Romain Tramoy
email: romain.tramoy@gmail.com

Johann Schnyder

T T Nguyen

J Yans

Jérémy Jacob

M Sebilo

S Derenne

M Philippe

A Huguet

D Pons

T T Nguyen Tu

F Baudin

The Pliensbachian-Toarcian paleoclimate transition: New insights from organic geochemistry and C, H, N isotopes in a continental section from Central Asia

Keywords: n-Alkanes, δ 2 H, δ 15 N org, Xenoxylon, brGDGT

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Major paleoenvironmental changes have been documented during the Early Jurassic and particularly in Western Tethyan marine sections during the Pliensbachian-Toarcian transition [START_REF] Jenkyns | The early Toarcian (Jurassic) anoxic event; stratigraphic, sedimentary and geochemical evidence[END_REF][START_REF] Bassoullet | Le Toarcien inférieur: Une période de crise dans les bassins et sur les plate-formes carbonatées de l'Europe du Nord-Ouest et de la Téthys[END_REF][START_REF] Bailey | Paleoceanographic changes of the Late Pliensbachian-Early Toarcian interval: a possible link to the genesis of an Oceanic Anoxic Event[END_REF][START_REF] Morard | A new scenario for the Domerian -Toarcian transition[END_REF][START_REF] Rosales | Paleotemperature variations of Early Jurassic seawater recorded in geochemical trends of belemnites from the Basque-Cantabrian basin, northern Spain[END_REF][START_REF] Hesselbo | Carbon-isotope record of the Early Jurassic (Toarcian) Oceanic Anoxic Event from fossil wood and marine carbonate (Lusitanian Basin, Portugal)[END_REF][START_REF] Suan | Secular environmental precursors to Early Toarcian (Jurassic) extreme climate changes[END_REF][START_REF] Hermoso | Dynamics of a stepped carbon-isotope excursion: Ultra high-resolution study of Early Toarcian environmental change[END_REF][START_REF] Korte | Jurassic climate mode governed by ocean gateway[END_REF]. During the late Pliensbachian, a 5°-6 °C decrease in sea surface temperatures has been evidenced by δ 18 O and Mg/Ca of carbonates, possibly related to ice sheet development at high latitudes [START_REF] Bailey | Paleoceanographic changes of the Late Pliensbachian-Early Toarcian interval: a possible link to the genesis of an Oceanic Anoxic Event[END_REF][START_REF] Rosales | Paleotemperature 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] Suan | Secular environmental precursors to Early Toarcian (Jurassic) extreme climate changes[END_REF], and a sea level drop [START_REF] Hallam | An Environmental Study of the Upper Domerian and Lower Toarcian in Great Britain[END_REF][START_REF] Hesselbo | British Lower Jurassic Sequence Stratigraphy[END_REF]. Then, a period of drastic warming (~8 °C) has been recorded in sea-surface temperatures during the early Toarcian [START_REF] Bailey | Paleoceanographic changes of the Late Pliensbachian-Early Toarcian interval: a possible link to the genesis of an Oceanic Anoxic Event[END_REF][START_REF] Rosales | Paleotemperature variations of Early Jurassic seawater recorded in geochemical trends of belemnites from the Basque-Cantabrian basin, northern Spain[END_REF][START_REF] Suan | Secular environmental precursors to Early Toarcian (Jurassic) extreme climate changes[END_REF], associated with a sea level rise [START_REF] Hallam | An Environmental Study of the Upper Domerian and Lower Toarcian in Great Britain[END_REF][START_REF] Hesselbo | British Lower Jurassic Sequence Stratigraphy[END_REF]. The global scale of paleoenvironmental disturbances during the Pliensbachian-Toarcian transition was confirmed by studies on sites located outside Europe in (i) the Neuquén Basin in Argentina [START_REF] Al-Suwaidi | First record of the Early Toarcian Oceanic Anoxic Event from the Southern Hemisphere, Neuquén Basin, Argentina[END_REF], (ii) the High Atlas, Morocco [START_REF] Bodin | Toarcian carbon isotope shifts and nutrient changes from the Northern margin of Gondwana (High Atlas, Morocco, Jurassic): Palaeoenvironmental implications[END_REF], (iii) the Qaidam Basin, Northwest China [START_REF] Wang | Early to Middle Jurassic vegetation and climatic events in the Qaidam Basin, Northwest China[END_REF], (iv) the Clarence-Moreton Basin, Eastern Australia [START_REF] Jansson | An Early Jurassic flora from the Clarence-Moreton Basin, Australia[END_REF] and (v) the Arctic [START_REF] Suan | Polar record of Early Jurassic massive carbon injection[END_REF]. However, only few studies have focused on non-marine records [START_REF] Wang | Early to Middle Jurassic vegetation and climatic events in the Qaidam Basin, Northwest China[END_REF][START_REF] Jansson | An Early Jurassic flora from the Clarence-Moreton Basin, Australia[END_REF], mainly because well-dated terrestrial records for the Early Jurassic are much less common than marine ones [START_REF] Sobel | Basin analysis of the Jurassic-Lower Cretaceous southwest Tarim basin, northwest China[END_REF].

The Taskomirsay section (South Kazakhstan, Central Asia) is made of non-marine, coalrich sediments that encompass the Pliensbachian-Toarcian transition (Schnyder et al., accepted). This section is a unique target for paleoclimatic reconstructions prior to and during the Pliensbachian-Toarcian transition in a continental setting. A multi-proxy approach was developed to document paleoclimatic changes in the area based on (i) palynological and paleobotanical records, (ii) bulk geochemistry (Rock-Eval and δ 15 N org ), (iii) n-alkane distribution and their hydrogen isotopic composition (δ 2 H), and (iv) branched glycerol dialkyl glycerol tetraethers (brGDGTs).

Palynological records (Schnyder et al., accepted), combined with paleobotanical evidences can help to decipher paleoclimatic background as some wood taxa are paleoecologically well constrained [START_REF] Philippe | Distribution and palaeoecology of the Mesozoic wood genus Xenoxylon: palaeoclimatological implications for the Jurassic of Western Europe[END_REF][START_REF] Philippe | Palaeoecologically significant wood genus Xenoxylon discovered in the East Stara Planina Mts. (East Bulgaria) Balaban Formation (Toarcian, Early Jurassic)[END_REF][START_REF] Philippe | Systematics of a palaeoecologically significant boreal Mesozoic fossil wood genus, Xenoxylon Gothan[END_REF][START_REF] Oh | Xenoxylon Synecology and Palaeoclimatic Implications for the Mesozoic of Eurasia[END_REF]. Recently, δ 15 N org measured in lignites and clayey layers was used to indicate wet/dry cycles during the Paleocene-Eocene transition [START_REF] Storme | Cycles of humid-dry climate conditions around the P/E boundary: new stable isotope data from terrestrial organic matter in Vasterival section (NW France)[END_REF] and the Eocene-Oligocene transition [START_REF] Tramoy | Stepwise palaeoclimate change across the Eocene-Oligocene transition recorded in continental NW Europe by mineralogical assemblages and δ 15 N org (Rennes Basin, France)[END_REF].

Early studies indeed showed that δ 15 N org values were positively correlated to temperature and negatively to precipitation in Quaternary and modern plants [START_REF] Austin | Nutrient dynamics on a precipitation gradient in Hawai'i[END_REF][START_REF] Handley | The 15 N natural abundance (δ 15 N) of ecosystem samples reflects measures of water availability[END_REF][START_REF] Amundson | Global patterns of the isotopic composition of soil and plant nitrogen[END_REF][START_REF] Swap | Natural abundance of 13 C and 15 N in C 3 and C 4 vegetation of southern Africa: patterns and implications[END_REF][START_REF] Liu | Nitrogen isotopic composition of plant-soil in the Loess Plateau and its responding to environmental change[END_REF].

Considering the potential of δ 15 N org as a paleoclimatic marker for pre-Quaternary sediments [START_REF] Storme | Cycles of humid-dry climate conditions around the P/E boundary: new stable isotope data from terrestrial organic matter in Vasterival section (NW France)[END_REF][START_REF] Tramoy | Stepwise palaeoclimate change across the Eocene-Oligocene transition recorded in continental NW Europe by mineralogical assemblages and δ 15 N org (Rennes Basin, France)[END_REF], it will be tested in the Early Jurassic of the Taskomirsay sequence. Similarly, compound-specific δ 2 H values are often used as a paleohydrological proxy [START_REF] Sachse | Molecular Paleohydrology: Interpreting the Hydrogen-Isotopic Composition of Lipid Biomarkers from Photosynthesizing Organisms[END_REF][START_REF] Sessions | Factors controlling the deuterium contents of sedimentary hydrocarbons[END_REF]. Major calibration efforts during the last decade allowed improving their use for paleoclimatic reconstructions in the Quaternary (e.g., [START_REF] Hou | Postglacial climate reconstruction based on compound-specific D/H ratios of fatty acids from Blood Pond, New England[END_REF][START_REF] Jacob | Paleohydrological changes during the last deglaciation in Northern Brazil[END_REF][START_REF] Mügler | Effect of lake evaporation on δD values of lacustrine n-alkanes: A comparison of Nam Co (Tibetan Plateau) and Holzmaar (Germany)[END_REF][START_REF] Aichner | δD values of n-alkanes in Tibetan lake sediments and aquatic macrophytes -A surface sediment study and application to a 16 ka record from Lake Koucha[END_REF], but also in the Cenozoic [START_REF] Andersen | Large and rapid climate variability during the Messinian salinity crisis: Evidence from deuterium concentrations of individual biomarkers[END_REF][START_REF] Pagani | Arctic hydrology during global warming at the Palaeocene/Eocene thermal maximum[END_REF][START_REF] Garel | Paleohydrological and paleoenvironmental changes recorded in terrestrial sediments of the Paleocene-Eocene boundary (Normandy, France)[END_REF], and even in the Paleozoic [START_REF] Dawson | Stable hydrogen isotopic composition of hydrocarbons in torbanites (Late Carboniferous to Late Permian) deposited under various climatic conditions[END_REF][START_REF] Izart | Palaeoclimate reconstruction from biomarker geochemistry and stable isotopes of n-alkanes from Carboniferous and Early Permian humic coals and limnic sediments in western and eastern Europe[END_REF]. However, to the best of our knowledge, compound-specific δ 2 H values have only been scarcely used in Mesozoic sediments and not for paleoclimatic purpose [START_REF] Radke | Correlation between hydrogen isotope ratios of lipid biomarkers and sediment maturity[END_REF]. Yet, the δ 2 H of n-alkanes preserved in ancient sediments has great potential because n-alkanes are much less prone to diagenetic effect than other compounds, because H is strongly bound to C and thus alkanes retain their original hydrogen isotopic composition (Yang and Huang, 2003;[START_REF] Izart | Palaeoclimate reconstruction from biomarker geochemistry and stable isotopes of n-alkanes from Carboniferous and Early Permian humic coals and limnic sediments in western and eastern Europe[END_REF][START_REF] Sessions | Factors controlling the deuterium contents of sedimentary hydrocarbons[END_REF]. During the last decade, proxies based on branched bacterial membrane lipids (brGDGTs; [START_REF] Schouten | The organic geochemistry of glycerol dialkyl glycerol tetraether lipids: A review[END_REF], have been developed to reconstruct Mean Annual Air Temperatures in terrestrial settings (MAAT;Weijers et al., 2007;[START_REF] Peterse | Revised calibration of the MBT-CBT paleotemperature proxy based on branched tetraether membrane lipids in surface soils[END_REF][START_REF] Coffinet | Potential of GDGTs as a temperature proxy along an altitudinal transect at Mount Rungwe (Tanzania)[END_REF] and their use for ancient sediments deserves evaluation.

