275 research outputs found
OntogenĂšse et covariation chez le genre toarcien Osperleioceras (Ammonoidea)
Les courbes de croissance des Osperleioceras de la sous-zone à Reynesi (Toarcien supérieur) du bassin des
Causses (Aveyron, France) sont toutes confondues du stade de la protoconque et de lâammonitelle, jusquâĂ un diamĂštre
de 7-8 mm. Elles forment ensuite un éventail de trajectoires résultant en un continuum morphologique allant de formes
Ă©volutes Ă tours larges et ornementation forte (O. reynesi) jusquâĂ des formes involutes, comprimĂ©es et faiblement ornĂ©es
(O. lapparenti). La covariation de divers paramĂštres morphologiques et ornementaux est observable tant au sein
dâassemblages provenant dâhorizons uniques quâau cours des dĂ©veloppements ontogĂ©nĂ©tiques individuels.
Il nâest pas possible dâinterprĂ©ter cette sĂ©rie continue et contemporaine, passant insensiblement des morphologies
de type reynesi Ă celles de type lapparenti, comme Ă©tant une tendance Ă©volutive due Ă des contraintes fonctionnelles ou
Ă des adaptations diffĂ©rentielles. LâinterdĂ©pendance complexe de la forme de la coquille (involution, section du tour) et
de son ornementation (force, densitĂ© et flexuositĂ© de la costulation) peut ĂȘtre simulĂ©e par un modĂšle oĂč lâintensitĂ© de
lâornementation dĂ©pend de la courbure du manteau [Guex, 1999].
La covariation observée chez les Osperleioceras dÚs la base du Toarcien supérieur trahit une augmentation de variabilité,
concomitante dâune rĂ©duction de taille, suite Ă des instabilitĂ©s environnementales. Elle se fait en plusieurs
poussĂ©es Ă partir dâun stock de formes ayant trĂšs peu variĂ© au cours du Toarcien moyen
Decaying shock studies of phase transitions in MgOSiO2 systems: implications for the Super-Earths interiors
We report an experimental study of the phase diagrams of periclase (MgO),
enstatite (MgSiO3) and forsterite (Mg2SiO4) at high pressures. We investigated
with laser driven decaying shocks the pressure/temperature curves of MgO,
MgSiO3 and Mg2SiO4 between 0.2-1.2 TPa, 0.12-0.5 TPa and 0.2-0.85 TPa
respectively. A melting signature has been observed in MgO at 0.47 TPa and 9860
K, while no phase changes were observed neither in MgSiO3 nor in Mg2SiO4. An
increasing of reflectivity of MgO, MgSiO3 and Mg2SiO4 liquids have been
detected at 0.55 TPa -12 760 K, 0.15 TPa - 7540 K, 0.2 TPa - 5800 K,
respectively. In contrast to SiO2, melting and metallization of these compounds
do not coincide implying the presence of poor electrically conducting liquids
close to the melting lines. This has important implications for the generation
of dynamos in Super-earths mantles
Invasion success of a Lessepsian symbiont-bearing foraminifera linked to high dispersal ability, preadaptation and suppression of sexual reproduction
Among the most successful Lessepsian invaders is the symbiont-bearing benthic foraminifera Amphistegina lobifera. In its newly conquered habitat, this prolific calcifier and ecosystem engineer is exposed to environmental conditions that exceed the range of its native habitat. To disentangle which processes facilitated the invasion success of A. lobifera into the Mediterranean Sea we analyzed aâ~â1400 bp sequence fragment covering the SSU and ITS gene markers to compare the populations from its native regions and along the invasion gradient. The genetic variability was studied at four levels: intra-genomic, population, regional and geographical. We observed that the invasion is not associated with genetic differentiation, but the invasive populations show a distinct suppression of intra-genomic variability among the multiple copies of the rRNA gene. A reduced genetic diversity compared to the Indopacific is observed already in the Red Sea populations and their high dispersal potential into the Mediterranean appears consistent with a bridgehead effect resulting from the postglacial expansion from the Indian Ocean into the Red Sea. We conclude that the genetic structure of the invasive populations reflects two processes: high dispersal ability of the Red Sea source population pre-adapted to Mediterranean conditions and a likely suppression of sexual reproduction in the invader. This discovery provides a new perspective on the cost of invasion in marine protists: The success of the invasive A. lobifera in the Mediterranean Sea comes at the cost of abandonment of sexual reproduction
A new scenario for the Domerian-Toarcian transition
Pour tenter de mieux comprendre lâĂ©vĂ©nement dâanoxie ocĂ©anique globale au Toarcien infĂ©rieur, il convient
de remonter aux changements environnementaux qui ont précédé cette phase paroxysmale. En effet, la comparaison des
séquences sédimentaires et biostratigraphiques du passage Domérien-Toarcien entre la Téthys occidentale (Maroc,
Espagne, Portugal) et lâEurope du Nord-Ouest (Causses, Allemagne, Angleterre) fait apparaĂźtre une importante lacune
dans la région septentrionale.
