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Marco Vecoli - One of the best experts on this subject based on the ideXlab platform.
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Abnormal forms of acritarchs (phytoplankton) in the upper Hirnantian (Upper Ordovician) of Anticosti Island, Canada
Review of Palaeobotany and Palynology, 2012Co-Authors: A. Delabroye, Thijs R.a. Vandenbroucke, Axel Munnecke, Thomas Servais, Marco VecoliAbstract:Abstract A detailed study of Late Ordovician–early Silurian acritarchs (Palaeozoic phytoplankton) from Anticosti Island (Quebec, Canada) revealed an unusually high abundance of abnormal forms from the upper Hirnantian carbonate strata (uppermost Ordovician) of the Ellis Bay Formation in the western part of the island (member 6, Spinachitina taugourdeaui chitinozoan Biozone). The objective of this paper is to describe these abnormal forms in detail. Two species are particularly affected: Disparifusa psakadoria Loeblich and Tappan, 1978 presents abnormally hypertrophied central vesicles, whereas Peteinosphaeridium laframboisepointense nov. sp. has appendices that are fused along their length. The abnormal forms of acritarchs occur in rocks deposited during periods that are near time-equivalents to those of maximum ice-sheet extensions on Gondwana during the Hirnantian glaciation. Although this stratigraphic level corresponds to an interval of strong perturbations of the global carbon cycle, the exact factors causing the observed malformations remain unknown.
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cryptospore assemblages from upper ordovician katian Hirnantian strata of anticosti island quebec canada and estonia palaeophytogeographic and palaeoclimatic implications
Review of Palaeobotany and Palynology, 2011Co-Authors: A. Delabroye, Marco Vecoli, Amalia Spina, Olle HintsAbstract:Abstract Rich palynological assemblages have been recovered from deposits of Hirnantian age in Anticosti Island (Quebec, Canada), and in borehole Valga-10 in southern Estonia. The assemblages are well preserved, and include acritarchs, chitinozoans, and cryptospores. The age of the deposits is well constrained by means of palynomorphs (acritarchs and chitinozoans) as well as sequence stratigraphic and chemostratigraphic correlations. Cryptospore assemblages from the two localities are similar and are also broadly comparable to the few known coeval assemblages described elsewhere. They include 11 genera and 20 species, and testify to the presence of an extended and diverse flora during Hirnantian times in Laurentia and, for the first time, also in Baltica. The present findings contribute to an improved knowledge of origin and early development of vegetative cover. The recovery of diverse and abundant cryptospores in Hirnantian deposits may be related to increased input of land-derived sediment during the global sea-level fall linked to the Late Ordovician glaciation, but it also demonstrates that the early land plants may have tolerated a wide range of climatic conditions.
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Cryptospore assemblages from Upper Ordovician (Katian–Hirnantian) strata of Anticosti Island, Québec, Canada, and Estonia: Palaeophytogeographic and palaeoclimatic implications
Review of Palaeobotany and Palynology, 2011Co-Authors: Marco Vecoli, A. Delabroye, Amalia Spina, Olle HintsAbstract:Abstract Rich palynological assemblages have been recovered from deposits of Hirnantian age in Anticosti Island (Quebec, Canada), and in borehole Valga-10 in southern Estonia. The assemblages are well preserved, and include acritarchs, chitinozoans, and cryptospores. The age of the deposits is well constrained by means of palynomorphs (acritarchs and chitinozoans) as well as sequence stratigraphic and chemostratigraphic correlations. Cryptospore assemblages from the two localities are similar and are also broadly comparable to the few known coeval assemblages described elsewhere. They include 11 genera and 20 species, and testify to the presence of an extended and diverse flora during Hirnantian times in Laurentia and, for the first time, also in Baltica. The present findings contribute to an improved knowledge of origin and early development of vegetative cover. The recovery of diverse and abundant cryptospores in Hirnantian deposits may be related to increased input of land-derived sediment during the global sea-level fall linked to the Late Ordovician glaciation, but it also demonstrates that the early land plants may have tolerated a wide range of climatic conditions.
