Search

Hardgrounds from the Upper Ordovician strata of eastern Laurentia exhibit a wide range of morphologies, though the development of their complex surface topographies remains poorly understood. These early lithified seafloors, which can be laterally very extensive, may provide key information about basin dynamics and global ocean fluctuations. Based on spectacular exposures and newly-excavated material, we examined a suite of hardgrounds from the Upper Ordovician (Katian) of southern Ontario and the Cincinnati Arch region and documented the processes of genesis, development, and modification of simple, complex, and block hardgrounds. In this study, we used extensive field work, petrology, and cathodoluminescence to determine the evolution and shaping of hardgrounds as a result of intense submarine erosion and prolonged exposure of these surfaces and reveal a progression from simple to complex hardgrounds and ultimately to isolated cobble- to boulder-sized block hardgrounds. We survey the distribution of hardgrounds in a sequence stratigraphic framework to determine allocyclic controls on hardground development, highlight the important role that mixed siliciclastic-carbonate environments play in promoting the development of heterogeneous hardground topographies, and analyze the effects of high surface reliefs on local depositional and erosional regimes. In addition, we interpret the impact of increasingly abundant complex hardgrounds throughout the Ordovician on hard-substrate community diversification and their contribution to the Great Ordovician Biodiversification Event. We also discuss time-specific aspects of the Late Ordovician that made it a peak interval of hardground proliferation.

The Evans Ferry section from the Appalachian Basin of eastern North America has been analyzed for chemostratigraphic trends to elucidate possible causal mechanisms facilitating the Great Ordovician Biodiversification Event (GOBE). Paired stable isotope (δ13C and δ34S) analyses were used in this carbonate-dominated locality from the Appalachian Basin to reconstruct the marine redox states during this key period of rapid biodiversification. This succession is one of the most expanded Sandbian Stage (Upper Ordovician) deposits known from North America, and it allows new high-resolution reconstructions of long-term, global carbon and sulfur cycle fluctuations. The integrated geochemical and sequence-stratigraphic investigation presented here from the Laurentian epeiric seaway allows for possible identification of the contraction and expansion of reducing water masses during the Middle–Late Ordovician that may have been linked to global-marine paleoredox dynamics. Utilizing new conodont-based 87Sr/86Sr isotope stratigraphy along with previous conodont biostratigraphy, we can confidently correlate our stable isotope profiles to other carbonate successions globally. Carbonate facies indicate that this area experienced partial restriction from open-marine conditions during certain intervals in the Sandbian. These semi-restricted environments record carbon (δ13Ccarb) isotope trends that are secular in nature, as they can be correlated to other successions across Laurentia and Baltica. We identify several intervals when δ13Ccarb and δ34SCAS (carbonate-associated sulfate) trends are decoupled. These inverse stratigraphic trends are, at times, followed by parallel positive shifts in δ13Ccarb and δ34SCAS. Causal mechanisms for the observed decoupled δ13C and δ34S trends may include a wide variety of factors such as more closed-system, local biogeochemical processes associated in part with diagenesis, or they could instead reflect variations in the global fluxes of organic matter and pyrite burial linked to changing marine paleoredox conditions. The positive covariation in trends presented here likely represents transient increases in organic carbon and pyrite burial in response to expansion of reducing marine environments.

The Ordovician Period records an extraordinary biodiversity increase known as the Great Ordovician Biodiversification Event (GOBE), which coincided with a series of environmental changes to the Earth System, notably a cooling global ocean, increased oxygenation, and increased nutrient supply from volcanism and continental weathering. The co-evolution of Earth and its biota during this interval has been studied in various contexts on multiple paleocontinents. Emerging patterns depend on the lens of investigation. Here we summarize the current state of understanding by reviewing and synthesizing the fossil and sedimentary records. Recent paleontological studies, mainly focused on rhynchonelliform (articulated) brachiopods, bryozoa, cephalopods, trilobites, graptolites, echinoderms, and reef organisms, have documented details of diversification, body size increase, development of ecosystem complexity, and intensification of inter-continental dispersal from the late Cambrian through Late Ordovician. Biomass increased markedly between the Early and Middle Ordovician. Furthermore, diversification rates increase statistically during the mid-Darriwilian Age both globally and regionally. Coincident with these biotic changes, geochemical proxies record significant changes to Earth's physical system. Oceanic temperatures decreased, and atmospheric oxygen levels increased to near modern levels in concert with the Middle Ordovician diversification of shelly fauna. Anoxic pulses ceased and evidence for deep ocean ventilation prevails in Middle Ordovician strata. Furthermore, a major Middle–Late Ordovician change in oceanic strontium isotopic composition indicates increased weathering of juvenile volcanic rocks and delivery of nutrients to marine settings. This multi-proxy dataset records near-simultaneous changes in fossil-rich shallow marine environments during exactly the interval of greatest diversification. By integrating biotic and geochemical datasets, a clear picture of the co-evolution of Earth and its biota emerges indicating that the Darriwilian was the critical interval facilitating increased capacity of ecosystems. We, therefore, recommend restricting the term “GOBE” to indicate this short interval of rapid diversification and ecosystem change, and using “Ordovician Radiation” when referencing the sum of diversifications that occurred throughout the Ordovician Period.

