Forcing mechanisms for mid-Cretaceous black shaleformation: evidence from the Upper Aptian andLower Albian of the Vocontian Basin (SE France)

Jens O. Herrle �, Jo«rg Pross, Oliver Friedrich, Peter Ko«Mler,Christoph Hemleben

Institut fu«r Geowissenschaften, Universita«t Tu«bingen, SigwartstraMe 10, D-72076 Tu«bingen, Germany

Received 10 April 2002; received in revised form 26 June 2002; accepted 18 October 2002

Abstract

Calcareous nannoplankton, palynomorph, benthic foraminifera, and oxygen isotope records from thesupraregionally distributed Niveau Paquier (Early Albian age, Oceanic Anoxic Event 1b) and regionally distributedNiveau Kilian (Late Aptian age) black shales in the Vocontian Basin (SE France) exhibit variations that reflectpaleoclimatic and paleoceanographic changes in the mid-Cretaceous low latitudes. To quantify surface waterproductivity and temperature changes, nutrient and temperature indices based on calcareous nannofossils weredeveloped. The nutrient index strongly varies in the precessional band, whereas variations of the temperature indexreflect eccentricity. Since polar ice caps were not present during the mid-Cretaceous, these variations probably resultfrom feedback mechanisms within a monsoonal climate system of the mid-Cretaceous low latitudes involving warm/humid and cool/dry cycles. A model is proposed that explains the formation of mid-Cretaceous black shales throughmonsoonally driven changes in temperature and evaporation/precipitation patterns. The Lower Albian NiveauPaquier, which has a supraregional distribution, formed under extremely warm and humid conditions whenmonsoonal intensity was strongest. Bottom water ventilation in the Vocontian Basin was diminished, probably due toincreased precipitation and reduced evaporation in regions of deep water formation at low latitudes. Surface waterproductivity in the Vocontian Basin was controlled by the strength of monsoonal winds. The Upper Aptian NiveauKilian, which has a regional distribution only, formed under a less warm and humid climate than the Niveau Paquier.Low-latitude deep water formation was reduced to a lesser extent and/or on regional scale only. The threshold for theformation of a supraregional black shale was not reached. The intensity of increases in temperature and humiditycontrolled whether black shales developed on a regional or supraregional scale. At least in the Vocontian Basin, theincreased preservation of organic matter at the sea floor was more significant in black shale formation than the role ofenhanced productivity.8 2002 Elsevier Science B.V. All rights reserved.

Keywords: black shale; Cretaceous; benthic foraminifera; calcareous nannofossils ; palynomorphs; monsoon

0031-0182 / 02 / $ ^ see front matter 8 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 3 1 - 0 1 8 2 ( 0 2 ) 0 0 6 1 6 - 8

* Corresponding author. Tel. : +49-7071-297-7509; Fax: +49-7071-295-766.E-mail address: j.o.herrle@uni-tuebingen.de (J.O. Herrle).

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1. Introduction

1.1. Mid-Cretaceous paleoceanographic andpaleoclimatic background conditions

The mid-Cretaceous has been characterized as atime of global warmth when the Earth’s climatewas in an extreme greenhouse mode. Based onoxygen isotopes from benthic foraminifera, thedeep oceans were signi¢cantly warmer than atpresent (e.g., Savin, 1977; Erbacher et al., 2001).The occurrence of warm and saline bottom watershas been taken as evidence that deep water for-mation occurred at low latitudes (Brass et al.,1982). This scenario has been veri¢ed by oceanicgeneral circulation simulations (Barron and Peter-son, 1990) and also suggested by several otherstudies (e.g., Bice et al., 1997; Voigt et al.,1999). More recent model experiments also dem-onstrate the signi¢cance of high-latitude deepwater formation during the mid-Cretaceous(Brady et al., 1998; Haupt and Seidov, 2001;Poulsen et al., 2001). Hence, although not exclu-sively responsible for mid-Cretaceous deep waterformation, low-latitude sites played an importantrole in this process.The general paleoclimatic conditions of the

mid-Cretaceous can be described as predomi-nantly humid in the northern hemisphere (NorthAmerica, Eurasia; e.g., Voigt, 1996) and both dryand humid in the southern hemisphere (SouthAmerica, Antarctica, India, Madagascar, Austra-lia, Africa). The northern and southern continentswere separated by the wide open eastern Tethysand the narrow western Tethys and AtlanticOcean. The land^sea con¢guration of the mid-Cretaceous rendered the low latitudes highly sen-sitive to monsoonal activity (Barron et al., 1985;Oglesby and Park, 1989; Wortmann et al., 1999).The Tethyan and Atlantic oceans provided a

circumglobal oceanic connection in the low lati-tudes (Hay et al., 1999), probably with a stable,westward-£owing circumglobal current through-out the Tethys (e.g., Roth, 1986; Barron, 1987).Recently, this view has been questioned by Poul-sen et al. (1998), who presented arguments for amore complicated circulation pattern dominatedby a clockwise gyre in the western Tethys.

The paleoceanographic and paleoclimatic con-ditions as described above favored the formationof regionally and supraregionally distributedblack shales. Mid-Cretaceous sections from thewestern Tethys and Atlantic Ocean contain nu-merous black shale horizons, some of which aremore or less synchronous in di¡erent basins andtherefore have been termed Oceanic AnoxicEvents (OAEs). These are, for instance, the OAE1a of Early Aptian age (e.g., Schlanger andJenkyns, 1976; Arthur et al., 1990; Bralower etal., 1994) and the OAE 1b of Early Albian age(e.g., Bre¤he¤ret, 1988; Bralower et al., 1993; Er-bacher et al., 1999; Herrle, 2002). OAEs representmajor perturbations of the ocean system de¢nedby massive deposition of organic matter in marineenvironments (e.g., Schlanger and Jenkyns, 1976;Arthur et al., 1990). The rapid changes in thecarbon cycle were accompanied by turnovers inmarine biota (e.g., Bralower et al., 1994; Erba,1994; Erbacher and Thurow, 1997).

1.2. Mid-Cretaceous black shale formation

After three decades of extensive research, theorigin and spatial distribution of mid-CretaceousOAEs are still a matter of debate. Di¡erent modelsof climate and ocean circulation have been pro-posed, either stressing the in£uence of productivityor preservation on black shale formation. Upwell-ing and elevated productivity, higher runo¡ result-ing in a thermohaline strati¢cation, and changes inrate or mode of deep water circulation and venti-lation all have been suggested as driving mecha-nisms for increasing organic carbon burial rates.Various authors suggested that the burial of

organic carbon was primarily a function of sedi-mentation rates and organic productivity (e.g.,Weissert, 1989; Pederson and Calvert, 1990; Cal-vert and Pederson, 1992; Erbacher et al., 1999;Hochuli et al., 1999; Premoli Silva et al., 1999).They attributed the formation of black shales toincreased surface water productivity during warm-er and more humid climate conditions. Increasingruno¡ accelerated the transfer of nutrients fromthe continents into the oceans, inducing higherprimary productivity in the surface waters.Bre¤he¤ret (1994) and Erbacher et al. (1996) sug-

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gested that Aptian to Albian black shale horizonsare linked to speci¢c positions of the sea level.They distinguished between black shales resultingfrom increased oceanic productivity (productivityoceanic anoxic events, P-OAEs of Erbacher etal., 1996; condensed-type of Bre¤he¤ret, 1994) andblack shales resulting from increased sedimen-tation of terrestrial organic matter (detrital oce-anic anoxic events, D-OAEs of Erbacher et al.,1996; fed-type of Bre¤he¤ret, 1994). P-OAEs werebelieved to be connected to transgressive periods,whereas D-OAEs were suggested to correlate withstill-stands or falling sea level (Erbacher et al.,1996).The so-called stagnant ocean model (e.g.,

Brumsack, 1980; Bralower and Thierstein, 1984;Herbin et al., 1986) is based on the assumptionthat high sea level and changes in thermohalinecirculation resulted in decreasing subsurface oxy-gen concentrations and an expansion of the oxy-gen minimum zone. This led to an increased buri-al of organic carbon (e.g., Schlanger and Jenkyns,1976; Bralower and Thierstein, 1984; de Gracian-sky et al., 1984; Arthur et al., 1990). In addition,organic-rich horizons have been interpreted to bethe result of local oceanographic and topographicfactors, resedimentation of organic matter fromshallow to deep water environments, increasedsupply of terrestrial organic carbon, or sea-levelfalls (e.g., Jenkyns, 1980; Habib, 1982; Thurowand Kuhnt, 1986). Most recently, Kuypers et al.(2001) introduced a new aspect of organic matteraccumulation at the sea £oor by highlighting therole of archaebacteria during formation of theOAE 1b black shale in the Atlantic Ocean andVocontian Basin.In addition to OAE black shales, regionally dis-

tributed black shales are common in the Aptian toAlbian sediments of the western Tethys (e.g., Tor-naghi et al., 1989; Bre¤he¤ret, 1994). These blackshale horizons are partly characterized by a cyclicoccurrence and have been interpreted as a resultof changes in seasonality (e.g., Herbert and Fisch-er, 1986; Erba and Premoli Silva, 1994). Accord-ing to these authors, productivity was high andbottom waters were well oxygenated during timesof high seasonality, whereas weak-seasonality in-tervals were characterized by low surface water

productivity and less well oxygenated bottomwaters.

