Palaeoenvironmental significance of Toarcian black shales and event deposits from southern Beaujolais, France

New sedimentological, biostratigraphical and geochemical data recording the Toarcian Oceanic Anoxic Event (T-OAE) are reported from a marginal marine succession in southern Beaujolais, France. The serpentinum and bifrons ammonite zones record black shales with high (1-10 wt%) total organic carbon contents (TOC) and dysoxia-tolerant benthic fauna typical of the ‘Schistes Carton' facies well documented in contemporaneous nearby basins. The base of the serpentinum ammonite zone, however, differs from coeval strata of most adjacent basinal series in that it presents several massive storm beds particularly enriched in juvenile ammonites and the dysoxia-tolerant, miniaturized gastropod Coelodiscus. This storm-dominated interval records a marked negative 5‰ carbonate and organic carbon isotope excursion being time-equivalent with that recording storm- and mass flow-deposits in sections of the Lusitanian Basin, Portugal, pointing to the existence of a major tempestite/turbidite event over tropical areas during the T-OAE. Although several explanations remain possible at present, we favour climatically induced changes in platform morphology and storm activity as the main drivers of these sedimentological features. In addition, we show that recent weathering, most probably due to infiltration of O2-rich meteoric water, resulted in the preferential removal of 12C-enriched organic carbon, dramatic TOC loss and total destruction of the lamination of the black shale sequence over most of the studied exposure. These latter observations imply that extreme caution should be applied when interpreting the palaeoenvironmental significance of sediments lacking TOC enrichment and lamination from outcrops with limited surface exposure

Pelagic carbonate production emergence during the Middle Jurassic (180-160 Ma) : the conquest of the oceans by the coccolithophorid genus Watznaueria

Les coccolithophoridés sont des algues marines photosynthétiques et planctoniques qui produisent des plaques micrométriques de carbonate de calcium (CaCO3) appelés coccolithes. Ces algues sont apparues il y 210 Ma et produisent actuellement la majeure partie du CaCO3 dans les océans modernes, jouant ainsi un rôle majeur dans le cycle du carbone. Cependant, l’émergence de la production de CaCO3 océanique par les coccolithophoridés au cours du Jurassique, ainsi que son impact sur le cycle du carbone, restent très mal compris. Cette étude s’est donc focalisée sur une période du Jurassique Moyen (Bajocien inférieur, -170 Ma) enregistrant la diversification de Watznaueria, un genre de coccolithophoridé qui a ensuite dominé la production de CaCO3 océanique pendant plus de 80 Ma. L’analyse des assemblages de coccolithes du Jurassique Moyen du Portugal et du sud de la France, réalisée à l’aide d’une méthode de reconnaissance automatique appliquée pour la première fois aux coccolithes du Jurassique, a permis de quantifier l’importance de cette période de diversification sur la production de CaCO3 pélagique. En outre, la durée de cet intervalle clé a été réévaluée grâce à l’analyse cyclostratigraphique des séries sédimentaires du Sud de la France. Les variations de production de CaCO3 pélagique ainsi reconstituées ont été comparées aux perturbations du cycle du carbone enregistrées par les rapports des isotopes du carbone, et indiquent un lien probable avec une augmentation marquée de la fertilité des océans. Par ailleurs, l’analyse paléontologique montre que cette diversification correspond à l’apparition successive de différentes espèces vraisemblablement opportunistes du genre Watznaueria. Enfin, les flux obtenus de CaCO3 pélagiques, largement inférieurs à ceux observés dans les océans actuels, semblent insuffisants pour avoir eu une influence significative sur le cycle global du carbone du Jurassique Moyen.Coccolithophorids are photosynthetic and planktonic marine algae that produce micrometric calcium carbonate (CaCO3) platelets called coccoliths. These algae appeared about 210 Ma ago and produce today most of the CaCO3 in the modern oceans, hence playing a major role in the carbon cycle. Nevertheless, the onset of oceanic CaCO3 production by these organisms during the Jurassic and its impact on carbon cycling remain poorly understood. This study therefore focused on the Middle Jurassic interval (Early Bajocian, -170 Ma) which records the diversification of Watznaueria, an evolutionary important coccolith genus that subsequently dominated oceanic CaCO3 production for more than 80 Myr. The analysis of coccolith assemblages from the Middle Jurassic of southern France and Portugal, based on an automaticcoccolith recognition device used for the first time on Jurassic coccoliths, allowed quantifying the impact of this diversification on CaCO3 production. In addition, the duration of this key interval has been revaluated by the cyclostratigraphic analysis of sedimentary strata from southern France. The reconstructed changes in CaCO3 production were compared to carbon cycle perturbations recorded by carbon isotope ratios and indicate a probable link with a marked increase of ocean fertility. Besides, paleontological analyses show that this diversification episode correspond to the successive appearance of different, probably opportunistic Watznaueria species. The obtained fluxes of pelagic CaCO3 production, by far lower than those recorded in modern oceans, seems too low to have significantly impacted theMiddle Jurassic carbon cycle

