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Reconstructing nutricline dynamics of mid-Cretaceous oceans: evidence from calcareous nannofossils from the Niveau Paquier black shale (SE France)
Marine Micropaleontology 47 (2003) 307^321 www.elsevier.com/locate/marmicro
Reconstructing nutricline dynamics of mid-Cretaceous oceans: evidence from calcareous nannofossils from the Niveau Paquier black shale (SE France) Jens O. Herrle Institut fu«r Geowissenschaften, Universita«t Tu«bingen, SigwartstraMe 10, D-72076 Tu«bingen, Germany Received 8 March 2002; received in revised form 28 August 2002; accepted 28 August 2002
Abstract A high-resolution calcareous nannofossil record is presented from the Lower Albian Niveau Paquier black shale from the Vocontian Basin (SE France). The Niveau Paquier black shale represents the regional equivalent of the supraregionally distributed Oceanic Anoxic Event 1b (OAE 1b). To reconstruct surface water fertility, a nutrient index based on Zeugrhabdotus erectus, Discorhabdus rotatorius (high fertility indicators), and Watznaueria barnesae (low fertility indicator) was established using principal component analysis. In addition, the distribution of Nannoconus spp. and absolute abundances of coccoliths (coccoliths per gram) were used for reconstructing nutricline dynamics of the surface waters. High surface water fertility coincides with low percentages of nannoconids and vice versa. Moreover, high percentages of nannoconids correlate with low absolute abundances of all other coccoliths. Based on the observed nannoplankton distribution pattern and a suggested similarity in ecological requirements between nannoconids and the modern taxon Florisphaera profunda, a model is proposed that couples nannoconid abundances with dynamics of the nutricline. Time series analyses of the nutrient index show fluctuations within the precessional band. The precession-controlled fluctuations of the surface water fertility may represent a monsoonal signal, with the nutrient supply in the surface waters depending 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 enhanced. During that time, the abundance of nannoconids decreased as a consequence of enhanced wind stress that improved vertical mixing and led to an entrainment of nutrients into the surface waters. This resulted in an increase of primary production. During periods of reduced monsoonal activity, marked by drier conditions and reduced wind stress, the surface waters were depleted in nutrients. As a result of a deep nutricline and a reduced nutrient supply to the upper photic zone in a stratified water column, percentages of nannoconids increased. According to the mechanisms outlined above, fluctuations of the nutrient index and nannoconid percentages have been used as a proxy for reconstructing the physical structure of mid-Cretaceous oceans steered by precession-forced monsoonal activity. The results of the study show that the formation of the Lower Albian OAE 1b from the Vocontian Basin occurred under strongly fluctuating surface waters fertility. 7 2002 Elsevier Science B.V. All rights reserved. Keywords: calcareous nannofossils; Cretaceous; productivity; black shale; nutricline
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1. Introduction Calcareous nannoplankton is an important component of the phytoplankton in the marine realm and has been one of the main open ocean primary producers since the Triassic. During the mid-Cretaceous, it was probably an even more important phytoplankton group than today because diatoms only became abundant as late as the Campanian (e.g., Burnett et al., 2000; Mitlehner, 2000). The distribution of calcareous nannoplankton is strongly in£uenced by climatic and hydrological conditions. Temperature has been considered the most important factor a¡ecting the distribution of calcareous nannoplankton, but trophic resources also play an important role in the distribution and abundance of calcareous nannoplankton in past and modern oceans (Chepstow-Lusty et al., 1989; Mol¢no and McIntyre, 1990; Aubry, 1992, 1998; Chepstow-Lusty and Chapman, 1995; Ziveri et al., 1995; Eshet and Almogi-Labin, 1996). In general, living calcareous nannoplankton exhibits a vertical zonation in the water column. Most species live in the upper photic zone (0^60 m). The deeper photic zone (60^200 m) is inhabited by fewer species among which Florisphaera profunda plays a dominant role in low latitudes (Okada and Honjo, 1973; Mol¢no and McIntyre, 1990). According to Mol¢no and McIntyre (1990), a high abundance of lower photic zone species is associated with a deep nutri- and thermocline, while low abundances are characteristic of a shallow nutri- and thermocline. A comparison of Early Cretaceous nannofossil assemblages with modern nannoplankton communities suggests that nannoconids possibly inhabited the lower photic zone similar to the modern F. profunda (Coccioni et al., 1992; Erba, 1994). Although the ecological preferences of nannoconids are still under debate (e.g., Mutterlose, 1987; Busson and Noe«l, 1991; Erba, 1994; Scarparo Cunha and Koutsoukos, 1998), there is increasing evidence that they may be comparable to those of the modern species F. profunda. This paper presents results from a high-resolution study of the Lower Albian Niveau Paquier black shale from the subtropical (30‡N) Vocon-
tian Basin (SE France) in order to reconstruct short-term surface water fertility changes. The Niveau Paquier black shale is the regional equivalent of the supraregionally distributed Oceanic Anoxic Event 1b (OAE 1b; e.g., Bralower et al., 1993; Erbacher et al., 2001). Based on time series analysis, the Niveau Paquier in the Vocontian Basin formed within V44 kyr (Herrle et al., in press a). 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 formation of OAE 1b is characterized by major perturbations of the ocean system de¢ned by a massive deposition of organic matter in marine environments (e.g., Schlanger and Jenkyns, 1976; Arthur et al., 1990; Bre¤he¤ret, 1988; Bralower et al., 1993; Erbacher et al., 1999; Herrle, 2002). The rapid changes in the carbon cycle were accompanied by turnovers in marine biota (e.g., Erbacher and Thurow, 1997). However, the trigger mechanisms as well as the role of increased productivity (e.g., Bre¤he¤ret, 1994; Erbacher et al., 1998, 1999) versus enhanced preservation (Tribovillard and Gorin, 1991; Erbacher et al., 2001) or a combination of both factors (Herrle et al., in press a,b) during OAE 1b formation is still under debate.
2. Study area The studied l’Arboudeysse section is located in the Vocontian Basin (SE France ; Figs. 1 and 2). During the mid-Cretaceous, the Vocontian Basin was located in the northern part of the Western Tethys and was surrounded by carbonate platforms. The eastern part of the Vocontian Basin was open to the Tethyan Ocean (Arnaud and Lemoine, 1993). The Vocontian Basin represents a marginal basin that was sensitive to both climatic and oceanographic changes. In the Aptian to Albian sedimentary record of the Vocontian Basin, extreme environmental conditions are re£ected by organicrich layers (black shales; Bre¤he¤ret, 1988, 1997; Fig. 3). The Aptian to Albian succession is dominated by marlstones and has been named Marnes Bleues Formation (Flandrin, 1963).
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Fig. 1. Paleogeographic reconstruction of the Vocontian Basin for the Late Aptian to Early Albian. Modi¢ed after Arnaud and Lemoine (1993). The rectangle indicates the studied area. The studied l’Arboudeysse section is located in the middle part of the Vocontian Basin (LA).
Fig. 2. Location map of the l’Arboudeysse section in SE France.
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2.1. Studied section The outcrop is situated 700 m southwest of l’Arboudeysse, about 5 km east of Rosans (Fig. 2). The investigated succession at l’Arboudeysse is up to 2.8 m thick and comprises the interval of the Niveau Paquier black shale reaching about 1.63 m in thickness (Fig. 3). The total organic carbon content is above 3% (maximum: 8%) and the organic matter consists of terrestrial and marine
components in variable proportions (Bre¤he¤ret, 1985; Tribovillard and Cotillon, 1989). The CaCO3 content within the Niveau Paquier reaches maximum values of 50% (Bre¤he¤ret, 1997). The Niveau Paquier black shale is rich in ammonites, with abundant leymeriellids (Fig. 3). In the upper part of this black shale an up to 3 cm thick carbonate layer rich in nannoconids can be recognized. This level has been called ‘K-bed’ (Bre¤he¤ret, 1983) and can be correlated throughout the Vocontian Basin.