The aim of the present study was to assess the paleoclimatic conditions during the Pliensbachian-Toarcian transition in the sedimentary succession of Taskomirsay using multiproxy analyses: paleobotany, δ 15 N org , n-alkane distributions and δ 2 H values, and, although restricted to a selected set of samples, brGDGTs.

Material and methods

Taskomirsay section

The Taskomirsay section is located in the Karatau (Leontiev Graben), South Kazakhstan, which is one of the numerous continental basins, in central Asia, aligned along the North Tethyan-paleomargin (Fig. 1). These basins were formed by collisions between Eurasia and several blocks (Tarim, Tian Shan and Pamir Kunlun) in a transtensional/transpressional context between the Late Triassic and the Early Jurassic [START_REF] Sobel | Basin analysis of the Jurassic-Lower Cretaceous southwest Tarim basin, northwest China[END_REF]. During the Early Jurassic, the paleolatitudinal position of the Karatau basin was estimated to 36° 8° N, based on paleomagnetic reconstructions (B. Vrielynck, pers. com. 2014), in agreement with paleofloral assemblages (40° N; [START_REF] Kirichkova | New data on the phytostratigraphy of the Jurassic deposits of Kazakhstan[END_REF], which is very close to present latitudes (Fig. 1). Thick piles of terrestrial sediments were deposited thanks to the coexisting high tectonic subsidence.

The study section is 53.5 m thick (0 m represents the base of the section) and shows 6 organic-rich sedimentary cycles (Fig. 2; Schnyder et al., accepted). Each cycle exhibits lignite beds (noted LB1 to LB6) overlaid by non-laminated clayey layers and silty-sandstone showing root and trunk surfaces at their top. The succession of lignites, clayey layers and silty-sandstones suggests a meandering river system, in which developed oxbow lakes and peat deposits thanks to the lateral migration of the river system. At the top of the section, a more open and less marginal lacustrine environment under storm influence is deduced from Botryococcus, clay/silt alternations and hummocky cross-beddings (HCS) (Schnyder et al., accepted). Wood fragments are frequent along the section and are particularly concentrated in three surfaces labeled TP1, TP2 and TP3 (Fig. 2). Macroscopically, they exhibit an excellent morphological preservation and are found in the tops and insides of silty-sandstones, and always in vertical position, suggesting in situ trunks or roots (Schnyder et al., accepted).

These sedimentological features are in agreement with peaty/swampy forests in lowland terrains and trees in sandy river banks and/or in upland forests, as might be expected from the paleoclimatic latitudinal patterns during the Early Jurassic, with warm to cool-temperate conditions prevailing from mid to high boreal latitudes [START_REF] Miao | Peat-forming materials and evolution of swamp sequencescase analysis of a Jurassic inland coal basin in China[END_REF][START_REF] Rees | Jurassic phytogeography and climates: new data and model comparisons[END_REF][START_REF] Wang | Early to Middle Jurassic vegetation and climatic events in the Qaidam Basin, Northwest China[END_REF].

Despite the common difficulty to date such continental settings, the Pliensbachian-Toarcian transition was identified between ~26 m and ~35 m, based on sporomorph associations and organic carbon isotopes (δ 13 C org ; Schnyder et al., accepted). The section was thus dated no younger than the early Toarcian and no older than the mid-Pliensbachian.

Analytical methods

Fossil wood analyses

Fossil wood samples were studied with collodion micro-casting, following a method described in [START_REF] Marynowski | Perylene as an indicator of conifer fossil wood degradation by wood-degrading fungi[END_REF]. Despite their good macroscopic morphological preservation, woody axes were revealed to be microscopically degraded by both bacterial and fungal rots. This condition is consistent with a relatively slow burial history, before diagenesis stopped biological decay. As a consequence, only one specimen was preserved enough for specific identification, although the others did not display significantly different features that could suggest a different systematic position. The wood specimen was identified at a generic level from the Philippe and Bamford ( 2008) key and at a species level with the Philippe et al. ( 2013) key.

Bulk organic analyses

Total organic carbon (TOC) values were obtained for 100 samples, sampled regularly along the study section, using a Leco automatic carbon analyzer (IR-212) that gives more reliable TOC values for samples containing large amounts of terrestrial OM than Rock-Eval pyrolysis [START_REF] Espitalié | La pyrolise Rock-Eval et ses applications[END_REF]. A total of 88 samples, including 3 replicates, were analyzed by Rock-Eval pyrolysis using a Rock-Eval 6. For this study, the following Rock-Eval parameters were used: (i) Hydrogen Index (HI, mg HC/g TOC), which is the amount of products released during pyrolysis normalized to TOC values; (ii) Oxygen Index (OI, mg CO 2 /g TOC), calculated from the amounts of CO and CO 2 released during pyrolysis, which represents the oxygen content of the OM; and (iii) T max , which is the temperature of the pyrolysis oven recorded at the maximum of HC production [START_REF] Espitalié | La pyrolise Rock-Eval et ses applications[END_REF]. After Rock-Eval pyrolysis, a few samples were suspected to contain siderite that compromised measurements of the Rock-Eval parameters, especially for OI values. This suspicion was supported by very low dissolution rate of carbonates in 6N HCl. Concerned samples were removed from the dataset. In addition, samples with low S2 peak values (S2 <0.3)leading to misestimation of HI values, were also removed. In total, 29 samples were removed from the dataset and will not be discussed further.

Bulk organic nitrogen isotopes (δ 15 N org )

A total of 52 samples were analyzed for δ 15 N org . Only samples with TOC values >1 wt. % were selected, because nitrogen content in Taskomirsay sediments is usually very low.

Because inorganic nitrogen (N bnd ) is strongly bound to clay minerals as ammonium [START_REF] Müller | CN ratios in Pacific deep-sea sediments: Effect of inorganic ammonium and organic nitrogen compounds sorbed by clays[END_REF], it is easier to remove organic nitrogen (N org ) and recalculate the δ 15 N org by massbalance, knowing total nitrogen (N tot ), N bnd and their respective isotopic signatures δ 15 N tot and δ 15 N bnd [START_REF] Storme | Cycles of humid-dry climate conditions around the P/E boundary: new stable isotope data from terrestrial organic matter in Vasterival section (NW France)[END_REF]. N tot and δ 15 N tot were measured on powdered/decalcified samples, whereas N bnd and δ 15 N bnd were measured on subsamples that were treated with KOBr-KOH solution (2 mol/l) to eliminate the N org [START_REF] Silva | Determination and Isotope-Ratio Analysis of Different Forms of Nitrogen in Soils: 5. Fixed Ammonium1[END_REF][START_REF] Schubert | Nitrogen and carbon isotopic composition of marine and terrestrial organic matter in Arctic Ocean sediments: implications for nutrient utilization and organic matter composition[END_REF]. The solution consisted of 6 ml pure bromine added at 0.5 mL/min to 200 ml of 2M KOH cooled with ice. Between 500 to 1000 mg of subsamples were immersed in 20 ml of the KOBr-KOH solution with magnetic stirring. After 2 hours, 60 ml of distilled water was added to each subsample before being boiled in an oil-bath at ~200 °C for at least 5 min.

With this treatment, organic nitrogen is liberated in ammonium form and stays in solution.

The mixture was washed the next day by repetitive centrifugation (2x; 4000 rpm; 1.5 min) with 0.5 M of KCl to remove the released ammonium. Potassium added to the solution process prevents the adsorption by clay minerals of the ammonium released during the previous oxidation. Then, subsamples were washed again with distilled water to remove nitrogen complex salts from the solution. Samples were finally dried and powdered again for isotope analyses.

N tot , N bnd of samples and their respective isotopic signature δ 15 N tot and δ 15 N bnd were determined by mass spectrometry using a Thermo Scientific Delta V plus mass spectrometer connected to a ConFlo IV dilution system, coupled with a Flash 2000 analyzer for elemental

analyses. The analytical accuracy and precision of the system were monitored using tyrosine (δ 15 N = 10.01 ‰) as an internal laboratory standard, that was calibrated on international standards IAEA-N-1 (δ 15 N = 0.4 ‰), IAEA-N-2 (δ 15 N = 20.3 ‰) and IAEA-NO-3 (δ 15 N = 4.7 ‰). Isotopic values were then expressed relative to the international N 2 air reference. The overall precision was better than 0.

n-alkane content = Σ[C i ] (1)
(2)

ACL = (3)
where is the concentration of the odd n-alkanes (µg/g TOC) with carbon number C i .

Apolar fractions were further eluted with heptane on silica gel-type silica columns in order to purify the n-alkane fraction before δ 2 H analysis by gas chromatography-isotope ratio mass spectrometry (GC-irMS). Silica was previously extracted with DCM, then activated at 120 °C for 24 h and finally deactivated with 5 wt. % H 2 O.

LC-MS analysis

Polar fractions obtained after separation of the TLEs over alumina columns were rotary evaporated, re-dissolved in 1 ml heptane and centrifuged using an Eppendorf Mini Spin centrifuge (1 min, 7000 rpm). The supernatant was collected and analyzed by high performance liquid chromatography-atmospheric pressure chemical ionization mass spectrometry (HPLC/APCI-MS). HPLC/APCI-MS analyses were performed with a Shimadzu LCMS-2020. GDGTs were analyzed using a procedure described in [START_REF] Huguet | Effects of a short-term experimental microclimate warming on the abundance and distribution of branched GDGTs in a French peatland[END_REF]. MAAT were calculated using Weijers et al. (2007) and [START_REF] Peterse | Revised calibration of the MBT-CBT paleotemperature proxy based on branched tetraether membrane lipids in surface soils[END_REF] calibrations, based on MBT and CBT indices as described in [START_REF] Coffinet | Potential of GDGTs as a temperature proxy along an altitudinal transect at Mount Rungwe (Tanzania)[END_REF].

GC-MS analysis

GC-MS analyses were performed on an Agilent 6890N gas chromatograph coupled with an Agilent 5973 Mass Selective Detector mass-spectrometer. The GC was fitted with a

Restek RTX-5Sil-MS column (30 m 0.25 mm i.d., 0.5 µm film thickness) under constant helium flow of 1 ml/min. Samples were dissolved in heptane and 1 µl injected in splitless mode with the injector temperature set at 320 °C. GC operating conditions were as follow:

initial temperature hold at 80 °C for 30 s, then increased from 80 to 100 °C at 10 °C/min and from 100 to 320 °C at 4 °C/min with a final isothermal hold at 320 °C for 20 min. The mass spectrometer was operated in the electron ionization (EI) mode at 70 eV ionization energy and scanned from 35 to 800 Da. Biomarkers were identified based on mass spectra and retention times. Quantification was performed thanks to the peak area in Total Ion Current (TIC) of each identified molecule. The peak areas were then normalized to the peak area of the internal standard C 12 -alkane, measured on the TIC and then normalized to the total organic carbon (TOC) of the sample.

GC-irMS analysis (δ 2 H)

The hydrogen isotopic composition of n-alkanes (δ 2 H) was determined by GC-irMS using a Trace GC chromatograph equipped with a Triplus autosampler, connected to a GC-Isolink pyrolysis interface, a ConFlo IV dilution system coupled to a DeltaV Advantage isotope ratio mass spectrometer, all from ThermoElectron (Bremen, Germany). A mixture of n-alkanes (n-C 16 to n-C 30 , Arndt Schimmelmann, Indiana University, Bloomington, IN, USA) was analyzed before and after each set of 5 injections, allowing monitoring the precision and accuracy of the system. The overall precision for the n-alkane standard δ 2 H values was better than 6 ‰ and were in good agreement with those measured off line. H 2 gas with known isotopic composition was used as an internal reference and δ 2 H values of samples were normalized to the VSMOW international isotopic scale by using the standard n-alkane mixture. The mean precision (standard deviation) for triplicate analyses was better than 6 ‰ for δ 2 H values of the C 17 to C 35 n-alkanes and of pristane, and better than 7 ‰ for phytane.

