Comment to “The Devonian-Carboniferous boundary in the Graz Paleozoic (Eastern Alps, Austria) and its global significance” by Kaiser and Hubmann

Reply to the Comment to “The Devonian-Carboniferous boundary in the Graz Paleozoic (Eastern Alps, Austria) and its global significance” by Ana-Voica Bojar and Franz Neubauer

For the first time, zircons from thin volcanic ash layers contained within organic-rich black shales (Domanik facies) at the Devonian–Carboniferous boundary have been dated using the U–Pb LA-ICP-MS method. The studied material comes from core samples of two boreholes located in the axial and marginal zones of the Kama–Kinel Trough System within the Volga–Ural Petroleum Province. In the axial zone, the base of the Siphonodella quadruplicata conodont zone yielded a concordant U–Pb age of 357.6 ± 1.7 Ma. In the marginal zone, the upper part of the Palmatolepis gracilis expansa Zone provided a concordant U–Pb age of 360.0 ± 1.2 Ma. These ages agree, within analytical uncertainty, with the current conodont-based chronostratigraphic framework and allow refinement of the onset of the Hangenberg Event in the studied basin.

The Strunian regional substage corresponding to the uppermost Famennian of Belgium and northern France (Avesnois area) is marked by the occurrence of trilobites after an almost complete absence that began at the top of the Frasnian. The Strunian phacopid trilobites represented by Omegops, i.e., O. accipitrinus and O. maretiolensis, are recorded and illustrated from two Belgian sections (Spontin and Chansin) situated in the central part of the Dinant Synclinorium. These belong to the youngest known phacopids and they rank among the victims of the Hangenberg Crisis that took place near the Devonian–Carboniferous boundary. The phacopid fauna formed a trilobite association inhabiting a shallow shelf together with diverse brachiopod but also coral, crinoid, mollusc communities. These Belgian trilobites are compared with Omegops bergicus encountered in the northern France (Avesnois area).

Lungfishes (Dipnoi, Sarcopterygii) initially radiated in the Early Devonian, and reached the apogee of their diversity during this period, especially with regard to their dentitions. Following the end-Devonian extinction, most of this diversity was lost and remained low throughout the Carboniferous and the rest of the Palaeozoic, mainly represented by the incredibly successful 'Sagenodus-like' dental morphology with sharp rows of fused teeth. Nevertheless, the exact scenario of lungfish evolution across the Devonian-Carboniferous boundary remains ambiguous. Recent work on new dipnoan assemblages from the Famennian (Upper Devonian) and Tournaisian (Lower Carboniferous) has challenged our understanding of lungfish evolution across this boundary. These studies suggest that the end-Devonian extinction did not impact lungfishes as strongly as other sarcopterygians, and that many Carboniferous lineages have origins that stretch much further back in time. However, concrete fossil evidence supporting these new hypotheses remained exceedingly rare. Here, we describe a highly derived lungfish tooth plate from the Famennian of the Tafilalt region in Morocco. This specimen's morphology is akin to that of the Carboniferous genus Sagenodus, often dubbed as the earliest 'modern' lungfish. Although the material is not sufficient for a precise taxonomic identification or placement with phylogenetic analyses, it shows that a Carboniferous lineage-the Sagenodontidae-extends deep roots into the Devonian. This specimen supports recently developed ideas about lungfish evolution blurring across the Devonian-Carboniferous boundary and pushes back the origin of the 'modern' lungfish dental morphotype by some 20 million years from the Visean to the Famennian.

