FIRST EVIDENCE OF PALAEO-TETHYAN UPPER TRIASSIC CALCAREOUS NANNOFOSSILS IN NORTH DOBROGEAN OROGEN (ROMANIA)

This study presents the analysis of Upper Triassic sediments from five locations in North Dobrogea(Romania) and the Black Sea. Microfacies analyses on thin sections from the Frecăţei log reveal a shallowing trend and likely an increase in energy upward into the section. The oldest part of the log is characterised by deposition of mudstones transitioning to wackestones, with an increase of bivalves. Similar wackestones are observed in the Izvoarele and Rândunica logs, i.e. microfacies with abundant bivalves, some foraminifera and echinoderms. The offshore boreholes 816 and 817 Lebăda Vest (core CM 9 and CM 31) seem to have been deposited in a basinal or distal marine shelf environment indicated by the presence of mudstones with rare bioclasts. In contrast, sample CM 10 from borehole 816 LV is a micritised grainstone suggesting a deposition in a shallower, higher energy environment.Scanning electron microscope observations reveal a moderate diagenetic alteration in all studied samples, mainly due to dissolution. Two calcareous nannofossil species: Prinsiosphaera triassica triassica and Eoconusphaera zlambachensis were commonly observed in sample F of the Frecăţei log, allowing for assignment of the sediments to the Rhaetian age. Two coccoliths were also observed in sample F of Frecăţei log and sample CM 9 of the borehole 816 LV indicating the presence of coccolithophorids within the study region. This discovery constitutes the first confirmed record of well-preserved, determinable Upper Triassic calcareous nannofossils in the Palaeo-Tethys Ocean

FIRST EVIDENCE OF PALAEO-TETHYAN UPPER TRIASSIC CALCAREOUS NANNOFOSSILS IN NORTH DOBROGEAN OROGEN (ROMANIA)

This study presents the analysis of Upper Triassic sediments from five locations in North Dobrogea (Romania) and the Black Sea. Microfacies analyses on thin sections from the Frecăţei log reveal a shallowing trend and likely an increase in energy upward into the section. The oldest part of the log is characterised by deposition of mudstones transitioning to wackestones, with an increase of bivalves. Similar wackestones are observed in the Izvoarele and Rândunica logs, i.e. microfacies with abundant bivalves, some foraminifera and echinoderms. The offshore boreholes 816 and 817 Lebăda Vest (core CM 9 and CM 31) seem to have been deposited in a basinal or distal marine shelf environment indicated by the presence of mudstones with rare bioclasts. In contrast, sample CM 10 from borehole 816 LV is a micritised grainstone suggesting a deposition in a shallower, higher energy environment. Scanning electron microscope observations reveal a moderate diagenetic alteration in all studied samples, mainly due to dissolution. Two calcareous nannofossil species: Prinsiosphaera triassica triassica and Eoconusphaera zlambachensis were commonly observed in sample F of the Frecăţei log, allowing for assignment of the sediments to the Rhaetian age. Two coccoliths were also observed in sample F of Frecăţei log and sample CM 9 of the borehole 816 LV indicating the presence of coccolithophorids within the study region. This discovery constitutes the first confirmed record of well-preserved, determinable Upper Triassic calcareous nannofossils in the Palaeo-Tethys Ocean

New constraints on the evolution of 87Sr/86Sr of seawater during the Upper Triassic

The Late Triassic time interval witnessed several important biological turnovers, extinctions and onset of new life forms. Therefore, it is crucial to better constrain the tectonic, climatic and oceanographic framework at this time. The radiogenic strontium isotopic composition (87Sr/86Sr) of seawater, recorded in marine carbonates is sensitive to variations in continental weathering and oceanic spreading rates. This makes it a powerful proxy for palaeo-environmental reconstructions and an excellent global stratigraphic correlation tool of marine sediment records. In this study, we present a new and extended 87Sr/86Sr dataset across the late Norian - Hettangian interval, established from carbonate successions in Austria and Turkey. The well-established biostratigraphy and high sampling resolution allow evaluating changes in the 87Sr/86Sr record with a better time constraint than in previous studies. An exceptionally sharp trend towards unradiogenic values has been observed in the latest Norian and in the early Rhaetian from 0.70800 to 0.70779. Considering Late Triassic climate change, updated tectonic models and actual dating of magmatic events, possible explanations for the observed trend in the 87Sr/86Sr record are: (i) enhanced dissolution of pre-Norian carbonates, (ii) dissolution of widespread Late Triassic evaporites and (iii) tectonic reorganization of sedimentary basins due to the break-up of the supercontinent Pangea. At the very end of the Choristoceras marshi Zone, in the late Rhaetian, the unradiogenic 87Sr/86Sr trend is reversed. This is likely related to environmental changes due to the continental emplacement of the Central Atlantic Magmatic Province