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The end-Ordovician glaciation and the Hirnantian Stage: A global review and questions about Late Ordovician event stratigraphy
Earth-Science Reviews, 2010Co-Authors: A. Delabroye, Marco VecoliAbstract:This paper proposes a global review of Hirnantian event stratigraphy. The Hirnantian GSSP in south China is tentatively correlated with latest Ordovician strata from the peri-Gondwanan “glacial” regions. Problems of biostratigraphical correlation are highlighted. At a worldwide scale, the major biostratigraphically useful fossil groups (graptolites, chitinozoans, brachiopods, conodonts, acritarchs) are analysed and their limits for global correlation of the uppermost Ordovician are discussed. Palaeobiogeographical disparities are invoked as the primary cause of the difficulty in establishing an effective Late Ordovician global biostratigraphical scheme. As an alternative correlative tool, the HICE (Hirnantian Isotopic Curve Excursion) event is often put forward in the literature. However, carbon isotope chemostratigraphy shows, like biostratigraphy, some limits to the present state of knowledge. No good independent biostratigraphical control of the HICE exists in both shallow carbonate deposits and deeper shaly ones. Recent studies have also demonstrated inconsistencies between carbon isotopic signals obtained from organic (δ13Corg) and inorganic (δ13Ccarb) carbon species, further complicating the use of the HICE as an isochronous benchmark. All of these difficulties for Hirnantian event stratigraphy are discussed in detail in order to enable them to be overcome in the future. Precise Late Ordovician and early Silurian event stratigraphies are essential for the understanding of the mechanisms linked to the first of the “Big Five” extinctions.
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The end-Ordovician glaciation and the Hirnantian Stage: A global review and questions about Late Ordovician event stratigraphy
Earth-Science Reviews, 2010Co-Authors: A. Delabroye, Marco VecoliAbstract:This paper proposes a global review of Hirnantian event stratigraphy. The Hirnantian GSSP in south China is tentatively correlated with latest Ordovician strata from the peri-Gondwanan "glacial" regions. Problems of biostratigraphical correlation are highlighted. At a worldwide scale, the major biostratigraphically useful fossil groups (graptolites, chitinozoans, brachiopods, conodonts, acritarchs) are analysed and their limits for global correlation of the uppermost Ordovician are discussed. Palaeobiogeographical disparities are invoked as the primary cause of the difficulty in establishing an effective Late Ordovician global biostratigraphical scheme. As an alternative correlative tool, the HICE (Hirnantian Isotopic Curve Excursion) event is often put forward in the literature. However, carbon isotope chemostratigraphy shows, like biostratigraphy, some limits to the present state of knowledge. No good independent biostratigraphical control of the HICE exists in both shallow carbonate deposits and deeper shaly ones. Recent studies have also demonstrated inconsistencies between carbon isotopic signals obtained from organic (delta(13)C(org)) and inorganic (delta(13)C(carb)) carbon species, further complicating the use of the HICE as an isochronous benchmark. All of these difficulties for Hirnantian event stratigraphy are discussed in detail in order to enable them to be overcome in the future. Precise Late Ordovician and early Silurian event stratigraphies are essential for the understanding of the mechanisms linked to the first of the "Big Five" extinctions. (C) 2009 Elsevier B.V. All rights reserved
Mohammad Ghavidel-syooki - One of the best experts on this subject based on the ideXlab platform.