1.3. Aim of the study

The models of enhanced organic matter accu-mulation outlined above may su⁄ciently describethe origin of mid-Cretaceous supraregional blackshales (OAEs). However, most paleoceanographicmodels for mid-Cretaceous black shale formationare based on data with a temporal resolution thatis by an order of magnitude coarser than the timescales on which many oceanographic processesoperate. Therefore, Milankovitch-scale tempera-ture, terrestrial input/humidity, and productivityvariations, which have been shown to play animportant role in Late Neogene and Quaternaryblack shale formation (e.g., Rossignol-Strick etal., 1982; Rossignol-Strick, 1985), are yet poorlydocumented and understood for the mid-Creta-ceous.To obtain insight into the development and pa-

leoenvironmental characteristics of supraregionaland more regional black shales, we studied sec-tions containing the Lower Albian Niveau Paqu-ier and the Upper Aptian Niveau Kilian fromthe Vocontian Basin, SE France. In contrast tothe regionally distributed Niveau Kilian, whichoccurs only in the northwestern part of the west-ern Tethys, the Niveau Paquier is known fromseveral sections of the Tethyan realm (Bre¤he¤retet al., 1986; Bre¤he¤ret, 1994; Arthur et al., 1990;Bralower et al., 1993) and is probably coevalwith organic-rich horizons in the North, South,and Central Atlantic named OAE 1b (Braloweret al., 1993; Erbacher et al., 1999; Herrle, 2002).Both the Niveau Paquier and the Niveau Kilian

were formed during relatively high sea level underbasically identical paleo-latitudes (cf. Bralower etal., 1994). Thus, a framework is provided to com-pare the formation of obviously di¡erent blackshales under partly identical boundary conditions.We used calcareous nannofossils as a proxy forsurface water temperature and productivity, paly-nomorphs as a proxy for terrestrial input and hu-midity, benthic foraminifera as a proxy for bot-tom water conditions, and oxygen isotopes toassess temperature trends to investigate the paleo-

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environmental dynamics of the Late Aptian andEarly Albian Tethyan Ocean on geologically shorttime scales. We have di¡erentiated between long-and short-term environmental changes as ob-served in the studied sections. For greater clarity,the data are presented and interpreted in the samesections.

2. Location, paleogeography, lithology, andchronostratigraphic framework

The studied l’Arboudeysse and Pre¤-Guittardsections are located in the northern part of theVocontian Basin, SE France (Fig. 1). The Vocon-tian Basin is located between the Vercors Massif,the river Rhone, the Ventoux^Lure Axis, and the

Nice^Castellane Arch of the southern SubalpineRanges.

2.1. Paleogeography

During the mid-Cretaceous, the Vocontian Ba-sin belonged to the northern part of the westernTethyan Ocean and was situated at a paleo-lati-tude of 25^30‡ North (Savostin et al., 1986; Fig.2). In the northwest, the boundary of the basinwas de¢ned by the Massif Central landmass (Figs.1, 2). The eastern part was open towards the Te-thys. Otherwise, the basin was surrounded byslopes and platforms with a hemipelagic faciesintercalating with shallow-water carbonates (Ar-naud and Lemoine, 1993), thus allowing an ex-change with the open Tethys.

Fig. 1. Depositional settings within the Vocontian Basin during the Upper Aptian to Lower Albian showing locations of studiedsections (PG, Pre¤-Guittard; LA, l’Arboudeysse). Modi¢ed after Arnaud and Lemoine (1993).

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The paleo-water depth of the Vocontian Basinis still under debate. Floral and faunal data sug-gested a depth of several hundred meters (Wilps-haar and Leereveld, 1994). Earlier authors, how-ever, argued that the basin was more than 2000 mdeep (Cotillon and Rio, 1984).

2.2. Lithology

The l’Arboudeysse and Pre¤-Guittard sectionsconsist of a monotonous sequence of marly hemi-pelagic to pelagic sediments of the Marnes BleuesFormation (Fig. 3). The lithology is characterizedby pale and dark bedded marlstones with inter-calated marly limestones, limestones, and blackshale horizons. The Marnes Bleues Formation iswidely characterized by the cyclic deposition ofpale and dark sediments (Bre¤he¤ret, 1988, 1994,1997; Ko«Mler et al., 2001).

2.2.1. Lower Albian l’Arboudeysse sectionThe Lower Albian l’Arboudeysse section, which

contains the supraregionally distributed NiveauPaquier (OAE 1b), is situated 5 km east of Ro-sans (topographic map 25 Rosans, No. 3239 Ou-est, Serie Bleues, Lambert III coordinates X: 854850, Y: 3238 825). Biostratigraphically, the sec-tion is located in the middle part of the Leymer-iella tardefurcata ammonite zone and Predisco-sphaera columnata nannoplankton zone of theLower Albian (middle part of the NC8B subzone;Fig. 3). Lithologically, the section consists of ca.

14 m of marlstones which are punctuated by theblack shale layers Haut Noir 7 (HN7) and HautNoir 8 (HN8) and by the Niveau Paquier whichreaches about 1.63 m in thickness (Fig. 3). Themarlstones show no pale/dark cyclicity. The HN7(thickness: 38 cm) and HN8 (thickness: 35 cm)black shales, named by Bre¤he¤ret (1997), are char-acterized by a total organic carbon (TOC) contentof up to 1.5%. In the Niveau Paquier, the TOCcontent generally exceeds 3% (maximum: 8%) andis derived from both terrestrial and marine organ-ic matter (Bre¤he¤ret, 1985, 1997; Tribovillard andCotillon, 1989).

2.2.2. Upper Aptian Pre¤-Guittard sectionThe Upper Aptian Pre¤-Guittard section, com-

prising the regionally occurring Niveau Kilian, issituated at the SE £ank of the Serre Sablon about1 km NE of Arnayon (topographic map 25 Dieu-le¢t, No. 3138 Ouest, Lambert III coordinates X:836 800, Y: 3248 825). Biostratigraphically, thesection comprises the upper Hypacanthoplites ja-cobi ammonite zone and the lower part of thePrediscosphaera columnata nannoplankton zoneof the Upper Aptian (upper part of the NC8Asubzone; Fig. 3). About 6 m of pale and darkbedded marlstones, marly limestones, and the Ni-veau Kilian were investigated (Fig. 3). The thick-ness of the pale beds is between 14 cm and 100cm, whereas that of the dark beds ranges from 15to 46 cm. The thickness of individual beds in-creases from the lower to the upper part of the

0°

30°N

Asia

Western Tethyan Ocean

Atlantic Ocean

AfricaSouth America

0°

30°N VB

North America

Fig. 2. Paleogeographic reconstruction of the Atlantic and Western Tethys for the Aptian^Albian, modi¢ed after Hay et al.(1999). The studied l’Arboudeysse and Pre¤-Guittard sections are located in the Vocontian Basin (VB).

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section. The contrast between pale and dark bedsis only weakly expressed. The upper part of thesection contains the 74-cm-thick Niveau Kilian,which has been named by Bre¤he¤ret et al. (1986).The TOC content of the Niveau Kilian reaches upto 3.3% and the organic matter consists mainly of

kerogen type III (Bre¤he¤ret, 1997), indicating aterrestrial origin.

2.3. Chronostratigraphic framework

Based on biostratigraphic data, sedimentologi-

Fig. 3. Compiled lithological column of the Aptian to Lower Albian of the Vocontian Basin (SE France) with regional andsupraregional key beds plotted against biostratigraphy. Lithology and stratigraphic ranges of the studied l’Arboudeysse and Pre¤-Guittard sections are indicated on the right. Planktonic foraminiferal and ammonite stratigraphy after Bre¤he¤ret (1997 and refer-ences therein), nannofossil zonation after Herrle and Mutterlose (2003). P. nut¢eld., Parahoplithes nut¢eldiensis ; H. nolani, Hypa-canthoplites nolani ; L. tardefurcata, Leymeriella tardefurcata; D. mammillatum, Douvilleiceras mammillatum ; FN, Faisceau Nolan;FF, Faisceau Fromaget; DC, De¤lits Calcaire; NJ, Niveau Jacob; NK, Niveau Kilian; HN, Haut Noire; NP, Niveau Paquier;LE, Niveau Leenhardt.

