Forcing mechanisms for mid-Cretaceous black shale formation: evidence from the Upper Aptian and Lower Albian of the Vocontian Basin (SE France)

Palaeogeography, Palaeoclimatology, Palaeoecology 190 (2003) 399^426 www.elsevier.com/locate/palaeo

Forcing mechanisms for mid-Cretaceous black shale formation: evidence from the Upper Aptian and Lower 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 the supraregionally distributed Niveau Paquier (Early Albian age, Oceanic Anoxic Event 1b) and regionally distributed Niveau Kilian (Late Aptian age) black shales in the Vocontian Basin (SE France) exhibit variations that reflect paleoclimatic and paleoceanographic changes in the mid-Cretaceous low latitudes. To quantify surface water productivity and temperature changes, nutrient and temperature indices based on calcareous nannofossils were developed. The nutrient index strongly varies in the precessional band, whereas variations of the temperature index reflect eccentricity. Since polar ice caps were not present during the mid-Cretaceous, these variations probably result from 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 through monsoonally driven changes in temperature and evaporation/precipitation patterns. The Lower Albian Niveau Paquier, which has a supraregional distribution, formed under extremely warm and humid conditions when monsoonal intensity was strongest. Bottom water ventilation in the Vocontian Basin was diminished, probably due to increased precipitation and reduced evaporation in regions of deep water formation at low latitudes. Surface water productivity in the Vocontian Basin was controlled by the strength of monsoonal winds. The Upper Aptian Niveau Kilian, 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 the formation of a supraregional black shale was not reached. The intensity of increases in temperature and humidity controlled whether black shales developed on a regional or supraregional scale. At least in the Vocontian Basin, the increased preservation of organic matter at the sea floor was more significant in black shale formation than the role of enhanced productivity. 8 2002 Elsevier Science B.V. All rights reserved. Keywords: black shale; Cretaceous; benthic foraminifera; calcareous nannofossils; palynomorphs; monsoon

* Corresponding author. Tel.: +49-7071-297-7509; Fax: +49-7071-295-766. E-mail address: [email protected] (J.O. Herrle).

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

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1. Introduction 1.1. Mid-Cretaceous paleoceanographic and paleoclimatic background conditions The mid-Cretaceous has been characterized as a time of global warmth when the Earth’s climate was in an extreme greenhouse mode. Based on oxygen isotopes from benthic foraminifera, the deep oceans were signi¢cantly warmer than at present (e.g., Savin, 1977; Erbacher et al., 2001). The occurrence of warm and saline bottom waters has been taken as evidence that deep water formation occurred at low latitudes (Brass et al., 1982). This scenario has been veri¢ed by oceanic general circulation simulations (Barron and Peterson, 1990) and also suggested by several other studies (e.g., Bice et al., 1997; Voigt et al., 1999). More recent model experiments also demonstrate the signi¢cance of high-latitude deep water formation during the mid-Cretaceous (Brady et al., 1998; Haupt and Seidov, 2001; Poulsen et al., 2001). Hence, although not exclusively responsible for mid-Cretaceous deep water formation, low-latitude sites played an important role in this process. The general paleoclimatic conditions of the mid-Cretaceous can be described as predominantly humid in the northern hemisphere (North America, Eurasia; e.g., Voigt, 1996) and both dry and humid in the southern hemisphere (South America, Antarctica, India, Madagascar, Australia, Africa). The northern and southern continents were separated by the wide open eastern Tethys and the narrow western Tethys and Atlantic Ocean. The land^sea con¢guration of the midCretaceous rendered the low latitudes highly sensitive 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 latitudes (Hay et al., 1999), probably with a stable, westward-£owing circumglobal current throughout the Tethys (e.g., Roth, 1986; Barron, 1987). Recently, this view has been questioned by Poulsen et al. (1998), who presented arguments for a more complicated circulation pattern dominated by a clockwise gyre in the western Tethys.

The paleoceanographic and paleoclimatic conditions as described above favored the formation of regionally and supraregionally distributed black shales. Mid-Cretaceous sections from the western Tethys and Atlantic Ocean contain numerous black shale horizons, some of which are more or less synchronous in di¡erent basins and therefore have been termed Oceanic Anoxic Events (OAEs). These are, for instance, the OAE 1a of Early Aptian age (e.g., Schlanger and Jenkyns, 1976; Arthur et al., 1990; Bralower et al., 1994) and the OAE 1b of Early Albian age (e.g., Bre¤he¤ret, 1988; Bralower et al., 1993; Erbacher et al., 1999; Herrle, 2002). OAEs represent major perturbations of the ocean system de¢ned by massive deposition of organic matter in marine environments (e.g., Schlanger and Jenkyns, 1976; Arthur et al., 1990). The rapid changes in the carbon cycle were accompanied by turnovers in marine 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, the origin and spatial distribution of mid-Cretaceous OAEs are still a matter of debate. Di¡erent models of climate and ocean circulation have been proposed, either stressing the in£uence of productivity or preservation on black shale formation. Upwelling and elevated productivity, higher runo¡ resulting in a thermohaline strati¢cation, and changes in rate or mode of deep water circulation and ventilation all have been suggested as driving mechanisms for increasing organic carbon burial rates. Various authors suggested that the burial of organic carbon was primarily a function of sedimentation rates and organic productivity (e.g., Weissert, 1989; Pederson and Calvert, 1990; Calvert and Pederson, 1992; Erbacher et al., 1999; Hochuli et al., 1999; Premoli Silva et al., 1999). They attributed the formation of black shales to increased surface water productivity during warmer and more humid climate conditions. Increasing runo¡ accelerated the transfer of nutrients from the continents into the oceans, inducing higher primary 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 horizons are linked to speci¢c positions of the sea level. They distinguished between black shales resulting from increased oceanic productivity (productivity oceanic anoxic events, P-OAEs of Erbacher et al., 1996; condensed-type of Bre¤he¤ret, 1994) and black shales resulting from increased sedimentation of terrestrial organic matter (detrital oceanic anoxic events, D-OAEs of Erbacher et al., 1996 ; fed-type of Bre¤he¤ret, 1994). P-OAEs were believed to be connected to transgressive periods, whereas D-OAEs were suggested to correlate with still-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 assumption that high sea level and changes in thermohaline circulation resulted in decreasing subsurface oxygen concentrations and an expansion of the oxygen minimum zone. This led to an increased burial of organic carbon (e.g., Schlanger and Jenkyns, 1976; Bralower and Thierstein, 1984; de Graciansky et al., 1984; Arthur et al., 1990). In addition, organic-rich horizons have been interpreted to be the result of local oceanographic and topographic factors, resedimentation of organic matter from shallow to deep water environments, increased supply of terrestrial organic carbon, or sea-level falls (e.g., Jenkyns, 1980; Habib, 1982; Thurow and Kuhnt, 1986). Most recently, Kuypers et al. (2001) introduced a new aspect of organic matter accumulation at the sea £oor by highlighting the role of archaebacteria during formation of the OAE 1b black shale in the Atlantic Ocean and Vocontian Basin. In addition to OAE black shales, regionally distributed black shales are common in the Aptian to Albian sediments of the western Tethys (e.g., Tornaghi et al., 1989; Bre¤he¤ret, 1994). These black shale horizons are partly characterized by a cyclic occurrence and have been interpreted as a result of changes in seasonality (e.g., Herbert and Fischer, 1986; Erba and Premoli Silva, 1994). According to these authors, productivity was high and bottom waters were well oxygenated during times of high seasonality, whereas weak-seasonality intervals were characterized by low surface water