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Stratigraphic evidence for Hirnantian glaciation in the Alborz Mountain Ranges, northeastern Iran
Palaeogeography Palaeoclimatology Palaeoecology, 2017Co-Authors: Mohammad Ghavidel-syookiAbstract:Abstract The author examined the high-latitude Hirnantian diamictites of the Ghelli Formation and the early Silurian olive-gray shales of the Niur Formation exposed in the northeastern Alborz Mountains, Iran. These 404 m thick glacial deposits can be divided into three progradational-retrogradational cycles, each potentially controlled by the regional advance and retreat of the Hirnantian ice sheet. The glaciated source area was west of the study area, in the Arabian Shield region, where numerous tunnel valleys have been reported. Time calibration was performed based on a high-quality biostratigraphic control, mainly derived from the chitinozoan biozones Tanuchitina fistulosa , Acanthochitina barbata , Armoricochitina nigerica , Ancyrochitina merga , Tanuchitina elongata , Spinachitina oulebsiri , and Spinachitina fragili s. The land-derived miospores present in most chitinozoan assemblages, often abundant, seem associated with global sea-level changes during the Late Ordovician glaciation. The Ghelli Formation's high abundance of terrestrial miospores and low abundance of marine palynomorphs suggests its deposition in a shallow marine environment. Additionally, rhythmic bedded tidalites of claystone and sandstone indicate the growth of early land plants producing cryptospores on adjacent flooded areas. Based on marine palynomorphs (chitinozoans and acritarchs), the northeastern Alborz Mountains glacial deposits were dated as Hirnantian; the Niur Formation was assigned to the earliest Llandovery (Rhuddanian). Glacial deposits and marine palynomorphs suggest the presence of Hirnantian ice caps in the Alborz Mountains, at the margin of the Arabian Plate, and indicate the peripheral extension of the Late Ordovician ice sheet. Several biostrome beds forming pelmatozoan-bryozoan mud-mounds, mainly comprising bryozoans, echinoderms, tentaculites, and subordinate trilobites, were found in the glacial deposits of the Ghelli Formation, suggesting carbonate deposition during the Hirnantian. These carbonate beds are not related to the Boda warming event, which happened over the Gondwana landmass in the Katian interval; the glacial deposits of the Alborz Mountain Ranges are thus not correlated to those of the Gondwana paleocontinent. Climatic amelioration is more likely than local reduction of clastic input to be responsible for the biostrome beds in the Glacial Member of the Ghelli Formation, namely a short-lived episode of global warming during the Hirnantian interval (the Milankovitch cycles). Four new chitinozoan species, namely Armoricochitina persianense n. sp., Tanuchitina alborzensis n. sp., Spinachitina iranense n. sp., and Hyalochitina jajarmensis n. sp., are described and illustrated. Biometric data are also provided for the Iranian S . oulebsiri and A . nigerica .
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Stratigraphic evidence for the Hirnantian (latest Ordovician) glaciation in the Zagros Mountains, Iran
Palaeogeography Palaeoclimatology Palaeoecology, 2011Co-Authors: Mohammad Ghavidel-syooki, J. Javier Álvaro, Leonid E. Popov, Mansoureh Ghobadi Pour, Mohammad H. Ehsani, Anna SuyarkovaAbstract:Abstract High-latitude Hirnantian diamictites (Dargaz Formation) and lower-Silurian kerogenous black shales (Sarchahan Formation) are locally exposed in the Zagros Mountains. The glaciogenic Dargaz deposits consist of three progradational/retrogradational cycles, each potentially controlled by the regional advance and retreat of the Hirnantian ice sheet. Glacial incisions of sandstone packages change laterally from simple planar to high-relief (
Thijs R.a. Vandenbroucke - One of the best experts on this subject based on the ideXlab platform.
