end-Ordovician Glaciation Research Articles

The Ordovician record of North and West Africa: unravelling sea-level variations, Gondwana tectonics, and the glacial impact

Abstract The Ordovician of North and West Africa comprises three main transgressive–regressive sequences understood as ‘second-order’ cycles of 10–15 myr duration. Tide- to wave-dominated shallow-marine clastic successions, preserving incidental bryozoan carbonates to the north, include fluvial deposits over the most proximal southern stretches of the platform. The boundary with Cambrian strata remains unclear but the latter are progressively less represented to the south in the undifferentiated ‘Cambro-Ordovician’. To the north, graptolites, brachiopods and trilobites combined with palynomorphs provide a robust biostratigraphic frame. Maximum flooding intervals occurred in the early to middle Tremadocian, middle Darriwilian and middle to late Katian. Two events interfered with an overall long-term transgressive trend. The ‘intra-Arenig’ (late Floian?) tectonic event highlighted palaeohighs coinciding with Paleoproterozoic basements. Gondwanan drainage basins were reorganized, which had an impact on sediment sourcing and distribution of detrital material (e.g. zircons) feeding the pre-Variscan Europe. The second event is the end-Ordovician glaciation. The domain supported the greatest part of the Hirnantian glaciers and may also have preserved pre-Hirnantian glacial archives. It is not until the very latest Ordovician that offshore conditions developed far inland; it is however suspected that this inundation benefited from a transient postglacial isostatic flexure.

Seawater signatures of Ordovician climate and environment

Abstract Tracking climatic changes throughout the Ordovician is crucial to a better understanding of the coevolution of life and environment on Earth. Ordovician climate fluctuations have been the subject of a vigorous and productive body of work over the past two decades. Here we present a synthesis of studies that have focused on reconstructing Ordovician climate and environment via direct geochemical proxy datasets and/or numerical modelling approaches. Many new insights have been gained on potential causes of events: the timing and potential causes of the major radiation of Ordovician marine life, changes in weathering and the transition from a greenhouse-to-icehouse state, and cooling and (de)oxygenation during the end-Ordovician Glaciation. Marked improvements in sample resolution/distribution of traditional palaeotemperature (oxygen isotopes), palaeoredox (sulfur isotopes) and weathering proxy records (strontium and neodymium isotopes), as well as the development of new palaeoenvironmental proxies (e.g. clumped, uranium, molybdenum and thallium isotopes; iodine, iron, trace metal geochemistry), have led to better constraints on Ordovician climate and environment. These recent works have led to a more nuanced understanding of the Ordovician Earth System, which allows the global community to focus future efforts on answering remaining questions regarding palaeoclimate and environment, as well as embark upon new investigations.

The Ordovician of Sardinia (Italy): from the ‘Sardic Phase’ to the end-Ordovician glaciation, palaeogeography and geodynamic context

Abstract This review illustrates the most important features of the Ordovician successions of the Sardinian basement. We focus on the stratigraphy and tectonic structures in the tectonic units of the External and Nappe zones of the Variscan basement. The Ordovician successions are characterized by unconformities related to tectonic events ascribed to the Sardic and Sarrabese phases. The different durations of the unconformity-related gaps in the External (17 myr) and Nappe (6 myr) zones, recent work on the trilobite fossil content, and the occurrence of a volcanic arc only in the Nappe Zone (Sarrabus and Gerrei units) highlight significant discrepancies suggesting that these domains did not share the same geodynamic setting and palaeogeographical position during the Ordovician. This implies they were amalgamated only in Variscan times. Whereas for the external and nappe zones the Ordovician features are clear, the high-grade metamorphic Inner Zone, where numerous Ordovician ortho- and para-gneiss occur, more detailed studies are needed to define a complete framework for the Ordovician evolution of Sardinia. The present revision of data for the best-preserved succession of Sardinian tectonic units suggests that at least two distinct terranes, which did not share the same Ordovician evolution, were only amalgamated during the Variscan Orogeny.

