Fabienne Giraud

Ammonoid Abundance Variations Related to Changes in Trophic Conditions Across the Oceanic Anoxic Event 1d (Latest Albian, SE France)

Abstract Macrofauna abundance has been quantified across the Breistroffer interval (OAE 1d, Latest Albian) in the Vocontian basin (southeast France). This interval is characterized by a high abundance of ammonoids showing significant morphologic disparity. Twenty ammonoid genera are recognized and assigned to seven morphostructural groups. The aims of this study are to: (1) interpret abundance variations of ammonoid taxa in terms of taphonomic processes, sedimentary dilution, and paleoenvironmental factors; and (2) discuss the habitat and mode of life of some Late Albian ammonoids with respect to their morphology and abundance variations. Most variations in ammonoid absolute abundance are not the result of post-mortem shell transport. High abundance within the Breistroffer interval is due partly to a low accumulation rate, which is related to a maximum-flooding stage. The different trends observed among ammonoid taxa can be interpreted partly in terms of response to changes in trophic conditions inferred from the study of calcareous nannofossils. Heteromorphs, which are dominant during mesotrophic conditions, could have been more competitive than involute/evolute planispirals (normal coiling) when paleoenvironmental conditions become more unstable. Lechites (orthocone), the most abundant taxon, probably inhabited epipelagic, distal paleoenvironments. This genus, interpreted as a vertical migrant in the water column, would have been able to move up when trophic conditions were good in surface waters, and down in order to avoid oligotrophic surface waters and to exploit food-rich layers in deeper waters. Turrilitoides and Mariella (torticones) may have been quasiplanktic, and inhabited mainly neritic paleoenvironments. These forms could have occupied more distal paleoenvironments when mesotrophic conditions prevailed in the surface water column. Anisoceras and Hamites (quasiplanktic U-shaped heteromorphs) mainly lived in distal, epipelagic habitats, and could have been more competitive compared with the torticones when oligotrophic conditions prevailed in surface waters. Among involute/ evolute planispiral ammonoids, Mortoniceras probably had a deep-nektonic mode of life and inhabited the lower part of the epipelagic zone. A stylized panorama of some Late Albian ammonoid habitats is proposed.

Paleoenvironmental and paleobiological origins of coccolithophorid genus Watznaueria emergence during the late Aalenian—early Bajocian

Abstract The latest Aalenian—early Bajocian time interval (ca. 171-169 Ma) is marked by a global reorganization of oceanic plates with the Central Atlantic opening and the formation of the Pacific plate. This time interval is also marked by a global geochemical perturbation of &dgr;13C with a negative excursion at the Aalenian/Bajocian boundary and a positive excursion during the early Bajocian. Evolutionary diversifications of marine invertebrate taxa, namely ammonites, radiolarians, and coccolithophorids, are recorded at that time. Concerning coccolithophorids, this interval witnesses the diversification and expansion of the most successful Mesozoic genus: Watznaueria. In this study, we explore the potential environmental, ecological, and biological forcing at the origin of Watznaueria diversification and its effect on the coccolith assemblages through quantification of the absolute and relative abundances of calcareous nannofossils in two Middle Jurassic key sections: Cabo Mondego (Portugal) and Chaudon-Norante (France). In both sections, we find an increase in nannofossil absolute abundance and flux at the beginning of the lower Bajocian, coeval with an increase in absolute and relative abundances of Watznaueria spp., followed by a plateau in the middle and upper part of the lower Bajocian. The increase of Watznaueria spp. is synchronous with a decrease in relative abundance of other major coccolith taxa, whereas the absolute abundance of these species did not decrease. During the climatically driven early Bajocian eutrophication event, Watznaueria spp. integrated into the calcareous nannoplankton community in two successive evolutionary steps involving first W. contracta and W. colaccicchii, and second W. britannica and W. aff. manivitiae. Step 1 was driven by an increase in niche carrying capacities linked to the early Bajocian eutrophication. Step 2 was driven by specific adaptation of the newly evolved Watznaueria species to bloom in nutrient-rich environments not exploited before. These evolutionary events have initiated the 100-Myr reign of Watznaueria over the calcareous nannoplankton community.