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Chitinozoan biostratigraphy and carbon isotope stratigraphy from the Upper Ordovician Skogerholmen Formation in the Oslo Region. A new perspective for the Hirnantian lower boundary in Baltica
Review of Palaeobotany and Palynology, 2017Co-Authors: Chloé E. A. Amberg, Thijs R.a. Vandenbroucke, Axel Munnecke, Arne Thorshøj Nielsen, Patrick I. MclaughlinAbstract:Abstract The end-Ordovician has received wide attention because it hosts major global events including mass extinctions, glaciations, significant sea-level fluctuations, and large-scale perturbations of the Earth's carbon cycle. Knowing the order and timing of these events and their components is crucial for understanding these environmental changes. Here, we use stable carbon isotope stratigraphy in combination with chitinozoan biostratigraphy to correlate the Upper Ordovician Belonechitina gamachiana chitinozoan Biozone. Its position has long been a matter of debate; some argue that it is of late Katian age and others that it is of early Hirnantian age. The Skogerholmen Formation from the Oslo-Asker District in Norway has been correlated with the lower-middle Pirgu Baltic Stage, hitherto believed to correspond to the international upper Katian Stage. Our study, however, reveals the presence of B . gamachiana , diagnostic of the eponymous Biozone, in the descending trend of a modest carbon isotope excursion in the lower part of this formation. This is strikingly similar to data from coeval end-Ordovician sections in North America, where the prevailing evidence suggests an early Hirnantian age for the B . gamachiana chitinozoan Biozone. This new correlation suggests that the lower Hirnantian boundary may be positioned within the Pirgu Baltic Stage.
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Chitinozoan biozonation in the upper Katian and Hirnantian of the Welsh Basin, UK
Review of Palaeobotany and Palynology, 2014Co-Authors: Thomas J. Challands, Thijs R.a. Vandenbroucke, Howard A. Armstrong, Jeremy DaviesAbstract:Abstract Here we present a chitinozoan biostratigraphical framework for the South Wales upper Katian and Hirnantian (Ashgill) succession. The current study indicates that three of the six Avalonian Ashgill chitinozoan biozones are recognised in the Welsh Basin; the bergstroemi , fossensis and umbilicata biozones. The Baltoscandian and Laurentian Hercochitina gamachiana biozone is suggested by the presence of Belonechitina cf. gamachiana and the Spinachitina taugourdeaui biozone is suggested by Spinachitina cf. taugourdeaui . Intervening between these is a newly erected lower Hirnantian regional biozone, the Belonechitina llangrannogensis n. sp. biozone. The late Katian (Cautleyan–Rawtheyan) Conochitina rugata biozone was not recognised, though the index taxon is recorded. The presence of B. cf. gamachiana below the lithological expression of the Hirnantian glacial maximum and alongside Rawtheyan graptolite and trilobite assemblages shows that the local base of the B. cf. gamachiana biozone lies beneath the Katian–Hirnantian boundary. Although at present in open nomenclature, the finds of B. cf. gamachiana and S . cf. taugourdeaui , from sites where these chitinozoans co-occur with graptolites, are potentially important; the area offers the potential to study how B. cf. gamachiana and S . cf. taugourdeaui are taxonomically and stratigraphically linked to the original index species. A composite Katian–Hirnantian chitinozoan biozonation for the Welsh Basin is presented and three new species are defined: Belonechitina llangrannogensis n. sp., Belonechitina ceregidionensis n. sp. and S pinachitina penbryniensis n. sp.
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Abnormal forms of acritarchs (phytoplankton) in the upper Hirnantian (Upper Ordovician) of Anticosti Island, Canada
Review of Palaeobotany and Palynology, 2012Co-Authors: A. Delabroye, Thijs R.a. Vandenbroucke, Axel Munnecke, Thomas Servais, Marco VecoliAbstract:Abstract A detailed study of Late Ordovician–early Silurian acritarchs (Palaeozoic phytoplankton) from Anticosti Island (Quebec, Canada) revealed an unusually high abundance of abnormal forms from the upper Hirnantian carbonate strata (uppermost Ordovician) of the Ellis Bay Formation in the western part of the island (member 6, Spinachitina taugourdeaui chitinozoan Biozone). The objective of this paper is to describe these abnormal forms in detail. Two species are particularly affected: Disparifusa psakadoria Loeblich and Tappan, 1978 presents abnormally hypertrophied central vesicles, whereas Peteinosphaeridium laframboisepointense nov. sp. has appendices that are fused along their length. The abnormal forms of acritarchs occur in rocks deposited during periods that are near time-equivalents to those of maximum ice-sheet extensions on Gondwana during the Hirnantian glaciation. Although this stratigraphic level corresponds to an interval of strong perturbations of the global carbon cycle, the exact factors causing the observed malformations remain unknown.
