Continental weathering as a driver of Late Cretaceous cooling : new insights from clay mineralogy of Campanian sediments from the southern Tethyan margin to the Boreal realm

21 pagesInternational audienceNew clay mineralogical analyses have been performed on Campanian sediments from the Tethyan and Boreal realms along a palaeolatitudinal transect from 45° to 20°N (Danish Basin, North Sea, Paris Basin, Mons Basin, Aquitaine Basin, Umbria-Marche Basin and Tunisian Atlas). Significant terrigenous inputs are evidenced by increasing proportions of detrital clay minerals such as illite, kaolinite and chlorite at various levels in the mid- to upper Campanian, while smectitic minerals predominate and represented the background of the Late Cretaceous clay sedimentation. Our new results highlight a distinct latitudinal distribution of clay minerals, with the occurrence of kaolinite in southern sections and an almost total absence of this mineral in northern areas. This latitudinal trend points to an at least partial climatic control on clay mineral sedimentation, with a humid zone developed between 20° and 35°N. The association and co-evolution of illite, chlorite and kaolinite in most sections suggest a reworking of these minerals from basement rocks weathered by hydrolysis, which we link to the formation of relief around the Tethys due to compression associated with incipient Tethyan closure. Diachronism in the occurrence of detrital minerals between sections, with detrital input starting earlier during the Santonian in the south than in the north, highlights the northward progression of the deformation related to the anticlockwise rotation of Africa. Increasing continental weathering and erosion, evidenced by our clay mineralogical data through the Campanian, may have resulted in enhanced CO2 consumption by silicate weathering, thereby contributing to Late Cretaceous climatic cooling

Évolution du climat et circulation océanique au Crétacé (utilisation de la composition isotopique ([delta]18O, [epsilon]Nd) de dents de poissons)