The H 3 + factor was calculated prior to analyses and was consistently below 3 ppm V -1 .

Because of the low amount of biomarkers in some samples, only 18 samples could be analyzed for δ 2 H out of 25 lipid extracts. Among the 18 samples, it was not possible to measure accurately the whole range of n-alkanes for all samples, depending on their respective concentration.

Results

Fossil woods

Many pieces of fossil woods are present along the section, although only one sample was positively identified as Xenoxylon fuxinense Ding in the TP3 trunk surface. The other samples (trunk surfaces 1, 2 and 3) did not seem to differ. However, their preservation state was too poor for safe identification. X. fuxinense is a secondary xylem documented from the Later Liassic (Negoya Fm.) of the Kuruma Group, Japan, to the Early Cretaceous (Aptian-Albian, Fuxin Fm.) of Liaoning, China. It is an Asian endemic taxon usually restricted to relatively high paleolatitudes [START_REF] Philippe | Distribution and palaeoecology of the Mesozoic wood genus Xenoxylon: palaeoclimatological implications for the Jurassic of Western Europe[END_REF][START_REF] Philippe | Systematics of a palaeoecologically significant boreal Mesozoic fossil wood genus, Xenoxylon Gothan[END_REF].

Bulk geochemistry

Exhaustive ). In an HI-OI diagram, the Taskomirsay samples plot in the Type-III zone (Fig. 3), suggesting an OM dominated by terrestrial plants [START_REF] Espitalié | La pyrolise Rock-Eval et ses applications[END_REF].

Total nitrogen contents range between 0.05 and 1.48 wt. % (Table 1). Considering high TOC values (>40 %) in lignites and the very low amount of clay in this lithology, we assume that N tot is almost equal to N org . In a plot of %N tot vs %TOC (Fig. 4), there is an intercept of 0.04 % N at 0 % TOC showing a significant fraction of inorganic N bound in clay minerals [START_REF] Schubert | Nitrogen and carbon isotopic composition of marine and terrestrial organic matter in Arctic Ocean sediments: implications for nutrient utilization and organic matter composition[END_REF] for samples below 20 % of TOC (clayey layers), whereas there is a negligible intercept of 0.002 % N when considering %N org vs %TOC. Thus, to calculate accurate atomic C/N ratios, we used N tot for lignites and N org for clayey layers. In Taskomirsay, C/N ratios range from 38 to 132 (Table 1).

Nitrogen isotopes

Together with the absence of clayprone to adsorb inorganic nitrogen [START_REF] Müller | CN ratios in Pacific deep-sea sediments: Effect of inorganic ammonium and organic nitrogen compounds sorbed by clays[END_REF] in lignites or coalified trunks, we argue for the use of the δ 15 N tot values as representatives of the δ 15 N org values. Indeed, [START_REF] Tramoy | Stepwise palaeoclimate change across the Eocene-Oligocene transition recorded in continental NW Europe by mineralogical assemblages and δ 15 N org (Rennes Basin, France)[END_REF] have shown that δ 15 N tot values were driven by δ 15 N org values in Eocene-Oligocene terrestrial sediments. In Taskomirsay, the δ 15 N tot values range between 2.5 and 3.8 wt. ‰ (3.2 ‰ on average; Table 1 and Fig. 2). Except for one sample (TP3-224), δ 15 N org values of clay samples were almost equal to their δ 15 N tot values 3.4. Alkanes n-Alkane amount range between 11 and 621 µg/g TOC (192 µg/g TOC on average;

Table 2), except TP1-65 (clay) that yielded ~3300 µg/g TOC. It is noteworthy that the amount of n-alkanes decreases from the base of the section to the base of LB5 at 32.3 m, before it increases to the top. These trends are even more marked within lignite beds, with values gradually decreasing from LB1 to LB5 (315 µg/g TOC to 18 µg/g TOC on average), while TOC values remain similar (Table 2; Fig. 5).

The distribution of n-alkanes is highly variable along the section, with patterns generally not being related to lithology or depth (Fig. 5). The distribution of n-alkanes in lignite beds from the bottom part (LB1 and LB2) is similar, with a bimodal distribution: a They often exhibit Carbon Preference Indices (CPI) higher than 2 and sometimes reaching 5.5 (Table 2). This odd predominance is mainly driven by the longest homologues (> C 25 ), which are usually attributed to higher plant leaf-wax input [START_REF] Eglinton | Hydrocarbon constituents of the wax coatings of plant leaves: A taxonomic survey[END_REF][START_REF] Eglinton | Leaf Epicuticular Waxes[END_REF][START_REF] Meyers | Organic geochemical proxies of paleoceanographic, paleolimnologic, and paleoclimatic processes[END_REF]. Shorter-chain n-alkanes (<C 21 )generally attributed to algal/bacterial sources [START_REF] Blumer | Hydrocarbons of marine phytoplankton[END_REF][START_REF] Cranwell | Lipids of aquatic organisms as potential contributors to lacustrine sediments-II[END_REF][START_REF] Giger | Aliphatic and olefinic hydrocarbons in recent sediments of Greifensee, Switzerland[END_REF][START_REF] Meyers | Organic geochemical proxies of paleoceanographic, paleolimnologic, and paleoclimatic processes[END_REF][START_REF] Ladygina | A review on microbial synthesis of hydrocarbons[END_REF] are in relatively low amounts (24 wt. % 15%) when compared to n-alkanes above C 21 .

The ACL values of the n-alkanes were calculated over the range C 21 -C 35 to cover the range of n-alkanes found in aquatic and terrestrial plants (e.g., [START_REF] Eglinton | Leaf Epicuticular Waxes[END_REF][START_REF] Ficken | An n-alkane proxy for the sedimentary input of submerged/floating freshwater aquatic macrophytes[END_REF][START_REF] Diefendorf | Production of n-alkyl lipids in living plants and implications for the geologic past[END_REF]. ACL values range between 22.4 and 31.8 (Table 2; Fig. 5). A similar trend is observed for the evolution of the ACL values and that of the amount of n-alkanes, i.e. the ACL and n-alkane amounts show maximal values in LB1 to decrease toward LB4 and LB5. Another minimal value is observed for LB6, before it increases again to the top of the section.

Pristane (Pr) and phytane (Ph) were detected in all samples in small amounts at level of ~9 and ~8 µg/g TOC on average, respectively (not shown). Pr/Ph ratios show large variations, ranging from 0.3 in clayey samples with poor organic matter content up to 7.6 in a lignite. The highest values are observed in lignites or clayey OM-rich samples (Table 2).

Compound-specific hydrogen isotopes (δ 2 H)

δ 2 H values were determined for C 17 to C 35 n-alkanes and for the isoprenoid pristane and phytane (Table 3; Fig. 6). Results for n-alkanes as well as pristane and phytane are reported along the section in Fig. 7. . This general increase in δ 2 H values occurs for all the n-alkanes. Third, lower δ 2 H values were measured in lignites, alternating with higher values measured in clayey layers and trunk/root samples (Fig. 6). However, this feature is less obvious for n-alkanes >C 27 .

Alkanes

Isoprenoids

The δ 2 H values of pristane and phytane range between 303 and 260 ‰ ( 283 ‰ on average) and between 311 and 233 ‰ ( 274 ‰ on average), respectively. Pristane and phytane are systematically 2 H-depleted when compared to n-alkanes with Δ 2 H = 80 ‰ on average, where Δ 2 H is the difference between the average δ 2 H value of the isoprenoids and the average δ 2 H value of the n-alkanes (Fig. 6).

brGDGT

The brGDGTs required for MAAT calculation were detected and could be quantified in only 7 samples (see supplementary data for details). They represent the oldest GDGTs ever recorded to date [START_REF] Schouten | The organic geochemistry of glycerol dialkyl glycerol tetraether lipids: A review[END_REF]. The use of this proxy is therefore attempted for the first time in the Early Jurassic and must be taken with caution. According to the calibration of Weijers et al. (2007), MAATs range between -1.1 °C and 18.0 °C, while they range between 4.0 °C and 15.9 °C based on the Peterse et al. ( 2012) calibration (Table 2). Although they show the same trends, negative MAATs values during the Early Jurassic are very unlikely.

Thus, the first set of MAATs values will not be further considered, and only trends will be considered for the second one. Here, an increasing trend was observed during the early Toarcian (cf. Fig. 9).

Discussion

Preservation of organic matter

The organic matter is mainly immature (T max <440 °C) and experienced a limited burial, allowing paleoenvironmental interpretations [START_REF] Espitalié | La pyrolise Rock-Eval et ses applications[END_REF], see supplementary data 1). Higher HI values in lignites as compared to clayey layers suggest a relatively better organic matter preservation in lignites, although a matrix mineral effect in clayey layers leading to lower HI values cannot be excluded [START_REF] Espitalié | La pyrolise Rock-Eval et ses applications[END_REF]. However, higher OI values in clayey layers relative to lignites are in agreement with a slightly less preserved organic matter in clayey layers, probably resulting from oxidative conditions at the watersediment interface or during transport processes for silty-sediments [START_REF] Espitalié | La pyrolise Rock-Eval et ses applications[END_REF]. In addition, no significant correlation was recorded between δ 15 N values and either TOC or TN values (n=52; p>>0.05), suggesting a relative independence between the isotopic composition and the organic content.

The δ 2 H values of individual lipid biomarkers are much less prone to diagenetic alteration when compared to bulk organic matter, because (i) specific compounds are not influenced by preferential degradation of less stable compounds [START_REF] Andersen | Large and rapid climate variability during the Messinian salinity crisis: Evidence from deuterium concentrations of individual biomarkers[END_REF] and (ii) hydrogen in lipids is strongly bound to carbon and considered non exchangeable, which allows the conservation of the primary δ 2 H values of lipids at temperatures above 150 °C [START_REF] Schimmelmann | D/H isotope ratios of kerogen, bitumen, oil, and water in hydrous pyrolysis of source rocks containing kerogen types I, II, IIS, and III[END_REF]. To this respect, [START_REF] Dawson | Stable hydrogen isotopic composition of hydrocarbons in torbanites (Late Carboniferous to Late Permian) deposited under various climatic conditions[END_REF] reported n-alkanes and isoprenoids that retained their original δ 2 H values in relatively immature torbanites as old as the Late Carboniferous.

It has been suggested that the transfer of hydrogen from water to organic matter is the most important mechanism leading to hydrogen-isotope exchange [START_REF] Schimmelmann | Hydrogen Isotopic (D/H) Composition of Organic Matter During Diagenesis and Thermal Maturation[END_REF], whereas hydrogen is thought to be totally exchanged on very long geological timescale (hundreds of million years, [START_REF] Koepp | D/H isotope exchange reaction between petroleum and water: A contributory determinant for D/H-isotope ratios in crude oils?[END_REF][START_REF] Sessions | Factors controlling the deuterium contents of sedimentary hydrocarbons[END_REF]. This mechanism leads to the homogenization of δ 2 H values of specific compounds, thus overprinting their primary δ 2 H values (Yang and Huang, 2003;[START_REF] Sessions | Isotopic exchange of carbonbound hydrogen over geologic timescales[END_REF][START_REF] Pedentchouk | Different response of δD values of nalkanes, isoprenoids, and kerogen during thermal maturation[END_REF]. In Taskomirsay, such a homogenization of δ 2 H values was not observed as indicated by both the large variability in δ 2 H values of homologous n-alkanes (up to 90 ‰) and between adjacent sedimentary layers (i.e. samples TP2-140 vs. TP3-151; Fig. 6 and7). The high CPI (>1) also argues against post-depositional alteration of the δ 2 H values [START_REF] Pedentchouk | Different response of δD values of nalkanes, isoprenoids, and kerogen during thermal maturation[END_REF]. Most importantly, an offset (Δ 2 H = 80 ‰ on average, Fig. 6) between the δ 2 H values of n-alkanes and isoprenoids was recorded. In Taskomirsay, this offset is similar to the offset recorded in modern biological samples [START_REF] Sessions | Fractionation of hydrogen isotopes in lipid biosynthesis[END_REF][START_REF] Chikaraishi | Carbon and hydrogen isotopic fractionation during lipid biosynthesis in a higher plant (Cryptomeria japonica)[END_REF] and rules out any extensive homogenization of δ 2 H values [START_REF] Andersen | Large and rapid climate variability during the Messinian salinity crisis: Evidence from deuterium concentrations of individual biomarkers[END_REF][START_REF] Dawson | Stable hydrogen isotopic composition of hydrocarbons in torbanites (Late Carboniferous to Late Permian) deposited under various climatic conditions[END_REF][START_REF] Sessions | Isotopic exchange of carbonbound hydrogen over geologic timescales[END_REF].

