Michel Moullade

Early Aptian δ 13 C and manganese anomalies from the historical Cassis-La Bédoule stratotype sections (S.E. France): relationship with a methane hydrate dissociation event and stratigraphic implications

Comparison of oxygen and carbon isotope and manganese evolution curves in bulk carbonate from the historical Bedoulian stratotype (Cassis-La Bedoule area, Provence, France) reveals an important geochemical event (negative δ 13 C and high Mn content) located within the D. deshayesi ammonite Zone and at the base of the R. hambrowi ammonite Subzone. This worldwide event, which can be observed in environments ranging from the fluvial to the pelagic realm (Selli/Goguel level), seems to be related to methane hydrate destabilization. Scenarios for manganese, carbon and oxygen evolutions are proposed for early Bedoulian oxic conditions and for dysoxic/anoxic conditions related to methane hydrate destabilization at the early/late Bedoulian transition. The impacts of this global event on the biosphere (nannoconid crisis) and its stratigraphic implications are considered. Comparison of geochemical and biostratigraphical data from the Cassis-La Bedoule stratotype with that of the Cismon- Apticore reference borehole shows that the La Bedoule sequence records geochemical evolution during the Goguel/Selli Event in more detail than that of any other previously published section.

Ecological structuring and evolution of deep sea agglutinated foraminifera — a review

Abstract Recent agglutinated foraminiferal communities exhibit both simple and complex ecological structuring within habitats. Simple ecological structuring (i.e. a limitation of the entire fauna at or close to the sediment-water interface and absence of erect suspension feeding forms) is mainly observed under low oxygen conditions and high organic flux or after severe substrate disturbance. Oligotrophic deep sea conditions with low vertical particle flux and little substrate disturbance as in the central areas of the modern oceans lead to the development of complex ecological structuring with various highly adapted “specialists” (i.e. mobile infaunal forms, a variety of epifaunal suspension feeders, and epifaunal deposit feeders with special mechanisms to avoid energy loss and use food resources not exploitable by other organisms). The paleontologic record of species diversity and habitat preferences since the late Jurassic exhibits two trends: (1) a general trend from simple to complex ecological structuring within habitats leads to an overall increase in diversity; (2) fluctuations in this trend coinciding with major paleoceanographic events, i.e. changes in deep water oxygenation and paleoproductivity. From a comparison of Recent and fossil communities from various trophic situations we conclude, that the ecological structuring of agglutinated foraminifera within habitats may have been one of the most important driving forces (selection mechanisms) in the evolution of this special group of protists.

Early history of the Atlantic Ocean and gas hydrates on the Blake Outer Ridge: Results of the Deep Sea Drilling Project Leg 76

Leg 76 of the Deep Sea Drilling Project achieved two major scientific objectives. The first objective was met at Site 533, where on the Blake Outer Ridge, gas hydrates were identified by geophysical, geochemical, and geological studies. Gas-hydrate decomposition produced a volumetric expansion of 20:1 of gas volume to pore-fluid volume; this expansion exceeded by about a factor of four the volume of gas that could be released from solution in pore water under similar conditions. The gas hydrate includes methane, ethane, propane, and isobutane but apparently excluded normal butane and higher molecular weight hydrocarbons as predicted from gas hydrate crystallography. For the first time, marine gas hydrates were tested with a pressure core barrel. The second objective was achieved when coring at Site 534 in the Blake-Bahama Basin sampled the oldest oceanic sediments yet recovered. The sequence of oceanic basement and overlying sediments documents the geologic history of the early stages of the opening of the North Atlantic Ocean in detail. The oldest oceanic sediments are red claystones and laminated green and brown claystones of middle Callovian age. This finding supports the interpretation that the beginning of the modern North Atlantic occurred in the early Callovian (∼ 155 m.y. B.P.), as much as 20 m.y. later in time than often previously thought.

The Deshayesitidae STOYANOV, 1949 (Ammonoidea) of the Aptian historical stratotype region at Cassis-La Bédoule (SE France)

One of the significant results of the multidisciplinary investigations carried out during recent years in the Lower Aptian historical stratotype of the Cassis-La Bedoule region (South-Eastern France) was a proposal to update the local Upper Barremian/Lower Aptian ammonite biozonation in order to be more consistent with the standard Mediterranean zonal subdivisions. So the lower and upper boundaries of the Lower Aptian (= Bedoulian substage of most French authors) as well as the boundaries of four biozones (Paradeshayesites tuarkyricus, P. weissi, Deshayesites deshayesi, Dufrenoyia furcata zones) and two subzones (Roloboceras hambrovi and Paradeshayesites grandis subzones) were identified and formally defined in the stratotype. However, to support this zonal scheme additional descriptions and illustrations of the ammonites collected bed by bed in the several sections studied were still required. In this paper we describe the members of the most significant ammonite family found in the Lower Aptian of Cassis-La Bedoule, i.e. the Deshayesitidae, and delimit precisely their stratigraphic positions. As regards zonation, we have replaced the index ammonite of the earliest Bedoulian Zone, i.e. Paradeshayesites tuarkyricus (Bogdanova, 1983), by Paradeshayesites oglanlensis (Bogdanova, 1983), and re-established the Pseudocrioceras waagenoides Zone as a subzone.

