Climate and sea-level variations along the northwestern Tethyan margin during the Valanginian C-isotope excursion: Mineralogical evidence from the Vocontian Basin (SE France)

Palaeogeography, Palaeoclimatology, Palaeoecology 302 (2011) 243–254

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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o

Climate and sea-level variations along the northwestern Tethyan margin during the Valanginian C-isotope excursion: Mineralogical evidence from the Vocontian Basin (SE France) Stéphanie Duchamp-Alphonse a,⁎, Nicolas Fiet a,b, Thierry Adatte c, Maurice Pagel a a b c

UMR CNRS 8148, IDES, University of Paris Sud-XI, 91405 Orsay Cedex, France AREVA, 75009 Paris, France Institut of Geology and Palaeontology, University of Lausanne, 1015 Lausanne, Switzerland

a r t i c l e

i n f o

Article history: Received 1 April 2010 Received in revised form 6 January 2011 Accepted 21 January 2011 Available online 26 January 2011 Keywords: Climate changes Sea-level changes Greenhouse conditions Valanginian Positive C-isotope shift Tethys

a b s t r a c t A high resolution mineralogical study (bulk-rock and clay-fraction) was carried out upon the hemipelagic strata of the Angles section (Vocontian Basin, SE France) in which the Valanginian positive C-isotope excursion occurs. To investigate sea-level fluctuations and climate change respectively, a Detrital Index (DI: (phyllosilicates and quartz)/calcite) and a Weathering Index (WI: kaolinite/(illite + chlorite)) were established and compared to second-order sea-level fluctuations. In addition, the mineralogical data were compared with the High Nutrient Index (HNI, based on calcareous nannofossil taxa) data obtained by Duchamp-Alphonse et al. (2007), in order to assess the link between the hydrolysis conditions recorded on the surrounding continents and the trophic conditions inferred for the Vocontian Basin. It appears that the mineralogical distribution along the northwestern Tethyan margin is mainly influenced by sea-level changes during the Early Valanginian (Pertransiens to Stephanophorus ammonite Zones) and by climate variations from the late Early Valanginian to the base of the Hauterivian (top of the Stephanophorus to the Radiatus ammonite Zones). The sea-level fall observed in the Pertransiens ammonite Zone (Early Valanginian) is well expressed by an increase in detrital inputs (an increase in the DI) associated with a more proximal source and a shallower marine environment, whereas the sea-level rise recorded in the Stephanophorus ammonite Zone corresponds to a decrease in detrital influx (a decrease in the DI) as the source becomes more distal and the environment deeper. Interpretation of both DI and WI, indicates that the positive C-isotope excursion (top of the Stephanophorus to the Verrucosum ammonite Zones) is associated with an increase of detrital inputs under a stable, warm and humid climate, probably related to greenhouse conditions, the strongest hydrolysis conditions being reached at the maximum of the positive C-isotope excursion. From the Verrucosum ammonite Zone to the base of the Hauterivian (Radiatus ammonite Zone) climatic conditions evolved from weak hydrolysis conditions and, most likely, a cooler climate (resulting in a decrease in detrital inputs) to a seasonal climate in which more humid seasons alternated with more arid ones. The comparison of the WI to the HNI shows that the nutrification recorded at the Angles section from the top of the Stephanophorus to the Radiatus ammonite Zones (including the positive C-isotope shift), is associated with climatic changes in the source areas. At that time, increased nutrient inputs were generally triggered by increased weathering processes in the source areas due to acceleration in the hydrological cycle under greenhouse conditions. This scenario accords with the widely questioned palaeoenvironmental model proposed by Lini et al., (1992) and suggests that increasing greenhouse conditions are the main factor that drove the palaeoenvironmental changes observed in the hemipelagic realm of the Vocontian Basin, during the Valanginian positive C-isotope shift. This high-resolution mineralogical study highlights short-term climatic changes during the Valanginian, probably associated to rapid changes in the C-cycle. Coeval Massive Paraña–Etendeka flood basalt eruptions may explain such rapid perturbations. © 2011 Elsevier B.V. All rights reserved.

1. Introduction

⁎ Corresponding author. Tel.: + 33 1 69 15 67 57; fax: +33 1 69 15 48 82. E-mail address: [email protected] (S. Duchamp-Alphonse). 0031-0182/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2011.01.015

Marine sediments of Valanginian age bear witness to significant changes in the ocean/atmosphere system associated with global palaeoceanographic and palaeobiological events. Valanginian sediments have been the focus for numerous authors in recent years due

