurassic System: applications, limitations and implications for palaeoceanography[END_REF][START_REF] Rosales | Palaeotemperature variations of Early Jurassic seawater recorded in geochemical trends of belemnites from the Basque Cantabrian basin, northern Spain[END_REF][START_REF] Van De Schootbrugge | Early Jurassic climate change and the radiation of organic walled phytoplankton in the Tethys Ocean[END_REF].

Amongst them, the Sinemurian/Pliensbachian interval is of particular interest because in some areas like the Lusitanian (Portugal), Basque-Cantabrian (Spain) and Bristol Channel and Wessex (England) basins, laminated OM-rich levels (black shales) are characterized by a very high (up to 20%) total organic carbon (TOC) content [START_REF] Jenkyns | Chemostratigraphy of the Jurassic System: applications, limitations and implications for palaeoceanography[END_REF][START_REF] Deconinck | Environmental controls on clay mineralogy of an Early Jurassic mudrock (Blue Lias Formation, southern England)[END_REF][START_REF] Rosales | Palaeotemperature variations of Early Jurassic seawater recorded in geochemical trends of belemnites from the Basque Cantabrian basin, northern Spain[END_REF][START_REF] Van De Schootbrugge | Early Jurassic climate change and the radiation of organic walled phytoplankton in the Tethys Ocean[END_REF][START_REF] Duarte | Organicrich facies in the Sinemurian and Pliensbachian of the Lusitanian Basin, Portugal: Total organic carbon and relation to transgressive-regressive facies cycles[END_REF]. In the Lusitanian Basin, the richness in organic matter makes the late Sinemurian-early Pliensbachian one of the most important intervals of potential oil source rocks (e.g., Duarte et al., 2010[START_REF] Duarte | High resolution stratigraphy, palynofacies and source rock potential of the Água de Madeiros formation (Lower Jurassic), Lusitanian Basin, Portugal[END_REF][START_REF] Silva | Optical and geochemical characterization of Upper Sinemurian (Lower Jurassic) fossil wood from the Lusitanian Basin (Portugal)[END_REF].

The Sinemurian/Pliensbachian boundary (~191 Ma) has been well documented for its macroinvertebrate record (e.g., [START_REF] Mouterde | Le Lias du Portugal. Vue d'ensemble et division en zones[END_REF][START_REF] Antunes | Faunule ichtyologique du Lias inférieur de São Pedro de Moel, Portugal[END_REF][START_REF] Dommergues | Endemism as a palaeobiogeographic parameter of basin history illustrated by early-and mid-Liassic peri-Tethyan ammonite faunas[END_REF][START_REF] Meister | Ammonites from the Apoderoceras beds (Early Pliensbachian) in São Pedro de Muel (Lusitanian Basin, Portugal)[END_REF][START_REF] Comas-Rengifo | Fácies carbonatadas ricas em matéria orgânica do Jurássico da Bacia Lusitânica. Novos contributos paleontológicos, sedimentológicos e geoquímicos[END_REF][START_REF] Duarte | Carbon isotope stratigraphy and ammonite biochronostratigraphy across the Sinemurian-Pliensbachian boundary in the western Iberian margin[END_REF], and a precise stratigraphic framework has allowed definition of the Global Stratotype Section and Point (GSSP) of the Pliensbachian Stage at Wine Haven, Yorkshire [START_REF] Meister | The Global Boundary Stratotype Section and Point (GSSP) for the base of the Pliensbachian Stage (Lower Jurassic)[END_REF].

This time interval is also recognized as one of the most important steps in the radiation of planktonic organisms, especially primary producers such as dinoflagellates (van de

A C C E P T E D M A N U S C R I P T

Schootbrugge et al., 2005) and coccolithophores [START_REF] Bown | Calcareous nannoplankton evolution and diversity through time[END_REF]. It is around this boundary that first occurred the placolith coccolith-type (for example Similiscutum), which marks an important event in the diversification history of the coccolithophores. This radiation is probably linked to major palaeoceanographic changes related to the opening of the Hispanic

Corridor described by some authors (van de Schootbrugge et al., 2005;[START_REF] Dera | Nd isotope constraints on ocean circulation, paleoclimate, and continental drainage during the Jurassic breakup of Pangea[END_REF]. A negative excursion in the carbon isotopic composition (δ 13 C) in carbonates has been recorded

in the Cleveland Basin (England) and interpreted as a major perturbation in the carbon cycle (e.g., [START_REF] Jenkyns | Chemostratigraphy of the Jurassic System: applications, limitations and implications for palaeoceanography[END_REF][START_REF] Korte | Shallow marine carbon and oxygen isotope and elemental records indicate icehouse-greenhouse cycles during the Early Jurassic[END_REF]. In spite of this, there is no detailed study documenting changes in planktonic assemblages across the Sinemurian/Pliensbachian boundary, while the palaeoceanographic and/or productivity changes occurring at that time are poorly understood. In addition, only few studies have provided isotopic data for the Sinemurian/Pliensbachian, and the main features and causes of the δ 13 C excursion remain unclear (e.g., [START_REF] Hesselbo | A potential global stratotype for the Sinemurian-Pliensbachian boundary (Lower Jurassic), Robin Hood's Bay, UK: ammonite faunas and isotope stratigraphy[END_REF][START_REF] Jenkyns | Chemostratigraphy of the Jurassic System: applications, limitations and implications for palaeoceanography[END_REF][START_REF] Duarte | Carbon isotope stratigraphy and ammonite biochronostratigraphy across the Sinemurian-Pliensbachian boundary in the western Iberian margin[END_REF].

The aim of this study is thus to appraise the palaeoenvironmental changes occurring across the Sinemurian/Pliensbachian boundary at the São Pedro de Moel section (Lusitanian Basin, Portugal) using micropalaeontology and organic geochemistry approaches at high resolution, and to relate these changes to the palaeoceanographic events characterizing the western Tethys in the early Jurassic. Sedimentological analyses combined to bulk organic matter characterization, calcareous nannofossil quantification, and lipid biomarker analysis allow for the first time discussing the changes in primary productivity, the major evolutionary events and their links with sea-level changes in Sinemurian/Pliensbachian sediments.

Geological setting

A C C E P T E D M A N U S C R I P T

The composite São Pedro de Moel section comprises the sections of Água de Madeiros (39°44´27˝ N; 9°02´20˝ W) and Polvoeira-Pedra do Ouro (39°43´18˝ N; 9°02´56˝ W) and outcrops along the coastline of central-western Portugal, approximately 110 km north of Lisbon [START_REF] Duarte | Litostratigrafia das séries margo-calcárias do Jurássico Inferior da Bacia Lusitânica (Portugal)[END_REF][START_REF] Duarte | High resolution stratigraphy, palynofacies and source rock potential of the Água de Madeiros formation (Lower Jurassic), Lusitanian Basin, Portugal[END_REF] (Fig. 1a). This locality was palaeogeographically located in the central-western part of the Lusitanian Basin. In the Jurassic, this narrow and north-south elongated basin developed at the western margin of the Tethys Ocean, and resulted from the aborted rifting of the proto-Atlantic [START_REF] Manspeizer | Triassic-Jurassic rifting, Continental breakup and the origin of the Atlantic Ocean and passive margins[END_REF][START_REF] Soares | Le sous bassin nord lusitanien (Portugal) du Trias au Jurassique moyen: Histoire d'un "rift avorté[END_REF]. The Lusitanian Basin was bounded to the East by the Iberian Meseta and to the West by the Variscan (granitic and metamorphic) Berlenga-Farilhões Horst (Fig. 1b andc). The Lusitanian Basin was connected to the north-western European epicontinental basins and to the Hispanic Corridor to the south, thus representing a key location between the proto-Atlantic and Tethyan oceanic domains (Fig. 1b). The peculiar setting of São Pedro de Moel in the central deepest part of the basin accounts for the completeness of the upper Sinemurianlower Pliensbachian sedimentary record in a relatively expanded 58 m-thick section.