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 (Herrle and Mutterlose, in press). Planktonic foraminiferal and ammonite stratigraphy after Bre¤he¤ret (1997 and references therein). 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 = Delits Calcaire; NJ = Niveau Jacob; NK = Niveau Kilian; HN = Haut Noire; NP = Niveau Paquier; LE = Niveau Leenhardt.
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3. Materials and methods Eighty-eight samples (sample intervals of 2^10 cm) were taken from freshly cut rock fragments and were analyzed for their calcareous nannofossil content using the random settling technique of Geisen et al. (1999) to calculate absolute abundances. At least 300 individuals were counted per sample in random traverses of each sample using an Olympus BX50 photomicroscope at U1250 magni¢cation. The taxonomic frameworks of Perch-Nielsen (1985) and Bown (1998 and references therein) have been followed. All coccoliths with more than half of the specimens preserved have been included in the counts. Among the 121 taxa identi¢ed four taxa (Discorhabdus rotatorius, Zeugrhabdotus erectus, Watznaueria barnesae and Nannoconus spp.) were chosen for a detailed analysis of the assemblage because of their special paleoecological and paleoceanographic signi¢cance. Nannoconus spp. includes N. dauvillieri, N. elongatus, N. fragilis, N. globulus, N. cf. inornatus, N. minutus, N. regularis, N. truitti, N. truitti frequens, N. truitti rectangularis, and N. vocontiensis. All presented data are available from the PANGAEA database (http:// www.Pangaea.de).
4. Biostratigraphy The Niveau Paquier black shale belongs to the Leymeriella tardefurcata ammonite Zone and Prediscosphaera columnata (upper Hayesites albiensis Subzone; NC8B) nannoplankton Zone (Bralower et al., 1993; Kennedy et al., 2000; Herrle and Mutterlose, in press ; Fig. 3). The Niveau Paquier has been recognized in several sections of the Tethys (Bre¤he¤ret, 1985, 1988, 1994; Arthur et al., 1990). It may correspond to the Livello Urbino in Italy (Bralower et al., 1993). Equivalent sections are also described by Bre¤he¤ret (1985) from South Germany (‘In der Ho«ll’) and Austria (‘Losenstein’). Several ODP/ DSDP legs from the Atlantic contain probably contemporaneous organic-rich sediments known as OAE 1b. These are DSDP Site 545 at the Mazagan Plateau, ODP Site 641 at the Galicia Bank,
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DSDP Site 511 at the Falkland Plateau and DSDP Site 386 at the Bermuda Ridge, and ODP Site 1047 at the Blake Nose Plateau (e.g., Bralower et al., 1993; Erbacher et al., 1999; Herrle, 2002).
5. Results The distribution of Watznaueria barnesae, Discorhabdus rotatorius, Zeugrhabdotus erectus, Nannoconus spp. as well as simple diversity and absolute abundance of coccoliths is shown in Fig. 4. Watznaueria barnesae ranges between 4% and 35.9% (mean: 15.3%) of the total assemblage. The lowest percentages of W. barnesae can be recognized within the Niveau Paquier, with a minimum in the lowest part followed by increasing percentages and again decreasing values towards the K-bed. From the K-bed to the top of the Niveau Paquier the percentages of W. barnesae increase. Discorhabdus rotatorius ranges between 2.7 and 15.1% (mean: 8.2%). The Niveau Paquier is characterized by maximum percentages of D. rotatorius at the base, below the K-bed and at the top of the black shale (Fig. 4). Zeugrhabdotus erectus ranges between 4.2 and 14.8% of the total assemblage, with a mean of 9.1%. The £uctuations show a similar trend as those of the D. rotatorius percentages (Fig. 4). The percentage of Nannoconus spp. ranges between 0 and 38.4% (mean: 8.3%). These taxa occur in elevated abundances only within the Niveau Paquier black shale and the upper part of the black shale below the Niveau Paquier (Fig. 4). Maximal percentages s 25% can be recognized in the lower part of the black shale and within the K-bed. The simple diversity lies between 26 and 46 taxa with a mean of 38 taxa per sample (Fig. 4). The lower part of the succession is characterized by the lowest simple diversity. Highest values occur within the lower to middle part of the Niveau Paquier. The total abundance of coccoliths ranges from 0.6U109 to 3.8U109 individuals per gram sediment (mean: 1.4U109 ind./g). Lowest numbers occur within the Niveau Paquier and in the upper part of the black shale below the Niveau Paquier ( 6 1.5U109 ind./g; Fig. 4). The low total abundance of coc-
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Fig. 4. Fluctuations of calcareous nannoplankton specimens, simple diversity and total coccolith abundance (individuals per gram sediment) of the Niveau Paquier black shale in the l’Arboudeysse section. Dashed lines indicate mean values. For lithological explanation see Fig. 3.
coliths within the Niveau Paquier corresponds to high percentages of Nannoconus spp.
6. Discussion 6.1. Preservation of calcareous nannofossils Dissolution and diagenesis can signi¢cantly alter the preservation of calcareous nannofossil assemblages and thus their utility as paleoenvironmental indicators for the surface waters. Four major abiotic processes control the preservation and accumulation of calcareous nannofossils dur-
ing the transition from the biocenosis to the thanatocenosis. These are (1) dissolution processes in the water column, (2) dissolution processes at the sediment surface and in the sediment, (3) dilution processes, and (4) lateral drifting of specimens (e.g., Honjo, 1976; Steinmetz, 1994; Samtleben et al., 1995; Andruleit, 1995, 1997). According to Levert and Ferry (1988), the sediment thickness above the studied outcrops in SE France amounted to about 700 m. These data may con¢rm that the studied succession has not been subject to deep burial and severe alteration (Weissert and Bre¤he¤ret, 1991). However, early diagenetic processes can operate even at these
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depths (e.g., Williams and Bralower, 1995). Therefore, to gain additional information regarding the preservation of the counted assemblages, a visual evaluation of etching (E) and overgrowth (O) was performed using the descriptive scheme of Roth and Thierstein (1972) and Roth (1983). In addition, the percentages of the most resistant nannoplankton species Watznaueria barnesae (e.g., Roth and Bowdler, 1981), the total abundance of coccoliths, and the simple diversity were used to assess the preservation of calcareous nannofossils (compare Williams and Bralower, 1995). All samples are characterized by etching and overgrowth categories E1 and O1. This indicates that some coccoliths are partly slightly etched and overgrown, but the nannofossil assemblage in total is well preserved. In the studied interval, Watznaueria barnesae occurs with maximum percentages of 35.9% (mean : 15.3%). Thierstein (1980), Roth and Bowdler (1981), and Roth (1984) considered that material containing more than 40% W. barnesae is frequently a¡ected by dissolution to the extent that the assemblages no longer yield a strong primary signal. Therefore, the low mean values of W. barnesae indicate that dissolution may not have a¡ected the primary assemblage composition. However, it is clear that as dissolution progresses beyond a certain point, more dissolution-sensitive nannofossils species will dissolve
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and species diversity will decrease (e.g., Williams and Bralower, 1995). Moreover, recent studies have shown that not all individuals and species of calcareous nannoplankton are represented in the sediment, even though calcareous nannofossils are more resistant to dissolution in the water column and sediment than, e.g., planktonic foraminifera because of the transport via fecal pellets (e.g., Andruleit, 1997 and references therein) and organic protection. The relationships between total coccolith abundances, Watznaueria barnesae percentages, and simple diversity is shown in Fig. 5. Dissolution should presumably lead to a reduction of total abundances of coccoliths, but should increase the percentage of W. barnesae. However, both the correlation coe⁄cients of W. barnesae percentages and total coccolith abundances (r2 = 0.05; n = 86) as well as W. barnesae and the simple diversity (r2 = 0.10; n = 86) are insigni¢cant for the studied interval. The lack of correlation of W. barnesae percentages, simple diversity, and total abundances of coccoliths, respectively, indicates that dissolution has not generally altered calcareous nannofossil assemblages of the study material. This indicates good preservation of calcareous nannofossils. This is corroborated by observations on Aptian to Albian nannoplankton assemblages from the nearby Col de Palluel sections (Thierstein, 1973).