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cal parameters, and nannoplankton assemblages,an age model was established in order to obtaintime control for the deposition of the Niveau Pa-quier and the Niveau Kilian. According to Bra-lower et al. (1995, 1997), the Aptian to LowerAlbian Rhagodiscus angustus (NC7A to NC7C)and Prediscosphaera columnata (NC8A to NC8B)nannofossil zones comprise about 11.7 Ma. In theVocontian Basin, sediments attributed to thesezones are up to 355 m thick (Bre¤he¤ret, 1997).This yields an average sedimentation rate of ap-proximately 3 cm/kyr. Time series analyses ofpale/dark color changes re£ecting precessionalcycles yielded a sedimentation rate of 3.5 cm/kyrfor the lowermost Upper Aptian of the VocontianBasin (Ko«Mler et al., 2001). The slightly lowersedimentation rate derived biostratigraphically isprobably due to condensed intervals of the Ni-veau Blanc and Faisceau Fromaget within thesuccession (Jacquin and de Graciansky, 1998).Spectral analyses of the calcareous nannofossil as-semblages from the l’Arboudeysse section showedthat they are in£uenced by precession and eccen-tricity (Herrle et al., 2003). These analyses indi-cate a mean sedimentation rate of 3.7 cm/kyr forthe l’Arboudeysse section, a value that is wellcompatible with the independently derived ratesas described above. Hence, the Niveau Paquierwith a thickness of 1.63 m formed within V44kyr. This is in good agreement with the estimatedduration of V46 kyr for OAE 1b formation inthe Central Atlantic (Erbacher et al., 2001;Herrle, 2002). The mean sedimentation rate ofthe uppermost Upper Aptian at the Pre¤-Guittardsection can be estimated at a minimum of 2.4 cm/kyr as pale and dark beds have been shown tore£ect the precessional cycle (18.4 kyr; Ko«Mler etal., 2001; Herrle, 2002). Hence, based on a thick-ness of 74 cm, the Niveau Kilian comprises a timeinterval of V31 kyr or less.

3. Materials and methods

3.1. Calcareous nannofossils

High-resolution studies with sample intervals of2^10 cm were carried out on 281 samples from the

l’Arboudeysse and Pre¤-Guittard sections. Sampleswere prepared following Geisen et al. (1999). Atleast 300 specimens were counted in random tra-verses of each slide. Calcareous nannofossils werestudied using an Olympus BX50 microscope atU1250 magni¢cation.

3.1.1. Calcareous nannofossil nutrient andtemperature indicesThe paleobiogeography and paleoecology of

mid-Cretaceous nannofossils have been investi-gated by many authors. Nutrient availabilityand temperature in the surface waters are amongthe most prominent parameters in£uencing thedistribution and composition of calcareous nan-noplankton (e.g., Brand, 1994; Roth, 1994; Win-ter et al., 1994; Burnett et al., 2000). To quantifythese parameters into nutrient and temperatureindices, R-mode (Varimax-rotated) PrincipalComponent Analyses (R-PCA) were performedon data sets from the l’Arboudeysse (194 samples)and Pre¤-Guittard sections (85 samples) using thestatistics software SYSTAT 51. Two samplesfrom the Pre¤-Guittard section were excludedfrom the evaluation due to poor nannoplanktonpreservation. PC loadings of s 0.4 were assignedto associated taxa (Malmgren and Haq, 1982)and PC loadings s 0.5 were assigned to domi-nant taxa of a calcareous nannofossil assem-blage. The R-PCA yielded four calcareous nanno-fossil assemblages for the l’Arboudeysse sectionand two for the Pre¤-Guittard section (Table 1).The multivariate model explains 57.5% and37.4% of the total variance of the data sets forthe l’Arboudeysse and Pre¤-Guittard section, re-spectively.In the nutrient and temperature indices, the ra-

tios between selected taxa are evaluated. The useof ratios with respect to surface water productiv-ity and temperature provides a clearer signal thanthe assessment of single species abundances. Thisis documented in thanatocoenoses of recent ma-terial where species ratios yield a better record ofthe surface water signal than the abundance ofsingle species (Andruleit, 1995).

3.1.1.1. Nutrient index (NI). Based on multi-variate statistics and other quantitative analyses,

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Roth and Bowdler (1981), Roth and Krumbach(1986), and Erba et al. (1992) identi¢ed calcareousnannofossil taxa closely related to surface waterproductivity. The cosmopolitan taxa Zeugrhabdo-tus erectus and Biscutum constans were assigned toareas of enhanced surface water productivity.Moreover, studies on Aptian to Albian materialfrom Italy indicate that Discorhabdus rotatorius isalso positively related to productivity (PremoliSilva et al., 1989; Erba, 1991; Coccioni et al.,1992). The above-mentioned species frequentlyoccur in upwelling areas (e.g., Roth and Bowdler,1981) and on the shelves where nutrients mayhave been added by terrestrial runo¡ (e.g., Streetand Bown, 2000). In contrast, the taxon Watz-naueria barnesae is widely interpreted as a low-productivity indicator (e.g., Roth and Krumbach,1986; Erba et al., 1992; Williams and Bralower,1995).In the present study, the positive and negative

loadings of the assemblages given by PrincipalComponent 2 (PC2; l’Arboudeysse, Pre¤-Guittard)were used to determine surface water productivity(Table 1). The PC2 assemblages comprise thehigh-productivity indicators Discorhabdus rotato-rius and Zeugrhabdotus erectus (positive loadings)as well as the low-productivity indicator Watz-naueria barnesae (negative loadings). Repagulumparvidentatum, which also appears in PC2, hasbeen excluded from the NI because its distribu-tion is controlled by cool and nutrient-depletedsurface waters (compare Erba et al., 1992). Thisspecies typically occurs in high latitudes and israrely observed in the Tethyan realm (Mutterloseand Wise, 1990; Mutterlose, 1992; Street andBown, 2000), indicating that its distribution isprimarily controlled by temperature. These ¢nd-ings suggest that its co-occurrence with the low-productivity indicator W. barnesae in the studymaterial is only indirect.

Table 1Principal component analyses (R-mode) of the l’Arboudeysse (A) and Pre¤-Guittard (B) sections

Dominant species Associated species Var.(%)

(A) Calcareous nannofossil assemblages from the l’Arboudeysse section (R-mode)PC1 Biscutum constans 0.69 20.4

Lithraphidites carniolensis 0.61Nannoconus spp. 30.87Orastrum spp. 30.74Broinsonia signata 30.50

PC2 Zeugrhabdotus erectus 0.55 Discorhabdus rotatorius 0.40 12.9Watznaueria barnesae 30.74Repagulum parvidentatum 30.73

PC3 Zeugrhabdotus trivectis 0.50 Discorhabdus rotatorius 0.44 11.2Zeugrhabdotus diplogrammus 30.73Staurolithites stradneri 30.58Seribiscutum spp. 30.52

PC4 Biscutum a¡. ellipticum 0.70 13.0Seribiscutum spp. 0.57Rhagodiscus asper 30.78

(B) Calcareous nannofossil assemblages from the Pre¤-Guittard section (R-mode)PC1 Zeugrhabdotus diplogrammus 0.61 Zeugrhabdotus erectus 0.47 19.4

Rhagodiscus asper 0.55Zeugrhabdotus trivectis 30.74 Repagulum parvidentatum 30.46Staurolithites stradneri 30.68

PC2 Discorhabdus rotatorius 0.80 Lithraphidites carniolensis 0.44 18.0Zeugrhabdotus erectus 0.50Watznaueria barnesae 30.82 Zeugrhabdotus diplogrammus 30.45

Repagulum parvidentatum 30.42

Taxa considered in statistical analysis have abundances s 0.5% in the l’Arboudeysse and s 1% in the Pre¤-Guittard section withrespect to total assemblages.

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Based on the information derived from theliterature as lined out above and the distributionof the index species in the study material, theNI

NI ¼ ZeþDrWbþ ZeþDr

U100 ð1Þ

was established in order to determine surfacewater productivity (with Ze=Zeugrhabdotus erec-tus, Dr=Discorhabdus rotatorius, and Wb=Watz-naueria barnesae).