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to a global positive δ13C excursion (Douglas and Savin, 1973; Patton et al., 1984; Weissert et al., 1985; Weissert and Channel, 1989; Weissert and Lini, 1991; Lini et al., 1992; Hennig et al., 1999; Adatte et al., 2001; Erba et al., 2004; Weissert and Erba, 2004; Gröcke et al., 2005; DuchampAlphonse et al., 2007; McArthur et al., 2007; Aguirre-Urreta et al., 2008; Bornemann and Mutterlose, 2008; Fozy et al., 2010; Nunn et al., 2010) at a time of widespread carbonate platform drowning (Schlager, 1981; Föllmi et al., 1994; Graziano, 1999; Wortmann and Weissert, 2000; Duchamp-Alphonse et al., 2007), eutrophication, and crisis for marine carbonate-producing biota (Lini et al., 1992; Bersezio et al., 2002; Erba and Tremolada, 2004; Erba et al., 2004; Duchamp-Alphonse et al., 2007). The intensification of the Paranà–Etendeka subaerial volcanism, which triggered excess CO2 in the atmosphere, has subsequently been related to these global palaeoenvironmental changes. This event occurred between ~ 138 and 130 Ma which, according to several geological time scales (Obradovich, 1993; Gradstein et al., 1994), places it within the Valanginian. In such an eventuality, it is assumed that the Paranà– Etendeka volcanic activity would be responsible for an acceleration of the hydrological cycle under greenhouse conditions, an increase in weathering, and a subsequent higher terrigenous and nutrient transfer from continents to oceans. This would lead to biocalcification crises in coastal marine ecosystems (Lini et al., 1992; Weissert et al., 1998; Erba et al., 2004; Duchamp-Alphonse et al., 2007). It is thought that the associated global increase of atmospheric CO2 would also generate chemical changes in the oceanic–sea-surface waters, acting either in neritic or open-sea pelagic environments to modify the biocalcification of carbonate producers (Weissert and Erba, 2004). However, in recent years this scenario has been questioned since: i) The temporal calibration of the Early Cretaceous is not accurate and it is difficult to demonstrate synchronicity, relationships, or interactions between major biological crises and magmatic emplacement. Large discrepancies in both absolute ages and relative duration of stages are observed between proposed time scales (Fiet et al., 2006). The Berriasian–Valanginian boundary (chron CM15) ranges between 131 Ma (Odin, 1994) and 140.2 Ma (Gradstein et al., 2004; Ogg et al., 2008), the duration of the Valanginian ranges between 3.8 and 8 Ma. According to these recent time scales, the highest Paranà–Etendeka volcanic activity either dated at 133–131 Ma (Courtillot et al., 1999), or more recently at 134. 6 ± 0.6 Ma with a rapid extrusion (b1.2 Ma; Thiede and Vasconcelos, 2010), would occur either during the (Early) Berriasian (Odin, 1994) or the (Early) Hauterivian (Gradstein et al., 2004; Ogg et al., 2008), but in any case, not during the Valanginian. ii) The Early Cretaceous global climate while assumed to have been durably warm, may contain cooler episodes. For example, the discovery in recent years of ice-rafted debris, coupled with new geochemical data and models, challenge the Valanginian greenhouse hypothesis. Further, Valanginian tillites and dropstones have been described in the Eromanga basin of Australia (Frakes et al., 1995; Alley and Frakes, 2003). In addition, stable isotope records from Valanginian belemnite rostra suggest cooler conditions compared to data from earlier and later time periods (Gröcke, 2001 in Gröcke et al., 2005; McArthur et al., 2004; Price et al., 2000). δ18Ocarb data obtained on high latitude belemnite rostra (Spitsbergen basin, 70°N) give an average low temperature of 8 °C (Ditchfield, 1997). δ18Ocarb data obtained from belemnite rostra and fish teeth located on the northwestern margin of Tethys both record a cooling of subtropical water-masses during the Late Valanginian, with average temperatures of 15 °C and 13 °C respectively (van de Schootbrugge et al., 2000; Pucéat et al., 2003). Based on δ13C data obtained from fossil plant material, Gröcke et al. (2005) documented a decrease in pCO2 during the Late Valanginian and suggests a cooler climate during the global C-isotope anomaly. McArthur et al. (2004, 2007) analysed

belemnite rostra from SE France and Italy that recorded both higher δ18Ocarb values and a decrease in the Mg/Ca ratio during the Late Valanginian (Verrucosum ammonite Zone). According to the authors, these results suggest a global cooling trend in response to the formation of substantial amounts of polar ice. Also, recently, Brassell (2009) interpreted the occurrence of steryl ethers in Early Valanginian sediments (NK3a calcareous nannofossil Zone) from the central Pacific as being a biological response to cooler temperatures that might suggest a global cooling during that time interval. Climate is the fundamental parameter of the model proposed by Lini et al. (1992) as it is linked to both geodynamic (Paranà–Etendeka volcanism) and stratigraphic events (eutrophication and biocalcification crises in marine ecosystems). In order to test the validity of this model and in the absence of a robust temporal calibration for the Early Cretaceous, it is crucial to define precisely the climatic conditions associated with the Valanginian C-isotope excursion. However, despite the ongoing importance of the debate on Valanginian climate variations, there are relatively few high-resolution studies that detail long-term climatic changes during the positive C-isotope shift. Moreover, climatic reconstructions are typically based on geochemical data while alternative climate proxies such as clay-mineral assemblages are less used to support interpretations. Geochemical data are usually obtained from belemnite rostra, however the precise stratigraphic position of these may be uncertain and diagenetic alteration may inhibit their use as temperature indicators. Thus, a high-resolution mineralogical approach may be useful for Valanginian climate reconstructions. Additionally, global climatic changes are usually associated with major sea-level fluctuations that may have a significant impact on both terrigenous/ nutrient transfer rates from continent to ocean and marine biocalcification. However, very few studies deal with sea-level fluctuations and associated palaeoenvironmental changes during the Valanginian, despite the occurrence of significant global second-order sea-level variations (Haq et al, 1987; Hardenbol et al., 1998). The objectives of this study are: i) to provide new insight into the weathering conditions of the north-western Tethyan realm during the Valanginian positive C-isotope shift, ii) to determine the factors that drove these palaeoenvironmental changes and iii) to test the model proposed by Lini et al. (1992) through a comparison of the palaeoclimatic record discussed herein and the palaeoceanographic data already obtained on the same samples (Duchamp-Alphonse et al. 2007). This study is based on bulk-rock and clay mineral data from the Angles section, which is located in the hemipelagic realm of the Vocontian Basin (SE France). Its palaeolocation close to the continent is ideal for such a study, because it provides intermediate sedimentological records between neritic carbonate platforms and pelagic realms characterized by a nearly continuous sedimentation. The essentially continuous nature of the succession offers a good opportunity to establish a high-resolution mineralogical record without any unconformity-induced artefacts. Since mineralogical data are sensitive to diagenetic processes, particular attention has been paid to the mineralogical assemblages in order to evaluate their potential diagenetic overprint.

2. Palaeogeographic setting of the Vocontian Basin (SE France) The Vocontian Basin is situated in the region of the “Alpes de Haute Provence” in south-eastern France (Fig. 1). During the Valanginian, it was a 150 km wide epicontinental sea situated in the north-western part of the Tethyan realm, located at a palaeolatitude of 25 to 30° N (Dercourt et al., 1993), and characterized by a palaeodepth of a few hundred metres (Donze, 1979; Wilpshaar et al., 1997). It was bounded by the Jura carbonate platform to the north and west and by the Provencal platform to the south. The Vocontian Basin was connected to the Ligurian Tethys to the east (Masse, 1993).

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Fig. 1. Late Jurassic to early Cretaceous paleogeographical map of the Subalpine Basin. The investigated site is the Angles section located in the hemipelagic realm of the Vocontian Basin.