Fig. 5. Scatter plots showing relationships between Watznaueria barnesae percentages, total coccolith abundance (individuals per gram sediment), and simple diversity in the studied interval of the Niveau Paquier black shale.
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6.2. Ecological signi¢cance of mid-Cretaceous calcareous nannofossils 6.2.1. Fertility indicators Detailed paleoceanographic studies based on Cretaceous calcareous nannofossils have been carried out by, e.g., Roth and Bowdler (1981), Roth and Krumbach (1986), Premoli Silva et al. (1989), Coccioni et al. (1992), Erba et al. (1992) and Williams and Bralower (1995). These authors investigated the paleobiogeography and paleoecology of Atlantic, Paci¢c and Indian ocean calcareous nannofossil assemblages using quantitative analysis and multivariate statistical techniques. According to these studies, the most prominent high fertility indicators are Zeugrhabdotus erectus, Biscutum constans (e.g., Roth and Bowdler, 1981; Roth and Krumbach, 1986; Premoli Silva et al., 1989; Erba et al., 1992), and Discorhabdus rotatorius (Premoli Silva et al., 1989; Erba, 1991; Coccioni et al., 1992). The above mentioned species are common in upwelling areas (e.g., Roth and Bowdler, 1981), but also in shelf areas where nutrients may have been added by freshwater runo¡ (e.g., Street and Bown, 2000). On the other hand, low fertility conditions have been shown to be indicated by Watznaueria barnesae (e.g., Roth and Krumbach, 1986; Erba et al., 1992; Williams and Bralower, 1995). In order to quantify surface water fertility, the species Zeugrhabdotus erectus and Discorhabdus rotatorius (high fertility indicators) and Watznaueria barnesae (indicative of low fertility) were used in a nutrient index (NI) based on R-mode (Varimax-rotated) Principal Component Analysis (see Herrle et al. (in press a) for a detailed discussion) : NI ¼ high fertility assemblage low fertility assemblage þ high fertility assemblage U100
ð1Þ
High NI values re£ect high fertility in surface waters and vice versa. In this index, the ratios between selected taxa are evaluated. This approach yields a clearer signal with respect to fertility conditions than the evaluation of single
species abundances (compare Figs. 4 and 6). Moreover, studies on Recent material have shown that species ratios, as documented in the thanatocenosis on the sea£oor, provide a better record of the surface water signal than that resulting from the abundance of single species (Andruleit, 1995; Gale et al., 2000). 6.2.2. Paleoecological signi¢cance of nannoconids Nannoconus spp. are important for biostratigraphic zonations of the Aptian to Albian interval in marginal and epicontinental basins (e.g., Deres and Ache¤rite¤guy, 1980; Erba, 1988). Moreover, they play an important role in the carbonate £ux during the mid-Cretaceous due to their compact structure and large size (e.g., Larson and Erba, 1999; Herrle and Mutterlose, in press). Over the last few decades, the paleoecological signi¢cance of nannoconids has been intensively discussed. It has become clear that the occurrence of nannoconids is positively correlated with lowlatitude shelf environments (e.g., Roth and Krumbach, 1986; Mutterlose, 1987; Busson and Noe«l, 1991; Street and Bown, 2000). Mutterlose (1989, 1992) and Street and Bown (2000) characterized nannoconids as typical Tethyan taxa. Based on increasing abundances of nannoconids accompanied by the occurrence of stenotherm warm water belemnites (Duvalia) in the boreal realm, nannoconids are interpreted as warm water taxa (Mutterlose, 1991). The trophic requirements of nannoconids, however, are still under debate. It has been proposed that nannoconids are characteristic of oligotrophic environments (Erba, 1994). On the other hand, Scarparo Cunha and Koutsoukos (1998) suggested a shallow, warm hypersaline, carbonate-saturated environment with intermediate, mesotrophic nutrient levels to be favorable for nannoconids. Busson and Noe«l (1991) documented nannoconids as meroplanktonic calcareous dino£agellate-like organisms that proliferated in epicontinental, oligotrophic seas with low terrigenous input. The inverse relationship between the abundance of nannoconids and coccolithophores is explained by these authors by red-tide poisoning. Moreover, Erba (1994) argued that nannoconids required similar conditions as the modern coccolithophorid Florisphaera profunda,
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Fig. 6. Nutrient Index (NI), distribution of Nannoconus spp., and absolute coccolith abundance in the l’Arboudeysse section. High NI values indicate high surface water fertility and vice versa. The broadly inverse relationship between nannoconids and the NI is explained by £uctuations of the nutricline. Percentages of nannoconids increase when in a strati¢ed water column the upper photic zone is impoverished in nutrients (deep nutricline). In contrast, the abundance of nannoconids decreases when enhanced wind stress improves vertical mixing, leading to an entrainment of nutrients into the surface waters (shallow nutricline). For lithological explanation see Fig. 3.
a deep photic-zone dweller. The inverse relationship between F. profunda and coccolithophore abundances is explained by a £uctuation of the nutricline depth (Mol¢no and McIntyre, 1990). However, disagreement exists regarding the similarity of trophic niches of nannoconids and F. profunda. Mol¢no and McIntyre (1990) demonstrated that £uctuations in the abundance of F. profunda re£ect variations in the nutricline (and thermocline) depth. A deep nutricline indicates mesotrophic conditions in the lower eu-
photic zone; consequently F. profunda occurs in higher abundances. In contrast, a shallow nutricline would favor higher abundances of other coccolithophorids. Similar distribution patterns of F. profunda were observed in surface sediments from the equatorial Atlantic (Kinkel et al., 2000), the Indian Ocean (Beaufort et al., 1997), the Tasman and Coral seas (Hiramatsu and De Decker, 1997), and around Japan (Ahagon et al., 1993; Okada and Wells, 1997; Hagino et al., 2000), although the latter authors correlated the distri-
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bution pattern of F. profunda with surface water transparency. Young (1994) suggests that £oriform coccolithophorids, a group including F. profunda, thrive in an environment characterized by low light and temperature levels and usually abundant nutrients. The present study indicates that the greatest concentrations of nannoconids coincide with low numbers of all other coccoliths within the Niveau Paquier black shale (Fig. 6). High percentages of nannoconids correspond to low values of the NI indicating low surface water fertility (Fig. 6). Conversely, during times of low percentages of nannoconids the NI values increase, re£ecting higher fertility in surface waters (Fig. 6). The results clearly demonstrate that in comparison to modern low-latitude environments nannoconids show a similar pattern as Florisphaera profunda and may be interpreted as deep-dwelling taxa as proposed earlier by Erba (1994). Considering all the arguments above, it can be assumed that nannoconids £ourished during deposition of the Niveau Paquier when the nutricline and the deep chlorophyll maximum (DCM) were located in the lower part of the photic zone. A shallowing of the nutricline/DCM obviously triggered blooms of other calcareous nannoplankton in the upper part of the photic zone. Therefore, it is assumed that the distribution of nannoconids may be used to determine the position of the nutricline/DCM during the studied interval.
deysse succession from the Vocontian Basin shows pronounced peaks at 74 cm (equivalent to V20 kyr, based on a mean sedimentation rate of 3.7 cm/kyr; see Herrle et al., in press b) and 68 cm (V16.8 kyr), re£ecting the orbital periods of the precession according to frequencies estimated for the mid-Cretaceous (Berger et al., 1992; Fig. 7). This indicates that the £uctuations of the NI values are predominantly controlled by the precessional cycle. Precession-controlled surface water fertility changes in the modern low latitudes (e.g., Arabian Sea) as well as during the Cretaceous (Tethyan Ocean) are mainly forced by monsoonal activity (e.g., Barron et al., 1985; Clemens and Prell, 1990; Wortmann et al., 1999; Herrle et al., in press b). The land^sea con¢guration of the midCretaceous rendered the low latitudes highly sensitive to monsoonal activity (e.g., Barron et al., 1985; Wortmann et al., 1999). This has also been emphasized by Oglesby and Park (1989) using general circulation model simulations (GCMs) and paleoceanographic studies (Wortmann et al., 1999; Herrle, 2002; Herrle et al., in press a,b).