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polar front shift and atmospheric co2 during the glacial maximum of the early paleozoic icehouse
Proceedings of the National Academy of Sciences of the United States of America, 2010Co-Authors: Thijs R.a. Vandenbroucke, Florentin Paris, Jaak Nolvak, Jan Zalasiewicz, Thomas J. Challands, Howard A. Armstrong, M Williams, Koen Sabbe, Jacques VerniersAbstract:Our new data address the paradox of Late Ordovician glaciation under supposedly high pCO2 (8 to 22× PAL: preindustrial atmospheric level). The paleobiogeographical distribution of chitinozoan (“mixed layer”) marine zooplankton biotopes for the Hirnantian glacial maximum (440 Ma) are reconstructed and compared to those from the Sandbian (460 Ma): They demonstrate a steeper latitudinal temperature gradient and an equatorwards shift of the Polar Front through time from 55°–70° S to ∼40° S. These changes are comparable to those during Pleistocene interglacial-glacial cycles. In comparison with the Pleistocene, we hypothesize a significant decline in mean global temperature from the Sandbian to Hirnantian, proportional with a fall in pCO2 from a modeled Sandbian level of ∼8× PAL to ∼5× PAL during the Hirnantian. Our data suggest that a compression of midlatitudinal biotopes and ecospace in response to the developing glaciation was a likely cause of the end-Ordovician mass extinction.
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chitinozoans and the age of the soom shale an ordovician black shale lagerstatte south africa
Journal of Micropalaeontology, 2009Co-Authors: Sarah E Gabbott, Thijs R.a. Vandenbroucke, Richard J. Aldridge, Florentin Paris, Johannes N. TheronAbstract:Isolated chitinozoans from the Soom Shale Member of the Cedarberg Formation, SW South Africa are described and provide a date of the latest Hirnantian–earliest Rhuddanian. The recovered chitinozoans are typical of the latest Ordovician Spinachitina oulebsiri Biozone, although an earliest Silurian age is possible. They indicate a very short time span (less than 1 Ma) across the Ordovician–Silurian boundary. This is currently the highest biostratigraphical resolution attainable for the Soom Shale Lagerstatte. Correlation of the Soom Shale chitinozoans with identical assemblages in post-glacial, transgressive deposits of Northern Africa is possible; both faunas occur in shales that overlie glacial diamictites of the Hirnantian glaciation. A new species, Spinachitina verniersi n. sp. is described.
David A. Fike - One of the best experts on this subject based on the ideXlab platform.
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a volcanic trigger for the late ordovician mass extinction mercury data from south china and laurentia
Geology, 2017Co-Authors: David S Jones, David A. Fike, Anna M Martini, Kunio KaihoAbstract:The Late Ordovician mass extinction (LOME), one of the five largest Phanerozoic biodiversity depletions, occurred in two pulses associated with the expansion and contraction of ice sheets on Gondwana during the Hirnantian Age. It is widely recognized that environmental disruptions associated with changing glacial conditions contributed to the extinctions, but neither the kill mechanisms nor the causes of glacial expansion are well understood. Here we report anomalously high Hg concentrations in marine strata from south China and Laurentia deposited immediately before, during, and after the Hirnantian glacial maximum that we interpret to reflect the emplacement of a large igneous province (LIP). An initial Hg enrichment occurs in the late Katian Age, while a second enrichment occurs immediately below the Katian-Hirnantian boundary, which marks the first pulse of extinction. Further Hg enrichment occurs in strata deposited during glacioeustatic sea-level fall and the glacial maximum. We propose that these Hg enrichments are products of multiple phases of LIP volcanism. While elevated Hg concentrations have been linked to LIP emplacement coincident with other Phanerozoic mass extinctions, the climate response during the LOME may have been unique owing to different climatic boundary conditions, including preexisting ice sheets. Our observations support a volcanic trigger for the LOME and further point to LIP volcanism as a primary driver of environmental changes that caused mass extinctions.