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productivity and less well oxygenated bottom waters. 1.3. Aim of the study The models of enhanced organic matter accumulation outlined above may su⁄ciently describe the origin of mid-Cretaceous supraregional black shales (OAEs). However, most paleoceanographic models for mid-Cretaceous black shale formation are based on data with a temporal resolution that is by an order of magnitude coarser than the time scales on which many oceanographic processes operate. Therefore, Milankovitch-scale temperature, terrestrial input/humidity, and productivity variations, which have been shown to play an important role in Late Neogene and Quaternary black shale formation (e.g., Rossignol-Strick et al., 1982; Rossignol-Strick, 1985), are yet poorly documented and understood for the mid-Cretaceous. To obtain insight into the development and paleoenvironmental characteristics of supraregional and more regional black shales, we studied sections containing the Lower Albian Niveau Paquier and the Upper Aptian Niveau Kilian from the Vocontian Basin, SE France. In contrast to the regionally distributed Niveau Kilian, which occurs only in the northwestern part of the western Tethys, the Niveau Paquier is known from several sections of the Tethyan realm (Bre¤he¤ret et al., 1986; Bre¤he¤ret, 1994; Arthur et al., 1990; Bralower et al., 1993) and is probably coeval with organic-rich horizons in the North, South, and Central Atlantic named OAE 1b (Bralower et al., 1993; Erbacher et al., 1999; Herrle, 2002). Both the Niveau Paquier and the Niveau Kilian were formed during relatively high sea level under basically identical paleo-latitudes (cf. Bralower et al., 1994). Thus, a framework is provided to compare the formation of obviously di¡erent black shales under partly identical boundary conditions. We used calcareous nannofossils as a proxy for surface water temperature and productivity, palynomorphs as a proxy for terrestrial input and humidity, benthic foraminifera as a proxy for bottom water conditions, and oxygen isotopes to assess temperature trends to investigate the paleo-

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

environmental dynamics of the Late Aptian and Early Albian Tethyan Ocean on geologically short time scales. We have di¡erentiated between longand short-term environmental changes as observed in the studied sections. For greater clarity, the data are presented and interpreted in the same sections.

2. Location, paleogeography, lithology, and chronostratigraphic framework The studied l’Arboudeysse and Pre¤-Guittard sections are located in the northern part of the Vocontian Basin, SE France (Fig. 1). The Vocontian Basin is located between the Vercors Massif, the river Rhone, the Ventoux^Lure Axis, and the

Nice^Castellane Arch of the southern Subalpine Ranges. 2.1. Paleogeography During the mid-Cretaceous, the Vocontian Basin belonged to the northern part of the western Tethyan Ocean and was situated at a paleo-latitude of 25^30‡ North (Savostin et al., 1986; Fig. 2). In the northwest, the boundary of the basin was de¢ned by the Massif Central landmass (Figs. 1, 2). The eastern part was open towards the Tethys. Otherwise, the basin was surrounded by slopes and platforms with a hemipelagic facies intercalating with shallow-water carbonates (Arnaud and Lemoine, 1993), thus allowing an exchange with the open Tethys.

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403

North America Asia 30°N

30°N

VB Atlantic Ocean

Western Tethyan Ocean 0°

0°

South America

Africa

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).

The paleo-water depth of the Vocontian Basin is still under debate. Floral and faunal data suggested a depth of several hundred meters (Wilpshaar and Leereveld, 1994). Earlier authors, however, argued that the basin was more than 2000 m deep (Cotillon and Rio, 1984). 2.2. Lithology The l’Arboudeysse and Pre¤-Guittard sections consist of a monotonous sequence of marly hemipelagic to pelagic sediments of the Marnes Bleues Formation (Fig. 3). The lithology is characterized by pale and dark bedded marlstones with intercalated marly limestones, limestones, and black shale horizons. The Marnes Bleues Formation is widely characterized by the cyclic deposition of pale and dark sediments (Bre¤he¤ret, 1988, 1994, 1997; Ko«Mler et al., 2001). 2.2.1. Lower Albian l’Arboudeysse section The Lower Albian l’Arboudeysse section, which contains the supraregionally distributed Niveau Paquier (OAE 1b), is situated 5 km east of Rosans (topographic map 25 Rosans, No. 3239 Ouest, Serie Bleues, Lambert III coordinates X: 854 850, Y: 3238 825). Biostratigraphically, the section is located in the middle part of the Leymeriella tardefurcata ammonite zone and Prediscosphaera columnata nannoplankton zone of the Lower Albian (middle part of the NC8B subzone; Fig. 3). Lithologically, the section consists of ca.

14 m of marlstones which are punctuated by the black shale layers Haut Noir 7 (HN7) and Haut Noir 8 (HN8) and by the Niveau Paquier which reaches about 1.63 m in thickness (Fig. 3). The marlstones show no pale/dark cyclicity. The HN7 (thickness : 38 cm) and HN8 (thickness: 35 cm) black shales, named by Bre¤he¤ret (1997), are characterized by a total organic carbon (TOC) content of up to 1.5%. In the Niveau Paquier, the TOC content generally exceeds 3% (maximum : 8%) and is derived from both terrestrial and marine organic matter (Bre¤he¤ret, 1985, 1997; Tribovillard and Cotillon, 1989). 2.2.2. Upper Aptian Pre¤-Guittard section The Upper Aptian Pre¤-Guittard section, comprising the regionally occurring Niveau Kilian, is situated at the SE £ank of the Serre Sablon about 1 km NE of Arnayon (topographic map 25 Dieule¢t, No. 3138 Ouest, Lambert III coordinates X: 836 800, Y: 3248 825). Biostratigraphically, the section comprises the upper Hypacanthoplites jacobi ammonite zone and the lower part of the Prediscosphaera columnata nannoplankton zone of the Upper Aptian (upper part of the NC8A subzone; Fig. 3). About 6 m of pale and dark bedded marlstones, marly limestones, and the Niveau Kilian were investigated (Fig. 3). The thickness of the pale beds is between 14 cm and 100 cm, whereas that of the dark beds ranges from 15 to 46 cm. The thickness of individual beds increases from the lower to the upper part of the

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Fig. 3. Compiled lithological column of the Aptian to Lower Albian of the Vocontian Basin (SE France) with regional and supraregional 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 references therein), nannofossil zonation after Herrle and Mutterlose (2003). P. nut¢eld., Parahoplithes nut¢eldiensis; H. nolani, Hypacanthoplites 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.

section. The contrast between pale and dark beds is only weakly expressed. The upper part of the section 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 up to 3.3% and the organic matter consists mainly of

kerogen type III (Bre¤he¤ret, 1997), indicating a terrestrial origin. 2.3. Chronostratigraphic framework Based on biostratigraphic data, sedimentologi-

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cal parameters, and nannoplankton assemblages, an age model was established in order to obtain time control for the deposition of the Niveau Paquier and the Niveau Kilian. According to Bralower et al. (1995, 1997), the Aptian to Lower Albian Rhagodiscus angustus (NC7A to NC7C) and Prediscosphaera columnata (NC8A to NC8B) nannofossil zones comprise about 11.7 Ma. In the Vocontian Basin, sediments attributed to these zones are up to 355 m thick (Bre¤he¤ret, 1997). This yields an average sedimentation rate of approximately 3 cm/kyr. Time series analyses of pale/dark color changes re£ecting precessional cycles yielded a sedimentation rate of 3.5 cm/kyr for the lowermost Upper Aptian of the Vocontian Basin (Ko«Mler et al., 2001). The slightly lower sedimentation rate derived biostratigraphically is probably due to condensed intervals of the Niveau Blanc and Faisceau Fromaget within the succession (Jacquin and de Graciansky, 1998). Spectral analyses of the calcareous nannofossil assemblages from the l’Arboudeysse section showed that they are in£uenced by precession and eccentricity (Herrle et al., 2003). These analyses indicate a mean sedimentation rate of 3.7 cm/kyr for the l’Arboudeysse section, a value that is well compatible with the independently derived rates as described above. Hence, the Niveau Paquier with a thickness of 1.63 m formed within V44 kyr. This is in good agreement with the estimated duration of V46 kyr for OAE 1b formation in the Central Atlantic (Erbacher et al., 2001; Herrle, 2002). The mean sedimentation rate of the uppermost Upper Aptian at the Pre¤-Guittard section can be estimated at a minimum of 2.4 cm/ kyr as pale and dark beds have been shown to re£ect the precessional cycle (18.4 kyr; Ko«Mler et al., 2001; Herrle, 2002). Hence, based on a thickness of 74 cm, the Niveau Kilian comprises a time interval of V31 kyr or less.

