Famennian Time Research Articles

Oxygen and sulfur stable isotope ratios of Late Devonian vertebrates trace the relative salinity of their aquatic environments

Late Devonian aquatic environments hosted the fin-to-limb transition in vertebrates. Upper Devonian (ca. 365−360 Ma) strata in Pennsylvania, USA, preserve a diversity of fishes and tetrapods in coastal marine to fluvial depositional environments, making this region ideal for investigating the ecology and evolution of Late Devonian vertebrates. A key unresolved issue has been reconstructing the specific aquatic habitats that hosted various vertebrates during this period. Specifically, the salinity of environments spanning fresh to shallow marine water is difficult to discern from sedimentological and paleontological analyses alone. Here, we analyze rare earth elements and yttrium (REY) as well as stable oxygen and sulfur isotope compositions (δ18O, δ34S) in fossil vertebrate bioapatite from late Famennian (ca. 362−360 Ma) strata of the Catskill and Lock Haven formations in the Appalachian Basin, USA, to determine the relative salinity of their aquatic environments. These results confirm the ecological euryhalinity of several taxa (Bothriolepis sp., tristichopterids, and Holoptychius sp.). Our results are the first demonstrating that some early tetrapod species occupied unequivocally freshwater habitats by late Famennian time (ca. 362−360 Ma). Our study shows that integrating sedimentological and paleontological data with combined oxygen and sulfur isotope analysis allows precise tracing of the relative salinity of vertebrate habitats deep in the past.

Upper Devonian environments and ostracods biofacies of the Kuznetsk Basin

The volcanogenic-carbonate-terrigenous Upper Devonian complex of deposits of the margin of the Kuznetsk Basin has been studied. The results of the studying of sedimentological processes, the composition of sedimentary rocks, the specifics of the petrographic composition of impurity pyroclastic material, make it possible to reconstruct the sedimentation conditions here in the Late Devonian. The predominance as an impurity and sometimes as the main component in sedimentary rocks of andesitic fine pyroclastics with a lower value of acidic and basic volcanic components has been established. From the Late Givetian — Early Frasnian to Famenmian, an uneven increase in the content of acid pyroclastics occurs in the rocks. The studied sections relate to the vast shelf and continental slope of the Siberian paleocontinent. The western direction of the paleoslope is determined. The studied sediments belong to the slope and basin type, they are represented by fine-grained tephroid and volcanomictic sandstones, fine-detrite limestones, olistostromes and slope landslide breccias, turbidites and rarer turbidite canalites, peculiar “conglomerate limestones” (paleoseismites). The composition of pyroclastics and volcanoclastics in sedimentary rocks indicates the manifestation of andesitic volcanism synchronously with sedimentation and the existence of paleogeodynamic conditions corresponding to the active continental margin possibly of the Andean type in the Late Devonian in the studied area. Based on the data obtained as a result of quantitative calculation of the generic composition of the ostracod associations from the studied sections of the Middle (upper part) and Upper Devonian within three structural-facies subdistricts of the margin of the Kuznetsk Basin, the analysis of ostracods was carried out. The features of the biofacial distribution of ostracods allowed us to identify 10 associations and establish their matching with certain parts of the basin from the Late Givetian to the Late Famennian time. The influence of volcanism on the composition of ostracod associations is traced, the appearance of pyroclastics in rocks reduces the taxonomic diversity of ostracod associations and quantitative composition. Variations in the taxonomic composition of ostracod associations correlate well with trends in T-R cycles, the genus is Bairdia is dominant during transgression.

The influence of palaeogeography and tectonic events on trilobite distributions in Morocco and northwestern Algeria

AbstractThe effects of sea-level changes and anoxia on Devonian trilobites have been studied for a long time, but the importance of palaeogeographic and tectonic events in this key-period is still not well understood. In the Devonian Period, trilobites invaded many different marine environments and areas in North Africa where important palaeogeographic changes occurred. Distribution patterns of trilobites through time and space have been analysed using a hierarchical cluster analysis and diversity indices. Our examination of the literature suggests that trilobites were scarce during Lochkovian time before an important diversification in Pragian time. Trilobites flourished in many North African regions without there being important taxonomic differences between basins, because of free migration in relatively flat palaeo-topography and homogeneous environments. During Middle Devonian time, early Variscan tectonic movements transformed the eastern Anti-Atlas area into a basin with a platform topography. Geographical barriers such as deep basins prevented trilobite migrations. At the beginning of Eifelian time, the reduction in migration between the different regions of this area coincided with a decrease in diversity. Consequently, tectonic events played an important role in the decline of trilobites during Middle Devonian time, especially when these were combined with sea-level changes and anoxic/hypoxic events. A recovery occurred in Famennian time involving only new genera. As at the global scale, cyrtosymbolines developed in shallow seas, whereas phacopids evolved in deeper environments. The basin and platform system still hampered migrations, although sea-level variations led to episodic exchanges. The late Famennian regression reduced trilobite diversity dramatically in the study area.