3. Material and methods 3.1. Material The Angles section is defined as the Valanginian stage stratotype by Busnardo et al. (1979), the advantages of its continuous nature and diverse fossil assemblage are also highlighted; 96 samples have been collected from this section. The studied section is composed of alternated hemipelagic marl–limestones, well dated by ammonoids (Cotillon, 1971; Busnardo et al., 1979; Bulot and Thieuloy, 1994; Reboulet, 1995; Reboulet and Atrops, 1999), calcareous nannofossils (Manivit, 1979; Bergen, 1994; Gardin et al., 2000; Duchamp-Alphonse et al., 2007), and calpionellids (Allemann and Remane, 1979). Micropalaeontological data have recently been complemented by a high resolution C-isotope stratigraphy (Duchamp-Alphonse et al., 2007) (Fig. 2). The Vocontian marl–limestone alternations are interpreted as representing variations in carbonate productivity vs clay content and it is now accepted that rhythmic bedding (calcareous beds and marly interbeds) is linked to cyclic variations in Earth's orbit (Boulila et al., 2008). At Angles, a high-resolution spectral analysis study of the variations in carbonate contents and colour intensity carried out over the Valanginian interval revealed the presence of orbital frequencies in precession and obliquity (Giraud et al., 1995). The precession signal (~21 ka) is the dominant forcing factor during the limestone-dominant alternations of the Lower Valanginian, and obliquity (~40 ka) is the most clearly defined signal during the marly-dominant alternations of the Upper Valanginian. As the aim of this work is to reconstruct long-term climatic variations superimposed on the astronomical signal, and as calcareous beds are absent at Angles during the Late Valanginian, samples were only taken from the bulk marly-interbeds (the darkest part of the interbeds), with a constant temporal step of around 100 ka over the whole section (see Duchamp-Alphonse et al., 2007 for more details). 2 samples were collected in certain 100 ka intervals in order to

better constrain mineralogical data, leading to a resolution of approximately 50 ka. This strategy allows analysis of a quasi-homogeneous lithology and therefore minimizes the lithological artefacts associated with astronomical climatic changes, mainly expressed by the 21 kyr and 40 kyr cyclicity in this study. 3.2. Methods For mineralogical analyses, approximately 20 g of each rock sample were coarsely crushed in a “jaw” crusher, dried at a temperature of 110 °C, and crushed again in an agate mortar to obtain a fine, homogeneous and random powder of the bulk rock with particles b40 μm. 3.2.1. Bulk mineralogy Bulk rock mineralogy samples were prepared and analysed at the Geological Institute of the University of Neuchâtel, following the procedure described by Adatte et al. (1996) after Ferrero (1965, 1966), Klug and Alexander (1974) and Kübler (1983). About 800 μg of bulk rock powder were pressed into a powder holder covered with blotting paper at a pressure of 20 bar, then analysed by XRD (SCINTAG XRD 2000 Diffractometer). The bulk mineralogy of sediments was determined by semi-quantitative measurements, using X-ray diffraction peak intensities of the main minerals present (Ferrero, 1966; Kübler, 1983) compared with external standards. Analytical uncertainties vary between 5 and 10% for phyllosilicates and 5% for grain minerals. 3.2.2. Clay mineralogy Clay mineralogy samples were prepared following the procedure described in detail by Colin et al. (1999) at the IDES Laboratory (Interaction et Dynamique des Environnements de Surface) of the University of Paris Sud. Samples were treated with diluted hydrochloric acid (HCl) and hydrogen peroxide (H202) to remove carbonate and organic matter. Clay deflocculation was done by successive washing in

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Fig. 2. Bulk rock mineralogy of the Angles section (%) plotted against: the lithological column of the Angles section from the Upper Berriasian to the Lower Hauterivian; Ammonite biostratigraphy from Bulot and Thieuloy (1994) in which Berrias p.p. = Berriasian p.p., H. p.p. = Hauterivian p.p., Bois. p.p. = Boissieri p.p., Pertrans. = Pertransiens, Ino = Inostranzewi, Cal. = Callidiscus, Rad = Radiatus, alp. = alpillensis, oto. = otopeta, th. = thieuloyi, hir. = Hirsutus, subcam. = Subcampylotoxus, campylo. = Campylotoxus, verruco. = Verrucosum, prone. = Pronecostratum, peregri. = Peregrinus, cal. = Callidiscus; and C-isotope stratigraphy (‰) from Duchamp-Alphonse et al. (2007). Note that bulk rock mineralogy is obtained from the marl interbeds and is composed, in decreasing quantities of: calcite, phyllosilicates quartz, plagioclase and feldspar. Plagioclase and feldspar contents are not represented in this figure since they are absent or very rare (respective averages of 1 and 0.5%). Periods 1, 2 and 3 (and related subperiods) are based on the vertical distribution of the bulk-rock minerals and the C-isotope stratigraphy (see text for details). Shaded bands highlight the periods described in the results (Section 4).

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distilled water. The b2 μm clay fraction was then separated from the bulk samples by settling according to the Stoke's law (Galehouse, 1971). Oriented slides were prepared from the concentrated clay suspensions by pipetting onto glass slides and allowing them to dry at ambient temperature. Three X-ray diagrams were measured using a PANalytical Diffractometer: one after air-drying, one after ethylene–glycol solvation for 24 h and one after heating at 490 °C for 2 h. Clay minerals were identified based on the position of the (001) series of basal reflections on the 3 X-ray diagrams. Semi-quantitative measurements were performed on the glycolated curve with the MacDiff software (Petschick, 2000) using the main X-ray diffraction peaks of each mineral [smectite, including mixed-layers (15–17 Å); illite (10 Å); and kaolinite/chlorite (7 Å)]. Relative proportions of kaolinite and chlorite were determined based on the ratio of the 3,5/3,54 Å peak areas. Analytical uncertainties are estimated to be 2%. 4. Results 4.1. Bulk mineralogy Minerals identified include calcite, phyllosilicates and quartz, with accessory minerals such as plagioclase and feldspar (Fig. 2). Carbonate and phyllosilicates are the dominant minerals with proportions ranging from 36 to 74% (average of 58%) and from 17 to 44% (average of 27%) respectively. Quartz is generally less abundant, ranging from 1.5 to 15%, with an average of 9%. Plagioclase and feldspar are rare or absent, with averages of 1 and 0.5% respectively. These two minerals will not be discussed further. Bulk rock mineral type shows a distinctive evolution during the Valanginian time interval. Three major successive periods (periods 1, 2 and 3) have been identified. Periods 1 and 3 have been further subdivided into 2 subperiods (Fig. 2): 1) Period 1 ranges from the top of the Boissieri ammonite Zone to the Stephanophorus ammonite Zone. It is characterized by variable proportions of calcite, phyllosilicates and quartz. Calcite content tends to decrease in the Pertransiens ammonite Zone (from 74 to 50%, subperiod 1a) then values significantly increase in the Stephanophorus Zone (to 68%, subperiod 1b). The phyllosilicates content curve shows an inverse trend with increased values in the Pertransiens Zone (from 19 to 35%; subperiod 1a) followed by a decrease in the Stephanophorus Zone (to 18%, subperiod 1b). Quartz content mimics the phyllosilicates trend with slight variations in amplitude (from 2 to 11%). 2) Period 2 corresponding to the Stephanophorus–Verrucosum ammonite Zones that includes the positive C-isotope shift shows the most striking feature in the bulk mineralogy evolution, with a long-term decreasing trend of the calcite content (Fig. 2). In particular, calcite proportion decreases pronouncedly from 68 to 46% at the top of the Stephanophorus Zone (Campylotoxus ammonite Subzone), increases in the Inostransewi ammonite Zone (to 69%) and finally drops off again at the base of the Verrucosum Zone (to 48%). Phyllosilicates proportion again mirrors carbonate content and increases from 18 to 30%. Quartz slightly increases from 6 to 10%. 3) Period 3 corresponds to the Verrucosum to Radiatus ammonite Zones (Fig. 2). It has a comparatively large sedimentary thickness marked by fairly abundant calcite, the proportion of which tends to increase up-section (from 49 to 68%), with the exception of the Furcillata– Callidiscus ammonite Zones in which it is less abundant (average of 47%, subperiod 3b). These calcite trends are accompanied by variable amounts of phyllosilicates, decreasing from 30 to 20% in the Verrucosum to Trinodosum ammonite Zones (subperiod 3a). Phyllosilicate content is significantly higher in the Furcillata–Callidiscus Zones to the detriment of calcite (average of 35%, subperiod 3b). Quartz proportion slightly but gradually increases from 9 to 15%, then decreases in the Callidiscus–Radiatus Zones (to 8%).