405

l’Arboudeysse and Pre¤-Guittard sections. Samples were prepared following Geisen et al. (1999). At least 300 specimens were counted in random traverses of each slide. Calcareous nannofossils were studied using an Olympus BX50 microscope at U1250 magni¢cation.

3.1. Calcareous nannofossils

3.1.1. Calcareous nannofossil nutrient and temperature indices The paleobiogeography and paleoecology of mid-Cretaceous nannofossils have been investigated by many authors. Nutrient availability and temperature in the surface waters are among the most prominent parameters in£uencing the distribution and composition of calcareous nannoplankton (e.g., Brand, 1994; Roth, 1994; Winter et al., 1994; Burnett et al., 2000). To quantify these parameters into nutrient and temperature indices, R-mode (Varimax-rotated) Principal Component Analyses (R-PCA) were performed on data sets from the l’Arboudeysse (194 samples) and Pre¤-Guittard sections (85 samples) using the statistics software SYSTAT 51. Two samples from the Pre¤-Guittard section were excluded from the evaluation due to poor nannoplankton preservation. PC loadings of s 0.4 were assigned to associated taxa (Malmgren and Haq, 1982) and PC loadings s 0.5 were assigned to dominant taxa of a calcareous nannofossil assemblage. The R-PCA yielded four calcareous nannofossil assemblages for the l’Arboudeysse section and two for the Pre¤-Guittard section (Table 1). The multivariate model explains 57.5% and 37.4% of the total variance of the data sets for the l’Arboudeysse and Pre¤-Guittard section, respectively. In the nutrient and temperature indices, the ratios between selected taxa are evaluated. The use of ratios with respect to surface water productivity and temperature provides a clearer signal than the assessment of single species abundances. This is documented in thanatocoenoses of recent material where species ratios yield a better record of the surface water signal than the abundance of single species (Andruleit, 1995).

High-resolution studies with sample intervals of 2^10 cm were carried out on 281 samples from the

3.1.1.1. Nutrient index (NI). Based on multivariate statistics and other quantitative analyses,

3. Materials and methods

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

Associated species

(A) Calcareous nannofossil assemblages from the l’Arboudeysse section (R-mode) PC1 Biscutum constans 0.69 Lithraphidites carniolensis 0.61 Nannoconus spp. 30.87 Orastrum spp. 30.74 Broinsonia signata 30.50 PC2 Zeugrhabdotus erectus 0.55 Discorhabdus rotatorius Watznaueria barnesae 30.74 Repagulum parvidentatum 30.73 PC3 Zeugrhabdotus trivectis 0.50 Discorhabdus rotatorius Zeugrhabdotus diplogrammus 30.73 Staurolithites stradneri 30.58 Seribiscutum spp. 30.52 PC4 Biscutum a¡. ellipticum 0.70 Seribiscutum spp. 0.57 Rhagodiscus asper 30.78 (B) Calcareous nannofossil assemblages from the Pre¤-Guittard section (R-mode) PC1 Zeugrhabdotus diplogrammus 0.61 Zeugrhabdotus erectus Rhagodiscus asper 0.55 Zeugrhabdotus trivectis 30.74 Repagulum parvidentatum Staurolithites stradneri 30.68 PC2 Discorhabdus rotatorius 0.80 Lithraphidites carniolensis Zeugrhabdotus erectus 0.50 Watznaueria barnesae 30.82 Zeugrhabdotus diplogrammus Repagulum parvidentatum

Var. (%) 20.4

0.40

12.9

0.44

11.2

13.0

0.47

19.4

30.46 0.44

18.0

30.45 30.42

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

Roth and Bowdler (1981), Roth and Krumbach (1986), and Erba et al. (1992) identi¢ed calcareous nannofossil taxa closely related to surface water productivity. The cosmopolitan taxa Zeugrhabdotus erectus and Biscutum constans were assigned to areas of enhanced surface water productivity. Moreover, studies on Aptian to Albian material from Italy indicate that Discorhabdus rotatorius is also positively related to productivity (Premoli Silva et al., 1989; Erba, 1991; Coccioni et al., 1992). The above-mentioned species frequently occur in upwelling areas (e.g., Roth and Bowdler, 1981) and on the shelves where nutrients may have been added by terrestrial runo¡ (e.g., Street and Bown, 2000). In contrast, the taxon Watznaueria barnesae is widely interpreted as a lowproductivity 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 Principal Component 2 (PC2; l’Arboudeysse, Pre¤-Guittard) were used to determine surface water productivity (Table 1). The PC2 assemblages comprise the high-productivity indicators Discorhabdus rotatorius and Zeugrhabdotus erectus (positive loadings) as well as the low-productivity indicator Watznaueria barnesae (negative loadings). Repagulum parvidentatum, which also appears in PC2, has been excluded from the NI because its distribution is controlled by cool and nutrient-depleted surface waters (compare Erba et al., 1992). This species typically occurs in high latitudes and is rarely observed in the Tethyan realm (Mutterlose and Wise, 1990; Mutterlose, 1992; Street and Bown, 2000), indicating that its distribution is primarily controlled by temperature. These ¢ndings suggest that its co-occurrence with the lowproductivity indicator W. barnesae in the study material is only indirect.

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Based on the information derived from the literature as lined out above and the distribution of the index species in the study material, the NI NI ¼

Ze þ Dr U100 Wb þ Ze þ Dr

ð1Þ

was established in order to determine surface water productivity (with Ze = Zeugrhabdotus erectus, Dr = Discorhabdus rotatorius, and Wb = Watznaueria barnesae). 3.1.1.2. Temperature index (TI). Within midCretaceous calcareous nannofossil assemblages, Repagulum parvidentatum and Seribiscutum spp. are known to bear a high-latitude and, thus, cold-water signal (Erba et al., 1992; Mutterlose and Wise, 1990; Street and Bown, 2000). Biscutum a¡. ellipticum (Biscutum constans large) is also interpreted to prefer cooler surface waters. The occurrence of Rhagodiscus asper, in contrast, re£ects warmer surface waters (e.g., Erba et al., 1992; Mutterlose, 1989, 1996). The paleoecological a⁄nities of Staurolithites stradneri and Zeugrhabdotus trivectis have not been determined previously, but our R-PCA results show that the distribution of these species is inversely correlated with that of the warm-water species R. asper. Consequently, they may represent cool-water markers. The distribution pattern of Zeugrhabdotus diplogrammus correlates with that of the warm-water species R. asper and therefore Z. diplogrammus is considered to represent a warmwater marker as well. To determine the TI, the positive and negative loadings of PC4 from the l’Arboudeysse section and PC1 from the Pre¤-Guittard section were used (Table 1). These factors comprise Biscutum a¡. ellipticum, Seribiscutum spp., Staurolithites stradneri, Zeugrhabdotus trivectis, and Repagulum parvidentatum (cold-water assemblage), and Rhagodiscus asper and Zeugrhabdotus diplogrammus (warm-water assemblage). They provide the basis for the TI TI ¼