Encrustation patterns on brachiopods from the Middle-Upper Devonian and their paleo-environmental implications

Epibionts and hosts as well as their interactions comprise a special ecosystem in the marine hard-substrate communities which could provide paleoecological implications for the critical events of Earth history. Abundant brachiopod hosts were sampled from the Givetian–Famennian of the Longmenshan region in South China. We describe the encrustation patterns on the brachiopods in terms of epibiont abundance, diversity, relationships between hosts and epibionts, and interaction between epibiont taxa. A total of 3067 brachiopod specimens are examined and identified into seven species, among which 683 specimens are encrusted by epibionts represented by eight taxa. The percentage of encrusted shells and the mean number of epibionts per shell of host brachiopods, are mainly related to the shell morphology of the host and ecological environments. The abundance of epibionts and the number of associations are quite variable among the Middle–Late Devonian stages. Most of epibionts have selective preferences on certain host taxa. The individual size of epibionts is an important factor in choosing the host. During the Givetian and Frasnian time, encrusters were much more abundant on the dorsal valves than on the ventral valves of host brachiopods. However, until the Famennian time, the biconvex spiriferoids have more encrusters on the ventral valves. Such an encrusting change from dorsal valves to ventral valves across the F/F boundary might not be related to ecological environments, yet only the change of host taxa. The epibionts encrusted on host brachiopods did not display a sudden collapse around the F-F boundary interval. The epibiont abundance, diversity, relationships between hosts and epibionts, as well as interaction between epibiont taxa do not show a significant change around the F/F boundary. It may indicate there was no dramatic environmental change in the Frasnian and Famennian boundary interval but long-term ecological damage during the Frasnian in the studied region. • There is a significant variation in abundance, composition, and association of epibionts encrusted on brachiopod taxa. • The epibionts show a selective preference for the hosts, which is related to the shape of host and the size of epibionts. • The dramatic decreases of abundance and association number of epibionts happened in early Frasnian. • The epibionts encrusted on brachiopods did not display a sudden collapse around the Frasnian/Famennian boundary interval. • There is long-term ecological damage during Late Devonian rather than a dramatic one in the Frasnian/Famennian boundary.

North-Gondwana – Laurussia dynamic paleogeography challenged by magnetic susceptibility through the Famennian

Large-scale magnetic susceptibility (MS) variations in ancient sediments are usually interpreted as related to sea-level and climate changes affecting the erosional regime and the amount of detrital input. To constrain such environmental changes during the Famennian, we compared MS records in representative sections of pelagic limestones of (1) the Avalonian margin of Laurussia [Sessacker (SES) and Beringhauser Tunnel (BHT), Rhenish Slate Mountains, Germany] and (2) in Gondwana related terranes [Erfoud (ERF), Tafilalt, Morocco; Col des Tribes (CT), Montagne Noire, France; Buschteich (BU), Thuringia, Germany; Corona Mizziu (CM I, II), Sardinia and Pizzul West (PZW) in the Carnic Alps, Italy]. Very low MS values throughout the Famennian characterize sections from Avalonia. In contrast, MS values are high and oscillating in Gondwana during the early Famennian; they drop significantly all together in the middle Famennian Palmatolepis marginifera marginifera Zone to match the pattern of low MS values of the Avalonian margin thereafter. To evaluate the importance of partial diagenetic overprint, MS data were compared to hysteresis parameters and geochemical proxies in sections BU, CM I, II and CT. The degree of thermal diagenesis is estimates taking into account the Color Alteration Indices (CAI) of conodonts. Striking dissimilarities in MS records between Laurussia and Gondwana are in favor for an existing remnant oceanic barrier between these continents during early Famennian times, when the amount of detrital supplies was obviously different. Uniformity is reached after the drop of MS values in N-Gondwana, when oceanic barriers could have vanished during the early middle Famennian T-R fluctuations of the sea-level. All sites remained under a rather similar regime of detrital supply through middle to uppermost Famennian. Comparison of MS trends with trends of conodont biofacies in five sections (CT, BU, CM I, II and SES) allows emphasizing the concordance of their relationship to sea level fluctuations.