3.1.1.2. Temperature index (TI). Within mid-Cretaceous calcareous nannofossil assemblages,Repagulum parvidentatum and Seribiscutum spp.are known to bear a high-latitude and, thus,cold-water signal (Erba et al., 1992; Mutterloseand Wise, 1990; Street and Bown, 2000). Biscu-tum a¡. ellipticum (Biscutum constans large) is alsointerpreted to prefer cooler surface waters. Theoccurrence of Rhagodiscus asper, in contrast, re-£ects warmer surface waters (e.g., Erba et al.,1992; Mutterlose, 1989, 1996). The paleoecologi-cal a⁄nities of Staurolithites stradneri and Zeug-rhabdotus trivectis have not been determined pre-viously, but our R-PCA results show that thedistribution of these species is inversely correlatedwith that of the warm-water species R. asper.Consequently, they may represent cool-watermarkers. The distribution pattern of Zeugrhabdo-tus diplogrammus correlates with that of thewarm-water species R. asper and therefore Z. dip-logrammus is considered to represent a warm-water marker as well.To determine the TI, the positive and negative

loadings of PC4 from the l’Arboudeysse sectionand PC1 from the Pre¤-Guittard section were used(Table 1). These factors comprise Biscutum a¡.ellipticum, Seribiscutum spp., Staurolithites strad-neri, Zeugrhabdotus trivectis, and Repagulum par-videntatum (cold-water assemblage), and Rhago-discus asper and Zeugrhabdotus diplogrammus(warm-water assemblage). They provide the basisfor the TI

TI ¼ Beþ SþRpþ Ssþ ZtRaþ Zdþ Beþ SþRpþ Ssþ Zt

U100 ð2Þ

(with Be=Biscutum a¡. ellipticum, S =Seribiscu-

tum spp., Rp=Repagulum parvidentatum, Ss =Staurolithites stradneri, Zt =Zeugrhabdotus trivec-tis, Ra=Rhagodiscus asper, Zd=Zeugrhabdotusdiplogrammus).The factors PC1 and PC3 from the l’Arbou-

deysse section comprise taxa considered to be in-dicative of open ocean environments (e.g., Lithra-phidites carniolensis ; Thierstein, 1976), unstablenear-shore environments (e.g., Broinsonia signata ;Roth and Bowdler, 1981), and yet unclear ecolog-ical a⁄nities (e.g., Orastrum spp.). As nutrientsand temperature represent the most importantparameters governing nannoplankton distribu-tion and these parameters are represented bythe factors PC2 and PC4 (l’Arboudeysse section),the factors PC1 and PC3 from l’Arboudeyssewere not used in the paleoenvironmental evalua-tion.

3.2. Palynomorphs

Palynomorphs were investigated from 28 sam-ples of the l’Arboudeysse section (covering theNiveau Paquier and the strata shortly belowand above) and 15 samples of the Pre¤-Guittardsection (covering the Niveau Kilian and thestrata shortly below and above). Sample prepara-tion followed standard palynological preparationtechniques (e.g., Wood et al., 1996). Knownweights of sample material between 5 and 8 gwere treated with hydrochloric and hydro£uoricacids. To facilitate the calculation of absolutepalynomorph abundances, samples were spikedwith Lycopodium marker spores. Due to thehigh amount of amorphous organic matter inthe residues, a short oxidation with nitric acidwas performed. For each sample, at least 300palynomorphs were counted from strew mounts.To quantify terrestrial input, the terrigenous/ma-rine ratio (TMR) of palynomorphs was calculated(e.g., Pross, 2001). The absolute abundance ofspores occurring in each sample and the ratio be-tween absolute spore abundances and absolutepollen abundances were used as proxies for hu-midity in the hinterland. Owing to an insu⁄cientdispersal of Lycopodium spores in the samplesfrom 10.06 m and 10.18 m from the l’Arbou-deysse section during preparation, absolute paly-

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nomorph abundances are not available for thesesamples.

3.3. Benthic foraminifera

Benthic foraminifera were studied in 60 samplesfrom the l’Arboudeysse section and 15 samplesfrom the Pre¤-Guittard section. After drying andweighing, a mixture of ethanol and detergent(REWOQUAD) was added. Subsequently, sam-ples were washed over a 63-Wm mesh. Foraminif-era from the 125^500-Wm fraction were deter-mined to the species level. For each sample, atleast 300 benthic foraminifera were counted.Benthic foraminiferal numbers were calculatedas individuals per gram of dry sediment.To reconstruct productivity changes at the sea

£oor, the productivity indicators Gyroidinoidesspp. and Valvulineria sp. were used (Erbacher etal., 1999; Holbourn et al., 2001). High abundan-ces of Gavelinella spp. were used as indicators oflow-oxygen conditions as this genus has been de-scribed as dominating foraminiferal assemblagesin mid-Cretaceous black shales (Koutsoukos etal., 1990; Holbourn et al., 2001).

3.4. Stable isotopes

Seventy-seven bulk samples from the l’Arbou-deysse and 85 bulk samples from the Pre¤-Guittardsections were analyzed for stable oxygen and car-bon isotopes. Samples were taken from freshly cutrock fragments, crushed, and thoroughly homog-enized in an agate mortar. Isotopes were analyzedusing a Finnigan MAT 251 mass spectrometer atthe ‘Leibniz-Labor fu«r Altersbestimmung und Iso-topenforschung’ at Kiel University. The instru-ment is coupled online to a Carbo-Kiel device Ifor automated CO2 preparation from carbonatesamples for isotopic analysis. The results are re-ported using the usual N-notation in per mill de-viation from the PDB standard. The system hasan accuracy of X 0.03x for oxygen and X 0.02xfor carbon isotopes.The dependence of the N

18O signal on temper-ature has been used to reconstruct sea surfacetemperatures based on the empirically derived

equation of Epstein et al. (1953) modi¢ed by An-derson and Arthur (1983):

T ¼ 16:034:14ðN18Oc3N18OwÞþ

0:13ðN18Oc3N18OwÞ2 ð3Þ

with T as temperature in ‡C, N18Oc as isotopic

composition of the calcite shell in x relative tothe PDB standard, and N

18Ow as the isotopic com-position of the ambient seawater in x (PDB).The N

18Ow of mid-Cretaceous seawater is esti-mated as 31.2x for an ice-free world excludingsalinity changes (Shackleton and Kennett, 1975).

3.5. Time series analysis

To identify potential cyclicities within the nan-noplankton record, spectral analysis was carriedout on the l’Arboudeysse section because it com-prises a su⁄ciently long time interval (V380 kyr)and a high number of samples (194 samples). Thecomputer program SPECTRUM 2.2, developedby Schulz and Stattegger (1997), has been used.The advantage of this spectral analysis is based onthe Lomb^Scargle^Fourier Transform (Lomb,1976; Scargle, 1982, 1989), which can be directlyapplied to unevenly spaced time series. The e¡ectof spectral leakage caused by the ¢nite length ofthe time series has been reduced by Welch-Over-lapped-Segment Averaging (WOSA; Welch, 1967;see also Ko«Mler et al., 2001).All presented data are available from the PAN-

GAEA database (http://www.Pangaea.de).

4. Preservation of calcareous nannofossil andstable isotope signals

4.1. Calcareous nannofossils

Dissolution and diagenesis can strongly alterthe preservation of calcareous nannofossil assem-blages. This can severely a¡ect their application inpaleoenvironmental reconstructions (e.g., Honjo,1976; Steinmetz, 1994; Andruleit, 1997). Hence,a visual evaluation of etching (E) and overgrowth

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(O) using a light microscope was performed tospecify the preservation state of the counted as-semblages following the method of Bown andYoung (1998). In addition, the percentages ofthe dissolution-resistant nannoplankton speciesWatznaueria barnesae (e.g., Roth and Bowdler,1981), total calcareous nannofossil abundances,and simple diversities were used to assess preser-vation (Williams and Bralower, 1995).In 194 samples from the l’Arboudeysse section,

156 calcareous nannofossil taxa were identi¢ed.The coccoliths are well preserved and character-ized by etching and overgrowth rankings of X(excellently preserved; terminology after Bownand Young, 1998) to E1 (slightly etched) andO1 (slightly overgrown) in all samples. In 87 sam-ples from the Pre¤-Guittard section, 119 calcareousnannofossil taxa were identi¢ed. With the excep-tion of two samples, the assemblages are well tomoderately preserved, with etching and over-growth rankings of E1 and O1.In general, Watznaueria barnesae is the most

common species in the studied sections (mean:18.2% at l’Arboudeysse and 25.6% at Pre¤-Guit-tard). Assemblages containing more than 40%W. barnesae are widely thought to be a¡ectedby dissolution to an extent that they no longerbear a strong primary signal (e.g., Thierstein,1980; Roth and Bowdler, 1981; Roth, 1984).This view is corroborated by the observationthat poorly preserved assemblages are character-ized by a low species richness (6 15 species) andhigh percentages of W. barnesae (Roth andKrumbach, 1986). The relationships between totalcalcareous nannofossil abundances, the percent-ages of W. barnesae, and simple diversity is shownin Fig. 4. Dissolution should presumably lead to areduction of total abundances of calcareous nan-nofossils and, at the same time, increase the per-centage of W. barnesae. However, correlation co-e⁄cients of W. barnesae percentages and totalcalcareous nannofossil abundances are insigni¢-cant for the l’Arboudeysse (r2 = 0.07, n=194)and Pre¤-Guittard (r2 = 0.08, n=87) sections (Fig.4A,B). Correlation coe⁄cients of W. barnesaepercentages and simple diversities are also insig-ni¢cant for both sections (r2 = 0.14, n=194 forl’Arboudeysse and r2 = 0.15, n=87 for Pre¤-Guit-

tard; Fig. 4C,D). Hence, the lack of correlationsbetween W. barnesae percentages, simple diver-sity, and total calcareous nannofossil abundancesindicates that diagenesis has not generally alteredthe examined calcareous nannofossil assemblages.A paleoenvironmental interpretation of calcare-ous nannofossils is therefore well justi¢ed forthe study material.

