Wenlock Boundary Research Articles

Marine redox dynamics and biotic response to the mid-Silurian Ireviken Extinction Event in a mid-shelf setting

The early Silurian Llandovery–Wenlock boundary interval is marked by significant marine perturbations and biotic turnover, culminating in the Ireviken Extinction Event and the Early Sheinwoodian Carbon Isotope Excursion. Here, we apply multiple independent redox proxies to the early Wenlock Buttington section, which was deposited in a mid-shelf location in the Welsh Basin, UK. To account for the regional geochemical variability in marine sediments due to factors such as sediment provenance, we first define oxic baseline values for the Welsh Basin, utilizing deeper water, well-oxygenated intervals of late Llandovery age. Our approach documents unstable, oscillating redox conditions on the mid-shelf at Buttington. We suggest that these dynamic redox fluctuations are likely to relate to changes in the position of the chemocline or a migrating oxygen minimum zone. Benthic biota, such as trilobites, brachiopods, bivalves and gastropods, appear to have been relatively unaffected by fluctuating oxic-ferruginous conditions, but were more severely impacted by the development of euxinia, highlighting the inhibiting role of toxic sulfides. By contrast, the redox perturbations appear to have placed extreme stress on graptolites, causing many extinction losses regardless of the specific development of euxinia. Supplementary material: The geochemical data for the Buttington section is available at https://doi.org/10.6084/m9.figshare.c.7165009 Thematic collection: This article is part of the Chemical Evolution of the Mid-Paleozoic Earth System and Biotic Response collection available at: https://www.lyellcollection.org/topic/collections/chemical-evolution-of-the-mid-paleozoic-earth-system

Age of the Silurian Wuxiahe Formation in Langao, Northwest China: New conodont data

A recent study of conodonts from the Wuxiahe Formation (lower to middle part) in the Ziyang-Langao region suggested its age of middle Telychian (Llandovery) to lower Sheinwoodian (Wenlock), contradicted by subsequent graptolite studies indicating an age of late Telychian for the same interval. New samples from the Qiaoxi section for conodonts to re-access the age of the Wuxiahe Formation collected in this study show that the lower to middle part of the formation belongs to the Pterospathodus amorphognathoides amorphognathoides Biozone, suggesting the age of late Telychian; thus, the Llandovery–Wenlock boundary in the section is most probably higher than previously estimated, but its precise position is not determined since the identification of the Wenlock graptolite Cyrtograptus cf. lundgreni in the section is to be further confirmed. Based on the conodont faunas recognized in the Qiaoxi and Tianwancun sections, the base of the Wuxiahe Formation in the Ziyang-Langao region is diachronous, i.e., not higher than the upper Telychian Pterospathodus amorphognathoides amorphognathoides Biozone at Qiaoxi, but not lower than the lower Sheinwoodian Kockelella ranuliformis Biozone at the Tianwancun setion.

Geochemical evidence for expansion of marine euxinia during an early Silurian (Llandovery–Wenlock boundary) mass extinction

Repeated biotic crises have become the hallmark for the Silurian with the three most significant marine turnover events being related to dramatic global environmental perturbations. Causal mechanisms linking these marine extinction events with positive carbon isotope (δ13C) excursions, paleoceanographic change, and climate remain poorly constrained. Here, we examine the positive δ13C excursion across the Llandovery/Wenlock boundary, and the associated Ireviken extinction event. This positive δ13C excursion has been interpreted to reflect a major change in global oceanographic-climate state and enhanced organic carbon burial. New geochemical data from two paleocean basins have been analyzed to determine local and global redox conditions using carbon- and sulfur-isotopes (δ34S), and iodine (I/Ca+Mg) proxies. The high-resolution δ13C and δ34S data from both sections show positive excursions indicative of global perturbations to each of these elemental cycles, with minimal temporal offsets between the two systems. Numerical box modeling of δ13C and δ34S data indicates that these isotopic shifts can be generated by significant increases in the burial of organic carbon and pyrite, which are most likely due to enhanced burial under euxinic (anoxic and sulfidic) conditions. Independently, I/(Ca+Mg) values point to locally anoxic bottom waters in the distal and deeper basinal setting in Nevada before, during, and after the Ireviken positive δ13C excursion. I/(Ca+Mg) values in the proximal shelf setting in Tennessee show relatively oxic waters during the onset of peak δ13C values, after which bottom-water oxygen concentrations dropped throughout the remainder of the excursion. This multiproxy paleoredox dataset provides the first direct evidence for local and global expansion of reducing marine conditions coincident with the Silurian biotic event and positive δ13C excursion. Integration of these geochemical data for local- and global-scale changes in marine redox conditions with the paleontological data and evidence for eustatic sea-level rise points toward a shoaling of anoxic and/or euxinic waters onto the shelf as a driver for the Ireviken extinction event.

