Miocene (Burdigalian) seawater and air temperatures estimated from the geochemistry of fossil remains from the Aquitaine Basin, France

Palaeogeography, Palaeoclimatology, Palaeoecology 481 (2017) 14–28

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Miocene (Burdigalian) seawater and air temperatures estimated from the geochemistry of fossil remains from the Aquitaine Basin, France Jean Goedert a,⁎, Romain Amiot a, Florent Arnaud-Godet a, Gilles Cuny a, François Fourel b, Jean-Alexis Hernandez a, Ulysse Pedreira-Segade a, Christophe Lécuyer a,c a b c

Univ Lyon, Université Lyon 1, Ens de Lyon, CNRS, UMR 5276 LGL-TPE, 69622 Villeurbanne, France Univ Lyon, Université Lyon 1, CNRS, UMR 5023 LEHNA, 69622 Villeurbanne, France Institut Universitaire de France, 103 boulevard Saint-Michel, 75005 Paris, France

a r t i c l e

i n f o

Article history: Received 18 November 2016 Received in revised form 9 March 2017 Accepted 3 April 2017 Available online 19 May 2017 Keywords: Oxygen isotopes Rare earth elements Vertebrates Invertebrates Neogene Palaeoclimate

a b s t r a c t Construction work of the Highway A65 led the top of the Burdigalian ‘Molasses de l'Armagnac’ Formation to outcrop in two close localities near the town of Bazas and Marimbault, situated on the western edge of the Aquitaine Basin, France. From this formation, a rich fossil assemblage has been recovered and includes both marine and terrestrial fossil remains which offered the opportunity to reconstruct both seawater and terrestrial air temperatures at a regional scale during the Burdigalian. For this purpose, two sets of fossil samples representative of the whole assemblage have been selected for each locality. They include teeth and bones of sharks, fish, rays, reptiles and mammals, as well as tests of sea urchins and shells of bivalves. First, we performed rare earth element (REE) analyses of 53 apatite samples from both localities in order to characterize the diagenetic history of the assemblage. We then analysed the oxygen isotope composition of 49 biogenic apatite phosphate samples and 6 biogenic carbonate samples representing both marine and terrestrial organisms. Using published isotopic fractionation equations, both seawater and terrestrial air temperatures were reconstructed. Calculated seawater temperatures using phosphate and carbonate isotopic thermometers are + 23 ± 4 °C and +25 ± 1 °C respectively for the locality of Mendouillet (Bazas) and + 24 ± 4 °C and + 26 ± 1 °C respectively for the locality of Monbalon-Miron (Marimbault). Calculated terrestrial air temperatures are +18 ± 2 °C for the locality of Mendouillet and +17 ± 2 °C for the locality of Monbalon-Miron. These regional temperatures are in good agreement with seawater and terrestrial temperatures calculated for other European localities of contemporaneous periods. These results also fit the global climatic context of the Mid-Miocene Climatic Optimum starting at the end of the Burdigalian. Even though the global climatic conditions were warmer than today during the end of the Burdigalian, calculated continental and seawater thermal gradient at mid-latitude appear to have been comparable to present-day ones. © 2017 Elsevier B.V. All rights reserved.

1. Introduction International scientific ocean drilling programs (Deep Sea Drilling Project (DSDP), Ocean Drilling Program (ODP) and Integrated Ocean Drilling Program (IODP)) have now operated over 50 years resulting in a huge collection of high-quality deep-sea sediments cores. Extraction of foraminifera from these cores and subsequent oxygen isotope analyses of their carbonated shells have been used to study past climatic variations through the Cenozoic (e.g. Flower and Kennett, 1993; Miller and Thomas, 1985; Shevenell et al., 2004; Woodruff et al., 1981) using the principle that the oxygen isotope composition of their carbonate (δ18Oc) depends both on the temperature and the ⁎ Corresponding author at: CNRS UMR 5276, Laboratoire de Géologie de Lyon: Terre, Planètes, Environnement, Université Claude Bernard Lyon 1, Campus de La Doua, Bâtiment Géode 2 rue Raphaël Dubois, 69622 Villeurbanne CEDEX, France. E-mail address: [email protected] (J. Goedert).

http://dx.doi.org/10.1016/j.palaeo.2017.04.024 0031-0182/© 2017 Elsevier B.V. All rights reserved.

oxygen isotope composition of seawater (δ18Oseawater) from which they have precipitated (e.g. Craig, 1965; Epstein et al., 1953; McCrea, 1950; O'Neil et al., 1969; Shackleton, 1974). These studies have resulted in the construction of a high-resolution composite oxygen isotope curve spanning the whole Cenozoic (Miller et al., 1987; Zachos et al., 2001). The composite curve generated by Zachos et al. (2001) represents the variations in δ18Oc values of benthic foraminifera. Therefore, their δ18Oc values partly reflect the δ18Oseawater values and partly the temperatures of bottom oceanic waters derived from the cooling and sinking of surface oceanic waters at the poles. Consequently, these long-term isotopic variations reflect temperature variations from high-latitude oceanic waters that may differ both in direction and amplitude from those experienced in continental areas of lower paleolatitudes. Those considerations may explain some inconsistency when using this marine isotopic record to explain the evolution of floral and faunal structures of terrestrial environments (e.g. Alroy et al., 2000).

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Climatic reconstructions of Cenozoic continental environments are contrarily scarce and fragmentary over space and time. They have used several proxies based on floral assemblages (Böhme et al., 2007; Cavagnetto and Anadón, 1996; Fauquette et al., 2007; Jiménez-Moreno, 2006; Jiménez-Moreno et al., 2010; Jiménez-Moreno and Suc, 2007; Mosbrugger et al., 2005; Mosbrugger and Utescher, 1997; Utescher et al., 2009; Utescher et al., 2007a; Utescher et al., 2007b; Utescher and Mosbrugger, 2007; Wolfe, 1994), faunal assemblages (ectothermic amphibians and reptiles: e.g. Böhme, 2003; Böhme et al., 2006; channid fish: Böhme, 2004; mammals body weight: e.g. Costeur and Legendre, 2008; Legendre, 1988, 1986) herbivorous mammals tooth (e.g. crown height Fortelius et al., 2002 or wear facet analysis Merceron et al., 2007) and the oxygen isotope composition of continental carbonates (paleolake carbonates: e.g. Bein, 1986; Bellanca et al., 1989; Talbot, 1990; meteoric carbonates: e.g. Hays and Grossman, 1991; speleothem carbonates: e.g. Harmon et al., 1978; Hopley et al., 2007; eggshell carbonates: e.g. Ségalen et al., 2006, Ségalen et al., 2002; earthworm granule carbonates: e.g. Prud'homme et al., 2016). Another way to reconstruct paleoclimates in continental areas is to use the oxygen isotope composition of phosphates (δ18Op) of biogenic apatites (bone, dentin or enamel). Indeed, Longinelli (1984), Luz et al. (1984) and Kohn (1996) have shown that the δ18Op values of biogenic apatites were linearly correlated to the oxygen isotope composition of drinking waters. Drinking waters mainly correspond to surface waters (rivers, lakes, ponds) that ultimately derive from meteoric waters. In turn, meteoric waters have oxygen isotope composition (δ18Ometeoric waters) reflecting both local air temperatures and humidity at mid- and high-latitudes (Dansgaard, 1964; Rozanski et al., 1993). Using this principle, several studies have intended to reconstruct Paleocene to Holocene terrestrial air temperatures during short-term time intervals (e.g. Bryant et al., 1996; Fricke et al., 1998; Kovács et al., 2015; Navarro et al., 2004; Rey et al., 2013; Tütken et al., 2006). Héran et al. (2010) proposed the first reconstruction of long-term continental air temperature variations from late Eocene (c.36 Ma) to the middle Miocene (c.10 Ma) based on the oxygen isotope composition of rodent tooth phosphates. Comparison with the long-term marine record of Zachos et al. (2001) reveals overall comparable trends: the gradual transition from greenhouse to icehouse conditions punctuated by intermediate phases of climatic optima (e.g. Late Oligocene Warming, Mid-Miocene Climatic Optimum) and abrupt climatic shifts (known as Oi- and Mi-events; Miller et al., 1991; Wright and Miller, 1993). The Mid-Miocene Climatic Optimum, occurring around 17–15 Ma, is a critical period regarding climatic evolution as it interrupted a global cooling trend, attributable to the development of the Antarctic icesheets (Anderson and Shipp, 2001; Zachos et al., 2001), which was initiated at the Eocene-Oligocene boundary (Liu et al., 2009), and accentuated by the development of northern hemisphere glaciations during the Pliocene (Maslin et al., 1998; Shackleton et al., 1988; Zachos et al., 2001). This period of global cooling trend is associated with profound and gradual turnovers of floras (e.g. Cerling et al., 1997; Fox and Koch, 2003; Jacobs et al., 1999; Potts and Behrensmeyer, 1992; Quade et al., 1994; Quade and Cerling, 1995; Strömberg, 2005) and faunas (e.g. Janis et al., 2004, Janis et al., 2002, Janis et al., 2000; Jernvall and Fortelius, 2002; Maridet et al., 2007; Raia et al., 2011). During the Mid-Miocene Climatic Optimum terrestrial floral and faunal structures underwent faster modifications caused by rapid colonization of mid-latitude terrestrial environments by thermophilic taxa (e.g. Böhme, 2003; Böhme et al., 2007; Sun and Zhang, 2008). We have analysed Miocene fossil samples from littoral deposits in an attempt to reconstruct both seawater and terrestrial air temperatures at a regional scale at the beginning of the Mid-Miocene Climatic Optimum. In France, several transgressive episodes took place during the Miocene leading to the deposition of marine sediments during the Aquitanian (23.03–20.44 Ma), the Burdigalian (20.44–15.97 Ma) and the LanghianSerravallian (15.97–13.82–11.63 Ma) in the north-western (Tréfumel-Le Quiou-St-Juvat Basin), southwestern (Aquitaine Basin) and southeastern (Rhodano-Provençal molassic Basin) parts of France. In the north-western

