Modeling evidences for global warming, Arctic seawater freshening, and sluggish oceanic circulation during the Early Toarcian anoxic event

International audienceThe paleoecological disturbances recorded during the Early Toarcian warming event (183 Myr ago), including marine anoxia, sea level rise, seawater acidification, carbonate production crisis, and species extinctions, are often regarded as past examples of Earth's possible responses to the rapid emergence of super greenhouse conditions. However, physical mechanisms explaining both the global and local expressions of paleoenvironmental events are still highly debated. Here we analyze the paleoclimatic and paleoceanographic consequences of increases in atmospheric pCO2 levels at a multiscale resolution using a fully coupled ocean–atmosphere model (FOAM). We show that, in association with stronger high-latitude precipitation rates and enhanced continental runoff, the demise of polar sea ice due to the global warming event involved a regional freshening of Arctic surface seawaters. These disturbances lead to progressive slowdowns of the global oceanic circulation accountable for widespread ocean stratification and bottom anoxia processes in deep oceanic settings and epicontinental basins. In agreement with very negative oxygen isotope values measured on fossil shells from the NW Tethys, our simulations also show that recurrent discharges of brackish and nutrient-rich Arctic surface waters through the Viking Corridor could have led to both vertical and geographical gradients in salinity and seawater δ18O in the NW Tethyan seas. Locally contrasted conditions in water mass density and rises in productivity rates due to strong nutrient supplies could partly explain the regional severity of the anoxic event in the restricted Euro-boreal domains, as it has been previously suggested and modeled regionally

Le rôle des changements paléoclimatiques sur l'évolution de la biodiversité au Pliensbachien et au Toarcien

La crise du Pliensbachien-Toarcien est souvent considérée comme l une des plus grandes extinctions du Phanérozoïque. Sa cause a fait l objet d hypothèses variées, incluant le volcanisme du Karoo, l OAE du Toarcien, ou des variations eustatiques. Dans ce travail, une étude transdisciplinaire est menée afin d explorer l influence des changements climatiques sur la biodiversité tout au long de l intervalle. Couplées aux données de 87Sr/86Sr et de cortèges argileux, nos données de 18O mesurées sur différents fossiles confirment la présence d un réchauffement et de phases humides au cours du Pliensbachien et du Toarcien inférieur. Les analyses de 13C et Nd suggèrent que ces épisodes pourraient être liés à des modifications océaniques, des éruptions volcaniques, ou des dégazages de méthane. En parallèle, les composantes taxonomiques, morphologiques, et biogéographiques de la diversité des ammonites ont été réévaluées. Nos résultats soulignent d importantes variations de la diversité, structurées par une crise polyphasée. Cinq pics d extinctions, présentant des dynamiques sélectives propres, sont identifiés et attribués à des contraintes environnementales différentes. De plus, nos analyses témoignent d un provincialisme faunique marqué au cours du Pliensbachien et de sa dislocation progressive pendant le réchauffement du Toarcien inférieur. En confrontant les données, il semble que le paléoclimat ait eu un impact indirect sur les faunes, en influençant les gradients climatiques, le glacio-eustatisme, la circulation océanique, ou l oxygénation des eaux. Néanmoins, le degré d interaction et de rétroaction entre contraintes peut induire des réponses biologiques multiples.The Pliensbachian-Toarcian biotic crisis is often regarded as one of the most important extinction events of the Phanerozoic. Its origins are debated and include hypotheses such as the Karoo volcanism, the Early Toarcian OAE, and major eustatic variations. In this study, a transdisciplinary approach was used in order to explore the influence of climate changes on the evolution of biodiversity. Associated to the 87Sr/86Sr and clay mineral data, our 18O data measured on fossils confirm warming events with more humid conditions during the Pliensbachian and the Early Toarcian. Our 13C and Nd analyses suggest that these events could be linked to oceanic modifications, volcanic events, and methane releases. In parallel, the taxonomic, morphological, and biogeographic components of the ammonite diversity were reappraised over the whole interval. Our results underline major diversity and disparity variations, partly structured by a polyphased crisis. Five main extinction peaks, with distinct spatial, taxonomic, and selective dynamics, are identified and attributed to various environmental constraints. In addition, our analyses show a strong faunal provincialism during the Pliensbachian, its progressive dislocation during the Early Toarcian warming, and its reappearance during the Middle-Late Toarcian. By confronting the different data, it seems that the palaeoclimatic changes had an indirect impact on faunas, by influencing the latitudinal climatic gradients, glacioeustatism, oceanic circulation, and seawater oxygenation. Nevertheless, the mechanisms are very complex and the degree of interaction or the feedback between constraints may induce multiple biological consequences.DIJON-BU Sciences Economie (212312102) / SudocSudocFranceF

