Kellwasser Horizons Research Articles

The World’s Highest-Grade Cobalt Mineralization at Bou Azzer Associated With Gondwana Supercontinent Breakup, Serpentinite and Kellwasser Hydrocarbon Source Rocks

Cobalt arsenide deposits associated with Neoproterozoic serpentinite in Morocco represent the highest-grade cobalt resource worldwide. Yet, genetic models for their origin remain controversial. We report here mineralogical and geochemical evidence for arsenide-calcite mineralization at Bou Azzer to constrain the temporal framework and identify the geodynamic trigger for mineralization mechanisms. To this end, radiometric ages for ore minerals are paramount for understanding the origin of the Bou Azzer cobalt arsenide deposit. New safflorite (CoAs2) rhenium-osmium (Re-Os) ages are Late Devonian in age: 380.4 ± 2.9 and 373.4 ± 1.2 to 368.1 ± 5.0 million years ago (Ma) for coarse-grained and fine-grained safflorite, respectively. These dates overlap with the timing of break-up of the supercontinent Gondwana, and the building of an arch-and-basin geometry from northern Africa to Arabia. Our findings temporally and spatially contextualize previous knowledge of fluid chemistry and mineralization mechanisms involving a two-fluid mixing. Arsenide mineralization resulted from mixing of a methane-dominated fluid with highly saline basinal brines that leached Os (187Os/188Osinitial = 0.120 ± 0.001), and by corollary cobalt, from Neoproterozoic serpentinite. Carbon and sulfur stable isotope data of ore-stage calcite and arsenides, respectively, show that hydrocarbons acted as the main reductant for mineralization. We speculate that the seawater-derived brines sank into the sedimentary basins adjacent to a carbonate platform with the Bou Azzer serpentinite in its basement in the Late Devonian. In the context of an enhanced geothermal gradient, such brines would have been involved in warm hydrothermal alteration of hydrocarbon source rocks of the local expression of the Kellwasser event in the geological record of present-day Morocco. This warm hydrothermal alteration of hydrocarbon source rocks may have taken place for coarse-grained safflorite mineralization (380.4 ± 2.9 Ma) shortly after ca. 382–381 Ma Lower Kellwasser horizons were deposited, or, for fine-grained safflorite mineralization (373.4 ± 1.2 to 368.1 ± 5.0 Ma) while the Upper Kellwasser horizons of present-day Morocco were being deposited.

Astronomical climate changes trigger Late Devonian bio- and environmental events in South China

One of the five great mass extinctions of the Phanerozoic is the Frasnian–Famennian (F F) mass extinction, for which the causes have not yet been identified. In this study, cyclostratigraphic analysis of two F F transition sections was carried out in South China: the Yangdi section, a marine slope facies, and the Lali section, a marine basin facies. Paleoclimate proxy data collected at high resolution along these sections include magnetic susceptibility and X-ray fluorescence geochemistry (Ca and Fe concentrations). Time series analysis and modeling of the proxy data reveal that frequencies comparable to those of the Earth's long and short orbital eccentricity, obliquity, and precession index characterize the two successions. Metronomic 405-kyr long orbital eccentricity cycles identified along the two sections were used to construct a floating astronomical time scale across the F F transition, revealing that 1000 kyr separates the Lower and Upper Kellwasser horizons (LKH and UKH), and ~1600 kyr separates the maximum values of the LKH and UKH δ 13 C excursions. The estimated duration of the UKH is 150 kyr, during which the first, second, and third extinctions of the F F biotic crisis lasted 120 kyr, 20 kyr, and 10 kyr, respectively. Sedimentary noise models of the magnetic susceptibility and Ca concentration time series indicate that changes in sedimentary noise correspond to sea level variations. Modeling suggests that the long orbital eccentricity cycles controlled sea surface temperatures, and that third-order eustatic changes were forced by the combined orbital eccentricity and obliquity variations. Finally, we propose an “astronomical climate change” model as a defining mechanism of the F F biotic crisis. • Milankovitch cycles were recognized from the Upper Devonian Yangdi and Lali sections. • Two FATSs (~4.2 and 4.8 myr) were constructed across the F F transition. • Timing and pacing of the F F mass extinction were determined based on the FATS. • Astronomical forced sea level and climate changes were detected in F F transition. • Astronomical forcing of climate triggers the F F mass extinction.

