The cretaceous/paleogene boundary and planktonic foraminifera in the flyschgosau (Eastern Alps, Austria)

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A search for iridium in the deccan traps and inter-traps

International audienceIt has been suggested that flood basalts in the Deccan (India) might be associated with events at the Cretaceous-Tertiary boundary (KTB). A search for iridium in 47 samples from lava flows and inter-trap sediments in the Deccan yields negative results. Concentrations are not statistically different from zero, with a minimum detection level on the order of 0.1 ng.g$^{-1}$ (ppb). This does not help to constrain the debate on the internal vs external origin of KTB boundary events, since for instance Ir could have been outgassed from the magma, and original concentrations of only 0.05 ng.g$^{-1}$ prior to outgassing aresufficient to account for the worldwide Ir excess mass at the KTB

Anomalous Surge of the White‐Light Corona at the Onset of the Declining Phase of Solar Cycle 24

International audienceIn late 2014, when the current Solar Cycle 24 entered its declining phase, the white‐light corona as observed by the LASCO‐C2 coronagraph underwent an unexpected surge that increased its global radiance by 60%, reaching a peak value comparable to the peak values of the more active Solar Cycle 23. A comparison of the temporal variation of the white‐light corona with the variations of several indices and proxies of solar activity indicate that it best matches the variation of the total magnetic field. The daily variations point to a localized enhancement or bulge in the electron density that persisted for several months. Carrington maps of the radiance and of the HMI photospheric field allow connecting this bulge to the emergence of the large sunspot complex AR 12192 in October 2014, the largest since AR 6368 observed in November 1990. The resulting unusually high increase of the magnetic field and the distortion of the neutral sheet in a characteristic inverse S‐shape caused the coronal plasma to be trapped along a similar pattern. A 3D reconstruction of the electron density based on time‐dependent solar rotational tomography supplemented by 2D inversion of the coronal radiance confirms the morphology of the bulge and reveals that its level was well above the standard models of a corona of the maximum type, by typically a factor of 3. A rather satisfactory agreement is found with the results of the thermodynamic MHD model produced by Predictive Sciences, although discrepancies are noted. The specific configuration of the magnetic field that led to the coronal surge resulted from the interplay of various factors prevailing at the onset of the declining phase of the solar cycles, which was particularly efficient in the case of Solar Cycle 24

Recherche de l'iridium à la limite Permien-Trias du site de Meishan, Changhsing (R. P. Chine).

La forte teneur en elements siderophiles des sediments de la limite Cretace-Tertiaire suggere que les principaJes disparitions d'especes ont ete provoquees par des catastrophes cosmiques. Cette hypothese pourrait etre confirmee par la decouverte d'une anomaJie similaire it la limite Permien-Trias, caracterisee par la plus grave crise biologique du Phanerozoique. L'etude du site de Meishan, en Republique populaire de Chine, n'apporte aucune confirmation de ce scenario. Aucune trace d'iridium, Ie meilleur traceur de la matiere extraterrestre, n'a ete trouvee dans les 18 echantillons preleves au voisinage de la transition Permien-Trias. Toute relation entre la crise biologique du Permien-Trias et une catastrophe cosmique doit donc, pour l'instant, etre consideree comme hypothetique. The presence of siderophile-enriched material at the Cretaceous-Tertiary boundary suggests that the major extinctions of living species could result from cosmic catastrophes. The finding of the same kind of material at the Permian-Triassic boundary would be important to confirm the influence of cosmic phenomena on extinctions. The study of the M eishan section, in China, does not provide any support to this view. Iridium, the best tracer of cosmic material, has not been detected in any of the 18 samples collected around the boundary. A relation between the Permian-Triassic extinction and a cosmic collision therefore remains hypothetical

The Cretaceous-Tertiary boundary at Beloc, Haiti: No evidence for an impact in the Caribbean area

International audienc

The stratigraphic distribution of Ni-rich spinels in Cretaceous-Tertiary boundary rocks at El Kef (Tunisia), Caravaca (Spain) and Hole 761C (Leg 122)

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An iridium rich layer at the Cretaceous/Tertiary boundary in the Bidart Section (southern France)

International audienc

The Cretaceous-Tertiary boundary at Gubbio revisited: vertical extent of the Ir anomaly

The classical Gubbio (Italy) Cretaceous-Tertiary boundary section has been resampled for both magnetostratigraphy and iridium. Paleomagnetic samples were taken over 7 m in the Maastrichtian and 6 m in the Paleocene. Previous results obtained a decade ago are confirmed. The reversal sequence is well defined and the individual reversals are somewhat more precisely located. A noticeable difference is the location of the position of the 29N/29R limit which may be lowered by 20-30 cm in the stratigraphic column. This would imply that the KTB occurs near the middle of chron 29R. Iridium measurements were made on samples from both shales and surrounding limestone beds from 2 m below to 3 m above the boundary: these measurements indicate that Ir is associated with clay minerals. Concentrations in the two types of samples are indeed compatible when reduced to a carbonate-free basis. Iridium concentrations stand above background over almost 3 m of section, corresponding to half a million years based on magnetostratigraphy. This is likely to indicate a protracted duration of the (external or internal) source of iridium, on top of which the main (short-lived) KTB anomaly proper stands. © 1990.link_to_subscribed_fulltex