Be þ S þ Rp þ Ss þ Zt U100 Ra þ Zd þ Be þ S þ Rp þ Ss þ Zt

ð2Þ

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

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tum spp., Rp = Repagulum parvidentatum, Ss = Staurolithites stradneri, Zt = Zeugrhabdotus trivectis, Ra = Rhagodiscus asper, Zd = Zeugrhabdotus diplogrammus). The factors PC1 and PC3 from the l’Arboudeysse section comprise taxa considered to be indicative of open ocean environments (e.g., Lithraphidites carniolensis ; Thierstein, 1976), unstable near-shore environments (e.g., Broinsonia signata; Roth and Bowdler, 1981), and yet unclear ecological a⁄nities (e.g., Orastrum spp.). As nutrients and temperature represent the most important parameters governing nannoplankton distribution and these parameters are represented by the factors PC2 and PC4 (l’Arboudeysse section), the factors PC1 and PC3 from l’Arboudeysse were not used in the paleoenvironmental evaluation. 3.2. Palynomorphs Palynomorphs were investigated from 28 samples of the l’Arboudeysse section (covering the Niveau Paquier and the strata shortly below and above) and 15 samples of the Pre¤-Guittard section (covering the Niveau Kilian and the strata shortly below and above). Sample preparation followed standard palynological preparation techniques (e.g., Wood et al., 1996). Known weights of sample material between 5 and 8 g were treated with hydrochloric and hydro£uoric acids. To facilitate the calculation of absolute palynomorph abundances, samples were spiked with Lycopodium marker spores. Due to the high amount of amorphous organic matter in the residues, a short oxidation with nitric acid was performed. For each sample, at least 300 palynomorphs were counted from strew mounts. To quantify terrestrial input, the terrigenous/marine ratio (TMR) of palynomorphs was calculated (e.g., Pross, 2001). The absolute abundance of spores occurring in each sample and the ratio between absolute spore abundances and absolute pollen abundances were used as proxies for humidity in the hinterland. Owing to an insu⁄cient dispersal of Lycopodium spores in the samples from 10.06 m and 10.18 m from the l’Arboudeysse section during preparation, absolute paly-

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nomorph abundances are not available for these samples.

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

3.3. Benthic foraminifera

T ¼ 16:034:14ðN 18 Oc 3N 18 Ow Þþ

Benthic foraminifera were studied in 60 samples from the l’Arboudeysse section and 15 samples from the Pre¤-Guittard section. After drying and weighing, a mixture of ethanol and detergent (REWOQUAD) was added. Subsequently, samples were washed over a 63-Wm mesh. Foraminifera from the 125^500-Wm fraction were determined to the species level. For each sample, at least 300 benthic foraminifera were counted. Benthic foraminiferal numbers were calculated as individuals per gram of dry sediment. To reconstruct productivity changes at the sea £oor, the productivity indicators Gyroidinoides spp. and Valvulineria sp. were used (Erbacher et al., 1999; Holbourn et al., 2001). High abundances of Gavelinella spp. were used as indicators of low-oxygen conditions as this genus has been described as dominating foraminiferal assemblages in mid-Cretaceous black shales (Koutsoukos et al., 1990; Holbourn et al., 2001). 3.4. Stable isotopes Seventy-seven bulk samples from the l’Arboudeysse and 85 bulk samples from the Pre¤-Guittard sections were analyzed for stable oxygen and carbon isotopes. Samples were taken from freshly cut rock fragments, crushed, and thoroughly homogenized in an agate mortar. Isotopes were analyzed using a Finnigan MAT 251 mass spectrometer at the ‘Leibniz-Labor fu«r Altersbestimmung und Isotopenforschung’ at Kiel University. The instrument is coupled online to a Carbo-Kiel device I for automated CO2 preparation from carbonate samples for isotopic analysis. The results are reported using the usual N-notation in per mill deviation from the PDB standard. The system has an accuracy of X 0.03x for oxygen and X 0.02x for carbon isotopes. The dependence of the N18 O signal on temperature has been used to reconstruct sea surface temperatures based on the empirically derived

0:13ðN 18 Oc 3N 18 Ow Þ2

ð3Þ

with T as temperature in ‡C, N18 Oc as isotopic composition of the calcite shell in x relative to the PDB standard, and N18 Ow as the isotopic composition of the ambient seawater in x (PDB). The N18 Ow of mid-Cretaceous seawater is estimated as 31.2x for an ice-free world excluding salinity changes (Shackleton and Kennett, 1975). 3.5. Time series analysis To identify potential cyclicities within the nannoplankton record, spectral analysis was carried out on the l’Arboudeysse section because it comprises a su⁄ciently long time interval (V380 kyr) and a high number of samples (194 samples). The computer program SPECTRUM 2.2, developed by Schulz and Stattegger (1997), has been used. The advantage of this spectral analysis is based on the Lomb^Scargle^Fourier Transform (Lomb, 1976; Scargle, 1982, 1989), which can be directly applied to unevenly spaced time series. The e¡ect of spectral leakage caused by the ¢nite length of the time series has been reduced by Welch-Overlapped-Segment Averaging (WOSA ; Welch, 1967; see also Ko«Mler et al., 2001). All presented data are available from the PANGAEA database (http://www.Pangaea.de).

4. Preservation of calcareous nannofossil and stable isotope signals 4.1. Calcareous nannofossils Dissolution and diagenesis can strongly alter the preservation of calcareous nannofossil assemblages. This can severely a¡ect their application in paleoenvironmental 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 to specify the preservation state of the counted assemblages following the method of Bown and Young (1998). In addition, the percentages of the dissolution-resistant nannoplankton species Watznaueria barnesae (e.g., Roth and Bowdler, 1981), total calcareous nannofossil abundances, and simple diversities were used to assess preservation (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 characterized by etching and overgrowth rankings of X (excellently preserved; terminology after Bown and Young, 1998) to E1 (slightly etched) and O1 (slightly overgrown) in all samples. In 87 samples from the Pre¤-Guittard section, 119 calcareous nannofossil taxa were identi¢ed. With the exception of two samples, the assemblages are well to moderately preserved, with etching and overgrowth 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¤-Guittard). Assemblages containing more than 40% W. barnesae are widely thought to be a¡ected by dissolution to an extent that they no longer bear a strong primary signal (e.g., Thierstein, 1980; Roth and Bowdler, 1981; Roth, 1984). This view is corroborated by the observation that poorly preserved assemblages are characterized by a low species richness ( 6 15 species) and high percentages of W. barnesae (Roth and Krumbach, 1986). The relationships between total calcareous nannofossil abundances, the percentages of W. barnesae, and simple diversity is shown in Fig. 4. Dissolution should presumably lead to a reduction of total abundances of calcareous nannofossils and, at the same time, increase the percentage of W. barnesae. However, correlation coe⁄cients of W. barnesae percentages and total calcareous 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. barnesae percentages and simple diversities are also insigni¢cant for both sections (r2 = 0.14, n = 194 for l’Arboudeysse and r2 = 0.15, n = 87 for Pre¤-Guit-