A Middle Ordovician limestone succession, representing part of the Alai Formation (middle part of the Chiatsun Group) of the Himalayan Orogenic Belt, is well exposed at Jiacun section in Nyalam County to the west of the Mount Jolmo Lungma (Mount Everest), southern Xizang (Tibet), China. The succession, though reported previously, has yet to be investigated in detail, especially in the aspects of precise dating, stratigraphic correlation and its paleogeographical affinities. In this contribution we document for the first time the conodont biostratigraphy, carbon isotope chemostratigraphy and microfacies analyses of this carbonate succession. The conodont fauna indicates a middle Darriwilian (Middle Ordovician) age corresponding to the Histiodella holodentata Biochronozone. The carbon isotope records have shown minor fluctuations of δ13C values, with only a small negative carbon excursion in the lower part. The microfacies analysis has resulted in the identification of seven microfacies, including micritic limestone, lithoclastic packstone, non-skeletal grainstone, bioclastic limestone, bioclastic-lithoclastic limestone, bioclastic-lithoclastic-oncoid limestone, and bioclastic-lithoclastic-oncoid-ooid limestone. From thin sections, fragments of crinoids, brachiopods, gastropods, trilobites, bryozoans and calcareous algae are identified. Abundant lithoclasts, cortoids, peloids, oncoids and ooids, together with the occurrences of dasyclad algae and hardgrounds, are also recognized, indicating a shallow- and warm-water marine environment, and a likely tropical or subtropical affinity. Comparison of the lithofacies and faunas of this limestone succession of the Alai Formation with coeval strata in the Zanskar-Spiti area (northern India), Nepal, Bhutan, Canning Basin (West Australia), Lhasa, Qiangtang, Sibumasu, Tarim and North China, suggests that during the Darriwilian these regions or terranes may have possessed a warm-water marine environment and were located in low-latitudinal zones. It seems likely that this region of the northeastern Gondwana and peri-Gondwana was protected from substantial influx of cool-water currents derived from high latitudes at that time.

In this study, systematic conodont biostratigraphy and cyclostratigraphy are carried out on Ordovician sections from the North China and South China blocks. The Liangjiashan section, situated on the North China Block margin, accumulated shallow marine carbonate during the Early Ordovician. The Huanghuachang section (GSSP for the Dapingian Stage), situated on the South China Block margin, accumulated thinly bedded carbonates and minor clastics in a deep shelf environment. Conodont biostratigraphy and 405-kyr long-eccentricity cycles interpreted from the Liangjiashan and Huanghuachang sections are used to establish an Astronomical Time Scale (ATS) for the Early–Middle Ordovician period. The ATS indicates that the duration of the Floian Stage is 7.08 ± 0.405 Myr. The temporal ranges of Lower–Middle Ordovician conodont biozones are also estimated: the durations of the Serratognathus bilobatus, Serratognathus extensus, Oepikodus communis, Oepikodus evae, and Baltoniodus triangularis zones are 1.0 Myr, 5.1 Myr, 0.2 Myr, 0.8 Myr, and 0.2 Myr, respectively. The evolution of the ratio of obliquity to total power and short-eccentricity power to total power depicts periodicities of 1 Myr for the s4–s3 term, and 1.9 Myr shifting to 1 Myr for the g4–g3 term. The shift in the g4–g3 term may indicate a chaotic transition in the Mars and Earth orbital motions from a (s4–s3)–2(g4–g3) to (s4–s3)–(g4–g3) resonance state during the Early Ordovician.

Middle to Late Ordovician cephalopod occurrence data from South China and adjacent terranes in the northeastern peri-Gondwana region are used to define biogeographic provinces. Several multivariate methods are adopted for biogeographic analysis, including cluster analysis, non-metric multidimensional scaling and network analysis. During the Middle Ordovician, three biogeographic provinces in this region are recognized, the Australia, the North China–Tibet–Sibumasu (NTS) and the South China–Altun (SA) provinces. In the Late Ordovician, three provinces with modified geographic ranges may also be recognized, as the cephalopods of the Tibetan region and Sibumasu terrane changed significantly and accordingly they have been included with South China–Altun to form a South China–Tarim–Tibet–Sibumasu Province (STTS), while the other two Late Ordovician provinces are the Australia and the North China provinces. The dynamic variation of cephalopod provincialism in the northeastern peri-Gondwana region, may have been controlled by the changing palaeolatitude and differentiated movement of these plates or terranes from the Middle through Late Ordovician.