Downsizing the pelagic carbonate factory: Impacts of calcareous nannoplankton evolution on carbonate burial over the past 17 million years

International audienceCenozoic deep-sea carbonates (“calcareous oozes”) are predominantly biogenic in origin and offer detailed records of the evolution of calcifying plankton groups, such as coccolithophores and foraminifera. The size and abundance of calcifying plankton determine the strength of the calcium carbonate “pump” in the open ocean, which acts as a short-term source of CO2, while the burial of pelagic carbonates serves as a long-term sink of carbon. Here, we show how the macroevolutionary size decrease in calcareous nannoplankton (coccoliths and calcareous nannoliths) has affected burial rates of calcareous ooze over the past 17 million years. We quantified nannofossil carbonate burial rates (g CaCO3/m2/yr) at five DSDP/ODP sites in the Atlantic, Indian, and Western Pacific oceans. The proportion of nannofossil-dominated fine fraction carbonate ( 38 μm fraction remained stable over the past 17 Myr. This suggests that changes in the deposition of calcareous ooze were primarily driven by calcareous nannoplankton, and that foraminifera did not compensate for the lower nannofossil-carbonate accumulation rates since the Pliocene. Despite a deepening of the lysocline over the past 4 Myr, global pelagic carbonate mass accumulation likely decreased. Whether, or how, this may relate to changes in weathering or other components within the long-term carbonate cycle remains unclear. Explanations for the macroevolutionary size decrease in calcareous nannoplankton focus on the physiological and ecological advantages of small, lightly calcified algal cells in a low-CO2 and more stratified marine environment

Statistical confidence intervals for relative abundances and abundance-based ratios: Simple practical solutions for an old overlooked question

International audienceMicropaleontologists often consider relative abundances of taxa to infer past ecological, environmental and climate conditions and dynamics. However, most published micropaleontological studies involving relative abundance data still do not routinely consider the counting uncertainty inherent to any sample, and thus simply ignore the statistical confidence interval (CI) related to a relative abundance or abundance-based ratio value. In an attempt to make this rather classic computation freely and easily available to the scientific community, we highlight here the calculation of binomial proportion CIs based on the 'exact' Clopper-Pearson method as implemented in the user-friendly PAST freeware. We also introduce a general solution for the computation of the CI related to any abundance-based ratio. In all cases, we strongly recommend that future studies involving taxonomic abundance-based data should systematically display the CIs associated to sample estimates, the only way to integrate sampling uncertainties into result interpretation