6.3. Paleoenvironmental interpretation Changing environmental conditions during deposition of the l’Arboudeysse section are indicated by the vertical distribution patterns of the NI and nannoconids (Fig. 6). Throughout the investigated succession, the NI generally shows an inverse trend with the percentages of nannoconids although both curves di¡er by a small o¡set. The opposing variation of the curves probably re£ects £uctuations of the nutricline position. In the following, a model utilizing the role of monsoonal forcing is presented to interpret the observed short-term changes in surface water conditions in the subtropical Vocontian Basin. Time series analysis of the NI of the l’Arbou-
Fig. 7. Power spectra of the NI on linear scale from the l’Arboudeysse section (based on calcareous nannofossils, 194 samples; modi¢ed after Herrle et al., in press a). The power spectrum of the NI shows peaks at: E, 74 cm (20 kyr); F, 68 cm (16.8 kyr), based on a mean sedimentation rate of 3.7 cm/ kyr. Peaks that did not pass the statistical signi¢cance tests (Fisher/Siegel test) are not marked. The cross depicts a 6-dB bandwidth and the 90% con¢dence interval of the spectra (Welch windows, WOSA: three segments).
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Therefore the dominant precessional signal governing the NI at the low-latitude site of the Vocontian Basin may indicate a strong monsoonal signal. Periods of enhanced monsoonal activity are characterized by humid conditions and stronger winds (Meyers and Doose, 1999; Herrle et al., in press b) allowing a better mixing of sea surface waters and the entrainment of nutrients in the upper photic zone (shallow nutricline). Intervals of reduced monsoonal activity are characterized by cooler, more arid conditions and reduced wind stress. The surface water is more strati¢ed and depleted in nutrients (deeper nutricline). In the Vocontian Basin, seasonally increased wind stress during increased monsoonal activity is considered to have caused a rise of the nutricline, leading to enhanced phytoplankton productivity. Enhanced wind stress during this time improved vertical mixing that led to an entrainment of nutrients into the surface waters. This caused a decrease in nannoconid productivity. At the same time, it resulted in an increase of the primary production (high values of the NI). Conversely, the percentages of nannoconids increased when in a strati¢ed water column during decreasing monsoonal activity, the upper photic zone was impoverished in nutrients (low values of the NI) and the nutricline was deep. The £uctuation of both the NI and the percentages of nannoconids give evidence that nutricline dynamics were strongly in£uenced by precessional forcing during the formation of the Niveau Paquier black shale in the Vocontian Basin. A similar precessional forcing on Florisphaera profunda percentages has been observed in the modern lowlatitude Indian and Atlantic Oceans where productivity is driven by wind intensity of the monsoonal and trade winds (e.g., Okada and Matsuoka, 1996; Beaufort et al., 1997; Kinkel et al., 2000). Here, the £uctuations of the wind intensity depend on precessionally controlled insolation changes. During times of high northern summer insolation, heating over the landmass is intense. The air over the continent rises and causes a strengthening of the monsoonal wind component.
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6.4. Paleoenvironmental signi¢cance for OAE 1b formation The established model as discussed above emphasizes the role of regional paleogeography and oceanographic conditions in controlling surface water productivity that ¢nally led to the enrichment of organic matter at the sea£oor during OAE 1b formation. Moreover, it highlights the role of precessionally forced monsoonal activity in controlling £uctuations of surface water fertility during OAE 1b formation in the Vocontian Basin. To date, explanations for productivity, climatic, and biotic changes during OAE 1b deposition are related to extremely warm and humid conditions (Herrle et al., in press a,b) and high freshwater runo¡ favoring the preservation of organic matter at the sea£oor (e.g., Tribovillard and Gorin, 1991; Erbacher et al., 2001). As stated above, the role of monsoonal activity and thus surface water fertility changes represents a regional climate signal restricted to the low latitudes. However, the OAE 1b occurs supraregionally. As proposed by Herrle et al. (in press b), the formation of the OAE 1b in areas other than those monsoonally in£uenced is indirectly linked to monsoonal activity through changes in low-latitude deep water formation. There, the enhanced preservation of organic matter contributed strongly to black shale formation. However, studies dealing with elevated productivity or preservation models alone (e.g., Bralower and Thierstein, 1984; Erbacher et al., 1999, 2001) as a cause for widespread black shale deposition during the midCretaceous may be too general to describe black shale deposition under dysoxic to anoxic conditions.
7. Conclusions Abundances of coccoliths and nannoconids studied in the Lower Albian Niveau Paquier succession of the Vocontian Basin provide useful information on mid-Cretaceous paleoceanography. The cyclic variations in the NI, total abundances of coccoliths, and percentages of the deep-dwelling nannoconids are correlated with a varying nu-
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trient supply to the upper photic zone. It is proposed that the cyclic variation depends on changes in monsoonal activity. Seasonally increased wind stress caused a rise of the nutricline that led to an entrainment of nutrients into the surface waters. This resulted in an increase of primary production in the upper photic zone and decreasing percentages of nannoconids. Times of reduced wind stress are characterized by an enhanced strati¢cation and a deeper nutricline. During these times, the percentages of nannoconids increased, whereas the upper photic zone was impoverished in nutrients. Based on the calcareous nannofossil approach it is possible to determine nutricline dynamics during the mid-Cretaceous. Changes in the position of the nutricline were probably controlled by changes in monsoonal activity. Moreover, the results of this study show that the formation of the OAE 1b may be more complex than previously suggested.
Acknowledgements Jo«rg Pross (Tu«bingen) is thanked for valuable suggestions that strongly improved the manuscript. I am grateful to Christoph Hemleben (Tu«bingen) for fruitful discussions and allowing me to use his laboratory facilities. The helpful comments of reviewers Marie-Pierre Aubry and Gregory D. Price are highly appreciated. This research was supported through the Sonderforschungsbereich 275 (Project A5) of the University of Tu«bingen and by the German Research Foundation (Grant He 697/32-1/2).