4.2. Stable isotopes

The sedimentary succession of SE France hasnot been subject to deep burial and severe alter-ation (V700 m; Levert and Ferry, 1988; Weissertand Bre¤he¤ret, 1991). However, early diageneticprocesses can operate even at these shallowdepths. Hence, we examined the ¢delity of ourstable isotope data with regard to yielding origi-nal trends.The biogenic carbonate fraction of the sedi-

ments sampled for this study is mainly composed(in descending order) of calcareous nannofossilsand planktic and benthic foraminifera. It re£ectspredominantly a surface water signal. Calcite ofdiagenetic origin, i.e., cements and micrite, is aminor component and its contribution to the iso-topic signal is small. This is indicated by the goodpreservation of calcareous nannofossils (cf. Sec-tion 4.1) and the relatively low amount of micriteobserved in the nannofossil samples. Moreover,Weissert and Bre¤he¤ret (1991) and Bre¤he¤ret (1997)point out that no dolomite was present in theirstudied sections from the Aptian/Albian of SEFrance. Hence, a paleoenvironmental interpreta-tion of the stable isotope record seems possible.Independently from these aspects, similar £uc-

tuations of the N18O curve from l’Arboudeysse

(Niveau Paquier) in the Vocontian Basin (SEFrance) and the Mazagan (DSDP Site 545) andBlake Nose Plateaus (ODP Site 1049C) also pointtowards a predominantly original trend (Herrle,2002). However, there is a positive correlationbetween N

18O and N13C for the l’Arboudeysse

(r2 = 0.71; n=77) and Pre¤-Guittard (r2 = 0.69;n=89) sections (Fig. 5A,B). Jenkyns and Clayton(1986) and Jenkyns (1996) suggest that a positivecorrelation between N

18O and N13C values re£ects

a signi¢cant alteration of the stable isotope com-

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position during burial diagenesis. Hence, the cor-relation of N18O and N

13C values may indeed pointto a slight diagenetic overprint of the isotope val-ues.However, Rao (1996) has shown that a positive

correlation of N13C and N18O values does not nec-

essarily indicate a diagenetic signal. The possiblemagnitude of resulting alterations in the N

18O sig-nal may be assessed through a comparison withLate Pleistocene (isotope stages 7^5) isotope re-cords from the eastern Mediterranean Sea. Here,a cross-plot of N

13C and N18O values from

planktic foraminifera (Schmiedl et al., 1998) alsoyields a weak positive correlation (r2 = 0.46;

n=56; Fig. 5C). The environmental conditionsin the eastern Mediterranean Sea were stronglyin£uenced by monsoonal climate conditions withsuperimposed glacial and interglacial cycles. Thecyclic impact of freshwater input triggered bymonsoonal circulation probably resulted in thedepletion of both the N

18O and N13C signals

(e.g., Schmiedl et al., 1998). The magnitude ofshort-term (V20 kyr) £uctuations ranges from 1to 4x for N

18O and from 0.6 to 2.5x for N13C

values (Schmiedl et al., 1998), similar to the stableisotope record from the studied sections of theVocontian Basin.Summarizing, the good preservation of calcar-

Fig. 4. Scatter plots showing the relationships between total calcareous nannofossil abundance and Watznaueria barnesae (A,B)and simple diversity and W. barnesae (C,D) in the l’Arboudeysse and Pre¤-Guittard sections.

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eous nannofossils, the low amount of micrite inthe sample material, the similarity of the oxygenisotope curves in the OAE 1b succession from thewestern Tethyan and Atlantic oceans (Erbacher etal., 2001; Herrle, 2002), and the analogy to stableisotope records from the Quaternary of the east-ern Mediterranean Sea indicate a su⁄cient ¢delityof our stable isotope data regarding relative £uc-tuations of the original trends.

5. Long-term environmental changes

5.1. Lower Albian l’Arboudeysse section

The NI of calcareous nannofossils exhibitshigh-frequency £uctuations between values of23.2 and 86.2, with high values re£ecting en-hanced productivity in the surface waters andvice versa (Fig. 6). Productivity maxima occurbelow and within the HN8 black shale, the blackshale horizon below the Niveau Paquier, andwithin the Niveau Paquier itself.The TI £uctuates between 18.5 and 81.0. In

comparison to the NI, the £uctuations are of low-er frequencies (Fig. 6). Cooler intervals are in thelower part of the studied section (HN7 blackshale) and below and above the Niveau Paquier.Both the Niveau Paquier and the HN8 blackshales occur within warm periods.Oxygen isotope values range from 34.6 to

32.4x (Fig. 6). N18O £uctuations correlate

rather well with those of the TI. A rapid decreaseof N18O values can be recognized 33 cm (V9 kyr;see Section 2.3 for chronostratigraphy) before theonset of Niveau Paquier formation. Above theNiveau Paquier, the N

18O values show a trend tomore positive values.Oxygen isotope values translated into paleo-

temperatures (neglecting possible salinity changesin order to describe maximum possible tempera-ture changes) vary by up to 6‡C in the interval

Fig. 5. Cross-plots of N18O and N

13C values from bulk rockcarbonate samples from the l’Arboudeysse and Pre¤-Guittardsections (A,B) and from Quaternary (isotope stages 7^5)planktic foraminifera (C) from the eastern MediterraneanSea (data from Schmiedl et al., 1998).

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below the Niveau Paquier and exhibit an increaseof up to 8‡C towards the Niveau Paquier (Fig. 6).A similar rapid temperature increase towards theOAE 1b has been recognized in Atlantic sections

from the Mazagan (4‡C; Herrle, 2002) and BlakeNose Plateaus (10‡C; Erbacher et al., 2001).Both the NI and TI suggest highly variable

temperature and productivity in the surface

Fig. 6. Nutrient and temperature indices from the l’Arboudeysse section based on calcareous nannofossils in comparison with theN18O record. High values of the NI indicate high productivity and vice versa. Low values of the TI indicate high temperaturesand vice versa. In the right column, thick lines indicate smoothed records based on the locally weighted least squared error meth-od after Chambers et al. (1983). For lithological explanations and abbreviations see Fig. 3. For a close-up of the TI data fromthe Niveau Paquier see Fig. 9A.

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waters. Time series analyses indicate orbital forc-ing of the NI and TI, with a dominant preces-sional signal for the NI and a dominance of theeccentricity signal for the TI (Herrle et al., 2003;Fig. 7). Spectral analysis of the NI shows pro-nounced peaks at 74 cm (equivalent to V20 kyrbased on the age control as presented in Section2.3) and 68 cm (V16.8 kyr), corresponding to theprecessional signal. The TI exhibits its strongestpeak at 460 cm (V124 kyr) and other pro-nounced peaks at 87 cm (V23.5 kyr) and 68 cm(V18.4 kyr) representing orbital periods of eccen-tricity and precession. The power spectrum ofobliquity at 210 cm (V57 kyr) is statistically in-signi¢cant.As pointed out in Section 1.1, the mid-Creta-

ceous land^sea distribution in the low latitudeswas highly sensitive to monsoonal activity (Bar-ron et al., 1985; Oglesby and Park, 1989; Wort-mann et al., 1999). Studies on the Quaternaryfrom the Arabian Sea have shown that humidityand wind stress changed within the precessionalsignal in this monsoonally in£uenced region (e.g.,Clemens and Prell, 1990). Hence, the precession-controlled productivity £uctuations as indicatedby the NI may represent a monsoonal signal.The nutrient supply in the surface waters de-pended on the strength of monsoonal activity.During periods of enhanced monsoonal activity,which were characterized by humid conditions

and stronger winds, mixing of the upper watercolumn was most pronounced. This led to an en-trainment of nutrients into the surface waters. Pe-riods of reduced monsoonal activity were charac-terized by cooler, drier conditions and reducedwind stress and the surface waters were more de-pleted in nutrients. Eccentricity-steered tempera-ture changes were superimposed on the short-term precessional cycles (Figs. 6, 7).During formation of the Niveau Paquier and

HN8 black shales, eccentricity-driven increasesin temperature and humidity coincided with in-creasing amplitudes of changes in surface waterproductivity driven by precession (Fig. 6). Highsurface water productivity was induced by stron-ger winds accompanied by increasing humidity,leading to a better mixing of the upper water col-umn that resulted in higher surface water produc-tivity. In contrast, episodes of reduced wind stressand decreasing humidity resulted in a depletion ofnutrients in the surface waters.Summarizing, the l’Arboudeysse section is char-

acterized by precession-controlled surface waterproductivity changes, whereas temperature/hu-midity £uctuations are dominated by eccentricity.The Niveau Paquier originated under extremelywarm and humid conditions that started up toV9 kyr before the onset of black shale formation,representing the transition from a long-term(i.e., eccentricity-controlled) cool/dry to warm/hu-

Fig. 7. Power spectra of the nannoplankton-derived temperature and nutrient indices on linear scales based on 194 samples fromthe l’Arboudeysse section (modi¢ed after Herrle et al., 2003). TI spectrum shows peaks at: A, 460 cm; B, 210 cm; C, 87 cm; D,68 cm. NI spectrum shows peaks at: E, 74 cm; F, 68 cm. The cross depicts a 6 dB bandwidth and the 90% con¢dence intervalof the spectra. The letters depict the orbital periods of eccentricity, obliquity, and precession according to frequencies estimatedfor the mid-Cretaceous (Berger et al., 1992).