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part of France, some of these marine deposits called ‘faluns’ are historically well-known for the rich vertebrate assemblages they have yielded, comprising both marine and terrestrial organisms (Ginsburg and Mornand, 1986; Gobé et al., 1980; Mornand, 1978). Terrestrial fossils from these deposits have been shown to be either reworked during marine transgression from underlying continental alluvial sand deposits or contemporaneous, being transported by rivers from the continent to these epicontinental seas (Ginsburg and Janvier, 2000). In the southwestern part of France contemporaneous deposits have yielded similar assemblages also comprising this unique association of marine and terrestrial organisms (e.g. Antoine et al., 1997; Grateloup, 1838). In 2009, the construction work of the Highway A65 in France crosscut these Miocene deposits in Mendouillet and Monbalon-Miron, located in the municipalities of Bazas and Marimbault, respectively. A rich fossil assemblage has been collected from the top of the Burdigalian ‘Molasses de l'Armagnac’ Formation which cropped out during the construction. This assemblage includes both marine and terrestrial vertebrates along with some molluscs and echinoderms providing a unique opportunity to reconstruct at a regional scale both seawater and terrestrial air temperatures using oxygen isotope analyses of their skeletons. First, we analysed biogenic apatite samples of both marine and terrestrial vertebrates for their rare earth element (REE) contents in order to characterize their diagenetic history and in particular to test whether or not terrestrial vertebrate samples have been reworked from underlying strata. We then analysed the oxygen isotope compositions of apatite phosphate from marine and terrestrial vertebrates along with those of carbonate from marine invertebrates and interpreted them in terms of seawater and air temperatures. Results were finally compared to near-contemporaneous published oxygen isotope data that were obtained from marine fish and mammals collected from the so-called ‘faluns’ (sedimentary deposits of marine origin generally consisting in friable sands rich in mollusc shells) from the Tréfumel-Le Quiou-St-Juvat Basin (Lécuyer et al., 1996) and rodent teeth collected from the southern part of Germany (Héran et al., 2010). 2. Geological setting During the Miocene, the Aquitaine Basin was located at a paleolatitude close to 40°N (Scotese et al., 1988). It corresponded to a large embayment opening westward into the Atlantic Ocean. Sedimentological studies of the historical stratotypes of the Aquitanian and the Burdigalian allow the recognition of at least six depositional sequences, four assigned to the Aquitanian, one to the Burdigalian and the last one to the Serravallian (Parize et al., 2008). In the Aquitaine Basin (Fig. 1A), the excavation of Mendouillet and Monbalon-Miron trenches during the construction work of the Highway A65 in 2009 (Fig. 1B), made Miocene and Pliocene sedimentary deposits temporarily accessible (Fig. 1C). In the locality of Mendouillet and Monbalon-Miron, the Burdigalian ‘Molasses de l'Armagnac’ Formation (Capdeville, 1992) defines the base of the section (Fig. 1C). Only half to one meter corresponding to the top of the ‘Molasses de l'Armagnac’ have been exposed, but drilling exploration revealed that they locally reach 25 to 40 m in thickness (Capdeville, 1992). The ‘Molasses de l'Armagnac’ consist in grey to dark silty fossiliferous clays. This formation lays on the Aquitanian ‘Calcaire gris de l'Agenais’ Formation which consists of 4 m of lacustrine limestones containing ramshorn snails (Capdeville, 1992). The ‘Calcaire gris de l'Agenais’ has not been exposed by the digging of the Highway A65 neither at the locality of Mendouillet nor at Monbalon-Miron. At the locality of Mendouillet, the ‘Molasses de l'Armagnac’ Formation is locally overlain by Langhian-Serravallian carbonated clays (‘faluns’) (Capdeville, 1992) appearing as an outlier (‘butte-témoin’) (Fig. 1C). These carbonated clays are very rich in mollusc shell debris that locally forms hard grey limestones (Fig. 1D). In turn, these carbonated clays are overlain by a thin unit of Serravallian tawny sands (‘sables fauves’) (Capdeville, 1992), which consist in yellow to ochre gravelbearing fossiliferous sands (Fig. 1C).

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Fig. 1. Geographic locations of Mendouillet and Monbalon-Miron. A – Aquitaine Basin showing the localisation of the town of Bazas and Marimbault; B – Satellite image obtained from Google earth showing the precise location of the town of Bazas and Marimbault and the trenches of Mendouillet (1) and (2) Monbalon-Miron along the construction work of the Highway A65 in 2009. C – Details of the trenches of Mendouillet showing the Miocene and Pliocene deposits. D – Details of the summit of the Burdigalian ‘Molasses de l'Armagnac’ Formation covered by the Langhian-Serravallian carbonated clays rich in mollusc shells.