TAPHONOMIC CONTROLS ON MICROBIALITE TEXTURES FROM THE STEAMBOAT POINT MEMBER, UPPER ORDOVICIAN BIGHORN DOLOMITE, WESTERN TETON MOUNTAINS, USA

ABSTRACT The Bighorn Dolomite is a massive, cliff-forming dolostone unit found across the state of Wyoming and adjacent areas that records the transition from greenhouse to icehouse conditions during the Late Ordovician. The basal Steamboat Point Member of the Bighorn Dolomite contains cm-scale mottled fabrics often attributed to the ichnogenus Thalassinoides, but their origin remains enigmatic in Upper Ordovician strata. The development of mottled fabrics may have significant implications for marine chemistry and paleoecology. We analyzed cm-scale mottled fabrics from the basal Steamboat Point Member of the western Teton Mountains to determine their origin and develop criteria for identification of similar mottled fabrics found in other Upper Ordovician strata. In vertical cross-section, massive, m-scale exposures display vertical columns (approximately 1 cm in diameter) that anastomose and maintain decimeter-scale vertical continuity. Microscopically, the columns are composed of micro-peloidal micritic textures surrounded by spar and microspar, whereas the intercolumnar matrix is composed of dolomicrite grains, skeletal clasts, and intraclasts. We interpret the microscopic peloidal texture as a product of benthic microbial calcification that coalesced to form mesoclots—diagnostic features of thrombolitic microbialites. Paragenetic analyses reveal evidence for early lithified microbial biofilms despite dolomite replacement and aggrading neomorphism. These observations indicate the Steamboat Point Member of the Bighorn Dolomite was deposited in seawater that was highly supersaturated with respect to carbonate, consistent with other studies that suggest deposition during greenhouse conditions prior to the end-Ordovician glaciation. We present a novel ternary diagram that contains criteria to help differentiate between burrows, microbialite textures, or physical processes that may result in mottled fabrics in the rock record.

Sedimentary and Geochemical Responses to the End Ordovician Glaciation in the Guanyinqiao Formation (Late Ordovician-Early Silurian Period) in the Sichuan Basin

Sedimentary and Geochemical Responses to the End Ordovician Glaciation in the Guanyinqiao Formation (Late Ordovician-Early Silurian Period) in the Sichuan Basin

Enhanced organic carbon burial intensified the end-Ordovician glaciation

Enhanced organic carbon burial intensified the end-Ordovician glaciation

Late Ordovician climate and sea-level record in a mixed carbonate-siliciclastic-evaporite lithofacies, Williston Basin, USA

The Late Ordovician late Katian was a period of generally warm, transitional greenhouse to icehouse climate with sea-level oscillations dominated by moderate-amplitude eustasy. This study aims to improve the understanding of the climatic and sea-level fluctuations prior to the end-Ordovician glaciation, using a comprehensive core, wireline log, and thin-section dataset spanning the topmost Red River-Stony Mountain Formation within the subsurface of North Dakota, Williston Basin, North America. Eleven facies are identified, interpreted, and grouped into four facies associations, including salina to peritidal evaporative, restricted-marine subtidal, open-marine subtidal, and open-marine deeper water. Stratigraphic trends, basin-wide facies variations, and correlation of individual surfaces for ~380 km across the study area, suggest that the facies were deposited under different climatic and sea-level regimes as part of the long-term Stony Mountain sequence. The sequence (37–52 m) is bounded by anhydrites (basin center) and their correlative thin exposure-related breccias (updip proximal basin), and contains a set of high-frequency sequences (HFS; 2–26 m) that can be traced across the study area. The lowstand systems tract (LST) anhydrites formed in a shallow and well‑oxygenated setting under an arid climate during relatively short periods of the basin restriction. The sea-level rise during the early transgressive system tract (TST) resulted in a higher evapotranspiration rate and an increasingly humid climate that peaked during the late TST with the input of siliciclastics off the Transcontinental Arch. The increased precipitation and runoff resulted in quasiestuarine circulation. Return to semi-arid conditions characterizes the highstand systems tract (HST), with increasingly more restricted, shallow-marine and oxygenated setting with antiestuarine circulation. The work provides insight into the interaction of siliciclastic input, climate, and sea-level changes on depositional dynamics and sequence architecture of a mixed carbonate-siliciclastic-evaporite succession that formed in a tropical epicontinental basin setting prior to the end-Ordovician glaciation.