On the basis of these lines of evidence, we can conclude that the organic proxies mainly reflect primary environmental signals, thus allowing paleoenvironmental interpretations.

Sources of organic matter

The sporomorph associations as well as fossil woods argue for a well-developed terrestrial macro-paleoflora. The presence of Xenoxylon from LB2 to the transition zone, inferred from fossil woods, is also confirmed by infra-millimeter wood fragments, belonging to Xenoxylon, observed in palynological slides (Schnyder et al., accepted). After the transition zone, lacustrine areas are suggested by sedimentology and the presence of Botryococcus sp.

and Ovoidites from ~45 m (Schnyder et al., accepted). However, the Type-III organic matter points to a major contribution of vascular plants to the organic matter through the entire section (Fig. 3; [START_REF] Espitalié | La pyrolise Rock-Eval et ses applications[END_REF], confirmed by (i) woods in situ, (ii) wood fragments associated with sporomorphs (Schnyder et al., accepted) and (iii) the combination of atomic C/N ratios and δ 13 C org values [START_REF] Tyson | Sedimentary Organic Matter[END_REF][START_REF] Meyers | Organic geochemical proxies of paleoceanographic, paleolimnologic, and paleoclimatic processes[END_REF]Fig. 8).

The terrestrial origin of the organic matter is confirmed at the molecular level by the distribution of n-alkanes dominated by >C 21 n-alkanes associated with CPI >1 [START_REF] Eglinton | Leaf Epicuticular Waxes[END_REF][START_REF] Ficken | An n-alkane proxy for the sedimentary input of submerged/floating freshwater aquatic macrophytes[END_REF][START_REF] Diefendorf | Production of n-alkyl lipids in living plants and implications for the geologic past[END_REF], whereas a minor algal/bacterial contribution is deduced from the relatively low amount of shorter chain n-alkanes, except in clayey and OM poor samples where n-C 18 usually dominates (Fig. 5; [START_REF] Blumer | Hydrocarbons of marine phytoplankton[END_REF][START_REF] Cranwell | Lipids of aquatic organisms as potential contributors to lacustrine sediments-II[END_REF][START_REF] Giger | Aliphatic and olefinic hydrocarbons in recent sediments of Greifensee, Switzerland[END_REF][START_REF] Meyers | Organic geochemical proxies of paleoceanographic, paleolimnologic, and paleoclimatic processes[END_REF][START_REF] Ladygina | A review on microbial synthesis of hydrocarbons[END_REF]. Despite various possible sources of pristane and phytane, Pr/Ph ratios <1 observed in those samples are consistent with the development of bacteria in anoxic/suboxic hypersaline environments, whereas Pr/Ph ratios >>1 in OM-rich sediments agree with a terrestrial organic matter deposited in oxic environments (Table 2; [START_REF] Didyk | Organic geochemical indicators of palaeoenviron-mental conditions of sedimentation[END_REF][START_REF] Peters | The Biomarker Guide: Biomarkers and isotopes in the environment and human history[END_REF][START_REF] Riboulleau | Environmental change during the Early Cretaceous in the Purbeck-type Durlston Bay section (Dorset, Southern England): A biomarker approach[END_REF].

Long-chain n-alkanes (C 27 , C 29 , C 31 ) are usually attributed to land-plant waxes [START_REF] Eglinton | Hydrocarbon constituents of the wax coatings of plant leaves: A taxonomic survey[END_REF][START_REF] Eglinton | Leaf Epicuticular Waxes[END_REF]. In Taskomirsay, they are likely produced by gymnosperms since the major diversification of angiosperms occurred much later, during the Early Cretaceous [START_REF] Gee | Shaking the Tree: Readings from Nature in the History of Life[END_REF], about 50 My after the study period. However, long-chain nalkanes (C 27 -C 31 ) with strong odd/even carbon predominance could also result from an algal input (e.g., Botryococcus spp., [START_REF] Lichtfouse | Possible algal origin of long chain odd n-alkanes in immature sediments as revealed by distributions and carbon isotope ratios[END_REF][START_REF] Riboulleau | Environmental change during the Early Cretaceous in the Purbeck-type Durlston Bay section (Dorset, Southern England): A biomarker approach[END_REF]. Thus, an algal influence on the long-chain n-alkanes in TP3-224 cannot be ruled out, in agreement with the lowest C/N ratio recorded (C/N = 38) in the section.

n-Alkanes C 21 to C 25 are generally predominant in lignites and trunk/root samples from LB3 to the base of LB6 (Fig. 5). They have been reported to originate from modern Sphagnum [START_REF] Baas | A comparative study of lipids in Sphagnum species[END_REF][START_REF] Nott | n-Alkane distributions in ombrotrophic mires as indicators of vegetation change related to climatic variation[END_REF] and aquatic macrophytes [START_REF] Ficken | An n-alkane proxy for the sedimentary input of submerged/floating freshwater aquatic macrophytes[END_REF][START_REF] Bechtel | Paleogeography and paleoecology of the upper Miocene Zillingdorf lignite deposit (Austria)[END_REF], thus representing an aquatic environment. But, sphagnums are unlikely important sources for those n-alkanes, because they radiated during the Miocene and only rare fossils and spores related to proto-sphagnums have been reported in the Mesozoic [START_REF] Shaw | Peatmoss (Sphagnum) diversification associated with Miocene Northern Hemisphere climatic cooling?[END_REF]. Alternatively, mosses might be a valuable source of these n-alkanes since they have been recognized at least from the lower Carboniferous [START_REF] Thomas | A Probable Moss from the Lower Carboniferous of the Forest of Dean, Gloucestershire[END_REF]. However, no sporomorph of mosses or sphagnums could be recognized (Schnyder et al., accepted).

The major contribution of the C 23 in most of the samples (Fig. 5) is more likely attributed to conifers as this distribution is close to the general trend observed in the latter, with maxima at C 23 and C 25 [START_REF] Oros | Epicuticular wax compositions of predominant conifers of Western North America[END_REF][START_REF] Nakamura | Aliphatic and aromatic terpenoid biomarkers in Cretaceous and Paleogene angiosperm fossils from Japan[END_REF]. This is supported by the unimodal distribution maximizing at C 23 observed in the two trunk/root samples TP1-65 and TP1-114.

Further evidence come from the high concentration of fossil trunks/roots in life positions through the section in agreement with peaty/swampy forestsinferred from the numerous coal-rich formations in the North Hemispherecharacterizing the Early Jurassic [START_REF] Miao | Peat-forming materials and evolution of swamp sequencescase analysis of a Jurassic inland coal basin in China[END_REF][START_REF] Rees | Jurassic phytogeography and climates: new data and model comparisons[END_REF][START_REF] Wang | Early to Middle Jurassic vegetation and climatic events in the Qaidam Basin, Northwest China[END_REF]. In addition, C 20 -C 23 alkanes have been reported in Carboniferous coalified paleoflora [START_REF] Disnar | Biological origin of tetracyclic diterpanes, n-alkanes and other biomarkers found in lower carboniferous Gondwana coals (Niger)[END_REF], in Carboniferous-Permian terrigenous/coaly sediments [START_REF] Scheffler | Global changes during Carboniferous-Permian glaciation of Gondwana: Linking polar and equatorial climate evolution by geochemical proxies[END_REF], and in the Late Oligocene Taxodium dubium (Cupressaceae s.l) fossil plant maximizing at C 21 [START_REF] Stefanova | Paleoenvironmental application of Taxodium macrofossil biomarkers from the Bobov dol coal formation, Bulgaria[END_REF]. The first Germanypaleoecologically related to environments with no water restriction, usually riparian forests and swamps [START_REF] Oh | Xenoxylon Synecology and Palaeoclimatic Implications for the Mesozoic of Eurasia[END_REF] also maximizes at C 23 [START_REF] Marynowski | Systematic relationships of the Mesozoic wood genus Xenoxylon: an integrative biomolecular and palaeobotanical approach[END_REF], although these authors attributed the presence of n-alkanes to contaminations by host clays. This is not the case here when compared the root/trunk TP1-65 to its host clays (TP1-65bis) as they show large differences in their n-alkane distribution (Fig. 5). To our opinion, all these lines of evidence support the hypothesis that C 21 -C 23 n-alkanes in our samples could result from conifers like Xenoxylon that thrived in flooded environments.

In summary, those evidences support the use of n-C 21 -n-C 23 as representing an environment under an almost permanent shallow water layer, and the use of n-C 27 -n-C 31 as strictly terrestrial vascular plant biomarkers (eg. [START_REF] Ficken | An n-alkane proxy for the sedimentary input of submerged/floating freshwater aquatic macrophytes[END_REF][START_REF] Sachse | Hydrogen isotope ratios of recent lacustrine sedimentary n-alkanes record modern climate variability[END_REF][START_REF] Mügler | Effect of lake evaporation on δD values of lacustrine n-alkanes: A comparison of Nam Co (Tibetan Plateau) and Holzmaar (Germany)[END_REF]. Accordingly, C 21 -C 23 n-alkanes will be considered as deriving from an "aquatic pool" and C 27 -C 31 n-alkanes from a "terrestrial pool", respectively, for this study. As δ 2 H values of C 23 and C 27 n-alkanes were accurately measured for all samples, they will be considered as representative of the aquatic and terrestrial pool, respectively.

The existence of these two different pools could explain the offset between the δ 2 H values of the 2 H-depleted aquatic pool and the 2 H-enriched terrestrial pool (Fig. 6 and7). 2 Henriched C 27 -C 31 n-alkanes suggest hydrogen loss by evaporation and/or vascular plant transpiration [START_REF] Sachse | Hydrogen isotope ratios of recent lacustrine sedimentary n-alkanes record modern climate variability[END_REF][START_REF] Sachse | δD values of individual n-alkanes from terrestrial plants along a climatic gradient -Implications for the sedimentary biomarker record[END_REF], whereas the 2 H-depleted C 21 -C 23 n-alkanes suggest that they originate from organisms that do not evapotranspire (e.g., aquatic macrophytes; [START_REF] Ficken | An n-alkane proxy for the sedimentary input of submerged/floating freshwater aquatic macrophytes[END_REF][START_REF] Nott | n-Alkane distributions in ombrotrophic mires as indicators of vegetation change related to climatic variation[END_REF][START_REF] Bechtel | Paleogeography and paleoecology of the upper Miocene Zillingdorf lignite deposit (Austria)[END_REF] or from organisms living in high relative humidity sub-environments (e.g., ombrotrophic peatlands; [START_REF] Nichols | Quantitative assessment of precipitation seasonality and summer surface wetness using ombrotrophic sediments from an Arctic Norwegian peatland[END_REF].