Le Crétacé a souvent été considéré comme une époque chaude et climatiquement stable, avec une pression partielle de CO2 dans l'atmosphère estimée à 3 à 10 fois la pression actuelle, qui aurait été à l'origine d'un effet de serre plus important qu'aujourd'hui. Cependant, des études récentes montrent que le climat chaud du Crétacé aurait pu être épisodiquement interrompu par des épisodes froids avec le développement possible de calottes glaciaires d'envergure limitée en hautes latitudes, notamment durant le Crétacé inférieur. L'objectif de cette étude est de préciser l'évolution des températures des eaux marines de surface au cours du Crétacé, de localiser dans le temps les éventuels épisodes froids, et d'étudier l'impact des variations climatiques sur la faune marine et la productivité carbonatée. Dans un premier temps, une courbe représentant l'évolution des températures des eaux marines de surface de la Téthys occidentale (Europe de l'ouest) au cours du Crétacé est construite, grâce à l'analyse des isotopes de l'oxygène de dents de poissons. En effet, le rapport 18O/16O (représenté par la notation d18O) mesuré dans les dents de poissons dépend d'une part du d18O (rapport 18O/16O) de l'eau environnante, et d'autre part de la température de cette eau. Ainsi, en faisant une hypothèse raisonnable sur le d18O de l'eau de mer du Crétacé, et ayant mesuré le d18O des dents de poissons d'une époque considérée, il est alors possible de calculer la température de l'eau environnant le poisson au moment où sa dent s'est formée. L'intérêt d'utiliser l'apatite de dents de poissons et non des coquilles calcitiques d'organismes marins est double: d'une part l'apatite est connue pour la résistance du groupement PO4 à la diagénèse (processus d'altération pouvant modifier la composition isotopique originelle des échantillons par des échanges post-mortem avec les fluides circulants dans les sédiments), et d'autre part l'équation de fractionnement, reliant le d18O mesuré dans l'organisme avec la température et le d18O de l'eau environnante, est la même quelle que soit l'espèce de poisson considérée. Ceci n'est pas le cas de la plupart des organismes à coquille calcitique (brachiopodes, foraminifères, ...) pour lesquels des effets vitaux importants et variables selon les espèces sont présents. Cependant, l'altération des dents de poissons reste possible, même si elle est bien moins fréquente que dans le cas des organismes à coquille carbonatée. Il a été proposé par certains auteurs d'utiliser la cristallinité des échantillons d'apatite pour discriminer les échantillons altérés des échantillons sains. Nous nous proposons donc dans le cadre de ce travail de nous intéresser à la relation existant entre cristallinité et altération de l'apatite, en vue d'obtenir un critère objectif d'évaluation de l'état de préservation des échantillons. La détermination de la crystallinité des apatites se fait à l'aide d'un spectromètre Raman, et est basée sur la vibration du pic d'étirement symétrique de la liaison P-O de l'apatite. Cette méthode présente l'avantage d'être très rapide et peu coûteuse, et un grand nombre d'échantillons peut ainsi être traité. La courbe de d18O construite à partir de l'analyse des dents de poissons est ensuite comparée à d'autres courbes obtenues par l'analyse de matériel différent (foraminifères, carbonate en roche totale) et déposé sous différentes latitudes, afin de déterminer si le signal enregistré est un signal climatique global ou si sa signification est uniquement régionale. L'évolution des températures est ensuite mise en relation avec les occurences d'événements anoxiques, locaux (sud-est de la France) et globaux. D'autre part, une compilation de valeurs de d18O est réalisée dans le but d'établir l'évolution des gradients latitudinaux de température au cours du Crétacé supérieur et d'affiner ainsi la compréhension de l'évolution du climat au cours du Crétacé. Une reconstitution de ces gradients est également réalisée à partir d'analyses de dents de poissons, et comparée à la compilation précédente. Les températures obtenues des divers endroits du monde seront également intégrées dans des modèles climatiques, qui permettent d'appréhender la circulation océanique de surface ainsi que la répartition des précipitations. Enfin, une étude des variations à court terme est réalisée sur un étage du Crétacé présentant des variations rapides de températures (Valanginien), qui seront mises en relation avec les crises et migrations de la faune marine. Les températures sont calculées grâce au d18O d'huitres fossiles, qui sont présentes en continu sur une coupe dilatée et bien contrainte biostratigraphiquement du Valanginien. Les huitres sont toutes au préalable examinées à la cathodoluminescence pour tester leur préservationLYON1-BU.Sciences (692662101) / SudocLYON1 - UFR Sciences de la terre (692662208) / SudocSudocFranceF

Relations entre les variations climatiques, les perturbations du cycle du carbone et les crises de la production carbonatée (application au Crétacé inférieur)

