Integrated stratigraphic study of the Rhuddanian-Aeronian (Llandovery, Silurian) boundary succession at Rheidol Gorge, Wales : a preliminary report

Rheidol Gorge, approximately 17 km west of Aberystwyth, mid Wales, exposes a continuous succession of strata from the middle part of the upper Rhuddanian Coronograptus Cyphus Biozone through the lower Aeronian Demirastrites triangulatus (= Monograptus triangulatus) Biozone. Parts of the Aeronian succession are well known for their beautiful lower Aeronian graptolites preserved as pyrite internal moulds. We measured this section and sampled for graptolites, palynomorphs, and for lithological and geochemical analyses. One of our objectives was to assess the section for its suitability as a candidate for a new Global Stratotype Section and Point for the base of the mid-Llandovery Aeronian Stage. The succession alternates between bioturbated grey mudstones lacking in graptolites and laminated, graptolitic black shales. The black shales commonly show thin, interbedded siltstones. The grey mudstones are interpreted to represent deposition under oxic to dysoxic conditions, the black shales an anoxic seafloor environment. The strata have undergone low-grade metamorphism, commonly with a weakly to moderately developed cleavage, and the graptolites often show ductile and/or brittle deformation. Strate of the middle to upper cyphus Biozone are c 10.2 m thick and yield graptolite faunas of varying diversity and preservation quality. 0.8 m below the base of the D. triangulatus Biozone there is a change from predominantly organic-poor mudrocks with interbeds of darker shales with sparse graptolites to an interval of predominantly black shales with a relatively rich graptolite fauna. The graptolites in the black shale interval, which spans the zonal boundary, are flattened or in partial relief, commonly deformed, and the strata tend to break along cleavage rather than bedding planes. Nevertheless, a distinctive graptolite fauna occurs through the boundary interval that allows good correlation with successions in other parts of the world. The base of the D. triangulatus Biozone is marked by the first appearance of D. triangulatus. Other species first appearing just below the base of the D. triangulatus Biozone that are useful for international correlation include Pristiograptus concinnus and Pseudorthograptus finneyi. Strata rich in well-preserved, pyritic graptolites become common about 2.3 m above the base of the triangulatus Biozone. Chitinozoans are poorly to moderately well preserved in the section and indicate the Spinachitina maennili Biozone through the boundary interval, without any significant faunal changes, as is the case in many other parts of the world

Metal-induced malformations in early Palaeozoic plankton are harbingers of mass extinction

Glacial episodes have been linked to Ordovician–Silurian extinction events, but cooling itself may not be solely responsible for these extinctions. Teratological (malformed) assemblages of fossil plankton that correlate precisely with the extinction events can help identify alternate drivers of extinction. Here we show that metal poisoning may have caused these aberrant morphologies during a late Silurian (Pridoli) event. Malformations coincide with a dramatic increase of metals (Fe, Mo, Pb, Mn and As) in the fossils and their host rocks. Metallic toxins are known to cause a teratological response in modern organisms, which is now routinely used as a proxy to assess oceanic metal contamination. Similarly, our study identifies metal-induced teratology as a deep-time, palaeobiological monitor of palaeo-ocean chemistry. The redox-sensitive character of enriched metals supports emerging ‘oceanic anoxic event’ models. Our data suggest that spreading anoxia and redox cycling of harmful metals was a contributing kill mechanism during these devastating Ordovician–Silurian palaeobiological events

New chitinozoans from the historical type area of the Hirnantian Stage and additional key sections in the Wye Valley, Wales, UK

The type locality for several core elements of the Hirnantia brachiopod fauna is a small disused quarry on the western slopes of Cwm Hirnant. There, the Hirnant Limestone Member of the Foel-y-Ddinas Mudstone Formation yields a new, well-preserved chitinozoan assemblage, attributed to the Spinachitina taugourdeaui Biozone. This allows tight correlation with the Hirnantian of Baltica and Laurentia and neatly ties the chitinozoan zonation with the classical brachiopod fauna. Nearby, the chitinozoan assemblage from the Caradoc Cymerig Limestone Member at Gelli-grîn belongs to the Spinachitina cervicornis Biozone?, and is identical to that recovered from the Burrellian in the Onny Valley, Welsh Borderland. A Silurian assemblage higher up section, discovered in the Cwm-yr-Aethnen Formation, is attributed to the globally recognized Eisenackitina dolioliformis Biozone. Attempts to integrate the chitinozoan and graptolite biozonation, in the central Welsh Rhayader area, were less successful

Ground-truthing Late Ordovician climate models using the paleobiogeography of graptolites.