The Assemblage of Late Devonian Primitive Parathuramminids (Foraminifera) from Carbonate Deposits of the Mendeleev Rise, Eastern Arctic

The results of a paleontological and stratigraphic study of an assemblage of Paleozoic primitive calcareous foraminifers from the central part of the Mendeleev Rise (Eastern Arctic) are presented. The obtained data allow us to refine and correct the age of strata containing these fossils. It is shown that the Parathurammina (s.l.) foraminifera are the most stratigraphically informative in this assemblage. The primitive foraminifers recorded in Sample 1601/8c from carbonate deposits of the central part of the Mendeleev Rise indicate the early Famennian time of formation of these deposits.

Basites of the Vilyui paleorift: geochemistry and sequence of intrusive events

We present results of study of the basites of the Vilyui paleorift. Their 40Ar/39Ar isotopic ages were used to establish the sequence of their intrusion, which lasted ~18 Myr. Dolerite sills of the Kontai-Dzherba zone were the first crystallizing phase (378.0-380.7 Ma), which was followed by the intrusion of dikes of the Vilyui-Markha zone (372-373 Ma). Dikes of the Chara-Sinsk zone were the last crystalizing phase (362-364 Ma). The prolonged plume-lithosphere interaction resulted in the successive enrichment of basaltic melt with titanium, phosphorus, REE, HSFE, and LILE. Accumulation of the melt and formation of a magmatic lens at the crust-mantle boundary led to early crystallization of the melt at a depth of 35-40 km from the paleosurface under reducing conditions at a temperature of 1450-1470 °C. We assume that the inception and sequent opening of the Vilyui paleorift were associated with the relative rotation of 22° of the Angara-Anabar block with respect to the Aldan block in Early Famennian time. The Euler pole of rotation was located in the southern part of the paleorift, and a melting zone was directly beneath it.

Nd isotope composition of conodonts: An accurate proxy of sea-level fluctuations

Analyses of the Nd isotope composition of conodonts from the Anti-Atlas (Morocco), the Montagne Noire (France) and the Rhenohercynian domain (Germany) provide insight into the temporal and lateral variations in seawater geochemistry in the western part of the Variscan Sea during the Late Devonian. Most of the isotopic excursions accurately record changes in sea level. The εNd values decreased during regression phases when erosion accelerated supply of unradiogenic Nd from old continental sources. A rise in sea level generated a positive shift in εNd values due to input of more radiogenic oceanic water into the seawater reservoir of the shelf areas. The method has great potential for sea-level research, paleoceanography and correlation because sea-level changes can be precisely recognized even in successions with uniform, monotonous lithologies. However, it has a significant limitation being valid only in epeiric seas and shelf areas. We have used the Nd isotope record to reconstruct the eustatic sea-level changes in late Frasnian–early Famennian time. The onset of the organic-rich Kellwasser units coincides with negative trends in εNd values. This accords with oxygen deficiency at the sea bottom being initiated by short-term regressive events, which in turn would be consistent with cooling periods of glacial origin, as previous authors have suggested.

Diversity and richness of the Devonian terrestrial plants in the Southeastern Mountainous Altay (Southern Siberia): Regional versus global patterns