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4.2. Clay mineralogy Clay mineral assemblages are mainly composed of illite, kaolinite, smectite, chlorite and illite-smectite mixed layers (Fig. 3). As for bulk rock mineralogy, the vertical distribution of clay-mineral assemblages is not random and the same 3 successive mineral assemblage time intervals (and associated sub-intervals) can be identified within the succession (Fig. 3): 1) Period 1 ranges from the top of the Boissieri to the Stephanophorus Zones. It is characterized by variable clay assemblages with large amounts of illite, as well as by the presence of significant amounts of kaolinite. Smectite, illite–smectite mixed layers and chlorite are less abundant. Illite gradually increases from 33% at the base of the Pertransiens Zone to 67% in the Stephanophorus Zone (Subcampylotoxus Subzone; subperiod 1a), it then decreases slightly (reaching 45% at the top of the Stephanophorus Zone (subperiod 1b)). Kaolinite mirrors illite's trend and increases from 20 to 45% in the Pertransiens Zone, achieves maximum values during the Subcampylotoxus Zone (subperiod 1a), and then decreases again to 20% in the Stephanophorus Zone (subperiod 1b). Smectite sporadically occurs with fairly high proportions in the Stephanophorus Zone, reaching 26% of the assemblages (Campylotoxus Zone; subperiod 1b). The illite–smectite mixed layer proportion drastically decreases from 44 to 3% (subperiod 1a), then stays relatively low (subperiod 1b). Chlorite is occasionally present with higher proportions (average of 5%) at the top of the Stephanophorus Zone. 2) Period 2 ranges from the top of the Stephanophorus Zone to the Verrucosum Zone and corresponds to the positive C-isotope shift (Fig. 3). It is marked by a significant increase in kaolinite content, ranging from 11 to 42%, with an average of 39%. Illite is generally less abundant compared to period 1, with a sharp decrease from 73 to 37%. Illite–smectite mixed layers slightly increase from 4 to 15%. Chlorite remains relatively low, never exceeding 12%. Smectite is absent. 3) Period 3 correlates with the Verrucosum to Radiatus Zones (Fig. 3). As for period 1, clay assemblages are variable but are generally relatively illite rich (highest value of 77% observed in the Radiatus Zone, subperiod 3b), contain moderate amounts of kaolinite (longterm decreasing trend from 42 to 6% in the Verrucosum– Trinodosum Zones, subperiod 3a), and low but constant proportion of illite–smectite mixed layers (that never exceed 10% except in the Furcillata Subzone (15%)). This time interval is marked by the presence of smectite, which significantly increases at the base of the Trinodosum Zone, and reachs 45% of the clay assemblages in the Furcillata and Radiatus Subzones (subperiod 3b). 5. Discussion 5.1. Diagenetic overprint In marine sediments, relative changes in clay mineral assemblages may record palaeoenvironmental (mainly climatic and eustatic) and/or diagenetic changes. Thus, before any palaeoenvironmental interpretation of clay mineral assemblages is made, it is necessary to distinguish the detrital and authigenic clays and estimate their potential diagenetic overprint. Authigenesis of clay minerals can be either a synsedimentary (e.g. glauconite formation) or post-sedimentary process (e.g. smectite into illite transformation), through fluid flow and burial diagenesis. The occurrence of authigenic clay minerals is more often observed in highly porous lithologies promoting fluid circulations, such as sandstones. Argillaceous sediments such as the Angles marls are less prone to authigenic mineral development. Previously published studies on the clay mineral distribution of the Mesozoic series of the Vocontian Trough document an increasing diagenetic impact eastward as a result of the Alpine orogeny (Deconinck and Chamley, 1983; Ferry et al., 1983).

248 S. Duchamp-Alphonse et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 302 (2011) 243–254 Fig. 3. Clay mineralogy of the Angles section (relative %) plotted against: the lithological column of the Angles section (marl–limestone alternations, SE France) from the Upper Berriasian to the Lower Hauterivian; Ammonite biostratigraphy from Bulot and Thieuloy (1994), see Fig. 2 for abbreviations; and C-isotope stratigraphy (‰) from Duchamp-Alphonse et al. (2007). Note that clay mineralogy is obtained from the marl layers which show the following composition, in decreasing order of: illite, kaolinite, smectite, illite–smectite mixed layers and chlorite. Periods 1, 2 and 3 (and related subperiods) are based on the vertical distribution of the clay-minerals and the C-isotope stratigraphy (see text for details). Shaded bands highlight the periods described in the results (Section 4).