Appendix. List of calcareous nannofossil taxa
Discorhabdus rotatorius (Bukry, 1969) Thierstein, 1973 Florisphaera profunda Okada and Honjo, 1973 Nannoconus elongatus Bro«nnimann, 1955 Nannoconus dauvillieri De£andre and De£andreRigaud, 1960 Nannoconus fragilis Deres and Ache¤rite¤guy, 1980 Nannoconus globulus Bro«nnimann, 1955
Nannoconus cf. inornatus Nannoconus minutus Bro«nnimann, 1955 Nannoconus regularis Deres and Ache¤rite¤guy, 1980 Nannoconus truitti Bro«nnimann, 1955 Nannoconus truitti frequens Deres and Ache¤rite¤guy, 1980 Nannoconus truitti rectangularis Deres and Ache¤rite¤guy, 1980 Nannoconus vocontiensis Deres and Ache¤rite¤guy, 1980 Watznaueria barnesae (Black in Black and Barnes, 1959) Perch-Nielsen, 1968 Zeugrhabdotus erectus (De£andre, 1954) Reinhardt, 1965
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Reconstructing nutricline dynamics of mid-Cretaceous oceans: evidence from calcareous nannofossils from the Niveau Paquier black shale (SE France) Jens O. Herrle Institut fu«r Geowissenschaften, Universita«t Tu«bingen, SigwartstraMe 10, D-72076 Tu«bingen, Germany Received 8 March 2002; received in revised form 28 August 2002; accepted 28 August 2002
Abstract A high-resolution calcareous nannofossil record is presented from the Lower Albian Niveau Paquier black shale from the Vocontian Basin (SE France). The Niveau Paquier black shale represents the regional equivalent of the supraregionally distributed Oceanic Anoxic Event 1b (OAE 1b). To reconstruct surface water fertility, a nutrient index based on Zeugrhabdotus erectus, Discorhabdus rotatorius (high fertility indicators), and Watznaueria barnesae (low fertility indicator) was established using principal component analysis. In addition, the distribution of Nannoconus spp. and absolute abundances of coccoliths (coccoliths per gram) were used for reconstructing nutricline dynamics of the surface waters. High surface water fertility coincides with low percentages of nannoconids and vice versa. Moreover, high percentages of nannoconids correlate with low absolute abundances of all other coccoliths. Based on the observed nannoplankton distribution pattern and a suggested similarity in ecological requirements between nannoconids and the modern taxon Florisphaera profunda, a model is proposed that couples nannoconid abundances with dynamics of the nutricline. Time series analyses of the nutrient index show fluctuations within the precessional band. The precession-controlled fluctuations of the surface water fertility may represent a monsoonal signal, with the nutrient supply in the surface waters depending 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 enhanced. During that time, the abundance of nannoconids decreased as a consequence of enhanced wind stress that improved vertical mixing and led to an entrainment of nutrients into the surface waters. This resulted in an increase of primary production. During periods of reduced monsoonal activity, marked by drier conditions and reduced wind stress, the surface waters were depleted in nutrients. As a result of a deep nutricline and a reduced nutrient supply to the upper photic zone in a stratified water column, percentages of nannoconids increased. According to the mechanisms outlined above, fluctuations of the nutrient index and nannoconid percentages have been used as a proxy for reconstructing the physical structure of mid-Cretaceous oceans steered by precession-forced monsoonal activity. The results of the study show that the formation of the Lower Albian OAE 1b from the Vocontian Basin occurred under strongly fluctuating surface waters fertility. 7 2002 Elsevier Science B.V. All rights reserved. Keywords: calcareous nannofossils; Cretaceous; productivity; black shale; nutricline
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1. Introduction Calcareous nannoplankton is an important component of the phytoplankton in the marine realm and has been one of the main open ocean primary producers since the Triassic. During the mid-Cretaceous, it was probably an even more important phytoplankton group than today because diatoms only became abundant as late as the Campanian (e.g., Burnett et al., 2000; Mitlehner, 2000). The distribution of calcareous nannoplankton is strongly in£uenced by climatic and hydrological conditions. Temperature has been considered the most important factor a¡ecting the distribution of calcareous nannoplankton, but trophic resources also play an important role in the distribution and abundance of calcareous nannoplankton in past and modern oceans (Chepstow-Lusty et al., 1989; Mol¢no and McIntyre, 1990; Aubry, 1992, 1998; Chepstow-Lusty and Chapman, 1995; Ziveri et al., 1995; Eshet and Almogi-Labin, 1996). In general, living calcareous nannoplankton exhibits a vertical zonation in the water column. Most species live in the upper photic zone (0^60 m). The deeper photic zone (60^200 m) is inhabited by fewer species among which Florisphaera profunda plays a dominant role in low latitudes (Okada and Honjo, 1973; Mol¢no and McIntyre, 1990). According to Mol¢no and McIntyre (1990), a high abundance of lower photic zone species is associated with a deep nutri- and thermocline, while low abundances are characteristic of a shallow nutri- and thermocline. A comparison of Early Cretaceous nannofossil assemblages with modern nannoplankton communities suggests that nannoconids possibly inhabited the lower photic zone similar to the modern F. profunda (Coccioni et al., 1992; Erba, 1994). Although the ecological preferences of nannoconids are still under debate (e.g., Mutterlose, 1987; Busson and Noe«l, 1991; Erba, 1994; Scarparo Cunha and Koutsoukos, 1998), there is increasing evidence that they may be comparable to those of the modern species F. profunda. This paper presents results from a high-resolution study of the Lower Albian Niveau Paquier black shale from the subtropical (30‡N) Vocon-
tian Basin (SE France) in order to reconstruct short-term surface water fertility changes. The Niveau Paquier black shale is the regional equivalent of the supraregionally distributed Oceanic Anoxic Event 1b (OAE 1b; e.g., Bralower et al., 1993; Erbacher et al., 2001). Based on time series analysis, the Niveau Paquier in the Vocontian Basin formed within V44 kyr (Herrle et al., in press a). 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 formation of OAE 1b is characterized by major perturbations of the ocean system de¢ned by a massive deposition of organic matter in marine environments (e.g., Schlanger and Jenkyns, 1976; Arthur et al., 1990; Bre¤he¤ret, 1988; Bralower et al., 1993; Erbacher et al., 1999; Herrle, 2002). The rapid changes in the carbon cycle were accompanied by turnovers in marine biota (e.g., Erbacher and Thurow, 1997). However, the trigger mechanisms as well as the role of increased productivity (e.g., Bre¤he¤ret, 1994; Erbacher et al., 1998, 1999) versus enhanced preservation (Tribovillard and Gorin, 1991; Erbacher et al., 2001) or a combination of both factors (Herrle et al., in press a,b) during OAE 1b formation is still under debate.
2. Study area The studied l’Arboudeysse section is located in the Vocontian Basin (SE France ; Figs. 1 and 2). During the mid-Cretaceous, the Vocontian Basin was located in the northern part of the Western Tethys and was surrounded by carbonate platforms. The eastern part of the Vocontian Basin was open to the Tethyan Ocean (Arnaud and Lemoine, 1993). The Vocontian Basin represents a marginal basin that was sensitive to both climatic and oceanographic changes. In the Aptian to Albian sedimentary record of the Vocontian Basin, extreme environmental conditions are re£ected by organicrich layers (black shales; Bre¤he¤ret, 1988, 1997; Fig. 3). The Aptian to Albian succession is dominated by marlstones and has been named Marnes Bleues Formation (Flandrin, 1963).
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Fig. 1. Paleogeographic reconstruction of the Vocontian Basin for the Late Aptian to Early Albian. Modi¢ed after Arnaud and Lemoine (1993). The rectangle indicates the studied area. The studied l’Arboudeysse section is located in the middle part of the Vocontian Basin (LA).
Fig. 2. Location map of the l’Arboudeysse section in SE France.
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2.1. Studied section The outcrop is situated 700 m southwest of l’Arboudeysse, about 5 km east of Rosans (Fig. 2). The investigated succession at l’Arboudeysse is up to 2.8 m thick and comprises the interval of the Niveau Paquier black shale reaching about 1.63 m in thickness (Fig. 3). The total organic carbon content is above 3% (maximum: 8%) and the organic matter consists of terrestrial and marine
components in variable proportions (Bre¤he¤ret, 1985; Tribovillard and Cotillon, 1989). The CaCO3 content within the Niveau Paquier reaches maximum values of 50% (Bre¤he¤ret, 1997). The Niveau Paquier black shale is rich in ammonites, with abundant leymeriellids (Fig. 3). In the upper part of this black shale an up to 3 cm thick carbonate layer rich in nannoconids can be recognized. This level has been called ‘K-bed’ (Bre¤he¤ret, 1983) and can be correlated throughout the Vocontian Basin.
