Catherine Girard

Magnetostratigraphy susceptibility of the Frasnian/Famennian boundary

Magnetosusceptibility event and cyclostratigraphy (MSEC) is used to establish a non-polarity-based magnetostratigraphy susceptibility (MSS) between the stratotype region for the Frasnian/Famennian (F/F) boundary sequence in the Montagne Noire of southern France, the eastern Rheinisches Schiefergebirge of Germany, Belgium, the eastern Anti-Atlas of Morocco, and the Arbuckle Mountains of southern Oklahoma (USA). Despite differences of depositional environment, the MSS is remarkably consistent and can be described in the context of a hierarchy of magnetozones that allow the extension of correlation away from the Montagne Noire reference section. The nature of the controls on the influx of iron into the marine system produces a natural hierarchy of at least seven orders or magnetozones designated MSZ1, MSZ2, MSZ3, MSZ4, MSZ5, MSZ6, and MSZ7. These are characterized in terms of the magnitude of their duration. The MSS reference section of choice for the F/F boundary is the well-known Trench C at La Serre (LSC) in the northeastern Montagne Noire of southern France. The F/F boundary is located at the lower boundary of magnetozone La Serre Iα3b at La Serre as well as in all other sections studied. MSZ4 and MSZ5 magnetozones are used to establish intra- and inter-regional correlation between the reference sequence and the other regions. The base of the Upper Kellwasser event is as variable within the MSS zonation as it is within the biostratigraphic zonation, as is the position of the Lower Kellwasser event.

Variations in Ce anomalies of conodonts through the Frasnian/Famennian boundary of Poland (Kowala – Holy Cross Mountains): implications for the redox state of seawater and biodiversity

Abstract Rare-earth element (REE) concentrations on Upper Devonian conodonts, distinguished by very low Color Alteration Index, were measured by inductively coupled plasma mass spectrometry from a stratigraphic section across the Frasnian/Famennian (F/F) boundary in the Holy Cross Mountains (Poland). The present data show a ‘hat-shaped’ pattern of the REE spectra similarly to other Devonian conodont spectra already documented. However, our sample composition suggests little to moderate alteration of the original oceanic pattern through the adsorption mechanism in a context of ‘weak’ or ‘early’ diagenesis. All along the section, the magnitude of the Ce anomaly is always negative, and there is an inverse correlation between the Ce anomaly and the (La/Sm) SN ratio. This has been interpreted as an La enrichment due to a terrestrial input. A mathematical procedure was used in order to restore the original Ce anomalies that would reflect the seawater environment not contaminated by terrestrial inputs. No noticeable change is observed in the Frasnian with homogeneous values of Ω (Ce) model but a sharp increase is initiated just below the F/F boundary with maximum values in the lowermost Famennian. The measured (La/Sm) SN ratios co-vary with the Ω (Ce) model in agreement with the hypothesis of a terrigenous input. Finally, the inverse correlation between the values of Ω (Ce) model and the relative abundance of Palmatolepis during the Famennian constitutes the first evidence known of a redox control on the presence of a conodont genus that is an indicator of deep-water environment.

Trace elements in conodont phosphates from the Frasnian/Famennian boundary

Abstract Rare-earth elements (REE) and Th concentrations on individual Devonian conodonts were measured by ICP-MS from stratigraphic sections across the Frasnian/Famennian ( F F ) boundary in the Montagne Noire (Southern France) and Morocco. The present data confirm the reversed concavity of the REE spectra found by Wright et al. (1986) which is a characteristic although poorly understood feature of the ancient oceanic sediments. Within the layers corresponding to the Upper Kellwasser and the F F events, the Ce Ce ∗ ratios show only minor changes with respect to younger and older sediments. The magnitude of the Ce anomaly is nevertheless consistently weak over all the sections. If, as in modern oceans, the biologically mediated Ce anomaly in particles and seawater originates in the uppermost water column, the present observations suggest a global decrease in the biological productivity. Lower productivity is supported by the decreasing abundance of conodont elements during the Upper Kellwasser, but continuing carbonate deposition through this period still makes evidence quite equivoqual. As a proxy to Th Pa fractionation in modern sediments, the Th La fractionation is a potential tracer of the rate of horizontal exchange of seawater between deep-sea and continental margins, which may be correlated with eustatic sea-level fluctuations. During the Upper Kellwasser crisis, the Th La ratios fall significantly in all the sections, this can reflect reduced exchanges between the deep-sea and continental margins. It is suggested that the F F extinction event was the result of the coincidence between a rather modest transgression event and global suboxia.