At the locality of Monbalon-Miron (Marimbault), the clays constituting the ‘Molasses de l'Armagnac’ Formation appear siltier and more light-grey to light-brown in colour than in Mendouillet. The carbonated clays and tawny sands are absent and the ‘Molasses de l'Armagnac’ Formation is directly covered by the Pliocene ‘Sables d'Onesse et de Belin’ formations (Capdeville, 1992) that consist in about 5 m of white sands and gravels (Fig. 1C). In the more northern localities of La Bourasse and Lassime, PontPourquey and Péloua (municipality of Saucats, Aquitaine Basin) Burdigalian deposits are about 2 m and 9 m thick respectively. At the locality of Lassime, Pont-Pourquey and Péloua, the summit of the Burdigalian deposits was dated at 19.2 ± 0.2 Ma by 87Sr/86Sr analysis; these deposits are directly covered by Serravallian deposits (Langhian deposits being absent), dated at their base as 12.9 ± 0.7 Ma, also using 87Sr/86Sr analysis (Cahuzac et al., 1997; Cahuzac and Turpin, 1999; Parize et al., 2008). Furthermore, the transition between the Burdigalian and the Serravallian deposits is clearly marked by an erosive unconformity (Parize et al., 2008). Comparatively, at the locality of Mendouillet and Monbalon-Miron, Burdigalian deposits corresponding to the ‘Molasses de l'Armagnac’ Formation have been estimated by drilling to vary locally from 25 m to 40 m in thickness (Capdeville, 1992). At least for the locality of Mendouillet, these deposits are covered by the Langhian-Serravallian ones, suggesting a more complete sedimentary sequence than in the locality of Saucats. Therefore, the top part of the Burdigalian deposits of the ‘Molasses de l'Armagnac’ Formation is likely older than the top part of the Burdigalian deposits cropping out at Saucats, dated at 19.2 ± 0.2 Ma, and which correspond to the upper part of the Burdigalian which lasted from 20.44 Ma to 15.97 Ma.

Basin, France (Fig. 1A, B). All the fossil remains were collected from the top part of the ‘Molasses de l'Armagnac’ Formation (Fig. 1C). This fossil assemblage comprises both marine and terrestrial vertebrates along with marine invertebrates (Fig. 2 and 3). Forty nine fossil biogenic apatite samples along with 6 fossil biogenic carbonate samples have been selected. They include 23 biogenic apatite samples and 3 biogenic carbonate from the locality of Bazas (reference: 1-BAZ; Table S1 and S2, note: all tables are available in Supplementary Information) and 26 biogenic apatite samples and 3 biogenic carbonate samples from the locality of Marimbault (reference: 2-BAZ; Table S1 and S2). Biogenic apatite remains consist in isolated teeth and bones of sharks, rays, marine teleosteans, marine and terrestrial mammals, turtles and crocodiles (Fig. 2; Table S1 and S2). Biogenic carbonates consist in tests of sea urchins and bivalve shells (Fig. 3; Table S2). All samples have been washed in distilled water using ultrasonic bath to remove any trace of sediment. Biogenic apatite and carbonate samples were then collected using a spherical diamond-tipped drill bit (1 mm in diameter). When possible, dentine and enamel powders were both collected separately from the same tooth. Dentine powder was dedicated to REE content analysis (Table S1) while enamel powder was used to perform oxygen isotope analyses (Table S2). A few enamel powders were also analysed for their REE content in order to compare them with those of dentine powders; particularly, 4 fossil teeth were selected (1-BAZ-8;17, 2-BAZ-8;17) to perform REE analyses on both dentine and enamel (Table S1). Enamel and carbonate powders were sampled along the whole growth axis of the teeth and bivalve shells, respectively, in order to avoid seasonal bias when measuring oxygen isotope ratios. 3.2. Raman spectroscopy

3. Material and methods 3.1. Sample collection A rich fossil assemblage was recovered from the localities of Mendouillet (municipality of Bazas) and of Monbalon-Miron (municipality of Marimbault) both situated in the western edge of the Aquitaine

Biogenic carbonate samples 1-BAZ-25:27 and 2-BAZ-29:31 corresponding respectively to specimens of the sea urchin Scutella leognanensis and to the bivalve Megacardita jouanneti were analysed using Raman spectroscopy to characterize the polymorph of calcium carbonate. Raman spectra were collected at the Laboratoire de Géologie de Lyon (UMR 5276, Ens de Lyon) using a LabRAM HR800 confocal

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Fig. 2. A – Carcharias taurus tooth in l. labial, m. lateral and r. lingual views; B – Araloselachus cuspidatus tooth in l. labial, m. lateral and r. lingual views; C – Cosmopolitodus hastalis tooth in l. labial and r. lingual views; D – Isurus desori tooth in l. labial, m. lateral and r. lingual views; E – Hemipristis serra tooth in l. labial and r. lingual views; F – Anotodus retroflexus tooth in l. labial, m. lateral and r. lingual views; G Otodus megalodon tooth in l. labial, m. lateral and r. lingual views; H?Rhinoptera sp. tooth in u. lingual, m. basal and l. apical views; I Aetobatus arcuatus tooth in l. basal and r. apical views; J Sparus cinctus tooth in l. apical and r. lateral views; K Sparus auratus tooth in l. apical and r. lateral views; L Tetrodon lecointrae tooth in l. apical, m. lateral and r. basal views; M Trigonodon juvleri tooth in l. lingual, lr. lateral and r. labial views; N Labrodon pavimentatum tooth in l. apical, u. lateral and r. basal views; O 1BAZ-16 Gomphoterium angustidens tooth in apical view; P 1BAZ-17 Rhinocerotidae indet. tooth in apical view; Q 1BAZ-21 Gomphoterium angustidens milk tooth in apical view; R Suidae indet. tooth in u. apical and lr lateral views; S Artiodactyla indet. talus bone in l. lateral and r. anterior view; T 1BAZ-22 Balaena sp. centrum bone in l. lateral and r. apical view U Metaxytherium sp. rib bone in lateral view. V Crocodylia indet. osteoderm in apical view. Scale bar equals 1 cm.

spectrometer and excitation provided by a Continuous Wave laser with 532 nm line.

Lu) were then analysed using an Agilent 7500™ Inductively Coupled Plasma-Mass Spectrometer (ICP-MS; UMR 5276, Ens de Lyon).

3.3. Rare earth element (REE) analysis

3.4. Oxygen isotope analysis of biogenic apatite phosphate samples

Fifty-three apatite samples (Table S1) weighing around 50 mg were dissolved overnight in screw-top Teflon bombs (Savillex™) using 2 mL of 14 M nitric acid (HNO3) at 150 °C. The solutions were rinsed and diluted to 25 mL with distilled water. Each sample solution was then diluted 100 times in 0.5 M HNO3 containing 2 ppb Indium as internal standard. REE elements (La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb,

The 49 biogenic apatite powders have been treated following the wet chemistry protocol described by Crowson et al. (1991) and slightly modified by (Lécuyer et al., 1993). This protocol consists in the isolation of phosphate (PO3− 4 ) from apatite as silver phosphate (Ag3PO4) crystals using acid dissolution and anion-exchange resin. For each sample, 20– 30 mg of enamel powder was dissolved in 2 mL of 2 M HF overnight.

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Fig. 3. A – 1BAZ-25 Scutella leognanensis test in apical view; B – 2BAZ-29 Megacardita jouanneti shell in apical view. Scale bar equals 1 cm.