During the Late Ordovician, large explosive volcanic eruptions deposited worldwide K-bentonites, including the Millbrig and Deicke K-bentonites in North America and the Kinnekulle K-bentonite in Scandinavia. We have studied a classical locality in Oslo containing one of the most complete sections of K-bentonites in Europe. In a 53 m section of Sandbian age, we discovered 33 individual K-bentonite beds, the most notable beds being the Kinnekulle and the upper Grimstorp K-bentonite. Magnetic susceptibility (MS) measurements on two intervals show significant periodicity peaks interpreted as Milankovitch cycles and thus astronomically forced changes in sediment supply and composition. These cycles fit remarkably well with both the expected Milankovitch periodicities for the Ordovician as well as the radiometric ages presented in this study and may represent one of the most convincing demonstrations of Milankovitch cycles from the lower Paleozoic so far. Five of the K-bentonites have been dated by high-precision chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) U-Pb zircon geochronology, where the Kinnekulle K-bentonite gives an age of 454.06 ± 0.43 Ma. We have integrated the new data and calculated an age model showing the sedimentation rates through the section and thereby the ages of each of the 33 K-bentonites. Using the age model, we further present a new age for the Sandbian-Katian stage boundary. The section in Oslo provides the highest resolution window into the Upper Ordovician K-bentonite succession so far and helps shed more light on the chronology of one of the most intense volcanic periods of the Paleozoic and the relationship with the global carbon cycle changes that followed.

Chitinozoans are a group of biostratigraphically valuable microfossils which appeared in the Tremadocian and diversified during the rest of the Great Ordovician Biodiversification Event, to become extinct in the Devonian. The early diversification of chitinozoans on the Baltica palaeocontinent has remained poorly known due to preservation and collecting bias. In this study we document the distribution of chitinozoans through the upper Tremadocian to lower Darriwilian strata in the Jagala waterfall section from northern Estonia, in order to better understand the regional diversity patterns and biogeographic links. Fifty samples were studied from the 7.5 m succession of sandstones, marls and carbonates revealing 47 chitinozoan species and 11 genera. The lowermost productive samples, attributed to the late Tremadocian, represent the earliest rich chitinozoan fauna from Baltica. Combined with previous reports this shows a diversity peak in the Tremadocian with balanced total diversity reaching 17, possibly followed by a decline in the Floian and then a rather gradual increase through the Dapingian and early Darriwilian. The new diversity curve shows more than three times higher values for the Tremadocian and Dapingian than indicated in previous studies, and thus less abrupt radiation in the Darriwilian. The Jagala section provided also new data on the regional biozonal species Conochitina cucumis and Cyathochitina regnelli, which characterize the middle and upper Volkhov and lower Kunda regional stages, respectively. Several other biostratigraphically significant taxa typical of other regions were recovered from the Jagala section, including Conochitina decipiens and Desmochitina bulla. A new short-ranging species Laufeldochitina toilaensis sp. nov. is introduced, having potential to become an index species for middle Dapingian strata. The early Darriwilian assemblage from Jagala includes Rhabdochitina sp. A with up to 2.7 mm long vesicle, making it the largest chitinozoan ever reported.

The Middle-Late Ordovician transition (Darriwilian to Sandbian Age) witnessed a major pulse of the Great Ordovician Biodiversification Event (GOBE) and distinctive oceanic geochemical fluctuations, such as coeval negative C and Sr isotope excursions. In this study, investigations into geochemical variations, notably the Hg abundance (or Hg/TOC), have been carried upon the organic-rich black shale of the Middle-Upper Ordovician Saergan Formation to unravel the causes of this pulse. Based on these data, three phases were identified. Phase 1 (0 to 3 m) is characterized by rising Hg/TOC (up to 138 ppb/wt%) and Ti/Al values as well as high CIAcorr (corrected chemical index of alteration) values (68.9–72.3) with negligible enrichment of redox sensitive elements (RSE) and nutrient elements (e.g. U ≤ 5.2 ppm, V ≤ 153 ppm, Mo ≤ 1.8 ppm, P2O5 ≤ 0.2%), suggesting intensified volcanism, which could have emitted significant amounts of greenhouse gases, thereby leading to climate warming. In contrast, Phase 2 (3 to 11 m) is characterized by decreasing Hg/TOC and Ti/Al ratios, relatively low though slightly fluctuating CIAcorr values, generally depleted in RSE (except moderately enriched U up to 14.6 ppm) and increased P/Al and Ba/Al ratios, implying weakening volcanic activity and subsequent climate cooling and the potential for improved seawater ventilation as a result of oceanic upwelling. Phase 3 (Sandbian Age: 11–13 m) witnessed continuous decrease in Hg/TOC ratio, an increase in Ti/Al and CIAcorr values, fairly low values of RSE enrichment and P/Al and Ba/Al ratios, indicating recurrent climate warming, and the potential for slowed oceanic circulation and attenuated upwelling of nutrient-rich deep waters onto the shallow shelf. These changes could have diminished bioproductivity and organic output onto the seafloor. This study offers insights into volcanic-climatic-oceanic interactions during a major pulse of the GOBE around the Middle-Late Ordovician transition while black shales were extensively deposited.