Significant fluctuation in the global sulfate reservoir and oceanic redox state during the Late Devonian event.

Ocean sulfate concentration might have fluctuated greatly throughout the Earth's history and may serve as a window into perturbations in the ocean-atmosphere system. Coupling high-resolution experimental results with an inverse modeling approach, we, here, show an unprecedented dynamic in the global sulfate reservoir during the Frasnian-Famennian (F-F) boundary event, as one of the "Big five" Phanerozoic biotic crises. Notably, our results indicate that, in a relatively short-time scale (∼200 thousand years), seawater sulfate concentration would have dropped from several mM before the Upper Kellwasser Horizon (UKH) to an average of 235±172 μM at the end of the UKH (more than 100 times lower than the modern level) as the result of evaporite deposition and euxinia, and returned to around mM range after the event. Our findings indicate that the instability in the global sulfate reservoir and nutrient-poor oceans may have played a major role in driving the Phanerozoic biological crises.

Mercury isotope evidence for regional volcanism during the Frasnian-Famennian transition

Mercury (Hg) concentrations and isotopes are widely used as proxies for volcanic inputs to paleodepositional systems. Enhanced volcanism during the Late Devonian was previously mainly inferred in the western Paleo-Tethys Ocean region from high Hg enrichments around the Upper Kellwasser horizon (UKW) and Frasnian-Famennian Boundary (FFB). In this study, we generated high-resolution Hg concentration data for four UKW-FFB sections (Walnut Creek and Dupont-GHS in the U.S.A., and Yangdi and Nandong in South China) and Hg isotope data for two of them (Walnut Creek and Nandong) in order to test the geographic distribution and stratigraphic range of volcanic inputs during the Frasnian-Famennian transition. Within the uppermost Frasnian Palmatolepis linguiformis Zone and the lowermost Famennian Lower Pa. triangularis Zone, Nandong (Paleo-Tethys Ocean) exhibits enrichment factors (HgEF > 2) and mass independent fractionations (MIF) of odd isotopes (Δ199Hg ∼0 ‰) that are consistent with significant volcanic inputs. Positive Δ199Hg values and an absence of Hg enrichments in the equivalent stratigraphic intervals at Walnut Creek and Dupont-GHS (peri-Panthalassic region) indicate that Late Devonian volcanism was a regional rather than a global phenomenon, and that it may have been associated with subduction zones and/or a large igneous province (LIP) of limited size in the Paleo-Tethyan region rather than with a major continental LIP of potentially global significance.

Isotopic and biogenic silica evidence for eutrophic conditions in South China during the Frasnian-Famennian biotic crisis

The Late Devonian Frasnian-Famennian (F F) transition at ca. 367 Ma was a key event in Phanerozoic Eon involving a massive biotic crisis, which coincided with a positive carbon isotope anomaly and large climate changes associated with eustatic fluctuations. The details underpinning the initiation of the mass extinction, however, remain problematic. To better understand this scientific issue, we conducted a time series of biogeochemical studies that included C N isotopes and biogenic silica (BSi) from the F F transition in the Yangdi section, South China. The carbon isotope data accurately constrain the Upper Kellwasser Horizon (UKH) and Lower Kellwasser Horizon (LKH) in this section. A short-term decrease in carbon isotope fractionation in the F F transition indicates a decreased p CO 2 level in the LKU and UKH, while the increased fractionation in the uppermost UKH was caused by the rapidly negative δ 13 C org excursion, likely as the result of enhanced inputs from chemoautotrophs. On the other hand, the nitrogen isotope data suggest that the surface water had limited availability of dissolved N before and after the F F boundary due to enhanced benthic denitrification; these conditions led to a supply of external nitrogen that was likely driven by persistent diazotrophy. Increased sedimentary δ 15 N (up to +7.3‰) during the UKH suggests a rapidly amplified nitrate pool associated with water column denitrification, implying intensification of marine productivity and expansion of dysoxic-anoxic conditions linked to intensified oceanic circulation. In addition, BSi in sediments has the potential to evaluate historic changes in primary productivity of oceans and the maxima of BSi mass values (up to 1.96%) are clearly documented in the upper part of the UKH. Insofar as the elevated abundance of biolimited nitrogen coincided with strongly increased marine phytoplankton productivity at the F F boundary, we speculate that the nutrient-driven eutrophic conditions in surface waters associated with attendant redox changes may have been the main killing mechanism in the mass extinction. • The cooling event in the UKH coincided with declined △ 13 C and elevated δ 15 N. • We attribute the low δ 15 N values to enhanced microbial nitrogen fixation. • Sedimentary δ 15 N increases are indicative of an amplified N pool. • Siliceous organism blooms are consistent with positive δ 13 C carb and δ 15 N shifts. • The F F crisis was likely the result of eutrophication in shallow water.