The CaF2 residue was separated by centrifugation and the solution was neutralized by adding 2.2 mL of 2 M KOH. 2.5 mL of Amberlite™ anion-exchange resin was added to the solution to separate the PO3− 4 ions. After 24 h, the solution was removed and the resin was eluted with 6 mL of 0.5 M NH4NO3. After 4 h, 0.5 mL of NH4OH and 15 mL of an ammoniacal solution of AgNO3 were added and the samples were placed in a thermostated bath at 70 °C during 7 h allowing the precipitation of Ag3PO4 crystals. Oxygen isotope compositions were measured using a high temperature elemental analyzer (EA)-pyrolysis interfaced in continuous flow (CF) mode to an isotopic ratio mass spectrometer (IRMS) (EA-Py-CFIRMS technique, Fourel et al., 2011; Lécuyer et al., 2007) at the Laboratoire de Géologie de Lyon (UMR 5276, Université Claude Bernard Lyon 1). For each sample, 5 aliquots of 300 μg of Ag3PO4 were mixed with 300 μg of pure graphite powder loaded in silver foil capsules. Pyrolysis was performed at 1450 °C using a varioPYROcube™ Elemental Analyzer interfaced in Continuous Flow mode with an Isotopic Ratio Mass Spectrometer Isoprime™. Measurements have been calibrated against the NBS120c (natural Miocene phosphorite from Florida). The value of NBS120c was fixed at 21.7‰ (V-SMOW) accordingly to Lécuyer et al. (1993) for correction of instrumental mass fractionation during CO isotopic analysis. Silver phosphate precipitated from standard NBS120c was repeatedly analysed (δ18Op = 21.8 ± 0.2‰, n = 11) along with the silver phosphate samples derived from fossil bioapatites to ensure that no fractionation occurred during the wet chemistry. The average standard deviation equals 0.32 ± 0.15‰. Data are reported as δ18O values vs. V-SMOW (in ‰ δ units). 3.5. Oxygen isotope analysis from biogenic carbonate samples Oxygen isotope compositions of biogenic carbonates were measured using a MultiPrep™ automated preparation system coupled to an isotopic ratio mass spectrometer Isoprime™ in dual-inlet mode at the Laboratoire de Géologie de Lyon (UMR 5276, Université Claude Bernard Lyon 1). For each sample, two aliquots of about 400 μg calcium carbonate were automatically reacted with anhydrous oversaturated phosphoric acid at 90 °C for 20 min, according to the method developed by Swart et al. (1991). Measurements have been calibrated against the internal reference ‘Carrara Marble’ along with the international reference NBS18 and NBS-19 according to the protocol given by Werner and Brand (2001). Data are reported as δ18O values vs. V-PDB (in ‰ δ units). External reproducibility is ±0.1‰ (note: all mean values are given with a one standard deviation). When analysing carbonate samples, the Ionvantage software (Isoprime UK Ltd) automatically corrects isotopic data from isotopic fractionation occurring during calcium carbonate reaction with phosphoric acid. Ionvantage software considers by default calcite to react

with phosphoric acid. However, the phosphoric acid fractionation factor differs between the two calcium carbonate polymorphs calcite and aragonite. Kim et al. (2007) determined experimentally the phosphoric acid fractionation factors for calcite and aragonite between 25 °C and 75 °C and calculated the two following equations:   1000lnαCO2ðacidÞ−Calcite ¼ 3:59  103 =T −1:79

ð1Þ

  1000lnαCO2ðacidÞ−Aragonite ¼ 3:39  103 =T −0:83

ð2Þ

Considering Eqs (1) and (2), the δ18O values of aragonitic samples are overestimated by 0.409‰ for a 90 °C temperature of reaction. Aragonitic samples (2BAZ-29:31) were corrected accordingly. Biogenic calcite may incorporate magnesium which substitute with calcium cation in their crystal lattice reaching up to 45 mol% of MgCO3 (Long et al. (2014) and references therein). Oxygen isotope fractionation between calcite and water has been found to be linearly correlated to Mg content (Jiménez-López et al., 2004; Tarutani et al., 1969). Jiménez-López et al. (2004) calculated the δ18Oc values to increase by 0.17‰ per mol% of MgCO3, suggesting that palaeotemperatures inferred from high Mg-calcite (i.e. N 3–4 mol% MgCO3; Long et al., 2014) may be significantly underestimated. Bischoff et al. (1985) showed that halfwidth of the different Raman peaks of the calcite spectrum of synthetic Mg-calcites increased linearly with Mg content reflecting increasing positional disorder. Using the halfwidth of the bending mode peak around 713 cm−1 which is easily identified from the Raman spectra baseline (Cf. 3.2 Raman spectroscopy section hereunder and Fig. 4), we calculated the calcitic shell of the sea urchins Scutella leognanensis to contain 7 to 9 mol% Mg (Table S5) which are common values reported in the literature for echinoids (e.g. Berman et al., 1990; Bischoff et al., 1985; Chave, 1954; Pilkey and Hower, 1960). Accordingly, we corrected oxygen isotope composition by 0.17‰ per mol% MgCO3 (Jiménez-López et al., 2004) for the biogenic calcite 1BAZ25:27 (Table S5). 3.6. Estimating temperatures from oxygen isotope composition of biogenic apatites and carbonates 3.6.1. Estimating seawater temperatures Oxygen isotope compositions of apatite phosphate from marine organisms (sharks, rays, fish 1BAZ-1:15; 2BAZ-1:15) and oxygen isotope compositions of marine carbonates (sea urchins 1BAZ-25:27 and bivalves 2BAZ-29:31) were used to reconstruct seawater temperature. For biogenic apatites, seawater temperature was calculated using the Eq. (3) established by Lécuyer et al. (2013) (T(°C) = 117.4(± 9.5) − 4.50(± 0.43) × (δ18Op − δ18Oseawater); Table 1) and assuming that the oxygen isotope composition (δ18Oseawater) of the sea covering

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Fig. 4. Normalised and baseline corrected Raman spectra of sea urchins Scutella leognanensis (1BAZ-25:27) and of bivalves Megacardita jouanneti (2BAZ-29:31).

the southwestern part of France during the upper Burdigalian was +1.5 ± 0.5‰. Indeed, such high δ18Oseawater values are characteristics of present-day tropical seas (e.g. Carribean Sea) and closed marine settings (e.g. Oriental Mediterranean Sea, Red sea) which experience more evaporation than precipitation and are therefore 18O-enriched relative to V-SMOW (e.g. LeGrande and Schmidt, 2006). Moreover, Lécuyer et al. (1996) showed that the mid-Miocene epicontinental sea of northwestern France also had high δ18Oseawater values that fluctuated

between 0‰ and 1.5‰ (V-SMOW). In this study, δ18Op values measured for a whale (Balaena sp.) and a dugong (Metaxytherium sp.) (1BAZ22:23) led us to calculate consistent δ18Oseawater values of + 2.1‰ and + 1.9‰ (see Section 5.3.1.). As these δ18Oseawater values only rely on two samples, we chose to consider an intermediate value of +1.5 ± 0.5‰ for the mean δ18Oseawater value of the embayment. For biogenic calcites (1BAZ-25:27), seawater temperature was calculated using the Eq. (4) established by Anderson and Arthur (1983)

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Table 1 Published mineral-water isotopic fractionation equations used to calculate seawater temperatures and oxygen isotope compositions of surface waters (δ18Osurface water). Mineral-Water fractionnation equation

Material used to construct the equation

References

Samples to which the equation was applied

(3) T(°C) = 17.4(±9.5) − 4.50(±0.43) × (δ18Op − δ18Oseawater) (4) T(°C) = 16.00 − 4.14 × (δ18Oc − δ18Oseawater) + 0.13 × (δ18Oc − δ18Oseawater)2 (5) T(°C) = 20.6 – 4.34 × (δ18Oc − δ18Oseawater) (6) δ18Osurface water = –22.43 + 0.93 × δ18Op

Modern lingulids and sharks

Lécuyer et al. (2013)

1BAZ-1:15; 2BAZ-1:15

Modern biogenic calcites

1BAZ-25:27

Modern biogenic aragonites Modern elephants (Loxondonta africana and Elephas maximus) Modern rhinoceros Modern pigs

Anderson and Athur (1983) recalculated after McCrea (1950) and Epstein et al. (1953) Grossman and Ku (1986) Kovács et al., 2015 recalculated after Ayliffe et al. (1992) Tütken et al. (2006) Longinelli (1984)

Modern deers (Cervus elaphus)

D'Angela and Longinelli (1990)

1BAZ-19 ; 2BAZ-19:20; 23

Modern cetaceans

Ciner et al. (2016)

1BAZ-22; 23; 2BAZ-24

Modern crocodilians Modern chelonians

Amiot et al. (2007) Pouech et al. (2014) recalculated after Barrick et al. (1999) Royer et al. (2013)

