Catherine Girard
email: catherine.girard@umontpellier.fr

Jean-Jacques Cornée
email: jean-jacques.cornee@gm.univ-montp2.fr

Anne- Lise Charruault
email: anne-lise.charruault@umontpellier.fr

Anne Dufour

-Béatrice

Paleogeographic differences in temperature, water depth and conodont biofacies during the Late Devonian

Keywords: Famennian, Montagne Noire, Saxo-Thuringia, Palmatolepis, oxygen isotopes, sedimentary facies

The Famennian (Late Devonian) started after and ended with two of the seven largest crises of the Phanerozoic, the Kellwasser and Hangenberg events, respectively. In between, global environmental trends have been identified, involving cooling and eustatic regression. Tropical and subtropical marine faunas were largely cosmopolitan.

Overall, this suggests that the Famennian was marked by long-term environmental changes occurring in a relatively homogeneous manner despite being punctuated by several short-term events of limited faunal impact. How these trends were modulated according to geographic location is investigated in this study by comparing two continuous outcrops in the Montagne Noire (Col des Tribes section, France) and in Saxo-Thuringia (Buschteich section, Germany). Both were located in the subtropical area during this period and belonged to two former microcontinents bracketed between the closing Rheic Ocean and Gondwana. Sedimentary facies, oxygen isotopes, and the generic conodont composition (biofacies) were studied in a high resolution and in the same rock samples. Sedimentary facies provided an estimate of water depth and oxygen

Introduction

The Famennian is well known for being bracketed by two of the most important biological crises in the Earth's history: the Upper Kellwasser Event just before the Frasnian-Famennian boundary (e.g. [START_REF] Mcghee | The Late Devonian Mass Extinction -The Frasnian/Famennian crisis[END_REF][START_REF] Schindler | Event-stratigraphic markers within the Kellwasser Crisis near the Frasnian/Famennian boundary (Upper Devonian) in Germany[END_REF][START_REF] Walliser | Global events and event stratigraphy[END_REF] and the Hangenberg Crisis punctuating the Devonian-Carboniferous boundary (e.g. [START_REF] Becker | Review of chrono-, litho-and biostratigraphy across the global Hangenberg Crisis and Devonian-Carboniferous Boundary[END_REF][START_REF] Kaiser | Environmental change during the Late Famennian and Early Tournaisian (Late Devonian-Early Carboniferous): implications from stable isotopes and conodont biofacies in southern Europe[END_REF][START_REF] Kaiser | The global Hangenberg Crisis (Devonian-Carboniferous transition): a review of a first order mass extinction[END_REF][START_REF] Walliser | Global events and event stratigraphy[END_REF]. Many studies were devoted to these crises, allowing a fine-scale characterization of the biotic and abiotic variations during the hundred thousand years involved in these worldwide perturbations [START_REF] Becker | The Devonian Period[END_REF][START_REF] Corradini | Late Devonian (Famennian) conodonts from the Corona Mizziu Sections near Villasalto (Sardinia, Italy)[END_REF][START_REF] Girard | Eustatic trends in conodont diversity across the Frasnian/Famennian boundary in the stratotype area, Montagne Noire, France[END_REF][START_REF] Joachimski | Devonian climate and reef evolution: Insights from oxygen isotopes in apatite[END_REF][START_REF] Mcghee | The Late Devonian Mass Extinction -The Frasnian/Famennian crisis[END_REF][START_REF] Sandberg | Late Frasnian mass extinction: conodont event stratigraphy, global changes, and possible causes[END_REF][START_REF] Walliser | Global events and event stratigraphy[END_REF]. Environmental changes also occurred during the time interval between the crises (ca. 13 Myr, [START_REF] Becker | The Devonian Period[END_REF], but deserved much less attention. Short-term events have nevertheless been described, with minor impact on the faunas (e.g. Annulata Events by [START_REF] House | Correlation of mid-Palaeozoic ammonoid evolutionary events with global sedimentary perturbations[END_REF][START_REF] Korn | The mid-Famennian ammonoid succession in the Rhenish Mountains: the "annulata Event" reconsidered[END_REF][START_REF] Hartenfels | Die globalen Annulata-Events und die Dasberg-Krise (Famennium, Oberdevon) in Europa und Nord-Afrika -hochauflösende Conodonten-Stratigraphie, Karbonat-Mikrofazies, Paläoökologie und Paläodiversität[END_REF][START_REF] Hartenfels | The global Annulata Events: review and new data from the Rheris Basin (northern Tafilalt) of SE Morocco[END_REF]; Dasberg Crisis by [START_REF] Becker | Anoxia, eustatic changes, and upper Devonian to lowermost Carboniferous global ammonoid diversity[END_REF][START_REF] Hartenfels | Timing of the global Dasberg Crisis -implications for Famennian eustasy and chronostratigraphy[END_REF][START_REF] Hartenfels | Die globalen Annulata-Events und die Dasberg-Krise (Famennium, Oberdevon) in Europa und Nord-Afrika -hochauflösende Conodonten-Stratigraphie, Karbonat-Mikrofazies, Paläoökologie und Paläodiversität[END_REF]Condroz Events by Becker, 1993;[START_REF] Becker | Sea-level changes in the Upper Devonian of the Canning Basin, Western Australia[END_REF]. A regressive trend punctuated by minor transgressive-regressive couplets has been evidenced for the Famennian [START_REF] Johnson | Devonian eustatic fluctuations in Euramerica[END_REF][START_REF] Johnson | Devonian transgressive-regressive cycles and biostratigraphy, Northern Antelope Range, Nevada: establishment of reference horizons for global cycles[END_REF]. Global cooling inferred from oxygen isotope data has been suggested for the same interval [START_REF] Joachimski | Oxygen isotope evolution of biogenic calcite and apatite during the Middle and Late Devonian[END_REF][START_REF] Joachimski | Devonian climate and reef evolution: Insights from oxygen isotopes in apatite[END_REF], an interpretation supported by independent evidence of Latest Famennian glacial deposits described from South America [START_REF] Caputo | Late Devonian and Early Carboniferous glacial records of South America[END_REF][START_REF] Isaacson | Late Devonian-earliest Mississippian glaciation in Gondwanaland and its biogeographic consequences[END_REF]. The congruence of climate cooling and a regressive trend is therefore coherent in this eustatic context. Tropical and subtropical marine faunas were cosmopolitan at the generic level [START_REF] Feist | Discovery of Famennian trilobites in Australia (Late Devonian, Canning Basin, NW Australia)[END_REF][START_REF] Korn | Biogeography of Paleozoic ammonoids[END_REF][START_REF] Mottequin | Biostratigraphic significance of brachiopods near the Devonian-Carboniferous boundary[END_REF][START_REF] Spalletta | Famennian (Upper Devonian) conodont zonation: revised global standard[END_REF] suggesting rather homogeneous environmental conditions and efficient exchanges between the faunas. Conodonts, small, nektonic animals of chordate affinities, were an abundant part of this marine fauna. Their taxonomic identification provides a detailed stratigraphic framework for this period [START_REF] Hartenfels | Die globalen Annulata-Events und die Dasberg-Krise (Famennium, Oberdevon) in Europa und Nord-Afrika -hochauflösende Conodonten-Stratigraphie, Karbonat-Mikrofazies, Paläoökologie und Paläodiversität[END_REF][START_REF] Spalletta | Famennian (Upper Devonian) conodont zonation: revised global standard[END_REF][START_REF] Ziegler | Taxionomy und Phylogenie oberdevonischer Conodonten und ihre stratigraphische Bedeutung[END_REF][START_REF] Ziegler | Palmatolepis-based revision of upper part of standard Late Devonian conodont zonation[END_REF][START_REF] Ziegler | The late Devonian standard conodont zonation[END_REF]. Conodonts can also give paleoenvironmental indications because conodont genera distribution was influenced by several factors [START_REF] Lüddecke | Conodont biofacies of a monotonous middle Famennian pelagic carbonate succession (Upper Ballberg Quarry, northern Rhenish Massif)[END_REF], notably water depth [START_REF] Sandberg | Conodont biofacies of late Devonian Polygnathus styriacus Zone in western United States[END_REF][START_REF] Seddon | An ecologic model for conodonts[END_REF]. The congruence of temporal variations in the conodont biofacies (i.e. assemblage composition of the different genera) in different sections supported that the Famennian was marked by long-term environmental changes that were not durably perturbed by the short-term events punctuating it [START_REF] Corradini | Late Devonian (Famennian) conodonts from the Corona Mizziu Sections near Villasalto (Sardinia, Italy)[END_REF][START_REF] Girard | Palaeoenvironmental changes at Col des Tribes (Montagne Noire, France), a reference section for the Famennian of north Gondwana-related areas[END_REF][START_REF] Core | R: A language and environment for statistical computing[END_REF]. This view, however, can also be the consequence of the paucity of detailed records in different paleogeographic areas. Famennian paleogeography was characterized by the closure of the Rheic Ocean bracketing microcontinents derived from Gondwana (e.g. [START_REF] Franke | The Palaeozoic Variscan oceans revisited[END_REF][START_REF] Stampfli | The formation of Pangea[END_REF] (Fig. 1). Discrepancies in relative sea-level changes have been reported from these microcontinents (e.g. in the Saxo-Thuringian area) and attributed to tectonic movements overprinting the global eustatic trend [START_REF] Girard | Conodont biostratigraphy and palaeoenvironmental trends during the Famennian (Late Devonian) in the Thuringian Buschteich section (Germany)[END_REF]. This raises the question whether the environmental trends evidenced so far are of global significance, and/or whether local/regional factors related to tectonics and paleogeographic changes interfered with these (presumed) global trends.