Numerous ammonite and calcareous nannofossil bioevents are recorded in this section, providing a detailed biostratigraphic framework (e.g. [START_REF] Mouterde | Le Lias du Portugal. Vue d'ensemble et division en zones[END_REF][START_REF] Antunes | Faunule ichtyologique du Lias inférieur de São Pedro de Moel, Portugal[END_REF][START_REF] Meister | Ammonites from the Apoderoceras beds (Early Pliensbachian) in São Pedro de Muel (Lusitanian Basin, Portugal)[END_REF][START_REF] Comas-Rengifo | Fácies carbonatadas ricas em matéria orgânica do Jurássico da Bacia Lusitânica. Novos contributos paleontológicos, sedimentológicos e geoquímicos[END_REF][START_REF] Mattioli | Calcareous nannofossil biostratigraphy: New data from the Lower Jurassic of the Lusitanian Basin[END_REF][START_REF] Duarte | Carbon isotope stratigraphy and ammonite biochronostratigraphy across the Sinemurian-Pliensbachian boundary in the western Iberian margin[END_REF][START_REF] Boussaha | Spatial characterization of the late Sinemurian (Early Jurassic) palaeoenvironments in the Lusitanian Basin[END_REF]. The studied time interval includes the Ammonite Oxynotum, Raricostatum and Jamesoni Zones and the Nannofossil NJT 3a, NJT 3b and NJT 4 Zones (Fig. 2).

In the upper Sinemurian/lower Pliensbachian, the sedimentation was characterized by marl-limestone alternations with common occurrence of black shales (organic carbon-rich sedimentary levels), belonging to the Água de Madeiros and Vale das Fontes formations. The Água de Madeiros Formation can be subdivided into the Polvoeira and Praia da Pedra Lisa members [START_REF] Duarte | Litostratigrafia das séries margo-calcárias do Jurássico Inferior da Bacia Lusitânica (Portugal)[END_REF][START_REF] Duarte | Early Jurassic carbonate evolution in the Lusitanian Basin: Facies, sequence stratigraphy and cyclicity[END_REF][START_REF] Duarte | Organicrich facies in the Sinemurian and Pliensbachian of the Lusitanian Basin, Portugal: Total organic carbon and relation to transgressive-regressive facies cycles[END_REF][START_REF] Duarte | Lithostratigraphy, sequence stratigraphy and depositional setting of the Pliensbachian and Toarcian series in the Lusitanian Basin (Portugal)[END_REF]. Interpretation of

A C C E P T E D M A N U S C R I P T
the depositional environments of the São Pedro de Moel section and its sequence stratigraphical evolution have been presented by [START_REF] Boussaha | Spatial characterization of the late Sinemurian (Early Jurassic) palaeoenvironments in the Lusitanian Basin[END_REF] and are summarized in Figure 2. Briefly, the first 42 meters of the section are characterized by a 2 nd -order transgression (see [START_REF] Duarte | Organicrich facies in the Sinemurian and Pliensbachian of the Lusitanian Basin, Portugal: Total organic carbon and relation to transgressive-regressive facies cycles[END_REF] indicated by facies evolution from mid to outer-ramp environments. Black shales interbedded with storm deposits are frequently recorded in this interval. Above, the facies succession suggests an evolution from outer to mid-to-inner-ramp, attesting for a major sea-level fall. The lowest sea level is recorded between 55 and 62 meters with a sedimentation dominated by amalgamated tempestites that show hummocky crossstratifications (HCS). The upper part of the section (62-89 m) is characterized by marllimestone alternations increasingly argillaceous upwards, testifying for a progressive rise in sea level in an outer-ramp environment (Fig. 2).

Methods

Calcareous nannofossils

Eighty-six samples were analysed for calcareous nannofossils, 45 from Pedra do Ouro section and 41 from Água de Madeiros section. Samples were prepared following the technique of [START_REF] Beaufort | Adaptation of the random settling method for quantitative studies of calcareous nannofossils[END_REF] modified by [START_REF] Geisen | Calibration of the random settling technique for calculation of absolute abundances of calcareous nannoplankton[END_REF] allowing absolute quantification of nannofossil abundances per gram of rock. Slides were then studied in a ZEISS Axioscope 40 polarising microscope, using a 1000 X magnification. As lower Jurassic nannofossil abundance is low, 250 nannofossils on average (both coccoliths and the incertae sedis Schizosphaerella spp. and Orthogonoides hamiltoniae) could be counted in each slide.

The intervals comprised between 20 and 26 meters (base of the section studied at Pedra do Ouro; [START_REF] Boussaha | Spatial characterization of the late Sinemurian (Early Jurassic) palaeoenvironments in the Lusitanian Basin[END_REF] and between 62 and 65 meters (after the tempestite interval in Água de Madeiros section) are extremely poor in terms of nannofossil abundance. In these

A C C E P T E D M A N U S C R I P T
intervals, total absolute abundances could be calculated but the scarcity of the specimens counted (less than 100) prevented us to confidently calculate the relative abundances of each coccolith taxa. Relative abundance of Schizosphaerella spp. and Orthogonoides hamiltoniae were calculated with respect to the total nannofossil abundance for 86 samples, while speciesspecific relative abundances of the coccolith species were calculated with respect to the counted coccolith number for the 49 richest samples.

Nannofossil preservation was also evaluated in each sample by taking into account the degree of etching and overgrowth according to [START_REF] Roth | Preservation of calcareous nannofossils and fine grained carbonate particles in mid cretaceous sediments from the southern Angola Basin[END_REF], as well as the degree of fragmentation. Three classes of preservation were thus used: poor for nannofossil assemblages showing strong etching, overgrowth or fragmentation, moderate for moderate etching, overgrowth or fragmentation, and good when Schizosphaerella spp. Fragmentation was limited or absent and delicate coccoliths displayed relatively pristine structures.

Rock-Eval pyrolysis

In order to obtain information on the type and thermal maturity of the bulk OM, Rock-Eval analyses were performed on 23 samples with a Rock-Eval device (Vinci Technologies) under standard conditions (see all details about Rock-Eval pyrolysis and parameters in [START_REF] Espitalié | Rock Eval pyrolysis[END_REF][START_REF] Behar | Rock-Eval 6 technology: performances and developments[END_REF]. Measurements made include S 1 (mg HC/g rock), S 2 (mg HC/g rock), S 3 (mg CO 2 /g rock) and T max (°C). Derived parameters such as the Total Organic Carbon (TOC) content (wt.%), the Hydrogen Index (HI = S 2 /TOCx100; mg HC/g TOC) and

the Oxygen Index (OI= S 3 /TOCx100; mg CO 2 /g TOC) were also determined.

Lipid biomarkers

From the 23 samples used for Rock-Eval analyses, 19 organic matter-rich marls were selected and studied for their aliphatic hydrocarbon composition. Samples (ca. ~30 g) were

A C C E P T E D M A N U S C
v/v),
and polars with DCM-methanol (1:1 v/v). Aliphatic hydrocarbons were then dried under a stream of nitrogen, and a known amount of squalane was added as an internal standard before analysis by gas chromatography (GC/FID) and gas chromatography/mass spectrometry (GC/MS).

Hydrocarbons were identified using a MD800 Voyager spectrometer interfaced to an HP6890 gas chromatograph equipped with an on-column injector and a DB-5MS column (30 m x 0.25 mm x 0.25 µm). The oven temperature was programmed from 60°C (1 min) to 130°C at 20°C min -1 , and then to 310°C (20 min) at 4°C min -1 . Helium was used as the carrier gas at constant flow (1 mL.min -1 ). Selective detection of hopanes and steranes was realized by ion extraction (191 and 217, respectively).

Compound abundances were determined by GC/FID using a HP6890 Series gas chromatograph configured as for GC-MS analyses.