Massive coralla representing the tabulate coral Agetolites occur on a lime mudstone bed in the Upper Ordovician Xiazhen Formation of southeastern China. Other fossils include solitary rugose corals, bryozoans, trilobites, and mollusks. In addition, abundant spicules and spicule networks suggest that sponges were widespread. The occurrence of intact, unabraded fossils in micritic matrix and the absence of high-energy sedimentary structures indicate deposition in low-energy conditions. Thin section analysis of disoriented specimens demonstrates that geopetal indicators are consistent with stratigraphic “up” and differ from the growth axes of the corals, implying that geopetal infillings formed after disorientation. The growth axes of coralla were not redirected during life, suggesting that the corals were either dead at the time of disorientation or died as a result of disorientation. An examination of cyclomorphism indicates that the corals died at different times, rather than during a single event. A close association between sponges and corals is suggested by the presence of spicule networks in calices and intracorallum spaces of Agetolites. It is hypothesized that disorientation of corals was related to growth on biodegradable substrates. We suggest that many coral larvae settled on sponges that formed “sponge meadows”. Disorientation of the resulting corals may have occurred when (1) a host sponge could no longer support the weight of a coral and collapsed, (2) increasing weight or imbalance of a coral caused it to fall off the sponge, or (3) a coral became detached when the sponge died and decomposed. If the coral was alive, sudden deposition in a disoriented position within muddy sediment resulted in its death. Disorientation of massive skeletons, such as colonial coralla, is most commonly attributed to water energy. This study suggests that ephemeral substrates may have been involved in some cases.

The Late Ordovician mass extinction (LOME) was a severe biocrisis during the Phanerozoic and in this extinction, trilobites, brachiopods, mollusks, graptolites, conodonts and other types suffered severe mortality, and 85% of marine invertebrate species were eliminated. The potential cause of this mass extinction remains unclear, although glaciation and marine anoxia have received the greatest attention. Herein, we present findings on pyrite morphology, pyritic sulfur isotopic composition and total organic carbon content (TOC) to analyze the evolution of redox conditions across the Ordovician-Silurian boundary in the Yangtze area of South China. Our results show that long-term euxinic conditions interrupted by episodic dysoxic/oxic events occurred from the upper Katian stage to the lower Rhuddanian stage. The two extinction pulses of LOME were coincident with euxinic conditions, suggesting that marine euxinia may have been the main cause of this mass extinction event.

Establishing a high-precision numerical geological time scale for the Paleozoic Era remains a considerable challenge. In this study, we conducted detailed cyclostratigraphic analysis on the Middle Ordovician of the Huangnitang section and the core Changjin 3 (CJ-3), Zhejiang Province, South China. Magnetic susceptibility (MS) series were obtained from a total of 3909 discrete samples and are used as a paleoclimate proxy indicative of variations in detrital input of magnetic minerals. Power spectral analyses show that both the section and core have recorded an excellent hierarchy of Milankovitch cycles of 405 kyr, 101–135 kyr, 31–34 kyr, and 15.8–21.3 kyr. An integrated floating astronomical time scale (ATS) for the section and core was established according to the extracted stable 405-kyr eccentricity cycles. This new floating ATS suggests that the durations of the Darriwilian and Dapingian stages were 8.38 ± 0.4 Myr and 1.97 ± 0.7 Myr, and the durations of complete graptolite zones of the Middle Ordovician were precisely estimated. The ratios of band (obliquity or short eccentricity) power to total power reveal periodicities of ~1.2 Myr for the s4–s3 term and ~1.9 Myr for the g4–g3 term, and ~1.2-Myr obliquity cycles may have controlled global third-order eustatic sequences during the late Darriwilian.

The Great Ordovician Biodiversification Event (GOBE) involved a remarkable rise of skeletal reefs during the late Middle Ordovician. A new type of skeletal patch reef built by labechiid stromatoporoids is reported from the Yeongheung Formation (Darriwilian) of the Korean Peninsula, part of the eastern North China Block, a hot-spot for late Middle Ordovician diversification of early labechiids. The attachment of laminar labechiids, subordinate peloidal micritic crusts and spiculate sponges, and minor bryozoans on peloidal–intraclastic packstone to grainstone substrates, and upward growth of these organisms by repeated encrustation, resulted in the construction of centimeter-scale domal tight frame structures. These skeletal frames were in turn veneered by larger labechiids, forming sub-meter-scale laminar skeletal reefs with primary cavities. Such skeletal reefs that formed by the encrustation and binding of labechiid stromatoporoids are analogous to the bindstone frameworks of coeval Laurentian reef mounds and skeletal reefs, though they differ markedly in composition, as is apparent in the absence of tabulate corals and markedly lower contribution of bryozoans, both of which were key elements of the Chazy reefs of Laurentia. The Yeongheung reefs described herein represent some of the oldest labechiid skeletal reefs known to date, and formed mainly by the aggregation of early stromatoporoids prior to the arrival of corals and other robust constructors in the region. This discovery provides new information on the evolutionary trends of early skeletal reefs in response to the appearance of new reef-building organisms and their regional distributions during the Ordovician radiation.