A Devonian age for the Sarhlef Formation (Jebilet Massif, Morocco) – evidence from new biostratigraphic data based on metamorphosed conodonts

The Sarhlef Formation, which hosts the massive sulphide deposits of the Moroccan Meseta, has been considered as Carboniferous in age. Moreover, palynological analysis of the black shales hosting the Draa Sfar deposit (Central Jebilet) has allowed to restrict the age of the Sarhlef Formation to the late Visean (Asbian). However, this age is not compatible with geochronological data of various magmatic intrusions present in Central Jebilet. New biostratigraphic results based on metamorphosed conodonts from 34 samples (three cores, DS170, DF306 and DS125) of the Draa Sfar deposits provide a new biostratigraphic framework of the Sarhlef Formation. The collected fauna is affected by pyritization, silicification and tectonic deformation. Some non-pyritised or partially pyritized conodonts show a conodont color alteration (CAI) change varying from black to translucent (CAI 5 to CAI 8). The conodont fauna comprise representatives of the genera Icriodus, Polygnathus, Ancyrodella, Palmatolepis, Ancyrognathus, Ancyroides, and Palmatolepis showing that age spans from the Frasnian to Late Famennian. This age is more compatible with the radiometric data of the intrusive magmatic rocks. In core DS170, the Upper Kellwasser Horizon at the Frasnian-Famennian boundary has been identified by lithological characteristics and stratigraphy. The proposed Late Devonian age of the Sarhlef Formation provides new insights on the geological evolution and stratigraphy of the entire region.

Anchoring the Late Devonian mass extinction in absolute time by integrating climatic controls and radio-isotopic dating

The Devonian Frasnian–Famennian (F–F) boundary marks one of the five main extinction intervals of the Phanerozoic Aeon. This time was characterized by two pulses of oceanic anoxia, named the Lower and Upper Kellwasser events, during which massive marine biodiversity losses occurred. This paper presents high-resolution magnetic susceptibility, X-ray fluorescence elemental geochemistry and carbon isotope datasets obtained from the Steinbruch Schmidt F–F boundary section (Germany). These records lead to an astronomical time calibration of the environmental changes associated with the two ocean anoxia pulses. Cyclostratigraphic interpretation indicates deposition of the black argillaceous Lower and Upper Kellwasser horizons over ~ 90 and ~ 110 kyr, respectively; approximately equivalent to the duration of one short eccentricity cycle. This study confirms that the succession of events within the Upper Kellwasser event is paced by obliquity, under a low-eccentricity orbit. Hence, astronomical insolation forcing likely contributed to the expansion of ocean anoxia and other environmental perturbations associated with these two crises. The new floating chronology established for the Steinbruch Schmidt section is anchored in numerical time by means of a radio-isotopic date, obtained from a bentonite layer interbedded between the two Kellwasser horizons. After anchoring, this time scale gives a high-precision age of 371.870 ± 0.108 Ma for the F–F boundary.