Biomarkers used in this study

To better characterize changes in primary productivity, we have selected some biomarkers to be analysed. These are:

(1) Phytane, a chlorophyll-derived (phytol) biomarker that can be used as an indicator of primary productivity [START_REF] Didyk | Organic geochemical indicators of palaeoenvironmental conditions of sedimentation[END_REF];

A C C E P T E D M A N U S C R I P T
(2) Sterols, produced by eukaryotic algae, protists and multicellular eukaryotes [START_REF] Volkman | Microalgal biomarkers: A review of recent research developments[END_REF]. Algae and metazoa are the main producers of C27 sterols, whereas C28 sterols are associated with chlorophyll c-containing phytoplankton such as diatoms, dinoflagellates and coccolithophores [START_REF] Knoll | The geological succession of primary producers in the oceans[END_REF]. C29 sterols are mostly attributed to higher land plants [START_REF] Volkman | Microalgal biomarkers: A review of recent research developments[END_REF] although contribution from marine green algae [START_REF] Knoll | The geological succession of primary producers in the oceans[END_REF] or freshwater microalgae [START_REF] Volkman | Eustigmatophyte microalgae are potential sources of C 29 sterols, C 22 -C 28 n-alcohols and C 28 -C 32 n-alkyl diols in freshwater environments[END_REF][START_REF] Kodner | Sterols in a unicellular relative of the metazoans[END_REF]) is also possible. Therefore, secular variations of different groups of steranes can be interpreted in terms of ecological successions among groups of organisms and/or varying terrestrial inputs to the sediments;

(3) C27-C35 hopanes, derived from C35 bacteriohopanepolyols synthesized by some members of the Bacteria including cyanobacteria [START_REF] Rohmer | Distribution of hopanoid triterpenes in prokaryotes[END_REF][START_REF] Ourisson | Prokaryotic hopanoids and other polyterpenoid sterol surrogates[END_REF].

The proportions of regular C27-29 steranes relative to C30-35 hopanes, here expressed as St/(St+Hop) ratio, can be used for estimating the relative input of eukaryotic (mostly algae and/or higher plants) and bacterial biomass to sediments [START_REF] Peters | The Biomarker Guide[END_REF].

Isotope analyses

In order to measure their organic carbon isotopic composition (δ 13 C TOC ), aliquots of the 19 marl samples studied for their aliphatic hydrocarbon composition were decarbonated using 2N

HCl for 24h at ambient temperature. They were then rinsed with deionized water, centrifuged, rinsed again until neutrality and dried in an oven at 60°C. About 0.2 mg of decarbonated sample were weighed in tin capsules and placed in a Eurovector Elemental Analyzer (EuroEA3028-HT) connected to a GV instrument Isoprime TM stable isotope-ratio mass spectrometer. The stable carbon isotope ratios are reported in the delta notation as the per mil (‰) deviation relative to the Vienna Pee Dee belemnite (V-PDB) standard. Analytical precision and accuracy were determined by triplicate analyses and by comparison with international (IAEA CH7; δ 13 C= -28.73‰) and in-house (TPAC, Phenyl/Amine; δ 13 C= -

A C C E P T E D M A N U S C R I P T 32.15‰
) standards, which were analysed continuously throughout the generation of each isotopic data set. Reproducibility was better than 0.1 ‰ for δ 13 C TOC values.

Finally, the carbon isotopic composition of carbonates (δ 13 C carb ) was determined for samples A to N and SPM-OU, by using an auto sampler Multiprep coupled to a Dual-Inlet GV-Isoprime TM mass spectrometer. For each sample, an aliquot of calcium carbonate (between 1150 and 500 µg according to the initial carbonate calcium content of the sample)

was reacted with anhydrous oversaturated phosphoric acid at 90°C during 20 min. Isotope values are quoted in the delta notation in per mil relative to V-PDB. Each sample analysis was duplicated and adjusted to the international reference NIST NBS19 (δ 13 C= +1.95‰).

Reproducibility was 0.03 ‰ for δ 13 C carb values.

Results

Calcareous nannofossils

Nannofossil preservation is generally moderate to good in the São Pedro de Moel composite section, with fragile structures such as the long and delicate spines of Parhabdolithus liasicus liasicus commonly observed.

Nannofossil absolute abundances (both coccoliths and nannoliths) greatly fluctuated in the studied interval (Fig. 3). On the long-term trend, very low values (<10*10 6 nannofossils/g of sediment) are recorded in the Oxynotum Ammonite Zone, but an increase up to 275*10 6 nannofossils/g of sediment is observed in the Raricostatum Ammonite Zone reaching the highest values around the maximum flooding surface (MFS) (Fig. 3). This is followed by a decrease to very low values in the base of the Jamesoni Zone (including the interval dominated by the HCS-bearing tempestites) where a forced regression (FR) occurred.

Absolute abundance progressively rises again up to 180*10 6 nannofossils/g of sediment

A C C E P T E D M A N U S C R I P T
afterwards, but never reaches again the high values observed around the MFS in the Raricostatum Zone. At a higher frequency, fluctuations are observed in link with lithology, namely higher values are recorded in marly intervals and lower values in limestones (Fig. 3).

Nannofossil assemblages are dominated by Schizosphaerella spp. in most of the studied interval, as testified by its very high proportions (between 75 and 100%) in the Oxynotum and Raricostatum Zones, except at the transition between these two zones where percentages drop down to ~25% (Fig. 3). In the Jamesoni Zone, Schizosphaerella spp. percentages tend to decrease especially in the upper part of the composite section. Fluctuations at a higher frequency are also observed in link with lithology, but contrary to nannofossil absolute abundances, higher values of Schizosphaerella relative abundances are recorded in limestones and lower values in marly intervals (Fig. 3).

The coccolith assemblages are mainly represented by the species Parhabdolithus liasicus distinctus, Parhabdolithus liasicus liasicus, Parhabdolithus robustus, Tubirhabdus patulus, Crucirhabdus primulus, Crepidolithus pliensbachensis, Crepidolithus crassus, Mitrolithus lenticularis, Mitrolithus jansae, and Mitrolithus elegans. Some taxa, namely P. liasicus liasicus, T. patulus, and C. pliensbachensis, show a trend similar to that of nannofossil absolute abundances (Fig. 3). On the contrary, P. liasicus distinctus shows an opposite trend, with lowest relative abundances recorded when nannofossil absolute abundance values are at the highest (Fig. 3). M. jansae and M. elegans peak in few peculiar intervals in some OM-rich layers deposited above storm events and after the HCS-bearing tempestite interval (62-70 m; Fig. 3).

TOC and Rock-Eval pyrolysis

A C C E P T E D M A N U S C R I P T

The studied samples are characterized by relatively high TOC content (4.5 wt.% on average). Higher values are recorded in the Raricostatum Zone and a trend to decreasing values, from 5.8 to 3.3 wt.%, is observed towards the Jamesoni Zone (Fig. 4).

Rock-Eval S 1 and S 2 parameters vary between 0.09 and 1.41 mg HC/g rock and between 4.25 and 42.77 mg HC/g rock, respectively. The HI and OI values range from 154 to 561 mg HC/g TOC and from 23 to 86 mg CO 2 /g TOC, respectively. T max values vary between 417 and 437°C, with an average of 426°C. The crossplot of HI/T max indicates that OM is immature and has undergone only minor thermal diagenesis (Fig. 5). The OM is of Type II and thus dominantly of marine origin, as shown by the plot HI versus OI data (Fig. 5). A general increase of OI is noted with decreasing HI and TOC, suggesting that, for some samples, the marine organic matter suffered a slight oxidation at the time of its deposition.

All these results are consistent with those presented by [START_REF] Duarte | High resolution stratigraphy, palynofacies and source rock potential of the Água de Madeiros formation (Lower Jurassic), Lusitanian Basin, Portugal[END_REF] and Poças Ribeiro et al. ( 2013), who studied the palynofacies and the organic geochemistry (TOC content, Rock-Eval pyrolysis) of the São Pedro de Moel composite section at a higher resolution.