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dynamic sulfur and carbon cycling through the end ordovician extinction revealed by paired sulfate pyrite δ34s
Earth and Planetary Science Letters, 2013Co-Authors: David S Jones, David A. FikeAbstract:Geochemical records of the end-Ordovician Hirnantian Stage show parallel positive excursions in the stable isotope compositions of sedimentary pyrite sulfur (δ34Spyr), organic carbon (δ13Corg), and carbonate carbon (δ13Ccarb); these isotope excursions coincide with the end Ordovician glaciation and mass extinction. A relative increase in pyrite burial (fpyr) attributed to marine anoxia has been invoked to explain the sulfur isotope excursion and link oceanic redox conditions to the extinction of marine fauna. An increase in fpyr would necessarily generate a parallel excursion of equal magnitude in the isotopic composition of coeval marine sulfate (δ34SSO4δ34SSO4). Here we present new high-resolution paired sulfur isotope data from carbonate-associated sulfate (δ34SCAS) and pyrite from the Hirnantian Stage of western Anticosti Island (Quebec, Canada). These data document a positive 20‰ enrichment in δ34Spyr (comparable in magnitude to previous reports), but no parallel excursion in δ34SCAS. This pattern provides new constraints on the origin of the δ34Spyr excursion and the nature of carbon–sulfur coupling through Hirnantian time. Specifically, these observations preclude enhanced pyrite burial as the cause of the Hirnantian δ34Spyr excursion and suggest the possible role of anoxia in the mass extinction may need to be reevaluated. Rather, the global δ34Spyr excursion is best explained by a transient reduction in the isotopic fractionations expressed during microbial sulfur cycling (epyr). The epyr record shows a strong inverse correlation with δ13C, suggesting a mechanistic link between carbon cycling and processes controlling epyr during the Hirnantian. Changes in sea level or marine redox state associated with glaciation could further impact the expression of the biological fractionation (e.g., through syndepositional sediment reworking and/or chemocline migration and resultant restricted exchange of porewater sulfate). The magnitude of isotopic fractionation during microbial sulfate reduction is partially controlled by metabolic rates, which are sensitive to the abundance, type, and lability of metabolically relevant substrates. Environmental change associated with the end Ordovician glaciation may have elevated the flux of organic material to marine sediments or caused an increase in physical reworking of sediments, leading to increased microbial sulfate reduction rates and reduced epyr. As such, the Hirnantian δ34Spyr excursion may be viewed as a dynamic biological response to global climate change, highlighting the connections between the carbon and sulfur biogeochemical cycles.
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Carbon- and sulfur-isotope geochemistry of the Hirnantian (Late Ordovician) Wangjiawan (Riverside) section, South China: Global correlation and environmental event interpretation
Palaeogeography Palaeoclimatology Palaeoecology, 2012Co-Authors: Paul Gorjan, Kunio Kaiho, David A. FikeAbstract:Detailed geochemical analyses (δ13Ccarb, δ13Corg, δ34Ssulfide, and abundance of sulfide, carbonate and organic carbon) were performed on samples from the Wangjiawan (Riverside) section, close to the GSSP (Global Stratotype Section and Point) for the Hirnantian stage of the Ordovician. New data show two increases in carbonate content coincident with two glacial pulses that reduced detrital input. The new δ34Ssulfide data show distinct changes in this section, with relatively high values in the Kuanyinchiao Formation, a pattern observed throughout the Yangtze Platform. However, there is no consensus on the cause of these changes. The new δ13Ccarb data show a sharp rise and peak in the extraordinarius zone, below the previously published δ13Corg peak in the persculptus zone. A compilation of the new results with other sections indicates the Hirnantian carbon-isotope excursion starts near the pacificus–extraordinarius boundary and elevated values remain until the end of the excursion in the persculptus zone for both δ13Corg and δ13Ccarb. The controversy over correlating Hirnantian graptolite zones with chitinozoan zones can also be addressed. The new δ13Ccarb data also allow direct comparison with Hirnantian δ13Ccarb data from Anticosti Island and the Baltic region, which are zoned by chitinozoan fossils. This comparison favors a correlation of the taugourdeaui and scabra chitinozoan zones with the extraordinarius graptolite zone.