Fossil coral communities are often simply considered as “coral reefs.” In fact, coral-bearing communities are not necessarily coral reefs. The key criterion is whether the corals are in situ and if they are abundant enough to form topographic relief. Two outcropped profiles in the western margin of the Ordos Basin, northwestern China, drill cores of the Lianglitag Formation from 4 wells in the Tarim Basin and 500 large thin sections were examined in this study to reveal the difference between coral reefs and coral-bearing carbonate open platform facies in Upper Ordivician carbonate succession. The second layer of the Shijiezigou section is massive in internal structure, mound-like in shape, 5.0 m in height and 33.6 m in width and is interpreted as a patch reef in an open platform. This layer has a variety of corals that are preserved in situ. Reef-dwelling organisms include dasycladaceans, crinoids and gastropods. Other layers of the section are bedded limestones, many of which bear corals and crinoids, while others bear dasycladaceans or only the Dasycladacea genus Vermiporella. Unlike in the reef, the corals in the open platform facies are sparse and generally occur as fragments, most of which are not preserved in situ. According to the dominant species, four communities are distinguished from the Shijiezigou section, namely, the Vermiporella community, coral community, crinoid-coral community and dasycladacean-crinoid community, each of which adapted to some special environmental conditions. The patch reef of the Shijiezigou section contain abundant and diverse tabulate corals, which form bafflestones, while stromatoporoids are the primary builders in the platform margin reefs of well TZ 822 and the Lijiapo section. The common biotic components of the open platform of the Shijiezigou section and well TZ 822 are crinoids and dasycladaceans. The restricted platform of the Shijiezigou section is dominated by the dasycladacean Vermiporella, while the organisms of the restricted platform facies in well TZ 35 are mainly composed of cyanobacteria, ostracods and Tetradium-like organisms.

The Middle to Late Ordovician was a time of profound biotic diversification, paleoecological change, and major climate shifts. Yet studies examining speciation mechanisms and drivers of dispersal are lacking. In this study, we use Bayesian phylogenetics and maximum likelihood analyses in the R package BioGeoBEARS to reanalyze ten published data matrices of brachiopods and trilobites and produce time-calibrated species-level phylogenetic hypotheses with estimated biogeographic histories. Recovered speciation and biogeographic patterns were examined within four time slices to test for changes in speciation type across major tectonic and paleoclimatic events. Statistical model comparison showed that biogeographic models that incorporate long-distance founder-event speciation best fit the data for most clades, which indicates that this speciation type, along with vicariance and traditional dispersal, were important for Paleozoic benthic invertebrates. Speciation by dispersal was common throughout the study interval, but notably elevated during times of climate change. Vicariance events occurred synchronously among brachiopod and trilobite lineages, indicating that tectonic, climate, and ocean processes affected benthic and planktotrophic larvae similarly. Middle Ordovician inter-oceanic dispersal in trilobite lineages was influenced by surface currents along with volcanic island arcs acting as “stepping stones” between areas, indicating most trilobite species may have had a planktic protaspid stage. These factors also influenced brachiopod dispersal across oceanic basins among Laurentia, Avalonia, and Baltica. These results indicate that gyre spin-up and intensification of surface currents were important dispersal mechanisms during this time. Within Laurentia, surface currents, hurricane tracks, and upwelling zones controlled dispersal among basins. Increased speciation during the Middle Ordovician provides support for climatic facilitators for diversification during the Great Ordovician Biodiversification Event. Similarly, increased speciation in Laurentian brachiopod lineages during the Hirnantian indicates that some taxa experienced speciation in relation to major climate changes. Overall, this study demonstrates the substantial power and potential for likelihood-based methods for elucidating biotic patterns during the history of life.

The Cambrian and Ordovician radiations were the most important biodiversification events in the history of animal life, yet they were separated by intervals of recurrent anoxic sedimentation and regional biomere extinction events in the Late Cambrian and Early Ordovician. Recent work linked biomere extinction events to positive carbon isotopic excursions interpreted to represent enhanced organic matter burial under anoxic conditions. This led to the hypothesis that an increase in oceanic oxygenation following the last of the biomere extinctions paved the way for the rapid diversification of the Ordovician Radiation. Here we test the hypothesis of increasing ocean oxygenation using Th/U preserved in carbonate rocks from the Lower–Middle Ordovician succession near Ibex, Utah, USA. The global seawater uranium inventory is expected to vary inversely with the amount of deep-water anoxic deposition due to uranium sequestration in black shales. Therefore in shallow water carbonates higher Th/U ratios may reflect increased global ocean anoxia. Our data show that the highest Th/U values are coincident with the heaviest carbon isotopic values reported from the base of this succession, which is latest Tremadocian in age. Th/U then decreases and remains low through the Floian up through the contact with the Kanosh Formation, a shale-dominated unit with interspersed limestone units. The geochemistry of the carbonates within the Kanosh Formation likely represents local rather than global conditions in the marine environment. We interpret the overall decrease in Th/U ratios throughout the Tremadocian to Floian interval to represent an increase in global seawater uranium concentration. These data are consistent with the hypothesis that an increase in ocean oxygenation occurred prior to the major pulses of diversification associated with the Ordovician Radiation.