Lipid biomarkers

The presence of biodegradable short chain n-alkanes and acyclic isoprenoids such as pristane (Pr) and phytane (Ph) suggests slight to moderate levels of biodegradation of the OM present in the samples [START_REF] Peters | The Biomarker Guide[END_REF]. This implies that more resistant biomarkers such as regular C27-29 steranes and C29-C32 hopanes used in this study have not been severely affected by biodegradation.

The concentration in phytane shows a decreasing trend, with a mean phytane content of

A C C E P T E D M A N U S C R I P T

The Steranes/Hopanes ratio (St/(St+Hop)) is low (<1) throughout the entire studied interval but a decreasing trend is observed (from 0.44 to 0.32), especially in the Jamesoni Zone above the HCS-bearing tempestite interval (Fig. 4).

The C 27 (marine plankton), C 28 (chlorophyll-c phytoplankton) and, especially, C 29 (mostly higher land plants) sterane concentrations show the same trend across the studied interval: an increase through the Raricostatum and Jamesoni Zones with a peak at the top of the HCSbearing interval, followed by a decrease during the transgressive trend of the Jamesoni Zone (Fig. 4). Through the whole studied interval, C 29 steranes show higher concentrations (17 µg/g TOC) compared to C 27 (7 µg/g TOC) and C28 steranes (4 µg/g TOC), with a much pronounced peak concentration following the tempestite interval. The sum of hopanes shows an increasing trend throughout the studied interval, rising from 36 µg/gTOC in the Rariscotatum Zone to 90 µg/gTOC in the Jamesoni Zone (Fig. 4).

Carbon isotopes

The δ 13 C carb values vary between -1.59‰ and +1.21‰ (Fig. 4). A negative shift of ~2‰ is observed around the Sinemurian/Pliensbachian boundary (Raricostatum/Jamesoni Zones).

Our results are similar to those presented by [START_REF] Duarte | Carbon isotope stratigraphy and ammonite biochronostratigraphy across the Sinemurian-Pliensbachian boundary in the western Iberian margin[END_REF] for the same section and the same time interval.

The δ 13 C TOC values determined across the studied section vary between -28.16‰ and -29.70‰. Two negative spikes are observed separated by a positive peak occurring at the Sinemurian/Pliensbachian transition (Fig. 4).

The δ 13 C TOC does not covary with the δ 13 C carb and an opposite trend is observed between δ 13 C TOC and phytane concentration (R 2 = -0.41; p = 0.0028), with more depleted values (-29.5 ‰) recorded when phytane concentration is high (Raricostatum Zone) and less depleted values (-28.5 ‰) for low phytane concentrations (Jamesoni Zone).

A C C E P T E D M A N U S C R I P T

Discussion

Changes in sea level and primary production at São Pedro de Moel

Sea-level changes have been described at the São Pedro de Moel composite section in previous sequence stratigraphy studies [START_REF] Duarte | Organicrich facies in the Sinemurian and Pliensbachian of the Lusitanian Basin, Portugal: Total organic carbon and relation to transgressive-regressive facies cycles[END_REF][START_REF] Boussaha | Spatial characterization of the late Sinemurian (Early Jurassic) palaeoenvironments in the Lusitanian Basin[END_REF]. In the present work, variations in calcareous nannofossil absolute abundances mirror those of the sea level that determined the eco-space available, namely the depth of the photic zone where calcareous phytoplankton could thrive (Fig. 6). This photic zone was reduced in times of low sea level and more expanded in times of transgression or maximum flooding. First, absolute abundance increased after the rapid flooding that occurred in the upper part of the Oxynotum Zone. Values were the highest around the maximum flooding surface of the Raricostatum Zone, and then dropped during the forced regression at the base of the Jamesoni Zone.

Finally, abundances rose again progressively after the sequence boundary of the Jamesoni

Zone when the sea level progressively increased.

Relative abundances of Schizosphaerella spp. also match with these sea-level changes. [START_REF] Mattioli | Nannoplankton productivity and diagenesis in the rhythmically bedded Toarcian-Aalenian Fiuminata section (Umbria-Marche Apennine, central Italy)[END_REF] and [START_REF] Mattioli | Spatial and temporal distribution of calcareous nannofossils along a proximal-distal transect in the Lower Jurassic of the Umbria-Marche Basin (central Italy)[END_REF] argued that this nannolith preferentially developed in proximal settings where water mixing during storms favoured the resuspension of the nutrients in surface waters. Schizosphaerella spp. has also been reported to present higher percentages in calcareous than in marly lithologies in different early to late Jurassic settings, when oligotrophic conditions occurred both in platform environments and basinal surface waters (e.g., [START_REF] Noël | Le nannoplancton calcaire et la formation des alternances calcaires-marnes dans le Lias des bassins de Marches-Ombrie (Italie)[END_REF][START_REF] Bucefalo Palliani | Phytoplankton response and geochemical evidence of the lower Toarcian relative sea level rise in the Umbria-Marche basin (Central Italy)[END_REF][START_REF] Mattioli | Contribution of calcareous nannoplankton to carbonate deposition: a new approach applied to the Early Jurassic of central Italy[END_REF][START_REF] Pittet | The carbonate signal and calcareous nannofossil distribution in an Upper Jurassic section (Balingen-Tieringen, Late Oxfordian, southern Germany)[END_REF]. All these observations suggest that this taxon preferentially inhabited proximal settings, and was abundant during periods favourable to carbonate

A C C E P T E D M A N U S C R I P T

deposition that commonly corresponded to relative oligotrophic conditions with occasional replenishment in nutrients following storm events.

Variations in abundances of each coccolith species and their link to sea-level changes are more difficult to interpret, since little is known about the ecology of Jurassic taxa. Some taxa, namely Parhabdolithus liasicus liasicus and Tubirhabdus patulus, seem to be more abundant during maximum flooding or transgressive phases, whereas P. liasicus distinctus shows higher relative abundances in the beginning of the transgressive intervals (Fig. 6). The apparent negative correlation between P. liasicus liasicus and P. liasicus distinctus may be explained by their differences of morphology. Both coccoliths bear spines but P. liasicus distinctus is more robust with a shorter spine than P. liasicus liasicus, which possesses a very long and thin spine. Although the presence of spines in coccoliths has been linked to protection from zooplankton predation (Young, 1994), the dominance of the long-spined P.

liasicus liasicus coccoliths may have also increased the buoyancy of the coccosphere in relatively more stratified and distal environments. In an analogous way, long-spined acritarchs (i.e., Micrhystidium) are interpreted by some authors as typical of relatively open and distal environments in Jurassic [START_REF] Bucefalo Palliani | Phytoplankton response and geochemical evidence of the lower Toarcian relative sea level rise in the Umbria-Marche basin (Central Italy)[END_REF] and Holocene [START_REF] Mudie | Non-pollen palynomorphs in the Black Sea corridor[END_REF] series. It is thus likely that P. liasicus liasicus was dwelling in a stratified upper photic zone and in relatively distal environments, where it was likely more competitive than P. liasicus distinctus.

When looking at variations in TOC and lipid biomarker data, it becomes clear that changes in primary productivity did occur in concert with sea-level changes at the São Pedro de Moel section (Fig. 6). OM preservation may have been enhanced when the basin was the most restricted, a condition that is attained during relatively low sea level in a narrow basin such as the Lusitanian Basin [START_REF] Boussaha | Spatial characterization of the late Sinemurian (Early Jurassic) palaeoenvironments in the Lusitanian Basin[END_REF]. However, high productivity is also recorded around the maximum flooding interval in the Raricostatum Zone (Fig. 6). This suggests that

A C C E P T E D M A N U S C R I P T

OM accumulation was not only favoured by restriction of the basin, but also by peculiar palaeoenvironmental conditions (oceanographic, climatic) triggering high productivity.