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 (Herrle and Mutterlose, in press). Planktonic foraminiferal and ammonite stratigraphy after Bre¤he¤ret (1997 and references therein). 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 = Delits Calcaire; NJ = Niveau Jacob; NK = Niveau Kilian; HN = Haut Noire; NP = Niveau Paquier; LE = Niveau Leenhardt.
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3. Materials and methods Eighty-eight samples (sample intervals of 2^10 cm) were taken from freshly cut rock fragments and were analyzed for their calcareous nannofossil content using the random settling technique of Geisen et al. (1999) to calculate absolute abundances. At least 300 individuals were counted per sample in random traverses of each sample using an Olympus BX50 photomicroscope at U1250 magni¢cation. The taxonomic frameworks of Perch-Nielsen (1985) and Bown (1998 and references therein) have been followed. All coccoliths with more than half of the specimens preserved have been included in the counts. Among the 121 taxa identi¢ed four taxa (Discorhabdus rotatorius, Zeugrhabdotus erectus, Watznaueria barnesae and Nannoconus spp.) were chosen for a detailed analysis of the assemblage because of their special paleoecological and paleoceanographic signi¢cance. Nannoconus spp. includes N. dauvillieri, N. elongatus, N. fragilis, N. globulus, N. cf. inornatus, N. minutus, N. regularis, N. truitti, N. truitti frequens, N. truitti rectangularis, and N. vocontiensis. All presented data are available from the PANGAEA database (http:// www.Pangaea.de).
4. Biostratigraphy The Niveau Paquier black shale belongs to the Leymeriella tardefurcata ammonite Zone and Prediscosphaera columnata (upper Hayesites albiensis Subzone; NC8B) nannoplankton Zone (Bralower et al., 1993; Kennedy et al., 2000; Herrle and Mutterlose, in press ; Fig. 3). The Niveau Paquier has been recognized in several sections of the Tethys (Bre¤he¤ret, 1985, 1988, 1994; Arthur et al., 1990). It may correspond to the Livello Urbino in Italy (Bralower et al., 1993). Equivalent sections are also described by Bre¤he¤ret (1985) from South Germany (‘In der Ho«ll’) and Austria (‘Losenstein’). Several ODP/ DSDP legs from the Atlantic contain probably contemporaneous organic-rich sediments known as OAE 1b. These are DSDP Site 545 at the Mazagan Plateau, ODP Site 641 at the Galicia Bank,
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DSDP Site 511 at the Falkland Plateau and DSDP Site 386 at the Bermuda Ridge, and ODP Site 1047 at the Blake Nose Plateau (e.g., Bralower et al., 1993; Erbacher et al., 1999; Herrle, 2002).
5. Results The distribution of Watznaueria barnesae, Discorhabdus rotatorius, Zeugrhabdotus erectus, Nannoconus spp. as well as simple diversity and absolute abundance of coccoliths is shown in Fig. 4. Watznaueria barnesae ranges between 4% and 35.9% (mean: 15.3%) of the total assemblage. The lowest percentages of W. barnesae can be recognized within the Niveau Paquier, with a minimum in the lowest part followed by increasing percentages and again decreasing values towards the K-bed. From the K-bed to the top of the Niveau Paquier the percentages of W. barnesae increase. Discorhabdus rotatorius ranges between 2.7 and 15.1% (mean: 8.2%). The Niveau Paquier is characterized by maximum percentages of D. rotatorius at the base, below the K-bed and at the top of the black shale (Fig. 4). Zeugrhabdotus erectus ranges between 4.2 and 14.8% of the total assemblage, with a mean of 9.1%. The £uctuations show a similar trend as those of the D. rotatorius percentages (Fig. 4). The percentage of Nannoconus spp. ranges between 0 and 38.4% (mean: 8.3%). These taxa occur in elevated abundances only within the Niveau Paquier black shale and the upper part of the black shale below the Niveau Paquier (Fig. 4). Maximal percentages s 25% can be recognized in the lower part of the black shale and within the K-bed. The simple diversity lies between 26 and 46 taxa with a mean of 38 taxa per sample (Fig. 4). The lower part of the succession is characterized by the lowest simple diversity. Highest values occur within the lower to middle part of the Niveau Paquier. The total abundance of coccoliths ranges from 0.6U109 to 3.8U109 individuals per gram sediment (mean: 1.4U109 ind./g). Lowest numbers occur within the Niveau Paquier and in the upper part of the black shale below the Niveau Paquier ( 6 1.5U109 ind./g; Fig. 4). The low total abundance of coc-
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Fig. 4. Fluctuations of calcareous nannoplankton specimens, simple diversity and total coccolith abundance (individuals per gram sediment) of the Niveau Paquier black shale in the l’Arboudeysse section. Dashed lines indicate mean values. For lithological explanation see Fig. 3.
coliths within the Niveau Paquier corresponds to high percentages of Nannoconus spp.
6. Discussion 6.1. Preservation of calcareous nannofossils Dissolution and diagenesis can signi¢cantly alter the preservation of calcareous nannofossil assemblages and thus their utility as paleoenvironmental indicators for the surface waters. Four major abiotic processes control the preservation and accumulation of calcareous nannofossils dur-
ing the transition from the biocenosis to the thanatocenosis. These are (1) dissolution processes in the water column, (2) dissolution processes at the sediment surface and in the sediment, (3) dilution processes, and (4) lateral drifting of specimens (e.g., Honjo, 1976; Steinmetz, 1994; Samtleben et al., 1995; Andruleit, 1995, 1997). According to Levert and Ferry (1988), the sediment thickness above the studied outcrops in SE France amounted to about 700 m. These data may con¢rm that the studied succession has not been subject to deep burial and severe alteration (Weissert and Bre¤he¤ret, 1991). However, early diagenetic processes can operate even at these
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depths (e.g., Williams and Bralower, 1995). Therefore, to gain additional information regarding the preservation of the counted assemblages, a visual evaluation of etching (E) and overgrowth (O) was performed using the descriptive scheme of Roth and Thierstein (1972) and Roth (1983). In addition, the percentages of the most resistant nannoplankton species Watznaueria barnesae (e.g., Roth and Bowdler, 1981), the total abundance of coccoliths, and the simple diversity were used to assess the preservation of calcareous nannofossils (compare Williams and Bralower, 1995). All samples are characterized by etching and overgrowth categories E1 and O1. This indicates that some coccoliths are partly slightly etched and overgrown, but the nannofossil assemblage in total is well preserved. In the studied interval, Watznaueria barnesae occurs with maximum percentages of 35.9% (mean : 15.3%). Thierstein (1980), Roth and Bowdler (1981), and Roth (1984) considered that material containing more than 40% W. barnesae is frequently a¡ected by dissolution to the extent that the assemblages no longer yield a strong primary signal. Therefore, the low mean values of W. barnesae indicate that dissolution may not have a¡ected the primary assemblage composition. However, it is clear that as dissolution progresses beyond a certain point, more dissolution-sensitive nannofossils species will dissolve
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and species diversity will decrease (e.g., Williams and Bralower, 1995). Moreover, recent studies have shown that not all individuals and species of calcareous nannoplankton are represented in the sediment, even though calcareous nannofossils are more resistant to dissolution in the water column and sediment than, e.g., planktonic foraminifera because of the transport via fecal pellets (e.g., Andruleit, 1997 and references therein) and organic protection. The relationships between total coccolith abundances, Watznaueria barnesae percentages, and simple diversity is shown in Fig. 5. Dissolution should presumably lead to a reduction of total abundances of coccoliths, but should increase the percentage of W. barnesae. However, both the correlation coe⁄cients of W. barnesae percentages and total coccolith abundances (r2 = 0.05; n = 86) as well as W. barnesae and the simple diversity (r2 = 0.10; n = 86) are insigni¢cant for the studied interval. The lack of correlation of W. barnesae percentages, simple diversity, and total abundances of coccoliths, respectively, indicates that dissolution has not generally altered calcareous nannofossil assemblages of the study material. This indicates good preservation of calcareous nannofossils. This is corroborated by observations on Aptian to Albian nannoplankton assemblages from the nearby Col de Palluel sections (Thierstein, 1973).