1BAZ-24; 2BAZ-25 2BAZ-26:27

(7) δ18Osurface water = –19.15 + 0.76 × δ18Op (8) δ18Osurface water = (δ18Op – 22.71) / 0.86(±0.05) (9) δ18Osurface water = (δ18Op – 25.55) / 1.13(±0.14) (10) δ18Osurface water = –17.971(±0.605) + 0.95317(±0.03293) × δ18Op (11) δ18Osurface water = –19.13 + 0.82 × δ18Op (12) δ18Osurface water = –21.197(±0.755) + 0.994(±0.046) x δ18Op (14) δ18Osurface water = (δ18Op – 24.76(±2.70)) / 1.22(±0.20)

Modern rodents

recalculated after McCrea (1950) and Epstein et al. (1953) (T(°C) = 16.00–4.14 x (δ18Oc − δ18Oseawater) + 0.13 × (δ18Oc − δ18 Oseawater)2; Table 1). For biogenic aragonites (2BAZ-29:31), seawater temperature was calculated using the Eq. (5) established by Grossman and Ku (1986) (T(°C) = 20.6–4.34 × (δ18Oc − δ18 Oseawater); Table 1) which was successfully tested by Lécuyer et al. (2004b) on modern molluscs shells (Bivalvia, Gastropoda and Polyplacophora) from Martinique Island. Temperatures were also calculated assuming a same δ18Oseawater value of +1.5 ± 0.5‰. 3.6.2. Estimating terrestrial air temperatures Oxygen isotope compositions of apatite phosphate (δ18Op) from terrestrial vertebrates were used to reconstruct terrestrial air temperatures. Oxygen isotope compositions of surface water (δ18Osurface water) ingested by vertebrates were calculated using appropriate phosphate-water isotopic fractionation equations available ((6) δ18Osurface water = −22.43 + 0.93 × δ18Op, Kovács et al. (2015) for proboscideans 1BAZ-16; 21; 2BAZ-16; (7) δ18Osurface water = − 19.15 + 0.76 × δ18Op, Tütken et al. (2006) for rhinocerotids 1BAZ-17; 2BAZ-17; 21; (8) δ18Osurface water = (δ18Op − 22.71)/0.86(± 0.05), Longinelli (1984) for suoids 1BAZ-18; 2BAZ-18; (9) δ18Osurface water = (δ18Op − 25.55)/1.13(±0.14), D'Angela and Longinelli (1990) for artiodactyls 1BAZ-19; 2BAZ-19:20; 23; (10) δ18Osurface water = − 17.971(± 0.605) + 0.95317(± 0.03293) × δ18Op, Ciner et al. (2016) for marine mammals 1BAZ-22; 2BAZ-23:24; (11) δ18Osurface water = −19.13 + 0.82 × δ18Op, Amiot et al. (2007) for crocodilians 1BAZ-24; 2BAZ-25; and (12) δ18Osurface water = − 21.197(± 0.755) + 0.994(± 0.046) × δ18Op, Pouech et al. (2014) for chelonians 2BAZ26:27; Table 1). Mean annual temperature (MAT) were then estimated from calculated oxygen isotope composition of ingested waters assumed to derive from meteoric water (δ18Ometeoric water) using a global relationship proposed by Rey et al. (2013) from the Global Network of Isotopes in Precipitations (GNIP) data provided by the International Atomic Energy Agency-World Meteorological Organization (IAEA-WMO 2013): MAT



°

 C ¼ 1:978ð0:008Þ  δ18 Ometeoric water þ 26:414ð0:595Þ

2BAZ-29:31 1BAZ-16; 21; 2BAZ-16 1BAZ-17; 2BAZ-17; 21 1BAZ-18; 2BAZ-18

Samples from Héran et al. (2010)

4. Results 4.1. Raman spectroscopy The shell of sea urchin Scutella leognanensis (1BAZ-25:27) is made up of calcite whereas the external part of the shell of the bivalve Megacardita jouanneti (2BAZ-29:31) is made up of aragonite. The two polymorphs of CaCO3 are readily identified from the low frequency part of the Raman spectra. The lattice mode is characterized by two peaks located around 156 cm−1 and 282 cm−1 in the case of calcite (Fig. 4A) whereas it is characterized by five peaks around 155 cm−1, 180 cm−1, 209 cm−1, 217 cm−1 and 275 cm−1 in the case of aragonite (Fig. 4B). The splitting of the symmetric bending mode that occurs as a single peak around 713 cm−1 in the case of calcite (Fig. 4A) and as a doublet located around 702 cm−1 and 706 cm−1 in the case of aragonite is also clearly visible (Fig. 4B). 4.2. REE contents The REE contents of fossil biogenic apatites are reported in Table S1 and were normalised to the modified Post Archean Australian Shale (PAAS) values of McLennan (1989) to generate REE patterns (Fig. 5). REE patterns present a wide range of enrichment relative to the PAAS from ten times less to hundred times more (Fig. 5). The total REE concentration (ΣREE) ranges from 4.5 ppm to 8166.0 ppm (Table S1). For both localities, all the samples have similar REE patterns with a slight enrichment in heavy-REE (HREE) relative to light-REE (LREE) (Cf. (La/Yb)N ratios hereunder and in Table S1 and see also Fig. 6). All the samples have REE patterns with a negative Cerium anomaly (ΩCe) ranging from −0.66 to −0.21 (Table S1). (La/Sm)N and (La/Yb)N ratios have been calculated to generate the (La/Sm)N-(La/Yb)N diagram proposed by Reynard et al. (1999) (Fig. 6). (La/Sm)N ratios range from + 0.21 to +1.42 and (La/ Yb)N ratios range from + 0.06 to + 1.43 (Table S1). The locality of Mendouillet has on average higher (La/Sm)N and (La/Yb)N ratios than the locality of Monbalon-Miron (+ 0.49 vs + 0.37; t-test: p-value = 0.0704, not quite statistically significant and +0.26 vs + 0.19; t-test: pvalue = 0.0027).

ð13Þ 4.3. Oxygen isotope compositions of Burdigalian fossils

This relationship was applied to δ18Ometeoric water values derived from Miocene Hipparion teeth δ18Op values and provided relevant mean air temperature for the Vallesian-Turolian interval (Rey et al., 2013).

4.3.1. Oxygen isotope compositions of apatite phosphate (δ18Op) δ18Op values (V-SMOW) range from +16.7‰ to +24.4‰ and from + 17.6‰ to + 24.4‰ for the vertebrates at Bazas and Marimbault

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21

Fig. 5. Post Archean Australian Shale (PAAS)-normalised REE patterns of biogenic apatites from Mendouillet (A) and Monbalon-Miron (B). The modern REE pattern of the Atlantic Ocean near the Amazon Estuary (sampling site 901; Rousseau et al. (2015)) is given for comparison.

respectively (Table S2; Fig. 7). Sharks, rays and fish (1BAZ-1:15; 2BAZ1:15) have δ18Op values ranging from + 21.2‰ to + 24.4‰ and from + 21.0‰ to + 24.4‰ with mean values of + 22.6 ± 0.9‰ and + 22.3 ± 0.9‰ for the locality of Mendouillet and Monbalon-Miron, respectively. Terrestrial mammals (1BAZ-16:21; 2BAZ-16:23) have comparatively lower δ18Op values ranging from +16.7‰ to +20.9‰ and from + 17.6‰ to + 21.5‰ with mean values of + 19.2 ± 1.7‰ and + 19.3 ± 1.3‰ for the locality of Mendouillet and Monbalon-Miron, respectively. Marine mammals (1BAZ-22:23) have intermediate δ18Op values of + 20.8‰ and + 21.1‰ for the locality of Mendouillet. The semiaquatic seal (2BAZ-24) has a δ18Op value of + 18.3‰. Crocodiles (1BAZ-24; 2BAZ-25) have δ18Op values of + 21.7‰ and + 17.7‰ and the three turtles (2BAZ-26:28) from Monbalon-Miron have a mean δ18Op value of +21.5 ± 0.0‰ (Table S2).