Cold feet: Trackways and burrows in ice-marginal strata of the end-Ordovician glaciation (Table Mountain Group, South Africa)

Abstract New observations from an outcrop of Upper Ordovician Table Mountain Group strata (Matjiesgoedkloof, Western Cape Province, South Africa) have revealed an unexpected ichnofauna that is hosted within diamictites and sandstones that were deposited by a retreating low-latitude (∼30°S) ice sheet during the Hirnantian glaciation. The locality provides a rare window onto animal-sediment interactions in an early Paleozoic ice-marginal shallow-marine environment and contains a trace fossil community with a surprising ichnodiversity and ichnodisparity of burrows, trackways, and trails (Archaeonassa, Diplichnites, Heimdallia, Metaichna, ?Multina, Planolites, Protovirgularia, Skolithos). Exceptional phenomena preserved in the strata include evidence for direct colonization of glacial diamictites by deep-burrowing Heimdallia infauna, and interactions between trackways and dropstones on substrates. Observations from the newly recognized outcrop dramatically expand our understanding of deep-time glacial habitats, demonstrating that deglaciating ice margins had already been colonized by the latest Ordovician. The freshwater influx that would have been associated with such settings implies that faunal associations that were tolerant of brackish water were also established by that time. The locality has further significance because it records the activity of a nearshore animal community contemporaneous with the fauna of the nearby Soom Shale lägerstatte. Combined, these features reveal a paleoecological transect of the diverse marine life that inhabited cold-climate, low-latitude shallow seas around the time of the end-Ordovician deglaciation.

Silurian records of carbon and sulfur cycling from Estonia: The importance of depositional environment on isotopic trends

The Llandovery–Wenlock carbonate-marl succession of the Baltoscandian Basin has been analyzed in the Viki drill core, western Estonia, for carbonate carbon (δ13Ccarb), organic carbon (δ13Corg), carbonate-associated sulfate (δ34SCAS) and pyrite (δ34Spyr) isotopes, along with trace element concentrations. Following the End-Ordovician glaciation and the Hirnantian carbon isotope excursion, δ13Ccarb values rise to an early Llandovery carbon isotope maximum (+2‰). Subsequently, δ13Ccarb and coeval δ13Corg record a large ∼+4.5‰ excursion in the early Wenlock, associated with a regression and biotic turnover known as the Ireviken bioevent (IBE). Overall, δ34SCAS values generally fall within a range (25‰ to 35‰) similar to that reported in other sections of comparable age. Two distinct trends are observed: low and stable δ34SCAS values are associated with deep-water facies, while δ34SCAS becomes higher and more variable in the shallow-water facies. Similarly, δ34Spyr values are low and invariant (∼−15‰) in deep-water facies but become higher and more variable (ranging between −35‰ and 16‰) in shallow-water deposits. Despite abundant recrystallization, no obvious evidence for large-scale alteration of proxies is apparent in concentrations of Fe, Sr, Mn and Mg, or in isotopic correlations, although increased variability and lower values of δ34SCAS are observed where CAS abundance is low. A large, well-resolved positive excursion in δ34SCAS observed during the IBE from a time-correlative section on Gotland (Sweden) is absent in the data presented here. This difference demonstrates that, counter to general understanding, local environmental variations can give rise to divergent δ34SCAS profiles in coeval sections. The S-isotope fluctuations in CAS and pyrite in the Viki drill core can be explained by local processes involving facies changes, early diagenetic processes and associated open vs. closed system behavior that can overprint and potentially mask basin or global signals.