Fig. 5. Scatter plots showing relationships between Watznaueria barnesae percentages, total coccolith abundance (individuals per gram sediment), and simple diversity in the studied interval of the Niveau Paquier black shale.
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6.2. Ecological signi¢cance of mid-Cretaceous calcareous nannofossils 6.2.1. Fertility indicators Detailed paleoceanographic studies based on Cretaceous calcareous nannofossils have been carried out by, e.g., Roth and Bowdler (1981), Roth and Krumbach (1986), Premoli Silva et al. (1989), Coccioni et al. (1992), Erba et al. (1992) and Williams and Bralower (1995). These authors investigated the paleobiogeography and paleoecology of Atlantic, Paci¢c and Indian ocean calcareous nannofossil assemblages using quantitative analysis and multivariate statistical techniques. According to these studies, the most prominent high fertility indicators are Zeugrhabdotus erectus, Biscutum constans (e.g., Roth and Bowdler, 1981; Roth and Krumbach, 1986; Premoli Silva et al., 1989; Erba et al., 1992), and Discorhabdus rotatorius (Premoli Silva et al., 1989; Erba, 1991; Coccioni et al., 1992). The above mentioned species are common in upwelling areas (e.g., Roth and Bowdler, 1981), but also in shelf areas where nutrients may have been added by freshwater runo¡ (e.g., Street and Bown, 2000). On the other hand, low fertility conditions have been shown to be indicated by Watznaueria barnesae (e.g., Roth and Krumbach, 1986; Erba et al., 1992; Williams and Bralower, 1995). In order to quantify surface water fertility, the species Zeugrhabdotus erectus and Discorhabdus rotatorius (high fertility indicators) and Watznaueria barnesae (indicative of low fertility) were used in a nutrient index (NI) based on R-mode (Varimax-rotated) Principal Component Analysis (see Herrle et al. (in press a) for a detailed discussion) : NI ¼ high fertility assemblage low fertility assemblage þ high fertility assemblage U100
ð1Þ
High NI values re£ect high fertility in surface waters and vice versa. In this index, the ratios between selected taxa are evaluated. This approach yields a clearer signal with respect to fertility conditions than the evaluation of single
species abundances (compare Figs. 4 and 6). Moreover, studies on Recent material have shown that species ratios, as documented in the thanatocenosis on the sea£oor, provide a better record of the surface water signal than that resulting from the abundance of single species (Andruleit, 1995; Gale et al., 2000). 6.2.2. Paleoecological signi¢cance of nannoconids Nannoconus spp. are important for biostratigraphic zonations of the Aptian to Albian interval in marginal and epicontinental basins (e.g., Deres and Ache¤rite¤guy, 1980; Erba, 1988). Moreover, they play an important role in the carbonate £ux during the mid-Cretaceous due to their compact structure and large size (e.g., Larson and Erba, 1999; Herrle and Mutterlose, in press). Over the last few decades, the paleoecological signi¢cance of nannoconids has been intensively discussed. It has become clear that the occurrence of nannoconids is positively correlated with lowlatitude shelf environments (e.g., Roth and Krumbach, 1986; Mutterlose, 1987; Busson and Noe«l, 1991; Street and Bown, 2000). Mutterlose (1989, 1992) and Street and Bown (2000) characterized nannoconids as typical Tethyan taxa. Based on increasing abundances of nannoconids accompanied by the occurrence of stenotherm warm water belemnites (Duvalia) in the boreal realm, nannoconids are interpreted as warm water taxa (Mutterlose, 1991). The trophic requirements of nannoconids, however, are still under debate. It has been proposed that nannoconids are characteristic of oligotrophic environments (Erba, 1994). On the other hand, Scarparo Cunha and Koutsoukos (1998) suggested a shallow, warm hypersaline, carbonate-saturated environment with intermediate, mesotrophic nutrient levels to be favorable for nannoconids. Busson and Noe«l (1991) documented nannoconids as meroplanktonic calcareous dino£agellate-like organisms that proliferated in epicontinental, oligotrophic seas with low terrigenous input. The inverse relationship between the abundance of nannoconids and coccolithophores is explained by these authors by red-tide poisoning. Moreover, Erba (1994) argued that nannoconids required similar conditions as the modern coccolithophorid Florisphaera profunda,
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Fig. 6. Nutrient Index (NI), distribution of Nannoconus spp., and absolute coccolith abundance in the l’Arboudeysse section. High NI values indicate high surface water fertility and vice versa. The broadly inverse relationship between nannoconids and the NI is explained by £uctuations of the nutricline. Percentages of nannoconids increase when in a strati¢ed water column the upper photic zone is impoverished in nutrients (deep nutricline). In contrast, the abundance of nannoconids decreases when enhanced wind stress improves vertical mixing, leading to an entrainment of nutrients into the surface waters (shallow nutricline). For lithological explanation see Fig. 3.
a deep photic-zone dweller. The inverse relationship between F. profunda and coccolithophore abundances is explained by a £uctuation of the nutricline depth (Mol¢no and McIntyre, 1990). However, disagreement exists regarding the similarity of trophic niches of nannoconids and F. profunda. Mol¢no and McIntyre (1990) demonstrated that £uctuations in the abundance of F. profunda re£ect variations in the nutricline (and thermocline) depth. A deep nutricline indicates mesotrophic conditions in the lower eu-
photic zone; consequently F. profunda occurs in higher abundances. In contrast, a shallow nutricline would favor higher abundances of other coccolithophorids. Similar distribution patterns of F. profunda were observed in surface sediments from the equatorial Atlantic (Kinkel et al., 2000), the Indian Ocean (Beaufort et al., 1997), the Tasman and Coral seas (Hiramatsu and De Decker, 1997), and around Japan (Ahagon et al., 1993; Okada and Wells, 1997; Hagino et al., 2000), although the latter authors correlated the distri-
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bution pattern of F. profunda with surface water transparency. Young (1994) suggests that £oriform coccolithophorids, a group including F. profunda, thrive in an environment characterized by low light and temperature levels and usually abundant nutrients. The present study indicates that the greatest concentrations of nannoconids coincide with low numbers of all other coccoliths within the Niveau Paquier black shale (Fig. 6). High percentages of nannoconids correspond to low values of the NI indicating low surface water fertility (Fig. 6). Conversely, during times of low percentages of nannoconids the NI values increase, re£ecting higher fertility in surface waters (Fig. 6). The results clearly demonstrate that in comparison to modern low-latitude environments nannoconids show a similar pattern as Florisphaera profunda and may be interpreted as deep-dwelling taxa as proposed earlier by Erba (1994). Considering all the arguments above, it can be assumed that nannoconids £ourished during deposition of the Niveau Paquier when the nutricline and the deep chlorophyll maximum (DCM) were located in the lower part of the photic zone. A shallowing of the nutricline/DCM obviously triggered blooms of other calcareous nannoplankton in the upper part of the photic zone. Therefore, it is assumed that the distribution of nannoconids may be used to determine the position of the nutricline/DCM during the studied interval.
deysse succession from the Vocontian Basin shows pronounced peaks at 74 cm (equivalent to V20 kyr, based on a mean sedimentation rate of 3.7 cm/kyr; see Herrle et al., in press b) and 68 cm (V16.8 kyr), re£ecting the orbital periods of the precession according to frequencies estimated for the mid-Cretaceous (Berger et al., 1992; Fig. 7). This indicates that the £uctuations of the NI values are predominantly controlled by the precessional cycle. Precession-controlled surface water fertility changes in the modern low latitudes (e.g., Arabian Sea) as well as during the Cretaceous (Tethyan Ocean) are mainly forced by monsoonal activity (e.g., Barron et al., 1985; Clemens and Prell, 1990; Wortmann et al., 1999; Herrle et al., in press b). The land^sea con¢guration of the midCretaceous rendered the low latitudes highly sensitive to monsoonal activity (e.g., Barron et al., 1985; Wortmann et al., 1999). This has also been emphasized by Oglesby and Park (1989) using general circulation model simulations (GCMs) and paleoceanographic studies (Wortmann et al., 1999; Herrle, 2002; Herrle et al., in press a,b).