Mass occurrence of the large solitary rugose coral Phaulactis angusta at the boundary Lower/Upper Visby Formation in the Silurian of Gotland, Sweden: palaeoecology and depositional implications

The boundary between the Lower and Upper Visby formations on Gotland (Sweden), which roughly correlates with the Llandovery–Wenlock boundary, is characterised by a mass occurrence of the large solitary rugose coral Phaulactis angusta. This coral-rich layer can be followed for some 50 km along the north-west coast of Gotland. The mass occurrence coincides with the onset of a pronounced, globally recognised δ13C and δ18O excursions in the lower Sheinwoodian. Altogether 31 specimens of P. angusta from eight localities were examined. We demonstrate that the corals preferred to grow in an upright position. Attachment structures were not observed, and therefore a liberosessile life strategy is suggested. Some corallites show an abrupt change in growth direction accompanied by a decrease in diameter. These features mostly occur in later growth stages and may be explained by disturbances in the growth due to toppling or tilting of the corals. Many specimens were able to change their growth axes, but finally died after such an event. Most specimens in the outcrops show a flattened upper surface, which is interpreted as a result of post-mortem abrasion. Epibiontic bryozoan encrustation took place mostly while the corals where alive, whereas stromatoporoid encrustation and Trypanites borings occurred post-mortem. The repeated growth, tilting and erosion of the corallites of P. angusta demonstrate that the Phaulactis layer represents a condensed interval, characterised by multiple pulses of sediment accumulation and erosion.

Microfossils across the Llandovery–Wenlock boundary in Ziyang–Langao region, Shaanxi, NW China

A total of 80 samples were collected primarily for chitinozoans from the Wuxiahe Formation (late Llandovery–early Sheinwoodian) in the Ziyang–Langao region, Shaanxi Province, Northwest China. Besides chitinozoans, several other microfossil groups have been recovered unexpectedly, including conodonts, melanosclerites, acritarchs, and scolecodonts. Colonized acritarchs and club-shaped melanosclerites appear sporadically. Among the rare recorded chitinozoans, Angochitina longicollis is a Telychian–early Sheinwoodian species occurring throughout the studied interval of the Wuxiahe Formation at the Bajiaokou A section of Ziyang County. Conodonts of the late Telychian Pterospathodus eopennatus, Pterospathodus celloni, Pterospathodus a. amorphognathoides biozones and the early Sheinwoodian Pterospathodus p. procerus Biozone are identified at the Qiaoxi section of Langao County, where they co-occur with graptolites. The occurrences of melanosclerites and turbidite deposits support an open marine environment during late Llandovery to early Wenlock in the study region. Based on the recovered microfossils, the Llandovery–Wenlock boundary seems to be confined within a 0.73m-thick interval at the Qiaoxi section.It is the first time that microfossils, especially conodonts, have been recorded in the Llandovery–Wenlock transitional interval in the Ziyang–Langao region, Northwest China. The discovery of these microfossil groups supplements biostratigraphic information in the Ziyang region, where a possible GSSP candidate for the base of the Wenlock Series is located. The potential input of these microfossil groups in defining the Llandovery–Wenlock boundary is discussed.

Пограничные отложения лландовери и венлока в опорном разрезе силура центральной части гряды Чернышева

Пограничные отложения лландовери и венлока в опорном разрезе силура центральной части гряды Чернышева

The Llandovery–Wenlock boundary interval in west-central continental Estonia: an example from the Suigu (S-3) core section