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Burial diagenesis would be responsible for the usual replacement of smectite by chlorite in the calcareous beds and illite in the marly interbeds (Chamley, 1989; Deconinck, 1993; Lanson and Meunier, 1995). However, at the Angles section, the diversity of the clay minerals and the lack of any continuous vertical trend in clay mineral composition within the 240 m-thick succession (Fig. 3) suggests that the clay mineral diagenesis driven by burial effects is weak (Kisch, 1983; Chamley et al., 1997) and that relative variations in the mineralogical assemblage most likely reflect a primary signal driven by palaeoenvironmental changes. The two sporadic appearances of smectite, reaching highs of 26% and 45% of the assemblages in the Stephanoporus and the Furcillata and Radiatus Zones respectively, support this interpretation (Fig. 3). Recently, Fesneau et al. (2009) recognized a bentonite horizon in the Vocontian Basin in the Subcampylotoxus Subzone; Campylotoxus Zone according to Hoedemaeker (1995). This bentonite is detected in both La Charce and Vergol sections and consists of a centimetre-thick ochre-layer, characterized by an enrichment of both well-crystallized smectite and trace elements that exhibit a magmatic affinity (Zr, Th, Y…). At the Angles section, the Subcampylotoxus Zone is associated with the appearance of smectite. A volcanic origin for this smectite is unlikely since: i) the coeval ochrecoloured horizon has not been sampled; ii) at Angles, during the Early Valanginian, the smectite is present from the base of the Subcampylotoxus Zone to the top of the Campylotoxus Zone, corresponding to a 33-m-thick sedimentological unit, lasting up to 1.5 Ma (according to a cyclostratigraphic study of the Angles section (Duchamp-Alphonse, unpublished data)); and iii) contrary to Fesneau et al. (2009), this smectite enrichment correlates with a decrease in the abundance of elements having a magmatic affinity, such as Zr, Th and Ti (Duchamp-Alphonse, unpublished data). Several other datasets obtained at the Angles section support a weak diagenetic overprint at the Angles section. For the Valanginian, Duchamp-Alphonse et al. (2007) present well-preserved geochemical data (C- and O- isotopes as well as major and trace elements) and calcareous nannofossil assemblages that are not significantly affected by etching or secondary calcite overgrowth. From the Late Hauterivian to the Early Aptian, Godet et al. (2008) demonstrate that clay-mineral assemblages from Angles did not suffer from a significant diagenetic overprint. From the Hauterivian to Aptian, Godet (2006) and Bodin et al. (2009) interpret bulk-rock δ18O signal variations as a palaeoenvironmental and palaeoclimatic signal, arguing that the long term δ18O trends are comparable to the ones recorded in other regions. Bodin et al. (2009) also document belemnites with well-preserved isotopic and elemental compositions. These results, combined with those presented in the present paper suggest that the clay mineral assemblages of the Angles section, and, notably, the kaolinite, did not undergo strong diagenetic alteration; therefore, their trends can be used as palaeoenvironmental proxies. 5.2. Palaeoenvironmental changes The distinct trends occurring in the bulk and clay mineralogies of the Angles section are interpreted as reflecting variations in the weathering that prevailed in the perivocontian source areas. During the Valanginian, the nature and intensity of weathering are mainly the products of interactions between climate (rainfall and temperature in particular), topography, tectonic activity linked to the structural evolution of margins and associated sea-level changes (Chamley, 1989; Weaver, 1989). 5.2.1. Sea-level changes Sea-level changes are commonly recognized in the field using a sequence stratigraphic approach combined with bulk rock and clay mineral analysis. A detailed sequence stratigraphic study of the Angles section has previously been published by Arnaud-Vanneau et al. (1982). It is herein compared to the global sea-level curve proposed by Hardenbol et al. (1998), and the mineralogical results obtained in this

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study. A Detrital Index (DI: detritus (phyllosilicates and quartz)/calcite ratio) and the relative proportions of illite and kaolinite compared to smectite have been used (Figs. 3 and 4). Both represent useful indicators of sea-level variations (Chamley et al., 1983; Deconinck et al., 1985; Adatte et al., 2002): 1) A decrease of the DI generally reflects a more distant detrital source and decreased erosion. Thus, it reflects a higher sea-level or deeper water conditions (Adatte et al., 2002). An increasing DI indicates a more proximal detrital source, increased erosion associated to increased accumulation of detrital matter, and, consequently, a lower sea-level or shallower water environment. 2) Differential flocculation and settling play an important role in determining the distribution of clay minerals in sediments. As illite and kaolinite are dense minerals, they settle more rapidly than other clay minerals and tend to be deposited nearest the shore in shallow water settings. Conversely, smectite tends to be particularly finegrained and settles much more slowly. It remains dispersed for longer and tends to settle in deeper waters in more offshore settings. As a consequence, the relative proportions of illite and kaolinite compared to smectite have been used to infer shallowing/regressive and deepening/transgressive episodes (Hallam, 1975; Chamley et al. 1983; Deconinck et al., 1985). In their sequence stratigraphic study of the Angles section, ArnaudVanneau et al. (1982) recognized major second order sea-level variations during the Early Valanginian (Pertransiens to Stephanophorus Zones, period 1, see Section 4), and a durable and stable highstand sea-level characterizing the late Early Valanginian up to the base of the Hauterivian (top of the Stephanophorus to the Radiatus Zones, periods 2 and 3, see Section 4 and Fig. 3). The Pertransiens Zone is marked by a major sea-level fall (subperiod 1a) followed by a major rise during the Stephanophorus Zone, the onset of which occurs at the Hirsutus–Subcampylotoxus Subzone transition (subperiod 1b). This interpretation is corroborated by the global sea-level curve obtained by Hardenbol et al. (1998) and is also supported by the mineralogical data obtained in this study. The most significant sealevel fluctuations recorded during the Early Valanginian are in agreement with the DI and the illite and kaolinite vs. smectite proportions (Figs. 4 and 5). During the Pertransiens–Stephanophorus Zones (subperiod 1a), the observed sea-level fall is associated with a sharp increase of the DI (from 0.33 to 0.83), an absence of smectite, and a progressive rise of illite and kaolinite contents (from 33% to 67% and from 20% to 45% respectively; Fig. 3). The sharp reduction of the calcite proportion relative to the terrigenous inputs, and the augmentation of the dense clay minerals relative to the fine-grained clay-minerals suggest shallower conditions and a more proximal terrigenous source (Adatte et al., 2002). This trend reflects an increase of continental erosion during a sea-level lowstand. Similarly, in the Stephanophorus Zone (subperiod 1b), mineralogical data fit the sedimentological and lithological observations. During this time interval, the major sea-level rise identified by Arnaud-Vanneau et al. (1982) and Hardenbol et al. (1998), corresponds to a relative decrease of the DI (from 0.83 to 0.36), the appearance of smectite (up to 26%) and a significant decrease in illite and kaolinite contents (to 45% and 20% respectively, Figs. 3 and 4). This distribution reflects a deeper depositional environment, and a more distal detrital source due to high sea-level. In conclusion, during the Pertransiens to Stephanophorus Zones (Lower Valanginian, Period 1), the distribution of the detrital material from the Angles section (bulk and clay minerals of the marl interbeds) is mainly influenced by second order sea-level changes, while climate influence is only of minor importance. In contrast, from the top of the Stephanophorus Zone to the Radiatus Zone, since second-order sealevel changes are not significant, the mineralogical record is inferred to mostly reflect climate changes in the source areas. 5.2.2. Climate changes In contrast to the Pertransiens–Stephanophorus Zones, during the Stephanophorus–Radiatus Zones interval (uppermost Lower Valanginian