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mid period. Surface water productivity variedthroughout the Niveau Paquier, with increasingamplitudes during black shale formation.

5.2. Upper Aptian Pre¤-Guittard section

The NI of calcareous nannofossils ranges from15 to 64.4 (Fig. 8). Due to the general dominanceof low-productivity indicators in comparison tothe l’Arboudeysse section, oligo- to mesotrophicconditions can be assumed for the surface waters.Surface water productivity started to increase 15cm (V6 kyr, see Section 2.3 for chronostratigra-phy) before the onset of Niveau Kilian formation.The upper part of the black shale is characterizedby a rapid drop in surface water productivity.Moreover, productivity changes based on the NIcorrelate with the lithology. Pale layers are char-acterized by slightly higher productivity in com-parison to dark beds.The TI ranges from 30.6 to 82.3 (Fig. 8). Sur-

face water temperatures based on the TI changedfrom relatively low in the lower part of the suc-cession to relatively high from 30 cm below (V13kyr; see Section 2.3 for chronostratigraphy) theNiveau Kilian to the top of the succession.Short-term temperature £uctuations correspond-ing to the pale/dark bedding are superimposedon these long-term trends.Pre¤-Guittard N

18O values range from 34.1 to32.2x (Fig. 8). The lower and middle parts ofthe succession are characterized by more positivevalues. The N18O record shows a clear relationshipto the pale/dark bedding, with higher N18O valuesoccurring in dark beds. Di¡erences of the N

18Ovalues between pale and dark layers amount toup to 1.0x (Fig. 8). Starting 30 cm below theNiveau Kilian, the N

18O values decrease by up to1.6x. From the lower part of the Niveau Kilianto the upper part of the succession, the N

18O val-ues increase slowly. They are slightly more nega-tive as compared to the lower part of the succes-sion. With respect to temperature changes, theN18O record yields a similar picture as the TI ex-cept for below the Niveau Kilian. The more pos-itive N

18O values in this interval may be inter-preted to re£ect increasing evaporation ratesthat resulted in higher salinities.

The N18O signal translated into temperature

(neglecting possible salinity changes in order todescribe maximum possible temperature changes)indicates a rapid temperature increase of up toV6‡C starting V13 kyr before the onset of theNiveau Kilian (Fig. 8). N

18O-based temperaturechanges between pale and dark beds amount toa maximum of V4‡C, with higher temperaturesduring the sedimentation of pale beds.As in the l’Arboudeysse succession, the high-

frequency productivity changes within the paleand dark bedding of the Pre¤-Guittard sectionare probably also related to orbital forcing. Thiscan be derived from observations of Bre¤he¤ret(1994) and Ko«Mler et al. (2001), who interpretedthe pale/dark bedding in the Vocontian Basin asbeing controlled by the precessional cycle.Summarizing, the Pre¤-Guittard section is char-

acterized by high-frequency productivity changesthat, in analogy to the l’Arboudeysse section, areprobably related to orbital forcing within the pre-cessional band. The Niveau Kilian formed undermoderately warm and humid conditions thatstarted to increase up to V13 kyr before the on-set of black shale formation. Surface water pro-ductivity was variable during the deposition of theNiveau Kilian.

6. Short-term environmental changes during blackshale formation

6.1. Lower Albian Niveau Paquier succession

Based on our micropaleontological data, short-term changes in temperature, terrigenous input,humidity, productivity, and bottom water oxygen-ation during formation of the Niveau Paquier canbe assessed.Calcareous nannofossil TI values decrease with

the onset of the Niveau Paquier black shale, in-dicating a warming trend. The TMR of palyno-morphs, re£ecting terrigenous in£uence, shows a¢rst weak peak low in the section, a second, morepronounced peak within the black shale horizonbelow the Niveau Paquier, and a very strong in-crease with the onset of Niveau Paquier formationand surface water warming as evidenced by the

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nannofossil TI (Fig. 9A). The curve of absolutespore abundances shows similar £uctuations asthat of the TMR.The NI of calcareous nannofossils as a proxy

for surface water productivity exhibits high-fre-quency £uctuations before and within the NiveauPaquier controlled by precession (Fig. 9A; Herrleet al., 2003). High surface water productivity as

Fig. 8. Nutrient and temperature indices from the Pre¤-Guittard section based on calcareous nannofossils in comparison with theN18O record. High values of the NI indicate high productivity and vice versa. Low values of the TI indicate high temperaturesand vice versa. In the right column, thick lines indicate smoothed records based on the locally weighted least squared error meth-od after Chambers et al. (1983). For lithological explanations and abbreviations see Fig. 3. For a close-up of the TI data fromthe Niveau Kilian see Fig. 9B.

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indicated by the NI tends to correlate with highTMR values (Fig. 9A).Eutrophic indicators within the benthic forami-

nifera reach maximum percentages shortly beforethe NI and TMR peaks. Afterwards, they declinedrastically, while TMR and NI values increasefurther. Subsequently TMR values remain highand £uctuate only little during Niveau Paquierdeposition, whereas NI values oscillate strongly,thus indicating further variations in surface waterproductivity.Anoxic conditions at the sea £oor prevailed

during most of the time of Niveau Paquier depo-sition, as can be deduced from the lack of benthicforaminifera. Temporary low-oxygen (dysoxic)bottom waters are indicated by the sporadic oc-currence of Gavelinella spp. in the lower part ofthe Niveau Paquier. A similar repopulation event,marking a short-term change towards better ven-tilation at the sea £oor (Fig. 9A), has also beendescribed from the Niveau Paquier in the Vocon-tian Basin (Col de Palluel section) and the OAE1b black shale at the Blake Nose Plateau (Erbach-er et al., 1999).According to our data on temperature and hu-

midity changes (i.e., N18O, TI, TMR, and absolutespore abundances), the formation of Niveau Pa-quier occurred under increasingly warm and hu-mid conditions. The humidity increase led to ahigher £ux of terrigenous palynomorphs throughriverine input, as is indicated by high TMR valuesand absolute spore abundances.Changes in surface and bottom water produc-

tivity as reconstructed through the calcareousnannofossil NI and the benthic foraminiferal rec-ord can be explained by the development of highproductivity in the surface waters as a result ofincreasing wind stress. Rising surface water pro-ductivity, in turn, caused a higher organic matter£ux to the sea £oor. This resulted in rising per-centages of high-productivity indicators withinbenthic foraminifera. With a further increase insurface water productivity, oxygen depletion oc-curred at the sea £oor, causing a drastic decreaseof eutrophic benthic foraminifera.Within the Niveau Paquier black shale, surface

water productivity £uctuated strongly as is indi-cated by the NI. This can probably be attributed

to £uctuations in wind stress steered by preces-sion-controlled monsoonal intensity. During for-mation of the Niveau Paquier, the TMR values£uctuated only little. This may suggest that TMR£uctuations during Niveau Paquier formationwere mainly controlled by changes in humidity(i.e. riverine input) rather than by changes inwind stress. The main environmental characteris-tics of the Niveau Paquier as derived from ourproxy data are summarized in Table 2.