A gap corresponding to several conodont and chitinozoan zones occurs in the Llandovery-Wenlock boundary interval in some sections in the western coastal region of Estonia and on islands in the Muhu Strait. New data from the Suigu (S-3) core section demonstrate that the gap has geographically wider distribution in western continental Estonia. Detailed study of faunas from that section revealed that here the gap corresponds to at least four conodont zones (from below): the Lower and Upper Pseudooneotodus bicornis and Lower and Upper Pterospathodus pennatus procerus zones, but probably also the upper(most) Upper Pt. amorphognathoides amorphognathoides Subzone and the Lower Kockelella ranuliformis Zone (or part of it) are missing. In sense of chitinozoan biostratigraphy, the gap correlates with most of the Margachitina margaritana Zone. Only the uppermost part of this zone is represented in the section. However, although the gap in the Suigu (S-3) core section is distinct and well dated biostratigraphically, the δ 13 C curve demonstrates no evidence of it. Causes of this controversy are still waiting to be revealed. Our data indicate that changes in sedimentation in the Llandovery-Wenlock boundary interval occurred earlier in the distal graptolite-bearing and later in the proximal environments. Most probably, sedimentation in the Baltic Palaeobasin in the late Telychian and early Sheinwoodian was strongly affected by tectonic evolution of the Baltoscandian foreland basin.

Conodonts from the Niur Formation (Silurian) of the Derenjal Mountains, Central Iran

AbstractA Llandovery to Ludlow age for the Niur Formation in the Derenjal Mountains (east-central Iran) is proposed based on new conodont data and previous work on other fossils. The uppermost part of the studied section yielded no diagnostic conodonts but may be Pridoli in age. Some intervals can be dated more precisely: Unit 11 (at least its upper part) is middle Telychian in age and corresponds to thePterospathodus amorphognathoides lennartiZone; the lowermost part of Unit 16 is earliest Ludlow in age and corresponds to theKockelella crassaZone; the uppermost Unit 16 is late Ludlow (Ludfordian) in age and corresponds to theOzarkodina snajdriInterval Zone. The Llandovery–Wenlock boundary lies between units 12 and 13 based on sedimentological evidence. The precise location of the Wenlock–Ludlow boundary in the section is not clear but lies below Unit 16. Present-day Iran was located far away from Baltica and Laurentia, on the other side of the Rheic Ocean. This ocean does not seem to have been a major migration barrier for most organisms including the conodonts.

A new early Silurian prioniodontid conodont with three P elements from Iran and associated species

A prioniodontid conodont Arianagnathus jafariani gen. et sp. nov. from the late Llandovery part of the Niur Formation of the Derenjal Mountains, East Central Iran had an apparatus bearing 3 pairs of P elements. Pa elements of its apparatus are closest to those of Icriodella sandersi (Llandovery—Wenlock boundary interval, Wales, Great Britain) in the weak development of an icrion. Due to the small sample size not all S-elements have been identified but those present are similar to those described in the Icriodella and Icriognathus apparatuses. Based on similarities with previously described apparatus Notiodella we suggest that Arianagnathus jafariani gen. et sp. nov. probably had an apparatus of 17 elements. Arianagnathus is therefore an important additional example that has potential for aiding the future revision of the palaeobiological arrangement of elements within and the phylogeny of conodont apparatuses with 3 P elements, one of which is icrion bearing. The completely known apparatus of associated O...

Faunal Dynamics Across the Silurian—Devonian Positive Isotope Excursions (δ13C, δ18O) in Podolia, Ukraine: Comparative Analysis of the Ireviken and Klonk Events

Two global isotopic events, the early Sheinwoodian (early Wenlock) and that at the Silurian—Devonian transition, have been comprehensively studied in representative carbonate successions at Kytayhorod and Dnistrove, respectively, in Podolia, Ukraine, to compare geochemistry and biotic changes related correspondingly to the Ireviken and Klonk events. These two large-scale isotope excursions reveal different regional ecosystem tendencies. The well-defined increasing trend across the Llandovery—Wenlock boundary in siliciclastic input, redox states and, supposedly, bioproductivity, was without strict correlative relations to the major 13C enrichment event. The environmental and biotic evolution was forced by eustatic sea-level fluctuations and two-step climate change toward a glaciation episode, but strongly modified by regional epeirogeny movements due to location near the mobile Teisseyre-Tornquist Fault Zone. Thus, the global early Sheinwoodian biogeochemical perturbation was of minor depositional signific...

Correlation of Silurian bentonites from Gotland and the eastern Baltic using sanidine phenocryst composition, and biostratigraphical consequences

Silurian bentonites from Sweden and the eastern Baltic are correlated using XRD analysis of sanidine composition. Identifying the same bentonite in different sections has constrained correlations between the conodont, graptolite and chitinozoan zonations. Three bentonites correlate the Cyrtograptus murchisoni Zone with the upper part of the Pterospathodus amorphognathoides Zone and both the Lower and the Upper Pseudooneotodus bicornis conodont biozones at their type locality. Thus the base of the Llandovery–Wenlock boundary stratotype can be correlated with a level in the upper part of the Cyrtograptus murchisoni graptolite Biozone. Identifying the Y Bentonite (Gotland, Sweden) in eastern Baltic drill-cores shows that the base of the Eisenackitina lagena chitinozoan Biozone is close to the base of the K. patula conodont Biozone, which is coeval with an upper part of the C. rigidus graptolite Zone. Sanidine data from the Grötlingbo Bentonite fit well with previous correlations. Some bentonites from Gotland with distinct sanidine compositions and bulk geochemistries have not yet been identified elsewhere.