Le Crétacé inférieur est ponctué de perturbations du cycle du carbone associées à des épisodes de préservation de matière organique et à des crises de la production carbonatée néritique et pélagique. Ces évènements coïncident également avec des refroidissements à très court terme (<1Ma) dont l origine et les conséquences sont encore imprécises en raison de la faible résolution des courbes de températures dans les eaux de surface. Les études récentes réalisées à partir de modèles climatiques couplés à des modèles géochimiques laissent penser qu une crise de la production carbonatée pourrait engendrer un refroidissement climatique sur une échelle de temps inférieure à 1 Ma (Donnadieu et al., accepté). Basées sur les dépôts d âge Valanginien des coupes de La Charce Vergol et d Ollioules (Sud-est de la France) et sur les sédiments de l Aptien du sous-Bassin de Galvé (Nord-est de l Espagne), des études stratigraphiques, paléoécologiques et géochimiques ont été menées pour établir les relations entre la production carbonatée et le climat au cours du Valanginien et de l Aptien. Pour cela, des courbes de température à haute résolution pour les eaux de surface ont été établies et mises en regard des évolutions des producteurs néritiques et pélagiques de carbonate. Le Valanginien et l Aptien présentent tous deux des arrêts polyphasés de la production carbonatée néritique. Certains arrêts précèdent des refroidissements à court terme, dont ceux datés du Valanginien supérieur, du début et de la fin de l Aptien inférieur. Cette succession suggérerait un lien de cause à effet entre les crises de la production et les fluctuations climatiques. Toutefois, les refroidissements du Valanginien supérieur et de la fin de l Aptien inférieur sont respectivement précédés par un enfouissement de matière organique continentale et océanique, qui est un processus pouvant également générer une diminution de CO2 et un refroidissement. Ce travail a permis de mettre en évidence les répercutions des changements climatiques sur la production carbonatée par l intermédiaire de changements de producteurs dans les domaines néritiques et pélagiques. Dans un premier temps, la mise en place de conditions froides au Valanginien supérieur et à la fin de l Aptien inférieur est suivie de remplacements floro-fauniques caractérisés par l évolution de communautés hétérozoaires à photozoaires. Les bouleversements observés suggèrent un changement drastique des conditions trophiques sous le développement de conditions arides relatives au refroidissement. Dans un second temps, l évolution des communautés pélagiques au Valanginien répondraient également aux changements climatiques : ces communautés marquées d un déclin depuis la fin du Valanginien inférieur présentent une courte reprise du début au milieu du Valanginien supérieur. Cette reprise coïncide avec le développement de conditions froides et plus arides occasionnant des conditions trophiques plus faiblesThe Early Cretaceous is punctuated by carbon cycle perturbations, associated with organic matter burial episodes and carbonate production crises. These events coincide with short-term cooling (<1Ma), yet the mechanisms are still unclear, because of low resolution in sea surface temperature reconstructions. Recent climatic models suggest that carbonate-platform-collapse events may trigger a short-term ocean cooling episode (Donnadieu et al., accepted). In order to establish relations between climates and carbonate productions, we performed stratigraphic, palaeoecologic and geochemical analyses on Valanginian sediments from the La Charce-Vergol and the Ollioules sections (South-East France) and Aptian sedimentary record of the Galvé subasin (North East Spain). Therefore, high-resolution sea surface temperature curves were reconstructed with regard to the pelagic and neritic carbonate producer evolution. Both Valanginian and Aptian stages are marked by polyphased neritic production drawdown. Moreover, the Late Valanginian, basal and latest Early Aptian carbonate-platform demises predate a short-term cooling occurrence. The chronology of these events may imply that carbonate production decrease may have affected the atmospheric CO2 pool and the climate. Nevertheless, the decreases of water temperature that took place in the Late Valanginian and the latest Early Aptian are also prior to episodes of continental and oceanic organic matter burial, respectively. This process is also known as a CO2 drawdown and cooling generator. The present study allowed establishing climate feedbacks on the pelagic and neritic carbonate producers triggering fluctuations of the carbonate production fluxes. At first, cooler conditions during the Late Valanginien and latest Early Aptian are posterior to floro-faunal changes characterised by heterozoan to photozoan replacements. These suggest a trophic level decrease relative to cool and dryer climatic condition. Subsequently, nannoconid communities seem to record a response to the Late Valanginian climatic change: these producers are characterised by a decline from the latest Early Valanginian onward, interrupted by a recovery from the earliest Late Valanginian up to the mid-Late Valanginian. This recovery coincides with the development of cooling and dryer conditions, triggering low trophic level and thus promoting a subsequent pelagic production recovery.DIJON-BU Doc.électronique (212319901) / SudocSudocFranceF

Late Cretaceous Antarctic fish diversity

International audienceNew material from the Santa Marta Formation (late Coniacian–?early Maastrichtian) of James Ross Island contributes significantly to the current knowledge of Late Cretaceous Antarctic fish diversity. The taxon list for the Santa Marta Formation is extended, and new records of neoselachians and teleosts are reported. The stratigraphic ranges of some previously known taxa are enlarged, and the palaeobiogeography and palaeoecology of Late Cretaceous Antarctic fishes are discussed. Top predators that occupied the higher levels in the food chain along with marine tetrapods dominate the marine faunas from the Santa Marta and López de Bertodano formations. The only fish adapted to crushing hard-shelled invertebrates were the chimeroids. Rays, an important component of marine fish associations, as well as fish from lower trophic levels, remain unknown from the Late Cretaceous of Antarctica