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Fig. 4. Sea-level, climate and terrigenous input variations during the Valanginian at the Angles section inferred from mineralogical ratios (this study) and sequence stratigraphy study (local sea-level curve from Arnaud-Vanneau et al., 1982) (A) compared to the global sea-level curve from Chart 1 of Hardenbol et al. (1998) (B)). The ratio of quartz and phyllosilicates (main terrigenous components) to calcite corresponds to the Detrital Index (DI). Palaeoclimatic reconstructions are based on the Weathering Index (WI: kaolinite/(illite + chlorite) ratio). During the Early Valanginian, both the second-order sea-level changes inferred from sequence stratigraphy and mineralogical data support high amplitude sea-level changes. During this time interval, a decrease in the DI reflects decreased detritus vs carbonate, a more distant detrital source and a higher sea-level; whereas an increase in the DI indicates an increase in detritus, a more proximal terrigenous source and a lower sea-level. As the sequence stratigraphic studies (sea-level curves from Arnaud-Vanneau et al., 1982 (A) and Hardenbol et al. (1998) (B)) document a durable and stable sea-level highstand for the Late Valanginian, it is assumed that mineralogical ratio variations are linked to palaeoclimatic changes during this time interval. The decreased detritus ratio testifies of lower terrigenous inputs whereas increasing trend indicates a higher terrigenous supply. At this time, increased WI indicates a wetter and probably warmer climate (higher hydrolysis), whereas a decreasing trend indicates a drier and probably cooler climate (lower hydrolysis). Shaded bands highlight the periods described in the results (Section 4).

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Fig. 5. Sea-level, climate and nutrient input variations during the Valanginian at the Angles section inferred from Weathering Index (WI, this study), Nutrient Index (HNI based on the calcareous nannofossil fertility indicators ratio from Duchamp-Alphonse et al., 2007), and sequence stratigraphy study (local sea-level curve from Arnaud-Vanneau et al., 1982) (A) compared to the global sea-level curve of Hardenbol et al. 1998) (B)). Increased WI indicates a wetter and probably warmer climate (higher hydrolysis conditions), whereas a decreasing trend documents a drier and probably cooler climate (lower hydrolysis conditions). Higher HNI values testify of higher water fertility and vice versa. Shaded bands highlight the periods of fairly good correlation between the WI and the HNI for the Late Valanginian.

to Lower Hauterivian, Periods 2 and 3) the Vocontian Basin is characterized by a steady and long-term sea-level highstand (ArnaudVanneau et al., 1982; Hardenbol et al., 1998; Fig. 4). Detrital input changes linked to sea-level fluctuations are therefore not significant, and mineralogical variations are mainly driven by palaeoclimatic variations which influence the type of weathering and the intensity of pedogenesis (Chamley, 1989). Thus, the DI is reflecting fluctuations in terrigenous supply linked with changes in hydrolysis conditions. In addition, the Weathering Index (WI: kaolinite/(illite + chlorite) ratio) and the occurrence vs absence of smectite (Figs. 3 and 4) are used. Kaolinite mainly forms during highly hydrolytic weathering reactions in warm humid climates (Chamley, 1989) whereas mica and chlorite are the common by-products under cool to temperate, dry conditions with low hydrolysis (Singer, 1984; Chamley, 1989). Abundant illite reflects minimum hydrolyzing conditions i.e. physical weathering (Chamley, 1989; Robert and Chamley, 1990) under either cold or dry conditions. WI will therefore show fluctuations between warm humid and cool to temperate, dry climate conditions. The relationship between smectite formation and climate is more uncertain. Smectite originates either from tropical soil under semi-arid and seasonal climatic conditions or as a weathering by-product of basalt (Singer 1984; Chamley, 1989). The presence or absence of smectite will therefore give detailed insights on climate seasonality. Based on the changes in vertical distribution of δ13C and bulk and clay mineralogies described above, two major climatic episodes can be distinguished in the Stephanophorus–Radiatus Zones (Fig. 4): 1) The first major climatic episode corresponds to the period 2, which shows the most striking feature of the C-isotope and mineralogical evolutions (Fig. 4). The bulk rock positive C-isotope excursion

(amplitude of + 1.5‰) coincides with a significant increase in both DI (from 0.42 to 0.83) and WI indices (from 0.18 to 0.95). Smectite is absent (Fig. 4). These results suggest a significant increase of terrigenous input under a warm and humid climate. Since both DI, and WI indices reach their highest values (0.83, and 0.95 respectively) during the δ13C shift, it is assumed that the peak of the C-isotope excursion coincides, at a regional scale, with the most humid and warmest conditions of the (Late) Valanginian. This result is not inconsistent with the discovery of ice-rafted debris in Australia, during the Valanginian (Frakes et al., 1995; Alley and Frakes, 2003) implying the presence of polar ice. Further, as shown by the palaeobiogeographical distributions of both foraminifera and calcareous nannofossils, more pronounced latitudinal temperature gradients might have characterized the Valanginian (Mutterlose and Kessels, 2000; Mutterlose et al., 2003), which is consistent with glaciation (Gröcke et al., 2005). It is thus assumed, as has already been proposed by Price (1999) for the Mesozoic, that the Earth's climate regime during the Valanginian, and especially during the Valanginian positive Cisotope excursion, could have been characterised by a relatively steep pole-to-equator temperature gradient, where low-latitude regions (the Angles section for example) were warmer than today, and high-latitudes regions experienced cold or sub-freezing conditions. 2) The second major climatic episode corresponds to period 3, representing a greater amount of time and marked by relatively high C-isotope values, with a gradual upward decreasing trend. Detrital inputs and weathering are variable, reflecting an overall unstable climate (Fig. 4). However, long-term general trends observed in subperiods 3a and 3b present some differences.