Based on the comparison between variations in calcareous nannofossil absolute and relative abundances, in organic matter contents and in biomarker concentrations, five main intervals of varying sea level and primary production can be identified across the studied time interval (Figs. 6 and7):

(1) The Oxynotum Zone is characterized by very high, although fluctuating, TOC contents (Figs. 4 and6). OM preservation was particularly effective when the basin was the most restricted, a condition that is attained at the sequence boundary around 20 m. The Lusitanian Basin was then an inner to outer-ramp, with relatively low sea level [START_REF] Boussaha | Spatial characterization of the late Sinemurian (Early Jurassic) palaeoenvironments in the Lusitanian Basin[END_REF].

Very low nannofossil absolute abundances are recorded in this interval, probably due to a reduced photic zone (Fig. 7).

(2) Calcareous nannofossil absolute abundance increased after the rapid flooding that occurred in the upper part of the Oxynotum Zone and values were the highest around the maximum flooding surface of the Raricostatum Zone, when the photic zone was more expanded thanks to the sea-level increase. High primary production also occurred around this maximum flooding interval, as indicated by high TOC [START_REF] Duarte | Organicrich facies in the Sinemurian and Pliensbachian of the Lusitanian Basin, Portugal: Total organic carbon and relation to transgressive-regressive facies cycles[END_REF]2012) and phytane concentrations. The St/(St+Hop) ratio further supports this observation, with higher input to sediments of eukaryotic relative to bacterial biomass (Fig. 6).

During this interval, depositional environment was an outer-ramp, and an important contrast is observed between periods during which the water column was well stratified and periods characterized by intense water mixing due to storms (Fig. 7). During storm events, surface waters were replenished in nutrients thus boosting primary production. The eventual water stratification and the respiration of the excess biomass produced during or following storm events regularly led to bottom water dysoxia/anoxia and the formation of black shales.

A C C E P T E D M A N U S C R I P T

This was interrupted by temporary re-oxygenation due to storms as indicated by the presence of several distal tempestites and numerous HCSs [START_REF] Boussaha | Spatial characterization of the late Sinemurian (Early Jurassic) palaeoenvironments in the Lusitanian Basin[END_REF]. The Lusitanian Basin was thus probably under the control of highly variable environmental conditions during the late Sinemurian. Although the mechanisms of these abrupt and frequent environmental changes are unknown, they appear to be linked to climatic variations and to a particular restricted configuration of the Lusitanian Basin, which likely led to the recurrent development of black shales [START_REF] Boussaha | Spatial characterization of the late Sinemurian (Early Jurassic) palaeoenvironments in the Lusitanian Basin[END_REF]. The interpretation of the water column stratification could be supported by peaks in the relative abundance of Mitrolithus jansae (Figs. 3 and6), considered as a deep-dweller species [START_REF] Bucefalo Palliani | Phytoplankton response and geochemical evidence of the lower Toarcian relative sea level rise in the Umbria-Marche basin (Central Italy)[END_REF][START_REF] Mattioli | Spatial and temporal distribution of calcareous nannofossils along a proximal-distal transect in the Lower Jurassic of the Umbria-Marche Basin (central Italy)[END_REF][START_REF] Tremolada | Early Jurassic schizosphaerellid crisis in Cantabria, Spain: Implications for calcification rates and phytoplankton evolution across the Toarcian oceanic anoxic event[END_REF] since its thick coccoliths probably optimized light intensity into the cell by refraction [START_REF] Mattioli | Calcareous nannoplankton across Early Toarcian anoxic event: Implications for paleoceanography within the western Tethys[END_REF]. [START_REF] Mattioli | Spatial and temporal distribution of calcareous nannofossils along a proximal-distal transect in the Lower Jurassic of the Umbria-Marche Basin (central Italy)[END_REF] studied calcareous nannofossils from the Umbria-Marche basin (Italy) during the Pliensbachian/Toarcian, and showed that the relative abundance of M. jansae tended to increase when the stratification of the water was more pronounced. In the present study, M. jansae peaks in a few marly intervals deposited above storm events. This observation suggests that, in the present case, the development of this deep-dweller might have been favoured by the return to water stratification after storm events.

(3) The base of the Jamesoni Zone is marked by a sharp drop in sea level. The following low sea level was characterized by amalgamated HCS-bearing tempestites (Fig. 6). Calcareous nannoplankton probably could not develop under such conditions, as indicated by a drop in absolute abundances (Fig. 6), and basin waters were likely too much oxygenated for OM to be preserved (Fig. 7).

(4) During the basal transgression that followed storm deposits (between 62 and 71 m), contents in TOC and phytane and the St/(St+Hop) ratio indicate that primary productivity progressively decreased (Fig. 6). The concentration peak in C 29 sterane recorded just above

A C C E P T E D M A N U S C R I P T

the HCS interval (Fig. 4) may testify for an input of land plants [START_REF] Volkman | Microalgal biomarkers: A review of recent research developments[END_REF], and thus higher terrestrial input to the sea. Alternatively, this C 29 sterane peak could result from the contribution of freshwater algae [START_REF] Volkman | Eustigmatophyte microalgae are potential sources of C 29 sterols, C 22 -C 28 n-alcohols and C 28 -C 32 n-alkyl diols in freshwater environments[END_REF]Rodner et al., 2008) induced by freshwater input. However, the contribution of freshwater algae seems unlikely since palynofacies analysis of the São Pedro de Moel section indicates that the OM is mainly of marine origin [START_REF] Duarte | High resolution stratigraphy, palynofacies and source rock potential of the Água de Madeiros formation (Lower Jurassic), Lusitanian Basin, Portugal[END_REF][START_REF] Poças Ribeiro | Palynofacies and organic geochemistry of the Sinemurian carbonate deposits in the western Lusitanian Basin (Portugal): Coimbra and Água de Madeiros formations[END_REF]. In addition, an episode of strong continental contribution is reflected by an increase in phytoclast content just above the HCS interval [START_REF] Poças Ribeiro | Palynofacies and organic geochemistry of the Sinemurian carbonate deposits in the western Lusitanian Basin (Portugal): Coimbra and Água de Madeiros formations[END_REF]. Thus, the C 29 sterane content rather testifies for higher land plant input, but it still might evidence subsequent water column stratification due to freshwater input (by rivers). This is in agreement with high proportions of the deep-dweller M. jansae (Fig. 6). This interval was thus characterized by a relatively shallow, stratified water column that did not necessarily lead to permanent bottom water dysoxia/anoxia and deposition of black shales, as only few OM-rich levels occur in this interval (Fig. 7).

(5) Transgression then intensified as testified by the increasingly argillaceous content in the upper part of the section (71-89 m) (Fig. 6). The absolute abundance of nannofossils shows that the eco-space available for nannoplankton development progressively increased (Fig. 7).

A trend to increased distality is also suggested by the net decrease in Schizosphaerella spp.

abundance relative to coccolith abundance (Fig. 6). In parallel, primary production decreased, likely due to enhanced distality with respect to continental input of nutrients (Figs. 6 and7).

Global palaeoceanographic changes around the Sinemurian/Pliensbachian boundary?