Chitinozoan bio-event in the Frasnian-Famennian boundary beds at La Serre (Montagne Noire, Southern France)

Abstract Chitinozoans and other palynomorphs have been investigated in Frasnian-Famennian boundary beds from trench C at La Serre (Montagne Noire, Southern France). This section is located some 30 km from the Coumiac quarry containing the Global Stratotype Section and Point (GSSP) for the base of the Famennian. At La Serre, the Frasnian-Famennian (F-F) boundary falls within interbedded anoxic dark shales and limestones. This lithology proves to be more suitable for palynological investigations than the reddish nodular limestones of the F-F stratotype in the Coumiac section, which are virtually devoid of organic matter. At La Serre, the uppermost meter of Frasnian deposits, corresponding to the Upper Kellwasser Event level, contains only a few chitinozoans but other palynomorphs, especially tasmanaceans ( Maranhites ) proliferate in some horizons. Conversely, the most basal Famennian bed yields an exceptional number of chitinozoans (up to 19,000 specimens per gram of rock), a few tracheids, but virtually no tasmanaceans. The absence—or the extremely low numbers—of miospores and acritarchs in these F-F boundary beds seems related to the distal position of the deposition site. We have compared the fluctuation of the chitinozoan abundance both with the abundance of the other palynomorphs, and with that of the conodonts present in the same samples. These data, collated with sedimentological and palaeoecological information document the relationships between the anomalous abundance of the chitinozoans and the latest Frasnian-earliest Famennian mass extinction. It is concluded that the exceptional chitinozoan concentration in the most basal Famennian bed at La Serre is not related to a sorting process generated by turbidity currents or by a temporary increase in the hydrodynamic energy at the water-sediment interface. It corresponds to the conjunction of a high production of chitinozoans, a very low rate of destruction of the vesicles and minimal dilution of the deposit by terrestrial mineral input. The environmental factors leading to such an exceptionally high record of chitinozoans are discussed with a particular attention to consequences of the late Frasnian mass extinction.

Chapter 7Subdivision of the terminal Frasnian linguiformis conodont Zone, revision of the correlative interval of Montagne Noire Zone 13, and discussion of stratigraphically significant associated trilobites

Abstract The terminal zone of the Frasnian standard conodont zonation, the linguiformis Zone has been subdivided into alower part defined by the total range of Palmatolepis linguiformis and an upper part defined by the absence of the nominal species together with the rare occurrences of Pa. ultima (= Pa. praetriangularis) and Ancyrognathus (= Ancyroides) ubiquitus . This subdivision has proved to be effective in sections on different continental blocks. Furthermore, the two subdivisions correlate exactly with subdivisions formally recognized herein of Montagne Noire Zone 13 (13b and 13c). The lower boundary of the linguiformis Zone lies closely below the base of the Upper Kellwasser event bed, the top of which coincides with the end-Frasnian extinction. Nonetheless, prominent gaps in the geographic distribution of Pa. linguiformis have led to some difficulty in the identification of the zone. These gaps have been partially closed by discoveries of the species in areas where it was unknown until now, notably the central Moroccan Meseta and especially the Frasnian/Famennian boundary-stratotype section at Coumiac Upper Quarry in the Montagne Noire, southern France. Nevertheless, anomalous areas of absence, such as the Anti-Atlas Mts. of southern Morocco and western Canada still exist. The trilobite Palpebralia brecciae is widespread in Europe, North Africa, and Australia, providing evidence for correlation of the lower part of the linguiformis Zone in sections where the nominal conodont species has not been found. Analysis of variation in the relative abundance of the principal conodont genera leads to the conclusion that no substantial difference exists in the conodont associations among representative sections in four of the continental blocks.

Strategies of survival during extreme environmental perturbations: evolution of conodonts in response to the Kellwasser crisis (Upper Devonian)

Abstract The size and shape evolution of two conodont genera surviving the Kellwasser period, Icriodus and Palmatolepis , has been quantified using a Fourier analysis of the outline of platform elements. Morphological variations of both genera in response to environmental fluctuations are comparable, although slightly shifted in time, suggesting a differential sensitivity to the environmental variations due to differences in favored habitats. The period of the crisis comprises two anoxic events of increasing importance and impact on the biosphere. For each event, the post-crisis conodont elements are smaller than those found before and exhibit extreme morphs, while the recovery period is characterized by an increase in size and inverse changes in shape; morphological trends are expressed with a lower amplitude during the first anoxic event. Strategies of morphological response are different for the two genera, implying heterochronic shifts or changes in the developmental pathway.