Marine anoxia is widely proposed as a kill mechanism for the Devonian-Carboniferous boundary mass extinction commonly linked to the end-Devonian Hangenberg Crisis (~359.3 Ma), but the exact connections between primary drivers, marine redox conditions, and biotic turnover are not well constrained. Mercury concentrations and isotopes from multiple sections in South China and western Canada show no significant signature of enhanced volcanism during the end-Devonian Hangenberg Crisis. Rather, the distinct negative shifts of Δ199Hg coupled with positive shifts of δ202Hg across the end-Devonian Hangenberg Crisis provide compelling evidence for development of photic zone euxinia, as supported by covariant pyrite framboid data and Ce/Ce* values of carbonate. The photic zone euxinia was likely driven by increased nutrient delivery given its coincidence with carbon isotopic perturbations and enhanced weathering immediately before the end-Devonian Hangenberg Crisis. Our data highlight the possibility that photic zone euxinia played a pivotal role in the extinction of marine biota during the end-Devonian Hangenberg Crisis by limiting the habitable space and by increased hydrogen sulfide toxicity, especially for organisms living in shallow water.

Conodonts from the Devonian–Carboniferous boundary deposits in two wells within the Sarailinskaya Depression of the Kama-Kinel Trough System were studied. The expansa conodont zone (Upper Famennian) was recognized, as well as the duplicata and quadruplicata conodont zones (Lower Tournaisian). Platform conodonts were absent in the intervals between the well-defined Famennian and Tournaisian deposits in both wells. These stratigraphic intervals span the praesulcata (uppermost Famennian) and sulcata (lowermost Tournaisian) conodont zones and are likely to be associated with the global Hangenberg Extinction Event. The conodont changes across the Devonian–Carboniferous boundary indicated a restructuring of the conodont assemblages between the Famennian and Tournaisian, with the dominant Devonian genera (Palmatolepis, Branmehla, and Bispathodus) replaced by the genus Siphonodella in the early Carboniferous. Data on the conodont distribution were presented, along with SEM images of index species.

Development of the Middle Devonian-Mississippian carbonate platform in Zonguldak Terrane (NW Anatolia, Türkiye) with special emphasis on the Devonian-Carboniferous Boundary

As a result of the study of core material in the volume of 211 thin sections from five boreholes of the Northern part of the Lviv Paleozoic Trough, a complex of fossil green, red, and charophyte algae was revealed, which consists of 20 species of 13 genera of green, red, and charophyte algae, among which five species of five genera are identified for the first time for the Devonian deposits of the Lviv Paleozoic Trough. The carbonate deposits belong to the Frasnian and Famennian stages and are represented mainly by sandstones, argillites, siltstones, limestones, dolomites, marls, and white chalk. Carbonate deposits are represented mainly by pelitomorphic wackestones, packstones, wackestone-packstones, grainstones, grainstone-packstones, micrite, bioclastic wackestones, and microbioclastic pelitomorphic wackestones. Limestones often have a pelitomorphic texture, enriched with sand material or dolomitized. The algae are confined primarily to pelitomorphic wackestones of the Waulsortian type, which indicates the intermediate facies of the shelf margins, which were formed by the open circulation of sea waters. To a lesser extent, algae occur in packstones, grainstones, wack-packstones, and are found predominantly along the remains of foraminifera, crinoids, bryozoans, ostracods, and sponge spicules. It is revealed that the algal assemblage of the Famennian stage is depleted and represented mainly by the green tubular algae of the families Palaeoberesellaceae Mamet et Roux and Dasycladaceae Kützing. Higher up the section, a few charophyte algae of the genus Planoumbella Platonov are found. The assemblage of the Famennian stage is significantly enriched, both numerous and systematic. Numerous charophyte and red algae appear, especially in the upper part of the section, which corresponds to the stratigraphic level of the zone C1ta of Donbass. These deposits were considered for a long time to be Tournaisian until at the All-Union Congress in 1986 the Devonian-Carboniferous boundary at the base of the Gattendorfia (Acutimoceras) goniatite zone and the Siphonodella sulcata conodont zone was drawn, which represents the top of the C1ta zone of Donbass. At this level, the richest and most varied algae assemblage is revealed. Above this level, representatives of the genus Parachaetetes Deninger and charophyte algae are not found. The prevalence of the green siphoncladal algae in the Frasnian association indicates a calm hydrodynamic regime below the level of the tide, as well as the depth of the basin, which does not exceed 3-5 m, slight fluctuation in the depth of the sea basin, good illumination, and warm water. The significant distribution of charophyte algae in the Famennian time indicates a decrease in salinity in the paleobasin due to the flow of fresh water from the Kovel uplift.