Calcareous nannofossil response to Late Cretaceous climate change in the eastern Tethys (Zagros Basin, Iran)

14 pagesInternational audienceConiacian to Maastrichtian changes in calcareous nannofossil assemblages have been investigated in the eastern Tethyan Shahneshin section (central Zagros Basin, Iran). The nannofossil assemblages are mainly composed of Watznaueria spp. (avg. 54%), Retecapsa spp (avg. 7.9%), Cribrosphaerella ehrenbergii (avg. 7.7%) and Micula spp. (avg. 5.7%). Throughout the late Campanian, there is a trend to lower abundances in Watznaueria spp. together with increasing abundances of C. ehrenbergii and Arkhangelskiella cymbiformis, which are considered in this basin as the main cool-water taxa. Our results reveal that, despite a diagenetic impact on calcareous nannoflora, a number of primary paleoecological trends are preserved which depict well features of the progressive Late Cretaceous cooling. The first pronounced cooling episode occurs across the late Campanian to early Maastrichtian. The onset of pronounced cooling in the eastern Tethys appears to occur prior to the Campanian/Maastrichtian Boundary event (CMBE) δ13C negative excursion, in contrast with the Boreal realm where pronounced cooling only occurs in the early Maastrichtian, postdating the onset of the CMBE. The coincidence of this earlier cooling in the Zagros Basin with an interval characterized by a significant increase in benthic foraminifera suggests an amplified response of the assemblage due to a change to shallower environments. Hence, the late Campanian calcareous nannofossil assemblage turnover in central Zagros is either a response to an early cooling trend in the eastern Tethys or to sea-level fall or both. The mid-Maastrichtian warming and late Maastrichtian cooling episodes are also delineated in the nannofossil assemblage of Shahneshin and likely correlate with similar episodes in the Boreal Realm

Chemical weathering response to extreme global warming during Paleocene-Eocene Hyperthermals, Southern Pyrenees, Spain

The Paleocene and early Eocene were periods yielding multiple hyperthermal events. The most pronounced of them was the Paleocene-Eocene Thermal Maximum (PETM), which was characterized by an abrupt increase in global temperature (5–8 °C) over a short time (20 ka). A negative carbon isotope excursion marks the onset of the PETM, which resulted in the fast injection of CO2 into the ocean-atmosphere system, triggering global climatic changes. Geochemical, mineralogical, and sedimentological markers record the resulting increase in continental weathering. This is important, as enhanced chemical erosion influences both the CO2 concentration in the atmosphere and ocean acidity, generating a feedback mechanism. Hence, constraining the rates and intensity of weathering response can further clarify the causes for the PETM and Eocene hyperthermals. This study focuses on the well-preserved Pyrenean foreland basin and intends to assess the continental chemical weathering response of the sediment routing system during the PETM. Clay mineralogy is a climate-sensitive proxy, which records changes in continental erosion. Therefore, clay mineral proportions will be analyzed using X-ray diffraction and major element chemistry on clay-rich samples from the entire source-to-sink system (continental to deep marine deposits). Kaolinite and smectite will be separated from the detrital clay fraction and further subjected to δ18O and δD isotopic analysis for paleoclimatic reconstruction. The combined Lu-Hf and Sm-Nd isotope systems in the clay fraction of the sediments will be used to track the evolution of chemical weathering intensity. The outcome of this project will serve to validate numerical models to understand erosion as a function of rapid climatic change. This topic is of keen interest, as the PETM and its sedimentological signal work as a natural analog for anthropogenically-induced climatic change. The project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Grant Agreement No 860383

