Laurent Emmanuel

Global and local forcing of Early Toarcian seawater chemistry: A comparative study of different paleoceanographic settings (Paris and Lusitanian basins)

[1]xa0The Early Toarcian was characterized by two large perturbations to the carbon cycle: a positive trend associated with increased organic matter burial and ocean anoxia, and a pronounced negative carbon isotope excursion (CIE). We contrast the geochemical evolution in the carbonate phases of two successions: one from the Paris Basin (Sancerre core, comprising black shales), the other from the Lusitanian Basin (Peniche section with very minor lithological expression of bottom water anoxia). Our aim was to identify whether these carbon cycle perturbations were related, and differentiate between the common (global) versus regional expressions of the biogeochemical response and ocean chemistry. Our results highlight contrasts in timing of different phases of anoxia in both locations through the widely documented negative CIE. Widespread anoxic conditions were not a necessary prerequisite for generating a pronounced CIE, as required by the recycling (so-called “Kuspert”) model. The production of carbonate simultaneously dropped during the δ13C negative shift in both locations, likely in response to lowered seawater saturation rate induced by substantial absorption of CO2 from the atmosphere. The recovery interval was accompanied by a rapid reestablishment of seawater alkalinity, and primary and carbonate productivity in epicontinental seas, as evidenced by high δ13C and Sr/Ca, in contrast with the more open ocean regime in the Lusitanian Basin. Our results confirm that parallels can be draw between the ocean productivity response and feedback during the Toarcian CIE and the PETM. Both events are characterized by ocean acidification and reduced pelagic calcification followed by a peak in nearshore coccolith productivity, which could have helped the recovery from the perturbation.

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.

Fracture‐controlled paleohydrogeology in a basement‐cored, fault‐related fold: Sheep Mountain Anticline, Wyoming, United States

New geochemical and microstructural data constrain the origins and pathways of paleofluids during the growth of Sheep Mountain Anticline, Wyoming, United States. Oxygen, carbon, and strontium isotope studies were performed on prefolding and fold-related calcite veins and their sedimentary host rocks and combined to fluid inclusion microthermometry results. We show that most of the cements precipitated from Paleogene meteoric fluid. Stable isotopes and fluid inclusion homogenization temperatures further indicate that most veins were mineralized from upward moving fluids after these fluids were heated at depth (T > 110°C). This implies that fluids migrated along the basement thrust underlying the fold and/or at the base of the cover. Above the fault tip, the fluids circulated rapidly in the diffuse synfolding (and early folding) fracture network. The zone of preferential migration of the warm fluids is currently located in the backlimb of the fold, which supports some of the previously published structural interpretation of the subsurface. This study also highlights the potential of combined fracture analysis and geochemical analyses of paleofluid flows in fractures to constrain both the deformation history and the fluid flow during basement-involved shortening in Laramide-style forelands.

Middle Lutetian climate in the Paris Basin: implications for a marine hotspot of paleobiodiversity

The present study reports the evolution of environmental conditions and seawater temperatures during the establishment of a marine hotspot of paleobiodiversity that took place in the Paris Basin during the Lutetian. The stable isotope compositions (δ18O and δ13C) of three species of molluscs (two bivalves: Cubitostrea plicata and Venericardia imbricata, and one gastropod: Sigmesalia multisulcata) collected along the reference section of Grignon (Falunière) are used for paleoenvironmental and paleoclimatic reconstructions. Additional high-resolution analyses on one specimen of Haustator imbricatarius allow the documentation of seasonal changes for temperature. The high-resolution profiles of the δ18O signatures of S. multisulcata reveal that these gastropods mineralized their shell during the warm months of the year, as did V. imbricata, which probably had a short life span (less than 1xa0year). These two species thus only yield temperatures for the summer period, from 22 to 30xa0°C. The δ18O of C. plicata shells indicate mean annual sea surface temperatures ranging between 15 and 23xa0°C during the Middle Lutetian, with minimal temperatures probably reflecting greater depth at the base of the section. The seasonal contrasts reconstructed in the upper part of the section, from the large gastropod H. imbricatarius, ranged between 18 and 30xa0°C. Comparison of the isotopic values of the species indicates that the δ13C of the three taxa seems to be mostly influenced by ecological features, leading to differences between endobenthic (V. imbricata) and epibenthic species (C. plicata); or the food habits. The paleoclimatic reconstructions show that the Lutetian climate was relatively stable in the Paris Basin with long-term cooling of the mean annual sea-surface temperatures. Nevertheless, this study shows that despite a context of colder conditions compared to the Early Eocene, the climate provided a favorable context for the increase of marine biodiversity in the Paris Basin during the Middle Lutetian.

Apport de la cathodoluminescence à la connaissance de la spéciation du manganèse dans les carbonates pélagiques

Mn contents analysis of pelagic carbonates from the Upper Jurassic of the Vocontian trough (Châteauneuf-dOze section, Hautes-Alpes) allowed the definition of six third-order eustatic sequences that agree with the chronostratigraphic chart [17]. Cathodoluminescence intensity enables a semi-quantitative approach of Mn trapped in the calcite lattice; thus, CL-results show that most of Mn is found in the pelagic limestones as Mn2+. The Mn content variations observed in the low stands (low Mn contents) and in the high-stand systems tract samples (high Mn contents) reflect the sea water Mn content variations, which appear to be linked to the hydrothermal activity, especially by the ocean ridge. In the Vocontian realm, third-order sequences of the Upper Jurassic seem to be controlled by tectono-eustatism.