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terminal ordovician carbon isotope stratigraphy and glacioeustatic sea level change across anticosti island quebec canada
Geological Society of America Bulletin, 2011Co-Authors: David S Jones, David A. Fike, Seth Finnegan, Woodward W Fischer, Daniel P Schrag, Dwight MccayAbstract:Globally documented carbon isotope excursions provide time-varying signals that can be used for high-resolution stratigraphic correlation. We report detailed inorganic and organic carbon isotope curves from carbonate rocks of the Ellis Bay and Becscie Formations spanning the Ordovician-Silurian boundary on Anticosti Island, Quebec, Canada. Strata of the Anticosti Basin record the development of a storm-dominated tropical carbonate ramp. These strata host the well-known Hirnantian positive carbon isotope excursion, which attains maximum values of ~4.5‰ in carbonate carbon of the Laframboise Member or the Fox Point Member of the Becscie Formation. The excursion also occurs in organic carbon, and δ^(13)C_carb and δ^(13)C_org values covary such that no reproducible Δ^(13)C (= δ^(13)C_carb – δ^(13)C_org) excursion is observed. The most complete stratigraphic section, at Laframboise Point in the west, shows the characteristic shape of the Hirnantian Stage excursion at the global stratotype section and point (GSSP) for the Hirnantian Stage in China and the Silurian System in Scotland. We therefore suggest that the entire Hirnantian Stage on Anticosti Island is confined to the Laframboise and lower Fox Point Members. By documenting discontinuities in the architecture of the carbon isotope curve at multiple stratigraphic sections spanning a proximal to distal transect across the sedimentary basin, we are able to reconstruct glacioeustatic sea-level fluctuations corresponding to maximum glacial conditions associated with the end-Ordovician ice age. The combined litho- and chemostratigraphic approach provides evidence for the diachroneity of the oncolite bed and Becscie limestones; the former transgresses from west to east, and the latter progrades from east to west. The sea-level curve consistent with our sequence-stratigraphic model indicates that glacioeustatic sea-level changes and the positive carbon isotope excursion were not perfectly coupled. Although the start of the isotope excursion and the initial sea-level drawdown were coincident, the peak of the isotope excursion did not occur until after sea level had begun to rise. Carbon isotope values did not return to baseline until well after the Anticosti ramp was reflooded. The sea-level–δ13Ccarb relationship proposed here is consistent with the “weathering” hypothesis for the origin of the Hirnantian δ^(13)C_carb excursion.
David A T Harper - One of the best experts on this subject based on the ideXlab platform.
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A nearshore Hirnantian brachiopod fauna from South China and its ecological significance
Journal of Paleontology, 2019Co-Authors: Huang, David A T Harper, Rong, Hang-hang ZhouAbstract:The brachiopods collected from the Kuanyinchiao Beds (Hirnantian, uppermost Ordovician) in Meitan and Zunyi counties, northern Guizhou, include 13 species and one undetermined taxon, dominated by Hirnantia sagittifera (M'Coy, 1851) (which accounts for over one-third of the specimens), together with common Eostropheodonta hirnantensis (M'Coy, 1851). They are assigned to the Hirnantia–Eostropheodonta Community, which probably inhabited a shallow-water, nearshore Benthic Assemblage (BA) 2 to upper BA 3 environment. Population analysis shows that the community was well adapted to this environment after the first phase of the end-Ordovician mass extinction. Representative specimens of all the species are illustrated, and a new species, Minutomena missa, is described herein. The variation in Hirnantia sagittifera was noted in many of previous studies but was not statistically evidenced. Here we have measured representative specimens of that famous species from the major paleoplates and terranes in the world, along with other species assigned to the genus from South China. Having used principal component analysis (PCA), significant variations in the species are documented statistically and revised, and three nominal species, one subspecies, and two morphotypes are now reassigned to Hirnantia sagittifera sensu stricto.