6.2. Upper Aptian Niveau Kilian succession

As it has been shown for the Niveau Paquier,short-term changes in temperature, terrigenous in-put, humidity, productivity, and bottom wateroxygenation can also be recognized during theformation of the Niveau Kilian.The calcareous nannofossil TI is characterized

by minor £uctuations, with generally warmer sur-face waters prevailing during formation of theNiveau Kilian (Fig. 9B). A short-term cooling oc-curred within the dark layer below the NiveauKilian. Terrigenous input as re£ected by theTMR curve of palynomorphs exhibits a slight in-crease in the dark layer below the Niveau Kilian,a rapid increase with the onset of black shaleformation, and a decrease in the upper part ofthe Niveau Kilian. Absolute spore abundancesshow a similar trend as the TMR curve.Sea surface productivity as indicated by calcar-

eous nannofossil NI exhibits a ¢rst peak belowthe Niveau Kilian, a second peak within the Ni-veau Kilian itself with a subsequent temporaryreduction to low values, and a minor increase atthe top of the black shale (Fig. 9B). High abun-dances of eutrophic indicators within the benthicforaminifera occur in the upper part of the Ni-veau Kilian and above the black shale. Gavelinellaspp. show highest percentages below and in theupper part of the Niveau Kilian, re£ecting anoxygen-poor environment at the sea £oor. Gave-linella spp. occur in all samples studied, indicatingdysoxic conditions during formation of the Ni-veau Kilian (Fig. 9B).Temperatures as documented in the N

18O andTI data rise V13 kyr before the onset of NiveauKilian formation (Fig. 8). This rise is accompa-

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nied by increasing TMR values and absolutespore abundances, indicating increasing terrestrialinput and, via enhanced runo¡, a rise in humidity.Surface and bottom water productivity, as

documented in the NI and the percentages ofbenthic foraminifera (Fig. 9B), increased beforethe onset of Niveau Kilian formation. This trendwas accompanied by increasing TMR values andabsolute spore abundances, re£ecting rising hu-midity. Eutrophic indicators are relatively rareamong benthic foraminifera and Gavelinella spp.dominate the benthic foraminiferal assemblage(Fig. 9B). The low abundances of eutrophic indi-

cators within benthic foraminifera despite highsurface water productivity are probably due topoor oxygenation. Further increasing surfacewater productivity within the Niveau Kilian tendsto correlate with increasing TMR values and ab-solute spore abundances. In the upper part of theNiveau Kilian, productivity decreases rapidly.This is accompanied by decreasing TMR valuesand absolute spore abundances, indicating a trendto reduced runo¡ and, thus to drier conditions.During this interval, the percentages of Gavelinel-la spp. increase, indicating that benthic oxygen-ation improved. The percentages of eutrophic in-

NiveauPaquier

0

1

2

3

mlithology

m

0

1

NiveauKilian

nutrient indexlow high

temperature indexcold warm

terrigenous/marine ratio

spores per gram

sediment (x10 )4

eutrophic indicators(% of assemblage)

Gavelinella spp.(% of assemblage)

calcareous nannofossils palynomorphs benthic foraminifera

25 50 75 0 2.5 5 0 5 0 2010 0 20 40

0 2.5 5 0 220 40 60 0 20 40 5025B

A 255075

4060

Fig. 9. (A) Detailed section of the supraregionally distributed Niveau Paquier from the l’Arboudeysse section with nutrient andtemperature indices based on calcareous nannofossils, TMR of palynomorphs and absolute spore abundances, and eutrophic indi-cators (Gyroidinoides spp., Valvulineria spp.) and Gavelinella spp. within the benthic foraminiferal record. High NI values, TMRof palynomorphs, and spore abundances re£ect high surface water productivity and increasing terrestrial input/humidity and viceversa. High values of the TI re£ect cool surface waters and vice versa. For lithological explanations and abbreviations see Fig. 3.(B) Detailed section of the regionally distributed Niveau Kilian from the Pre¤-Guittard section with nutrient and temperature indi-ces based on calcareous nannofossils, TMR of palynomorphs, absolute spore abundances, and eutrophic indicators and Gavelinel-la spp. within the benthic foraminifera record. For lithological explanations and abbreviations see Fig. 3.

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dicators within benthic foraminifera also increase,followed by decreasing values towards the topof the Niveau Kilian and above. A similar trendis shown by the NI and palynomorph records.Hence, slightly increasing surface water produc-tivity corresponds to increasing percentages ofeutrophic indicators derived from the benthic fo-raminifera. Table 2 summarizes the main environ-mental characteristics of the Niveau Kilian as de-rived from our proxy data.

6.3. Environmental similarities and di¡erencesbetween the Niveau Paquier and Niveau Kilian

The supraregionally distributed Niveau Paquierand the regionally occurring Niveau Kilian fromthe Vocontian Basin show similar characteristicsregarding general temperature, terrestrial input,humidity, and productivity changes (Table 2).Di¡erences, however, exist in the amplitude ofchanges within these factors and sea £oor oxygen-ation during black shale deposition and in theduration of black shale formation.Temperature, terrestrial input, and humidity

started to rise up to several thousand years before

the onset of both the Niveau Paquier (33 cm,equivalent to V9 kyr) and Niveau Kilian forma-tion (30 cm, equivalent to V13 kyr; Figs. 6, 8, 9).In both cases, surface water productivity washighly variable during black shale deposition,probably owing to £uctuations in wind stresssteered by changes in the monsoonal climate sys-tem.Di¡erences between both black shales are re-

lated to the extent of temperature and humiditychanges. Based on N

18O values and neglectingpossible salinity changes, the temperature increaseof 8‡C with the onset of the Niveau Paquier is upto 2‡C higher as compared to the Niveau Kilian.Moreover, the ratio between absolute spore abun-dances and absolute pollen abundances is muchhigher in the Niveau Paquier than in the NiveauKilian, indicating extremely humid conditionsduring the deposition of the Niveau Paquier incomparison to the Niveau Kilian (Fig. 10; Table2). This is because spore-producing plants requirehumid conditions in order to proliferate. Alterna-tively, the lower spore/pollen ratio in the NiveauKilian could also be due to an increased distanceto the shoreline as most spores have a relatively

Table 2Main paleoenvironmental characteristics of the Early Albian Niveau Paquier and Late Aptian Niveau Kilian black shales fromthe Vocontian Basin based on calcareous nannofossil, palynomorph, benthic foraminiferal, and oxygen isotope records

Supraregional black shale Regional black shale(Niveau Paquier) (Niveau Kilian)

GeochemistryTOC up to 8% up to 3.5%Temp. increase (N18 O) up to 8‡C up to 6‡CCalcareous nannofossilsNI (productivity) variable (mean: 49.9) variable (mean: 37.2)TI (temperature) increasing (mean: 51.3) increasing (mean: 50.6)PalynomorphsTMR high (mean: 2.7) high (mean: 2.9)Spore abundances very high (mean: 26 175/g) high (mean: 15 974/g)Benthic foraminiferaEutrophic indicators high highOpportunistic species mostly absent mostly presentPaleoenvironmental interpretationEstimated duration V44 kyr V31 kyrSurface water fertility variable (meso- to eutrophic) variable (mesotrophic)Surface water mixing variable variableO2 at the sea £oor predominantly anoxic predominantly dysoxicHumidity extreme moderate

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low buoyancy, resulting in a deposition center inrelatively proximal settings (e.g., Traverse, 1988).For the studied section, however, this scenariocan be ruled out owing to the very similar sealevel during deposition of the Niveau Paquierand the Niveau Kilian (compare Bralower et al.,1994; Bre¤he¤ret, 1997).Bottom water conditions as indicated by

benthic foraminifera were predominantly anoxicto dysoxic during Niveau Paquier formation,whereas they were dysoxic during Niveau Kilianformation. Regarding the duration of black shaleformation, the Niveau Paquier was deposited overa longer interval (163 cm, equivalent to V44 kyr)than the Niveau Kilian (74 cm, equivalent to upto V31 kyr).

7. Driving mechanisms for supraregional andregional black shale formation duringthe mid-Cretaceous

Our micropaleontological approach yields a

consistent data set that can be used to establishthe main driving factors for the formation of thesupraregional Niveau Paquier and the regionalNiveau Kilian. In the following, the roles of tem-perature, terrestrial input/humidity, productivity,and runo¡ in black shale formation will be dis-cussed. Moreover, a model for the formation ofthe Niveau Paquier and Niveau Kilian blackshales will be proposed.

7.1. Temperature and humidity/terrestrial input

The formation of both the Niveau Paquier andthe Niveau Kilian is linked to increasingly warmand humid conditions. The supraregional NiveauPaquier is characterized by an extreme tempera-ture and humidity increase, whereas temperatureand humidity conditions were more moderateduring the formation of the Niveau Kilian (Figs.6, 8^10).The observation that temperature, terrestrial in-

put, and humidity increase during mid-Cretaceousblack shale formation is in agreement with pre-vious studies that connected the deposition ofblack shales to an accelerated hydrological cycle(e.g., Weissert et al., 1979, 1998; Weissert, 1989;Fo«llmi et al., 1994; Hochuli et al., 1999). As pro-posed by these authors, black shale formationmay have been triggered by the strengtheningof continental weathering and increased runo¡which caused enhanced surface water productiv-ity. According to several authors (e.g., Haber-mann and Mutterlose, 1999; Erbacher et al.,2001), the increased runo¡ probably contributedto a better preservation of organic matter at thesea £oor as a result of increased thermohalinestrati¢cation.

