Silurian Climatic Zonation of Cryptospore, Trilete Spore and Plant Megafossils, with Emphasis on the Přídolí Epoch

peer reviewe

Persistent global marine euxinia in the early Silurian

The second pulse of the Late Ordovician mass extinction occurred around the Hirnantian-Rhuddanian boundary (~444 Ma) and has been correlated with expanded marine anoxia lasting into the earliest Silurian. Characterization of the Hirnantian ocean anoxic event has focused on the onset of anoxia, with global reconstructions based on carbonate δ238U modeling. However, there have been limited attempts to quantify uncertainty in metal isotope mass balance approaches. Here, we probabilistically evaluate coupled metal isotopes and sedimentary archives to increase constraint. We present iron speciation, metal concentration, δ98Mo and δ238U measurements of Rhuddanian black shales from the Murzuq Basin, Libya. We evaluate these data (and published carbonate δ238U data) with a coupled stochastic mass balance model. Combined statistical analysis of metal isotopes and sedimentary sinks provides uncertainty-bounded constraints on the intensity of Hirnantian-Rhuddanian euxinia. This work extends the duration of anoxia to >3 Myrs – notably longer than well-studied Mesozoic ocean anoxic events

Osmium and lithium isotope evidence for weathering feedbacks linked to orbitally paced organic carbon burial and Silurian glaciations

The Ordovician (∼487 to 443 Ma) ended with the formation of extensive Southern Hemisphere ice sheets, known as the Hirnantian glaciation, and the second largest mass extinction in Earth History. It was followed by the Silurian (∼443 to 419 Ma), one of the most climatically unstable periods of the Phanerozoic as evidenced by several large scale (> 5‰) carbon isotope (δ13C) perturbations associated with further extinction events. Despite several decades of research, the cause of these environmental instabilities remains enigmatic. Here, we provide osmium (187Os/188Os) and lithium (δ7Li) isotope measurements of marine sedimentary rocks that cover four Silurian δ13C excursions. Osmium and Li isotope records resemble those previously recorded for the Hirnantian glaciation suggesting a similar causal mechanism. When combined with a new dynamic carbon-osmium-lithium biogeochemical model we suggest that astronomical forcing of the marine organic carbon cycle, as opposed to a decline in volcanic arc degassing or the rise of early land plants, resulted in drawdown of atmospheric CO2, triggering continental scale glaciation, intense global cooling and eustatic sea-level lows recognised in the geological record. Lower atmospheric pCO2 and temperatures during the Hirnantian and Silurian glaciations suppressed CO2 removal by silicate weathering, driving 187Os/188Os and δ7Li variability, supporting the existence of climate-regulating feedbacks

First record of Rhabdoceras suessi (Ammonoidea, Late Triassic) from the Transylvanian Triassic Series of the Eastern Carpathians (Romania) and a review of its biochronology, paleobiogeography and paleoecology

Abstract The occurrence of the heteromorphic ammonoid Rhabdoceras suessi Hauer, 1860, is recorded for the first time in the Upper Triassic limestone of the Timon-Ciungi olistolith in the Rarău Syncline, Eastern Carpathians. A single specimen of Rhabdoceras suessi co-occurs with Monotis (Monotis) salinaria that constrains its occurrence here to the Upper Norian (Sevatian 1). It is the only known heteromorphic ammonoid in the Upper Triassic of the Romanian Carpathians. Rhabdoceras suessi is a cosmopolitan species widely recorded in low and mid-paleolatitude faunas. It ranges from the Late Norian to the Rhaetian and is suitable for high-resolution worldwide correlations only when it co-occurs with shorter-ranging choristoceratids, monotid bivalves, or the hydrozoan Heterastridium. Formerly considered as the index fossil for the Upper Norian (Sevatian) Suessi Zone, by the latest 1970s this species lost its key biochronologic status among Late Triassic ammonoids, and it generated a controversy in the 1980s concerning the status of the Rhaetian stage. New stratigraphic data from North America and Europe in the subsequent decades resulted in a revised ammonoid biostratigraphy for the uppermost Triassic, the Rhaetian being reinstalled as the topmost stage in the current standard timescale of the Triassic. The geographic distribution of Rhabdoceras is compiled from published worldwide records, and its paleobiogeography and paleoecology are discussed

A silicified Early Triassic marine assemblage from Svalbard

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Carbon isotope chemostratigraphy of the Llandovery in northern peri-Gondwana: new data from the Barrandian area, Czech Republic; pp. 220–226

The first complete δ13Corg record of the uppermost Hirnantian to lower Telychian strata of the Barrandian area (northern peri-Gondwana) is presented based on 168 new samples. The new data from the study area reveal that the evolution of the Llandoverian organic carbon isotope reservoir was similar to that on other palaeoplates, but it differs from the development of the coeval carbonate carbon isotope reservoir in the absence of two δ13C excursions (i.e. the early Aeronian positive excursion in the upper part of the Demirastrites triangulatus graptolite Biozone and a negative excursion occurring close to the boundary between the Cystograptus vesiculosus and Coronograptus cyphus graptolite biozones)