4.3.2. Oxygen isotope compositions of carbonates (δ18Oc) δ18Oc values range from + 0.42‰ to + 0.61‰ for the sea urchins (1BAZ-25:27) collected at the locality of Mendouillet (Table S2), and from + 0.25‰ to + 0.42‰ for the bivalves (2BAZ-29:31) collected at the locality of Monbalon-Miron (Table S2). 5. Discussion 5.1. Diagenetic history REE contents of fossil marine biogenic apatites have been considered to reliably represent REE composition of past-seawaters, based on the assumption that biogenic apatites incorporate REE post-mortem from the overlaying water column, at the sediment-water interface, without

Fig. 6. Post Archean Australian Shale (PAAS)-normalised La/Sm vs La/Yb of biogenic apatites from Mendouillet and Monbalon-Miron. Modified after Reynard et al. (1999) and Trotter et al. (2016).

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J. Goedert et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 481 (2017) 14–28

Fig. 7. Oxygen isotope compositions of apatite phosphate samples from Mendouillet and Monbalon-Miron.

significant fractionation (e.g. Armstrong et al., 2001; Bertram et al., 1992; Felitsyn et al., 1998; Girard and Albarède, 1996; Girard and Lécuyer, 2002; Grandjean-Lécuyer et al., 1993; Grandjean et al., 1987; Grandjean and Albarède, 1989; Holser, 1997; Lécuyer et al., 2004a, Lécuyer et al., 1998; Picard et al., 2002; Reynard et al., 1999; Song et al., 2012; Wright et al., 1987, Wright et al., 1984; Zhao et al., 2013). However, this view has been recently challenged by Trotter et al. (2016) who emphasized the importance of pore-water control on the REE content of biogenic apatites. Indeed, several studies have shown that REE composition of fluids circulating within pore-waters shows a large variability related to the nature of the host-sediment and to the REE recycling process taking place close to the sediment-water interface and further within the subsea sediments. Consequently, the REE content of biogenic apatites does not always reflect the REE composition of the overlying seawater column (e.g. Abbott et al., 2015; Elderfield and Sholkovitz, 1987; German and Elderfield, 1990; Haley et al., 2004). All studied samples are characterized by HREE-enriched REE patterns that mimic those of present-day open seawater or waters in the mixing zones of estuaries (e.g. Elderfield et al., 1990; Elderfield and Greaves, 1982; Rousseau et al., 2015), while a negative Cerium anomaly reflects a well-oxygenated seawater column. This observation indicates that at a first order the fluids circulating in pore-waters have retained the REE signature of the overlying seawater column and that no critical fractionation took place between these two aquatic reservoirs. REE patterns of our biogenic apatite samples resemble those of the Amazon River at 901 sampling point of Rousseau et al. (2015), located on the coast at the seawater-influenced end of the Amazon estuary transect (Fig. 5). At both Miocene localities, vertebrates of terrestrial or marine origin have similar REE patterns, thus suggesting that they share a common early diagenetic history dominated by aqueous fluids of marine origin (Fig. 5). If vertebrate material of terrestrial origin had been reworked from older continental deposits, it would have likely exhibited a different REE pattern, as observed in several case studies (e.g. Fadel et al., 2015; Staron et al., 2001; Suarez et al., 2007; Trueman and Benton, 1997). This observation argues in favour of a contemporaneous origin for both terrestrial and marine vertebrate remains and preclude a possible reworking from underlying continental deposits as it has been observed in the mid-Miocene deposits of the Tréfumel-Le Quiou-St-Juvat Basin (Ginsburg and Janvier, 2000). It likely corresponds to carcasses of terrestrial animals that have been transported by rivers to the embayment. Reynard et al. (1999) proposed a model to account for the incorporation of REE in biogenic apatites either by substitution or adsorption mechanisms. REE incorporation by substitution mechanism was recognized to explain bell-shaped patterns resulting from extensive diagenesis, i.e. mineral recrystallization in the presence of an aqueous fluid.

These authors proposed a diagram representing the (La/Yb)N ratios in function of (La/Sm)N ratios in order to visualize samples having incorporated REE by substitution and presenting the so-called ‘bell-shaped’ REE patterns (Fig. 6). This adsorption-substitution model predicts that samples having (La/Sm)N ratios b 0.3 have incorporated REE during late diagenesis. Most of our samples have (La/Sm)N ratios N0.3. Some of them have (La/Sm)N ratios lower than 0.3, but always higher than 0.2, indicating that incorporation of REE by a substitution mechanism remained a process of minor importance. This interpretation is also supported by the absence of correlation between the total REE concentration (ΣREE) and the (La/Sm)N ratio (Fig. 8). Indeed, the adsorption mechanism only operates at mineral surfaces whereas the substitution mechanism involves the total volume of the crystal regarding the mode of incorporation of REE in apatite. Therefore, the substitution mechanism should result in the incorporation of a larger amount of REE than the adsorption mechanism. All of our samples have (La/Sm)N ratios in the range of coastal to oceanic waters and most of them also have (La/Yb)N ratios in the range of coastal to oceanic waters. The majority of samples coming from the locality of Monbalon-Miron has lower (La/Yb)N ratios (t-test: p-value = 0.0027) than those from the locality

Fig. 8. (La/Sm)N vs the sum of REE concentrations of biogenic apatites from Mendouillet and Monbalon-Miron.

J. Goedert et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 481 (2017) 14–28

of Mendouillet, suggesting that they could have incorporated REE from aqueous fluids reflecting more coastal or estuarine conditions (Fig. 6).

5.2. Preservation of original oxygen isotope compositions in biogenic apatites and carbonates 5.2.1. Biogenic apatites Secondary precipitation of apatite and isotopic exchange during microbially-mediated reactions may alter the primary isotopic signal (Blake et al., 1997; Zazzo et al., 2004a, 2004b). However, apatite samples selected in the framework of this study mainly consist in tooth enamel (see Section 3), which has a very low porosity and hence limited pore space for the precipitation of secondary minerals (Zazzo et al., 2004a, 2004b). Apatite enamel also has a very low organic matter content that makes it unsuitable for bacterial development. Apatite bone or dentine should be more prone to diagenetic alteration because hydroxyapatite crystallites of bones are smaller and less densely intergrown than those of enamels (Kolodny et al., 1996). However, even on a geological time scale, inorganic isotopic exchanges might not affect the oxygen isotope composition of phosphates as the covalent bond between oxygen and phosphorus in the phosphate group requires a lot of energy to break (Lécuyer et al., 1999). In this study, the main argument supporting the preservation of the pristine isotopic compositions of biogenic apatite samples is the systematic difference between the mean δ18Op value of marine vertebrates (sharks, rays and fish, 1BAZ-1:15; 2BAZ-1:15) and the mean δ18Op value of terrestrial vertebrates (1BAZ-16:21; 2BAZ-16:23), the former being significantly higher for both localities (+ 22.6 ± 3.8‰ vs + 19.2 ± 1.7‰ t-test: p-value b 0.0001 for the locality of Mendouillet and 23.6 ± 4.1‰ vs + 19.5 ± 1.3‰ t-test: p-value b 0.0001 for the locality of Monbalon-Miron; Fig. 7). This distribution of data is in good agreement with the oxygen isotope composition of environmental water from which apatite has precipitated and reflects the higher δ18Oseawater value compared to δ18Ometeoric water values. Furthermore, for the locality of Mendouillet, the mean δ18Op value of marine mammals (1BAZ22:23) is lower than those of coexisting sharks, rays and fish (+21.0 ± 0.2‰ vs +22.6 ± 3.8‰ t-test: p-value = 0.0225), a difference already observed by Lécuyer et al. (1996) for a near-contemporaneous marine assemblage collected from the Tréfumel-Le Quiou- St-Juvat Basin (France) and by Amiot et al. (2008) for a younger marine assemblage collected from the Pisco Formation (Peru). These observations are consistent with the fact that marine mammals, as endotherms, precipitate their apatite at higher body temperatures than coexisting sharks, rays and fish. It is worthy to note that the beaver sample has the lowest δ18Op value of terrestrial mammals which is consistent with a semi-aquatic ecology as it has already been observed in other vertebrates (e.g. Cerling et al., 2008; Goedert et al., 2016a). Semi-aquatic lifestyle implies enhanced water fluxes flushing through the body and less water loss through evaporation than fully terrestrial vertebrates. As expected, the beaver apatite phosphate is less 18 O-enriched than that of associated terrestrial vertebrates. The preservation of pristine oxygen isotopic signal is not inconsistent with the fact that biogenic apatite samples possibly incorporated a small fraction of their REE by a mechanism of substitution implying at least a partial recrystallization of the apatite (see Section 5.1.). Indeed, within the apatite lattice (Ca5(PO4)3(OH)) the incorporation of REE occurs at the cationic site [Ca2+] which is different from the anionic site [PO3− 4 ]. Goedert et al. (2016b) have measured the sulfur isotope composition (δ34S) of sulfates [SO2− 4 ] naturally occurring at the anionic site [PO34 −] in biogenic apatite samples coming from Mendouillet (1BAZ-8e; 8d; 17; 19; 23). They concluded that these sulfates have also retained their pristine oxygen isotope compositions, thus indicating the robustness of this anionic site regarding diagenetic processes.