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In subperiod 3a, the mineral assemblages are characterized by: i) a decrease in the DI from 0.83 to 0.43 in the Pronecostratum Subzone (Verrucosum Zone) and fairly steady low DI values (average of 0.6) in the Peregrinus–Nicklesi Subzone; ii) a significant decrease of the WI, from 0.95 to 0.11 in the Trinodosum Zone; and iii) a near absence of smectite (Fig. 4). These trends reflect a gradual change from a warm humid climate to a more temperate climate with lower hydrolysis conditions in the source areas and, consequently, lower terrigenous inputs into the Vocontian Basin. This interpretation implies a decrease in moisture/rainfall, and, most likely, a drop in temperature; this latter is supported by previous geochemical and paleontological data from the Vocontian Basin that point to decreasing temperatures during the Verrucosum Zone (van de Schootbrugge et al., 2000; Pucéat et al., 2003; McArthur et al., 2004, 2007) and highlight the presence of Boreal ammonites and calcareous nannofossils respectively (Reboulet and Atrops, 1999; Reboulet, 1995; Melinte and Mutterlose, 2001). In subperiod 3b, the mineral assemblages are characterized by: i) an increase of the DI to 1.66 in the Callidiscus Zone; ii) an increase in WI values (to 0.83, Nicklesi Zone); and iii) the presence of fairly abundant smectite, reaching 45% of the clay assemblages in the Trinodosum and Radiatus Zones (Fig. 4), implying that a seasonally contrasted climate (more humid vs more arid periods) prevailed during the Trinodosum to Radiatus Zones. At the smaller scale, DI and WI show similar trends which can be explained by alternations of humid seasons triggering higher detrital inputs with more arid periods characterized by lower terrigenous inputs. Therefore, the mineralogical patterns described for period 3 probably reflect a period of unstable climate, linked to an overall chain of feedback mechanisms induced by the strong greenhouse conditions which prevailed during the positive δ13C excursion. Given that the duration of such climatic changes does not exceed a few millions of years, they are assumed to be closely linked to “short-term” perturbations in the C-cycle that could be induced by the Paranà–Etendeka volcanism and the associated CO2 degassing in the atmosphere as proposed by Lini et al. (1992). Moreover, this assumption is not inconsistent with the record of a cooler episode during the Verrucosum Zone. Indeed, rapid cooling events have already been documented in the aftermath of greenhouse warming associated with the formation of subsequent Large Igneous Provinces. In particular, the Early Aptian and Cenomanian–Turonian greenhouse episodes that exhibit positive C-isotope excursions are usually interpreted as the consequence of a pCO2 rise in the atmosphere triggered by the emplacement of the Ontong-Java/ Manihiki and Caribbean Plateaus respectively. Such events could be followed by cooler episodes (Arthur et al., 1988; Menegatti et al., 1998). Such cooling events are recorded during Oceanic Anoxic Events (OAEs) and are thus associated with the accumulation and burial of large amounts of organic carbon (OC) in the sediments; a process that can lead to a significant drop in atmospheric pCO2 and a subsequent climate cooling (Arthur et al., 1988; Menegatti et al., 1998; Kuypers et al., 1999). During the Valanginian organic matter (OM) rich layers are only described from restricted basins within the North Atlantic and Weddell Sea, and are generally rare in the Tethyan Realm (Westermann et al., 2010). However, the coeval formation of individual black shale layers, and the increased storage of OM on continents, as proposed by Westermann et al. (2010), could have triggered climatic cooling in the aftermath of the Valanginian positive C-isotope shift. It is thus assumed that these general “forced” feedbacks have the potential to enable the atmosphere–ocean system to weaken greenhouse conditions and thus return to more stable conditions (Föllmi et al., 1994). 5.2.3. Palaeoenvironmental model Using a high-resolution calcareous nannofossil analysis in association with geochemical data, Duchamp-Alphonse et al. (2007) documented a calcareous nannofossil biocalcification crisis at the Angles section, associated with the eutrophication of the photic zone during the Stephanophorus–Verrucosum Zones. Trophic conditions were in part inferred using a High Nutrient Index (HNI) to distinguish oligo- meso-