Population dynamics of modern planktonic foraminifera in the western Barents Sea

International audienceThis study reports on diversity and distribution of planktonic foraminifera (PF) in the Barents Sea Opening (BSO). Populations of PF living in late summer (collected by means of stratified plankton tows) and recently deposited individuals (sampled by interface corer) were compared. High abundances reaching up to 400 unit ind.m-3 in tow samples and 8000 unit ind.m-3 in surface sediments were recorded in the centre of the studied area while low abundances were observed in coastal areas, likely due to continental influences. The living and core-top assemblages are mainly composed of the same four species Neogloboquadrina pachyderma, Neogloboquadrina incompta, Turborotalita quinqueloba and Globigerinita uvula. The two species G. uvula and T. quinqueloba dominate the upper water column, whereas surface sediment assemblages display particularly high concentrations of N. pachyderma. The unusual dominance of G. uvula in the water sample assemblages compared to its low proportion in surface sediments might be the signature of (1) a seasonal signal due to summer phytoplankton composition changes at the BSO, linked to the increase in summer temperature at the study site, and/or (2) a signal of a larger timescale and wider geographical reach phenomenon reflecting poleward temperate/subpolar species migration and consecutive foraminiferal assemblage diversification at high latitudes due to global change. Protein concentrations were measured on single specimens and used as a proxy of individual carbon biomass. Specimens of all species show the same trend, a northward decrease in their size-normalized-protein concentration. This suggests that foraminiferal biomass is potentially controlled by different constituents of their organelles (e.g. lipids). The coupling of data from plankton tows, protein measurements and surface sediments allows us to hypothesize that PF dynamics (seasonality and distribution) are decoupled from their metabolism

Fate of calcareous nannofossils during the Rhaetian (Late Triassic) : evidence from the Northern Calcareous Alps, Austria

Calcareous nannofossils evolved in the global ocean from the Carnian (early Late Triassic) and have contributed to the accumulation of biogenic calcium carbonate in marine sediments since then. Bio-diversification and bio-productivity became more significant in the Rhaetian (Late Triassic), representing an important period to understand the dynamics of calcareous nannofossil evolution. The calcareous nannofossil content of the Zlambach Formation, Northern Calcareous Alps, Austria, was qualitatively and quantitatively investigated using both scanning electron and light microscopy. The nannolith, Prinsiosphaera triassica triassica, dominates the assemblage in most samples and increases slightly in abundance in the lower Rhaetian, followed by a small-scale short-term increase during the middle Rhaetian and reaches rock-forming abundance in the upper Rhaetian. A systematic size decrease is recorded from the lower Rhaetian onwards, possibly due to changes in the palaeo-environment. The abundance of P. triassica triassica is slightly affected by the occurrence of Eoconusphaera hallstattensis and strongly by E. zlambachensis. E. hallstattensis is constrained to a relatively short interval, from the upper Paracochloceras suessi Zone (lower Rhaetian) and disappeared during the lower Vandaites stuerzenbaumi Zone (middle Rhaetian), after the occurrence of a second Eoconusphaeraceae species: E. zlambachensis. The last occurrence of E. hallstattensis comes along with the subspecies Prinsiosphaera triassica crenulata showing characteristic parallel-oriented calcite lamellae. Those three species are suggested as good biostratigraphical markers for the Upper Triassic. The coccolithophorids are present in low abundance, increasing slightly in the middle Rhaetian. After the first record of coccoliths in the middle Norian (Alaunian), the oldest Crucirhabdus minutus and Archaeozygodiscus koessenensis were observed in the upper Norian (Sevatian) and the first occurrence of Crucirhabdus primulus was recorded in the lower Rhaetian. These observations suggest a rather slow temporal diversification of the first coccolithophorids

Development of early calcareous nannoplankton in the Northern Calcareous Alps (Austria) in the Upper Triassic

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Development of early calcareous nannoplankton in the late Triassic (Northern Calcareous Alps, Austria)