The Late Ordovician Hirnantian glaciation coincided with a major perturbation of the global carbon cycle, characterized by large positive excursions of δ13Ccarb and δ13Corg known as the Hirnantian carbon isotopic excursion (HICE). Previous studies from South China have reported a positive shift in δ13Corg but ambiguous trends in carbonate carbon isotopes. To better define δ13Ccarb variation through the Ordovician-Silurian (O-S) transition in South China, we investigated multiple components (brachiopods, micritic matrix, and microspar cement) in five study sections representing shallow-shelf to deep-basin facies across the Yangtze Block. All sections exhibit negative δ13Ccarb shifts near the base of the Hirnantian stage, with small shifts in two sections (to between − 2 and − 1‰) and large shifts in three sections (mostly

During the Early-Middle Ordovician, the Yangtze Platform in South China was drowned, as indicated by the replacement of pure grey grainstones and packstones of the Hunghuayuan Formation by purple red, argillaceous limestone of the Zitai Formation and correlative units, which are mixed siliciclastic and carbonate deposits. The Xiangshuidong Section, located in Songzi City, southwestern Hubei Province, was selected to study the process in detail. Cluster analysis of point-count groups of the Hunghuayuan and the Zitai formations, indicates that the former contains six microfacies: MF-1 peloidal grainstone, MF-2 thrombolitic grainstone, MF-3 oolitic grainstone, MF-4 bioclastic grainstone, MF-5 lithoclastic grainstone, MF-6 interreef bioclastic grain-packstone, indicating a shelf setting before drowning, while the latter contains seven microfacies: MF-7 bioclastic grainstone, MF-8 bioclastic grain- to packstone, MF-9 bioclastic packstone, MF-10 bioclastic wackestone, MF-11 argillaceous bioclastic wackestone, MF-12 open-marine bioclastic wackestone, MF-13 calcimudstone, indicating the ramp setting during and after the drowning. The drowning was gradual and fluctuating, starting as a marginal shoal, through the inner ramp, and becoming a stable ramp setting. This drowning is mainly attributed to a global sea-level rise, and an increase of terrigenous supply that resulted from local tectonic movement (i.e. the initiation of collision between the South China and Cathaysia blocks). After drowning, red offshore marine facies were well developed along the platform margin, indicating an oxic sea bottom environment during the first acme of GOBE in South China. The changes in environments associated with this transition may have provided new evolutionary opportunities that contributed to the Middle Ordovician diversification.

As an obconical macrofossil with porous double-wall, Calathium was commonly present in reefs of Early to early Middle Ordovician age. The Calathium -bearing reefs thrived globally during the Early Ordovician, but this ecosystem collapsed in Middle Ordovician. A rare case of Calathium -microbial reefs was found from the middle part of the Yijianfang Formation (Darriwilian, late Middle Ordovician) of the Bachu area, located in the northwestern margin of the Tarim Basin, northwestern China. Surrounded by bioclastic grainstones, patch reefs here are well-developed and vary in size, with 1–4 m in thickness and 2–18 m in diameter. Three facies types are distinguished within the reef limestones: (1) Calathium -microbial framestone, (2) echinoderm- Calathium bafflestone, and (3) bryozoan-microbial bindstone. As a dominant type, the Calathium -microbial framestone shows a three-dimensional skeletal framework that is mainly constructed by Calathium and stabilized by microbialites. Although most specimens are toppled, Calathium displays well-developed lateral outgrowths, which connected individuals of the same species. Morphological characters of Calathium in thin sections show that calathids are hypercalcified sponges rather than receptaculitid algae. Unlike the Early Ordovician lithistid sponge- Calathium reefs, the Tarim reefs studied herein contain very few lithistid sponges. Instead, bryozoans are fairly common and act as the most important non-microbial encrusters, attaching to the walls of Calathium . Noticeably, the co-occurrence of pelmatozoans and Calathium is similar to those reported from Late Ordovician calathids-echinoderm reef communities from Tennessee. The presence of Calathium -microbial reefs in Tarim indicates that locally calathids were major reef builders, at least before the latest Middle Ordovician.

In the Central Uplift of the Tarim Basin, northwestern China, the Katian (Upper Ordovician) limestones of the Lianglitag Formation extend from outcrops in the Bachu area to the subsurface in the Tazhong Oil Field (TZOF) area. Reefs and inner platform deposits are the dominant facies throughout the formation. The reef composition illustrates a palaeoecologic gradient from microbial-dominated buildups on the northwestern margin to metazoan-dominated reefs on the eastern and southern margins of the platform. Between these areas no outcrops are present. In the present study, data including litho- and biofacies logs and correlations from eight wells are presented, six of them of the region between Bachu and Tazhong, and two wells along the former platform margin from the TZOF. Calcimicrobial carbonates are common in core from the inner platform part. Benthic biota documented here resembles that of the microbial mounds and bioclastic limestones in the Bachu outcrop area, and thus is considered as the eastward extension of this realm. However, metazoan-dominated boundstones formed by corals, stromatoporoids, sponges together with calcareous algae are abundant along the southern platform margin, and are similar in composition to the reefs of TZOF in the easternmost part of the platform. The palaeoecologic differentiation between microbial- and metazoan-dominated reefs suggests a strong environmental gradient from a well-oxygenated windward eastern and southern side of the platform, which favoured the open ocean and which was dominated by metazoan reefs, and the inner platform as well as the northwestern, leeward margin of the platform with warmer, less-oxygenated water, which favoured the growth of microbial carbonates. Assuming the trade winds as cause for the contrasting facies on both sides of the platform a palaeo-position of Tarim south of the ITCZ seems probable because a position north of it would require a post-Ordovician clockwise rotation of Tarim of about 90° which is possible but less probable.