Reconstitution of dune morphologies and palaeowind regimes in the Proto-Namib since the Miocene

In the Namib Desert, indurated fossil dunes are found between the active sands. Establishment of a biostratigraphic scale based on associated mammal faunas and ratite eggshells allows the study of palaeowind fluctuations that have controlled these palaeodunes since the Middle Miocene with a temporal resolution of between 1 and 3 Ma. In the southern Namib, the Proto-Namib aeolian systems consist of crescent dunes (barkhan type). In the eastern part of the Namib Desert, the aeolianite cross-bedding corresponds to longitudinal or pyramidal dunes. These aeolian structures are also controlled by local winds, resulting from high relief and the southern trade winds : this regime is identical to that which currently sweeps, through the Namib Desert, and is related to the presence of anticyclonic conditions in the Atlantic Ocean. A regime of seasonal winds from the northeastern sector is also recorded starting in the late Miocene.

Les Ostracodes de la falunière de Grignon (Lutétien du Bassin de Paris): implications stratigraphiques

ABSTRACT The ostracod fauna from the outcroup “la falunière” of Grignon (Lutetian, Paris Basin): stratigraphical implications. The rehabilitation of the outcroup “la falunière” of Grignon (shell marl pit) allowed for the first time to take an almost complete section of the Paris Basin Middle Lutetian under its uncimented facies. It was the opportunity to undertake sedimentological and paleontological studies. This publication presents results of the ostracode study. More than 6000 valves or carapaces were collected and 87 species was distinguished. Three new species are described : Paracypris keiji n. sp., Neocytherideis labyrinthoidea n. sp. and Grinioneis pachycosta n. sp. The ostracodes vertical distribution in the faluns of Grignon is probably without stratigraphical signification. On the contrary, the Lutetian ostracodes associations are characteristic, as well in comparison with those of the Ypresian that with those of the Auversian or of the Barton beds. Specific diversification reflects that of the environmental niches and the wealth in individuals testify of the abundance of the food. Ostracodes indicate that the Lutetian sea was shallow, from some decimetres to some metres, probably, and its waters limpid, not much agitated. The summer temperature of water was equal at least to that of present Mediterranean Sea. From a paleogeographic point of view, the similarity between gallic fauna seems much more narrow than between whatever of this gallic fauna and those of the Aquitanian basin; so, the majority of English Lutetian species are known in the Paris Basin while the proportion of common species of the Paris Basin and of the North Aquitain basin is weak.

A sequence analyzed from the basin to the platform : the Middle Oxfordian calcareous succession in southeastern France

Middle Oxfordian sedimentation is very homogeneous in southeastern France. It is characterized by a specific alternation of marls and fine-grained limestones (G. transversarium Zone). This work shows that sets of calcareous beds allow accurate stratigraphic correlations in various paleogeographic areas, from the shallow Jura platform to the deep Dauphinois basin. Following a rifting period, this sedimentation illustrates a doming period with decreasing subsidence and water depth. This corresponds to the establishment of a wide marine area more favourable to carbonate sedimentation. Considering sequence stratigraphy, some slight variations occur in the sediment record and lead to interesting comparisons between the platform and the basin. The geochemical evolution, mainly the manganese content, seems to be dependant of the activity of the Ligurian Tethys oceanic ridge. This fact shows that sedimentation in southeastern France could be significantly controlled by tectono-eustasy.

Stratigraphie intégrée du Toarcien stratotypique (coupes de Thouars et Airvault, Deux-Sèvres, France)

Resume Letude stratigraphique pluridisciplinaire du Toarcien dans sa region stratotypique (essentiellement la coupe dereference dAirvault) se revele riche denseignements dordre chronostratigraphique et dordre paleoenvironnemental. La confrontation des donnees biostratigraphiques (Ammonites, Ostracodes, Foraminiferes, Nannofossiles, Palynomorphes) et magnetostratigraphiques permet daffiner la resolution chronostratigraphique de letage: des lacunes sont mises en evidence dans les zones a Bifrons et Insigne. Les teneurs en carbone organique total sont faibles: entre 0,1 et 1,9%. La matiere organique a une origine mixte: marine et continentale, plutot continentale a la base de la section dAirvault (Toarcien moyen compris), plutot marine au Toarcien superieur. Les resultats geochimiques (elements traces et isotopes stables de la phase carbonatee) traduisent levolution de lenvironnement marin: le passage de la zone a Pseudoradiosa vers la zone a Aalensis est, en particulier, marque par un rechauffement. Une interpretation en terme de stratigraphie sequentielle est proposee.

Fluctuations of sea-water chemistry during Gargasian (Middle Aptian) time. Data from trace-element content (Mg, Sr, Mn, Fe) in hemipelagic carbonates from La Marcouline Quarry (Cassis, SE France)

In the Lower Aptian historical stratotype area (Cassis-La Bedoule, SE France), a geochemical study of the Gargasian (Middle Aptian) marl-limestone alternations of the La Marcouline quarry complements data already obtained from Bedoulian (Early Aptian) sediments there. Nannoconids are the main carbonate producers in both limestones and marls. Although diagenetic minerals, such as ankerite (2.5%) are present in small amounts, the trace-element content of bulk carbonate is very close to that of Nannoconus spp. so geochemical sequences can be defined. The long-term evolution of trace-element content was not affected by diagenetic processes, variations in carbonate mineralogy, or a change of carbonate producers. An increase of around 500 ppm in the strontium content of bulk carbonate occurs between the base of the Cabri zone (late Bedoulian) and the Algerianus zone (late Gargasian). This evolution is linked to fluctuations in seawater Sr/Ca ratios caused by variability in the influx of hydrothermal and river waters, by changes in the ratio of aragonite/calcite production and by shifts in sea level. The eustatic sequence Aptian 4, its parasequences and its key surfaces (sequence boundaries, maximum flooding surface) are clearly reflected in the evolution of the bulk-carbonate contents of manganese.