Astronomical time calibration of the Upper Devonian Lali section, South China

Abstract The Late Devonian Period was a critical geological interval, affected by numerous significant global bio- and environmental events, including the Kellwasser, annulata and Hangenberg events, and a major climate transition leading to the Carboniferous ice ages. The timing and pattern of these events are unclear due to lack of the precise time scale. In this study, an analysis of calcium concentration series was carried out on the Upper Devonian cyclostratigraphic slope-facies of the Lali section, South China. Objective testing of the section indicates the presence of astronomical forcing: long orbital eccentricity, short eccentricity, obliquity, and precession index. Two independent astrochronologies were developed for the Ca series and compared with spectral analysis. The age of the Devonian–Carboniferous boundary (358.9 ± 0.4 Ma) was selected as an “anchor”, and an Astronomical Time Scale (ATS) was constructed from the late Frasnian to early Tournaisian based on 405-kyr cycles. The ATS indicates a duration of the Famennian Stage of 14.4 ± 0.28 myr, and assigns 373.3 ± 0.49 Ma to the Frasnian–Famennian boundary. The durations of the Famennian conodont zones are also indicated. The timing of Famennian bio- and environmental events also can be established by the ATS. The Upper Kellwasser horizon starts at 373.4 Ma and is constrained to a duration of 100-kyr; a sharp positive carbon isotope excursion CIE, +3‰ recognized across the Frasnian–Famennian boundary lasts 300 kyr. The Lower and Upper annulata events each last 50-kyr with positive CIEs; and the ages of two CIE peaks are 361.7 Ma and 361.3 Ma, respectively. Five transgressive events, including the Nehden, Enkeberg, Dasberg, Strunian, and Hangenberg transgressions occur at 370.6 Ma, 368 Ma, 360.4 Ma, 360.1 Ma, and 358.9 Ma, respectively. Five regressive events i.e., the linguiformis, prima, Condroz, velifer, and Derwer regressions occur at 373.4 Ma, 371 Ma, 369 Ma, 362.3 Ma, and 359.2 Ma, respectively.

Phosphorus-cycle disturbances during the Late Devonian anoxic events

The Late Devonian was marked by repeated faunal crises and episodes of geographically widespread marine anoxia, and featured one of the ‘Big Five’ mass extinctions of the Phanerozoic Aeon during the Frasnian–Famennian transition. However, the processes responsible for causing the numerous anoxic events remain unclear. This study highlights the occurrence of disturbances to the phosphorus cycle during several Late Devonian crises by investigating sedimentary concentrations of the element (Ptot) as a tracer of nutrient influx, as well as its ratio with total organic carbon (TOC) to infer the recycling of the element from marine sediments. Increased TOC/Ptot ratios in the Frasnian–Famennian Lower and Upper Kellwasser horizons and upper Famennian Annulata and Hangenberg levels suggest that such nutrient recycling occurred across extensive areas of the marine shelf in Laurentia and both Rheic Ocean margins at those times, helping to sustain reducing conditions in those environments. Elevated Ptot values in the Upper Kellwasser, Annulata, and Hangenberg levels are consistent with an enhanced nutrient influx as the initial trigger for the anoxia. Correlation of phosphorus trends with other geochemical indicators of weathering/detrital influx (osmium-isotope, silicon/aluminum, and titanium/aluminium ratios) support a scenario in which terrestrial runoff provided these nutrients both to marine shelves and the oceanic inventory. Upwelling of oceanic deep-water bodies may have then brought the phosphorus to areas that had not featured major direct inputs of terrigenous material. The exception is the Lower Kellwasser Event, during which there was no increase in phosphorus delivery to marine areas and no evidence for terrestrial influx at the studied sections, invoking a different mechanism for the development of water-column anoxia. Clearly, the Late Devonian marine realm was unusually susceptible to becoming anoxic through various possible triggers, including nutrient influx from land and/or deep-water upwelling, and the recycling of phosphorus from newly deposited sediments.

The micrometeorite flux to Earth during the Frasnian–Famennian transition reconstructed in the Coumiac GSSP section, France

We have reconstructed the distribution of extraterrestrial chrome spinels in a marine limestone section across the Frasnian–Famennian stratotype section at Coumiac in southern France, providing the first insights on the types of micrometeorites and meteorites that fell on Earth at this time. The data can test whether the small cluster of roughly coeval, large impact structures is related to an asteroid breakup and shower with possible bearings also on the late Devonian biodiversity crisis. A total of ∼180 extraterrestrial spinel grains (>32 μm) were recovered from 957 kg of rock. Noble-gas measurements of individual grains show high solar-wind content, implying an origin from decomposed micrometeorites. Element analyses indicate a marked dominance of ordinary chondritic over achondritic grains, similar to the recent flux. The relation between H, L and LL meteorites is ∼29–58–13%, similar to the late Silurian flux, ∼31–63–6%, but different from the distribution, ∼45–45–10%, in the recent and the Cretaceous flux. Our data show no indication of a generally enhanced late Devonian micrometeorite flux that would accompany an asteroid shower. However, in a single limestone bed that formed immediately before the Upper Kellwasser horizon, that represents the main end-Frasnian species-turnover event, we found an enrichment of ∼10 ordinary chondritic grains (>63 μm) per 100 kg of rock, compared to the ∼1–3 grains per 100 kg that characterise background. The anomalously abundant grains are of mixed H, L and LL types and may be related to an enhanced flux of extraterrestrial dust during postulated minima in both the 405 ka and 2.4 Ma Earth-orbit eccentricity cycles at the onset of the Upper Kellwasser event. In the present solar system the dust accretion at Earth is the highest at eccentricity minima because of the spatial distribution of dust bands of the zodiacal cloud. Besides this small grain anomaly the data here and in previous studies support a stable meteorite flux through the late Silurian and Devonian, in contrast to the mid-Ordovician, when achondritic meteorites that are rare on Earth today were common, followed by the influx of a flood of debris related to the breakup of the L-chondrite parent body. Our accumulated data for six time windows through the Phanerozoic indicate that the ordinary chondrites make up a major fraction in the meteorite flux since at least the mid-Ordovician. We note that the sources in the asteroid belt of the H and L meteorites, the two most common types of meteorites today and through much of the Phanerozoic, remain elusive.