The sharp sea-level fall followed by the major transgression recorded in the Lusitanian Basin around the Sinemurian/Pliensbachian boundary seems to have been a global feature since similar sea-level variations have been evidenced in other sections of England [START_REF] Hesselbo | Mesozoic and Cenozoic Sequence Stratigraphy of European Basins[END_REF], Poland [START_REF] Pieńkowski | The epicontinental Lower Jurassic of Poland[END_REF]), France (de Gracianski et al., 1998),

A C C E P T E D M A N U S C R I P T
Greenland, and south Andes [START_REF] Hallam | A review of the broad pattern of Jurassic sea level changes and their possible causes in the light of current knowledge[END_REF]. In these sections, the dramatic sea-level fall is however dated from the latest Sinemurian, followed by an important flooding across the Sinemurian/Pliensbachian transition (beginning of the Jamesoni Zone). This slight timing difference in sea-level changes could be explained by local tectonic conditions at São Pedro de Moel, as suggested by the observation of lateral facies variation in the Lusitanian Basin (see [START_REF] Duarte | Carbon isotope stratigraphy and ammonite biochronostratigraphy across the Sinemurian-Pliensbachian boundary in the western Iberian margin[END_REF]. However, it should be noted that the Sinemurian/Pliensbachian interval is much better defined in São Pedro de Moel section thanks to a continuous ammonite record [START_REF] Comas-Rengifo | Fácies carbonatadas ricas em matéria orgânica do Jurássico da Bacia Lusitânica. Novos contributos paleontológicos, sedimentológicos e geoquímicos[END_REF] than in other well-studied basins. In fact, either the ammonite record is discontinuous below the boundary (for example the well-dated Yorkshire sections; see Hesselbo et al., 2000, fig. 4), the ammonite record is inferred to indicate a range interval comprised between the uppermost Sinemurian to basal Pliensbachian interval (such as in Morocco; Wilmsen et al., 2002), or a condensation or hiatus occurred across the upper Sinemurian/lower Pliensbachian (such as in France; de [START_REF] De Graciansky | Ammonite biostratigraphic correlation and Early Jurassic sequence stratigraphy in France: comparison with some U.K. sections[END_REF].

In spite of the problem of exactly dating the Sinemurian/Pliensbachian boundary, on a long-term trend, these sea-level changes are however very similar, and the temporal difference is actually minor as the important flooding is recorded at the base of the Jamesoni Zone at São Pedro de Moel in a way similar to that reported by [START_REF] Pieńkowski | The first Early Jurassic ammonite find in central Poland[END_REF] for the Poland Basin. Then, sea-level changes at São Pedro de Moel do not seem to be strongly affected by local tectonic conditions and may reflect widespread sea-level variations.

This transgression at the base of the Pliensbachien could be related to tectono-eustatic sealevel changes linked to the breakup of the supercontinent Pangea, which led to the establishment of epeiric seaways such as the Hispanic Corridor that connected the Tethys and palaeo-Pacific oceans [START_REF] Hallam | Tectonism and eustasy in the Jurassic[END_REF][START_REF] Hallam | An outline of phanerozoic biogeography[END_REF][START_REF] Smith | Paleobiogeography of the ancient Pacific[END_REF][START_REF] Smith | Atlas of Mesozoic and Cenozoic Coastlines[END_REF][START_REF] Aberhan | Bivalve palaeobiogeography and the Hispanic Corridor: time of opening and effectiveness of a proto-Atlantic seaway[END_REF]. Several studies based on the comparison of Tethyan and eastern Pacific ammonites [START_REF] Venturi | Comparison between western Tethys and eastern Pacific ammonites: further evidence for a possible late Sinemurian-early Pliensbachian trans-Pangaean marine connection[END_REF][START_REF] Venturi | Ammonites from the early Pliensbachian of the Furlo Pass (Marche, Italy): biostratigraphic and paleobiogeographic implications[END_REF], bivalve palaeobiogeography [START_REF] Aberhan | Bivalve palaeobiogeography and the Hispanic Corridor: time of opening and effectiveness of a proto-Atlantic seaway[END_REF] and geochemical data [START_REF] Porter | Opening of a trans Pangaean marine corridor during the Early Jurassic: insights from osmium isotopes across the Sinemurian-Pliensbachian GSSP, Robin Hood's Bay, UK[END_REF][START_REF] Dera | Nd isotope constraints on ocean circulation, paleoclimate, and continental drainage during the Jurassic breakup of Pangea[END_REF] suggest that the opening of the Hispanic Corridor was progressive, but point the latest Sinemurian-earliest Pliensbachian as the time during which it became a true seaway. This opening may have induced palaeoceanographic and palaeoclimatic changes, with more pronounced influence of warm water masses from the equatorial Tethys (e.g., [START_REF] Van De Schootbrugge | Early Jurassic climate change and the radiation of organic walled phytoplankton in the Tethys Ocean[END_REF] and the establishment of a mega- A global ~2‰ negative shift recorded in δ 13 C bulk carbonate, marine benthic and nektobenthic molluscs and brachiopods and terrestrial wood also marks the Sinemurian/Pliensbachian boundary in various areas [START_REF] Jenkyns | Chemostratigraphy of the Jurassic System: applications, limitations and implications for palaeoceanography[END_REF]Woodfine et al., 2008;[START_REF] Korte | Shallow marine carbon and oxygen isotope and elemental records indicate icehouse-greenhouse cycles during the Early Jurassic[END_REF]. This shift suggests an increase in pCO 2 , probably related to enhanced mantle-related hydrothermal activity associated with the opening of the Hispanic

Corridor [START_REF] Porter | Opening of a trans Pangaean marine corridor during the Early Jurassic: insights from osmium isotopes across the Sinemurian-Pliensbachian GSSP, Robin Hood's Bay, UK[END_REF][START_REF] Franceschi | Early Pliensbachian (Early Jurassic) C-isotope perturbation and the diffusion of the Lithiotis Fauna: Insights from the western Tethys[END_REF]. At São Pedro de Moel, the δ 13 C of and6). This chemostratigraphic event seems to be correlative with δ 13 C negative excursion recorded at Robin Hood's Bay (England; [START_REF] Korte | Shallow marine carbon and oxygen isotope and elemental records indicate icehouse-greenhouse cycles during the Early Jurassic[END_REF].

According to [START_REF] Duarte | Carbon isotope stratigraphy and ammonite biochronostratigraphy across the Sinemurian-Pliensbachian boundary in the western Iberian margin[END_REF], the δ 13 C carb curve across the Sinemurian/Pliensbachian of São Pedro de Moel is strongly influenced by the sedimentation of OM, which may play an important role in the control of the local seawater carbon-isotope composition.

The δ 13 C TOC does not covary with the δ 13 C carb and an opposite trend is observed between The present work highlights the necessity of using a multi-proxy approach to obtain a more comprehensive picture of the environmental context in which placoliths first occurred. Still, more studies in other epicontinental basins of the western Tethys would be needed to better constrain these palaeoceanographic/palaeoclimatic changes and to understand how they may have induced phytoplankton radiation. Also, in future works, the degree of restriction in anoxic marine basins, such as the Lusitanian Basin, should be quantified, using for example trace-metal concentration data. et al., 1993). C) Distribution of the studied section on a schematic proximal-distal profile through the Lusitanian Basin during the early Jurassic; BF means Berlenga-Farilhões (modified after [START_REF] Vanney | La plateforme continentale du Portugal et les provinces adjacentes: Analyse géomorphologique[END_REF]. indicates that organic matter is immature and has undergone only minor diagenesis. B) The plot of HI (mgHC/gTOC) vs. OI (mgCO 2 /gTOC) shows that organic matter is of Type II, dominantly of marine origin. The boundary lines that denote the different kerogen types are from [START_REF] Espitalié | La pyrolyse Rock-Eval et ses applications. Troisième partie[END_REF]. The different symbols indicate the samples that are in the "outerramp-offshore", "mid to outer-ramp" and the "mid to inner-ramp" environments. This shows that there is no distinct grouping of organic matter type versus palaeoenvironment.