7.2. Productivity

The enhanced £ux of marine organic matterduring the formation of both the Niveau Paquierand the Niveau Kilian is partly due to increasedproductivity in the surface waters as discussedabove. However, surface water productivity var-ied during the formation of the black shales. Wesuggest that productivity £uctuations during for-mation of the Niveau Paquier and the Niveau

0·104

sporespergramsediment

pollen per gram sediment

1·104

2·104

3·104

4·104

5·104

6·104

0·105

0.5·105

1.0·105

1.5·105

2.0·105

high humidity

low humidity

Fig. 10. Scatter plot showing the ratios between absolutespore and absolute pollen abundances from the Niveau Pa-quier of the l’Arboudeysse section and the Niveau Kilian ofthe Pre¤-Guittard section. Highest spore/pollen ratios re£ectmost humid conditions and vice versa. Samples from the Ni-veau Paquier are marked by black dots and samples fromthe Niveau Kilian by circles.

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Kilian are linked to changes in wind stress trig-gered by a monsoonal climate system. This modelis similar to the Quaternary Arabian monsoonsystem. Paleoproductivity records of this regionare strongly controlled by changes in precession-ally driven £uctuations in summer surface waterproductivity (e.g., Reichart et al., 1997; Den Dulket al., 1998). Increasing wind speeds during en-hanced monsoonal activity may have allowed abetter mixing of the sea surface waters, resultingin increased nutrient supply from subsurfacewaters. Reduced monsoonal activity is character-ized by reduced wind stress, leading to nutrientdepletion in the surface waters. However, basedon the strong £uctuations of surface water pro-ductivity during Niveau Paquier and Niveau Ki-lian formation, enhanced productivity alone canbe excluded as a cause for the large amount oforganic matter buried in both black shales.

7.3. Runo¡ during black shale formation

The rapid TMR increase during formation ofthe Niveau Paquier and the Niveau Kilian indi-cates enhanced terrigenous input into the Vocon-tian Basin. High TMR values during black shaleformation have often been invoked as indicatorsof increased riverine runo¡, causing a densitystrati¢cation and a productivity rise in the surfacewaters (e.g., Rossignol-Strick, 1985; Rohling,1991; Below and Kirsch, 1997). For the mid-Cre-taceous, a runo¡-induced density strati¢cation hasbeen proposed for the formation of the OAE 1bblack shale from the Atlantic Ocean (Erbacher etal., 2001) and the coeval Niveau Paquier from theVocontian Basin (Tribovillard and Gorin, 1991).According to the latter authors, the OAE 1b (Ni-veau Paquier) formation was mainly a result ofenhanced preservation. However, a calculationof the freshwater budget for the western Tethysshows that the riverine discharge required to es-tablish a persistent density strati¢cation (s 1x)is nearly four times higher than that of the mod-ern Nile River which drains up to 91 km3/yr(Foucault and Stanley, 1989) into the Mediterra-nean Sea. During the mid-Cretaceous, the north-ern borderlands of the western Tethys were toosmall to provide enough freshwater to trigger

black shale formation (Fig. 11). Although ourpalynological data would be compatible withsuch a scenario both for the formation of thesupraregional Niveau Paquier and the regionalNiveau Kilian, a persistent density strati¢cationcaused by riverine runo¡ can therefore be ruledout for that region. Hence, in the following analternative model for the formation of the NiveauPaquier and Niveau Kilian black shales is pro-posed.

7.4. Model for black shale formation

Both the supraregional Niveau Paquier andthe regional Niveau Kilian formed shortly afterthe onset of a long-term, eccentricity-controlledwarm and humid period as indicated by theN18O, TI, TMR, and absolute spore abundancedata.We suggest that the supraregional signi¢cance

of the Niveau Paquier is due to the reduction oflow-latitude deep water formation during ex-tremely warm and humid conditions. Under ex-treme monsoonal forcing, deep water productionat the main sites of deep water formation in thelow latitudes (such as SW Asia; Barron and Pe-terson, 1990; Fig. 11) was probably diminishedbecause of the enhanced precipitation rates inthese areas under extremely humid conditions(Herrle et al., 2003). This is because a decreasein evaporation rates resulted in a decrease of sur-face water densities and, when a threshold valuewas reached, in a drastic reduction of deep waterformation (e.g., Bice et al., 1997). Longer periodsduring which the precipitation rate exceeded evap-oration are characterized by a sluggished or di-minished oceanic circulation mode. This favoredthe widespread preservation of organic matter dueto anoxic to dysoxic conditions at the sea £oor asthey are indicated by our benthic foraminiferaldata.The formation of the regionally distributed Ni-

veau Kilian is also linked to warm and humidconditions resulting from monsoonal forcing.However, the threshold value for a supraregionaldistribution was not reached. This is indicated bya weaker temperature increase and less humidconditions in comparison to the Niveau Paquier

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as suggested by the N18O data (Figs. 6, 8, 9) and

spore/pollen ratios (Fig. 10). Therefore, deepwater formation and bottom water ventilationwere reduced to a lesser extent and/or on a re-gional scale only (Fig. 11). Moreover, bottomwater oxygenation remained on a dysoxic levelthroughout black shale formation. This is re-£ected in the persistent occurrence of benthic fo-raminifera within the black shale.

Based on our data, both the Niveau Paquierand the Niveau Kilian in the Vocontian Basinare characterized by varying surface water pro-ductivity controlled by changes in a monsoonalclimate system. Productivity was an importantfactor in the formation of both studied blackshales. Much more signi¢cant, however, was theenhanced preservation of organic matter due toreduced ventilation at the sea £oor, probably as

Fig. 11. Proposed model for the formation of the supraregionally distributed Niveau Paquier and the regionally distributed Ni-veau Kilian, with schematic mean annual atmospheric circulation pattern for the low latitudes at insolation maximum during themid-Cretaceous. Principal elements of the mid-Cretaceous climate in the low-latitude region as depicted by climate models (e.g.,Oglesby and Park, 1989; Barron and Peterson, 1990; Price et al., 1995; Poulsen et al., 1998). Paleogeography of the mid-Creta-ceous with high sea-level stand shorelines (modi¢ed after Hay et al., 1999). During extremely warm and humid conditions andstrong monsoonal climate (as prevailing during Niveau Paquier formation), deep water formation was restricted in the northernand eastern Tethyan area. During Niveau Kilian formation, in contrast, the temperature and humidity increase was more moder-ate. During that time, deep water formation was restricted only in the area of the Vocontian Basin. Dark gray areas indicateland, areas of restricted deep water formation are hatched. H, high-pressure system; T, low-pressure system; VB, Vocontian Ba-sin.

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a function of decreasing deep water formation inthe low latitudes.

8. Conclusions

Calcareous nannofossil, palynomorph, benthicforaminiferal, and oxygen isotope data from thesupraregionally distributed Niveau Paquier andregionally distributed Niveau Kilian black shalesin the Vocontian Basin provide information onthe driving mechanisms of mid-Cretaceous blackshale formation. Environmental changes recon-structed from our internally consistent proxydata comprise variations in temperature, terres-trial input, humidity, and productivity. The mostsalient results of our study are the following:(1) The studied sections show strong signatures

of a mid-Cretaceous monsoonal climate as is in-dicated by the dominance of precession-controlledvariations in surface water productivity.(2) Eccentricity-controlled changes in tempera-

ture and evaporation/precipitation patterns werethe most important climate factors in£uencingthe hydrological cycle and deep water formationin the low latitudes and therefore in the deposi-tion of the Niveau Paquier and Niveau Kilian.(3) The supraregionally distributed Niveau Pa-

quier formed under strongly increasing tempera-tures and humidity, whereas the regionally dis-tributed Niveau Kilian formed under moremoderately increasing temperatures and humidity.Extremely increasing temperatures and humidity(with precipitation prevailing over evaporation)had a dramatic impact on low-latitude deep waterformation on a supraregional scale, whereas amoderate increase in these conditions probablyin£uenced deep water formation to a lesser degreeand/or on a regional scale only. Therefore, theintensity of temperature and humidity increasecontrolled if mid-Cretaceous black shales devel-oped supraregionally or regionally. Increased run-o¡ rates that caused a persistent density strati¢-cation and therefore favored the preservation oforganic matter at the sea £oor can most likely beruled out as an important driving mechanism forblack shale formation in the Vocontian Basin.(4) The monsoonally forced productivity

changes only represent a regional climate signal.Because productivity increased during the forma-tion of both the Niveau Paquier and Niveau Ki-lian, this parameter must certainly have played arole in the formation of these black shales. How-ever, the majority of organic matter buried duringblack shale formation was probably a result ofenhanced organic matter preservation at the sea£oor.

Acknowledgements

Phil Meyers and Alessandra Negri are thankedfor the editorial work. Helmut Erlenkeuser (Kiel)is thanked for stable isotope measurements andMichaela Blessing for technical assistance. Help-ful comments of the reviewers Helmut Weissertand Sherwood Wise are highly appreciated. Thisstudy was funded by the Deutsche Forschungsge-meinschaft within the SFB 275 (project A5) of theUniversity of Tu«bingen and through Grant He697/32.

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