Diversity and morphological evolution of Jurassic belemnites from South Germany

International audienc

Adaptive radiation in the fossil record: a case study among Jurassic ammonoids.

15 pagesInternational audienceEvolutionary radiations have been extensively studied especially in the fossil record and in the context of postcrisis recoveries. The concept of adaptive radiation that emerges from this very broad topic explicitly involves the effect of adaptation driven by ecological opportunity and is considered to be of the foremost importance. It is essential to be able to detect adaptive radiation because it points up factors that predispose a clade to radiate. Adaptive radiation has received much attention in recent decades based mostly on studies dealing with recent clades, but data from the fossil record are still scarce. This study begins to fill this gap with the example of Lower Jurassic ammonoids (through c. 8 Myr of history). A survey of several clades, using both taxonomic and disparity-based approaches, shows that they diversified successively through time, but not systematically, in terms of species numbers and morphological variety. Some clades seem to have exhibited adaptive radiation and to have become rapidly extinct. One clade (which engendered nearly all post-Lower Jurassic ammonoids) has a fossil record that begins with low diversity and disparity but is superseded by a sustained radiation pattern. The results are discussed in the light of the Modern Synthesis and its continuation into an Extended Evolutionary Synthesis

Quoi de neuf au Jurassique en France ?

7 pagesNational audienc

The flourishing diversity of models in theoretical morphology: from current practices to future macroevolutionary and bioenvironmental challenges.

17 pagesInternational audienceFor decades, theoretical morphological studies of different groups of organisms have been successfully pursued in biological, paleontological, and computational contexts, often with distinct modeling approaches and research questions. A regular influx of new perspectives and varied expertise has contributed to the emergence of a veritable multidisciplinary outlook for theoretical morphology. The broadening of this discipline is reflected in a substantial increase in the number of models, leading to a bewildering diversity that has yet to be scrutinized. In this work, we tackle this issue in a synthetic fashion, with a quantitativemeta-analysis that allows an objective comparison of theoretical morphological models treated as entities. By analogy with empirical morphospace analyses of actual organisms, we performed a multivariate ordination of a representative sample of models, producing a metaspace of models in which patterns of similarity and difference are visualized. A phenetic tree was used to characterize the relationships between models. Four major groups have been identified, and their disparity analyzed. We suggest this typology as a useful starting point to identify a core set of fundamental principles and protocols for better interpretation of the plethora of current models and for more efficient construction of models in the future. This in turn can help in diversifying the scope of macroevolutionary, developmental, and bioenvironmental questions in theoretical morphology

Ammonite paleobiogeography during the Pliensbachian-Toarcian crisis (Early Jurassic) reflecting paleoclimate, eustasy, and extinctions.