The Douvilleiceratidae (Ammonoidea) of the Lower Aptian historical stratotype area at Cassis-La Bédoule (SE France)

Recent biostratigraphic research in the marly limestones of the Cassis-La Bedoule area (SE France) provided a rich macrofauna of Douvilleiceratidae Parona & Bonarelli, 1897. From the uppermost Barremian (Pseudocrioceras waagenoides Subzone) to the middle Aptian (Parahoplites melchioris Zone), specimens of Procheloniceras, Cheloniceras, Roloboceras, Megatyloceras and Epicheloniceras were collected in succession. In this paper we describe the various genera and species from this material and delimit precisely their stratigraphic positions. Our study shows that each genus or subgenus characterizes a discrete stratigraphic interval. In addition, the Cheloniceras meyendorffi (upper Bedoulian), Epicheloniceras debile, Epicheloniceras gracile, and Epicheloniceras buxtorfi (Gargasian = middle Aptian) subzones, originally defined in England by Casey (1961a), are identified for the first time in the Lower Aptian stratotypic area of Cassis-La Bcdoule.

Lower cretaceous planktonic foraminiferal biostratigraphy of the Western North Atlantic (ODP Leg 171B), and taxonomic clarification of key index species

Abstract An upper Aptian to Cenomanian sedimentary succession containing unusually well-preserved planktonic foraminifers was documented during ODP Leg 171B, which was devoted to the study of the Blake Plateau off Florida. With the exception of two zones within the Albian, an almost continuous composite section permits detailed investigation of the evolution of the foraminiferal assemblages. Emphasis in this paper is placed on the Aptian/Albian and Albian/Cenomanian transitions. In addition, the excellent preservation of the material enables some taxonomic revision, mainly on the basis of wall microstructural characters, of several planktonic foraminiferal lineages and taxa, such as the Hedbergella praetrocoidea — H. trocoidea — Ticinella eubejaouaensis phylogenetic relationship and the Rotalipora globotruncanoides or Hedbergella rischi species concepts.

Upper cretaceous abyssal claystones in the North Atlantic and Western Tethys: current status of biostratigraphical correlation using agglutinated foraminifers and palaeoceanographic events

Abstract The biostratigraphic correlation scheme of Upper Cretaceous multicoloured claystones in the Polish Romanian. External Carpathians using agglutinated deep-water foraminifers can be extended to onshore localities in the Gibraltar Arch area (Morocco, Spain) and DSDP/ODP sites in the North Atlantic. In all studied areas, taxonomic turnovers in deep-water agglutinated foraminifers are observed at the Cenomanian-Turonian boundary and in the early/middle Campanian. Additionally, a remarkable decrease in abundance and diversity of benthic agglutinated foraminifers is observed at the same levels in continuous sections of North Atlantic DSDP sites. These datum levels correspond to inter-regional and time-constant palaeoceanographic events, and may facilitate the direct correlation of the biozonation of agglutinated foraminifers to the standard geomagnetic polarity time scale.

Occurrence of marine mid-Cretaceous sediments along the Guinean slope (Equamarge II cruise): their significance for the evolution of the central Atlantic African margin

Abstract During the Equamarge II cruise (1988), several coring attempts made along the slope edge of the southern Guinean Plateau led to the recovery of middle Cretaceous sediments in two cores. One contained a carbonate block, dated as middle Albian (H. rischi Zone); the micropaleontological and sedimentological data suggest an open marine environment and paleodepths of approximately 150–200 m. Another core included a dark shale level corresponding to the global anoxic event “CTBE” (Cenomanian/Turonian Boundary Event, W. archaeocretacea Zone). As shown by seismic reflection profiles, these mid-Cretaceous beds lie very close to a major unconformity, which distinctly divides the sedimentary cover into two units (the lower one folded and faulted, the upper one undisturbed). Geodynamic and paleoenvironmental implications of these new data better constrain models of evolution for this crucial area of the Atlantic realm. The compressive and extensional events which affected the Equatorial Atlantic during the progressive separation of Africa from South America are thus more accurately dated. These results also strengthen the hypothesis of an open seaway between the Central and South proto-Atlantic as early as the middle Albian.

The 'Pseudocrioceras beds' at La Bédoule (SE France) and the position of the Barremian-Aptian boundary in the historical lower Aptian stratotype

Marly limestones from La Bedoule (Bouches-du-Rhone, SE France), historical type-locality of the lower Aptian, contain in their lowermost part a rich ammonite fauna belonging to the heteromorphic genus Pseudocrioceras. Up to the present this level, corresponding to the Pseudocrioceras coquandi Zone of Busnardo (1984), was considered by ammonitologists as early Aptian. However, a detailed study of newly collected faunas from both this level and immediately under- and overlying beds leads us to reconsider the age assignment of this interval. Our results show that the Pseudocrioceras are first accompanied, in the upper part of the Martelites sarasini Zone, by a heteroceratid fauna showing a Barremian signature, and then coexist briefly with the first Deshayesitidae of the Aptian Deshayesites tuarkyricus Zone. Therefore, our proposal is to place the Barremian/Aptian boundary in the historical type-locality at the FAD (First Appearance Datum) of Deshayesites, being better defined and having a wider geographic extension in the Mesogean realm.

About the stratigraphic position of the Lower Aptian Roloboceras hambrovi (Ammonoidea) level

In the stratotype of the Lower Aptian substage the position of the assemblage that includes Roloboceras spp. and Megatyloceras spp. has been clearly established as being in the middle part of the upper Bedoulian. Some authors have erected this horizon as a subzone, or even a zone, with R. hambrovi as species-index. In other areas (southern England, eastern Spain, the Ardeche in southeastern France), where taphonomic conditions are not always as favourable as they are in the stratotype, the level at which this assemblage occurs seems to be confined to the upper part of the lower Bedoulian. Various hypotheses are considered in an attempt to explain this divergence.