The δ18O records obtained from conodonts suggest that during the Mid‐Ordovician, equatorial temperatures stabilized at close to the present day, a hypothesis broadly supported by published climate models. However, the degree to which equatorial temperatures represent global climate state and varied between different climatic modes (greenhouse/icehouse, interglacial/glacial) and to what extent Ordovician δ18O and climate models truly reflect the global climate remain to be tested. Here we present paleobiogeographical data for the planktonic graptolites, from the gracilis time slice (i.e., early Sandbian Stage) that just postdates the postulated onset of climate stabilization. TWINSPAN analysis and constrained seriation provide robust ecological groupings in paleobiogeographical presence/absence data. The highest‐level groups reflect tropical‐subtropical assemblages and a high–southerly latitude assemblage. Constrained seriation defines latitude‐distinct biotopes that are considered equivalent to modern zooplanktonic provinces. The distribution pattern of graptolite biotopes in the gracilis time slice show (1) that models explaining local graptolite ecological assemblages using lateral differentiation (e.g., upwelling) are to be favored above those using depth stratification and (2) a steep faunal gradient from equator to pole, which is typical of a cool, nongreenhouse world and comparable to the modern situation. We therefore broadly support the climate stabilization hypothesis. Paradoxically the climate of the early Late Ordovician appears similar to that of the present day despite the higher atmospheric pCO2 levels. Graptolite species were indicative of temperature‐controlled ocean water masses, in much the same way as the modern zooplankton

Epipelagic chitinozoan biotopes map a steep latitudinal temperature gradient for earliest Late Ordovician seas: Implications for a cooling Late Ordovician climate

International audienceThe Early-Mid Ordovician has long been considered a super-greenhouse world, based largely on high relative global sea levels and light stable oxygen isotope data from bulk carbonates. An alternative and largely untested hypothesis has suggested that, at least in equatorial palaeolatitudes, there was a steady cooling trend through the Early Ordovician reaching the range of modern equatorial sea surface temperatures by the Mid Ordovician. This hypothesis, though controversial, is supported by palaeobiogeographical studies of the early Late Ordovician (Sandbian) zooplankton (graptolite) biotopes. These are comparable to the modern planktonic foraminifera ¡®provinces¡¯ and suggest a latitudinal temperature gradient that is similar to the modern one. Chitinozoans are also an important group of Palaeozoic marine microfossils, but with a poorly known biological affinity. Here we present an analysis of chitinozoan species palaeobiogeography for the early Late Ordovician (Sandbian c. 460 Ma), which confirms that these microfossils, and likely their parent organisms, were epipelagic. Unlike the graptolites, chitinozoans had their highest diversity and abundance south of ~35°S during this time, which strongly suggests adaptation to temperate to cold waters. The distribution of chitinozoan biotopes during the gracilis time-slice allows us to identify the austral Subtropical, Subpolar and Polar belts; key oceanographic boundaries are identified as the Subtropical-Subpolar Transition at ~35°S and tentatively, the Polar Front at ~55°-70°S. Again, these are in a similar position to those in the modern Southern Ocean. Chitinozoan biotopes also map a steep latitudinal faunal gradient that is comparable to that of the graptolites and that of modern plankton. This likely indicates a steep latitudinal temperature gradient for early Late Ordovician seas and questions the notion that this interval of Earth history had a greenhouse climate

A Cenozoic-Style Scenario for the End-Ordovician Glaciation

International audienceThe end-Ordovician (Hirnantian) was an enigmatic interval in the Phanerozoic, known for massive glaciationpotentially at elevated CO2 levels, biogeochemical cycle disruptions recorded as large isotope anomalies and adevastating extinction event. Such linkage of eustatic, biological and isotopic records to the climatically forceddevelopment of an ice sheet can only be contemplated within a framework of high-resolution sequence stratigraphythat integrates allo-, chemo- or biostratigraphic markers. We develop sequence stratigraphic correlations fortwo superbly exposed and exceptionally well-developed latest Ordovician successions, the Anti-Atlas of Moroccoand Anticosti Island in Canada. Both offer sections, on a 100-km scale, from the basin edge to the axis of activesedimentary depocentres. Relative to the end-Ordovician ice-sheet centre (present-day north-central Africa), theyprovide a near-field (Anti-Atlas, siliciclastic platform) and a far-field (Anticosti Island, mixed carbonate and siliciclastic)stratigraphic records. These two successions, up to 300 and 100m thick, respectively, were deposited inbasins with notable subsidence rates and significant (ca. 100 m) initial water depths, enabling the development ofcomprehensive archives of the latest Ordovician glaciation. This framework, driven by glacio-eustatic cycles tiedto the evolution of polar continental-scale ice sheets over west Gondwana, enables the correlation of eustatic cyclesat a level that is beyond the resolution capability of most absolute dating methods and of biozones, the latter typicallyof Myr duration. A proposed Cenozoic-style scenario including three main glacial cycles and higher-orderphenomena necessitates the revision of the end-Ordovician, glaciation-related sequence of events