409

tard; Fig. 4C,D). Hence, the lack of correlations between W. barnesae percentages, simple diversity, and total calcareous nannofossil abundances indicates that diagenesis has not generally altered the examined calcareous nannofossil assemblages. A paleoenvironmental interpretation of calcareous nannofossils is therefore well justi¢ed for the study material. 4.2. Stable isotopes The sedimentary succession of SE France has not been subject to deep burial and severe alteration (V700 m; Levert and Ferry, 1988; Weissert and Bre¤he¤ret, 1991). However, early diagenetic processes can operate even at these shallow depths. Hence, we examined the ¢delity of our stable isotope data with regard to yielding original trends. The biogenic carbonate fraction of the sediments sampled for this study is mainly composed (in descending order) of calcareous nannofossils and planktic and benthic foraminifera. It re£ects predominantly a surface water signal. Calcite of diagenetic origin, i.e., cements and micrite, is a minor component and its contribution to the isotopic signal is small. This is indicated by the good preservation of calcareous nannofossils (cf. Section 4.1) and the relatively low amount of micrite observed in the nannofossil samples. Moreover, Weissert and Bre¤he¤ret (1991) and Bre¤he¤ret (1997) point out that no dolomite was present in their studied sections from the Aptian/Albian of SE France. Hence, a paleoenvironmental interpretation of the stable isotope record seems possible. Independently from these aspects, similar £uctuations of the N18 O curve from l’Arboudeysse (Niveau Paquier) in the Vocontian Basin (SE France) and the Mazagan (DSDP Site 545) and Blake Nose Plateaus (ODP Site 1049C) also point towards a predominantly original trend (Herrle, 2002). However, there is a positive correlation between N18 O and N13 C 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 positive correlation between N18 O and N13 C values re£ects a signi¢cant alteration of the stable isotope com-

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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.

position during burial diagenesis. Hence, the correlation of N18 O and N13 C values may indeed point to a slight diagenetic overprint of the isotope values. However, Rao (1996) has shown that a positive correlation of N13 C and N18 O values does not necessarily indicate a diagenetic signal. The possible magnitude of resulting alterations in the N18 O signal may be assessed through a comparison with Late Pleistocene (isotope stages 7^5) isotope records from the eastern Mediterranean Sea. Here, a cross-plot of N13 C and N18 O values from planktic foraminifera (Schmiedl et al., 1998) also yields a weak positive correlation (r2 = 0.46;

n = 56; Fig. 5C). The environmental conditions in the eastern Mediterranean Sea were strongly in£uenced by monsoonal climate conditions with superimposed glacial and interglacial cycles. The cyclic impact of freshwater input triggered by monsoonal circulation probably resulted in the depletion of both the N18 O and N13 C signals (e.g., Schmiedl et al., 1998). The magnitude of short-term (V20 kyr) £uctuations ranges from 1 to 4x for N18 O and from 0.6 to 2.5x for N13 C values (Schmiedl et al., 1998), similar to the stable isotope record from the studied sections of the Vocontian Basin. Summarizing, the good preservation of calcar-

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eous nannofossils, the low amount of micrite in the sample material, the similarity of the oxygen isotope curves in the OAE 1b succession from the western Tethyan and Atlantic oceans (Erbacher et al., 2001; Herrle, 2002), and the analogy to stable isotope records from the Quaternary of the eastern Mediterranean Sea indicate a su⁄cient ¢delity of our stable isotope data regarding relative £uctuations of the original trends.

5. Long-term environmental changes 5.1. Lower Albian l’Arboudeysse section The NI of calcareous nannofossils exhibits high-frequency £uctuations between values of 23.2 and 86.2, with high values re£ecting enhanced productivity in the surface waters and vice versa (Fig. 6). Productivity maxima occur below and within the HN8 black shale, the black shale horizon below the Niveau Paquier, and within the Niveau Paquier itself. The TI £uctuates between 18.5 and 81.0. In comparison to the NI, the £uctuations are of lower frequencies (Fig. 6). Cooler intervals are in the lower part of the studied section (HN7 black shale) and below and above the Niveau Paquier. Both the Niveau Paquier and the HN8 black shales occur within warm periods. Oxygen isotope values range from 34.6 to 32.4x (Fig. 6). N18 O £uctuations correlate rather well with those of the TI. A rapid decrease of N18 O values can be recognized 33 cm (V9 kyr; see Section 2.3 for chronostratigraphy) before the onset of Niveau Paquier formation. Above the Niveau Paquier, the N18 O values show a trend to more positive values. Oxygen isotope values translated into paleotemperatures (neglecting possible salinity changes in order to describe maximum possible temperature changes) vary by up to 6‡C in the interval Fig. 5. Cross-plots of N18 O and N13 C values from bulk rock carbonate samples from the l’Arboudeysse and Pre¤-Guittard sections (A,B) and from Quaternary (isotope stages 7^5) planktic foraminifera (C) from the eastern Mediterranean Sea (data from Schmiedl et al., 1998).

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Fig. 6. Nutrient and temperature indices from the l’Arboudeysse section based on calcareous nannofossils in comparison with the N18 O record. High values of the NI indicate high productivity and vice versa. Low values of the TI indicate high temperatures and vice versa. In the right column, thick lines indicate smoothed records based on the locally weighted least squared error method after Chambers et al. (1983). For lithological explanations and abbreviations see Fig. 3. For a close-up of the TI data from the Niveau Paquier see Fig. 9A.

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

from the Mazagan (4‡C; Herrle, 2002) and Blake Nose Plateaus (10‡C; Erbacher et al., 2001). Both the NI and TI suggest highly variable temperature and productivity in the surface

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Fig. 7. Power spectra of the nannoplankton-derived temperature and nutrient indices on linear scales based on 194 samples from the 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 interval of the spectra. The letters depict the orbital periods of eccentricity, obliquity, and precession according to frequencies estimated for the mid-Cretaceous (Berger et al., 1992).

waters. Time series analyses indicate orbital forcing of the NI and TI, with a dominant precessional signal for the NI and a dominance of the eccentricity signal for the TI (Herrle et al., 2003 ; Fig. 7). Spectral analysis of the NI shows pronounced peaks at 74 cm (equivalent to V20 kyr based on the age control as presented in Section 2.3) and 68 cm (V16.8 kyr), corresponding to the precessional signal. The TI exhibits its strongest peak at 460 cm (V124 kyr) and other pronounced peaks at 87 cm (V23.5 kyr) and 68 cm (V18.4 kyr) representing orbital periods of eccentricity and precession. The power spectrum of obliquity at 210 cm (V57 kyr) is statistically insigni¢cant. As pointed out in Section 1.1, the mid-Cretaceous land^sea distribution in the low latitudes was highly sensitive to monsoonal activity (Barron et al., 1985; Oglesby and Park, 1989; Wortmann et al., 1999). Studies on the Quaternary from the Arabian Sea have shown that humidity and wind stress changed within the precessional signal in this monsoonally in£uenced region (e.g., Clemens and Prell, 1990). Hence, the precessioncontrolled productivity £uctuations as indicated by the NI may represent a monsoonal signal. The nutrient supply in the surface waters depended 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 water column was most pronounced. This led to an entrainment of nutrients into the surface waters. Periods of reduced monsoonal activity were characterized by cooler, drier conditions and reduced wind stress and the surface waters were more depleted in nutrients. Eccentricity-steered temperature changes were superimposed on the shortterm precessional cycles (Figs. 6, 7). During formation of the Niveau Paquier and HN8 black shales, eccentricity-driven increases in temperature and humidity coincided with increasing amplitudes of changes in surface water productivity driven by precession (Fig. 6). High surface water productivity was induced by stronger winds accompanied by increasing humidity, leading to a better mixing of the upper water column that resulted in higher surface water productivity. In contrast, episodes of reduced wind stress and decreasing humidity resulted in a depletion of nutrients in the surface waters. Summarizing, the l’Arboudeysse section is characterized by precession-controlled surface water productivity changes, whereas temperature/humidity £uctuations are dominated by eccentricity. The Niveau Paquier originated under extremely warm and humid conditions that started up to V9 kyr before the onset of black shale formation, representing the transition from a long-term (i.e., eccentricity-controlled) cool/dry to warm/hu-