23

5.2.2. Biogenic carbonates Comparatively, the covalent bond between oxygen and carbon in carbonate group is less strong and thus more prone to suffer from isotopic exchange. Apatite has also a very low solubility product (Ksp = 10– 54.45 at 25 °C) compared to calcite and aragonite polymorphs (Ksp = 10–8.48 and Ksp = 10–8.34 at 25 °C respectively). However, Raman analyses revealed that the bivalves Megacardita jouanneti preserved their original aragonitic mineralogy, which is a strong argument in favour of the preservation of the pristine isotopic signal. Indeed, aragonite crystal is highly unstable under higher temperature and pressure conditions than those of the Earth's surface, transforming it by epigenisation into its calcite crystal polymorph. Another argument is the good agreement between the phosphate and carbonate thermometers. Indeed, calculated seawater temperatures from δ18Op and δ18Oc are comparable within the uncertainties (cf. Section 5.3). 5.3. Interpretation of isotopic data in terms of temperatures 5.3.1. Seawater δ18O values and temperatures δ18Oseawater values calculated from the δ18Op of marine mammals are +2.1‰ and +1.9‰ for the Balaena sp. and Metaxytherium sp. samples, respectively (1BAZ-22:23). The seal Paleophoca nysti (2BAZ-24) is characterized by an intermediate δ18Osurface water value of −0.5‰. δ18 Osurface water values calculated for crocodile samples (1BAZ-24; 2BAZ-25) are –1.4‰ and −4.6‰ (Table S3). The two aquatic turtle samples (2BAZ-27; 28) whose living environment is unknown give positive δ18Osurface water values averaging +0.2 ± 0.0‰ which may be compatible with those of lagoon or estuarine environments (Table S3). Seawater temperatures calculated from the δ18Op values of sharks, rays and fish (all being marine species Cappetta, 2012; Nelson, 1994) using Eq. (3) range from 14 °C to 29 °C with a mean value of 23 ± 4 °C and from 15 °C to 30 °C with a mean value of 24 ± 4 °C for the locality of Mendouillet and Monbalon-Miron, respectively (Table S4). As shark teeth grow very fast over a reduced period of time (from one week to three months, Whitenack et al., 2011), the analysed teeth have probably grown during different periods of the year and thus have recorded different seawater temperature conditions. Moreover, as the selachian assemblage strongly suggests a shallow coastal environment, Carcharias and Rhinoptera being mostly found nowadays along the coasts at a depth not exceeding 25 m, the difference of water temperatures between seasons could be well marked (Ebert et al., 2013; Last and Stevens, 2009). The large lamniforms (Cosmopolitodus, Isurus, Anotodus and Otodus), as well as Aetobatus, having a more pelagic mode of life, could have been more occasional visitors of these shores. Seawater temperatures calculated from the δ18Oc values of the sea urchins Scutella leognanensis (1BAZ-25:27) and the bivalves Megacardita jouanneti (2BAZ-29:31) average 25 ± 1 °C and 26 ± 1 °C, respectively (Table S5). The sea urchin Scutella leognanensis is an extinct species belonging to the Order of Clypeasteroida which is an echinoid form with extremely flattened test, like the so-called ‘sand dollars’. Present-day flattened sea urchins generally live just beneath the surface of sandy or muddy areas of shadow coastal to deep water marine environments. Bivalves Megacardita jouanneti is also an extinct species belonging to the Family of Carditidae whose present-day genera are found in shallow coastal to deep water marine environments. It is worth noting that for the two localities, seawater temperatures derived from both the oxygen isotope compositions of sharks, rays and fish phosphate and urchins and bivalve carbonate are consistent with each other within the uncertainties. This is not surprizing as the phosphate-water, calcite-water and aragonite-water oxygen isotope fractionation equations were previously properly intercalibrated for temperature calculations (Lécuyer et al., 2013). Keeping in mind the related uncertainties in seawater temperatures, the slight difference between the two sites might reflect difference in distance to the coast, with Mendouillet displaying lower water temperature because of its more distal position than Monbalon-Miron. This difference in coastal

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proximity is also observed with La/Sm ratios which tend to be higher for the locality of Mendouillet, reflecting oceanic conditions (Fig. 6). Nonetheless, the geological exposition was not continuous between these two sites. We therefore cannot rule out the possibility that the fossils coming from these two sites record temperatures corresponding to slightly different period of times. For both sites, these high seawater temperatures are compatible with those of present-day tropical seas (e.g. Northern part of the Red Sea, Persian Gulf). They are in good agreement with seawater temperatures of the ‘Helvetian’ (Langhian-Serravallian) sea of northwestern France calculated by Lécuyer et al. (1996). Also based on oxygen isotope analyses of phosphate from sharks, fish and coexisting aquatic mammals, Lécuyer et al. (1996) calculated a seawater temperature of +20 ± 2 °C compatible with a sub-tropical climate. These data are therefore in good agreement with the Mid-Miocene Climatic Optimum taking place at the end of the Burdigalian and ending at the very beginning of the Serravallian (e.g. Zachos et al., 2001). The difference in regional seawater temperatures between the Aquitaine Basin and the Tréfumel-Le Quiou-St-Juvat Basin (Δ = 4 ± 6 °C; Table S6; Fig. 9) may be due to a difference in the geological age; the Miocene deposits from the localities of the Tréfumel-Le Quiou-St-Juvat Basin being younger than the Burdigalian ‘Molasses de l'Armagnac’ Formation. However, it may also correspond to the marine latitudinal thermal gradient of the mid-Miocene that is comparable to the present-day one existing between Arcachon and Brest seas (Δ = 2.3 °C; Table S6).

5.3.2. Air temperatures δ18Osurface water values calculated from the δ18Op of terrestrial vertebrates (1BAZ-16:19; 21; 2BAZ-16:23) range from −5.3‰ to −3.3‰ with a mean value of −4.3 ± 0.9‰ for the locality of Mendouillet and from −5.8‰ to −2.4‰ with a mean value of −4.7 ± 1.1‰ for the locality of Monbalon-Miron (Table S3). It is noteworthy that the terrestrial tortoise 2BAZ-28 attributed to Testudo sp. has a similar δ18Op value than measured aquatic turtles, but its terrestrial lifestyle results in important body water 18O–enrichment and δ18Op value that mimics the δ18Op values of estuarine turtles (Fig. 5). As no phosphate-water fractionation equation has been established for terrestrial turtles, sample 2BAZ-28 cannot be interpreted in terms of ingested local water δ18O value and will not be considered in subsequent discussion.