Sequestration of 12C in the deep ocean during the early Wenlock (Silurian) positive carbon isotope excursion

Marine carbonates from three biostratigraphically well-dated Silurian sections from the mid-continent of North America were analyzed for carbon isotopic (δ 13C carb) study. The early Wenlock positive carbon isotope excursion (Ireviken Excursion) is recorded in three sections from Tennessee, Iowa, and Ohio, with values of >+4.5‰. This global shift in the carbon isotopic ratio of marine waters began during a protracted extinction event (Ireviken Event) that spanned the Llandovery–Wenlock boundary. Although several oceanographic models have been developed to account for the coincident changes in lithology, biology, and carbon isotope stratigraphy during this interval of the Silurian, their proposed causal connections among glaciation, oceanography, primary productivity, and the global carbon cycle remain a matter of debate. This investigation provides carbon isotopic and lithologic evidence that the Ireviken Excursion occurred during a time of relatively oligotrophic conditions, a third order transgressive–regressive cycle, and increased carbonate production throughout the mid-continent of North America. The Ireviken Excursion is recorded in relatively pure carbonate sequences and is immediately preceded by a significant unconformity in each of the measured sections. It is proposed that the Ireviken Excursion was the product of sequestration and burial of 12C in deep water as a result of altered deep ocean circulation.

Chitinozoans of the Margachitina margaritana Biozone and the Llandovery–Wenlock boundary in West Estonian drill cores

Chitinozoans of the Margachitina margaritana Biozone and the Llandovery–Wenlock boundary in West Estonian drill cores

Taxonomy and Diversity History of Paleozoic Radiolarians

The Paleozoic Radiolaria, a major zooplankton group with a siliceous skeleton, comprises 147 genera, 29 families, and 5 suborders. The compilation of 270 literatures on secular diversity change of Paleozoic radiolarians clarifies 7 distinct radiolarian extinction events : Event 1 at the Tremadoc/Arenig boundary (Early Ordovician), Event 2 at the Llandovery/ Wenlock boundary (Early Silurian), Event 3 at the Late Ludlow, Event 4 at the Early/Middle Devonian boundary, Event 5 at the Frasnian/Famennian (F/F) boundary, Event 6 at the Devonian/Carboniferous (D/C) boundary, and Event 7 at the Early/Late Carboniferous boundary.The Paleozoic radiolarians are divided into 3 major groups : a spherical-shell group composed of Spumellaria and Entactinaria ; a cylindrical to conical-shell group composed of Albaillellaria and Nassellaria; and, a flattened-shell group composed of Stauraxonaria. The spherical-shell group declined remarkably at Events 1, 2, 3, 4, and 5, while the cylindrical to conical-shell group declined at Events 6 and 7. The spherical-shell group dominated the Late Paleozoic epicontinental realm, while the cylindrical to conical-shell group dominated the pelagic realm. Events 1-5 represent diversity loss of radiolarians in the epicontinental habitat, while Events 6 and 7 represent diversity loss of pelagic inhabitants. Events 6 and 7 coincide with the timings of continental glaciation, suggesting that the decline of the pelagic radiolarians may have been related to global cooling.It is noteworthy that a significant extinction of radiolarians occurred at the F/F boundary (Event 5), although Radiolaria has previously been regarded to have flourished even across the boundary.Events 1 and 4 are the most significant radiolarian extinction events among 7 events, and occurred synchronously with faunal changes of other planktonic biota. Although terrestrial and benthic records suggest minor crises at these events; a global turnover is emphasized for marine planktonic biota.

Stasis and extinction of Silurian (Llandovery–Wenlock) trilobite associations related to oceanic cyclicity

Silurian trilobites of the central United States belong to a series of temporally-successive associations which appeared abruptly, maintained taxonomic stasis for a time, and then disappeared abruptly. Their disappearance resulted from global perturbations of short-term duration and moderate magnitude, which caused substantial taxonomic replacement but no reorganization of major ecosystems. The most significant extinction and replacement in Silurian trilobite associations in the study area occurs near the Llandovery–Wenlock boundary. This turnover in trilobite associations appears to correspond to Jeppsson's Ireviken Event in his model of oceanic and climatic cyclicity. Major sea-level changes earlier in the Llandovery did not have a similar impact on trilobite associations.