The impact of the end-Ordovician glaciation on sediment routing systems: A case study from the Meseta (northern Morocco)

Assessment of sediment redistribution by end-Ordovician ice sheets is crucial for the reconstruction of Lower Paleozoic source-to-sink patterns. Focusing on the ice-distal, deepwater Tazekka depocenter (Moroccan Meseta), we performed a provenance study that combined whole-rock geochemistry, petrography and insights from high-resolution detrital zircon ages. The results show that the glacigenic sediments are compositionally — mineralogically and geochemically — more mature than preglacial strata. This observation points to a preferential cannibalization of the “great Lower Paleozoic quartz-rich sandstone sheet”, with a limited input of first-cycle, far-travelled clastic sediments. Differentiation of glacial units is not straightforward, yet the glaciation acme is typified by a highly mature sedimentary source and an age spectrum lacking Mesoproterozoic zircon grains, both features strongly indicating derivation from the Cambrian–Lower Ordovician cover of the Tuareg Shield. More regional sources are expressed during the earlier glaciation stages, during which lowstand remobilizations unrelated to subglacial erosion are also suspected. Subordinate but notable late Tonian (∼0.8 Ga) and latest Stenian to early Tonian (∼1 Ga) zircon populations are also evidenced in Morocco, which may have implications for future paleogeographic reconstructions.

Redox conditions and marine microbial community changes during the end-Ordovician mass extinction event

The end-Ordovician (Hirnantian) crisis is the first globally distinct extinction during the Phanerozoic, but its causes are still not fully known. Here, we present an integrated geochemical and petrographic analysis to understand the sedimentary conditions taking place before, during and after the Late Ordovician ice age. New data from the Zbrza (Holy Cross Mountains) and Gołdap (Baltic Depression) boreholes shows that, like in other worldwide sections, the total organic carbon (TOC) content is elevated in the upper Katian and uppermost Hirnantian to Rhudannian black shales, but depleted (below 1%) during most of the Hirnantian. Euxinic conditions occurred in the photic zone in both TOC-rich intervals. This is based on the maleimide distribution, occurrence of aryl isoprenoids and isorenieratane, as well as a dominance of tiny pyrite framboids. Euxinic conditions were interrupted by the Hirnantian regression caused by glaciation. Sedimentation on the deep shelf changed to aerobic probably due to intense thermohaline circulation. Euxinia in the water column occurred directly during the time associated with the second pulse of the mass extinction with a termination of the end-Ordovician glaciation and sea level rise just at the Ordovician/Silurian (O/S) boundary. In contrast, we suggest based on inorganic proxies that bottom water conditions were generally oxic to dysoxic due to upwelling in the Rheic Ocean. The only episode of seafloor anoxia in the Zbrza basin was found at the O/S boundary, where all inorganic indicators showed elevated values typical for anoxia (U/Th>1.25; V/Cr>4.25; V/(V+Ni): 0.54–0.82 and Mo>10–25 ppm). Significant differences in hopanes to steranes ratio and in C27–C29 sterane distribution between the Katian, Rhudannian and Hirnantian deposits indicate changes in marine microbial communities triggered by sharp climate change and Gondwana glaciation. The increase from biomarkers of cyanobacteria (2α-methylhopanes) after the O/S boundary implied enhanced microbial activity following the mass extinction event.

Coral faunal turnover through the Ordovician–Silurian transition in South China and its global implications for carbonate stratigraphy and macroevolution