  monsoonal system above northwestern Europe with intensified summer monsoons (southeast trade winds;[START_REF] Röhl | The Posidonia Shale (Lower Toarcian) of SW Germany: An oxygen depleted ecosystem controlled by sea level and palaeoclimate[END_REF]. Draining of freshwater into the Tethys, resulting from intense summer monsoons and high runoff, led to increased water column stratification (van deSchootbrugge et al., 2005) as suggested here for the interval following the forced regression at the base of Pliensbachian.Interestingly, the opening of the Hispanic Corridor may have played a role in the first occurrence of the genus Similiscutum with placolith-type coccoliths, through the development of environmental conditions favourable for placolith-coccoliths that became dominant over murolith-coccoliths by the end of the early Jurassic. It has indeed already been hypothesized that the palaeoceanographic/palaeoclimatic changes subsequent to the opening of this gateway may have induced the global radiation of dinoflagellates (van deSchootbrugge et al., 2005) and of the Lithiotis Fauna (peculiar genera of bivalves;[START_REF] Franceschi | Early Pliensbachian (Early Jurassic) C-isotope perturbation and the diffusion of the Lithiotis Fauna: Insights from the western Tethys[END_REF] during the early Pliensbachian. Alternatively, placolith-coccoliths may have originated in the Pacific Realm and colonized Tethyan waters following the opening of the Hispanic Corridor.

  show the sudden negative shift recorded in other settings. This is unlikely due to the lack of the δ 13 C shift record subsequent to erosion or hiatus at the base of the forced regressive system, since a negative carbon-isotope trend could be observed, with lowest values around the Sinemurian/Pliensbachian boundary and an increase in values in the earliest Pliensbachian(Figs. 4

  δ 13 C TOC and phytane concentration. During photosynthesis, isotopically light carbon ( 12 C) is preferentially incorporated by phytoplankton, leading to more depleted δ 13 C values in the produced OM. The opposite trend observed between δ 13 C TOC and phytane concentration thus testifies for a major contribution of phytoplankton to OM, which supports a decrease in primary production from the Raricostatum Zone to the Jamesoni Zone. Then, it is likely that the δ 13 C records at São Pedro de Moel are influenced by local environmental conditions and do not reflect global climatic/oceanographic changes.6. ConclusionsCombined sedimentological, TOC, lipid biomarker and calcareous nannofossil data allow a precise characterization of the palaeoenvironmental changes occurring across the Sinemurian/Pliensbachian boundary in the Lusitanian Basin. During the Oxynotum and Raricostatum Zones (late Sinemurian), the basin was particularly restricted with a relatively low sea level. This peculiar configuration of the basin, along with climatic variations, could A C C E P T E D M A N U S C R I P T account for the regular formation of black shales due to water stratification and bottom water dysoxia/anoxia. This was interrupted by storm events that allowed temporary re-oxygenation and replenishment in nutrients of surface waters thus boosting primary production. The earliest Jamesoni Zone (earliest Pliensbachian) is marked by a sharp sea-level fall followed by a major transgression. During the basal transgression, the basin was relatively shallow, with a stratified water-column that did not necessarily lead to permanent bottom water dysoxia/anoxia and deposition of black shales, and became progressively deeper and more open with the intensification of the transgression. In parallel, primary production decreased, likely due to enhanced distality with respect to continental input of nutrients. The major transgression in the earliest Pliensbachian is probably related to tectono-eustatic sea-level changes linked to the breakup of the Pangea, which led to the opening of the Hispanic Corridor that occurred during latest Sinemurian-earliest Pliensbachian. The palaeoceanographic and palaeoclimatic changes induced by this opening may have played a role in the diversification of coccolithophores with the first occurrence or colonization of Tethyan waters by placolith-type coccoliths, which were eventually very successful and became dominant over murolith-coccolith by the end of early Jurassic. The δ 13 C records at São Pedro de Moel are likely influenced by local environmental conditions, and do not reflect in the present case global climatic/oceanographic changes.

  ., 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 Paléobiol. Genève 21, 149-175 ISSN 0253-6730. Woodfine, R.G., Jenkyns, H.C., Sarti, M., Baroncini, F., Violante, C., 2008. The response of two Tethyan carbonate platforms to the early Toarcian (Jurassic) oceanic anoxic event: environmental change and differential subsidence. Sedimentology 55, 1011-1028. Young, J.R., 1994. Functions of coccoliths. In: Winter, A., Siesser, W. (Eds.), "Coccolithophores". Cambridge Univ. Press, Cambridge, pp. 63-82.

  Figure captions

Figure 2 .

 2 Figure 2. Stratigraphic log of the São Pedro de Moel composite section. Biostratigraphic data

Figure 3 .

 3 Figure 3. Absolute (nannofossils/g of sediment) and relative (%) species-specific nannofossil

Figure 4 .

 4 Figure 4. Bulk geochemistry and lipid-biomarker data during the Sinemurian/Pliensbachian

Figure 5 .

 5 Figure 5. Rock-Eval pyrolysis data. A) The plot of HI (mgHC/gTOC) vs. Tmax (°C)

Figure 6 .

 6 Figure6. Synthesis of the main data that allow distinguishing five intervals, noted (1) to (5),

Figure 7 .

 7 Figure 7. Schematic representations of the palaeoceanographic evolution at São Pedro de

Figure 1 A

 1 Figure 1

Figure 2 A

 2 Figure 2

Figure 3 A

 3 Figure 3

Acknowledgments

We would like to thank Thomas Algeo, Darren Gröcke and an anonymous reviewer for their constructive comments that helped to significantly improve the quality of the manuscript. We thank François Martineau and François Fourel (Université Lyon 1, France)

for their help and expertise in isotopic analyses. LVD had the support of Fundação para a Ciência e Tecnologia (FCT), through the strategic project UID/MAR/04292/2013 granted to MARE. EM and BP were funded by the PAUILF and Syster-Intervie (INSU) projects. Slides used for calcareous nannofossil study are curated at the Collections de Géologie de Lyon, Université Lyon 1 (reference number: FSL 765001 to FSL 765110).

A C C E P T E D

M A N U S C R I P T

Highlights

We discuss environmental changes across the Sinemurian/Pliensbachian in Portugal Calcareous nannofossil and lipid biomarker show a decrease in primary production This is associated with a major sea-level rise occurring above the boundary This could be related to the opening of the Hispanic Corridor becoming a true seaway This opening may have induced the first occurrence of placolith-type coccoliths


Bruno Michel
email: bruno.michel@cea.fr

Thomas H Helfer
email: bthomas.helfer@cea.fr

Isabelle Ramière
email: cisabelle.ramiere@cea.fr

Coralie Esnoul
email: dcoralie.esnoul@edf.fr

3D continuum damage approach for simulation of crack initiation and growth in ceramic materials

Keywords: Crack initiation and growth, numerical simulation, damage model

published or not. The documents may come   L'archive ouverte pluridisciplinaire

Introduction

Numerical simulation of crack initiation and growth in nuclear fuel ceramics is an important issue to assess mechanical integrity of the first confinement barrier for radio elements. Under irradiation fuel pellet fragmentation has to be considered because it will have an impact on the loading level and can increase the risk of rupture for the fuel element cladding. Since years 2000 this has been studying [START_REF] Michel | 3D fuel cracking modelling in pellet cladding mechanical interaction[END_REF], [START_REF] Helfer | Introducing the open-source MFront code generator : application to mechanical behaviours and material knowledgemanagement within the PLEIADES fuel element modelling plateform[END_REF] for Pellet Cladding Interaction (PCI) phenomenon [START_REF] Michel | Simulation of the Pellet Cladding Interaction phenomenon with the PLEIADES fuel performance software environment[END_REF] in fuel rods of Pressurized Water Reactors (PWRs) in order to take into account the impact of pellet fragmentation in Stress Corrosion Cracking cladding rupture [START_REF] Michel | A new phenomenological criterion for pellet-cladding interaction rupture[END_REF]. Pellet fragmentation at microscopic scale is also an important issue for fuel dispersal risk under Loss Of Coolant Accident (LOCA) transient loading condition [START_REF] Esnoul | Use of micromechanical approach to investigate transient fuel fragmentation mechanisms[END_REF]. A smeared crack model [START_REF] Michel | 3D fuel cracking modelling in pellet cladding mechanical interaction[END_REF], [START_REF] Helfer | Introducing the open-source MFront code generator : application to mechanical behaviours and material knowledgemanagement within the PLEIADES fuel element modelling plateform[END_REF] has been proposed and implemented in the fuel performance software environment PLEIADES [START_REF] Michel | Simulation of the Pellet Cladding Interaction phenomenon with the PLEIADES fuel performance software environment[END_REF] developed in the framework of a cooperative program, between EDF, AREVA and the CEA. This approach has been used for many years to simulate crack extension in the 1D, 2D and 3D fuel models of the PWR fuel performance code ALCYONE [START_REF] Sercombe | 1D and 3D analyses of the Zy2 SCIP BWR ramp tests with the fuel codes METEOR and ALCYONE[END_REF] of the PLEIADES platform.