To address these questions, we studied two recently described sections having a detailed record from the Late Frasnian to the end-Famennian [START_REF] Girard | Palaeoenvironmental changes at Col des Tribes (Montagne Noire, France), a reference section for the Famennian of north Gondwana-related areas[END_REF][START_REF] Girard | Conodont biostratigraphy and palaeoenvironmental trends during the Famennian (Late Devonian) in the Thuringian Buschteich section (Germany)[END_REF].

The Col des Tribes (CT) section in the Montagne Noire was part of the Armorica microcontinent, whereas the Buschteich section (BU) was located on the Saxo-Thuringian microplate. Hence, both sections were located on former microcontinents bracketed between the closing Rheic Ocean and Gondwana (Fig. 1). Water depth and approximate position relative to the shore were assessed based on the macroscopic description of the sediments as well as microfacies analysis (e. g. [START_REF] Flügel | Microfacies of carbonate rocks. Analysis, Interpretation and Application[END_REF]. Despite limitations due to the scarcity of preserved bioclasts in the sediments (e.g. [START_REF] Brett | Paleontological Approaches to the Environmental Interpretation of Marine Mudrocks[END_REF], this approach provided a paleoenvironmental estimate directly based on the depositional environment. Water depths derived from sedimentological analysis were compared with conodont biofacies, which have been proposed to react more sensitive to water-depth changes than microfacies [START_REF] Lüddecke | Conodont biofacies of a monotonous middle Famennian pelagic carbonate succession (Upper Ballberg Quarry, northern Rhenish Massif)[END_REF]. Nevertheless, as for any biotic record, evolutionary changes through time (changes in water depth preferences, decrease or increase in response to biotic factors such as predation or competition) may interfere with the paleoenvironmental signature [START_REF] Belka | Conodont biofacies patterns in the Kellwasser Facies (upper Frasnian/lower Famennian) of the eastern Anti-Atlas, Morocco[END_REF]. A multivariate approach was developed to quantitatively compare the conodont assemblages in the studied sections. In addition, paleotemperatures were reconstructed using oxygen isotopes measured on conodont apatite.

A strength of this study is that the three proxies are derived from the same rock samples taken at a comparable temporal resolution, allowing for detailed and quantitative comparisons. The following questions are addressed: (1) Did environmental changes affect similarly and at the same pace both areas, and if not, how to interpret any discrepancies? (2) Did water depth and temperature covary, as expected for eustatic trends during a global cooling culminating with the Devonian-Carboniferous boundary glaciation [START_REF] Buggisch | Mississippian δ 13 Ccarb and conodont apatite δ 18 O records -Their relation to the Late Palaeozoic Glaciation[END_REF][START_REF] Caputo | Late Devonian and Early Carboniferous glacial records of South America[END_REF][START_REF] Isaacson | Late Devonian-earliest Mississippian glaciation in Gondwanaland and its biogeographic consequences[END_REF]? (3) Was the forcing of water depth changes recorded as expected on conodont biofacies? In that context, did conodont biofacies rather react to local deviations from, or to globally recorded paleoenvironmental trends?

Geological setting

During the Late Devonian, the Col des Tribes (CT; Montagne Noire/France; [START_REF] Girard | Palaeoenvironmental changes at Col des Tribes (Montagne Noire, France), a reference section for the Famennian of north Gondwana-related areas[END_REF] and Buschteich (BU; Thuringia/Germany; [START_REF] Girard | Conodont biostratigraphy and palaeoenvironmental trends during the Famennian (Late Devonian) in the Thuringian Buschteich section (Germany)[END_REF] sections were located in subtropical latitudes on different microcontinents (Fig. 1). These microcontinents progressively migrated towards the equator and Laurussia during this period due to the closure of the Rheic Ocean [START_REF] Franke | The Palaeozoic Variscan oceans revisited[END_REF]. The absence of provincialism in benthic faunas compared to Laurussian faunas suggests that the Rheic Ocean was, at that time, already reduced [START_REF] Feist | Discovery of Famennian trilobites in Australia (Late Devonian, Canning Basin, NW Australia)[END_REF][START_REF] Mottequin | Biostratigraphic significance of brachiopods near the Devonian-Carboniferous boundary[END_REF]. It has been proposed that the Saxo-Thuringia microcontinent, on which BU was located, was situated in the middle of the sea between the remnant Rheic Ocean and the Paleotethys [START_REF] Franke | The Palaeozoic Variscan oceans revisited[END_REF] during the Late Frasnian (375 Ma). By the latest Famennian (~360 Ma), this seaway was almost or completely closed, locating BU at the margin of the remnant Rheic Ocean (Fig. 1). The Montagne Noire, including CT, was located on the Armorica microcontinent which was presumably positioned further towards the Paleotethys Ocean. Nevertheless, the precise location of these microcontinents relative to Laurussia as well as their position relative to each other is debated (e.g. [START_REF] Eckelmann | Plate interactions of Laurussia and Gondwana during the formation of Pangaea -Constraints from U-Pb LA-SF-ICP-MS detrital zircon ages of Devonian and Early Carboniferous siliciclastics of the Rhenohercynian zone, Central European Variscides[END_REF][START_REF] Franke | The Palaeozoic Variscan oceans revisited[END_REF].