AbstractA complete coral succession through the Ordovician–Silurian transition in South China reveals an adaptive phase during the Hirnantian glaciation, followed by an early survival phase and finally a late survival phase that persisted into the early Silurian. We demonstrate that a coral assemblage of latest Hirnantian to earliest Silurian age, remarkably similar to those from the Edgewood fauna known from Laurentia, occurs stratigraphically above the typical Hirnantian fauna. This, in combination with other evidence (e.g. brachiopods, lithology and chemostratigraphy), suggests the Edgewood fauna probably post-dated the early–middle Hirnantian glaciation, rather than being coeval with the older glacial-relatedHirnantiafauna. Evidence from South China shows that the Edgewood fauna appeared in the very latest Hirnantian and extended into the middle Rhuddanian, considerably younger than previously believed. Such a new correlation necessitates a reassessment of the influence of the end-Ordovician glaciation on biotas. We argue that this major glaciation probably would have substantially affected the ecosystem even in tropical regions, as shown by the development there of theHirnantiafauna or, alternatively, the presence of a conspicuous stratigraphic hiatus. This suggests a surprisingly rapid biotic recovery during the subsequent postglacial transgression, represented by the flourishing of comparatively diverse shelly faunas (e.g. the Edgewood fauna and theCathaysiorthisbrachiopod fauna) in nearshore shallow water environments from Laurentia to eastern peri-Gondwana terranes or blocks (e.g. South China).

Echinoderms from the lower Silurian Brassfield Formation of east-central Kentucky

AbstractA new echinoderm fauna is reported from the Brassfield Formation (Rhuddanian, Silurian) of Bath County, Kentucky. The Brassfield Formation was the first extensive marine unit to be deposited following the end-Ordovician glaciation and extinctions and represents several shallow, open-marine facies. These facies supported a diverse pelmatozoan fauna. This report not only extends the geographic distribution of this fauna, but also the temporal range of the fauna back to Rhuddanian time. Six pelmatozoans are reported, including the crinoids Browerocrinus arthrikos n. gen. n. sp., Temnocrinus americanus n. sp., Stereoaster sp., and Dendrocrinus sp.; and the glyptocystitids Brockocystis nodosarius Foerste, 1919, and Anartiocystis whitei Sumrall, 2002. In addition, the asteroid Gordonaster brassfieldensis Blake and Ettensohn, 2009, was reported previously from this locality. Browerocrinus increases the diverse calceocrinid fauna from the Brassfield Formation; Temnocrinus was previously only known from the Homerian (Silurian) of England; and this is the first known occurrence of Stereoaster beyond the greater Dayton, Ohio, region. Furthermore, this is the first Brassfield locality known with two glyptocystitid taxa.

Changes in marine productivity and redox conditions during the Late Ordovician Hirnantian glaciation

Changes in marine productivity and redox conditions during the end-Ordovician (Hirnantian) glaciation and Ordovician–Silurian transition were investigated through Mo-isotope and major- and trace-element analyses of the Wangjiawan (Hubei Province) and Nanbazi (Guizhou Province) sections from the Yangtze Platform of South China. Katian shales of the Wufeng Formation, which yield the graptolites Dicellograptus complanatus, Dicellograptus complexus, and Paraorthograptus pacificus, were deposited under euxinic conditions at both localities, as shown by high MoEF, UEF, and δ98Mo values. A major sea-level regression during the Hirnantian glaciation resulted in shallowing and a shift toward better-oxygenated conditions within the Yangtze Sea, as well as deposition of thin-bedded siliceous sediments, calcirudite debris flows, and limestone turbidites of the Kuanyinchiao Formation, the base of which correlates with the first phase of the end-Ordovician mass extinction. The termination of the Hirnantian glaciation at the top of the Kuanyinchiao Formation was associated with a major sea-level transgression, a rapid expansion of euxinia in the Yangtze Sea (as documented by a return of high MoEF, UEF, and δ98Mo values), and the second phase of the mass extinction, during which the cool-adapted Hirnantian Fauna went extinct.The long-term cooling trend of the Middle and Late Ordovician, which culminated in the Hirnantian glaciation, was driven by enhanced burial of organic carbon, as documented by δ13Ccarb, δ13Corg, and δ34Spy records. Increased organic carbon burial was linked to high rates of marine productivity, as shown by high TOC and biogenic Ba concentrations especially at the deeper, less-restricted Wangjiawan locale, producing the HICE (Hirnantian Isotopic Curve Excursion) δ13Ccarb excursion. The locus of organic carbon burial during the Hirnantian crisis shifted to deeper-water environments that were located outside the study region. The relatively rapid onset and termination of the Hirnantian glaciation were probably due to crossing of tipping points in the Late Ordovician climatic–oceanic system.