New constraints on chemical weathering in the aftermath of the PETM: Insights from the Spanish Pyrenees

Rapid climatic changes and their impact on the geological record is a topic of utmost importance for understanding ongoing global warming and the associated feedback mechanisms on environmental conditions. This study focuses on the Paleocene Eocene Thermal Maximum (PETM), a hyperthermal event characterized by a fast increase in global temperature (5–8°C) over a period of 20 ka. The aim is to understand the chemical weathering response to the PETM and its role as a feedback mechanism for a climatic recovery after the perturbation. Our focus is on the continental section of the sediment routing system near the locality of Esplugafreda in the Southern Pyrenean foreland basin. The changes in the clay mineral assemblages in two size-fractions (&lt;0.5 μm and 0.5-2 μm) were examined as paleoclimatic proxies. Oxygen and hydrogen stable isotope compositions of the clays were measured as indicators of paleo-precipitation, temperature, as well as the paleo-elevation of the catchment. Furthermore, we investigated the Sm-Nd and Lu-Hf radiogenic isotope systems to help constrain the provenance and chemical weathering intensity of the clays. Unlike other weathering proxies, this approach may provide important insights into the pedogenetic processes prevailing in the floodplains and the transported signal to the basin. The clay mineralogy shows an interplay between reworked clays during extreme events, together with more hydrolyzing conditions reflected in the presence of pedogenetic smectites during the onset and body of the PETM. The stable isotope geochemistry supports an increase in temperature with small variations in rainfall intensity over the year, disturbed by extreme events over short time windows. This is evidenced by two negative excursions in the δ18O record before the pre-onset excursion and during the body of the PETM. The ΔεHf results show no variation throughout the profile, indicating no increase in the chemical weathering intensity. Our results suggest that, in this mid-latitude catchment, chemical weathering remained limited by rainfall availability, while physical erosion played a primary control in the sediment routing system due to extreme events under semi-aridic conditions. This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant Agreement No. 860383.</p

Continental weathering using combined Hf-Nd isotope system and clay mineralogy: new insights for the Late Cretaceous climate

International audienceThe Late Cretaceous period records a pronounced decrease in marine temperatures at a global scale initiating the last greenhouse-icehouse transition, whose origin still remains enigmatic. Continental weathering represents a major sink of atmospheric CO2 through silicate weathering reactions yet the importance of this process in the Late Cretaceous cooling has only been scarcely explored.In this study we explore the impact of the eastern South American margin uplift, concomitant to the long-term Late Cretaceous cooling, on the evolution of chemical weathering of the Brazilian margin, using a new proxy of silicate weathering based on the coupled Lu-Hf and Sm-Nd isotope systems in clays. This proxy, expressed as ΔɛHf, has been recently calibrated in modern environments (Bayon et al., 2016) but has only been scarcely applied to deep-time environments. This proxy, applied on sediments from DSDP site 356 on the São Paulo Plateau, highlights a marked increase in silicate chemical weathering of the southeastern Brazilian margin from the Santonian to the Maastrichtian, also supported by the evolution of the chemical index of alteration (CIA) and clay mineralogy.This increase follows an episode of enhanced mechanical erosion of the margin revealed in the Turonian to Santonian by an increase of primary clay mineral (illite, chlorite) and Ti/Al ratio, linked to the tectonic uplift of the margin. Clay mineral assemblages additionally point to an evolution of local climatic conditions from arid to a more hydrolysing climate following this episode, that we link to a "rain shadow effect" affecting the eastern side of the newly formed relief that would have enhanced chemical weathering of the margin.Importantly the temporal coincidence of the increase in chemical weathering depicted here with the marked acceleration of the global cooling recorded worldwide during the Campanian points to a potentially important role of this process on the overall climate decline initiating the descent into our icehouse climate mode. Although records from additional sites are needed to establish the spatial extent of the margin affected by this process, our new dataset brings new insights about the impact of tectonic forcing on climate.Bayon et al. (2016) EPSL 438, p. 25-36