Both sections expose Late Frasnian to Late Famennian deposits, although with a gap during the earliest Famennian in BU. Their stratigraphical context has been formerly described [START_REF] Girard | Palaeoenvironmental changes at Col des Tribes (Montagne Noire, France), a reference section for the Famennian of north Gondwana-related areas[END_REF][START_REF] Girard | Conodont biostratigraphy and palaeoenvironmental trends during the Famennian (Late Devonian) in the Thuringian Buschteich section (Germany)[END_REF] using the zonation of [START_REF] Ziegler | The late Devonian standard conodont zonation[END_REF]; and has been updated applying the zonation of [START_REF] Spalletta | Famennian (Upper Devonian) conodont zonation: revised global standard[END_REF] in this study (Fig. 2). The CT section [START_REF] Girard | Palaeoenvironmental changes at Col des Tribes (Montagne Noire, France), a reference section for the Famennian of north Gondwana-related areas[END_REF] rhenana to the Scaphignathus velifer velifer Zone, followed by nodular limestones of the Pa. rugosa trachytera to the middle of the Bispathodus ultimus Zone and the Hangenberg shales. Depositional environments ranged from distal slope to outer ramp [START_REF] Girard | Conodont biostratigraphy and palaeoenvironmental trends during the Famennian (Late Devonian) in the Thuringian Buschteich section (Germany)[END_REF].

Materials

Sixty-four and forty samples were collected from the CT and BU sections, respectively (Supplementary Table ). Thin sections were used for microfacies analysis.

Conodonts were extracted by dissolving a fraction of the samples using 10% formic acid.

All conodonts were manually picked from the insoluble residues and determined at the generic and species level. The taxonomic identification allowed establishing a detailed biostratigraphic scheme. The abundance of conodonts is high to exceptionally high, with about 600 platform elements per kg at CT and about 7000 platform elements per kg at BU. Entire and broken platform elements representing at least half of an element were counted per genus to characterize conodont biofacies in each sample [START_REF] Sandberg | Conodont biofacies of late Devonian Polygnathus styriacus Zone in western United States[END_REF].

A fraction of these conodonts was thereafter used for oxygen isotope analyses. All approaches (microfacies, conodont biofacies, and oxygen isotope geochemistry) were performed on the same rock sample.

Methods

Age model

The stratigraphy of Late Devonian deposits is mostly based on conodont biostratigraphy. A revised global standard of the Famennian (Late Devonian) conodont zonation has been recently published [START_REF] Spalletta | Famennian (Upper Devonian) conodont zonation: revised global standard[END_REF]. Zonal boundaries and duration are essentially identical to those of [START_REF] Ziegler | The late Devonian standard conodont zonation[END_REF], which was used in the Global Time Scale 2012 [START_REF] Becker | The Devonian Period[END_REF] to date in absolute ages some zonal boundaries. In this study, the zonal boundaries dated in the Global Time Scale 2012 were used as anchor points to establish local linear regressions between absolute age and depth along the sections, thus providing age estimates for each sampled level.

Anchor points are the basis of the following zones: Palmatolepis triangularis, Pa. crepida, Pa. rhomboidea, Pa. marginifera marginifera, Pa. rugosa trachytera, Polygnathus styriacus, Palmatolepis gracilis expansa (adapted to the zonation of [START_REF] Spalletta | Famennian (Upper Devonian) conodont zonation: revised global standard[END_REF], and the base of the Early praesulcata sensu [START_REF] Ziegler | The late Devonian standard conodont zonation[END_REF].

Sedimentary facies description

The description of the sedimentary facies in the Col des Tribes and Buschteich sections has been published by [START_REF] Girard | Palaeoenvironmental changes at Col des Tribes (Montagne Noire, France), a reference section for the Famennian of north Gondwana-related areas[END_REF] and [START_REF] Girard | Conodont biostratigraphy and palaeoenvironmental trends during the Famennian (Late Devonian) in the Thuringian Buschteich section (Germany)[END_REF], respectively.

The different facies were characterized by the sedimentological and biological features from outcrops as well as thin sections, allowing an interpretation of the different depositional settings. For each section, this led to an ordering ('score') of the different samples along a theoretical proximal-distal gradient. In order to compare BU and CT, the scoring of the sedimentary facies (later on in the text: 'facies') in the two outcrops have been compiled into a single scheme.

Oxygen isotope analyses

Oxygen isotope ratios of conodont apatite can be used as a paleotemperature proxy [START_REF] Joachimski | Conodont apatite δ 18 O signatures indicate climatic cooling as a trigger of the Late Devonian mass extinction[END_REF][START_REF] Joachimski | Devonian climate and reef evolution: Insights from oxygen isotopes in apatite[END_REF][START_REF] Luz | Oxygen isotope variations in phosphate of biogenic apatites, III. Conodonts[END_REF]. Twentyseven conodont samples from the Buschteich section and 26 conodont samples from the Col des Tribes section have been studied for oxygen isotopes. Chemical conversion of apatite bound phosphate into trisilverphosphate (Ag 3 PO 4 ) was performed according the method described in [START_REF] Joachimski | Devonian climate and reef evolution: Insights from oxygen isotopes in apatite[END_REF]. Oxygen isotope analyses were performed using a TC-EA (high-temperature conversion-elemental analyzer) coupled online to a ThermoFinnigan Delta V Plus mass spectrometer. 0.2 to 0.3 mg Ag 3 PO 4 was weighed into silver foil and transferred to the sample carousel of the TC-EA. At 1450 °C, the silver phosphate is reduced and CO forms as the analyte gas [START_REF] Vennemann | Oxygen isotope analysis of phosphates: a comparison of techniques from analysis for Ag3PO4[END_REF]. CO was transferred in a helium stream through a gas chromatograph via a Conflo III interface to the mass spectrometer. All values are reported in ‰ relative to V-SMOW. Samples as well as standards were generally measured in triplicate. The measurements were calibrated by performing a two-point calibration [START_REF] Paul | Normalization of measured stable isotope composition to isotope reference scale -a review[END_REF] using NBS 120c (δ 18 O = 21.7‰) and a commercial Ag 3 PO 4 (9.9‰). A laboratory standard was used as a control standard and processed together with the samples. All standards were calibrated to TU1 (δ 18 O = +21.11‰) and TU2 (+5.45‰; [START_REF] Vennemann | Oxygen isotope analysis of phosphates: a comparison of techniques from analysis for Ag3PO4[END_REF]. The average oxygen isotope composition of the internationally distributed standard NBS 120c was measured as +21.67 ± 0.09 (1 σ; n= 5).

Conodont biofacies

Late Devonian conodont taxa were supposed to have different water depth preferences [START_REF] Klapper | Conodont ecology: pelagic versus benthic[END_REF][START_REF] Seddon | An ecologic model for conodonts[END_REF]. Hence, the proportion of the different genera within a given assemblage was interpreted as a proxy of water depth [START_REF] Sandberg | Conodont biofacies of late Devonian Polygnathus styriacus Zone in western United States[END_REF][START_REF] Seddon | An ecologic model for conodonts[END_REF]. Refined biofacies interpretations based on conodont subgenera or even species have been proposed (e.g. [START_REF] Lüddecke | Conodont biofacies of a monotonous middle Famennian pelagic carbonate succession (Upper Ballberg Quarry, northern Rhenish Massif)[END_REF] but due to species evolution and replacement through time, this approach appeared difficult to apply to the relatively long record considered in the present study. During the late Frasnian to late Famennian, at least seven genera are present and can provide information about depositional environments (e.g. [START_REF] Corradini | Late Devonian (Famennian) conodonts from the Corona Mizziu Sections near Villasalto (Sardinia, Italy)[END_REF][START_REF] Morrow | Shelf-to-basin lithofacies and conodont paleoecology across Frasnian-Famennian (F-F, mid-Late Devonian) boundary, Central Great Basin[END_REF][START_REF] Sandberg | Conodont biofacies of late Devonian Polygnathus styriacus Zone in western United States[END_REF][START_REF] Seddon | An ecologic model for conodonts[END_REF]. The genera that have been most clearly associated to water depth preferences are the following. ( 1) For deep waters, Palmatolepis and Bispathodus/Branmehla/Mehlina. It should be noted that whereas Palmatolepis is considered to be a specialist of offshore environments, Bispathodus presents wider tolerance to water depth and/or proximity to the shore [START_REF] Ziegler | Palmatolepis-based revision of upper part of standard Late Devonian conodont zonation[END_REF]. ( 2) Polygnathus for intermediate water depth. (3) Icriodus, Scaphignathus, and Alternognathus for shallow waters. Due to their low abundance and similar environmental preferences, the two latter genera have been grouped for biofacies representation. Hence, in this study, the following biofacies were recognized: the palmatolepid biofacies, the palmatolepid-polygnathid biofacies, and the bispathodidpolygnathid biofacies.