The origin of the C 33 and C 35 n-alkanes that are dominant at the base and at the top of the section (Fig. 5) is uncertain as such a predominance has never been reported in sediments to date. However, these homologues were found to be major n-alkanes in some modern

Cupressaceae [START_REF] Dodd | Ecotypic adaptation in Austrocedrus chilensis in cuticular hydrocarbon composition[END_REF][START_REF] Diefendorf | Production of n-alkyl lipids in living plants and implications for the geologic past[END_REF][START_REF] Street | n-Alkane evidence for the onset of wetter conditions in the Sierra Nevada, California (USA) at the mid-late Holocene transition, ~ 3.0 ka[END_REF], suggesting a terrestrial higher plant origin for these compounds. The relatively high δ 2 H signal of those compounds is in agreement with a vascular plant origin that would be subject to evapotranspiration.

Paleoclimate evolution

The Taskomirsay section is separated into three parts for discussion purposes. The first part includes the base of the section to the base of the transition zone at ~26 m and is named "Phase 1". It corresponds to the mid-to late Pliensbachian (Fig. 9; Schnyder et al., accepted). The second part is the transition zone, corresponding to the transition between Pliensbachian and Toarcian, based on sporomorph associations and δ 13 C org (Fig. 9; Schnyder et al., accepted). The δ 13 C org curve is described in Schnyder et al., accepted. The transition zone ends at ~35 m. The rest of the section, corresponding to the early Toarcian is named "Phase 2" (Fig. 9).

Paleoflora

The morphogenus conifer Xenoxylon Gothan is biogeographically related to cool/humid settings and thus, usually to high latitudes [START_REF] Philippe | Distribution and palaeoecology of the Mesozoic wood genus Xenoxylon: palaeoclimatological implications for the Jurassic of Western Europe[END_REF][START_REF] Philippe | Palaeoecologically significant wood genus Xenoxylon discovered in the East Stara Planina Mts. (East Bulgaria) Balaban Formation (Toarcian, Early Jurassic)[END_REF][START_REF] Philippe | Structure and diversity of the Mesozoic wood genus Xenoxylon in Far East Asia: implications for terrestrial palaeoclimates[END_REF]. Recent isotopic studies confirmed these paleoclimatic requirements for Xenoxylon [START_REF] Amiot | Environment and ecology of East Asian dinosaurs during the Early Cretaceous inferred from stable oxygen and carbon isotopes in apatite[END_REF]. In addition, Xenoxylon occurred at lower latitudes during the Jurassic and even thrived in Western Europe during cold episodes, such as in the late Pliensbachian or in the early Oxfordian [START_REF] Philippe | Distribution and palaeoecology of the Mesozoic wood genus Xenoxylon: palaeoclimatological implications for the Jurassic of Western Europe[END_REF]. Therefore, the occurrence of Xenoxylon at mid-latitudes in Taskomirsay in Phase 1 and in the transition zone suggests a cool/humid period, followed by less humid and/or warmer conditions in the early Toarcian (Phase 2), deduced from the demise of Xenoxylon (Fig. 9).

During the Jurassic, the Euro-Sinian (West-Kazakhstan) and the West-Siberian (East-Kazakhstan) floral provinces were separated by a NW/SE axis through Kazakhstan [START_REF] Vakhrameev | Jurassic and Cretaceous Floras and Climates of the Earth[END_REF] that may have fluctuated in the Jurassic [START_REF] Kirichkova | New data on the phytostratigraphy of the Jurassic deposits of Kazakhstan[END_REF]. Sporomorph associations (Schnyder et al., accepted) prior to the transition zone suggest an influence of the West-Siberian province with a monotonous flora dominated by ferns, few Cycadales, Ginkgoales, Czekanowskiales, Bennettitales and Coniferales, characteristic of a moderate-warm climate [START_REF] Vakhrameev | Jurassic and Cretaceous Floras and Climates of the Earth[END_REF], in agreement with the occurrence of Xenoxylon in the same part of the section (Fig. 9). The influence of the Euro-Sinian province (warm-temperate) progressively increased through the section with the emergence of Manumia delcourtii (Pocock) [START_REF] Dybkjaer | Palynological zonation and palynofacies investigation of the Fjerritslev Formation (Lower Jurassic-basal Middle Jurassic) in the danish subbasin[END_REF] and later of Ischyosporites variegatus (Couper) [START_REF] Schulz | Sporenpaläontologisce Untersuchungen rätoliassischer Schichten im Zentralteil des germanischen Beckens[END_REF] in Phase 2, which is one of the taxon used to identify the early Toarcian (Schnyder et al., accepted). Then, the early Toarcian (Phase 2) is marked by the occurrence of more thermophilic taxa such as Callialasporites spp. or Ischyosporites variegatus, while a slight decrease in humidity is recorded by rare Corollina spp., associated with the demise of Xenoxylon (Fig. 9; Schnyder et al., accepted). Accordingly, a similar climatic trend has been evidenced regionally by other palynological studies during the Early Jurassic in Central Asia [START_REF] Iljina | Jurassic palynology of Siberia. Academy of Sciences of the USSR, Siberian Branch[END_REF][START_REF] Vakhrameev | Jurassic and Cretaceous Floras and Climates of the Earth[END_REF][START_REF] Mogutcheva | Main phytostratigraphic boundaries in the Jurassic deposits of Western Siberia[END_REF]. Thus, globally humid conditions along the section probably favored high production of biomass that enabled the formation of the numerous lignite beds [START_REF] Flores | Organic facies and depositional palaeoenvironment of lignites from Rio Maior Basin (Portugal)[END_REF][START_REF] Kalaitzidis | A short-term establishment of forest fen habitat during Pliocene lignite formation in the Ptolemais Basin, NW Macedonia, Greece[END_REF].

Alternatively, changes in depositional environments or taphonomic conditions could have driven the occurrence of the wood Xenoxylon and the mentioned sporomorphs. But, there is no evidences to such hypothesis as no drastic changes in depositional environments occurred until the top of the section, where lacustrine conditions took place. Most importantly, the other organic markers do support a climatic interpretation of the data (see below) together with other regional paleobiogeographical studies mentioned above.

What do δ 15 N values record?

Many studies have concluded that the δ 15 N org values in terrestrial environments are mainly driven by climate via water-availability and nutrient cycling, the δ 15 N of plants and soils being correlated negatively with precipitation and positively with temperatures [START_REF] Austin | Nutrient dynamics on a precipitation gradient in Hawai'i[END_REF][START_REF] Handley | The 15 N natural abundance (δ 15 N) of ecosystem samples reflects measures of water availability[END_REF][START_REF] Amundson | Global patterns of the isotopic composition of soil and plant nitrogen[END_REF][START_REF] Swap | Natural abundance of 13 C and 15 N in C 3 and C 4 vegetation of southern Africa: patterns and implications[END_REF][START_REF] Liu | Nitrogen isotopic composition of plant-soil in the Loess Plateau and its responding to environmental change[END_REF]. In brief, δ 15 N org values in humid environments are driven by intense Nrecycling between dead and alive biomass through a rapid turnover of a small N-mineral pool, whereas this cycle is interrupted in arid environments (low biomass activity) leading to N-excess in a large mineral pool [START_REF] Austin | Nutrient dynamics on a precipitation gradient in Hawai'i[END_REF][START_REF] Handley | The 15 N natural abundance (δ 15 N) of ecosystem samples reflects measures of water availability[END_REF]Swap et al., 2004). The latter is then subjected to N-loss by leaching and gas emission leading to high δ 15 N values in the remaining N-pool [START_REF] Handley | The 15 N natural abundance (δ 15 N) of ecosystem samples reflects measures of water availability[END_REF][START_REF] Aranibar | Nitrogen cycling in the soil-plant system along a precipitation gradient in the Kalahari sands[END_REF]. In turn, δ 15 N org values are controlled by the relative openness of the N-cycle by changes in water-or N-availability as illustrated by [START_REF] Martinelli | Nitrogen stable isotopic composition of leaves and soil: Tropical versus temperate forests[END_REF], who reported significant higher δ 15 N org (+6.5 ‰) values in humid tropical forests (N-excess) than in temperate forests (N-limited).

Considerable variations exist in δ 15 N values from one site to another, ranging from around 8 to +22 ‰ in actual soil organic matter (Craine et al. 2015).

In Taskomirsay, δ 15 N org values were calculated for samples in which TOC is <20 % and do not strongly differ from the δ 15 N tot values, justifying the use of δ 15 N tot values as representative of the δ 15 N org values (Fig. 2), except for TP3-224 that might be influenced by an algal contribution and hence will not be considered for paleoclimate reconstructions. In the light of the previous interpretations, the relatively low and stable δ 15 N org values (~+3 ‰)when compared to those compiled by [START_REF] Handley | The 15 N natural abundance (δ 15 N) of ecosystem samples reflects measures of water availability[END_REF] in arid to semi-arid sites (>+5 ‰)

are in agreement with a globally closed N-cycle, suggesting humid conditions and/or limited nutrients in the system.

Although δ 15 N org values remain in a rather narrow range (~2.5 to ~3.5 ‰), the small excursions recorded in δ 15 N org values, either negative (LB1, LB2 and LB6) or positive (LB5), together with the scattered feature in LB4 and LB3 might reflect the tenuous equilibrium between water-availability and nutrient cycling. This pattern would imply that (i) despite paleoflora indices in Phase 2, no drastic changes occurred along the section whether in the nutrient dynamics or regarding water-availability (Fig. 9), (ii) the vegetation has adapted to environmental changes ensuring an efficient N-recycling and thus almost constant δ 15 N org values. The latter hypothesis suggests a strong influence of the local environment on δ 15 N org values as suspected by [START_REF] Amundson | Global patterns of the isotopic composition of soil and plant nitrogen[END_REF], who compiled δ 15 N values of modern plants and soils. The influence of local environments is supported by the high variability of δ 13 C org values, especially in LB4 and fossil woods (Fig. 9), which could result from changes of vegetation assemblages or changes of 13 C-fractionation rate tuned by temperature, altitude and/or relative humidity [START_REF] Warren | Water availability and carbon isotope discrimination in conifers[END_REF][START_REF] Bowen | Interpolating the isotopic composition of modern meteoric precipitation[END_REF][START_REF] Bowling | Carbon isotopes in terrestrial ecosystem pools and CO2 fluxes[END_REF]. A better sampling resolution for biomarkers studies would help to decipher the real causes of variability of the δ 13 C org values, while a global and long-term influence is suggested by Schnyder et al. (accepted) to highlight the recognition of the Pliensbachian-Toarcian transition.

To sum up, no drastic changes in the δ 15 N org are recorded as compared to those reported in the Paleocene-Eocene [START_REF] Storme | Cycles of humid-dry climate conditions around the P/E boundary: new stable isotope data from terrestrial organic matter in Vasterival section (NW France)[END_REF] or in the Eocene-Oligocene [START_REF] Tramoy | Stepwise palaeoclimate change across the Eocene-Oligocene transition recorded in continental NW Europe by mineralogical assemblages and δ 15 N org (Rennes Basin, France)[END_REF] that exhibit much higher variations in amplitude of up to 6 ‰.