Biogeographical patterns of Ordovician ostracods

The biogeography of marine shelf ostracod genera is analysed for two Ordovician time slabs, the earliest Late Ordovician and the terminal Ordovician, that have been considered to reflect comparatively warmer and cooler global climate states, respectively. The earlier time slab is equivalent to the Nemagraptus gracilis graptolite interval (centred about 460 Ma), and defined as the total range of the eponymous species. The Hirnantian time slab comprises the Normalograptus extraordinarius and Normalograptus persculptus graptolite biozones (445.6–443.7 Ma). The ostracod dataset consists of 160 taxa from 24 early Late Ordovician localities and 86 taxa from 10 Hirnantian localities. Ordination and variation partitioning analyses show that patterns in ostracod distribution in the gracilis time slab are largely related to palaeocontinental affinity of the samples and to a lesser degree to palaeolatitude. Some decrease of provincialism can be suggested for the Hirnantian, although the ostracod dataset is limited for this interval

Precession-driven climate cycles and time scale prior to the Hirnantian glacial maximum

Paleozoic astrochronologies are limited by uncertainties in past astronomical configurations and the availability of complete stratigraphic sections with precise, independent age control. We show it is possible to reconstruct a robust Paleozoic ~104-yr-resolution astrochronology in the well-preserved and thick Upper Ordovician reference record of Anticosti Island (Canada). The clear imprint of astronomical cycles, including ~18 k.y. precession, potential obliquity, and short and long eccentricity, constrains the entire Vauréal Formation (~1 km thick) to only ~3 m.y. in total, representing ~10 times higher accumulation rates than previously suggested. This ~104 yr resolution represents an order of magnitude increase in the current standard temporal resolution for the Katian and even allows for the detection of sub-Milankovitch climate-scale variability. The loss of a clear precession signal in the uppermost Vauréal Formation might be related to contemporaneous global cooling prior to the Hirnantian glacial maximum as indicated by the δ18O record. Complementary to the study of cyclostratigraphy of longer and often simplified records, it is important to recognize stratigraphic hiatuses and complexities on the ~104 yr scale to achieve robust sub-eccentricity-scale Paleozoic astrochronologies

Epipelagic chitinozoan biotopes map a steep latitudinal temperature gradient for earliest Late Ordovician seas: Implications for a cooling Late Ordovician climate

The Early–Mid Ordovician has long been considered a super-greenhouse world, based largely on high relative global sea levels and light stable oxygen isotope data from bulk carbonates. An alternative and largely untested hypothesis has suggested that, at least in equatorial palaeolatitudes, there was a steady cooling trend through the Early Ordovician reaching the range of modern equatorial sea surface temperatures by the Mid Ordovician. This hypothesis, though controversial, is supported by palaeobiogeographical studies of the early Late Ordovician (Sandbian) zooplankton (graptolite) biotopes. These are comparable to the modern planktonic foraminifera ‘provinces’ and suggest a latitudinal temperature gradient that is similar to the modern one.Chitinozoans are also an important group of Palaeozoic marine microfossils, but with a poorly known biological affinity. Here we present an analysis of chitinozoan species palaeobiogeography for the early Late Ordovician (Sandbian c. 460 Ma), which confirms that these microfossils, and likely their parent organisms, were epipelagic. Unlike the graptolites, chitinozoans had their highest diversity and abundance south of ~ 35°S during this time, which strongly suggests adaptation to temperate to cold waters. The distribution of chitinozoan biotopes during the gracilis time-slice allows us to identify the austral Subtropical, Subpolar and Polar belts; key oceanographic boundaries are identified as the Subtropical–Subpolar Transition at ~ 35°S and tentatively, the Polar Front at ~ 55°–70°S. Again, these are in a similar position to those in the modern Southern Ocean. Chitinozoan biotopes also map a steep latitudinal faunal gradient that is comparable to that of the graptolites and that of modern plankton. This likely indicates a steep latitudinal temperature gradient for early Late Ordovician seas and questions the notion that this interval of Earth history had a greenhouse climate

Ground truthing ordovician climate models using spatial analyses of chitinozoans and graptolites

International audienc