We investigate the expression of the Middle Darriwilian isotope carbon excursion (MDICE) across marine shelf environments in the Argentine Precordillera. Previous work identified the MDICE in the Las Chacritas Formation in Argentina, but did not recognize the expression of the MDICE in time-equivalent strata of the deeper-water Las Aguaditas Formation (Albanesi et al., 2013; Palaeogeography, Palaeoclimatology, Palaeoecology, v. 398, p. 48?66). Recent biostratigraphic investigations of these units have, for the first time, provided the opportunity for high-resolution correlation, which suggest that the MDICE, or at least the initiation of the MDICE, should be observed in both the Las Chacritas and Las Aguaditas formations. Here we present new paired carbon isotope data of carbonate and organic carbon from the Las Chacritas and Las Aguaditas formations. We identify a 2 ? positive shift in the isotopic composition of marine carbonate in the Las Chacritas Formation, whereas values abruptly fall to < −1 ? in equivalent strata of the Las Aguaditas Formation. This is the first record of divergence from the globally recognized MDICE event. There are also small, yet distinct differences in the isotopic composition of marine organic matter between these two sections. We suggest that the divergent C-isotope trends of carbonate in the Las Chacritas and Las Aguaditas formations represent deposition in fundamentally different parts of the water column above and below the marine chemocline, respectively during the MDICE interval. This interpretation is consistent with data from the Las Aguaditas Formation that shows elevated Mn and Fe concentrations in carbonate phases with little evidence for recrystallization, and with a growing consensus for regionally anoxic conditions for the Middle Ordovician. Our data indicate that redox gradients can play a critical role in the behavior of marine carbon isotope excursions. Fil: Henderson, Miles A.. University of Tennessee; Estados Unidos Fil: Serra, Fernanda. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina Fil: Feltes, Nicolás Alexis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Museo de Paleontología; Argentina Fil: Albanesi, Guillermo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Museo de Paleontología; Argentina Fil: Kah, Linda C.. University of Tennessee; Estados Unidos

Paired measurements of bulk carbonate (δ13Ccarb), organic matter (δ13Corg), and their difference (Δ13C) can be used to estimate changes in isotopic fractionation through time as a function of O2/CO2 in the atmosphere. However, because local scale processes can also affect Δ13C, it is essential to compare sections from widely separated water masses. Here we present new δ13Corg data from Ordovician carbonate rocks from the Great Basin, Oklahoma, and Appalachian Basin and compare with published δ13Ccarb records from these sections and paired δ13C values from other carbonate successions around North America. These new data complement previous studies that focused on Upper Ordovician δ13Ccarb excursions and now provide a composite Ordovician δ13Corg record. New Lower Ordovician (Tremadocian Stage) δ13Corg data range from ca. − 26 to − 28‰, decreasing throughout the Lower–Middle Ordovician (Floian–Dapingian Stages) to ca. − 29 to − 31‰. δ13Corg values remain at their lowest throughout the Sandbian and are similar to other published Upper Ordovician (Sandbian–Katian) δ13Corg data from North America. Δ13C values from well-preserved intervals generally vary between + 26 to + 28‰ throughout the Lower to Middle Ordovician (Tremadocian to early Darriwilian), but increase to + 31‰ during the mid–late Darriwilian and mid Sandbian, similar to published data from younger Late Ordovician positive δ13C excursions known as the Guttenberg (GICE) and Hirnantian (HICE) events. The overall Δ13C trend shows a ~ 3‰ increase throughout the Early–Middle Ordovician and coincides with a previously interpreted period of ocean cooling and some of the earliest pulses of global biodiversity of marine invertebrates and planktonic organisms. Modeling studies predict that pCO2 decreased during this time, suggesting that the effect of pCO2 on Δ13C may have been overwhelmed by other controls, such as an in increase in pO2 or a higher O2/CO2 ratio during this biodiversification event.

The controls of regional depositional environment on stable isotope records spanning the Ordovician-Silurian boundary on the Yangtze Platform have seldom been investigated. The objectives of this study include reconstruction of regional depositional settings and assessment of how sedimentary processes may have influenced carbon- and sulfur-isotopic fractionation. Seven shale facies have been recognized in the Wufeng-Longmaxi interval from two locations. Completely bioturbated claystone of the basal Wufeng Formation accumulated on a shallow oxygenated muddy shelf. Overlying faintly banded black siliceous shale suggests deposition under deep anoxic conditions interrupted by episodes of dysoxia. Muddy fossiliferous facies of the Guanyinqiao Formation reflects a glacial sea-level lowstand setting, and the observed proximal to distal heterogeneity of facies matrix supports shallowing in the proximal area. Overlying faintly banded black shale of the Longmaxi Formation tells of a post-glacial transgression. The carbonaceous deposits are overlain by banded gray and dark gray muddy siltstones and suggest a shoaling upward trend. The shallow-water facies display relatively heavier δ 13 C org values, whereas deep-water facies are characterized by lower δ 13 C org values. Discrepancies between Hirnantian positive δ 13 C org excursions in proximal and distal areas probably reflect spatial gradient in seawater δ 13 C DIC induced by glacioeustasy. Deposits accumulated under oxic and physically dynamic conditions display strong positive δ 34 S sulfide excursions, perhaps a consequence of diagenetic sulfate reduction occurred at some depth below the sediment-water interface and removed from the sulfate pool of overlying water column. Negative relationship between δ 34 S sulfide and TOC indicates that redox conditions and physical reworking of sediment at the sediment-water interface controlled the rate of microbial sulfate reduction and sulfur isotope fractionation. Overall, co-variation of lithofacies and isotopic records suggests that local heterogeneity of seawater chemistry and sedimentary dynamics may exert partial controls on biogeochemical processes and amplify the magnitude of the Hirnantian isotopic excursions in the proximal area.

