380 research outputs found
Metapolygnathus parvus Kozur, 1972 (Conodonta): a potential primary marker for the Norian GSSP (Upper Triassic)
No full textThe base of the Norian Stage (Carnian/Norian boundary, Upper Triassic) is still awaiting a formal designation by the International Commission on Stratigraphy. At present, two stratigraphic sections, Pizzo Mondello (Sicily, Italy) and Black Bear Ridge (British Columbia, Canada), have been proposed as GSSP (Global Stratotype Section and Point) candidates for the base of the Norian, but a bio- or physical primary marker event has not been yet selected by the Working Groups. We show here that conodonts represent the best choice to define the Carnian/Norian boundary. In the Upper Triassic, in fact, conodonts have been studied in great detail in the last 15 years providing a very high resolution biostratigraphic tool. We illustrate here the phylogenetic lineage Metapolygnathus praecommunisti - Metapolygnathus dylani - Metapolygnathus parvus, demonstrating that this lineage is present in both the GSSP candidate sections. Thus, we propose the First Occurrence (FO) of Metapolygnathus parvus (the last representative of its lineage) as a potential primary marker to define the base of the Norian Stage. Metapolygnathus parvus is in fact the most reliable and useful biomarker, because this species is morphologically simple and easy to recognise and its First Appearance Datum (FAD) is globally recognisable within its phylogenetic lineage.
The FO of M. parvus also coincides with an important global conodont faunal turnover (known as T2), that corresponds to the disappearance of the typically Carnian conodont genus Carnepigondolella and the rise of the genus Metapolygnathus. In addition, the FO of M. parvus occurs very close to a brief positive δ13Ccarb shift, documented in the two GSSP candidate sections and in the uppermost part of the magnetozone PM4n (ca. at 3.20 m below the top) at Pizzo Mondello
New Upper Triassic Conodont Biozonation of the Tethyan Realm
No full textConodonts are biostratigraphically very important microfossils in the Upper Triassic, occurring in different marine habitats, from deep-ocean to shallow- shelf waters. Because of their great abundance, worldwide distribution, strong resis- tance to rock metamorphism, and mineralogical composition that makes them reliable tools for biostratigraphic and geochemical studies, conodonts have proven to be important tools in de ning the Geological Time Scale (GTS) and Global Stratotype Section and Points (GSSPs). We present here an original Upper Triassic conodont biozonation for the Tethyan Realm integrated, where possible, with ammonoid and radiolarian zones, providing also numerical ages for stages and sub- stages. Based on the most recent conodont biostratigraphic and systematic studies, we propose a subdivision of the Upper Triassic interval into 22 conodont zones (nine for the Carnian, ten for the Norian, and three for the Rhaetian), correlated, where possible, with the most recent North American conodont zonations. Discussions on the most biostratigraphically important conodont taxa are also pro- vided, in particular for the stratigraphic intervals around the base of the Norian and Rhaetian stages, the GSSPs of which have yet to be de ned. In this view, we provide data supporting the validity of conodonts as reliable tools for global correlations, recommending two conodont biovents as possible primary biomarkers: the FAD (First Appearance Datum) of Metapolygnathus parvus for the base of the Norian and the FAD of Misikella posthernsteini for the base of the Rhaetian. The conodont species Norigondolella carlae n. sp. from the upper Tuvalian (Carnian) is also define
Triassic stratigraphy in the island of Hydra (Greece)
No full textThe Triassic sedimentary succession, cropping out in the island of Hydra, is described. A typical passive continental margin succession is recorded in the Triassic of Hydra, suggesting its affinity with the Subpelagonian domain. -from Author
Long-term cycles of Triassic climate change: a new δ18O record from conodont apatite
No full textA new oxygen isotope (δ18O) record derived from conodont apatite reveals variable long-term climate trends throughout the Triassic period. This record shows several major, first order, negative shifts reflecting intense warming episodes, not only the well-known extreme PTB–Early Triassic event (∼5h), but also two large cycles of similar magnitude (∼1.5, ∼1.7h) and duration (∼7 Myrs) during the late Carnian and late Norian. Between the PTB–Early Triassic and Carnian major episodes, three rapid shorter- term warming events of decreasing magnitude punctuate the mid–late Anisian, early Ladinian, and latest Ladinian, with distinct cooler (i.e. favourable) intervals characterising the early Anisian and early Carnian, indicating a fluctuating but ameliorating Middle Triassic climate trend. Two long periods of sustained cooler conditions occurred during the Late Triassic, for much of the Norian and Rhaetian. The five humid events previously recognised from the geological record, including the Carnian Pluvial Episode, are associated with the low δ18O warming phases. The magnitudes of these first order warming cycles, together with widespread geological and palaeontological evidence, suggest they were at least Tethyan- wide events