Spore assemblages from Upper Ordovician and lowermost Silurian sediments recovered from the Qusaiba-1 shallow core hole, Qasim region, central Saudi Arabia

Palynological analysis of a sequence of Upper Ordovician to lowermost Silurian sediments from the Qusaiba-1 core hole drilled in the Qasim region of central Saudi Arabia has yielded rich palynomorph assemblages. The palynomorphs are abundant, exceptionally well preserved and of low thermal maturity. The sequence represents marine sediments that encompass glacial deposits representing the end Ordovician (Hirnantian) glaciation. All of the palynomorph assemblages are dominated by marine elements (acritarchs, chitinozoans, scolecodonts), although land-derived spores are present in most assemblages and are often moderately abundant. There is clear evidence for reworking into some of the glacial deposits. The strata are tightly age constrained based on chitinozoan and acritarch biostratigraphy, in addition to graptolite biostratigraphy in the lowermost Silurian deposits. In this paper we systematically describe the spores and erect two new taxa: a cryptospore permanent tetrad Tetraplanarisporites laevigatus gen. et sp. nov. and a trilete spore Chelinospora prisca sp. nov. We discuss the biostratigraphical implications of these tightly age constrained spore assemblages and recognise the need to establish regional rather than global spore biostratigraphic schemes in the Ordovician–early Silurian. We also discuss the palaeobotanical implications of the spore assemblages. The presence of true trilete and hilate spores, some of which are ornamented, suggests that a clade of plants characterised by trilete/hilate spore production may have evolved earlier in Gondwana than elsewhere. We develop evolutionary and palaeophytogeographical hypotheses to explain this early occurrence of trilete and hilate spores.

A Cenozoic-style scenario for the end-Ordovician glaciation.

The end-Ordovician was an enigmatic interval in the Phanerozoic, known for massive glaciation potentially at elevated CO2 levels, biogeochemical cycle disruptions recorded as large isotope anomalies and a devastating extinction event. Ice-sheet volumes claimed to be twice those of the Last Glacial Maximum paradoxically coincided with oceans as warm as today. Here we argue that some of these remarkable claims arise from undersampling of incomplete geological sections that led to apparent temporal correlations within the relatively coarse resolution capability of Palaeozoic biochronostratigraphy. We examine exceptionally complete sedimentary records from two, low and high, palaeolatitude settings. Their correlation framework reveals a Cenozoic-style scenario including three main glacial cycles and higher-order phenomena. This necessitates revision of mechanisms for the end-Ordovician events, as the first extinction is tied to an early phase of melting, not to initial cooling, and the largest δ13C excursion occurs during final deglaciation, not at the glacial apex.

Mean grain size fluctuations of the siliciclastic component in the Aizpute-41 core: implication for end-Ordovician glaciation; pp. 257–263

The article deals with grain size distribution of the insoluble residue of Upper Ordovician carbonate rocks of the Baltoscandian Basin in the West Latvian Aizpute-41 core. Great fluctuations in mean grain size in the sections of the Pirgu (uppermost Katian) and Porkuni (Hirnantian) stages are considered as indications of eustatic sea level fluctuations, pointing to the glaciation in the Hirnantian and its prelude in Pirgu time. An alternative cause of grain size fluctuations might be the up-and-down tectonic movements. Nevertheless, the synchronicity of cessation of fluctuations with the end of the Hirnantian glaciation supports the glaciation-related reasons.

Dynamic sulfur and carbon cycling through the end-Ordovician extinction revealed by paired sulfate–pyrite δ34S

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 (δ34SSO4). 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 (Québec, 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 (εpyr). The εpyr record shows a strong inverse correlation with δ13C, suggesting a mechanistic link between carbon cycling and processes controlling εpyr 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 εpyr. 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.