Molecular response of plants

Leaf waxes are produced by plants as a protection against external environment, especially to prevent water loss. Accordingly, lower production of n-alkanes along with shorter chain lengths is believed to reflect a decreasing need for protection against water loss and/or a deficiency of energy for biosynthesizing long chain n-alkanes [START_REF] Weete | Lipid and Surface Wax Synthesis in Water-stressed Cotton Leaves[END_REF][START_REF] Gagosian | The importance of atmospheric input of terrestrial organic material to deep sea sediments[END_REF][START_REF] Gauvrit | Effect of low temperatures on 2,4-D behaviour in maize plants[END_REF][START_REF] Shepherd | The effects of stress on plant cuticular waxes[END_REF], indicative of humid conditions. Conversely, higher amounts of n-alkanes and higher ACL values suggest water stress for plants, drier conditions and higher temperatures. In this respect, decreasing ACL values and n-alkane amounts from Phase 1 to the transition zone suggest increasing humidity and decreasing temperatures (Fig. 9). Then, increasing ACL values and n-alkane amounts point to drier conditions and higher temperatures in the early Toarcian (Phase 2). MAATs, reconstructed based on brGDGTs distributions, depict the same trends with decreasing temperatures in Phase 1 and increasing temperatures in Phase 2, although the number of samples is limited. It is noteworthy that relations between ACL values and n-alkane amounts are particularly robust in lignites, whereas no correlation exists for clayey layers (Fig. 10). In order to explain this discrepancy, we hypothesize that the organic matter in swampy forest is dominated by autochthonous vascular plant inputs, responding promptly to relative changes in humidity, whereas organic matter in clayey layers could contain significant part of allochthonous plant products or algal/bacterial organic matter (n-C 18 ; Fig. 5) that is less influenced by water loss [START_REF] Hoffmann | Abundance and distribution of leaf wax n-alkanes in leaves of Acacia and Eucalyptus trees along a strong humidity gradient in northern Australia[END_REF]. Notice that the highest amount of n-alkanes was recorded in the clayey layer TP1-65, which is very poor in organic matter (<0.05 %). As the amount of n-alkanes is expressed relative to %TOC, the very low %TOC results in a very high concentration of n-alkanes in this specific sample.

Interpretation of n-alkane δ 2 H values

According to [START_REF] Sachse | Molecular Paleohydrology: Interpreting the Hydrogen-Isotopic Composition of Lipid Biomarkers from Photosynthesizing Organisms[END_REF], the δ 2 H values of higher plant n-alkanes are mainly influenced by (i) the δ 2 H values of precipitation [START_REF] Huang | Hydrogen isotope ratios of individual lipids in lake sediments as novel tracers of climatic and environmental change: a surface sediment test[END_REF][START_REF] Sachse | Hydrogen isotope ratios of recent lacustrine sedimentary n-alkanes record modern climate variability[END_REF][START_REF] Sachse | Molecular Paleohydrology: Interpreting the Hydrogen-Isotopic Composition of Lipid Biomarkers from Photosynthesizing Organisms[END_REF], (ii) the extent of evapotranspiration from soils and leaves [START_REF] Smith | Influence of physiology and climate on δD of leaf wax nalkanes from C 3 and C 4 grasses[END_REF][START_REF] Feakins | Controls on the D/H ratios of plant leaf waxes in an arid ecosystem[END_REF] and (iii) to a minor extent, by the interspecies variability [START_REF] Smith | Influence of physiology and climate on δD of leaf wax nalkanes from C 3 and C 4 grasses[END_REF]Chikaraishi and Naraoka, 2007). δ 2 H values of precipitations are positively correlated with temperature at the precipitation site and negatively correlated with the amount of precipitation and with the distance from the shore line [START_REF] Dansgaard | Stable isotopes in precipitation[END_REF][START_REF] Epstein | Climatic implications of the D/H ratio of hydrogen in C-H groups in tree cellulose[END_REF][START_REF] Gat | Oxygen and Hydrogen Isotopes in the Hydrologic Cycle[END_REF][START_REF] Sachse | Hydrogen isotope ratios of recent lacustrine sedimentary n-alkanes record modern climate variability[END_REF]. Also, the amount of precipitation is the most important factor in low latitudes, whereas the temperature effect predominates in higher latitudes [START_REF] Sachse | Molecular Paleohydrology: Interpreting the Hydrogen-Isotopic Composition of Lipid Biomarkers from Photosynthesizing Organisms[END_REF]. At the global scale, a latitudinal/climatic distribution pattern of δ 2 H values has been reported in modern precipitation patterns and also for geological times with δ 2 H values being negatively correlated with latitudes [START_REF] Bowen | Interpolating the isotopic composition of modern meteoric precipitation[END_REF][START_REF] Dawson | Stable hydrogen isotopic composition of hydrocarbons in torbanites (Late Carboniferous to Late Permian) deposited under various climatic conditions[END_REF]. [START_REF] Smith | Magnitude of the carbon isotope excursion at the Paleocene-Eocene thermal maximum: The role of plant community change[END_REF] and at high latitudes in the Arctic ( 210 to 160 ‰, [START_REF] Pagani | Arctic hydrology during global warming at the Palaeocene/Eocene thermal maximum[END_REF], than those reported for the same period in coastal regions from mid-to low-latitudes (-165 to 140 ‰; [START_REF] Handley | Changes in the hydrological cycle in tropical East Africa during the Paleocene-Eocene Thermal Maximum[END_REF]174 to 112 ‰;[START_REF] Garel | Paleohydrological and paleoenvironmental changes recorded in terrestrial sediments of the Paleocene-Eocene boundary (Normandy, France)[END_REF]. They are also closer to early Permian δ 2 H values recorded in high latitudes with glacial to cool-temperate environments (ca. 200 ‰), than those recorded for the same period in tropical regions (Fig. 6; ca. 150 ‰; [START_REF] Dawson | Stable hydrogen isotopic composition of hydrocarbons in torbanites (Late Carboniferous to Late Permian) deposited under various climatic conditions[END_REF]ca. 100 ‰;[START_REF] Izart | Palaeoclimate reconstruction from biomarker geochemistry and stable isotopes of n-alkanes from Carboniferous and Early Permian humic coals and limnic sediments in western and eastern Europe[END_REF]. These results are in agreement with the estimated mid-paleolatitudes of the Taskomirsay section (36° N 8°,

Bruno Vrielynck, pers. comm., 2014), associated with a warm to cool-temperate global climate during the Early Jurassic at those paleolatitudes [START_REF] Rees | Jurassic phytogeography and climates: new data and model comparisons[END_REF]. It may also point to the intra-continental character of the Karatau Basin during the Pliensbachian-Toarcian.

Considering these paleolatitudinal/paleoclimatic settings, the location of the Karatau Basin during the Jurassic (~40° N) and the large excursions of the δ 2 H values, air temperatures and precipitation amount were probably both important factors driving precipitation δ 2 H values. Then, δ 2 H values of n-alkanes were more or less affected by evapotranspiration with respect to their environment whether aquatic or terrestrial [START_REF] Sachse | δD values of individual n-alkanes from terrestrial plants along a climatic gradient -Implications for the sedimentary biomarker record[END_REF][START_REF] Smith | Influence of physiology and climate on δD of leaf wax nalkanes from C 3 and C 4 grasses[END_REF][START_REF] Feakins | Controls on the D/H ratios of plant leaf waxes in an arid ecosystem[END_REF]. To constrain the concomitant thermophilic paleoflora and increasing temperatures recorded by GDGTs in the early Toarcian (Fig. 9).

At high frequency, higher δ 2 H values intercalated in clayey layers may represent local changes in sub-environments between lignites and clayey layers with more humid conditions for lignites than for clays (Fig. 7 and9). Whereas paleoclimatic variations may have overprinted local changes at a broader scale when taking into account differences in δ 2 H values between the different sedimentary cycles.

Paleoclimate reconstructions based on Δ 2 H ter-aq

The influence of relative humidity and temperature on δ 2 H values is also recorded by the evapotranspiration of the ecosystem [START_REF] Sachse | δD values of individual n-alkanes from terrestrial plants along a climatic gradient -Implications for the sedimentary biomarker record[END_REF][START_REF] Mügler | Effect of lake evaporation on δD values of lacustrine n-alkanes: A comparison of Nam Co (Tibetan Plateau) and Holzmaar (Germany)[END_REF]. The ecosystem evapotranspiration can be evaluated by the hydrogen isotopic difference between terrestrial nalkanes and aquatic n-alkanes, based on two assumptions: (i) in a given ecosystem, water source is the same for terrestrial and aquatic organisms and (ii) aquatic and terrestrial organisms synthesize their n-alkanes with the same biosynthetic fractionation (ε = 157 ‰; [START_REF] Sachse | δD values of individual n-alkanes from terrestrial plants along a climatic gradient -Implications for the sedimentary biomarker record[END_REF]. However, terrestrial flora synthetize their n-alkanes from 2 H-enriched leaf-water after soil evaporation and leaf-transpiration, leading to 2 H-enrichment of longchain n-alkanes (eg. [START_REF] Sachse | δD values of individual n-alkanes from terrestrial plants along a climatic gradient -Implications for the sedimentary biomarker record[END_REF]. Indeed, n-alkanes from surface sediments of European lakes showed an isotopic difference as large as +30 ‰ between the terrestrial and the aquatic pool [START_REF] Sachse | Hydrogen isotope ratios of recent lacustrine sedimentary n-alkanes record modern climate variability[END_REF][START_REF] Sachse | δD values of individual n-alkanes from terrestrial plants along a climatic gradient -Implications for the sedimentary biomarker record[END_REF].

In the Taskomirsay samples, evapotranspiration was estimated from the isotopic difference between δ 2 H values of the C 27 and the C 23 n-alkanes and was noted as Δ 2 H ter-aq (Fig. 9). Notice that the aquatic pool defined for this study differs from that of the other studies by the origin of the aquatic n-alkanes, which are attributed here to swampy or riparian vegetation with no water restriction (e.g., Xenoxylon vegetation type; [START_REF] Oh | Xenoxylon Synecology and Palaeoclimatic Implications for the Mesozoic of Eurasia[END_REF].

Contributions from mosses and aquatic plants are also possible. The Δ 2 H ter-aq was always positive in lignites, up to +76 ‰ in LB4, whereas it remained close to 0 ‰ or slightly negative in clayey layers. According to [START_REF] Mügler | Effect of lake evaporation on δD values of lacustrine n-alkanes: A comparison of Nam Co (Tibetan Plateau) and Holzmaar (Germany)[END_REF], who calculated Δ 2 H ter-aq values in a Tibetan lake, positive Δ 2 H ter-aq values are typical for humid environments. They result from evapotranspiration of terrestrial vegetation leading to 2 H-enrichment in the terrestrial pool, whereas 2 H-enrichment of the aquatic pool is limited by low evaporation rates. Δ 2 H ter-aq values in the section are thus in agreement with humid conditions. In contrast, under arid to semi-arid conditions, significant 2 H-enrichment occurs in the aquatic pool relatively to the terrestrial pool, resulting from long term lake water evaporation, which counterbalances plant evapotranspiration: a negative Δ 2 H ter-aq is thus recorded (~ 60 ‰; [START_REF] Mügler | Effect of lake evaporation on δD values of lacustrine n-alkanes: A comparison of Nam Co (Tibetan Plateau) and Holzmaar (Germany)[END_REF][START_REF] Rao | Assessment of the difference between mid-and long chain compound specific δD n-alkanes values in lacustrine sediments as a paleoclimatic indicator[END_REF]. Because the most negative Δ 2 H ter-aq recorded did not exceed 6 ‰, arid to semiarid conditions during deposition of clayey layers are unlikely and generally humid conditions are suggested, in agreement with paleoflora, δ 15 N org and absolute δ 2 H values.

In addition, maximal Δ 2 H ter-aq positive values (+41 and +76 ‰ in LB4), suggest a considerable evapotranspiration rate (Fig. 9). This interpretation is in apparent contradiction with the coolest/most humid interval deduced from low δ 2 H values of n-alkanes from the aquatic pool and low δ 15 N org values. A possible explanation for this discrepancy is that higher input of allochthonous terrestrial n-alkanes from distant areas with a heavier isotopic composition of meteoric water might have led to higher Δ 2 H ter-aq (~70 ‰; [START_REF] Sachse | Hydrogen isotope ratios of recent lacustrine sedimentary n-alkanes record modern climate variability[END_REF]. Alternatively, high Δ 2 H ter-aq could be explained by seasonal differences in water uptake and n-alkane production between the aquatic and terrestrial pools [START_REF] Jacob | Paleohydrological changes during the last deglaciation in Northern Brazil[END_REF], which is typical under temperate climate-regime. Indeed, higher plant n-alkanes are mostly produced during the growing season in spring and summer [START_REF] Sachse | Hydrogen isotope ratios of recent lacustrine sedimentary n-alkanes record modern climate variability[END_REF][START_REF] Sachse | Significant seasonal variation in the hydrogen isotopic composition of leaf-wax lipids for two deciduous tree ecosystems (Fagus sylvativa and Acer pseudoplatanus)[END_REF], i.e.

under relatively drier/warmer conditions, which could account for the production of 2 H-enriched long-chain n-alkanes. Conversely, the aquatic pool of n-alkanes might have been produced under more humid/cool conditions during a wet season, which could explain their low δ 2 H values (Fig. 9).