Record of climate-driven morphological changes in 376 Ma Devonian fossils

The Lower and Upper Kellwasser horizons represent two anoxic events that mark the mass extinction at the Frasnian-Famennian (F-F) boundary. Among other groups, conodont animals were severely affected, but the genus Palmatolepis survived with a complete turnover at the F-F boundary. Here the fine morphological variations of the genus Palmatolepis and the sea-surface temperature evolution are quantified in two F-F boundary sections using morphometrics and oxygen isotopic composition of apatite, respectively. In accordance with other F-F sections, the isotope records show two positive excursions of ~1‰ during the Lower and Upper Kellwasser anoxic events. The conodont shape and the oxygen isotopic composition of the genus Palmatolepis are significantly correlated within the Frasnian and Famennian Stages, suggesting a strong environmental influence on the morphology of the feeding apparatus of the conodont animal. We propose that the morphological differences are linked to changes in the trophic position of Palmatolepis: enhanced organic carbon burial, which is supported by global positive carbon isotope excursions in inorganic and organic carbon during both the Lower and Upper Kellwasser events altered the primary biomass production and thus, the subsequent nutrient supply to higher trophic levels. While the carbon and oxygen isotopic shifts are of similar amplitude during Lower and Upper Kellwasser events, the variation of the shape of Palmatolepis during the Upper Kellwasser, i.e., the F-F boundary, is more pronounced than during the Lower Kellwasser.

Search for impact remains at the Frasnian–Famennian boundary in the stratotype area, southern France

In order to detect whether the end-Frasnian worldwide biotic crisis is related to an extraterrestrial impact, the global stratotype section of the Frasnian-Famennian boundary and auxiliary sections within the stratotype area have been examined for impact indicators: iridium. Ni-rich spinel bearing spherules and glassy microtektites. This area is particularly well suited to the search for discrete events because it exhibits biostratigraphically continuous sections of sedimentologically homogenous off-shore deposits. Different environmental settings on oxygenated deep-water seamounts, such as the stratotype section at Coumiac, and in oxygen-depleted depressions (La Serre section) are available. The latter is investigated in more detail because it is the least condensed across the boundary, which is determined by the first occurrence of the typical morphotype of Palmatolepis triangularis, the indicator of the first Famennian conodont biozone. Samples from the biostratigraphically defined boundary and adjacent levels failed to provide significantly high Ir values and no Ni-rich spinel or microtektite has been recovered. This is in contradiction with the results of earlier investigations carried out by H. Geldsetzer on the same section. In contrast, the values of Ir concentrations that we measured are always very low or not detectable. The small overabundances observed in some samples, which are about two orders of magnitude lower than what is currently observed at the Cretaceous-Tertiary boundary, are probably due to the accumulation of the normal flux of cosmic dust during periods of relatively low depositional rates or to a terrestrial origin. At present, we have no evidence that an extraterrestrial impact occurred at the F-F transition.

Palaeoenvironmental changes at Col des Tribes (Montagne Noire, France), a reference section for the Famennian of north Gondwana-related areas

We present detailed biostratigraphy based on conodonts and palaeoenvironmental trends deduced from microfacies and conodont abundance through the Famennian (Late Devonian) at Col des Tribes (Montagne Noire, France). The succession is characterized by micritic limestones deposited in settings oscillating between mid to outer ramp. Facies contain poor fauna, widely dominated by nektonic organisms. This section is complete and one of the most conodont-rich for the Famennian of the north Gondwana-related area. The Upper Kellwasser event (Frasnian–Famennian boundary) and the Hangenberg (Devonian–Carboniferous boundary) have been lithologically identified. They are characterized by decimetre-thick black dysoxic to anoxic argillaceous sediments. The Condroz and annulata events, although not materialized by lithological changes, have been positioned due to the precise stratigraphy. The first event occurred during the deposition of condensed ferruginous facies (griotte limestones) and the second event during the deposition of micrites barren of benthic fauna. The combination of information from both facies and conodont biofacies changes allows a general sea-level curve through the entire Famennian for north Gondwana to be proposed for the first time. At Col des Tribes, the general trend is a slight deepening upwards from triangularis to trachytera zones, then a pronounced shallowing-upwards trend from upper trachytera to praesulcata zones. This curve correlates with the well-known reference curve from Euramerica concerning the late Famennian ( trachytera to praesulcata Zones). There are some discrepancies in minor cycles which can be explained by tectonical phenomena at the onset of the edification of the Variscan belt in Europe.

Adaptation to environmental changes: communities need more time than species

Effects of environmental changes on communities and on species have been studied with two different types of animals: mammals and conodonts. This analysis was conducted on mammals from the ’Ubeidiya Sequence (Israel) dated to the Lower Pleistocene, and on conodonts from the quarry of Coumiac (France) dated to the Frasnian/Famennian boundary (Devonian). The main result of the analysis is that an ecological event can be subdivided. First of all there is a change in the environment that corresponds to a physical signal, the species react to this change, and afterwards the communities respond. Thus, a delay in time can be observed between the reaction of the species and the reaction of the community to an environmental signal.