14 pagesInternational audienceThe Pliensbachian-Toarcian crisis (Early Jurassic) is one of the major Mesozoic paleoecological disturbances when ca. 20% of marine and continental families went extinct. Contemporaneously, profound paleobiogeographical changes occurred in most oceanic domains including a disruption of ammonite provincialism during the Early Toarcian. Here, we quantitatively reappraise the structure and evolution of paleobiogeographical patterns displayed by ammonite faunas before, during, and after the biological crisis, over a time-interval including 13 biochronozones. The high-resolution study presented here involves the use of hierarchical Cluster Analyses, non-metric Multi-Dimensional Scaling methods, and Bootstrap Spanning Network approaches that we apply to a large database including 772 ammonite species from 16 northwestern Tethyan and Arctic basins. Our results confirm a robust faunal dichotomy between Euro-Boreal and Mediterranean areas throughout the Pliensbachian, with the first emergence of an Arctic biome during the cooling regressive event of the Spinatum Zone. Whatever its complexity, Pliensbachian provincialism could be directly linked to paleogeographical barriers and to latitudinal paleoclimatic and paleoecological contrasts. During the Early Toarcian, this pattern was progressively lost, with northward expansions of Mediterranean ammonites during the Tenuicostatum Zone, followed by a strong interprovincial mixing during the Falciferum Zone. This faunal homogenization results from the combination of several parameters including a major sealevel rise facilitating basinal connections, a global warming event stretching the spatial range limits of southern taxa, and a mass extinction preferentially removing endemic species. Ammonite provincialism, although slightly different, was progressively re-established during the cooling regressive trend of the Middle Toarcian. These results therefore suggest a paramount influence of paleoclimatic, eustatic, and extinction constraints on the paleobiogeography of Early Jurassic ammonites, even if some threshold effects or independent biological factors may sporadically complicate the patterns

De grands stades isotopique dans le cycle du carbone au Crétacé inférieur

National audienceLe Crétacé inférieur est ponctué de plusieurs événements d'accélération du cycle hydrologique intimement lié au cycle du carbone. La diversité de l'enregistrement sédimentaire de ces événements, de leur amplitude observée dans les fluctuations du δ13C et le manque de fiabilité des âges calendaires dans le Crétacé inférieur ont longtemps été un frein à la compréhension des mécanismes menant à ces événements. Nous avons réalisé la calibration astronomique des étages du Valanginien, de l'Hauterivien et du Barrémien à travers l'analyse spectrale de signaux de gamma-ray total et de susceptibilité magnétique, tous deux inversement corrélés à la teneur en CaCO3. Les séries sédimentaires analysées sont les dépôts cycliques marne-calcaire de bassin du Bassin vocontien (SE de la France), du Domaine subbétique (SE de l'Espagne) et du Bassin de Neuquén (W de l'Argentine). Nous proposons ici une révision du cadre chronologique sur la base de l'identification du cycle de l'excentricité de 405 ka. Ce cadre astrochronologique est ancré sur plusieurs âges U-Pb obtenus dans le Bassin de Neuquén et corrélés dans le Bassin vocontien par biostratigraphie. Par ailleurs, nous avons synthétisé les données de δ13C sur roche totale de carbonates marins et sur rostres de bélemnites. De ces données, il ressort que les signaux de δ13C ont des périodicités marquées à 1,2 Ma et 2,4 Ma, ce qui correspond à de longs cycles d'obliquité et d'excentricité. Ces signaux sont en phase entre plusieurs bassins et avec les variations d'argilosité observées dans le Bassin vocontien et le Domaine subbétique. Ceci suggère l'existence de grands stades isotopiques contrôlés par des cycles de productivité primaire en domaine marin, eux-mêmes liés à des fluctuations cycliques du climat et du niveau marin, peut-être contrôlés par glacio-eustatisme. Enfin, notre cadre chronologique montre que l'excursion isotopique du δ13C de plus forte amplitude de cet intervalle de temps, l'événement Weissert, au début du Valanginien supérieur, est synchrone à la mise en place des trapps du Paraná-Etendeka tout en continuant de s'inscrire dans les longs cycles orbitaux observés. Ainsi, les trapps du Paraná-Etendeka, bien loin d'effacer la réponse du cycle du carbone au forçage orbital, l'ont probablement amplifiée