The hypothesis of alternating wet/dry season responsible for high Δ 2 H ter-aq could also explain the scatter in δ 15 N org values in LB4 by the dual influences of wet and dry seasons [START_REF] Swap | Natural abundance of 13 C and 15 N in C 3 and C 4 vegetation of southern Africa: patterns and implications[END_REF]): (i) low N-recycling rate during dry season, leading to N mineralization and 15 N-enrichment, and (ii) intense N-recycling by microbial biomass during the growing season that re-processes the accumulated N, leading to 15 N-depletion. In other words, if the dry season effects of 15 N-enrichment exceed the wet season effects of 15 N-depletion, δ 15 N values of the remaining organic matter would increase and vice versa. Following the interpretation of Δ 2 H ter-aq values as a seasonal indicator, Δ 2 H ter-aq values close to 0 ‰ in Phase 2 might suggest low seasonality (Fig. 9). Precisely, recent spectral analysis of δ 13 C values measured in carbonates, used in cyclostratigraphy, revealed that astronomical configuration of the Earth would have enhanced seasonality at the Pliensbachian-Toarcian transition and favored low seasonality in the early Toarcian [START_REF] Martinez | Orbital pacing of carbon fluxes by a ∼9-My eccentricity cycle during the Mesozoic[END_REF].

Reconciling proxies

Proxies used in this study yield contrasting responses to the inferred climate changes.

An absence of drastic changes were recorded by δ 15 N org values and paleoflora revealed globally humid conditions apart from slightly less humid and warmer conditions in the early Toarcian. This warming was confirmed by MAATs reconstructed from brGDGTs. In contrast, molecular proxies (n-alkane amount, ACL, δ 2 H values, Δ 2 H ter-aq values and brGDGTs) were highly variable along the section, especially δ 2 H values. These proxies argue for maximal humidity and minimal temperatures in the Pliensbachian-Toarcian transition.

This period could thus correspond to the cool late Pliensbachian [START_REF] Bailey | Paleoceanographic changes of the Late Pliensbachian-Early Toarcian interval: a possible link to the genesis of an Oceanic Anoxic Event[END_REF][START_REF] Morard | A new scenario for the Domerian -Toarcian transition[END_REF][START_REF] Suan | Secular environmental precursors to Early Toarcian (Jurassic) extreme climate changes[END_REF]. Molecular proxies also supported drier/warmer conditions at the base of the section (middle Pliensbachian?) and in the early Toarcian as well as humid/dry cycles between lignites (humid) and clayey layers (drier).

The absence of a correlation between δ 15 N org and δ 2 H values, which are both influenced by water availability, might point to different spatial integration of those proxies, δ 15 N org and δ 2 H reflecting local [START_REF] Szpak | Complexities of nitrogen isotope biogeochemistry in plant-soil systems: implications for the study of ancient agricultural and animal management practices[END_REF] and regional (precipitation regimes, air-mass temperatures; [START_REF] Sachse | Molecular Paleohydrology: Interpreting the Hydrogen-Isotopic Composition of Lipid Biomarkers from Photosynthesizing Organisms[END_REF] influences, respectively. Sea level variations may influence δ 2 H values of precipitation by modifying the distance between shore lines and the study area [START_REF] Gat | Oxygen and Hydrogen Isotopes in the Hydrologic Cycle[END_REF]. Consequently, low sea level may increase the distance from the shore line leading to lower δ 2 H values of the precipitation and vice versa, and concomitant temperature/humidity variations could lead to over-or underestimation of paleohydrological changes (e.g., [START_REF] Garel | Paleohydrological and paleoenvironmental changes recorded in terrestrial sediments of the Paleocene-Eocene boundary (Normandy, France)[END_REF]. It is therefore likely that the low sea level and the low temperatures (higher 2 H-fractionation) recorded in the late Pliensbachian led to an overestimation of humidity and reversely for the early Toarcian, which is marked by global warming and sea-level rising [START_REF] Hallam | An Environmental Study of the Upper Domerian and Lower Toarcian in Great Britain[END_REF][START_REF] Hesselbo | British Lower Jurassic Sequence Stratigraphy[END_REF][START_REF] Bailey | Paleoceanographic changes of the Late Pliensbachian-Early Toarcian interval: a possible link to the genesis of an Oceanic Anoxic Event[END_REF][START_REF] Rosales | Paleotemperature variations of Early Jurassic seawater recorded in geochemical trends of belemnites from the Basque-Cantabrian basin, northern Spain[END_REF][START_REF] Suan | Secular environmental precursors to Early Toarcian (Jurassic) extreme climate changes[END_REF]. The recognition of Botryococcus in the uppermost part of the section and the alternation between silts and clay suggest a lacustrine/deltaic environment (Schnyder et al., accepted). This kind of environment would have been favored by the early Toarcian global transgression [START_REF] Hallam | An Environmental Study of the Upper Domerian and Lower Toarcian in Great Britain[END_REF][START_REF] Hesselbo | British Lower Jurassic Sequence Stratigraphy[END_REF]) that increased the regional water level base and consequently buried the older sediments. Thus, variations of sea level may reconcile the drastic paleohydrological changes recorded by the δ 2 H values and the rather stable climatic conditions implied by the δ 15 N org values. This hypothesis is strongly supported by [START_REF] Korte | Jurassic climate mode governed by ocean gateway[END_REF] who demonstrated the influence of the ocean dynamic on the global climate in the Early and Middle Jurassic.

Conclusions

The Taskomirsay section is a unique continental record that encompasses the Pliensbachian-Toarcian transition and contains well-preserved Type-III organic matter.

Sources of organic matter were separated into two pools: (i) a purely terrestrial and (ii) an "aquatic" pool constituted of vegetation that thrived under almost permanent water supply, with possible contributions from mosses and aquatic plants.

Our multi-proxy analysis argues for globally humid condition through the section, probably under a warm-to cool-temperate regime, as constrained by Xenoxylon wood.

However, paleoflora and δ 2 H values suggest slightly less humid and warmer conditions starting from the early Toarcian, as also usually recorded in the marine realm. This interval was preceded by the probably coolest/most humid period that may correspond to the late Finally, this study sheds new light on the use of compound-specific δ 2 H values in ancient sediments and stresses the complexity of the δ 15 N org as a paleoclimatic proxy. Those proxies still need to be combined with other parameters like clay mineralogy, palynofacies or other biomarker molecules (e.g., terpenoids). Paleoclimate changes should also be constrained at the regional scale with other continental sections in Central Asia to link paleoclimate changes recorded in the marine realm to those in continental settings, even though inter-linkages could be challenging.

  strong maximum at C 35 and a sub-maximum at C 29 . In LB3, distribution is also bimodal with a maximum at C 21 or C 23 and strong sub-maximum at C 35 . The lignites from the middle part of the section in LB4 and LB5 have a unimodal distribution maximizing at C 21 or C 23 . In LB6, three different patterns are recorded: a unimodal distribution maximizing at C 25 in TP3-195, at C 21 in TP3-203 and pattern similar to those reported in LB1 and LB2 in TP3-216. Clayey samples TP1-22, TP3-157 and TP3-176 show a unimodal distribution maximizing at C 25 or C 27 , whereas the distribution is affected by an unusual amount of C 18 in most of the others, leading to a bimodal and even trimodal distribution in TP3-224 that results from a strong contribution of C 35 (Fig. 5). In particular, the two trunk/root samples (TP1-65 and TP1-114) show similar patterns clearly maximizing at C 23 . Most of the samples have a marked odd/even predominance in the range C 21 to C 35 .

δ 2 H

 2 values range between 248 and 151 ‰. Maximal variability is observed for n-C 27 with 90 ‰ between minimum ( 242 ‰) and maximum ( 152 ‰) values. The variability of δ 2 H values is also high for n-C 19 to n-C 25 (Fig.6). In addition, the longer the chain, the more 2 H-enriched are the n-alkanes, except for the n-C 35 . The latter is 2 H-depleted when compared to common vascular plant n-alkanes (C 27 to C 31 ,[START_REF] Eglinton | Leaf Epicuticular Waxes[END_REF], but is still 2 Henriched when compared to <C 27 n-alkanes.Along the section, δ 2 H values exhibit generally parallel trends (Fig.7). In detail, three main features are recorded. First, the offset between δ 2 H values of two groups of n-alkanes increases from the base of LB1 to the base of LB5: the most negative values are recorded for n-alkanes <C 25 , whereas longer chain n-alkanes show more positive values. Second, δ 2 H values of all the n-alkanes increase drastically in the intercalated organic-rich clayey layer in LB5 (TP3-157) and then remain at almost constant high values ( 174 ±11 ‰) until LB6. At this point, δ 2 H decreases to lower values ( 191 ±17 ‰) before increasing again in the overlaying clayey layer ( 169 ±10 ‰)

  authors attributed the Carboniferous coals to Paleozoic Gondwana swamp flora made of conifer precursors, whereas Stefanova et al. (2013) emphasized that Taxodium dubium was known in marshy environments and flooded lowland terrains. Finally, the distribution of nalkanes in Xenoxylon fossil woods collected in the Bajocian/Callovian of Poland and

  Pliensbachian. In addition, enhanced seasonality has been hypothesized, starting from the mid-late Pliensbachian and maximizing at the Pliensbachian-Toarcian transition, in agreement with a temperate climate regime. The combination of sedimentological and organic analyses provide the most probable picture of landscapes in Taskomirsay: fluvial systems cutting through swampy forests from the base of the section (mid-to late Pliensbachian) to the transition zone (Pliensbachian-Toarcian transition) followed by relatively open lacustrine conditions in the upper part (early Toarcian).

  Figure 1-revised

Figure

  Figure 7-revised

Fig. 1 .

 1 Fig. 1. Geological and geographical situation of the Taskomirsay section (modified from Schnyder et al. accepted).

Fig. 2 .

 2 Fig. 2. Total organic carbon (TOC), Total Nitrogen (TN), δ 15 N tot and δ 15 N org values along the Taskomirsay section with respect to lithology. Lignites are in black, clayey layers in gray, silts-sandstones in orange triangles and coalified wood trunks or roots in brown. Transition zone, Pliensbachian-Toarcian transition.

Fig. 3 .

 3 Fig. 3. Taskomirsay samples plotted in a HI (Hydrogen Index)/OI (Oxygen Index) diagram. Arrows indicate pathways of OM alteration.

Fig. 4 .

 4 Fig. 4. TOC values of clayey layers plotted against corresponding N tot and N org values.

Fig. 5 .

 5 Fig. 5. ACL values, n-alkane amount and n-alkane distribution in lipid extracts of the Taskomirsay section. Blue bars refer to the "aquatic pool" (C 23 ) and green bars to the "terrestrial pool" (C 27 ) (see text, section 4.2).

Fig. 6 .

 6 Fig. 6. Variability of δ 2 H values of n-alkanes and isoprenoid pristane (Pr) and phytane (Ph). Also represented are compiled typical values under tropical, cool temperate and glacial regime from Dawson et al. (2004) and Izart et al. (2012).

Fig. 7 .Fig. 8 .Fig. 9 .

 789 Fig. 7. δ 2 H values of n-alkanes and isoprenoid pristane and phytane along the Taskomirsay section. Error bars (based on triplicates) are smaller than the symbol if not visible, except for red points, which correspond to single analyses.