Devonian deposits of the Southeastern Mountainous Altay, a large region in Southern Siberia, contain abundant remains of terrestrial plants. A semi-quantitative analysis of this regional palaeobotanical record reveals how both the diversity (number of taxa) and richness (number of taxa with an account of their abundances) of floristic assemblages changed during the Emsian–early Famennian time interval. A total of 60 species, representing 42 genera, are known from 8 regional assemblages. Changes in diversity of species and genera occurred simultaneously. The number of taxa was high in the early Emsian, declined in the late Emsian, rose again in the Eifelian–middle Givetian, dropped in the late Givetian, reached the highest values in the early Frasnian, and experienced the greatest decline taking place in the late Frasnian–early Famennian. The standing diversity (number of taxa crossing the time boundaries) was maximal in the Middle Devonian. The dynamics of terrestrial plant richness was similar to that of diversity with an exception of middle Givetian decline in richness despite growth of diversity. The floras were dominated by pteridophytes. Propteridophytes were less abundant, and pinophytes were very rare. Propteridophyte extinctions were high in the middle Givetian, concurrent with a brief decline in pteridophytes. Some regional and global patterns of floral dynamics were similar. However, propteridophyte decline was not abrupt globally. The main abiotic driving factor influencing species richness and diversity appears to have been regional shoreline shifts. It does not appear that climate changes was important for regional changes in phytodiversity, although both regional and global phytodiversity was at its high during the Middle Devonian cooling phase. A comparison of palaeobotanical data from the Southeastern Mountainous Altay and Kazakhstan suggests palaeogeographic proximity and proves an idea of Altay-Mongolian terrane wandered between Gondwana, Kazakhstan, and Siberia. The early Emsian and the early Frasnian, when floras of the study region were diverse and rich, are characterized by the very high degree of similarity. Thus, interregional floral exchanges would facilitate plant radiations.

Diagenetic versus detrital origin of the magnetic susceptibility variations in some carbonate Frasnian–Famennian boundary sections from Northern Africa and Western Europe: implications for paleoenvironmental reconstructions

To provide a new insight into the diagenetic versus detrital origin of the magnetic susceptibility variations in ancient carbonate sequences, a study was conducted within four Frasnian–Famennian platform carbonate sections from Germany, France and Morocco. The study includes along-section magnetic susceptibility and carbonate content measurements complemented by analyses of magnetic hysteresis parameters, inorganic geochemistry and clay mineralogy. Our results show that the magnetic susceptibility evolution is dominantly controlled by the variations in the concentration of low-coercivity ferromagnetic magnetite grains and, to a lesser extent, of paramagnetic clays. In more detail, hysteresis ratios suggest the coexistence of two magnetite populations with significantly different grain size: (1) a dominantly coarse-grained detrital fraction including a mixture of multi-domain and single-domain particles (2) an authigenic fine-grained fraction composed of a mixture of single-domain and superparamagnetic particles. Despite a diagenetic imprint on the clay assemblages, no relationship is established between magnetic susceptibility and illite crystallinity, therefore discarding a noticeable distortion of primary within-section magnetic susceptibility evolution. The overall inherited character of the magnetic susceptibility fluctuations is corroborated by a significant correlation of magnetic susceptibility with terrigenous proxies (Zr, Th). The poorer correlation of magnetic susceptibility with the Fe content is consistent with the existence of a very fine-grained authigenic magnetite component that possibly induces a global magnetic susceptibility increase at the section scale, but no distortion of the within-section evolution. The magnetic susceptibility curves presented here provide a general record of climate-driven detrital influx and carbonate productivity through Frasnian–Lower Famennian times.

Water column euxinia and wildfire evidence during deposition of the Upper Famennian Hangenberg event horizon from the Holy Cross Mountains (central Poland)

A palynological study of the uppermost Famennian section from Kowala Quarry (Holy Cross Mountains, central Poland) allowed recognition of two miospore zones: LV (Retispora lepidophyta–Apiculiretusispora verrucosa) and LN (Retispora lepidophyta–Verrucosisporites nitidus). Based on palynology and sedimentology, the black shale within the upper part of the section is identified as equivalent to the Hangenberg Black Shale, which is known globally. This black shale contains compounds characteristic of photic zone euxinia, including isorenieratane and its derivatives. Such compounds are absent in the organic-poor marls and shales occurring below the LN Zone, and are present only as traces in the layers just above the black shale, indicating fluctuations in the oxygen minimum zone during uppermost Famennian sedimentation. Palynofacies show high amounts of amorphous organic matter and prasinophyte concentrations in the black shale, and a subsequent significant decrease of amorphous organic matter concomitant with a rapid increase of terrestrial input (mainly miospores with common tetrads) in the layers above the black shale. This supports the relatively rapid change in the taxonomic composition of phytoplankton caused by fluctuations of the chemocline. The whole succession corresponds to one sea-level rise and fall. The presence of high concentrations of peri-condensed polycyclic aromatic hydrocarbons and large amounts of small charcoal particles at the Hangenberg event horizon indicate the occurrence of wildfires. Such observations suggest that atmospheric O22 levels had exceeded the critical threshold of 13 %, above which wildfires may occur, by latest Famennian time.