The objective of this paper is to present an overview of this modelling approach and a set of new results of 3D simulation for complex crack path extension. In a first part, the model and numerical approach will be detailed. Then, 3D simulation results will be discussed for crack extension in fuel pellet under thermal gradient and for microscopic fuel fragmentation under gaseous fission products internal pressure.

Smeared crack model

The formulation of the smeared crack model is given with equations (1) to [START_REF] Michel | Simulation of the Pellet Cladding Interaction phenomenon with the PLEIADES fuel performance software environment[END_REF]. The function () g is defined has a cohesive zone model but it is used as a continuous damage approach where the crack strain is derived from the displacement discontinuity with a characteristic length L. The cohesive zone model () g is able to represent softening from microscopic crack growth up to a macroscopic rupture where the material lost its mechanical stiffness. Crack closure due to unloading is also possible with the model. If unloading occurred during the softening stage, the unloading path is assumed to be elastic with an affine linear stiffness. It means that local plasticity due microscopic cracks is negligible, which seems reasonable for fragile rupture. After a complete crack closure the material can recover its elastic stiffness under a compressive stress state defined in the fracture basis. In this paper the fracture basis is derived from the stress tensor's eigenvalues at crack initiation. At this stage we didn't introduce constitutive equations to model material softening under a shear strain loading occurring in the fracture basis after crack initiation. In this formulation it is assumed that the stress eigen basis doesn't change during the softening process (radial stress relaxation) and that shear stresses can be neglected.

        crack tot E       : (1)
   i i i crack i crack w w   .   (2) 0 ) . , : (  crack i i L n g   ( 3
)
Where E is the fourth order elastic tensor, crack  is the crack strain rate tensor, i w  is the unit vectors used to define an orthonormal crack basis, L is the characteristic length for the regularization method and () g is the cohesive zone model for fragile rupture.

Numerical approach

As presented in reference [START_REF] Helfer | Introducing the open-source MFront code generator : application to mechanical behaviours and material knowledgemanagement within the PLEIADES fuel element modelling plateform[END_REF] the numerical integration of the local smeared crack model is based on an implicit approach implemented in the Cast3M Finite Element code [7] through the MFront material knowledge integrator [8]. A quasi-static approach of the equilibrium under unstable crack extension leads to a reverse displacement solution (snap back) needed to decrease elastic energy restitution rate up to the dissipated energy rate. In order to be able to solve the numerical problem associated to unstable crack extension a new approach has been proposed. This methodology is based on a modified Newton algorithm (MNA) to solve the nonlinear problem associated to the quasi static equilibrium formulation. The main idea of this methodology is based on the fact that the solution reached after unstable crack extension will be consistent again with a static equilibrium if external loading is based on Dirichlet boundary conditions. If we compare to a full dynamic solution it means that the post-instability static solution is consistent with a state where the kinetic energy stored during the damage process is vanished because of material dumping coupling with energy transport by stress waves. In order to be able to find this post-instability static solution a specific algorithm is needed because the MNA can't converge towards this solution if the iterative process is initialized with the elastic solution at crack initiation. This specific algorithm is based on two different aspects, first an adaptative time step algorithm, and secondly a fictive path loading algorithm. Thanks to the time step refinement method the unstable crack extension stage starts from the crack initiation loading. This time step refinement method is based on the convergence of the MNA, in order to detect if the unstable crack extension stage has been reached or not. The time step reduction ratio can be set in order to reach a given precision for the crack initiation criterion at the beginning of the unstable crack extension stage; this precision has to be consistent with the convergence criterion used in the MNA. To find the post instability static solution, intermediate updating of the mechanical state will be validated in the MNA even if these solutions are only consistent with the material behaviour and not with the static equilibrium. The residual equilibrium error associated to these intermediate up-dating can be considered as the dynamic nodal forces needed to reach a dynamic equilibrium.

Crack initiation and growth at microscopic scale

In nuclear fuel under irradiation gaseous fission products tend to precipitate in small bubbles and can induce internal stresses in the porous material. To study the risk of fuel pellet microscopic fragmentation we have developed a 3D volume element model of the nuclear fuel [START_REF] Esnoul | Use of micromechanical approach to investigate transient fuel fragmentation mechanisms[END_REF]. This finite element model, derived from a MOX fuel microstructure, is based on a periodic volume element of the porous material. Bubble sizes statistical distribution is defined with post irradiation micrographic examinations, and bubble spatial localization statistical distribution is derived from a Random Sequential Addition algorithm. Input parameter are given in Table 1, the effective material tenacity is an equivalent value taking into account a reduction of the real material tenacity due to the pressure driven forces on the crack surfaces. In order to avoid an indeterminacy of the static solution under Neumann boundary condition, the pressure loading is replaced by a strain control loading. The latter is defined through a fictive thermal expansion of the bubble gas, itself considered as a linear elastic material with a shear modulus much lower than the bulk modulus. As presented in equation ( 8) this leads to a pure hydrostatic stress state in the gas, and the stress vector at the gas-solid material interface is equivalent to a pressure loading boundary condition. Gas equivalent properties used in our study are given in Table 1 and the results obtained for both types of loading conditions are given in Figure 1. As shown in the latter, the fictive thermal loading leads to the same stress sate in the solid material and the local error between the two stress fields is lower than 1%.

I p I tr . ). ( . . . 2               (4)
Numerical results for microscopic fuel fragmentation are presented in Figure 2 with an amplification of the displacement field and a suppression of damaged element. The mean pressure in bubbles increases up to 120 MPa when a first unstable crack extension occurred. After the first crack extension the pressure is no more homogeneous in bubbles. Bubbles crossed by or near a crack plane have a reduced pressure due to the softening of damaged material. This leads to a mean pressure drop for each new crack extension has shown in Figure 2. Through these results we can compute the fuel fragment size distribution or the amount of gas released has a function of the material microstructure and the pressure induced by gaseous fission products.

Conclusion

A smeared crack model has been proposed to simulate continuously crack initiation and growth with or without an initial flaw. Thanks to a new methodology unstable crack extension can be simulated with a quasi-static nonlinear mechanical approach. This methodology is based on a modified Newton algorithm and uses an adaptive time step strategy coupling to a pseudo dynamic approach to find the post instability solution. Performances and robustness of the method are demonstrated for a full 3D computation to assess nuclear fuel microscopic fragmentation. In this application internal pressure loading is replaced by a fictive thermal loading in order to avoid convergence problem due to the indetermination of the quasi static formulation of a softening material equilibrium under Neumann boundary conditions.

To improve computation time further perspectives can be considered for numerical and computational aspects such as convergence acceleration or parallel computing.

Figure 1 :Figure 2 :

 12 Figure 1 : Maximal principal stress induced by internal pressure.

Table 1 :

 1 Input data for fuel microscopic fragmentation

	Fuel Volume element Size Bubble volumique fraction ratio	Young modulus	Solid material properties Poisson ratio Rupture stress Effective material tenacity	Gas equivalent properties  
	9 m	8 %	200000 MPa	0.3	100 MPa	0.001 J/m 2	83 GPa	0.07 GPa