Data analysis and statistics

The proportion of each genus was estimated in each sampled level based on counting [START_REF] Girard | Palaeoenvironmental changes at Col des Tribes (Montagne Noire, France), a reference section for the Famennian of north Gondwana-related areas[END_REF][START_REF] Girard | Conodont biostratigraphy and palaeoenvironmental trends during the Famennian (Late Devonian) in the Thuringian Buschteich section (Germany)[END_REF]. The description of these proportions corresponds to the conodont biofacies analysis itself. A principal component analysis (PCA on the variance-covariance matrix) was performed on these percentages to summarize the variation of all genera on few main axes. This analysis also provided a representation of the records of CT and BU on the same synthetic axes, allowing a direct comparison of the biofacies variations in the two outcrops. The contribution of each genus percentage to the axes allowed their interpretation in relation to the biofacies. The PCA was performed using the R package ade4 [START_REF] Dray | The ade4 package: implementing the duality diagram for ecologists[END_REF]. The different proxies (microfacies, oxygen isotope ratios and the first axis of the PCA on the conodont percentages) were compared using linear regression analysis in CT and BU separately, and in the combined dataset (CT+BU). To test for a secular climate evolution, the relationship between oxygen isotope values and time was tested using linear regressions in CT, BU and CT+BU. In addition, this relationship was also investigated using a linear model including as terms the age of the levels, the section and the interaction between the two former terms. If the interaction between two terms, here age and section, is significant, this means that the relationship of the dependent variable (here, δ 18 O values) to age have a different slope in the two sections. To test if δ 18 O values were different in two phases of the record, t-tests were also used. All these tests were performed under R (R [START_REF] Core | R: A language and environment for statistical computing[END_REF]). An analysis of variance (ANOVA) on a linear model combining microfacies and oxygen isotopes as explanatory variables was used to assess their relative influence on conodont assemblages, estimated by the scores along the first axis of the PCA on conodont percentages. A type II ANOVA was used, because this method is invariant to ordering of the model terms and is powerful even with unbalanced datasets. This test was done using the R package ffmanova [START_REF] Langsrud | ffmanova: Fifty-fifty MANOVA[END_REF].

Results

Age model

The absolute ages estimated in [START_REF] Becker | The Devonian Period[END_REF] for some zonal boundaries served us as anchor points. A correlation with sediment thickness allowed establishing an age model for each section (Fig. 2). The observed sediment thickness includes postdepositional processes such as compaction, erosion, and diagenesis, that may affect the estimates of accumulation rates through time and between sections. With this limitation, Famennian mean accumulation rates were low in CT (5 to 7 mm/kyr -1 ) and extremely low in BU (1 to 3 mm/kyr -1 ). Two periods of more or less constant accumulation rates can be identified in BU: one from the Pa. crepida Zone to the Sc. velifer velifer Zone, and a second, with even lower rates, from the Palmatolepis rugosa trachytera Zone to the end of the Famennian. In CT, these two intervals can also be recognized, with possibly a temporary acceleration of deposition during the Pa. marginifera marginifera and marginifera utahensis Zones. The ages estimated for each sample were thereafter used to plot the variations of the conodont assemblages and of the environmental proxies.

This allowed a comparison of the timing of changes in both sections.

Sedimentary facies

The sedimentary facies of BU and CT have been compiled into a single scheme.

Five facies types have been distinguished based on microfacies and faunal content (Fig. 3). The interpolated scheme is summarized below and compared to the published facies at CT [START_REF] Girard | Palaeoenvironmental changes at Col des Tribes (Montagne Noire, France), a reference section for the Famennian of north Gondwana-related areas[END_REF] and BU [START_REF] Girard | Conodont biostratigraphy and palaeoenvironmental trends during the Famennian (Late Devonian) in the Thuringian Buschteich section (Germany)[END_REF]. Details are provided in the Supp. Table 1. Based on these facies characteristics, the temporal variation in the depositional setting can be reconstructed. Note that all depositional environments are relatively deep, and correspond to the deepest environments found on the shelf, and that the terms 'shallow' and 'deep' are used in the context of relative shallowing or deepening. At BU only minor variations can be observed (Fig. 4A). The depositional environment, mostly corresponding to Facies 2, remained relatively deep throughout the record.

Intervals with the even deeper Facies 1 can be identified during the Late Frasnian, the Pa. crepida and termini Zones, the Pa. marginifera marginifera Zone, the Sc. velifer velifer Zone, and the top of Bispathodus ultimus Zone. The CT section shows evidence for more important changes in water depth. A first shallowing episode occurred during the latest Frasnian and earliest Famennian, followed by a second, shallower episode during the Pa. gracilis gracilis and the Pa. marginifera marginifera Zones. From the top of the Pseudopolygnathus granulosus Zone to the end of the Famennian, conditions were constantly shallower than during the two former episodes. These shallowing trends are separated by two deepening episodes, from the Pa. minuta minuta to the Pa. rhomboidea Zone and from the Pa. marginifera marginifera to the Pseudopolygnathus granulosus Zone.

Oxygen isotopes

Oxygen isotope ratios measured on conodont apatite from CT are between +17.1 and +17.7‰ from the end Frasnian to the Pa. crepida zone (Fig. 4B). Values thereafter more or less gradually increase to values around 18‰ in the Pa. marginifera utahensis Zone. From the Sc. velifer velifer to the Pa. gracilis expansa, δ 18 O increases to values above +19‰. A slight decrease is observed to the end of the Famennian. Oxygen isotope values of conodont apatite from BU are generally higher than values observed at CT. The offset in δ 18 O between the two sections is relatively small (+0.5 to +1‰) from the Pa. crepida Zone to Pa. marginifera utahensis Zone, but increases after the Sc. velifer velifer Zone (+1 to +1.5 ‰). This difference in δ 18 O translates into +6 to +6.5°C cooler temperatures in CT in case the offset is solely explained by temperature.

Conodont biofacies

The conodont biofacies follow overall comparable trends in both sections (Fig. 5; Supp. Table 1), sharing a dominance of Palmatolepis for most of the time up to a maximum at the base of the Pa. marginifera marginifera Zone (up to more than 95% of the assemblage), followed by a progressive decrease towards the end Famennian. This

Multivariate analysis of the conodont taxa abundances

The principal component analysis performed on the proportions of the different genera allowed summarizing the biofacies variations on one main axis (PC1) expressing more than 75% of the total variance. The record of the two sections CT and BU can be directly compared on these axes. The contribution of the variables (here, the percentages of the different genera) to the axes (Fig. 6A In CT, a shift towards negative PC1 values is launched between the Pa. marginifera marginifera and Pa. marginifera utahensis Zones (Fig. 4C,6E), corresponding to the onset of the decrease in the abundance of Palmatolepis (Fig. 5).