The biostratigraphical and palaeogeographical framework of the earliest diversification of tetrapods (Late Devonian)

Abstract The earliest diversification of tetrapods is dated as Late Devonian based on 10 localities worldwide that have yielded bone remains. At least 18 different species are known from these localities. Their ages span the ‘middle’–late Frasnian to latest Famennian time interval, with three localities in the Frasnian, one at the F/F transition (though this one is not securely dated) and six in the Famennian. These localities encompass a wide variety of environments, from true marine conditions of the nearshore neritic province, to fluvial or lacustrine conditions. However, it does not seem possible to characterize a freshwater assemblage in the Upper Old Red Sandstone based upon vertebrates. Most of the tetrapod-bearing localities (8 of 10) were situated in the eastern part of Laurussia (=Euramerica), one in North China and one in eastern Gondwana (Australia), on a pre-Pangean configuration of the Earth, when most oceanic domains, except Palaeotethys and Panthalassa, had closed.

Lead and zinc in formation waters, Alberta Basin, Canada: Their relation to the Pine Point ore fluid

The source of the ore fluid for the Pine Point Zn–Pb deposit has been controversial for decades. Both formation waters from deep in the basin and geothermal fluids from the Precambrian basement have been considered as sources of the ore in this carbonate-hosted deposit. A database of 694 analyses of Pb and Zn in formation waters from the Alberta Basin bears on this controversy. These waters have a mean content of 4.66 mg L −1 Pb and 6.88 mg L −1 Zn. For each of these metals, the maximum content is 360 mg L −1 and the median content is 0.9 mg L −1. In 71% of the samples Pb > Zn, with a mean ratio of 2.3:1. This is in contrast to the Pb:Zn ratio of formation waters in other sedimentary basins and geothermal waters but is consistent with that in brines from the Canadian Shield. Geochemical modelling using SOLMINEQ.88 shows that Pb and Zn occur mainly as the MeCl 4 2 - complex. It is not possible to produce a sulphide precipitate with Zn > Pb, such as the ore at Pine Point, from a formation water with Pb > Zn, even when the formation water carries the metals in the absence of H 2S, and the metals are precipitated by H 2S-rich brines. Therefore, the present formation waters are a poor candidate for the source of the metals at Pine Point. It is more likely that geothermal fluids rising along the Great Slave Lake Shear Zone were the source of the metals. Additional evidence for this alternate source includes: (1) the very homogeneous, non-radiogenic character of the Pb isotopes in galena; (2) the absence of a thermal anomaly along the Great Slave Lake Shear Zone; (3) a Famennian time of deposition (based on Rb–Sr dating of sphalerite), when overburden was thin and reservoir temperature low; and (4) the distribution of sphalerite and galena in the subsurface of the Alberta Basin in relation to major crustal discontinuities and normal faults cutting the crystalline basement. Exploration for other Zn–Pb deposits should therefore focus on the location of suitable shear zones and faults up which geothermal fluids might have migrated, rather than the flow and composition of formation waters.

Mountain building‐enhanced continental weathering and organic carbon burial as major causes for climatic cooling at the Frasnian–Famennian boundary (c. 376 Ma)?

AbstractThe Late Devonian was a period of drastic environmental changes, as exemplified by a major biotic crisis at the Frasnian–Famennian boundary (FFB) and the onset in Famennian times of glaciations across southern Gondwana. Worldwide evidence for the coeval development of the major Acadian–Eovariscan belt led us to propose a model relating the Late Frasnian–Famennian environmental perturbations to extensive continental uplift through two atmospheric CO2‐depleting mechanisms: (1) the intensification of silicate weathering on the continental areas as attested by a major rise in the 87Sr/86Sr composition of sea water at the FFB; and (2) the massive burial of organic carbon (Kellwasser events) in partially confined basins due to the collisional‐induced reduction of equatorial oceanic communications between the Palaeotethysian and Panthalassic oceans. This process is also suggested to have been controlled by an important primary productivity connected to an increased nutrient availability triggered by the enhanced continental run‐off.