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mid period. Surface water productivity varied throughout the Niveau Paquier, with increasing amplitudes during black shale formation. 5.2. Upper Aptian Pre¤-Guittard section The NI of calcareous nannofossils ranges from 15 to 64.4 (Fig. 8). Due to the general dominance of low-productivity indicators in comparison to the l’Arboudeysse section, oligo- to mesotrophic conditions can be assumed for the surface waters. Surface water productivity started to increase 15 cm (V6 kyr, see Section 2.3 for chronostratigraphy) before the onset of Niveau Kilian formation. The upper part of the black shale is characterized by a rapid drop in surface water productivity. Moreover, productivity changes based on the NI correlate with the lithology. Pale layers are characterized by slightly higher productivity in comparison to dark beds. The TI ranges from 30.6 to 82.3 (Fig. 8). Surface water temperatures based on the TI changed from relatively low in the lower part of the succession to relatively high from 30 cm below (V13 kyr; see Section 2.3 for chronostratigraphy) the Niveau Kilian to the top of the succession. Short-term temperature £uctuations corresponding to the pale/dark bedding are superimposed on these long-term trends. Pre¤-Guittard N18 O values range from 34.1 to 32.2x (Fig. 8). The lower and middle parts of the succession are characterized by more positive values. The N18 O record shows a clear relationship to the pale/dark bedding, with higher N18 O values occurring in dark beds. Di¡erences of the N18 O values between pale and dark layers amount to up to 1.0x (Fig. 8). Starting 30 cm below the Niveau Kilian, the N18 O values decrease by up to 1.6x. From the lower part of the Niveau Kilian to the upper part of the succession, the N18 O values increase slowly. They are slightly more negative as compared to the lower part of the succession. With respect to temperature changes, the N18 O record yields a similar picture as the TI except for below the Niveau Kilian. The more positive N18 O values in this interval may be interpreted to re£ect increasing evaporation rates that resulted in higher salinities.

The N18 O signal translated into temperature (neglecting possible salinity changes in order to describe maximum possible temperature changes) indicates a rapid temperature increase of up to V6‡C starting V13 kyr before the onset of the Niveau Kilian (Fig. 8). N18 O-based temperature changes between pale and dark beds amount to a maximum of V4‡C, with higher temperatures during the sedimentation of pale beds. As in the l’Arboudeysse succession, the highfrequency productivity changes within the pale and dark bedding of the Pre¤-Guittard section are probably also related to orbital forcing. This can be derived from observations of Bre¤he¤ret (1994) and Ko«Mler et al. (2001), who interpreted the pale/dark bedding in the Vocontian Basin as being controlled by the precessional cycle. Summarizing, the Pre¤-Guittard section is characterized by high-frequency productivity changes that, in analogy to the l’Arboudeysse section, are probably related to orbital forcing within the precessional band. The Niveau Kilian formed under moderately warm and humid conditions that started to increase up to V13 kyr before the onset of black shale formation. Surface water productivity was variable during the deposition of the Niveau Kilian.

6. Short-term environmental changes during black shale formation 6.1. Lower Albian Niveau Paquier succession Based on our micropaleontological data, shortterm changes in temperature, terrigenous input, humidity, productivity, and bottom water oxygenation during formation of the Niveau Paquier can be assessed. Calcareous nannofossil TI values decrease with the onset of the Niveau Paquier black shale, indicating a warming trend. The TMR of palynomorphs, re£ecting terrigenous in£uence, shows a ¢rst weak peak low in the section, a second, more pronounced peak within the black shale horizon below the Niveau Paquier, and a very strong increase with the onset of Niveau Paquier formation and surface water warming as evidenced by the

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Fig. 8. Nutrient and temperature indices from the Pre¤-Guittard section based on calcareous nannofossils in comparison with the N18 O record. High values of the NI indicate high productivity and vice versa. Low values of the TI indicate high temperatures and vice versa. In the right column, thick lines indicate smoothed records based on the locally weighted least squared error method after Chambers et al. (1983). For lithological explanations and abbreviations see Fig. 3. For a close-up of the TI data from the Niveau Kilian see Fig. 9B.

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

for surface water productivity exhibits high-frequency £uctuations before and within the Niveau Paquier controlled by precession (Fig. 9A ; Herrle et al., 2003). High surface water productivity as

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indicated by the NI tends to correlate with high TMR values (Fig. 9A). Eutrophic indicators within the benthic foraminifera reach maximum percentages shortly before the NI and TMR peaks. Afterwards, they decline drastically, while TMR and NI values increase further. Subsequently TMR values remain high and £uctuate only little during Niveau Paquier deposition, whereas NI values oscillate strongly, thus indicating further variations in surface water productivity. Anoxic conditions at the sea £oor prevailed during most of the time of Niveau Paquier deposition, as can be deduced from the lack of benthic foraminifera. Temporary low-oxygen (dysoxic) bottom waters are indicated by the sporadic occurrence of Gavelinella spp. in the lower part of the Niveau Paquier. A similar repopulation event, marking a short-term change towards better ventilation at the sea £oor (Fig. 9A), has also been described from the Niveau Paquier in the Vocontian Basin (Col de Palluel section) and the OAE 1b black shale at the Blake Nose Plateau (Erbacher et al., 1999). According to our data on temperature and humidity changes (i.e., N18 O, TI, TMR, and absolute spore abundances), the formation of Niveau Paquier occurred under increasingly warm and humid conditions. The humidity increase led to a higher £ux of terrigenous palynomorphs through riverine input, as is indicated by high TMR values and absolute spore abundances. Changes in surface and bottom water productivity as reconstructed through the calcareous nannofossil NI and the benthic foraminiferal record can be explained by the development of high productivity in the surface waters as a result of increasing wind stress. Rising surface water productivity, in turn, caused a higher organic matter £ux to the sea £oor. This resulted in rising percentages of high-productivity indicators within benthic foraminifera. With a further increase in surface water productivity, oxygen depletion occurred at the sea £oor, causing a drastic decrease of eutrophic benthic foraminifera. Within the Niveau Paquier black shale, surface water productivity £uctuated strongly as is indicated by the NI. This can probably be attributed

to £uctuations in wind stress steered by precession-controlled monsoonal intensity. During formation of the Niveau Paquier, the TMR values £uctuated only little. This may suggest that TMR £uctuations during Niveau Paquier formation were mainly controlled by changes in humidity (i.e. riverine input) rather than by changes in wind stress. The main environmental characteristics of the Niveau Paquier as derived from our proxy 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 input, humidity, productivity, and bottom water oxygenation can also be recognized during the formation of the Niveau Kilian. The calcareous nannofossil TI is characterized by minor £uctuations, with generally warmer surface waters prevailing during formation of the Niveau Kilian (Fig. 9B). A short-term cooling occurred within the dark layer below the Niveau Kilian. Terrigenous input as re£ected by the TMR curve of palynomorphs exhibits a slight increase in the dark layer below the Niveau Kilian, a rapid increase with the onset of black shale formation, and a decrease in the upper part of the Niveau Kilian. Absolute spore abundances show a similar trend as the TMR curve. Sea surface productivity as indicated by calcareous nannofossil NI exhibits a ¢rst peak below the Niveau Kilian, a second peak within the Niveau Kilian itself with a subsequent temporary reduction to low values, and a minor increase at the top of the black shale (Fig. 9B). High abundances of eutrophic indicators within the benthic foraminifera occur in the upper part of the Niveau Kilian and above the black shale. Gavelinella spp. show highest percentages below and in the upper part of the Niveau Kilian, re£ecting an oxygen-poor environment at the sea £oor. Gavelinella spp. occur in all samples studied, indicating dysoxic conditions during formation of the Niveau Kilian (Fig. 9B). Temperatures as documented in the N18 O and TI data rise V13 kyr before the onset of Niveau Kilian formation (Fig. 8). This rise is accompa-