This shift went on to reach extreme negative PC1 values as early as the Po. styriacus Zone (Fig. 6G,H). This change started more or less at the same time at BU, but it did not occur in a consistent manner for a while. Only towards the end of the studied interval, during the Bispathodus ultimus Zone, PC1 values in BU reach a range similar to the one observed in CT (Fig. 6I). This corresponded to the final decrease of Palmatolepis at BU, close before its extinction near the Devonian-Carboniferous boundary (Fig. 5). The consequence of this delay in the decrease of Palmatolepis at BU was a temporary offset between the two records of conodont genera proportions, starting during the Ps.

granulosus Zone and persisting until the base of the Bi. ultimus Zone (Fig. 4C).

Comparison of paleoenvironmental proxies and conodont biofacies

As a first step, we compared the different proxies (Table 2). Facies, oxygen isotope ratios, and the first axis of the principal component analysis on the conodont percentages (later on mentioned as conodont PC1) are all significantly correlated when considering the total dataset (CT+BU). In contrast, none of the correlations is significant when considering BU. At CT, conodont PC1 is significantly correlated to facies and δ 18 O, but the two latter variables are not correlated.

The variation in the oxygen isotope ratios shows a significant secular trend, the relationship of δ 18 O and age being significant in the CT, BU and CT+BU datasets.

In the linear model investigating the variation of δ 18 O depending on age in the two sections, the interaction term between δ 18 O and age is not significant. This means that the slope of the regression is the same in the two sections. While the two sections display a similar increase in δ 18 O when considering the whole record, from the end Frasnian to the Late Famennian, the records of CT and BU do not follow a similar pattern after the Pa. marginifera utahensis Zone (Fig. 4B). The oxygen isotope ratios increase in CT but remain stable in BU. To test for such a discrepancy between the records of the two sections, we performed t-tests between oxygen isotope values of the period preceding and following the apparent shift in CT. The difference in δ 18 O is indeed statistically significant in CT (t-test [from Pa. rhomboidea to Sc. velifer velifer, n =8] vs.

[from Pa. rugosa trachytera to Bi. ultimus, n = 7], P = 0.0026). In contrast, the δ 18 O values of the same periods do not differ in BU (t-test [n = 6 and 11], P = 0.3443).

Finally, to investigate whether δ 18 O or facies better explains the variation of the conodont genera, linear models with conodont PC1 as dependent variable, and δ 18 O and facies as independent factors were considered (Table 3). The interaction term between both factors (δ 18 O and facies) is weak (p = 0.0311 for CT+BU) or not significant (for CT and BU separately). This term was therefore discarded from the models. Facies and δ 18 O are significant explanatory variables of conodont biofacies when considering CT and BU together, and CT alone. Facies explains more than three times more variance than δ 18 O in both cases. None has significant effect in BU.

Discussion

Our detailed oxygen isotope records generally show an increase in δ 18 O in both sections from the end Frasnian to end Famennian, coherent with described global cooling during this time interval [START_REF] Joachimski | Devonian climate and reef evolution: Insights from oxygen isotopes in apatite[END_REF]. However, in details, the two records show different temperature variations from the Pa. rugosa trachytera Zone to end Famennian. While temperature remained more or less stable throughout the Famennian in the BU section (Saxo-Thuringia), it decreased in the CT section (Montagne Noire) towards Sc. velifer velifer Zone. This difference in δ 18 O between CT and BU is mirrored by different sea-level variations in CT and BU, as pointed out by facies description. Given these differences between the paleoenvironmental settings of the two areas, conodont biofacies delivered remarkably similar records, questioning the role of the paleoenvironmental forcing on conodont assemblages.

Paleotemperature variations: a global trend mitigated by paleogeography?

A global temperature trend has been sketched for the Late Devonian [START_REF] Mcghee | When the invasion on land failed: the legacy of the Devonian extinctions[END_REF], based on variations in δ 18 O of conodont apatite [START_REF] Joachimski | Devonian climate and reef evolution: Insights from oxygen isotopes in apatite[END_REF]. This composite record showed a rapid succession of cooling and warming pulses around the Frasnian-Famennian boundary [START_REF] Balter | Record of climate-driven morphological changes in 376 Ma Devonian fossils[END_REF][START_REF] Joachimski | Conodont apatite δ 18 O signatures indicate climatic cooling as a trigger of the Late Devonian mass extinction[END_REF][START_REF] Huang | Did climate changes trigger the Late Devonian Kellwasser crisis? Evidence from a high-resolution δ 18 OPO4 record from South China[END_REF]. After a warm period during the triangularis Zone, a progressive longterm cooling was proposed for the rest of the Famennian [START_REF] Joachimski | Devonian climate and reef evolution: Insights from oxygen isotopes in apatite[END_REF], culminating into a period with stable ice caps during the early Carboniferous [START_REF] Buggisch | Mississippian δ 13 Ccarb and conodont apatite δ 18 O records -Their relation to the Late Palaeozoic Glaciation[END_REF]. As for some other oxygen isotope studies in the Paleozoic, our isotope records are based on conodont apatite, thus tracing the environment where the conodont lived. Furthermore, all analyses were performed on the same genus, Palmatolepis, avoiding the problem that different oxygen isotope values in the two sections may be due to analyses on genera with different habitat preferences. Our results support a long-term cooling, since the two sections displayed a parallel increase in the oxygen isotope ratio from the end Frasnian to end Famennian, although more pronounced in CT than in BU. An offset, however, exists between the records of the two sections, with BU constantly showing lower δ 18 O values than CT. Based on the paleogeographic reconstruction (Fig. 1), BU and CT were both in the subtropical zone. Beyond the cooling trend registered in both sections from the Pa. crepida to Pa. marg. marginifera Zone, the oxygen isotope records from BU and CT show differing trends after the Pa. marginifera utahensis Zone. This pattern is also seen in other sections studied by [START_REF] Joachimski | Devonian climate and reef evolution: Insights from oxygen isotopes in apatite[END_REF]. For example, in the La Serre section (Montagne Noire) an increase in δ 18 O is reported between the marginifera marginifera and Sc. velifer velifer Zones, similar and synchronous to the one documented in CT. In contrast in the Kahlleite section (Saxo-Thuringian area) δ 18 O is relatively stable as in BU (Fig. 4). The difference in the δ 18 O records of the BU and CT sections after the Pa. marginifera utahensis Zone is thus confirmed at the regional scale, suggesting that the paleogeographic position of the two microcontinents may have controlled the temperature changes.

Paleogeographic reconstructions suggest that Saxo-Thuringia was located close to the first zone of closure of the Rheic Ocean [START_REF] Franke | The Palaeozoic Variscan oceans revisited[END_REF]. As the closure of the Rheic Ocean progressed during the Famennian, the area might have been influenced by a relative restriction of the water masses exchanges between the Rheic Ocean and the Paleotethys Ocean. The upper water column temperatures might have been warmer

than in the open sea, as e.g. nowadays in the Caribbean Sea, the Persian Gulf, or the eastern Mediterranean (see for instance [START_REF] Schlitzer | Ocean data view -3[END_REF][START_REF] De Schmidt | Morphological response of planktic foraminifers to habitat modifications associated with the emergence of the Isthmus of Panama[END_REF], dampening the effects of the global cooling, especially in a subtropical area. In contrast, the CT section and more generally the Armorica area, being opened towards the Paleotethys Ocean, would have been exposed to the Paleotethys water circulation and global temperature trends, as established by [START_REF] Joachimski | Devonian climate and reef evolution: Insights from oxygen isotopes in apatite[END_REF]. This interpretation is corroborated by the fact that Late Famennian δ 18 O values recorded in the Carnic Alps (Paleo-Adria, Fig. 1; Pa. gracilis expansa and Bi. ultimus Zones: ~18,3 ‰; [START_REF] Kaiser | Geochemical evidence for major environmental change at the Devonian-Carboniferous boundary in the Carnic Alps and the Rhenish Massif[END_REF][START_REF] Kaiser | Environmental change during the Late Famennian and Early Tournaisian (Late Devonian-Early Carboniferous): implications from stable isotopes and conodont biofacies in southern Europe[END_REF], and hence also in an environment open towards the Paleotethys (Paleo-Adria on Fig. 1), were closer to those recorded at CT than at BU.