Chemical weathering linked to global warming during the PETM: Insights from the Spanish Pyrenees

We aim to determine the intensity of chemical weathering of detrital clays, as well as the lag time between the onset of the Paleocene-Eocene Thermal Maximum (PETM) and the chemical weathering response in a source-to-sink system. The PETM was a hyperthermal event characterized by an abrupt increase in global temperature (5–8 °C) over a short period (20 ka). A negative carbon isotope excursion marks the onset of the PETM, which reflects the fast injection of light carbon into the ocean-atmosphere system, triggering global climatic changes. Thus, physical and chemical erosion acted as feedback mechanisms to recover the global climate to pre-onset conditions. We focus on the continental section of the source-to-sink system, near the locality of Esplugafreda in the Southern Pyrenean foreland basin. We analyzed the evolution of the clay mineral assemblages in two clay-sized fractions (&lt;0.5 µm and &lt;2 µm), which can provide insights into the hydrolyzing conditions on the continents. We then measured oxygen and hydrogen stable isotopes as indicators of paleo-precipitation, temperature, and elevation of the catchment areas. The clay mineralogy results show an interplay of reworked clays during the extreme events, together with more hydrolyzing conditions marked by the production of authigenic clay during the onset and body of the PETM. The stable isotope geochemistry results point towards a climatically controlled response, where temperature fluctuations, as opposed to precipitation, played the main role in shaping the climatic regime. This is evidenced in a negative δ18O excursion at the onset and body of the PETM found in both size fractions. Further, we combine hafnium and neodymium isotope analyses of both clay fractions to track the silicate weathering intensity. This method will help us constrain the weathering regime and its response time relative to the onset and the body of the PETM. The results obtained in this project will serve to test numerical models of landscape evolution incorporating the chemical weathering response to climatic changes. This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant Agreement No. 860383

Chemical weathering during the PETM: Insights from stable and radiogenic isotopes in the Spanish Pyrenees

We explore the feedback mechanisms at play during the climate recovery of the Paleocene Eocene Thermal Maximum (PETM) in a continental section of the Spanish Pyrenees, the Esplugafreda section. We combine isotopic and mineralogical proxies to assess the changes in the chemical weathering intensity and regime and explore their implications in this mid-latitude catchment. The changes in the clay mineral assemblages in two size fractions (&lt;0.5 µm and 0.5-2 µm) were examined as paleoclimatic proxies. We also used these fractions to study the combined Sm-Nd and Lu-Hf radiogenic isotope systems to constrain the clay provenance and chemical weathering intensity. Furthermore, we measured lithium isotopes in the bulk clays to assess local changes in the weathering regime and clay neoformation during the PETM. The clay mineralogy reveals an interplay between detrital clay input during extreme events and more hydrolyzing conditions reflected in the presence of pedogenetic smectites during the onset and body of the PETM. The extent to which the clay samples deviate from the clay array (ΔεHf) show no variations throughout the section, indicating no increase in the chemical weathering intensity. Nonetheless, the δ7Li values show a distinct negative excursion during the recovery of the PETM in the Esplugafreda section. This excursion is coeval with an increase in the proportions of illite and chlorite relative to smectite, an indicator of increasing physical erosion. Our results suggest that the low surface reactivity of the floodplain deposit sediments resulted in negligible variations in the chemical weathering intensity recorded in the clays. Yet, the floodplain soils saw more fractionated lithium isotope compositions, contemporary with an increase in the detrital nature of the clay minerals. Climate change in the region was characterized by a substantial temperature increase with massive but short precipitation events, resulting in efficient sediment transport. Although weathering in the Pyrenees was kinetic-limited, both chemical weathering and physical erosion played a role in the climate recovery to pre-PETM conditions. Rapid climatic changes and their impact on the geological record are of utmost importance for understanding ongoing global warming and the associated feedback mechanisms on environmental conditions.</p