Pulses of enhanced continental weathering associated with multiple Late Devonian climate perturbations: Evidence from osmium-isotope compositions

Anomalously high rates of continental weathering have frequently been proposed as a key stimulus for the development of widespread marine anoxia during a number of Late Devonian environmental and biospheric crises, which included a major mass extinction during the Frasnian–Famennian transition (marked by the Upper and Lower Kellwasser horizons). Here, this model is investigated by presenting the first stratigraphic record of osmium-isotope trends (187Os/188Os) in upper Devonian strata from the Kowala Quarry (Holy Cross Mountains, Poland). Changes in reconstructed 187Os/188Os seawater values to more radiogenic compositions are documented at the base of both the Lower (~0.42 to ~0.83) and Upper (~0.31 to ~0.81) Kellwasser horizons characteristic of the Frasnian–Famennian transition, and additionally within upper Famennian shales that record a more minor environmental perturbation known as the Annulata Event (~0.20 to ~0.53). These shifts indicate the occurrence of extremely enhanced continental weathering rates at the onsets of the Kellwasser crises and during the later Annulata Event. The similarity of 187Os/188Os values in this study from Frasnian–Famennian boundary and lower Famennian strata (between 0.4 and 0.5) to those from North American stratigraphic equivalents suggests that the 187Os/188Os values record global trends. These findings support a causal relationship between increased continental weathering (and thus, nutrient supply to the marine shelf) and the environmental perturbations that occurred during numerous Late Devonian events, including both of the biospherically catastrophic Kellwasser crises as well as other, less severe, oceanic anoxic events.

Did climate changes trigger the Late Devonian Kellwasser Crisis? Evidence from a high-resolution conodont [formula omitted] record from South China

The impact of climate change on biodiversity has been the focus of studies on the Phanerozoic biotic crises. However, it is still controversial whether climate change has caused the Frasnian–Famennian (F–F) biotic crisis since there is no unequivocal view on the global climate change during this critical period. In order to reconstruct palaeotemperatures during the F–F transition in South China (eastern Palaeotethys), a high-resolution oxygen isotope (δ18OPO4) record was obtained based on 104 measurements of conodont apatite from the Yangdi section. The oxygen isotope record based on mono-generic samples reveals an increase in δ18OPO4 by 0.7‰ in the Lower Kellwasser horizon (LKH) and 1.4‰ in the Upper Kellwasser horizon (UKH), translating into low-latitude surface water cooling of ∼3 °C and ∼6 °C, respectively. These two δ18OPO4 shifts agree well with the time-equivalent δ18OPO4 records from the western Palaeotethys, suggesting that two climate cooling pulses in conjunction with the deposition of the Kellwasser horizons are a global signal during the F–F transition. The positive shift of δ18OPO4 coincides with positive excursion in carbon isotope of carbonates (δ13Ccarb) in the UKH, indicating that enhanced burial of organic carbon resulted in a drop in atmospheric pCO2 and thus global climate cooling. Cooling started immediately before the Upper Kellwasser biotic crisis, with the lowest temperature documented at the top of the UKH. Climate cooling during the deposition of the LKH is observed in conjunction with the sharp decline in metazoan reefs. The coincidence of cooling and the Kellwasser biotic crisis suggests that global cooling played a major role in the collapse of ecosystems.