and eutrophic conditions (Duchamp-Alphonse et al., 2007). This HNI index has been compared with the WI index measured on the same samples, to better understand the link between climatic variations recorded in the surrounding continents and the trophic conditions inferred in the Vocontian photic zone (Fig. 5). Furthermore, it is also a valuable and independent test of the palaeoclimatic and palaeoceanographic “greenhouse model” proposed by Lini et al. (1992) for environments quite similar to the hemipelagic realm of the Vocontian Basin. From the Boissieri to the Stephanophorus Zones, e.g., during most of the Early Valanginian (Period 1), the WI and the HNI indices do not show any correlation (Fig. 5). This result is not surprising, since the WI is based on clay mineral associations which are mainly influenced by second order sea-level changes during the earliest Valanginian. Therefore, the WI cannot be used as a climate proxy and its relationships with nutrient inputs (e.g., HNI) are difficult to decipher. In contrast, periods 2 and 3 (Stephanophorus to Radiatus Zones) are characterized by clay-mineralogical variations that mainly reflect continental weathering associated to climate changes, as this interval corresponds to a steady and durable second-order highstand sealevel. Here, the WI and the HNI long-term trends show a remarkable similarity in shape (Fig. 5); generally an increase or decrease in the HNI coincides with an increase or decrease in the WI long-term trends. This data highlights the significant relationship that exists between climate and the nutrient supply in a hemipelagic environment such as the Vocontian Basin. The most significant correlation between WI and HNI indices occurs in the Inostransewi–Verrucosum Zones, which include the positive C-isotope shift, and in the Furcillata Subzone to the Radiatus Zone (both intervals corresponding to strong hydrolysis conditions and high terrigenous inputs). Consequently, it is proposed that at times of significantly high hydrolysis, nutrients are rapidly transferred from continent to ocean. A direct hydrolysis/element leaching relationship is then established as hydrolysis conditions are the more powerful forcing factor in climatic and pedogenetic processes. Thus, this study clearly demonstrates that during the late Valanginian, nutrification in the Vocontian Basin is directly caused by increased weathering linked to acceleration in the hydrological cycle and thus greenhouse conditions. These nutrification events probably represent a local response to the global increase of pCO2 in the atmosphere. Furthermore, these results support the hypothesis of Föllmi et al. (1994), Lini et al. (1992), and Weissert et al. (1998), which suggests an intensification of the greenhouse conditions as the main cause of the palaeoenvironmental changes during the Valanginian positive C-isotope shift, in that part of the Tethys realm. The WI is out of phase with (even inversely proportional to) the HNI only once, during the Nicklesi Subzone, indicating that, at a time of weaker hydrolysis, a pulse of nutrient supplies is nevertheless recorded in the Vocontian Basin. This result highlights a more complex system including a larger number of interacting factors within the weathering processes, and/or the occurrence of thresholds and delays in the interaction processes, which is expected in the aftermath of an extreme episode such as the Weissert event. A better understanding of these factors and processes will assist future interpretations. 6. Conclusions This high resolution multiproxy study of the Valanginian interval of the Angles section (SE France) shows that: 1) Weathering processes along the northwestern Tethyan margin are mainly influenced by sea-level changes during the Early Valanginian (Pertransiens to Stephanophorus Zones), and by climate variations from the late Early Valanginian to the base of the Hauterivian (top of the Stephanophorus Zone to the Radiatus Zone). 2) The positive C-isotope excursion is associated with a warm and humid climate and greenhouse conditions which lead to increased terrigenous supplies to the Vocontian Basin. The highest hydrolysis

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activity, and thus the most pronounced greenhouse conditions coincide with the positive C-isotope excursion peak. 3) An overall unstable climate is recorded in the aftermath of this greenhouse climate followed by a rapid cooling event; this latter occurred, most likely, during the Verrucosum–Trinodosum Zones. The overlying Trinodosum–Radiatus interval is marked by a seasonally contrasted climate in which more humid periods alternate with more arid ones. 4) During the Late Valanginian, (e.g. during and in the aftermath of the greenhouse conditions) the increased nutrient transfer to the Vocontian Basin and the associated nutrification responsible for the calcareous nannofossil biocalcification crisis (Duchamp-Alphonse et al., 2007) are generally triggered by periods of acceleration in the hydrological cycle. This high resolution mineralogical study demonstrates more precisely that greenhouse conditions during the Valanginian C-isotope excursion (as proposed Lini et al., 1992) are coupled with a drier episode during the Verrucosum Zone. It provides a mineralogical record consistent with previous geochemical studies, which depict a cooling event during this drier episode (van de Schootbrugge et al., 2000; Pucéat et al., 2003; McArthur et al. 2007). This work furthermore highlights rapid changes in the C-cycle during the Valanginian. Since such processes are mostly linked with the formation of Large Igneous Province in the Lower Cretaceous world, the intensification of the Paranà–Etendeka volcanism may be considered as the main driver of such rapid climatic changes. Acknowledgements The authors acknowledge Professor Jean-François Deconinck for stimulating discussions and Domenico Lodola and Dr. Alice Thomas for their helpful assistance with the English version. We thank the Editor F. Surlyk and the two anonymous reviewers for their constructive comments that improved the manuscript. References Adatte, T., Stinnesbeck, W., Keller, G., 1996. Lithostratigraphic and mineralogic correlations of near K/T boundary sediments in northeastern Mexico: implications for origin and nature of deposition. In: Ryder, G., Fastovsky, D., Gartner, S. (Eds.), The Cretaceous–Tertiary Event and Other Catastrophes in Earth History: Geological Society of America Special Paper, 307, pp. 211–226. Adatte, T., Stinnesbeck, W., Hubberten, H., Remane, J., Guadalupe, L.O., 2001. Correlation of a Valanginian stable isotopic excursion in Northeastern Mexico with the European Tethys. In: Bartolini, C., Buffler, R.T., Cantu-Chapa, A. (Eds.), The Western Gulf of Mexico Basin: Tectonics, Sedimentary Basins, and Petroleum Systems: American Association of Petroleum Geologists Memoir, 75, pp. 371–388. Adatte, T., Keller, G., Stinnesbeck, W., 2002. Late Cretaceous to Early Palaeocene climate and sea-level fluctuations: the Tunisian record. Palaeogeography, Palaeoclimatology, Palaeoecology 178, 165–196. Aguirre-Urreta, M.B., Price, G.D., Ruffell, A.H., Lazo, D.G., Kalin, R.M., Ogle, N., Rawson, P.F., 2008. Southern Hemisphere Early Cretaceous (Valanginian-Early Barremian) carbon and oxygen isotope curves from the Neuquen Basin, Argentina. Cretaceous Research 29, 87–99. Allemann, F., Remane, J., 1979. Les faunes de calpionelles du Berriasien supérieur/ Valanginien. In: Busnardo, R., Thieuloy, J.P., Moullade, M. (Eds.), Hypostratotype Mésogeen de l'Etage Valanginian (Sud-Est de la France): Les Stratotypes Français. C.N.R.S., 6, pp. 99–109. Paris. Alley, N.F., Frakes, L.A., 2003. First know Cretaceous glaciation: Livingston tillite member of the Cadna-owie Formation, South Australia. Australian Journal of Earth Sciences 50, 139–144. Arnaud-Vanneau, A., Arnaud, H., Boisseau, T., Darsac, C., Thieuloy, J.P., Vieban, F., 1982. Synchronisme des crises biologiques et paléogéographiques dans le Crétacé inférieur du S.E. de la France: un outil pour les corrélations plate-forme–bassin. Géologie Méditerranéenne IX (3), 153–165. Arthur, M.A., Dean, W.E., Pratt, L.M., 1988. Geochemical and climatic effects of increased marine organic carbon burial at the Cenomanian/Turonian boundary. Nature 335, 714–717. Bergen, J.A., 1994. Berriasian to Early Aptian calcareous nannofossils from the Vocontian trough (SE France) and deep-sea drilling site 534: new nannofossil taxa and summary of low-latitude biostratigraphic events. Journal of Nannoplanton Research 16 (2), 59–69.

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