Palynofacies and calcareous nannofossils in the Upper Kimmeridgian, southeastern Paris basin (France)

The Upper Kimmeridgian Members “Calcaires blancs superieurs” and the “Marnes a exogyres superieures” of the southeastern Paris basin were investigated for their palynofacies and calcareous nannofossils. These members display alternating limestone marl lithotypes and represent shallow marine paleoenvironments. The lower carbonate member is interpreted as a proximal palaeoenvironment (palaeobathymetry = 5 to 10m), where storm and swell deposits were prevalent and the salinity was occasionally weak. The relative richness of brown phytoclasts in this part is favoured by good preservation related to restricted conditions. These conditions would explain the dominance of the nannofossil Cyclagelosphaera margeretti in the nannofossil assemblages. The palynological data as those of the nannofossil assemblages show variations in the shift from carbonate member to marly member. The dominance of brown phytoclast over black phytoclast, the presence of amorphous organic matter (AOM), and the highest abundance and diversity observed within the nannofossil assemblages suggest that the maximum of distality occurred during this transition, at the basal part of the Marnes a Exogyres superieures. The upper part of the section (marly member) characterized by storm deposits an storm coquina beds is deeper (palaeobathymetry probably between 10 and 40 m depth) than the lower part. Oxidizing depositional conditions prevailed and explained the abundance of black particles found in this opper part., while proximate cysts and elevated non-placolith coccolith abundances indicate that relationships with the open sea were probably more significant compared to the lower part. Towards the top of the section, recurrences of restricted conditions are reflected by relatively elevated amounts of AOM and the abundance peak of the nonnofossil Biscutum ellipticum. This study shows that micropalaeontological signals can be well recorded in vast lagunal domains. We speculate that salinity , nutrient supply, and oxygenation of the waters control microfossil associations. In shallow environments, these parameters are particularly fluctuating, especially when freshwater dilutes marine waters in surface, on the occasion of rainy periods or of intense arrival of continental waters.

Comparative evolution of material flux through two DSDP successions from Central Atlantic and Gulf of Mexico. Preliminary results of a new approach to sedimentary dynamics in pelagic environments

Abstract A method for gaining knowledge of sedimentary fluxes is applied to Early Cretaceous sequences containing high-frequency sedimentary cycles. The cycles are used to determine the relative fluctuations of (1) the sedimentation rate and (2) carbonate and silicate terrigenous fluxes expressed as linear values proportional to the real fluxes. Visual and automatic image analysis of black-and-white core photographs have been used to calculate the masses of different materials deposited in equal time intervals and to determine the distribution of the material within lithological units of cores according to a scale of grey intensities. Characters of these distributions allow definition of a flux variability index, assumed to be negatively correlated to ocean-continent distance related itself to sea level. At DSDP Sites 534 (western Atlantic off Florida) and 535 (Gulf of Mexico), the Valanginian-Hauterivian interval has been chosen for determining the relative variations of carbonate and silicate terrigenous linear fluxes, their variability and their distribution within lithological cycles. These data provide information about the relative importance of carbonate sources (lands, platforms and plankton) and their evolution, and of other forcings upon planktonic productivity such as supply of nutrients from the continents and upwelling. By comparing the data in the two sites, it is possible to detect common (climate and eustasy) and local factors (tectonics and upwelling) which have controlled the material fluxes. Compared with Site 534, Site 535 was supplied by more numerous, active and close carbonate sources and less active but closer terrigenous sources. At the two sites, carbonate flux was controlled by common factors, sea level being the most prominent one. Terrigenous flux was submitted to more local controls. This new method for investigating pelagic deposits should be tested on other deep-sea sedimentary sequences.

Guidebook for the post-congress fieldtrip in the Vocontian Basin, SE France (September 11-13, 2008)- 12th Meeting of the International Nannoplankton Association (Lyon, September 7-10, 2008)

Guidebook for the post-congress fieldtrip in the Vocontian Basin, SE France (September 11-13, 2008)