The influence of the Late Devonian Kellwasser events on deep-water ecosystems: Evidence from palaeontological and geochemical records from South China

The Late Devonian Frasnian–Famennian (F–F) event, consisting of the Lower and Upper Kellwasser events, had a severe impact on shallow-water ecosystems, while the effect on deep-water ecosystems is ambiguous. In order to recognize the Kellwasser events and to evaluate their influences on the deep-water ecosystems, a newly refined conodont biostratigraphic framework spanning the Lower Palmatolepis rhenana Zone to the Pa. delicatula platys Zone has been established in the Nandong section (basinal facies), Guangxi, South China. The Upper Devonian Lower and Upper Kellwasser horizons (LKH and UKH) have been recognized in the deep-water facies on the basis of lithological variations, biological turnovers, and positive carbon isotope (δ13Ccarb) excursions. The LKH is characterized by the enrichment of marine nektonic communities (e.g. conodonts, homoctenids, and entomozoacean ostracods), whereas the top of the UKH is barren of fossil organisms, suggesting that the Upper Kellwasser event had a more severe effect on nekton than the Lower Kellwasser event. The Upper Kellwasser (UK) δ13Ccarb positive excursion lags behind the enrichment of Cu, Ni, Mo and V, indicating that the UK δ13Ccarb positive excursion was probably related to elevated primary productivity, which induced oxygen depletion in the water column. The UK δ13Ccarb positive excursion peak prior to the F–F boundary indicates a more limited effect of riverine influx on the deep-water setting, which suggests that the regional palaeogeographic configuration played an important role in the carbon cycle.

Zinc and strontium isotope evidence for climate cooling and constraints on the Frasnian-Famennian (~372 Ma) mass extinction

The Frasnian-Famennian (F-F) mass extinction is one of the “Big Five” biotic crises during the Phanerozoic, whereas the cause of this catastrophic event still remains debated. Here we present for the first time high-resolution Zn and Sr isotope data obtained by a sequential leaching procedure for carbonate rocks across the F-F boundary from the Fuhe section in South China. Both δ66Zn and 87Sr/86Sr values rapidly increase in the Lower and Upper Kellwasser Horizons and Palmatolepis delicatula platys to P. minuta minuta conodont zones, and the positive isotope shifts are exactly located in the same horizons characterized by low ∆13C values (equivalent to low atmospheric PCO2 level, reflecting climate cooling) and sea-level fall stages. The strong temporal coupling of δ66Zn values raise with elevated 87Sr/86Sr ratios is most likely to have been caused by input of isotopically heavy Zn and radiogenic Sr from carbonate-dominated weathering, and/or uptake of isotopically light Zn into primary producers during climate cooling. Climate cooling could cause relative sea-level falls resulting in widespread exposure of carbonate platform, enhance carbonate influx proportion through differential weathering of carbonate and silicate rocks, and cause greater productivity by promoting oceanic thermohaline circulation and enhancing upwelling of bio-limiting nutrients. All of these processes would have contributed to the positive shifts of δ66Zn as observed. This proposed scenario is in accordance with the fact that the F-F mass extinction was almost limited to the shallow-water tropical stenothermal faunas, whereas the high latitude and deep-water faunas were less affected. Our data support and represent the composite effect of a series of events linked to climate cooling which may have induced this catastrophic event.

A multiproxy analysis of the Frasnian-Famennian transition in western New York State, U.S.A