The Devonian-Carboniferous boundary in the Graz Paleozoic (Eastern Alps, Austria) and its global significance

Research subject. Devonian-Carboniferous boundary beds in the Berchogur Depression (Western Kazakhstan). New data on the fauna of ammonoids, conodonts, and foraminifers from the Berchogur sections were obtained during fieldwork studies in 2018–2019. Aim. Identification of episodes of the Global Hangenberg event (crisis) in the Berchogur section and correlation of the potential markers of the base of the Carboniferous in this section. Materials and Methods. The research was conducted using paleontological and lithological methods. Results. The study of fossil assemblages and rock lithology exposed by new excavations suggests that the markers of the Hangenberg event are present in the section; moreover, they can be correlated with the phases of this event in the sections of Western Europe. A correlation of ammonoid, conodont, and foraminiferal zones is proposed. It is shown that the conodont S. (Eo.) praesulcata in the Berchogur section first appears in Member 2b, and the first appearance of S. (Eo.) sulcata is recorded in Member 3b. Based on the findings of ammonoids of the genus Acutimitoceras, Members 3b and 3c can be correlated with the upper part of the prorsum ammonoid Zone, which corresponds to the level of appearance of S. (Eo.) sulcata. The intervals corresponding to episodes of the global Hangenberg event (pre-crisis, crisis, and partly post-crisis) are provisionally identified in the section. Conclusions. The pre-crisis interval corresponds to the abundance of foraminifers in the Q. kobeitusana substricta Zone and the Upper expansa and lower part of the Lower praesulcata conodont zones. The so-called crisis prelude corresponds to the extinction of the quasiendothyrid foraminiferal community and the upper part of the Lower praesulcata Zone (Member 2a2). The Hangenberg Extinction, or the lower crisis interval, corresponds to the clays of Member 2b in, and to the interval of most of the Middle praesulcata Zone – the lower part of the Upper praesulcata Zone, and to the Hangenberg black shales in Europe. The subsequent regressive episode (Member 3a) corresponds to the upper part of the Upper praesulcata Zone. The members with ammonoids (3c) and conodonts of the sulcata Zone (3b and 3c) and Member 4 correspond to the transgressive phase and are considered equivalent to the Stoсkum Limestone of the Rhenish Massif.

The palaeoenvironmental record through a Devonian-Carboniferous boundary super-monsoon lake

The last episode of coral-stromatoporoid reef and faunal changes across a Devonian-Carboniferous Boundary section of South China

The Devonian-Carboniferous (D-C; 359 Ma) boundary is marked by widespread deposition of organic-matter-rich black shales associated with the Hangenberg mass extinction event. The Exshaw Formation spans the D-C boundary in the Western Canada Sedimentary Basin (WCSB) and includes the basal Exshaw Shale deposited under broadly anoxic waters. The sediments at the base of the Exshaw Shale were deposited synchronously during a transgressive event across the WCSB, spanning the geographic variability of the basin. The variable Corg content of the shale was affected by local nutrient upwelling and paleotectonic features impacting water depth and circulation. To characterize the link between paleogeography and nutrient cycling, geographic (N = 20 locations) and stratigraphic (N = 6 locations) trends of δ13Corg and δ15Nbulk were examined throughout the WCSB, representing a range of depositional settings. The δ15Nbulk values range between 0.0 and 6.3‰ and δ13Corg from −29.5 to −26.8‰. Phytoplankton production in focused upwelling zones acquired a relatively 15N-depleted signature through isotopic fractionation during nutrient assimilation, and the residual nutrient pool was 15N-enriched. The advection of surface waters away from the location of upwelling supported additional phytoplankton growth and the deposition of sediments with higher δ15N values. The stratigraphic sections include black laminated and burrowed mudrock sequences that record changes in paleoredox conditions, water depth, and tectonism over time. Up-core from the base of the Exshaw, the Corg content decreases and simultaneously δ15Nbulk increases, suggesting a decrease in eutrophic conditions. Variable δ13Corg and δ15Nbulk trends demonstrate that there is no “type” isotopic profile spanning the D-C boundary in the WCSB.