Figure Caption 2 trend

 2 Figure Caption

Fig. 10 .

 10 Fig. 10. ACL values vs. n-alkane amounts. Significant logarithmic correlation occurs in lignites (black points). See Fig. 2 for color caption.

  calculated by TN N bnd . * TOC/N org for clayey samples.

  The terrestrial signal in our samples is expressed by the absolute δ 2 H values of the nalkane C 27 ( 242 to -151 ‰; 192 ± 21 ‰), which are closer to those reported from the Paleocene-Eocene at mid-latitudes in the intracontinental BighornBasin ( 195 to 185 ‰;

  Weijers, J.W.H.,Schouten, S., van den Donker, J.C., Hopmans, E.C., Sinninghe Damsté, J.S., 2007. Environmental controls on bacterial tetraether membrane lipid distribution in soils. Geochim. Cosmochim.Acta 71, 703-713. doi:10.1016/j.gca.2006.10.003 Yang, H., Huang, Y., 2003. Preservation of lipid hydrogen isotope ratios in Miocene lacustrine sediments and plant fossils at Clarkia, northern Idaho, USA. Org. Geochem.

	34, 413-423. doi:10.1016/S0146-6380(02)00212-7

Table 1 Click here to download Table: Table 1.pdf

 1

	Xenoxylon indices	Callialasporites spp.	Ischyosporites	variegatus	Corollina spp.

Table 2 .

 2

	Sample	Lithology	Position (m)	TOC (%)	a Lipid (%) content	(max; sub-maxs) n-alkane range	(µg/gTOC) n-alkane content	(C21-C35) CPI	(C21-C35) ACL	ratio Pr/Ph	b MAAT (°C)
	TP3-234	Clay	52.02	0.2	0.01 C16-C37 (C21; C25; C35)	51	1.7	26.4	0.3	12.4
	TP3-231	Clay	49.85	0.1	<0.01	C14-C37 (C23)	268	1.1	27.2	0.3	-
	TP3-226	Clay	48.43	0.1	<0.01	C14-C35 (C18; C27)	78	1.1	26.1	0.3	-
	TP3-224	Clay	45.17	0.6	0.13 C14-C37 (C27; C35; C18)	621	3.2	27.9	0.7	11.9
	TP3-216	Lignite	43.41	59.6	1.85 C13-C37 (C35; C25; C29)	172	4.0	29.8	3.9	-
	TP3-203	Lignite	41.98	65.1	0.86	C14-C28 (C21)	11	2.0	22.4	2.0	10.5
	TP3-195	Lignite	41.12	47.9	2.08	C13-C37 (C23; C33)	154	3.2	27.1	5.7	-
	TP3-190	Clay	40.34	0.4	0.01	C13-C36 (C18; C25)	29	1.8	26.4	1.0	-
	TP3-176	Clay	36.85	1.5	0.03	C17-C35 (C27)	247	2.3	26.8	2.1	-
	TP3-169	Clay	35.34	1.1	<0.01	C14-C37 (C18; C27)	40	2.1	27.5	0.8	-
	TP3-157	OM-rich Clay	33.32	6.2	0.21	C14-C36 (C29; C27)	92	2.6	27.3	7.6	6.8
	TP3-151	Lignite	32.32	73.1	2.49	C14-C35 (C23)	13	5.5	23.7	2.7	-
	TP2-140	Lignite	27.94	69.6	1.27	C15-C31 (C21)	8	1.6	23.0	2.6	-
	TP2-127	Lignite	26.36	65.6	2.06	C14-C35 (C23)	46	2.1	24.8	6.7	-
	TP1-114	Coalified root/trunk	23.39	67.4	3.64	C14-C35 (C23; C18)	11	2.3	24.0	6.0	-
	TP1-107	Clay	21.24	0.1	<0.01	C14-C37 (C18; C27)	311	1.1	27.2	0.6	-
	TP1-99	Coalified root/trunk	18.72	75.4	2.35 C13-C37 (C21; C35; C29)	26	2.7	26.3	6.1	-
	TP1-87	Lignite	16.54	38.6	1.90 C13-C37 (C23; C27; C35)	262	3.8	27.5	3.3	-
	TP1-65	Coalified root/trunk	13.20	70.6	2.23	C13-C29 (C23)	129	1.6	23.6	1.7	11.5
	TP1-65bis	Clay	13.20	0.02	<0.01	C14-C37 (C27; C18)	3313	1.6	26.7	0.9	-
	TP1-50	Clay	10.00	3.0	0.1 C14-C37 (C23; C35; C29)	53	2.4	27.3	3.2	4.0
	TP1-38	Lignite	7.89	67.0	3.90 C11-C37 (C35; C29; C19)	218	3.5	30.4	4.5	-
	TP1-22	Clay	4.36	9.4	0.58	C14-C37 (C25)	166	2.6	26.2	2.9	15.9
	TP1-15	Lignite	1.70	62.9	2.65	C14-C37 (C35; C29; C23)	189	3.4	30.5	6.6	-
	TP1-5	Lignite	0.34	59.7	2.43	C14-C37 (C35; C29; C23)	374	4.4	31.8	5.4	-
	Average							2.5	26.7	3.1

Table 2 Click here to download Table: Table 2.pdf

 2

	Table 3.
	Sample	Lithology	C17	SD	C18	SD	C19	SD	C21	SD	C23	SD	C25	SD
				(1σ)		(1σ)		(1σ)		(1σ)		(1σ)		(1σ)
	TP3-224	Clay	-183	6	-175	2	-153	3	-163	2	-176	2	-175
	TP3-216	Lignite	-169	-	-177	30	-206	10	-191	2	-205	8	-213
	TP3-195	Lignite	-186	2	-193	1	-212	3	-204	3	-213	2	-215
	TP3-190	Clay	-	-	-177	-	-161	-	-166	12	-168	18	-172	19
	TP3-176	Clay	-	-	-	-	-	-	-171	10	-181	10	-178
	TP3-157	OM-rich clay	-158	5	-172	3	-191	3	-185	4	-188	4	-190
	TP3-151	Lignite	-190	13	-192	7	-231	3	-237	7	-248	1	-244	10
	TP2-140	Lignite	-	-	-218	1	-230	1	-234	2	-228	<1	-195
	TP2-127	Lignite	-204	4	-220	3	-234	2	-230	4	-233	1	-226
	TP1-114	Coalified root/trunk	-166	3	-191	2	-191	2	-184	2	-189	3	-188
	TP1-99	Coalified root/trunk	-206	2	-216	3	-224	1	-220	2	-215	2	-213
	TP1-87	Lignite	-207	2	-222	2	-237	1	-246	3	-245	<1	-240
	TP1-65	Coalified root/trunk	-196	2	-180	5	-210	4	-225	2	-232	1	-232	<1
	TP1-50	Clay	-	-	-172	20	-195	20	-196	6	-192	9	-188
	TP1-38	Lignite	-183	3	-206	6	-217	4	-205	11	-204	7	-202
	TP1-22	Clay	-157	6	-162	15	-183	6	-181	10	-192	5	-208
	TP1-15	Lignite	-175	13	-193	12	-213	8	-194	8	-222	3	-207
	TP1-5	Lignite	-166	18	-172	15	-201	5	-194	16	-199	5	-193
		Average	-182	6	-190	8	-205	5	-201	6	-207	5	-204

Table 3 Click here to download Table: Table 3.pdf

 3

Table 1 .

 1 Bulk organic results. Isotopic values are reported with their standard deviation (1σ) calculated on replicates analyses (2 at least). Standard deviation calculated on tyrosines (±0.3‰) was applied for samples below 40 µg N (highlighted in grey).

Table 2 . Molecular geochemistry of the lipid extracts.

 2 Relative abundances of brGDGTs used for MAAT calculations are in supplementary data.

a Percentage of lipids in dry sediments b brGDGT-derived temperatures calculated from

[START_REF] Peterse | Revised calibration of the MBT-CBT paleotemperature proxy based on branched tetraether membrane lipids in surface soils[END_REF]

Table

3

. Hydrogen isotopes (δ 2 H) of n-alkanes. δ 2 H values and their standard deviation (SD) are expressed in per mille (‰) from C 17 to C 35 and of pristane and phytane.

Table Caption Schnyder et al. text-Final revised paper Click here to download Supplementary Interactive Plot Data (CSV): Schnyder-Cent-As-Bas-Revision 2.docx

 Caption

Acknowledgments

We thank the geological department of Namur and their members for their technical assistance with treatments for nitrogen isotopes analyses and especially Gaëtan Rochez. We are also grateful to Véronique Vaury (IEES-UPMC) for nitrogen isotopes analyses, Christelle Anquetil for GC-MS analyzes, Nicolas Brossart for IRMS, Paula Iacumin and Jean Yves Storme for preliminary nitrogen analyzes. We also thank Alexandre Lethiers for help in drawing, and Arndt Schimmelmann and the anonymous reviewer for their constructive reviews. This study was supported by EMERGENCE project from UPMC, DARIUS program and by Agence Nationale de la Recherche PalHydroMil project (ANR JCJC, 2011-2013).

evapotranspiration effect of terrestrial plants, the aquatic n-C 23 will be considered for paleohydrological reconstructions.

δ 2 H values of n-alkanes are usually well correlated with that of the source water used by organisms and thus constitute good markers for paleohydrological reconstructions [START_REF] Sachse | Hydrogen isotope ratios of recent lacustrine sedimentary n-alkanes record modern climate variability[END_REF][START_REF] Mügler | Effect of lake evaporation on δD values of lacustrine n-alkanes: A comparison of Nam Co (Tibetan Plateau) and Holzmaar (Germany)[END_REF][START_REF] Aichner | δD values of n-alkanes in Tibetan lake sediments and aquatic macrophytes -A surface sediment study and application to a 16 ka record from Lake Koucha[END_REF][START_REF] Nichols | Differential hydrogen isotopic ratios of Sphagnum and vascular plant biomarkers in ombrotrophic peatlands as a quantitative proxy for precipitation-evaporation balance[END_REF][START_REF] Sachse | Molecular Paleohydrology: Interpreting the Hydrogen-Isotopic Composition of Lipid Biomarkers from Photosynthesizing Organisms[END_REF].

Because the n-C 23 is associated with the aquatic pool in the study section, its low δ 2 H values ( 248 to 168 ‰; 201 ± 25 ‰, similar to n-C 21 δ 2 H values) argue for very humid conditions either in a cool-temperate or glacial climate regime (Fig. 6). However, no sedimentological evidence has been found for a glacial environment, and this kind of environment is highly inconsistent with plant assemblages in Taskomirsay (Schnyder et al., accepted). Thus, cool-temperate conditions are most likely, whereas the very negative δ 2 H values could reflect a long distance from the source water [START_REF] Gat | Oxygen and Hydrogen Isotopes in the Hydrologic Cycle[END_REF]. Although the exact distance from the shore line is currently unknown (either to the South or West), it is most probably on the order of tens to hundreds of kilometers (B. Vrielynck, pers. Comm., 2014).

Along the section, the decrease in n-C 23 δ 2 H values from the base of the section to the transition zone could result from a decrease in source-water δ 2 H values, indicating an increase in humidity and decrease in temperature, the coolest/most humid interval being recorded from LB4 to the base of LB5 within the transition zone ( 236 ± 11 ‰; Fig. 9).

Then, a strong increase in n-C 23 δ 2 H values ( 188 ± 18 ‰) suggests relatively drier conditions in the early Toarcian, associated with higher temperatures. ACL values are in agreement with those two general trends. Nevertheless, slightly lower n-C 23 δ 2 H values in LB6 ( 209 ± 6 ‰) suggest a more humid period that favored swampy environments. But, those δ 2 H values are ~27 ‰ higher than in LB4/LB5, probably resulting from the deposition of LB6 in warmer conditions instead of drier conditions. This interpretation is supported by