The Late Devonian Frasnian-Famennian transition is well exposed along Walnut Creek in the western New York State region of the Appalachian Basin. The studied stratigraphic interval encompasses approximately 29m of section from the organic-rich Pipe Creek Formation, the equivalent of the Lower Kellwasser carbonaceous horizon of Europe, to a thin black shale layer at the top of the overlying Hanover Formation that correlates with the Upper Kellwasser horizon. The Walnut Creek section comprises five depositional cycles that are likely eustatic in nature, perhaps modulated by tectonic dynamics of the basin. Clastic proxy trends (Ti/Al, Zr/Al) suggest that deposits spanning the Frasnian-Famennian boundary (FFB) accumulated in association with rising sea level, a trend documented from elsewhere in the basin. Increasing Si/Al across the FFB may reflect elevated post-extinction biosiliceous productivity documented from Lower Famennian deposits elsewhere. The Walnut Creek section includes two positive δ13Corg excursions of >3‰, the stratigraphically lower event associated with the Pipe Creek Formation and the highest excursion approximately coincident with the Upper Kellwasser equivalent black shale layer. These isotopic events are interpreted to record episodes of increased global productivity and organic carbon sequestration at the end of the Frasnian Stage. Redox proxy trends (Mo and U enrichment and pyrite framboid size-distributions) documented from black shale associated with the positive excursions suggest sediment accumulation under suboxic to intermittently anoxic or euxinic bottom-water conditions in an unrestricted open-marine setting. The paleoclimate history of the end of the Frasnian based on Th/K data is one of fluctuating atmospheric conditions, including a brief cooling trend at the FFB. Though the Walnut Creek section sheds no light on the triggering mechanism or mechanisms of the Late Devonian extinction, inferred climate fluctuations associated with the FFB reflected in the Th/K data may have contributed to the severity of the biological crisis.

Depositional conditions during the Lower Kellwasser Event (Late Frasnian) in the deep-shelf Łysogóry Basin of the Holy Cross Mountains Poland

Integrated biostratigraphical, microfacial and geochemical studies of the Lower Kellwasser Event in the Plucki succession (southern Poland) provide details about redox conditions during the deposition of this horizon in the deep-shelf Łysogory basin of the Holy Cross Mountains. The environment is characterized by calm sedimentation and soft, muddy carbonate substrate. However, microfacies changing from wackestones to grainstones, the presence of crushed or current-oriented nautiloid shells and the occurrence of redeposited material from shallow-water Dyminy Reef environments (such as calcispheroids, algae and girvanellid cyanobacteria) suggest episodes of a higher-energy regime. Uranium/thorium ratios indicate that bottom-water redox conditions changed periodically from being mainly anoxic in the middle part of the Lower Kellwasser Horizon to dysoxic in the lower and upper parts. During a short-term episode of bottom-water ventilation, the seafloor was rapidly colonized by a dense assemblage of opportunistic buchiolid bivalves, which suffered mass mortality upon the return to anoxic conditions. A very rich concentration of cephalopods and homoctenids may be regarded as reflecting a bloom of high-density populations during high-productivity events. Similarly, they suffered mass mortality when episodically increasing anoxia/euxinia reached the upper part of the water column. The Late Frasnian inorganic carbon isotope records in the Plucki section show a positive shift with a maximum amplitude of 3‰. This enrichment in δ13C can be correlated with the deposition of the Lower Kellwasser Horizon and reflects the expansion of anoxic and probably high-productivity regimes.

Timing and patterns of the Frasnian–Famennian event: Evidences from high-resolution conodont biostratigraphy and event stratigraphy at the Yangdi section, Guangxi, South China

Continuous centimeter-scale sampling for conodonts with microfacies and carbon isotope analysis has been carried out across the Frasnian–Famennian (F–F) transition at the Yangdi section, Guangxi, South China. The newly revised conodont biostratigraphy reveals a succession of six zones, ranging from the upper Palmatolepis rhenana zone to lower Palmatolepis crepida zone in ascending order. Based on a detailed analysis of conodont abundance and diversity changes through the section, two intervals involving significant conodont faunal changes were recognized in the uppermost Palmatolepis linguiformis zone at the Yangdi section. The first conodont change occurred at the transition between the lower and upper Pa. linguiformis zone and is marked by the disappearance of Pa. linguiformis, accompanied by the successive disappearances of Pa. eureka, Pa. rhenana, Pa. nasuta, Pa. gigas, Ad. ioides, and Ad. nodosa. The second conodont change appeared in a horizon equivalent to the uppermost Upper Kellwasser Horizon (UKH) and is indicated by the elimination of nearly all Frasnian palmatolepids, ancyrodellids, and polygnathids. Furthermore, the second conodont change is coincident with the decimation of ostracods and tentaculitids and is synchronous with the turnovers of conodont biofacies. In addition, the onset of the δ13Ccarb perturbation is a little earlier than the second conodont change, with the maximum amplitude of excursion right at the start of the Famennian. Therefore, we conclude that the second conodont change represents the F–F mass extinction based on the temporal correlation among δ13Ccarb excursions, pelagic faunal changes, and conodont biofacies turnovers during the F–F boundary interval. According to the high-resolution conodont biostratigraphy together with cyclostratigraphy, the uppermost UKH and whole UKH contain 1 and 12 bundle cycles, respectively. Thus, the time span of the mass extinction and the entire Upper Kellwasser crisis lasted 16–18kyr and ~200kyr, respectively.