A NEW TRIASSIC CLIMATE (d18O) RECORD FROM CONODONT APATITE AND IMPLICATIONS FOR MAJOR BIO-EVENTS
No full textWe have established a new and comprehensive record of long-term climate change for the Triassic period. The record was determined from the oxygen isotope compositions of conodont apatite (d18Ophos) using secondary ion mass spectrometry (SHRIMP II). Notably, this method permits spatially targeted and multiple in-situ spot (30μm) analyses of individual conodont specimens, as well as species-based comparisons, the latter being useful for discriminating
taxon-specific effects (eg. biofacies). The d18Ophos record was determined from 98 samples (>1600 spot analyses from >500 conodonts) from the western and central Tethys, which span the Early-Late Triassic. Our d18Ophos record reveals that the Triassic climate was not as static or gradual as previously thought. There were three major, first-order negative shifts reflecting intense warming episodes, not only the well-known PTB-Early Triassic event, but also two large cycles of similar magnitude and duration during the late Carnian and late Norian. A series of alternating short-term cooling-warming cycles occurred throughout the Middle Triassic that decreased in magnitude reflecting an unstable but ameliorating climate. Three distinct short-term warming events occurred during the mid-late Anisian, early Ladinian, and latest Ladinian. In contrast, following each of the two major Late Triassic warming cycles, the Norian and Rhaetian experienced cooler (ie. mild) periods of sustained climate stability representing much more favourable climatic conditions. Notably, the five humid episodes recognized from the geological record are all associated with negative d18Ophos (warming) events. Our d18Ophos record thus provides a very new perspective on Triassic climate, which will also be discussed in the context of known geological and biological events
STENOTHERMAL HABITS OF GLADIGONDOLELLA AND NORIGONDOLELLA: CONSTRAINTS FROM !18O OF CONODONT APATITE
No full textThe stable mineralogy of conodont microfossils (fluorapatite) is a robust archive of palaeoseawater temperatures, which can be estimated from their oxygen isotope compositions. Using secondary ion mass spectrometry (SHRIMP II), multiple spot (30μm) in situ analyses of single conodont specimens enables comparative analyses at species level, which can be useful to discriminate taxon-specific isotopic variability that reflects palaeoecological (eg. biofacies) controls.
Cooler water Gladigondolella and Norigondolella biofacies have been distinguished by careful comparative analyses of different species within and between samples. Gladigondolella
consistently yielded high !18Ophos values (between ~21 and 21.5‰) compared to coeval
eurythermal genera (e.g. Paragondolella and Budurovignathus, 20 and 20.5‰), which have been found in different environments, from marginal to proximal to open ocean settings, during the Middle and early Late Triassic. The consistent behavior of Gladigondolella at different sites and time intervals supports the notion that Gladigondolella was stenothermal, and thus could not tolerate higher SST’s during particularly warm periods so opted to migrate to deeper/cooler waters. A similar behavior is suggested for the genus Norigondolella (Late Triassic), which also preferred cooler conditions (~21‰)
(Table 2) Distribution of early late Eocene fossil content in ODP Hole 115-714A
No full textAbundances: A = abundant, C = common, P = present
(Table 1) Distribution of early late Eocene fossil content in ODP Hole 115-715A
No full textAbundances: A = abundant, C = common, P = present
A new Metapolygnathus platform conodont species and its implications for Upper Carnian global correlations
No full textA rich conodont fauna from two Neotethyan sections, Pizzo Mondello (western Sicily, Italy) and Pignola 2 sections (southern Apennines, Italy) includes conodonts described herein as a new species. Metapolygnathus praecommunisti sp. nov. is transitional between Paragondolella noah and Metapolygnathus communisti. The genus Metapolygnathus (including M. praecommunisti) is now characterised by posterior prolongation of the keel termination, associated with a centrally located pit and with a weak ornamentation confined to the anterior part of the platform margins. The establishment of M. praecommunisti addresses the problems related to the origin and the peculiar, probably facies-controlled, distribution of its descendant species M. communisti. Since M. praecommunisti occurs in the entire Tethys and in North America, we propose the species as a good guide fossil for global correlations, characterised by a short temporal range limited to the uppermost Tuvalian (upper Carnian). The stratigraphic occurrence of the genus Metapolygnathus is restricted to the Tuvalian-Lacian (upper Carnian-lower Norian), excluding its presence in the Julian substage (lower Carnian)
CONODONTS FROM THE LOWER TRIASSIC SEQUENCE OF CENTRAL DOLPO, NEPAL
No full textIn the present paper, the conodont fauna from three detailed sections surveyed in the Lower Triassic sequence of Central Dolpo, Nepal (Tarap-Atali area) is illustrated. Combining faunas from the three sections, it was possible to recognize a succession of faunal events that covers most of the Scythian and the Lower Anisian. In the whole, 11 faunas have been recognized and discussed
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