Devonian fluvial strata in and adjacent to the Emsian‐Eifelian Moona Vale Trough, western New South Wales

The lower part of the mid‐Emsian ‐ mid‐Eifelian strata exposed near Moona Vale Station are here called Units 2 and 3 of the Muckabunnya beds (new) whereas the mid‐upper part of these previously unnamed Emsian‐Eifelian strata are represented by the fining‐upwards Menamurtee Sandstone (new). Within this formation lies the largely debris‐flow‐deposited Moona Vale Conglomerate Member (new) which comprises boulders of monomictic sandstone of Devonian aspect derived from an alluvial fan lying west of the Koonenberry Fault. It is inferred that the Menamurtee Sandstone and Unit 3 of the Muckabunnya beds were deposited in the 20 km‐wide active Moona Vale Trough (new), which extended at least 40 km northwards as a pullapart between a northwest‐trending, sinistral Koonenberry Fault and a north‐northeast‐trending sinistral basement fault lying along the trend of the Lake Wintlow Line. In mid‐Eifelian time at the onset of the Tabberabberan Orogenic Pulse, trough growth ceased and continuing sinistral motion (till ?late Givetian time) along the Koonenberry Fault (but probably not along the eastern branch of the Lake Wintlow Line Fault) caused the Menamurtee Anticline (and at least eight other structures) to form in the mapped area east of the Koonenberry Fault. The anticline was then truncated by erosion by mid‐Givetian time and sandstone (Gum Creek beds: new) were deposited unconformably across its truncated crest. In late Givetian to Famennian time the fine‐grained fluvial Ravendale Formation, which crops out west of the Koonenberry Fault in the study area, was deformed during the Kanimblan Orogenic Pulse when the fault was dextral.

Evolution of the southern margin of the Donbas (Ukraine) from Devonian to Early Carboniferous times

Abstract A Devonian-Early Carboniferous succession comprising thick clastic and carbonate sediments with interbedded volcanics was examined along the southern margin of the Donbas fold belt, Ukraine. Following initial rifting and subsidence, a continental (fluvial, lacustrine) succession was established. This first phase of synrift activity (Eifelian) was accompanied by the extrusion of basalts. In Late Givetian-early Famennian times, half-graben development was pronounced and a series of E-W-trending half-grabens were formed, along with coeval volcanic activity. Subsequent basin subsidence led to the establishment of a broad marine carbonate platform across the region (Late Famennian-Tournaisian-early Viséan). Renewed uplift led to partial exposure and karstification of the platform. This was accompanied by trachyte intrusion and extrusion. The overlying chert-rich unit was probably deposited under lacustrine conditions, although a shelf environment has also been suggested. Renewed tectonic activity along the main basin-bounding fault resulted in the synsedimentary deformation of this unit.

Renalcids as Fossilized Biofilm Clusters

Abstract Renalcids are a problematic group of mostly Paleozoic microbial fossils. In the Upper Devonian reef complex of the Canning Basin, Western Australia, they grew in cryptic reef environments, such as the undersides of laminar stromatoporoids during Frasnian time and within arborescent thrombolites during Famennian time. Renalcids preferred a pendant growth habit and were often the final phase of microbial encrustation in reef crypts. The micritic chambers of the Upper Devonian renalcids, Shuguria and Izhella, have carbon isotope values 0.3–1.8‰ lower than contemporaneous early marine cements, suggesting incorporation of respired CO2 into the micrite. Based on growth environment and carbon-isotope geochemistry, renalcids are neither fossilized cyanobacteria nor marine algae. Renalcids may be fossilized biofilm clusters, which calcified due to heterotrophic bacterial activity within biofilm microenvironments. The dendritic clots of the renalcids are similar in size and morphology to reported microbial...