Coccolith production through coccolithophore growth experiment

Experiment measuring mass of a single coccolithophore cell every minute for near 9h. The coccolithophore is a Gephyrocapsa huxleyi culture strain RCC1216 from the Roscoff Culture Collection. A multi-well plate containing a low-density culture of calcifying cells grown in K/2 culture medium (Keller et al., 1987) was placed on the stage of an inverted microscope Leica DMi8 with a Smarteck GC3851MP camera, installed in a climate-controlled room (20°C) at the Biological Station of Roscoff and illuminated with large-spectrum white LED lights with an intensity of ca. 100 µmol.m-2.s-1. The coccolithophore produced 9 coccoliths during the experiment. The mass of individual coccospheres (in pg CaCO3) was estimated from their birefringence quantified from the images (Beaufort et al., 2014). Each produced coccolith was heavier than the previous one. The cell was most probably in G1 interphase and thus growing during the experiment

Pelagic carbonate production across the K/Pg boundary

International audienceThe Cretaceous/Paleogene (K/Pg) boundary is one of the most important mass extinction crisis during the Phanerozoic. Caused by the volcanism of Deccan trapps and/or the impact of a meteorite, this mass extinction had a disastrous impact on calcareous nannofossils diversity leading to the most important turnover in their evolutionary history. The K/Pg mass extinction led also to an important change in the pelagic carbonate production and accumulation which has deeply perturbed the carbonate system. In this study, we aim to quantify nannofossil carbonate accumulation rates before and after the K/Pg boundary to discuss i) the impact of the volcanism on calcareous nannofossil size and carbonate production; ii) the impact of the mass extinction event s.s. on the pelagic carbonate accumulation rate and iii) the timing of recovery of a stable and efficient pelagic carbonate production and accumulation after the crisis. Ultimately, we aim to estimate the impact of the K/Pg mass extinction on the carbonate and carbon cycles. Three deep-sea drilling sites were studied: ODP Site 762C from the Eastern Indian Ocean, IODP Site 1209 from the Northwestern Pacific Ocean and IODP Site 1267 from the Southeastern Atlantic. These sites have been precisely dated by cyclostratigraphy and biostratigraphy (Husson et al., 2011; Westerhold et al., 2011; Dinarès-Turell et al., 2014) and presents continuous sedimentation in the studied interval. We studied around 100 samples by site covering about 3 Myr before and 3 Myr after the K/Pg boundary. On each sample, the total carbonate content was measured and its accumulation rate estimated. We performed biometry of nannofossils (size and mass) in order to trace the morphometric variability of nannofossil and species specific carbonate accumulation rates through the K/Pg boundary. ReferencesDinarès-Turell, J., Westerhold, T., Pujalte, V., Röhl, U. and Kroon, D., 2014. Astronomical calibration of the Danian stage (Early Paleocene) revisited: Settling chronologies of sedimentary records across the Atlantic and Pacific Oceans, Earth and Planetary Science Letters, 405: 119-131.Husson, D., Galbrun, B., Laskar, J., Hinnov, L.A., Thibault, N., Gardin, S. and Locklair, R.E., 2011. Astronomical calibration of the Maastrichtian (Late Cretaceous). Earth and Planetary Science Letters, 305: 328-340.Westerhold, T., Röhl, U., Donner, B., McCarren, H.K. and Zachos, J.C., 2011. A complete high-resolution Paleocene benthic stable isotope record for the central Pacific (ODP Site 1209), Paleoceanography, 26: PA2216

Extraterrestrial 3He shows that Mesozoic marl-limestone alternations are mainly driven by CaCO3 variations at the astronomical timescale