Water depth variations, combination of eustatic trends and local tectonics

Sea-level variations correspond to a high-stand during the end of the Frasnian, followed by a progressive sea-level fall during the Famennian [START_REF] Haq | A chronology of Paleozoic sea-level changes[END_REF][START_REF] Johnson | Devonian eustatic fluctuations in Euramerica[END_REF][START_REF] Johnson | Devonian transgressive-regressive cycles and biostratigraphy, Northern Antelope Range, Nevada: establishment of reference horizons for global cycles[END_REF][START_REF] Sandberg | Late Devonian sea-level changes, catastrophic events, and mass extinctions[END_REF]. These variations appear to have occurred world-wide, thus pointing to eustatic trends [START_REF] Johnson | Devonian eustatic fluctuations in Euramerica[END_REF][START_REF] Johnson | Devonian transgressive-regressive cycles and biostratigraphy, Northern Antelope Range, Nevada: establishment of reference horizons for global cycles[END_REF]. In this context, a decrease in water depth should coincide with increasing ice volume, a glacio-eustatic sea-level fall and a temperature decrease. In the CT section, a regressive trend and a cooling are indeed registered, but they do not occur in parallel with the shallowing clearly postponing the shift towards colder temperature. Another discrepancy with the expectation is that both the temperature and water depth decrease do not seem to have occurred in a progressive manner, but rather abruptly. Possibly, paleogeographic and/or tectonic events might have temporarily mitigated the expression of the long-term trend in CT. Water depth variations in BU do not follow this global trend. In BU, the depositional environment remained deep throughout the Famennian, and tectonic activity has been invoked to explain this discrepancy [START_REF] Girard | Conodont biostratigraphy and palaeoenvironmental trends during the Famennian (Late Devonian) in the Thuringian Buschteich section (Germany)[END_REF]. The paleogeographic position of Saxo-Thuringia, close to Baltica and hence one of the first microcontinent to collide with it, matches well with active tectonics causing a local departure from the eustatic trend.

Congruent variations of the conodont assemblages despite discrepant paleoenvironmental settings

In contrast to the different patterns of temperature variation after the Pa. marginifera utahensis Zone, and differences in water depth records in CT and BU, trends in conodont biofacies are quite comparable. Both sections reflect a dominance of Palmatolepis for most of the Famennian with a decline starting in the Pa. marginifera marginifera Zone. If the difference between the environments at CT and BU had any impact on the conodont assemblages, it was only minor and corresponded to a transient offset between the two conodont records. However, conodont biofacies are advocated to be mostly driven by water depth variations, due to the habitat preferences of the different conodont genera [START_REF] Sandberg | Conodont biofacies of late Devonian Polygnathus styriacus Zone in western United States[END_REF][START_REF] Seddon | An ecologic model for conodonts[END_REF]. Initially, two ecological models, i.e. depth stratification vs. lateral segregation, were proposed [START_REF] Klapper | Conodont ecology: pelagic versus benthic[END_REF][START_REF] Seddon | An ecologic model for conodonts[END_REF]. The depth-stratification model implies that specific conodont taxa inhabited different depth zones within the water column, whilst the lateral segregation model proposes that bottom-dwelling taxa were dominantly controlled by habitat properties near the sediment-water interface. The abundant occurrence of conodonts during anoxic events precludes a benthic or nektobenthic lifestyle. During the Frasnian and the Famennian, genera supposed to have "shallow preferences "(e.g. Icriodus) and "preferences for deep environments" (e.g.

Palmatolepis)

have been shown to share similar oxygen isotope signatures, suggesting that they were living in the same water-mass [START_REF] Joachimski | Devonian climate and reef evolution: Insights from oxygen isotopes in apatite[END_REF]. This supports the interpretation that all genera were at that time living close to the surface, but occupying proximal or distal locations. The higher proportion of the offshore surface dweller genus Palmatolepis in the sediments from BU than from CT is coherent with the more off-shore depositional setting in BU. The decrease of the genus Palmatolepis after the Pa. marginifera marginifera Zone is also coherent with the regressive trend characterizing the Famennian. Possibly, before the Sc. velifer velifer Zone, water depth was constantly deep enough to favor the dominance of this offshore surface dweller genus. However, if a direct forcing of water depth on conodont distribution is assumed, conodont biofacies at BU should have been relatively stable through time, and depart more clearly from the trend observed at CT. This suggests that the response of conodont assemblages to water depth was mitigated by other factors. Possibly, the eustatic regressive trend was disadvantageous to the offshore surface dweller genus Palmatolepis at a worldwide scale, triggering its global decline. Ultimately, it would have even declined in constantly deep environments such as BU, although with a time lag suggesting that it temporarily found there a refuge of favorable environments.

Relationship between the different paleoenvironmental proxies

The three paleoenvironmental proxies considered here (sedimentary facies, oxygen isotopes, and conodont biofacies) are potentially interrelated: oxygen isotopes are temperature proxies; facies traces water depth and distance to the shore, related to temperature trends through eustatic variations; and conodont biofacies are interpreted as related to water depth due to habitat preferences of the different conodont genera along a proximal-distal gradient in the surface waters. No relationship between these proxies is evident in BU, but this absence of a significant correlation is probably due to the reduced range of variation in the oxygen isotope ratios and facies observed in this section. Oxygen isotopes and facies are correlated when considering the total data set. However, conodont PC1 displays a stronger relationship with oxygen isotopes on the one hand, and with facies on the other hand, than oxygen isotopes with facies: the percentage of variance explained is larger, leading to significant relationships in CT as well (Table 2). Considering the percentage of variance explained, facies furthermore appears as a better explanatory variable for conodont variations than δ 18 O. The sedimentary facies trace directly the depth of the depositional environment but the relevance of this approach has been discussed due to the paucity of preserved biotas in old sediments (e.g. see [START_REF] Flügel | Microfacies of carbonate rocks. Analysis, Interpretation and Application[END_REF][START_REF] Lüddecke | Conodont biofacies of a monotonous middle Famennian pelagic carbonate succession (Upper Ballberg Quarry, northern Rhenish Massif)[END_REF]. The correlation evidenced between facies and conodont biofacies is nevertheless coherent with the interpretation of conodont genera distribution being controlled by water depth, along a proximal-distal gradient [START_REF] Klapper | Conodont ecology: pelagic versus benthic[END_REF]. The fact that conodont biofacies respond earlier and more progressively to the global regressive trend may be due to the fact that CT and BU being located in outer shelf environments, the decrease in water depth should be important enough to cause a change in the facies.

