Contrasting origins of Early Cretaceous black shales in the Vocontian basin: Evidence from palynological and calcareous nannofossil records
Introduction
The Aptian to Turonian interval (∼120–90 Ma, Gradstein et al., 1995) has been described as a time of exceptionally warm climates (Hallam, 1985, Wilson and Norris, 2001), low equator-to-pole thermal gradients (Barron, 1983, Huber et al., 1995, Jenkyns et al., 2004) and high levels of atmospheric carbon dioxide (Freeman and Hayes, 1992, Berner, 1994, Beerling and Royer, 2002). This period of greenhouse conditions was accompanied by the episodic formation of organic carbon-rich sediments (“black shales”) in the world oceans, which have been interpreted to reflect major perturbations of the ocean-atmosphere system (Arthur et al., 1988, Bralower et al., 1994, Jenkyns, 1999, Herrle et al., 2003b, Erba and Tremolada, 2004, Wagner et al., 2004, Weissert and Erba, 2004). The relatively short-lived intervals (∼50 to 500 ka) of organic carbon (OC) accumulation were confined to marine pelagic and hemipelagic environments and have been termed oceanic anoxic events (OAEs) by Schlanger and Jenkyns (1976). Besides the occurrence of globally distributed OC-rich deposits generated during OAEs, additional black shale intervals of only regional distribution are observed in Aptian to Albian sediments from the Atlantic and western Tethyan Oceans (Erba, 1991, Bréhéret, 1994, Erbacher et al., 1996, Friedrich et al., 2003, Herrle et al., 2004).
The processes that controlled the formation of the OC-rich deposits during the mid-Cretaceous are still a matter of debate. During the last decades, a variety of different palaeoceanographic models have been proposed, most of which can be assigned to one of two contrasting hypotheses. (1) The productivity model is based on the observation that enhanced fertility in ocean surface waters results in an increased flux of OM to the seafloor. This in turn causes increasing oxygen deficiency within the water column and hence, increased OM preservation under dysoxic to anoxic bottom waters. The important role of enhanced oceanic palaeoproductivity for the increase in OM burial during the mid-Cretaceous OAEs has been emphasised in various studies (e.g. Arthur et al., 1987, Pedersen and Calvert, 1990, Erba, 1994, Weissert et al., 1998, Jenkyns, 1999, Premoli Silva et al., 1999, Erba and Tremolada, 2004). (2) In contrast to this, the stagnant ocean model argues for a reduction of deep-water renewal and/or enhanced water column stratification. The decline in oxygen-rich deep water production prevents the aerobic degradation of OM within the water column and at the sediment–water interface, resulting in its accumulation at the seafloor and in sediments (e.g. Schlanger and Jenkyns, 1976, Bralower and Thierstein, 1984, Arthur et al., 1990, Tyson, 1995, Erbacher et al., 2001). In addition, sea-level fluctuations have been suggested by various authors to play a key role in the formation of black shales in hemipelagic and pelagic settings (e.g. Bréhéret, 1994, Erbacher et al., 1996, Strasser et al., 2001). A number of studies have emphasised the important role of climatic fluctuations and the complex interplay between various factors (e.g. palaeoceanographic setting, basin configuration) in controlling the formation of OC-rich deposits (Kuypers et al., 2002, Herrle et al., 2003a, Wagner et al., 2004).
In order to obtain an understanding of the mechanisms controlling the formation of black shales during the mid-Cretaceous, both marine and terrestrial inputs need to be considered. In this study, the supraregionally distributed Niveau Goguel (OAE1a) and the regionally distributed Niveau Jacob, both deposited in the Vocontian basin (southeastern France) during the Aptian are chosen for comparison (Fig. 1). Spore–pollen records are combined with data based on calcareous nannofossils, organic-walled plankton, particulate organic matter and with geochemical results. The main objectives are: (i) to trace changes in terrestrial vegetation patterns across black shale episodes; (ii) to reconstruct variations in terrigenous input and sedimentation rates, and; (iii) to obtain information on changes in palaeoproductivity during periods of OM accumulation.
Section snippets
Palynology
Sixteen samples from the Serre Chaitieu section (Niveau Goguel) and thirteen samples from the Tarendol section (Niveau Jacob) were prepared for palynological analysis. Cleaned and weighed (10 to 12 g) samples were treated with hydrochloric and hydrofluoric acid following standard palynological preparation techniques (Traverse, 1988). The residue was sieved with an 11-μm mesh sieve and a first set of strew mounts was prepared for palynofacies analysis. A short oxidation with HNO3 was performed
Geological setting
During the mid-Cretaceous, the Vocontian basin was situated at a palaeolatitude of 25° to 30°N (Hay et al., 1999), forming part of the northern continental margin of the Alpine Tethyan Ocean (Fig. 1). The Marnes Bleues formation, a thick monotonous succession of grey to dark-grey marls intercalated with calcareous marls, limestones and numerous black shale horizons was deposited in the basin between early Aptian and early Cenomanian times (Flandrin, 1963). Accumulation of fine-grained sediments
Preservation of the particulate OM
Although the sporopollenin of pollen and spore walls is relatively resistant to degradation, chemical and biological processes during transport and deposition as well as post-depositional alteration can corrode or even destroy palynomorphs. Generally, palynomorphs are more affected by degradation processes than refractory organic material (e.g. phytoclasts), which can result in an enrichment of the latter in the sediment (Tyson, 1995). Furthermore, thin-walled pollen grains are less resistant
Conclusions
Except for several similarities, our results highlight the different nature of the globally distributed Niveau Goguel (OAE1a) and the regionally occurring Niveau Jacob, both deposited in hemipelagic environments of the Vocontian basin during the Aptian. Both black shale intervals are characterised by enriched TOC-contents compared to the overlying and underlying strata and exhibit laminated sedimentary textures. The absence of intense bioturbation and the abundant occurrence of AOM indicates
Acknowledgements
Financial support from ETH-project TH-34./99-4 is gratefully acknowledged. This manuscript was significantly improved thanks to suggestions and comments by S.P. Hesselbo and helpful reviews by Susanne Feist-Burkhardt and Elisabetta Erba.
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2022, Global and Planetary ChangeMarine paleoecological variations during the mid-Cretaceous oceanic anoxic event 1a in the Vocontian Basin, southeastern France
2022, Palaeogeography, Palaeoclimatology, PalaeoecologyCitation Excerpt :TOC values are affected not only by redox conditions but also by productivity, terrestrial organic matter input, sedimentation rate, and carbonate flux. Heimhofer et al. (2004, 2006) reported that terrestrial input was reduced during OAE 1a compared to other black shale intervals (especially in OAE 1b) in the Vocontian Basin. Therefore, the higher TOC content in the upper black shale layers (GO2 to GO6-I) suggest that oceanic anoxia and/or primary production strengthened after the deposition of black shale layer GO1.
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