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De Wever P.,French Natural History Museum | O'Dogherty L.,University of Cadiz | Gorican T.,Paleontoloski Institute Ivana Rakovca
Comptes Rendus - Geoscience | Year: 2014

The radiolaritic facies (red/green cherts with radiolarians) is a very characteristic feature of the Tethyan realm. For a long time, its presence has been interpreted as a consequence of depth of an oceanic environment. It is now preferable to consider it as high productivity sediment. We here underline the interpretation inferring the role of monsoons for such productivity according to the relative position of lands at that time. © 2014 Académie des sciences. Source

Carter E.S.,Portland State University | Gorican S.,Paleontoloski Institute Ivana Rakovca | Guex J.,University of Lausanne | O'Dogherty L.,University of Cadiz | And 5 more authors.
Palaeogeography, Palaeoclimatology, Palaeoecology | Year: 2010

Jurassic radiolarians from 220 samples in Queen Charlotte Islands, B.C., Williston Lake, B.C., east-central Oregon, Baja California Sur, southern Spain, Austria, Slovenia, Turkey, Oman, Japan and Argentina were studied in order to construct global zonation for the Pliensbachian, Toarcian and Aalenian stages. Well-preserved faunas from continuous stratigraphic sections in Queen Charlotte Islands provide the most detailed record for this time interval, and all collections are tied to North American ammonite zones or assemblages. Collections from nearly all other areas lack independent dating except for early Toarcian carbon-isotope dating in Slovenia and late Aalenian ammonites in Spain.A database of 197 widely distributed updated taxonomic species was used to construct a Unitary Association (UA) zonation for the interval. A global sequence of 41 UAs was obtained for the top of the Sinemurian to the base of the Bajocian. The first and the last UAs represent the Late Sinemurian and the Early Bajocian respectively. The remaining 39 UAs were merged into nine zones (four Early Pliensbachian, one Late Pliensbachian, one Early Toarcian, one Middle-Late Toarcian, and two Aalenian) according to prominent radiolarian faunal breaks and ammonite data. The new zones are the Canutus tipperi - Katroma clara Zone (latest Sinemurian/earliest Pliensbachian); Zartus mostleri - Pseudoristola megaglobosa, Hsuum mulleri - Trillus elkhornensis and Gigi fustis - Lantus sixi zones (Early Pliensbachian); Eucyrtidiellum nagaiae - Praeparvicingula tlellensis Zone (Late Pliensbachian); Napora relica - Eucyrtidiellum disparile Zone (Early Toarcian); Elodium pessagnoi - Hexasaturnalis hexagonus Zone (Middle and Late Toarcian); Higumastra transversa - Napora nipponica Zone (early Aalenian); and Mirifusus proavus - Transhsuum hisuikyoense Zone (late Aalenian). These zones can be correlated worldwide and link previously established UA zonations for the Hettangian-Sinemurian and the Middle to Upper Jurassic. The new zonation allows high-resolution dating in the studied interval and provides a solid basis for analyzing faunal turnovers and the paleobiogeography of Jurassic radiolarians. © 2010 Elsevier B.V. Source

Gorican T.,Paleontoloski Institute Ivana Rakovca | Carter E.S.,Portland State University | Guex J.,University of Lausanne | O'Dogherty L.,University of Cadiz | And 5 more authors.
Palaeogeography, Palaeoclimatology, Palaeoecology | Year: 2013

Recent studies on the global distribution of Pliensbachian and Toarcian polycystine radiolarians allowed us to examine faunal turnovers and the biogeography through this critical time interval around a major ecologic and biotic crisis. The analysis is based on the distribution of 167 species belonging to 69 genera. Significant variations in the ratio between the number of originating and extinct species have been recognized. During the early Early Pliensbachian FADs greatly exceeded LADs and the maximum diversity was reached in the late Early Pliensbachian. The trend then reversed with the number of LADs exceeding FADs throughout the Late Pliensbachian and Early Toarcian (extinction interval). Recovery started in the Middle and Late Toarcian, when the number of FADs again surpassed the number of LADs.Three differing evolutionary patterns are observed amongst radiolarian genera through the studied time interval. The largest group diversified rapidly in the Early Pliensbachian and experienced higher extinction rates in the Late Pliensbachian and Early Toarcian; a second group exhibited no major changes; and a third group of mainly spongy spumellarians was successful during the extinction interval. The overall trend of radiolarian diversity is in a fairly good agreement with that of other marine faunas (ammonites and also benthos), but shows an inverse correlation with diversity trends of phytoplankton.Correlation with concomitant environmental changes indicates that radiolarian radiation/extinction rates were not consistently linked with temperature fluctuations or sea-level changes. It is also evident that the diversity decrease started well before the Early Toarcian negative δ13C peak and the Oceanic Anoxic Event (OAE). The extinction interval corresponds well to the duration of a short-term anomaly in the strontium-isotope record, including the rapid decrease of 87Sr/86Sr values in the Late Pliensbachian as well as the rapid increase in the Early Toarcian. This coincidence supports the hypothesis that the predominance of extinctions over originations was caused by a series of climate and environmental changes related to intensified magmatic activity.Some distinct biogeographic differences have been observed. Generic differences are most strongly displayed by the presence or absence of a particular genus or by changes in abundance while species differences range from greater variability to having completely different species in separate palaeolatitudinal realms. Two groups of genera are distinguished: those that are common to abundant in the Tethys (low latitudes) and rare to absent in mid to high latitudes, and those common to abundant in mid to high latitudes and rare to absent in the Tethys. © 2013 Elsevier B.V. Source

Gawlick H.-J.,University of Leoben | Gorican S.,Paleontoloski Institute Ivana Rakovca | Missoni S.,University of Leoben | Lein R.,University of Vienna
Bulletin de la Societe Geologique de France | Year: 2012

A short-term radiolarite interval of Late Anisian (Late Illyrian) age was dated by means of radiolarians and conodonts in the Dinarides, Montenegro. The radiolarites, with intercalations of filament-rich limestone layers in the higher part, were deposited in deeper-water shelf areas, as documented by the underlying and overlying shallow-water carbonate succession. This Illyrian radiolarite interval is widespread in the northwestern Tethyan realm, especially in the Dinarides, and was deposited in a phase when the carbonate production decreased. The radiolarite event occurred shortly after the main subsidence pulse and the final break-up of the Neotethys ocean, and was concomitant with a prominent sea-level highstand. Source

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