Predominance of stratified anoxic Yangtze Sea interrupted by short-term oxygenation during the Ordo-Silurian transition

The Ordo-Silurian transition was a critical interval in geological history, during which profound biotic, climatic and oceanic changes occurred. In order to explore the oceanic palaeoredox changes, multiple geochemical proxies, including ratios of S/C, FeHR/FeT, FeP/FeHR and DOP values, are presented here from three sections (Wangjiawan, western Hubei, Sanjiaguan, northern Hunan, and Nanbazi, northern Guizhou) across the Ordo-Silurian boundary on the Yangtze Platform. These palaeoredox data indicate a predominance of stratified, anoxic (ferruginous) ocean on the Yangtze block during this transition, which was interrupted by a brief episode of oceanic oxygenation in the early Hirnantian. This oxygenation, temporally coinciding with the end-Ordovician glaciation and global glacio-eustatic sea level fall, likely resulted from enhanced circulation of polar cold, dense oxygen-rich water onto the low-latitude shelf. The prior and subsequent longer-term episodes of anoxic ocean, particularly the later one which started in the late Hirnantian, occurred in parallel with the rapid climatic warming and sea level rise, which could have slowed down oceanic circulation, thereby enhancing oceanic stratification, anoxia and organic preservation. Oceanic redox changes, together with rapid climatic and sea-level fluctuations, were likely responsible for the stepwise massive demise of the Ordo-Silurian biotic crisis.

A far-field record of the end Ordovician glaciation: The Ellis Bay Formation, Anticosti Island, Eastern Canada

The end Ordovician was a critical period in Earth history. It was a time of global climatic change with large-scale continental glaciations. A wealth of recent work has documented the stratigraphical records from the near-field areas, i.e., from various Gondwanan continental relics where glaciation was more severely felt. There is an urgent need to re-examine the stratigraphic record from the far-field areas at greater levels of resolution than it has been hitherto possible in order to understand the timing, duration, and character at the glaciation centred around the Ordovician/Silurian (O/S) boundary. One of the best exposed and most complete stratigraphic records from paleotropical areas near the boundary is on Anticosti Island in eastern Canada. The east–west trending, ∼ 200 km long Ellis Bay (Hirnantian) outcrop belt, with superb coastal exposures at both ends of Anticosti Island, is slightly oblique to the paleoshoreline with near-shore siliciclastic-dominated facies restricted to the eastern part of the island and more offshore carbonate-dominated facies present along the central and western parts. These mixed carbonate–siliciclastic facies accumulated along a storm-dominated beach-to-offshore profile. They are organized into a stacked set of shallowing-upward offshore to shoreline facies successions (typically coarsening upward) separated by deepening upward facies successions (typically fining upward) formed during intervening transgressions. High-frequency glacio-eustatic fluctuations were the dominant control on the development of TR sequences observed in the Ellis Bay succession of Anticosti Island, although tectonic subsidence had a substantial secondary effect on thickness. Five TR sequences are recognized allowing for precise correlation along the Ellis Bay outcrop belt. In the eastern sections, relatively thin TR sequences are locally bounded by unconformities. A typical sequence begins with thin transgressive limestone overlain by a maximum flooding, calcareous shale grading up into regressive, storm-influenced, proximal to transitional zone fine sandstone/grainstone and shale, which are sharply overlain by shoreface to foreshore sandstone/grainstone. In the central and western sections, the thicker TR sequences have more symmetrical transgressive–regressive components reflecting greater subsidence compensated by a higher carbonate sediment supply. Two discrete glacial periods are recognized during the deposition of the Hirnantian Ellis Bay succession with the younger one recording large sea level changes (> 50 m in magnitude) caused by the development of very large ice sheets over western Gondwana. Abundant ice conditions on the globe during the end Ordovician, and the resulting high-amplitude eustasy, appear to have left distinctive stratigraphical imprints on mixed, paleotropical, siliciclastic–carbonate ramps.