Tectonic control on sedimentation during transgression: a case study from Silurian successions in Ireland and Scotland

The Silurian succession of North Galway is relatively well constrained in terms of environmental analysis and, in its lower half, palaeontologically. The initiation of a late Llandovery marine transgression can be demonstrated over fluviatile red sandstones. The deposition of shallow-water conglomerates at the base of a turbidite sequence within the succession indicates the long-lived presence of a channel system that was probably fault controlled. The back-stripping method allows a subsidence curve to be constructed for this succession. It demonstrates an initial period of rapid rift-related subsidence followed by a short-lived hiatus that may be due to the cessation of subduction in this part of the Caledonides. A comparison of the Galway subsidence curve with that of the Silurian succession at Girvan in Scotland shows strong, but diachronous, similarities. This tends to support an earlier suggestion that they formed part of a single but partitioned basin throughout most of the Silurian period. Although the eustatic fall in sea level at the Llandovery–Wenlock boundary can be recognized, the influence of tectonic regime and sedimentation rates were the controlling factors in determining relative sea levels.

Integrated biostratigraphy of the lower Silurian of the Ohesaare core, Saaremaa, Estonia

Integrated graptolite, conodont and chitinozoan biostratigraphical data are presented from the lower Silurian of the Ohesaare core, Saaremaa, Estonia. A precise correlation of graptolite and conodont biozonations for much of the Telychian and Sheinwoodian is presented; this differs significantly from correlations suggested previously, which were based on inadequate graptolite data. The boundary between the celloni and amorphognathoides conodont biozones lies close to that between the spiralis and lapworthi graptolite biozones. The boundary between the bicornis and procerus conodont superzones lies mid-way through the murchisoni graptolite Biozone. The top of the procerus conodont Superzone lies at or very close to the boundary between the murchisoni and firmus graptolite biozones. The boundary between the Lower and Upper ranuliformis conodont biozones lies close to the boundary between the firmus and riccartonensis graptolite biozones. The precise position of the Llandovery–Wenlock boundary cannot be identified in the core. The newly demonstrated biozonal correlations are significant for studies of Silurian chronostratigraphy (previous estimates of the duration of the epochs and ages of the Silurian were based on inaccurate correlations) and sea-level changes. Palaeogeographically, the setting of Ohesaare is considered to be relatively deep shelf. Perhaps surprisingly, there is a major unconformity in the Llandovery of the core: much, if not all, of the Aeronian is missing, as is the lower part of the Telychian.

Basal Wenlock biofacies from the Girvan district, SW Scotland

Synopsis Basal Wenlock shelly biofacies from the Knockgardner Formation (middle Straiton Group) of the Girvan district are dominated by brachiopods and trilobites assigned, respectively, to the Howellella–Protochonetes and the Podowrinella faunas. The Podowrinella fauna is consistently developed across the Midland Valley in Llandovery–Wenlock boundary rocks but the association is not known outside Scotland. In contrast the Howellella–Protochonetes fauna is more variably developed and restricted to the western part of the Midland Valley; but the components of this type of fauna occur in Wenlock and Ludlow rocks elsewhere. In Scotland both associations occur in nearshore, commonly storm-dominated sediments. The contrasting distributional patterns of the mobile and sessile benthos may reflect the differing life strategies of both the larval and adult stages in each group. Lithofacies within the upper Llandovery–lower Wenlock Straiton Group are revised within the regressive framework of the region.

Graptolite extinction at the Llandovery—Wenlock boundary

Combined graptolite range data through the late Llandovery and early Wenlock of the Canadian Arctic Islands, the British Isles, and Lithuania reveal that survivorship rates from one zone to the next are between 56 and 85%, except at the Llandovery—Wenlock boundary, where only 19% (8 of 42) of the taxa of the upper sakmaricus—crenulata Zone survive into the centrifugus Zone. The high rate of graptolite extinction appears to coincide with glacioeustatic sea-level fall and may be attributed to changing oceanic temperatures, salinity, circulation patterns, oxygenation, and/or productivity associated with the onset of a glacial maximum event. □Graptolite, extinction, Silurian, Llandovery, Wenlock.