The 1st order mass extinction at the Devonian/Carboniferous transition, known as the Hangenberg Crisis, is characterized by major transgressive/regressive cycles which led to widespread ocean anoxia during the Hangenberg Black Shale Event, as well as to a global major sea-level fall and the worldwide deposition of regressive Hangenberg Sandstone equivalents. The Devonian/Carboniferous transition at the Tuye-Darvar section in the eastern Alborz Range is studied in terms of conodont biostratigraphy, litho-, microfacies and sequence stratigraphy. In order to examine the biostratigraphical framework, forty conodont samples were systematically taken from the studied interval. Thirty-two conodont species belonging to ten genera led to the discrimination of twelve conodont zones, ranging from the Pseudopolygnathus granulosus Zone to the Scaliognathus anchoralis-Doliognathus latus Zone. Due to facies, the conodont record also exhibits some hiatuses. Field observations and sedimentological and microfacies studies led to the identification of thirteen facies types from sub-tidal environments to the fully marine environments, including seven microfacies types. The sediments deposited in a mixed carbonate-siliciclastic platform, revealed four third-order sequences. The Hangenberg Black Shale is not recorded in the Tuye-Darvar section as a result of depositional facies. In comparison with other studied Devonian/Carboniferous Boundary (DCB) sections of the central and northern Iran, the Tuye-Davar section suggests a tectonic position, which is most likely placed on a separate tectonic block.

Geochronological constraints on the Hangenberg Event of the latest Devonian in South China

Devonian-Carboniferous boundary beds in the Kuz-Yama section (Northern Cis-Urals) - Conodonts, ostracods, and carbon isotope record

Devonian–Carboniferous transition in various facies of Northeast Laurussia (North Urals)

Among the important geological attributes of the small Cumbraes Basin in the southwestern off-shore Midland Valley of Scotland, the 4 km-long, fragmented but well-exposed section through the Devonian–Carboniferous boundary sequence in southern Great Cumbrae island ranks highly. In a continuous short stretch of the south and west coasts, large portions of this sequence are superbly preserved in tide-washed strandline surfaces and in old post-glacial sea cliff. This enhanced exposure leaves no doubt that the outcrops belong to two, immediately successive, members of the Kinnesswood Formation – the lower the Doughend Sandstone, the upper the Foul Port Mudstone – and that they provide some of the best available exposures on which to base description and interpretation. The facies of the fluviatile Doughend Sandstone shares many characteristics with the underlying Kelly Burn Sandstone of the Upper Old Red Sandstone, whereas that of the Foul Port Mudstone is distinct and unusual, attributed to local tectonics, influenced by movements on older and more major fractures such as the Highland Boundary Fault, with sediment accumulation taking place in a playa-lake complex of considerable extent.

The boundary between the Devonian and the Carboniferous has been drawn using ammonoids, conodonts and miospores, while geochemical data have only rarely been used. The sampling of eight sections (Aprath, Apricke, Effenberg, Wocklum, Stockum, Drewer WJ, Drewer WA, Marsberg) at the northern margin of the Rhenish Mountains demonstrates the potential of the isotope ratio of organic carbon to regionally correlate sections in different facies areas. With the help of the carbon isotopes, a distinct stratigraphic succession is documented, which is characterised by several positive and negative excursions; this succession was recorded in all examined sections. The carbon isotopes can therefore make a significant contribution to the stratigraphic subdivision of sections and complement the biostratigraphic methods.