Paleogeography and paleoenvironment: a complex interplay

Many studies focused on global environmental trends in the Devonian (e.g. [START_REF] Goddéris | Global change in the Late Devonian: modelling the Frasnian-Famennian short-term carbon isotope excursions[END_REF][START_REF] Haq | A chronology of Paleozoic sea-level changes[END_REF][START_REF] Miller | The Phanerozoic Record of Global Sea-Level Change[END_REF][START_REF] Sandberg | Late Frasnian mass extinction: conodont event stratigraphy, global changes, and possible causes[END_REF][START_REF] Wilson | Warm tropical ocean surface and global anoxia during the mid-Cretaceous period[END_REF], especially during the Late Devonian for which environmental conditions have been suggested to be homogeneous within the subtropical zone, and the marine faunas to be cosmopolitan. However, our results, based on two detailed 13 My long records, indicated different paleoenvironmental changes in BU and CT. After the end of the Pa. marginifera utahensis Zone, the cooling and the regressive trend found little expression in BU compared to CT, but these differences are only weakly mirrored in the conodont assemblages. The differences in the temperature records of CT and BU are not only observed in the two studied sections, but registered as well in two other sections (Kahlleite and La Serre) from the Saxo-Thuringian and Montagne Noire areas [START_REF] Joachimski | Devonian climate and reef evolution: Insights from oxygen isotopes in apatite[END_REF], respectively. We propose that the [START_REF] Bartoli | Stable isotope analysis and temperature reconstruction data from DSDP Hole 94-609B and ODP Hole 162-984B[END_REF]Haug andTiedemann, 1998), but this remains speculative and challenges further studies.

Increasing the geographic coverage of a multi-proxy approach, such as the one outlined here, may thus provide relevant information to better constrain paleogeographic reconstructions characterized by a complex distribution of land and water masses, as e.g. in the Late Devonian. This key step will contribute to a better understanding of the complex interplay between paleogeography, climate, and biotic response and go beyond the tempting, but misleading, view of global trends affecting the different regions in a relatively homogeneous manner.

Conclusions

This study presents a detailed comparison of two Famennian successions in the Montagne Noire (France) and Saxo-Thuringia (Germany). During the Famennian, both areas were located in the subtropical realm and were part of two microcontinents bracketed between the remnant Rheic Ocean and Gondwana. Based on the same rock samples, paleotemperatures were estimated using oxygen isotopes of conodont apatite, depositional environments were assessed based on sedimentary facies, and the response of conodont assemblages was evaluated using conodont biofacies. Applying a principal component analysis to biofacies data provided a synthetic axis summarizing the biofacies variations along the two sections. This allowed for a quantitative comparison of the biofacies records with paleoenvironmental proxies.

During the Famennian, both the Montagne Noire and Saxo-Thuringia display a parallel increase in the oxygen isotope ratios translating into long-term cooling.

However, during the Late Famennian, temperatures remained stable in Saxo-Thuringia, leading to a temperature offset compared to the Montagne Noire record. The Late and latest Famennian are also characterized by a regressive trend which was found to be well expressed in the sedimentary facies in the Montagne Noire, but not in Saxo-Thuringia, where the depositional environment remained deep throughout. Despite these environmental differences, conodont biofacies were rather comparable in the Montagne Noire and Saxo-Thuringia. A temporary offset however suggested that Palmatolepis, the typical off-shore surface dwelling taxon, was abundant for a longer time period in the deep Saxo-Thuringian environment. The environmental differences between the two areas are tentatively explained by their paleogeographic location. The Saxo-Thuringian microplate is interpreted as located close to the point of first closure of the Rheic Ocean and thus might have been influenced by active tectonism and a restricted water mass exchange, blurring the impact of the long-term trends in this setting. 2017) on the left, absolute ages based on [START_REF] Becker | The Devonian Period[END_REF]. Each symbol corresponds to a sampled level. FRS: Frasnian. Pa.: Palmatolepis; del.: delicatula; gl.: glabra; marg.: marginifera; gr.: gracilis; Sc.: Scaphignathus; v.: velifer; r.: rugosa; Ps.: Pseudopolygnathus; Bi.: Bispathodus, ac.: aculeatus. In grey, zones which were not recognized in both sections. Correlations between two variables using the Pearson's product-moment correlation, coefficient of correlation (R) and probability (p) are given. Analyses were performed for CT and BU combined, and CT and BU alone. Significant correlations marked in bold (p < 0.05).

Figure captions

GONDWANA R H E I C O C E A N P A L E O T E T H Y S O C E A N

Age is estimated based on the age model.

  is ca. 80 m thick. It consists of fine-grained bioclastic limestones from the Palmatolepis rhenana to the Pa. glabra pectinata conodont Zone, followed by typical reddish griottes from the Pa. rhomboida to the Pa. marginifera marginifera Zone. Lime mudstones were deposited from the Pa. marginifera utahensis to the Pa. rugosa trachytera Zone and are overlain by wackestones with an age from the Polygnathus styriacus to the middle of the Bispathodus ultimus Zone. The Famennian carbonate deposits are overlain by the Hangenberg shales. Depositional environments ranged from mid to outer ramp. The BU section is ca. 35 m thick and consists of lime mudstones and fine-grained turbiditic wackestones from the Pa.

δ

  18 O values range from +16.4 to +16.9 ‰ from the end Frasnian to the Pa. crepida zone. From a minimum of +16.4 ‰ at the base of the Pa. crepida Zone, a trend to higher values occurs up to the Pa. marginifera marginifera Zone, reaching δ 18 O values of +17.8 ‰. A brief decrease marks the first part of the Pa. marginifera utahensis Zone, with δ 18 O values of around +16.7 ‰. Thereafter, up to the end of the Famennian, δ 18 O values remain close to +17.5 ‰.

  decrease of Palmatolepis was compensated almost exclusively by a rise of Bispathodus in CT and by a rise of both Polygnathus and Bispathodus in BU. The Frasnian assemblages were complemented by the occurrence of other genera such as Polygnathus and Ancyrodella. The Pa. triangularis to Pa. minuta minuta Zones, which are missing in BU, were marked by an increase in the abundance of Icriodus in CT. Another maximum in the abundance of Icriodus in CT is recognized in the Pa. glabra prima Zone. Except for the Pa. crepida Zone, Icriodus was rare in BU. In CT, a short-term rise of Scaphignathus/Alternognathus occurred around the Ps. granulosus Zone. A similar but more discrete rise is present at the base of the Pa. rugosa trachytera Zone in BU.

  ) shows that positive values along the first axis correspond to a high proportion of Palmatolepis whereas negative values correspond to a high proportion of Bispathodus. Variations along the PC1 axis thus correspond to the replacement of Palmatolepis by Bispathodus. The second axis represents only less than 20% of variance, and mostly corresponds to variations in the proportion of Polygnathus. Other genera contribute very little to the two main axes, due to their rare occurrence in the records.Representing the different levels in the first principal plane of this PCA allows for a comparison of BU and CT conodont biofacies variations (Fig.6B-I). Most of the time, the range of values covered by samples from the two sections largely overlaps. The oldest samples (mostly Frasnian) are characterized by a peculiar diagonal distribution (Fig.6B). Their shift in the PCA space compared to Famennian samples is due to the occurrence of genera such as Ancyrodella that went extinct during the Frasnian -Famennian crisis. Early Famennian levels of both sections are tightly clustered, sharing extreme positive values along the first axis, due to the overwhelming dominance of Palmatolepis.

  The constant offset between the two records can thus hardly be explained by a latitudinal difference. Possibly, CT being positioned further towards the Paleotethys[START_REF] Franke | The Palaeozoic Variscan oceans revisited[END_REF], it might have been more exposed to the colder water masses of the open Paleotethys Ocean and Paleo-Adria epicontinental sea.

  deviation of the BU record from what we interpret as the global trend may be attributed to the paleogeography of the Saxo-Thuringian area, where a progressive restriction of water-mass exchange possibly accompanied the closing of the seaway between the remnant Rheic and Paleotethys oceans. This restricted paleogeography together with a subtropical location might have favored the persistence of warm conditions in the seawaters where conodonts lived despite the onset of global cooling. Such setting should also have been prone to active tectonics overprinting the eustatic trend. Not mutually exclusive, the closure of the Rheic Ocean itself might have contributed to the global climate change, by interrupting the circum-equatorial circulation (see for instance for the closure of the Panama Isthmus:

Figure 1 .