Geophysical records of dispersed weathering products on the Frasnian carbonate platform and early Famennian ramps in Moravia, Czech Republic: proxies for eustasy and palaeoclimate

An exceptionally high sea level in Frasnian times (early Late Devonian) changed many peneplained, cratonised and tectonically undisturbed continental margins of tropical belts into carbonate platforms which became covered by thick banks of limestone. The stages marked by very pure carbonates developed during partial sea-level highstands since, when the flooding reached its maximum, the shoreline was distant and conditions for reef growth were good. An increased extension of emerged, flat, coastal areas was coupled with an opposite situation, when oscillating sea level fell to its momentary minimum and the accommodation space available for accumulation of carbonate material was low. Subaerially exposed surfaces were subject to weathering and the products were dispersed by aeolian or aquatic means to form layers when these weathering materials were trapped in carbonates. This overall scenario relating to the trapping of weathering products in pure carbonates was tested at geographically isolated platform with a stable and only slowly subsiding basement (Eastern Moravia, Czech Republic). The amounts of weathering products dispersed in limestone were documented using the combined methods of γ-ray logging, γ-ray spectrometry and magnetic susceptibility measuring, i.e. according to their radioactivity (K, Th and U) and magnetism (Fe). Six third-order Frasnian eustatic cycles have been inferred from the data, with an estimated period of 1 Ma. The Late Frasnian and especially the Frasnian/Famennian (F/F) transition brought the total destruction or at least a strong overlay on this hypothetical background setting. The record of strong sea-level fluctuations in the F/F interval is shown in strong oscillations and offsets related to pronounced hiatuses and flooding surfaces. Such rapid sea-level fluctuation is typical for glacioeustatic control, although the present estimates of the maximum amplitudes are larger than realisable by glaciation events and may be explained in combination with tectonoeustatic models. Comparison of sections between the centre and periphery of the platform substantiates the concept that oscillating Late Frasnian shorelines were rapidly prograding. Very low sea-level stages of early Famennian times changed the depositionary systems from extensive platforms to narrow carbonate ramps which were positioned out and down on the extinct reef bank slopes. The central parts of the Eastern Moravian and also other Frasnian platforms in the world were not normally covered by F/F sediments, the transgressive onlaps above any crystalline highs were not observed and therefore the drowned Frasnian reefs on their periphery, although emphasised by many geologists, may reflect sliding or tectonic subsidence on the margins of platform blocks rather than eustatic rise of sea level.

Archaeopteris from the Upper Famennian of Belgium: heterospory, nomenclature, and palaeobiogeography

New fertile specimens of Archaeopteris roemeriana from Upper (but not uppermost) Famennian strata of Belgium have been studied. Crowded sporangia are borne on non-laminate, three times bifurcate fertile leaves helically arranged around the fertile ultimate axes. Vegetative leaves occur both proximal and distal to the fertile leaves. Heterospory is demonstrated with miospores belonging to the Geminospora– Aneurospora complex and macrospores most probably to Contagisporites genus. The established morphological traits allow us to substantiate a synonymy between A. roemeriana and A. halliana. The apparent dominance of A. halliana on the eastern side of the Acadian mountains during Late Famennian times is discussed.

Water column anoxia, enhanced productivity and concomitant changes in δ13C and δ34S across the Frasnian–Famennian boundary (Kowala — Holy Cross Mountains/Poland)

The investigation of the trace element and organic geochemistry of the Frasnian–Famennian boundary section at Kowala (Holy Cross Mountains/Poland) shows that the lower water column was oxygen-deficient during late Frasnian and early Famennian times. The abundance and carbon isotopic composition of diaryl isoprenoids, biomarkers indicative for green sulfur bacteria, prove that euxinic waters reached into the photic zone, at least episodically. Total organic carbon (TOC) contents show two maxima that are time-equivalent to the Kellwasser horizons deposited in shallower water settings. Enhanced TOC concentrations are explained by a higher primary productivity, presumably as a consequence of an enhanced nutrient supply from the continent. The increase in the abundance of hopanes and bituminite suggests that the bacterial contribution to TOC increased at the Frasnian–Famennian transition. The sulfur isotopic composition of pyritic- and organically bound sulfur shows a +27‰ excursion across the boundary. The observation that the δ 34S values of organic-bound sulfur closely resemble that of pyrite sulfur indicates a common sulfur source, likely early diagenetic sulfide. A change in the δ 13C of total dissolved inorganic carbon as a consequence of an enhanced burial of 12C-enriched organic carbon is indicated by a +3‰ excursion measured for TOC as well as for individual n-alkanes and isoprenoids. The burial of large amounts of organic carbon is expected to result in a decrease in pCO 2 and should affect the photosynthetic carbon isotope fractionation ( ε p). The fact that we observe no change in ε p can be explained by the circumstance that ε p was most probably at maximum values, as a consequence of high atmospheric and oceanic-dissolved CO 2 concentrations during the Devonian.