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calcareous nannofossils

Niveau Paquier

A

lithology m

25

nutrient index

temperature index

low

cold 75 50

50

high 75

palynomorphs terrigenous/ marine ratio

warm 25

0

2.5

5

0

40

0

2. 5

5

0

417

benthic foraminifera

spores per gram 4 sediment (x10 ) 5

eutrophic indicators (% of assemblage) 0 10 20

Gavelinella spp. (% of assemblage) 0 20 40

3

2

1

0

Niveau Kilian

B

m 20

40

60

60

2

0

20

40

25

50

1

0

Fig. 9. (A) Detailed section of the supraregionally distributed Niveau Paquier from the l’Arboudeysse section with nutrient and temperature indices based on calcareous nannofossils, TMR of palynomorphs and absolute spore abundances, and eutrophic indicators (Gyroidinoides spp., Valvulineria spp.) and Gavelinella spp. within the benthic foraminiferal record. High NI values, TMR of palynomorphs, and spore abundances re£ect high surface water productivity and increasing terrestrial input/humidity and vice versa. 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 indices based on calcareous nannofossils, TMR of palynomorphs, absolute spore abundances, and eutrophic indicators and Gavelinella spp. within the benthic foraminifera record. For lithological explanations and abbreviations see Fig. 3.

nied by increasing TMR values and absolute spore abundances, indicating increasing terrestrial input and, via enhanced runo¡, a rise in humidity. Surface and bottom water productivity, as documented in the NI and the percentages of benthic foraminifera (Fig. 9B), increased before the onset of Niveau Kilian formation. This trend was accompanied by increasing TMR values and absolute spore abundances, re£ecting rising humidity. Eutrophic indicators are relatively rare among benthic foraminifera and Gavelinella spp. dominate the benthic foraminiferal assemblage (Fig. 9B). The low abundances of eutrophic indi-

cators within benthic foraminifera despite high surface water productivity are probably due to poor oxygenation. Further increasing surface water productivity within the Niveau Kilian tends to correlate with increasing TMR values and absolute spore abundances. In the upper part of the Niveau Kilian, productivity decreases rapidly. This is accompanied by decreasing TMR values and absolute spore abundances, indicating a trend to reduced runo¡ and, thus to drier conditions. During this interval, the percentages of Gavelinella spp. increase, indicating that benthic oxygenation improved. The percentages of eutrophic in-

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dicators within benthic foraminifera also increase, followed by decreasing values towards the top of the Niveau Kilian and above. A similar trend is shown by the NI and palynomorph records. Hence, slightly increasing surface water productivity corresponds to increasing percentages of eutrophic indicators derived from the benthic foraminifera. Table 2 summarizes the main environmental characteristics of the Niveau Kilian as derived from our proxy data. 6.3. Environmental similarities and di¡erences between the Niveau Paquier and Niveau Kilian The supraregionally distributed Niveau Paquier and the regionally occurring Niveau Kilian from the Vocontian Basin show similar characteristics regarding general temperature, terrestrial input, humidity, and productivity changes (Table 2). Di¡erences, however, exist in the amplitude of changes within these factors and sea £oor oxygenation during black shale deposition and in the duration 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 formation (30 cm, equivalent to V13 kyr; Figs. 6, 8, 9). In both cases, surface water productivity was highly variable during black shale deposition, probably owing to £uctuations in wind stress steered by changes in the monsoonal climate system. Di¡erences between both black shales are related to the extent of temperature and humidity changes. Based on N18 O values and neglecting possible salinity changes, the temperature increase of 8‡C with the onset of the Niveau Paquier is up to 2‡C higher as compared to the Niveau Kilian. Moreover, the ratio between absolute spore abundances and absolute pollen abundances is much higher in the Niveau Paquier than in the Niveau Kilian, indicating extremely humid conditions during the deposition of the Niveau Paquier in comparison to the Niveau Kilian (Fig. 10; Table 2). This is because spore-producing plants require humid conditions in order to proliferate. Alternatively, the lower spore/pollen ratio in the Niveau Kilian could also be due to an increased distance to the shoreline as most spores have a relatively

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

Geochemistry TOC Temp. increase (N18 O) Calcareous nannofossils NI (productivity) TI (temperature) Palynomorphs TMR Spore abundances Benthic foraminifera Eutrophic indicators Opportunistic species Paleoenvironmental interpretation Estimated duration Surface water fertility Surface water mixing O2 at the sea £oor Humidity

Supraregional black shale (Niveau Paquier)

Regional black shale (Niveau Kilian)

up to 8% up to 8‡C

up to 3.5% up to 6‡C

variable (mean: 49.9) increasing (mean: 51.3)

variable (mean: 37.2) increasing (mean: 50.6)

high (mean: 2.7) very high (mean: 26 175/g)

high (mean: 2.9) high (mean: 15 974/g)

high mostly absent

high mostly present

V44 kyr variable (meso- to eutrophic) variable predominantly anoxic extreme

V31 kyr variable (mesotrophic) variable predominantly dysoxic moderate

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4

spores per gram sediment

6·10

419

consistent data set that can be used to establish the main driving factors for the formation of the supraregional Niveau Paquier and the regional Niveau Kilian. In the following, the roles of temperature, terrestrial input/humidity, productivity, and runo¡ in black shale formation will be discussed. Moreover, a model for the formation of the Niveau Paquier and Niveau Kilian black shales will be proposed.

high humidity

4

5·10

4

4·10

4

3·10

4

2·10

7.1. Temperature and humidity/terrestrial input 4

1·10

low humidity 4

0·10

0·10

5

0.5·10

5

1.0·10

5

1.5·10

5

2.0·10

5

pollen per gram sediment Fig. 10. Scatter plot showing the ratios between absolute spore and absolute pollen abundances from the Niveau Paquier of the l’Arboudeysse section and the Niveau Kilian of the Pre¤-Guittard section. Highest spore/pollen ratios re£ect most humid conditions and vice versa. Samples from the Niveau Paquier are marked by black dots and samples from the Niveau Kilian by circles.

low buoyancy, resulting in a deposition center in relatively proximal settings (e.g., Traverse, 1988). For the studied section, however, this scenario can be ruled out owing to the very similar sea level during deposition of the Niveau Paquier and the Niveau Kilian (compare Bralower et al., 1994; Bre¤he¤ret, 1997). Bottom water conditions as indicated by benthic foraminifera were predominantly anoxic to dysoxic during Niveau Paquier formation, whereas they were dysoxic during Niveau Kilian formation. Regarding the duration of black shale formation, the Niveau Paquier was deposited over a longer interval (163 cm, equivalent to V44 kyr) than the Niveau Kilian (74 cm, equivalent to up to V31 kyr).