International audienceIntroduction Marl-limestone alternations are well known rhyth-mical inter-bedded deposits that commonly occur inmany hemipelagic to pelagic deposits of the Phanerozoic. It is quite well established that the origin of these lithological variations are astronomically-driven climatic variations (22, 41, 100 and 405 ka being the main periods) e.g. [1]. However, the exact sedimentological control is not clear: several models attribute these alternations to cyclic changes in the carbonate flux, whereas the terrigenous silicoclastic flux remained relatively constant. On the opposite, some models suggest that the carbonate flux was constant while the silicoclastic flux changed cyclically. Material and methods To disentangle these different scenarios, we collected marlstone and limestone samples from two sedimentary succession of Bajocian (3 marl-limestone couplets over 3.5 m) and Valanginian (1 marl-limestone couplet over 1 m) age from the Southern French Alps (Barles). We analyzed their carbonate contents as well as their extraterrestrial 3He (3HeET) concentrations in ~200 mg decarbonated aliquots. Results and discussion The carbonate content ranges from 45% in marls to 86% in limestones. Importantly, for all samples, measured 3HeET concentrations are constant in the silicoclastic fractions, within uncertainties. Hence, our results indicate that sedimentation rates at the astronomical timescale in the examined examples were mainly controlled by large changes in the CaCO3 fluxes, leading to variable dilution of the terrigenous and 3HeET fractions. Finally, assuming a constant 3HeET flux of 0.1 pcc/cm2/ka [2], and the whole thickness of Bajocian and Valanginan strata in this region, the measured 3HeET concentrations imply sedimentation rates that are broadly compatible with current duration estimates of these two stages. References: [1] Eldrett J. S. et al. (2015) Earth. Plan. Sci. Let., 423, 98-113. [2] Farley K.A. et al. (2012) GCA, 84, 314-328

Pelagic carbonate production across the K/Pg boundary

International audienceThe Cretaceous/Paleogene (K/Pg) boundary is one of the most important mass extinction crisis during the Phanerozoic. Caused by the volcanism of Deccan trapps and/or the impact of a meteorite, this mass extinction had a disastrous impact on calcareous nannofossils diversity leading to the most important turnover in their evolutionary history. The K/Pg mass extinction led also to an important change in the pelagic carbonate production and accumulation which has deeply perturbed the carbonate system. In this study, we aim to quantify nannofossil carbonate accumulation rates before and after the K/Pg boundary to discuss i) the impact of the volcanism on calcareous nannofossil size and carbonate production; ii) the impact of the mass extinction event s.s. on the pelagic carbonate accumulation rate and iii) the timing of recovery of a stable and efficient pelagic carbonate production and accumulation after the crisis. Ultimately, we aim to estimate the impact of the K/Pg mass extinction on the carbonate and carbon cycles. Three deep-sea drilling sites were studied: ODP Site 762C from the Eastern Indian Ocean, IODP Site 1209 from the Northwestern Pacific Ocean and IODP Site 1267 from the Southeastern Atlantic. These sites have been precisely dated by cyclostratigraphy and biostratigraphy (Husson et al., 2011; Westerhold et al., 2011; Dinarès-Turell et al., 2014) and presents continuous sedimentation in the studied interval. We studied around 100 samples by site covering about 3 Myr before and 3 Myr after the K/Pg boundary. On each sample, the total carbonate content was measured and its accumulation rate estimated. We performed biometry of nannofossils (size and mass) in order to trace the morphometric variability of nannofossil and species specific carbonate accumulation rates through the K/Pg boundary. ReferencesDinarès-Turell, J., Westerhold, T., Pujalte, V., Röhl, U. and Kroon, D., 2014. Astronomical calibration of the Danian stage (Early Paleocene) revisited: Settling chronologies of sedimentary records across the Atlantic and Pacific Oceans, Earth and Planetary Science Letters, 405: 119-131.Husson, D., Galbrun, B., Laskar, J., Hinnov, L.A., Thibault, N., Gardin, S. and Locklair, R.E., 2011. Astronomical calibration of the Maastrichtian (Late Cretaceous). Earth and Planetary Science Letters, 305: 328-340.Westerhold, T., Röhl, U., Donner, B., McCarren, H.K. and Zachos, J.C., 2011. A complete high-resolution Paleocene benthic stable isotope record for the central Pacific (ODP Site 1209), Paleoceanography, 26: PA2216

Extraterrestrial 3He shows that Mesozoic marl-limestone alternations are mainly driven by CaCO3 variations at the astronomical timescale