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Figure S3 from Identifying the most surprising victims of mass extinction events: an example using Late Ordovician Brachiopods
2017Co-Authors: Seth Finnegan, Christian Ø. M. Rasmussen, David A T HarperAbstract:Crossplots of median predicted risk estimates from models based on the full dataset and from models based on the dataset with single-interval and single-region genera removed. Median predicted risk estimates are based on all background intervals for the full dataset but exclude interval S1 (due to edge effects, see figure S2 caption) for the culled dataset. Both sets of risk estimates are strongly correlated for both the latest Katian (Katian4) interval and the Hirnantian interval (Katian 4 R2 = 0.76, Hirnantian, R2 = 0.76)
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Table S1 from Identifying the most surprising victims of mass extinction events: an example using Late Ordovician Brachiopods
2017Co-Authors: Seth Finnegan, Christian Ø. M. Rasmussen, David A T HarperAbstract:Genus name, status (extinct by the end of interval or survives into the next interval) and risk preditions from all five background intervals for all genera in the latest Katian (Katian 4) and Hirnantian
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Hirnantian (Late Ordovician) brachiopod faunas across Baltoscandia: A global and regional context
Palaeogeography Palaeoclimatology Palaeoecology, 2016Co-Authors: David A T Harper, Linda HintsAbstract:Abstract A diverse, typical Hirnantia brachiopod fauna from terminal Ordovician strata in Latvia extends the distribution of the Kosov Province across much of the Baltic Palaeoplate into the deeper-water facies of the East Baltic. The new data emphasise the co-occurrence of the core elements of the fauna, Eostropheodonta , Dalmanella , Cliftonia , Hindella , Plectothyrella and Hirnantia , in a siliclastic shelfal setting. The fauna has close similarities with faunas elsewhere on Baltica, including Jamtland, the Oslo Region and Vastergotland and plot within the Kosov Province in analyses of the global distribution of Hirnantian assemblages. Carbonate facies, first, in Estonia (lower Hirnantian) and, second, in Oslo together with Ostergotland (upper Hirnantian) support quite different faunas related to the margins of Laurentia and the Edgewood Province of the midcontinent, respectively. Stable isotope curves, when used with caution, have helped correlate the sections across the eastern Baltic into Sweden.
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An earth system approach to understanding the end-Ordovician (Hirnantian) mass extinction
Volcanism Impacts and Mass Extinctions: Causes and Effects, 2014Co-Authors: Howard A. Armstrong, David A T HarperAbstract:The Hirnantian mass extinction is recognized as the first of the "big three" extinctions and, along with the end-Permian and end-Cretaceous events, is the result of an acceleration in biotic extinctions concomitant with a rise in originations. The Hirnantian mass extinction is characterized by high taxonomic impact and within-community extinctions. The Hirnantian mass extinction is also unusual in that (1) it is associated with glaciation, but there is little evidence elsewhere in the younger Phanerozoic that glaciations have been a cause of mass extinction, and (2) there is limited understanding of how glaciation could directly cause mass extinction, particularly in the marine realm. In this review, we argue that coordinated extinctions occurred at the onset and termination of glaciation and were due to climatically induced changes in relative sea level, ocean redox stratification, and sea-surface temperature gradients. These earth system changes resulted in a reduction in prospective niche space, both in the water column and on the seafloor, which in turn led to increased competition and selection pressures, resulting in extinctions where the carrying capacities of particular ecological niches were exceeded. The long-term ventilation of the oceans broke the link between glaciation and mass extinction.