La faune dâArieticeratinae (Emaciaticeras, Canavaria, Fontanelliceras) et dâHarpoceratinae (Lioceratoides, Neolioceratoides),
accompagnée ensuite de Dactylioceras particuliers (groupe mirabile-polymorphum = sous-genre Eodactylites),
fait presque totalement dĂ©faut en Europe du Nord-Ouest, alors quâelle abonde dans les derniĂšres alternances
marno-calcaires, sans changement lithologique notable avec le Domérien des coupes téthysiennes. Cette faune est intercalée
entre les derniers Pleuroceras et les Dactylioceras du groupe tenuicostatum. Ces derniers apparaissent dans les argiles
succĂ©dant immĂ©diatement au dernier banc calcaire Ă Dactylioceras mirabile au Portugal notamment. Câest lĂ le
diachronisme déjà reconnu entre les limites lithostratigraphique (disparition des bancs calcaires) et biostratigraphique
(apparition du genre Dactylioceras) au passage Domérien-Toarcien.
Cette observation peut sâintĂ©grer dans un nouveau scĂ©nario palĂ©o-ocĂ©anographique prenant en compte Ă la fois la
tendance rĂ©gressive majeure du DomĂ©rien supĂ©rieur (conduisant Ă une lacune rĂ©gionale importante), lâabondance de matiĂšre
charbonneuse dans les premiers dĂ©pĂŽts transgressifs du Toarcien et lâĂ©vĂ©nement anoxique global subsĂ©quent.
Dans la partie supĂ©rieure du DomĂ©rien, lâexistence dâun fort volcanisme peut ĂȘtre dĂ©duite des donnĂ©es relatives
aux variations des isotopes du strontium [McArthur et al., 2000]. A ce pic de strontium sont associées une faible anomalie
négative du Ύ13
C à la limite Domérien-Toarcien et des valeurs particuliÚrement élevées du Ύ18
O [Morettini et Bartolini,
1999]. Nous pensons que cette activité volcanique débute par des émissions massives de SO2 induisant des pluies
acides, un obscurcissement et un refroidissement. A cette phase de refroidissement correspond une augmentation de
lâenglacement des pĂŽles et une rĂ©gression responsable de la lacune majeure Ă©voquĂ©e plus haut, particuliĂšrement sensible
dans les mers Ă©picontinentales. Bien que les preuves directes dâune glaciation fini-domĂ©rienne fassent actuellement dĂ©faut
[Hallam, 2001], le glacio-eustatisme nous semble le seul mĂ©canisme permettant dâexpliquer une oscillation marine
importante mais de courte durée [Brandt, 1986 ; Dewey et Pitman, 1998]. En effet, le cycle régression-transgression
sâĂ©tale sur environ deux zones dâammonites, la lacune sĂ©dimentaire en elle-mĂȘme recouvrant essentiellement les
sous-zones Ă Elisa et Mirabile.