International audienceIntroduction Marl-limestone alternations are well known rhyth-mical inter-bedded deposits that commonly occur inmany hemipelagic to pelagic deposits of the Phanerozoic. It is quite well established that the origin of these lithological variations are astronomically-driven climatic variations (22, 41, 100 and 405 ka being the main periods) e.g. [1]. However, the exact sedimentological control is not clear: several models attribute these alternations to cyclic changes in the carbonate flux, whereas the terrigenous silicoclastic flux remained relatively constant. On the opposite, some models suggest that the carbonate flux was constant while the silicoclastic flux changed cyclically. Material and methods To disentangle these different scenarios, we collected marlstone and limestone samples from two sedimentary succession of Bajocian (3 marl-limestone couplets over 3.5 m) and Valanginian (1 marl-limestone couplet over 1 m) age from the Southern French Alps (Barles). We analyzed their carbonate contents as well as their extraterrestrial 3He (3HeET) concentrations in ~200 mg decarbonated aliquots. Results and discussion The carbonate content ranges from 45% in marls to 86% in limestones. Importantly, for all samples, measured 3HeET concentrations are constant in the silicoclastic fractions, within uncertainties. Hence, our results indicate that sedimentation rates at the astronomical timescale in the examined examples were mainly controlled by large changes in the CaCO3 fluxes, leading to variable dilution of the terrigenous and 3HeET fractions. Finally, assuming a constant 3HeET flux of 0.1 pcc/cm2/ka [2], and the whole thickness of Bajocian and Valanginan strata in this region, the measured 3HeET concentrations imply sedimentation rates that are broadly compatible with current duration estimates of these two stages. References: [1] Eldrett J. S. et al. (2015) Earth. Plan. Sci. Let., 423, 98-113. [2] Farley K.A. et al. (2012) GCA, 84, 314-328

De l’apport de la pédagogie numérique en Sciences de la Terre par numérisation d’objet en 3D et mise en place de la collection numérique collaborative de la SGF

International audienceLes Sciences de la Terre, discipline scientifique naturaliste, sont basées sur l’observation. L’enseignement des Sciences de la Terre se traduit donc par une pratique importante de l’étude de nombreux objets géologiques que sont les roches, fossiles et minéraux. Malheureusement, le temps dédié à l’observation est souvent trop restreint et ne permet pas aux étudiants d’être formés ou de s’auto-former sur une collection exhaustive. La pédagogie par le numérique est une alternative prometteuse qui limite cette restriction temporelle et permet le travail d’observation en autonomie sur des objets géologiques. Depuis plusieurs années, un peu partout dans le monde, de vastes efforts de numérisation 3D de la surface et/ou de la structure interne d'objets géologiques ont été réalisés par diverses technologies (photogrammétrie, laser, projection de franges de lumière, ou tomographie par rayon X). Aujourd’hui, un grand nombre d’universités proposent des échantillons en support numérique. La création d’un compte sur la plate-forme de visualisation 3D Sketchfab pour l’université d’Aix-Marseille a permis de constituer une collection de fossiles annotés en ligne. Des leçons pédagogiques ont également été créées dans moodle. Les étudiants apprécient l’accès aux ressources numériques, échantillons annotés, et leçons. En revanchele temps qu’ils dédient à l’observation reste bien inférieur à celui mobilisé pour la numérisation. Aussi, afin de mutualiser les efforts de numérisation réalisés partout dans le monde francophone, la SGF a créé un compte Sketchfab, donnant accès libre à l’ensemble des contributions des membres du consortium. Le choix de la plate-forme Sketchfab repose sur la possibilité d’intégrer les modèles dans les plates-formes de pédagogie numérique moodle et claroline. Cette initiative collaborative a pour objectif d’offrir à l’ensemble des enseignants une collection libre d’accès, exhaustive et référencée. Dans le futur, une collaboration avec Unisciel pourrait permettre d’élargir la ressource moodle type vers des travaux pratiques en ligne au travers de Socles3