Using Eq. (13), continental air temperatures calculated from the δOsurface water values of terrestrial vertebrates (1BAZ-16:20; 21; 2BAZ-16:23) range from 16 °C to 20 °C with a mean value of 18 ± 2 °C for the locality of Mendouillet, and from 15 °C to 22 °C with a mean value of 17 ± 2 °C for Monbalon-Miron (Table S3). These average temperatures are observed today at subtropical to tropical latitudes. It is noteworthy that terrestrial material may have been transported from far inland areas and could correspond to animals living in different geographic areas. For both localities, mean annual air temperatures calculated from the δ18Op values of terrestrial vertebrates are not statistically different (18 ± 2 °C vs 17 ± 2 °C; t-test: p-value = 0.5886). Héran et al. (2010) have analysed the oxygen isotope compositions of phosphate from rodent teeth coming from southwestern German localities (near Munich) covering a long time period from Late Eocene to Late Miocene. For comparisons, we have selected the localities of Sandelzhausen (Sa), Puttenhausen (Pu), Adelschlag (Ad) and Bellenberg (Bb) all assigned to the basal mammalian stage MN5, corresponding to the end of the Burdigalian. For each locality, we have recalculated the corresponding δ18Osurface water values ingested by the rodents by using the recently established phosphate-water fractionation Eq. (14) based on extant rodents (δ18Osurface water = (δ18Op − 24.76(± 2.70))/1.22(±0.20), Royer et al., 2013; Table 1). The mean MAT calculated for the four German localities corresponding to the end of the Burdigalian is 14 ± 1 °C which is 4 ± 6 °C lower than the mean MAT calculated from the two French localities (Fig. 9; Table S7). This calculated continental thermal gradient is comparable to the present-day one existing between the cities of Bordeaux and Munich (Δ = 3.7 °C; Table S6). Reconstructed palaeotemperatures for the Late Burdigalian suggest that western Europe was warmer than today by about 9 ± 4 °C and 4 ± 2 °C in seawater and air temperatures, respectively. Moreover, continental thermal gradients seems to have been more pronounced than previously proposed (e.g. Costeur and Legendre, 2008; Jiménez-Moreno and Suc, 2007). According to our study and previous ones (e.g. Böhme, 2004, 2003; Böhme et al., 2006; Costeur and Legendre, 2008; Fauquette et al., 2007; Jiménez-Moreno and Suc, 2007), sizable thermal and humidity gradients characterized the environments of western Europe around 18 Ma, and would at least partly explain the distributions of faunas 18

Fig. 9. Mid-Miocene western Europe paleogeography (after Kowalewski et al. (2002)) showing reconstructed seawater and terrestrial air temperature of Aquitaine Basin, compared to seawater temperature of Tréfumel-Le Quiou-St-Juvat Basin (Lécuyer et al., 1996) and terrestrial air temperature of southern Germany Basin (Héran et al., 2010).

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and floras, and their inter-regional affinities (e.g. Costeur et al., 2004; Maridet et al., 2007). 6. Conclusions Rare earth element and stable oxygen isotope analyses of the mixed marine and continental fauna of the Late Burdigalian of the Aquitaine Basin led to the following considerations: REE analyses have shown that in the two localities of Mendouillet and Monbalon-Miron, marine and terrestrial fossil remains are contemporaneous and share a common diagenetic history. Calculated δ18Osurface water values clearly distinguish marine vertebrates (selachians, actinopterygians and marine mammals) living in warm and evaporated seawater (e.g. Oriental Mediterranean Sea, Red Sea) from freshwater terrestrial mammals whose values are compatible with tropical δ18Ometeroric values. Intermediate values calculated for seal, crocodiles and aquatic turtles may reflect brackish water and correspond to lagoon or estuary environments. Calculated seawater temperatures are +24 ± 4 °C respectively for the locality of Mendouillet (Bazas) and + 25 ± 4 °C for the locality of Monbalon-Miron (Marimbault), whereas terrestrial air temperatures are +18 ± 2 °C for the locality of Mendouillet and +17 ± 2 °C for the locality of Monbalon-Miron. These elevated palaeotemperatures calculated for mid latitude environments in Western Europe (paleolatitude ~ 45°N) are in agreement with the global Mid Miocene Climatic Optimum. Compared to other temperature estimates in northwestern France and southern Germany, continental and seawater thermal gradients seems to have been similar to present-day ones, at least at the western Europe scale, and likely played a dominant role in the distribution pattern of faunas and floras during the Late Burdigalian. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.palaeo.2017.04.024. Acknowledgments The authors would like to thank Claire, Louis, Marie and Paul Goedert for their help in collecting the fossil material and Thibault Gauduchon for helpful discussions concerning the ‘Big Teeth’. The authors also thank the SEPANSO for providing photos of the locality of Mendouillet during construction work of the Highway A65, Gilles Montagnac for his help to interpret Raman spectra and Philippe Telouk for the ICPMS facility. This study was supported by the CNRS INSU program InterVie, and the Institut Universitaire de France (CL). The Raman facility in Lyon is supported by the Institut National des Sciences de l'Univers (INSU). References Abbott, A.N., Haley, B.A., McManus, J., Reimers, C.E., 2015. The sedimentary flux of dissolved rare earth elements to the ocean. Geochim. Cosmochim. Acta 154:186–200. http://dx.doi.org/10.1016/j.gca.2015.01.010. Alroy, J., Koch, P.L., Zachos, J.C., 2000. Global Climate Change and North American Mammalian Evolution. Amiot, R., Lécuyer, C., Escarguel, G., Billon-Bruyat, J.-P., Buffetaut, E., Langlois, C., Martin, S., Martineau, F., Mazin, J.-M., 2007. Oxygen isotope fractionation between crocodilian phosphate and water. Palaeogeogr. Palaeoclimatol. Palaeoecol. 243:412–420. http:// dx.doi.org/10.1016/j.palaeo.2006.08.013. Amiot, R., Göhlich, U.B., Lécuyer, C., De Muizon, C., Cappetta, H., Fourel, F., Héran, M.-A., Martineau, F., 2008. Oxygen isotope compositions of phosphate from Middle Miocene–Early Pliocene marine vertebrates of Peru. Palaeogeogr. Palaeoclimatol. Palaeoecol. 264, 85–92. Anderson, T.F., Arthur, M.A., 1983. Stable isotopes of oxygen and carbon and their application to sedimentologic and paleonvironmental problems. In: Arthur, M.A., Anderson, T.F., Kaplan, I.R., Veizer, J., Land, L.S. (Eds.), Stable Isotopes in Sedimentary Geology: SEPM Short Course, pp. 1–151. Anderson, J.B., Shipp, S.S., 2001. Evolution of the West Antarctic ice sheet. West Antarct. Ice Sheet Behav. Environ. 45–57. Antoine, P.-O., Duranthon, F., Tassy, P., 1997. L'apport des grands mammifères (Rhinocérotidés, Suoidés, Proboscidiens) à la connaissance des gisements du Miocène d'Aquitaine (France). In: Biochro, M. (Ed.), Actes Du Congres, pp. 581–590. Armstrong, H.A., Pearson, D.G., Griselin, M., 2001. Thermal effects on rare earth element and strontium isotope chemistry in single conodont elements. Geochim. Cosmochim. Acta 65:435–441. http://dx.doi.org/10.1016/S0016-7037(00)00548-2.

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