 1 Figure 1. Paleogeographic reconstruction during the latest Famennian (simplified after Franke et al. 2017). BU: Buschteich; CT: Col des Tribes. Rectangles: investigated areas.

Figure 2 .

 2 Figure 2. Age model allowing comparison of carbonate production rates between Buschteich (BU; red circles) and Col des Tribes (CT; green diamonds). Depth is given in meters. Conodont zonations according Ziegler and Sandberg (1990) on the right and Spalletta et al. (2017) on the left, absolute ages based on Becker et al. (2012). Each

Figure 3 .

 3 Figure 3. Depositional model for the Col des Tribes (CT) and Buschteich (BU) sections during the Famennian.

Figure 4 .

 4 Figure 4. Famennian environmental changes and conodont assemblage variations in the Buschteich section (BU; Saxo-Thuringia) and Col des Tribes section (CT; Montagne Noire). A: Facies distribution for CT and BU from the deepest (1) to the shallowest (5). B: δ 18 O in CT and BU samples, compared with data of Joachimski et al. (2009) from Thuringia (Kahlleite, blue circles) and the Montagne Noire (La Serre; grey diamonds). C. Variations of conodont assemblage through time depicted as variations along the first axis of the PCA on conodont genera percentages. Green diamonds: CT; red circles: BU. Abbreviations: see Figure 2.

Figure 5 .

 5 Figure 5. Conodont biofacies in CT and BU (after Girard et al. 2014; 2017). The proportions have been plotted again the age model. Note the absence of data (Gap) for the Pa. linguiformis to Pa. crepida zones at BU (white rectangle). Scaphignathus and Alternognathus: Sc., Siphonodella: Si. on the figure.

Figure 6 .

 6 Figure 6. Principal component analysis of the variations in the percentages of the conodont genera in BU and CT. The first two axes of the PCA are represented (PC1 = 77.5%; PC2 = 18.3%). Upper left panel: contribution of the percentages of the different genera to the first two PC axes. Next panels: distribution of the different levels in the plane defined by the first two PC axes (PC1 = 77.5%; PC2 = 18.3%). The representation has been split by temporal zones (because of their limited sampling, Pa. triangularis to Pa. gl. pectinata Zones have been grouped). Each dot represents the assemblage in a level. CT: green diamonds, green diamonds with black outlines: levels from the triangularis and minuta minuta Zones; BU: red circles. Pa.: Palmatolepis; gl.: glabra; gr.: gracilis; marg.: marginifera; Sc.: Scaphignathus; v.: velifer; r.: rugosa; Ps.: Pseudopolygnathus; Po.: Polygnathus; Bi.: Bispathodus.

Table 1 .

 1 Facies and environmental settings for the Col des Tribes and the Buschteich sections

	FACIES	DIAGNOSTIC	SEDIMENTARY	STRATAL	BIOCLASTS	DEPOSITIONAL	Girard et al.
		FEATURES	FEATURES	PATTERN		SETTING	*2014 (CT)
							**2017 (BU)
	Facies 1 :	Structureless		Well	Pelagic	Outer ramp	Facies 1 at CT
	pelagic	lime mudstone		stratified,	ostracods and		and BU
	mudstone	with few		dms thick	bivalves,
		bioclasts (2 -			goniatites,
		10%)			radiolarians,
					rare benthic
					ostracods,
					trilobites and
					crinoids
	Facies 2:	Laminated	Mm to cm scale	dm thick	Pelagic	Distal	Facies 2 and
	fine-grained	wackestone	thick graded	beds	ostracods and	turbidites in	3 at BU
	turbidites	with pelagic	bedding,		bivalves,	outer ramp
		fauna and	erosional		radiolarians,	interbedded
		scarce	surfaces at		some	within F1
		hardparts of	bottom		transported
		benthic			benthic
		organisms			ostracods,
					blind
					trilobites
					brachiopods
					and crinoids
	Facies 3:	Structureless	Pseunodular	cm to dm	Pelagic	Outer ramp,	Facies 2 at CT
	pelagic	wackestone	structure	thick beds	bivalves and	shallower than
	wackestone	dominated by			ostracods,	F1 and F2
		pelagic fauna			diversified
		and with 5 %			cephalopods,
		benthic			radiolarians,
		organisms			crinoids,
					benthic
					ostracods and
					trilobites
	Facies 4:	Reddish	cm sized lime	Cm to dm	Goniatites,	Slope in the	Facies 3 at CT
	cephalopod	nodular to	nodules coated	thick beds	pelagic	upper outer
	wackestone	wavy	by argillaceous		ostracods and	ramp
	to floatstone	laminated	and iron		bivalves,
	(griottes)	wackestone to	hydroxydes,		trilobites,
		floatstone with	intraformational		brachiopods,
		abundant	breccias, rotated		crinoids and
		goniatites	geopetal		foraminifers
			structures
	Facies 5:	Structureless	Pseudonodular	cm to dm	Pelagic	Mid ramp	Facies 4 at CT
	wackestone	to	structure with cm	thick beds	ostracods and
	with	pseudonodular	to dm thick beds		abundant
	diversified	well stratified			goniatites,
	fauna	wackestone			trilobites,
		with pelagic			crinoids,
		and benthic			brachiopods,
		organisms			gastropods,
					bivalves,
					foraminifers

Table 2 .

 2 Relationship between oxygen isotopes, microfacies, and conodont assemblages.

				CT+BU			CT			BU
			n	R	p	n	R	p	n	R	p
	δ 18 O	Facies	41 0.5023	0.0008	22 0.4107	0.0576	19	0.3149	0.1891
	Conodont	δ 18 O	51 -0.5304	6.2e-5	25 -0.5665	0.0031	26	-0.3039	0.1312
	PC1
	Conodont	Facies	78 -0.6576	6.1e-11	52 -0.7058	5.1e-09	26	0.1799	0.3791
	PC1
	δ 18 O	Age	51 -0.5150	0.0001	25 -0.7532	1.388e-05	26	-0.7018	6.455e-05

Table 3 .

 3 Analyses of variance on oxygen isotopes and conodont proportions. Two models were considered. (1) A model considered δ 18 O as dependent variable, and age, section, and interaction (age * section) as effects. (2) A model considered PC1 based on conodont proportions in the assemblages as the dependent variable. The effects tested were δ δ 18 O and facies score. Percentage of variance explained (pve) and probability (p) are given. Significant correlations marked in bold (p < 0.05).

				CT+			CT			BU
				BU
		Dependent	Independent	n	Pve	p	n	Pve	p	n	Pve	p
		variable	variable
	(1)	δ 18 O	Age	51	30.5%	1.83E-09
			Section		47.0%	4.06E-12
			Age * Section		0.4%	0.375
	(2)	Conodont	δ 18 O	41	6.1%	0.0311	22 9.0%	0.0316	19	11.2%	0.175
		PC1
			Facies		22.3%	0.0001		33.6%	0.0003		0.5%	0.769

Acknowledgments

We thank D. Delmas and C. Nevado (Lyon, France) who produced the thin-sections and Raimund Feist (Montpellier, France), Dieter Weyer (Berlin, Germany) and Konrad Bartzsch (Saafeld, Germany) for their contribution during field trips. The research was supported by the ANR Project ECODEV (ANR-13-BSV7-005). This is publication ISEM 2018-XXX.