6.3. Paleoenvironmental interpretation Changing environmental conditions during deposition of the l’Arboudeysse section are indicated by the vertical distribution patterns of the NI and nannoconids (Fig. 6). Throughout the investigated succession, the NI generally shows an inverse trend with the percentages of nannoconids although both curves di¡er by a small o¡set. The opposing variation of the curves probably re£ects £uctuations of the nutricline position. In the following, a model utilizing the role of monsoonal forcing is presented to interpret the observed short-term changes in surface water conditions in the subtropical Vocontian Basin. Time series analysis of the NI of the l’Arbou-
Fig. 7. Power spectra of the NI on linear scale from the l’Arboudeysse section (based on calcareous nannofossils, 194 samples; modi¢ed after Herrle et al., in press a). The power spectrum of the NI shows peaks at: E, 74 cm (20 kyr); F, 68 cm (16.8 kyr), based on a mean sedimentation rate of 3.7 cm/ kyr. Peaks that did not pass the statistical signi¢cance tests (Fisher/Siegel test) are not marked. The cross depicts a 6-dB bandwidth and the 90% con¢dence interval of the spectra (Welch windows, WOSA: three segments).
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Therefore the dominant precessional signal governing the NI at the low-latitude site of the Vocontian Basin may indicate a strong monsoonal signal. Periods of enhanced monsoonal activity are characterized by humid conditions and stronger winds (Meyers and Doose, 1999; Herrle et al., in press b) allowing a better mixing of sea surface waters and the entrainment of nutrients in the upper photic zone (shallow nutricline). Intervals of reduced monsoonal activity are characterized by cooler, more arid conditions and reduced wind stress. The surface water is more strati¢ed and depleted in nutrients (deeper nutricline). In the Vocontian Basin, seasonally increased wind stress during increased monsoonal activity is considered to have caused a rise of the nutricline, leading to enhanced phytoplankton productivity. Enhanced wind stress during this time improved vertical mixing that led to an entrainment of nutrients into the surface waters. This caused a decrease in nannoconid productivity. At the same time, it resulted in an increase of the primary production (high values of the NI). Conversely, the percentages of nannoconids increased when in a strati¢ed water column during decreasing monsoonal activity, the upper photic zone was impoverished in nutrients (low values of the NI) and the nutricline was deep. The £uctuation of both the NI and the percentages of nannoconids give evidence that nutricline dynamics were strongly in£uenced by precessional forcing during the formation of the Niveau Paquier black shale in the Vocontian Basin. A similar precessional forcing on Florisphaera profunda percentages has been observed in the modern lowlatitude Indian and Atlantic Oceans where productivity is driven by wind intensity of the monsoonal and trade winds (e.g., Okada and Matsuoka, 1996; Beaufort et al., 1997; Kinkel et al., 2000). Here, the £uctuations of the wind intensity depend on precessionally controlled insolation changes. During times of high northern summer insolation, heating over the landmass is intense. The air over the continent rises and causes a strengthening of the monsoonal wind component.
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6.4. Paleoenvironmental signi¢cance for OAE 1b formation The established model as discussed above emphasizes the role of regional paleogeography and oceanographic conditions in controlling surface water productivity that ¢nally led to the enrichment of organic matter at the sea£oor during OAE 1b formation. Moreover, it highlights the role of precessionally forced monsoonal activity in controlling £uctuations of surface water fertility during OAE 1b formation in the Vocontian Basin. To date, explanations for productivity, climatic, and biotic changes during OAE 1b deposition are related to extremely warm and humid conditions (Herrle et al., in press a,b) and high freshwater runo¡ favoring the preservation of organic matter at the sea£oor (e.g., Tribovillard and Gorin, 1991; Erbacher et al., 2001). As stated above, the role of monsoonal activity and thus surface water fertility changes represents a regional climate signal restricted to the low latitudes. However, the OAE 1b occurs supraregionally. As proposed by Herrle et al. (in press b), the formation of the OAE 1b in areas other than those monsoonally in£uenced is indirectly linked to monsoonal activity through changes in low-latitude deep water formation. There, the enhanced preservation of organic matter contributed strongly to black shale formation. However, studies dealing with elevated productivity or preservation models alone (e.g., Bralower and Thierstein, 1984; Erbacher et al., 1999, 2001) as a cause for widespread black shale deposition during the midCretaceous may be too general to describe black shale deposition under dysoxic to anoxic conditions.
7. Conclusions Abundances of coccoliths and nannoconids studied in the Lower Albian Niveau Paquier succession of the Vocontian Basin provide useful information on mid-Cretaceous paleoceanography. The cyclic variations in the NI, total abundances of coccoliths, and percentages of the deep-dwelling nannoconids are correlated with a varying nu-
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trient supply to the upper photic zone. It is proposed that the cyclic variation depends on changes in monsoonal activity. Seasonally increased wind stress caused a rise of the nutricline that led to an entrainment of nutrients into the surface waters. This resulted in an increase of primary production in the upper photic zone and decreasing percentages of nannoconids. Times of reduced wind stress are characterized by an enhanced strati¢cation and a deeper nutricline. During these times, the percentages of nannoconids increased, whereas the upper photic zone was impoverished in nutrients. Based on the calcareous nannofossil approach it is possible to determine nutricline dynamics during the mid-Cretaceous. Changes in the position of the nutricline were probably controlled by changes in monsoonal activity. Moreover, the results of this study show that the formation of the OAE 1b may be more complex than previously suggested.
Acknowledgements Jo«rg Pross (Tu«bingen) is thanked for valuable suggestions that strongly improved the manuscript. I am grateful to Christoph Hemleben (Tu«bingen) for fruitful discussions and allowing me to use his laboratory facilities. The helpful comments of reviewers Marie-Pierre Aubry and Gregory D. Price are highly appreciated. This research was supported through the Sonderforschungsbereich 275 (Project A5) of the University of Tu«bingen and by the German Research Foundation (Grant He 697/32-1/2).
Appendix. List of calcareous nannofossil taxa
Discorhabdus rotatorius (Bukry, 1969) Thierstein, 1973 Florisphaera profunda Okada and Honjo, 1973 Nannoconus elongatus Bro«nnimann, 1955 Nannoconus dauvillieri De£andre and De£andreRigaud, 1960 Nannoconus fragilis Deres and Ache¤rite¤guy, 1980 Nannoconus globulus Bro«nnimann, 1955
Nannoconus cf. inornatus Nannoconus minutus Bro«nnimann, 1955 Nannoconus regularis Deres and Ache¤rite¤guy, 1980 Nannoconus truitti Bro«nnimann, 1955 Nannoconus truitti frequens Deres and Ache¤rite¤guy, 1980 Nannoconus truitti rectangularis Deres and Ache¤rite¤guy, 1980 Nannoconus vocontiensis Deres and Ache¤rite¤guy, 1980 Watznaueria barnesae (Black in Black and Barnes, 1959) Perch-Nielsen, 1968 Zeugrhabdotus erectus (De£andre, 1954) Reinhardt, 1965
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