Ce premier Ă©pisode serait suivi, dans la zone Ă Tenuicostatum, par une importante perturbation du cycle du carbone
responsable dâun effet de serre. Le rĂ©chauffement, provoquerait alors la transgression bien connue du Toarcien infĂ©rieur,
cachetant le hiatus sĂ©dimentaire dans la province nord-ouest europĂ©enne. Lâintervalle de temps correspondant Ă
cette lacune aurait permis Ă la vĂ©gĂ©tation de coloniser les immenses surfaces nouvellement Ă©mergĂ©es. Câest le lessivage
et lâoxydation de la matiĂšre organo-humique et bactĂ©rienne accumulĂ©e pendant cette pĂ©riode, associĂ©e Ă une Ă©lĂ©vation
de la tempĂ©rature, qui aurait enclenchĂ© le mĂ©canisme dâanoxie lors du paroxysme de la transgression
Experimental investigation of the stability of Fe-rich carbonates in the lower mantle
International audienceThe fate of carbonates in the Earth's mantle plays a key role in the geodynamical carbon cycle. Although iron is a major component of the Earth's lower mantle, the stability of Fe-bearing carbonates has rarely been studied. Here we present experimental results on the stability of Fe-rich carbonates at pressures ranging from 40 to 105 GPa and temperatures of 1450-3600 K, corresponding to depths within the Earth's lower mantle of about 1000-2400 km. Samples of iron oxides and iron-magnesium oxides were loaded into CO2 gas and laser heated in a diamond-anvil cell. The nature of crystalline run products was determined in situ by X-ray diffraction, and the recovered samples were studied by analytical transmission electron microscopy and scanning transmission X-ray microscopy. We show that Fe-(II) is systematically involved in redox reactions with CO2 yielding to Fe-(III)-bearing phases and diamonds. We also report a new Fe-(III)-bearing high-pressure phase resulting from the transformation of FeCO3 at pressures exceeding 40 GPa. The presence of both diamonds and an oxidized C-bearing phase suggests that oxidized and reduced forms of carbon might coexist in the deep mantle. Finally, the observed reactions potentially provide a new mechanism for diamond formation at great depth
Disconnection between genetic and morphological diversity in the planktonic foraminifer Neogloboquadrina pachyderma from the Indian sector of the Southern Ocean
Eight SSU rDNA genetic types have been described in the planktonic foraminifera Neogloboquadrina pachyderma, but the level of correlation between genetic diversity and morphological variation remains unknown in this morphospecies. In this study, we combine molecular and morphometric analyses of specimens of N. pachyderma sampled during two consecutive years across a latitudinal gradient in the Indian sector of the Southern Ocean. We observe that three genetic types of N. pachyderma inhabit the (sub-)polar waters of the southern Indian Ocean where they have equivalent regional distributions to those previously observed in the South Atlantic. The geographic ranges of these genetic types are largely overlapping. Our morphometric data show that contrary to other planktonic foraminiferal morphospecies, there is no relationship between genetic diversity and morphological differentiation in at least two of the austral representatives of N. pachyderma (Type III and Type IV) despite a high morphological variability and large genetic distance between these types. These genetic types of N. pachyderma in the southern Indian Ocean thus constitute true cryptic species of planktonic foraminifera
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Direct Observation of Shock-Induced Disordering of Enstatite Below the Melting Temperature
We report in situ structural measurements of shock-compressed single crystal orthoenstatite up to 337 ± 55 GPa on the Hugoniot, obtained by coupling ultrafast X-ray diffraction to laser-driven shock compression. Shock compression induces a disordering of the crystalline structure evidenced by the appearance of a diffuse X-ray diffraction signal at nanosecond timescales at 80 ± 13 GPa on the Hugoniot, well below the equilibrium melting pressure (>170 GPa). The formation of bridgmanite and post-perovskite have been indirectly reported in microsecond-scale plate-impact experiments. Therefore, we interpret the high-pressure disordered state we observed at nanosecond scale as an intermediate structure from which bridgmanite and post-perovskite crystallize at longer timescales. This evidence of a disordered structure of MgSiO3 on the Hugoniot indicates that the degree of polymerization of silicates is a key parameter to constrain the actual thermodynamics of shocks in natural environments. © 2020. The Authors
Patterns of eukaryotic diversity from the surface to the deep-ocean sediment
Remote deep-ocean sediment (DOS) ecosystems are among the least explored biomes on Earth. Genomic assessments of their biodiversity have failed to separate indigenous benthic organisms from sinking plankton. Here, we compare global-scale eukaryotic DNA metabarcoding datasets (18S-V9) from abyssal and lower bathyal surficial sediments and euphotic and aphotic ocean pelagic layers to distinguish plankton from benthic diversity in sediment material. Based on 1685 samples collected throughout the world ocean, we show that DOS diversity is at least threefold that in pelagic realms, with nearly two-thirds represented by abundant yet unknown eukaryotes. These benthic communities are spatially structured by ocean basins and particulate organic carbon (POC) flux from the upper ocean. Plankton DNA reaching the DOS originates from abundant species, with maximal deposition at high latitudes. Its seafloor DNA signature predicts variations in POC export from the surface and reveals previously overlooked taxa that may drive the biological carbon pump
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