7. Driving mechanisms for supraregional and regional black shale formation during the mid-Cretaceous Our micropaleontological approach yields a

The formation of both the Niveau Paquier and the Niveau Kilian is linked to increasingly warm and humid conditions. The supraregional Niveau Paquier is characterized by an extreme temperature and humidity increase, whereas temperature and humidity conditions were more moderate during the formation of the Niveau Kilian (Figs. 6, 8^10). The observation that temperature, terrestrial input, and humidity increase during mid-Cretaceous black shale formation is in agreement with previous studies that connected the deposition of black 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 proposed by these authors, black shale formation may have been triggered by the strengthening of continental weathering and increased runo¡ which caused enhanced surface water productivity. According to several authors (e.g., Habermann and Mutterlose, 1999; Erbacher et al., 2001), the increased runo¡ probably contributed to a better preservation of organic matter at the sea £oor as a result of increased thermohaline strati¢cation. 7.2. Productivity The enhanced £ux of marine organic matter during the formation of both the Niveau Paquier and the Niveau Kilian is partly due to increased productivity in the surface waters as discussed above. However, surface water productivity varied during the formation of the black shales. We suggest that productivity £uctuations during formation of the Niveau Paquier and the Niveau

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Kilian are linked to changes in wind stress triggered by a monsoonal climate system. This model is similar to the Quaternary Arabian monsoon system. Paleoproductivity records of this region are strongly controlled by changes in precessionally driven £uctuations in summer surface water productivity (e.g., Reichart et al., 1997; Den Dulk et al., 1998). Increasing wind speeds during enhanced monsoonal activity may have allowed a better mixing of the sea surface waters, resulting in increased nutrient supply from subsurface waters. Reduced monsoonal activity is characterized by reduced wind stress, leading to nutrient depletion in the surface waters. However, based on the strong £uctuations of surface water productivity during Niveau Paquier and Niveau Kilian formation, enhanced productivity alone can be excluded as a cause for the large amount of organic matter buried in both black shales. 7.3. Runo¡ during black shale formation The rapid TMR increase during formation of the Niveau Paquier and the Niveau Kilian indicates enhanced terrigenous input into the Vocontian Basin. High TMR values during black shale formation have often been invoked as indicators of increased riverine runo¡, causing a density strati¢cation and a productivity rise in the surface waters (e.g., Rossignol-Strick, 1985; Rohling, 1991; Below and Kirsch, 1997). For the mid-Cretaceous, a runo¡-induced density strati¢cation has been proposed for the formation of the OAE 1b black shale from the Atlantic Ocean (Erbacher et al., 2001) and the coeval Niveau Paquier from the Vocontian Basin (Tribovillard and Gorin, 1991). According to the latter authors, the OAE 1b (Niveau Paquier) formation was mainly a result of enhanced preservation. However, a calculation of the freshwater budget for the western Tethys shows that the riverine discharge required to establish a persistent density strati¢cation ( s 1x) is nearly four times higher than that of the modern Nile River which drains up to 91 km3 /yr (Foucault and Stanley, 1989) into the Mediterranean Sea. During the mid-Cretaceous, the northern borderlands of the western Tethys were too small to provide enough freshwater to trigger

black shale formation (Fig. 11). Although our palynological data would be compatible with such a scenario both for the formation of the supraregional Niveau Paquier and the regional Niveau Kilian, a persistent density strati¢cation caused by riverine runo¡ can therefore be ruled out for that region. Hence, in the following an alternative model for the formation of the Niveau Paquier and Niveau Kilian black shales is proposed. 7.4. Model for black shale formation Both the supraregional Niveau Paquier and the regional Niveau Kilian formed shortly after the onset of a long-term, eccentricity-controlled warm and humid period as indicated by the N18 O, TI, TMR, and absolute spore abundance data. We suggest that the supraregional signi¢cance of the Niveau Paquier is due to the reduction of low-latitude deep water formation during extremely warm and humid conditions. Under extreme monsoonal forcing, deep water production at the main sites of deep water formation in the low latitudes (such as SW Asia; Barron and Peterson, 1990; Fig. 11) was probably diminished because of the enhanced precipitation rates in these areas under extremely humid conditions (Herrle et al., 2003). This is because a decrease in evaporation rates resulted in a decrease of surface water densities and, when a threshold value was reached, in a drastic reduction of deep water formation (e.g., Bice et al., 1997). Longer periods during which the precipitation rate exceeded evaporation are characterized by a sluggished or diminished oceanic circulation mode. This favored the widespread preservation of organic matter due to anoxic to dysoxic conditions at the sea £oor as they are indicated by our benthic foraminiferal data. The formation of the regionally distributed Niveau Kilian is also linked to warm and humid conditions resulting from monsoonal forcing. However, the threshold value for a supraregional distribution was not reached. This is indicated by a weaker temperature increase and less humid conditions in comparison to the Niveau Paquier

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421

Fig. 11. Proposed model for the formation of the supraregionally distributed Niveau Paquier and the regionally distributed Niveau Kilian, with schematic mean annual atmospheric circulation pattern for the low latitudes at insolation maximum during the mid-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-Cretaceous with high sea-level stand shorelines (modi¢ed after Hay et al., 1999). During extremely warm and humid conditions and strong monsoonal climate (as prevailing during Niveau Paquier formation), deep water formation was restricted in the northern and eastern Tethyan area. During Niveau Kilian formation, in contrast, the temperature and humidity increase was more moderate. During that time, deep water formation was restricted only in the area of the Vocontian Basin. Dark gray areas indicate land, areas of restricted deep water formation are hatched. H, high-pressure system; T, low-pressure system; VB, Vocontian Basin.

as suggested by the N18 O data (Figs. 6, 8, 9) and spore/pollen ratios (Fig. 10). Therefore, deep water formation and bottom water ventilation were reduced to a lesser extent and/or on a regional scale only (Fig. 11). Moreover, bottom water oxygenation remained on a dysoxic level throughout black shale formation. This is re£ected in the persistent occurrence of benthic foraminifera within the black shale.

Based on our data, both the Niveau Paquier and the Niveau Kilian in the Vocontian Basin are characterized by varying surface water productivity controlled by changes in a monsoonal climate system. Productivity was an important factor in the formation of both studied black shales. Much more signi¢cant, however, was the enhanced preservation of organic matter due to reduced ventilation at the sea £oor, probably as

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a function of decreasing deep water formation in the low latitudes.

8. Conclusions Calcareous nannofossil, palynomorph, benthic foraminiferal, and oxygen isotope data from the supraregionally distributed Niveau Paquier and regionally distributed Niveau Kilian black shales in the Vocontian Basin provide information on the driving mechanisms of mid-Cretaceous black shale formation. Environmental changes reconstructed from our internally consistent proxy data comprise variations in temperature, terrestrial input, humidity, and productivity. The most salient results of our study are the following: (1) The studied sections show strong signatures of a mid-Cretaceous monsoonal climate as is indicated by the dominance of precession-controlled variations in surface water productivity. (2) Eccentricity-controlled changes in temperature and evaporation/precipitation patterns were the most important climate factors in£uencing the hydrological cycle and deep water formation in the low latitudes and therefore in the deposition of the Niveau Paquier and Niveau Kilian. (3) The supraregionally distributed Niveau Paquier formed under strongly increasing temperatures and humidity, whereas the regionally distributed Niveau Kilian formed under more moderately increasing temperatures and humidity. Extremely increasing temperatures and humidity (with precipitation prevailing over evaporation) had a dramatic impact on low-latitude deep water formation on a supraregional scale, whereas a moderate increase in these conditions probably in£uenced deep water formation to a lesser degree and/or on a regional scale only. Therefore, the intensity of temperature and humidity increase controlled if mid-Cretaceous black shales developed supraregionally or regionally. Increased runo¡ rates that caused a persistent density strati¢cation and therefore favored the preservation of organic matter at the sea £oor can most likely be ruled out as an important driving mechanism for black shale formation in the Vocontian Basin. (4) The monsoonally forced productivity

changes only represent a regional climate signal. Because productivity increased during the formation of both the Niveau Paquier and Niveau Kilian, this parameter must certainly have played a role in the formation of these black shales. However, the majority of organic matter buried during black shale formation was probably a result of enhanced organic matter preservation at the sea £oor.

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