Calcareous nannofossils are considered to be the most productive calcifying organisms, but the impact of their emergence on the chemistry of the ocean in the Late Triassic is not understood yet. Evolutionary details of this bio-event are missing due to the scarcity of well-preserved outcrops worldwide. The current study focuses on the calcareous nannofossil assemblage of the middle Norian to lower Rhaetian at Sommeraukogel and Steinbergkogel in the Northern Calcareous Alps (Austria). The sediments were deposited on a topographic high on a deeper shelf at a palaeolatitude around 20–30°N. The steps in the emergence of pelagic calcifiers are documented by a scanning electron microscope (SEM) study. The First Occurrence (FO) of coccolith, not identified at a species level, was recorded in the middle Norian (Alaunian 3). The oldest Crucirhabdus minutus and Archaeozygodiscus koessenensis were observed in the late Norian (Sevatian) and were followed by the FO of Crucirhabdus primulus in the early Rhaetian. These observations suggest a rather slow temporal evolution of the first coccolithophorids, with millions of years from the ancestor C. minutus to C. primulus, and in between the evolution of the new genus A. koessenensis. Diagenetic overprinting of the sedimentary succession has affected the preservation degree of the calcareous nannofossils but not the trend of their quantity in the studied samples, as proven by petrographic studies and trace element signatures of the calcareous sediments. This supports our quantitative estimates of the CaCO3 volume- and palaeo-fluxes due to the export productivity of the calcareous nannofossils, Prinsiosphaera triassica. Our results show the dominance and the increase in abundance of Prinsiosphaera triassica slightly above the Norian/Rhaetian boundary. However, calcareous nannofossils did not reach rock-forming abundances at this time and thus did not significantly influence the geochemical composition of the Western Tethys Ocean

The Calcium Isotope (δ44/40Ca) Record Through Environmental Changes : Insights From the Late Triassic

Calcium isotopes (δ44/40Ca) are particularly useful in palaeo-environmental studies due to the key role of carbonate minerals in continental weathering and their formation in seawater. The calcium isotope ratio can provide hints on past changes in the calcium fluxes, environmental shifts, ecological factors and alternatively diagenesis of carbonate rocks. The investigation of the Late Triassic calcium isotope record offers a great opportunity to evaluate such factors in a time interval that witnessed important environmental and ecological turnovers, such as the first appearance of calcareous nannoplankton, ocean acidification and periods of elevated extinction rates. In this study, we present a δ44/40Ca data set from the upper Norian (Upper Triassic) through the lower Hettangian (Lower Jurassic) interval. The isotope records reveal two globally significant signals: a ∼ 0.20‰ decrease through the early Rhaetian (Upper Triassic) and a small, negative (∼0.14‰) excursion corresponding to the emplacement of the Central Atlantic Magmatic Province, at the end of the Triassic. The possible explanations for these signals are changes in the isotopic ratio of the continental calcium influx to the ocean due to the high chemical weathering rate of carbonates and possibly ocean acidification, respectively. The considerable (∼0.15–0.30‰) offset in δ44/40Ca between study areas is likely the combined result of local differences in lithology and early marine diagenesis. The major evolutionary step represented by the first occurrence of calcareous nannoplankton did not have at this time a determining role on the calcium isotopic signature of the marine carbonates

Mid-Norian to Hettangian record and time-specific oolites during the end-Triassic Mass Extinction at Wadi Milaha, Musandam Peninsula, United Arab Emirates

The end-Triassic Mass Extinction (ETME) is generally regarded as a consequence of the environmental changes associated with the emplacement of the Central Atlantic Magmatic Province (CAMP) and ranks among the ‘big five’ mass extinctions in Earth history. A notable feature of the ETME is a halt in marine carbonate deposition followed by the formation of unusual facies such as carbonate cement fans and oolites in the early aftermath of the event. The ETME time interval has been well studied over the last few decades, in contrast to a few minor extinction events that preceded it, among them the extinctions associated with the Norian-Rhaetian boundary (NRB). This study provides new insights into these extinction events with complete mid-Norian to Hettangian δ18Ocarb and δ13Ccarb record from a key section at Wadi Milaha (Ras Al Khaimah Emirate, United Arab Emirates). Ooids are important proxies for palaeoenvironmental reconstruction. The post ETME oolite horizon is documented providing morphological classification as well as a detailed modal analysis of rock components and different types of coated grains. Through a multi-technique approach, we argue for the stability of the carbon cycle across the NRB extinction event and the existence of a hiatus at the TJB (Triassic-Jurassic Boundary) in Wadi Milaha. Our new morphological classification of post-extinction ooids is compatible with a major role for seawater geochemistry with respect to sedimentological processes, by example in the peculiar way ooids diversify and alternate with other kinds of coated grains