Kellwasser horizons, sea-level changes and brachiopod–coral crises during the late Frasnian in the Namur–Dinant Basin (southern Belgium): a synopsis

Abstract In Belgium, the Lower Kellwasser Event (LKW) corresponds to the relative sea-level maximum of the first (‘Aisemont sequence’ (AS)) of the two late Frasnian third-order sequences that are recognized here, but the Upper Kellwasser Event (UKW) may have been triggered by a series of tsunamites. The end of the middle Frasnian carbonate platform and reefs is caused by the sea-level drop and emersion of the last middle Frasnian third-order sequence (‘Lion sequence’) in the Lower rhenana Zone. The end of the ‘Petit-Mont’ mudmound growth during the transgressive (TST) and highstand (HST) systems tracts of the AS was caused by sea-level fall and emersion at the top of this sequence. The coral and brachiopod extinction in the Upper rhenana Zone, during the second late Frasnian third-order sequence (‘Lambermont sequence’ (LS)), is progressive and due to the widespread development of the dysoxic and anoxic facies, before the UKW. Only the LS TST has been identified. No sea-level fall has been recognized in relation to the UKW or near the Frasnian–Famennian boundary. The late Frasnian extinctions are more likely to be related to the decrease in the atmospheric oxygen rate and its impact on marine environments and, to complete, the UKW.

Frasnian (Late Devonian) conodonts and environment at the northern margin of the Algerian Sahara platform: the Ben Zireg section

AbstractThe Ben Zireg anticline NW of Bechar yields the currently most-important Frasnian succession on the northern margin of the Algerian Sahara. It represents 26.5 m of calcilutites which are attributed to the middle–late Frasnian. As the early Frasnian is not represented, the succession rests conformably on undifferentiated, probably late Givetian, substrate. Fine-scaled conodont biostratigraphy reveals a continuous sequence of Montagne Noire Zones 5–13, superseded by the earliest Famennian LowertriangularisZone. The deposits are organized into 15 sedimentary cycles that are initially condensed and become dilated upwards. Conodont biofacies indicate an overall deepening towards the top of the section with an episode of slight shallowing at the MN11/12 transition marked by an increase of ancyrodellids, which is also seen in the Marhouma section of the Ougarta region (SW Algeria). Homogeneous calcareous microfacies with only a few shaly intercalations determine a predominantly oxygenated depositional environment on an outer platform, submarine rise or ramp setting. This is also indicated by relatively low sedimentation rates which are similar in selected sections from the Tafilalt platform, and contrast with those from the Marhouma trough and the Maider basin. Unlike other Frasnian successions south of the Atlas Fault, but similar to the Moroccan Meseta, the Upper Kellwasser horizon is clearly discriminated at Ben Zireg by an outstanding occurrence of black shales on top of the oxygenated latest Frasnian deposits.

Late Devonian conodonts and event stratigraphy in northwestern Algerian Sahara

Conodonts recovered from the Late Devonian South Marhouma section comprise 5 genera with 31 species (3 undetermined). The fauna establishes the presence of MN Zones 5, undifferentiated 6/7, 8/10 for the Middle Frasnian, the MN Zones 11, 12, 13 for the Upper Frasnian as well as the Early through Late triangularis Zones in the basal Famennian. The outcropping lithological succession is one of mostly nodular calcilutites alternating with numerous marly and shaly deposits, which, in the lower and upper part, comprise several dysoxic dark shale intervals. Among these the Upper Kellwasser horizon can be precisely dated and as such the presence of the terminal Frasnian Kellwasser Event is recognized for the first time in Algeria. Both the Middlesex and Rhinestreet Events cannot yet be precisely located, but supposedly occur among the dark shale horizons in the lower part of the section. However, their assignment to a precise level has so far not been established. Though poor in conodont abundance the South Marhouma section provides first evidence of the presence of several Montagne Noire conodont zones within the so far widely unstudied Frasnian of the Ougarta Chain. As such it is considered representative for the northwestern Algerian Saoura region.