Sea-level change influences carbon isotopic trends in both modern and ancient carbonate depositional environments. Generally, this relationship is manifested as a positive carbon isotopic excursion where the rising limb of the excursion is associated with transgression. These excursions exist because sea level can influence 1) local/regional/global carbon cycling, 2) basin restriction and meteoric influence, and 3) carbonate sedimentation. The first two processes are influenced, in part, by the large carbon reservoir represented by the terrestrial biosphere. Our goal is to explore the relationship between carbon isotopes and sea level prior to the evolution of vascular plants at a time when the terrestrial carbon reservoir was far smaller. This study focuses on the Upper Ordovician Union Furnace section in Pennsylvania, where a previously documented ~1‰ increase in δ13C values in the upper Sandbian (458.4–453.0 Ma; referred to as the “baseline shift”) has been interpreted to reflect a perturbation to the global carbon cycle due to the proliferation of non-vascular plants. By focusing on the interval deposited before the “baseline shift”, we were able to examine the relationship between sea level, carbon isotopes, and some of the earliest terrestrial ecosystems. We present a high-resolution δ13C record directly tied to the sedimentological and sequence stratigraphic framework. By sampling genetically related packages of rock we were able to document a relationship between carbon isotopes and sea level at the sequence and parasequence level. Our results show that carbon isotopic values decreased during transgressions and increased during regressions. This seemingly inverse relationship between δ13C values and sea level is interpreted to reflect increased terrestrial fluxes during regression in a world devoid of vascular plants. Our results suggest that the early colonization of land by non-vascular plants impacted regional carbon cycling in the Appalachian Basin and eventually the global carbon cycle in the Late Ordovician.

Lower Ordovician stromatolite-like columns and thrombolite-like mounds, composed of fossilised keratose sponges (keratolites) and microbial carbonates, are reported from the Tremadocian Mungok Formation, Yeongwol, Korea. The stromatolite-like columns, which are up to 10 cm wide and high, consist of an inner core with low-angled (10–45°) layers that are covered by high-angled (>45°) layers. The inner core is made up of millimetric layers of alternating keratolite and microbial carbonate, and microbial carbonate dominantly comprises the outer cover. The entire columns are surrounded by bioclastic packstone to grainstone. The thrombolite-like mounds are domes a maximum of 100 cm high and 40–60 cm wide embedded within lime mud and shale. These mounds consist of keratolite–microbial carbonate clots and minor lithistid sponge–microbial carbonate clots. The stromatolite-like columns were formed in a high-energy subtidal setting, in which laminoid keratolite and microbial carbonate formed the tight laminar frame columns. Continued growth of the column narrowed the intercolumnar space, resulting in higher-energy hydrodynamic conditions that limited the growth of sponges but promoted growth of microbial organisms. In contrast, thrombolite-like mounds developed in a low-energy environment below fair-weather wave base, where irregular to bulbose keratose sponges with minor lithistid sponge–microstromatolite associations formed cluster reefs. There appear to have been ecological and/or environmental factors that affected the distribution of these sponges; keratose and lithistid sponges rarely occur together in the Mungok reefs, whereas lithistids are pervasive within coeval intermediate-energy microbial reefs elsewhere. These results demonstrate the importance of hydrodynamic controls on overall reef morphology and configurations during the Early Ordovician, and suggest that keratosan–microbial consortia may have been an integral component of the Great Ordovician Biodiversification Event, together with the lithistid sponge–microbial consortium.

Results of conodont-based δ18Ophos studies from two core sections in Estonia (Mehikoorma-421 and Valga-10) indicate that the pre-Hirnantian Late Ordovician was characterized by a general cooling trend: a rapid climate cooling in the early Sandbian and a less intense cooling in the late Sandbian through Katian. During this time interval, sea-surface temperatures probably decreased by up to 9o C. This fits with earlier ideas of a colder Late Ordovician climate before the Hirnantian. On the background of general cooling trend in the pre-Hirnantian Late Ordovician, tendencies in the temperature changes were quite variable. Several episodes of SST decrease and increase of different magnitudes and durations are evident from the δ18Ophos curves in the studied sections. In the interval sampled in detail, 7 cooling events are recognized and tentatively called (in stratigraphical order): Early Kukruse, Haljala, Keila, Early Nabala, Vormsi, Early Pirgu and Middle Pirgu CE-s. Duration of these events and amplitudes of changes in seawater temperatures increased from Sandbian to late Katian. All cooling events may have triggered the waxing of polar and subpolar Gondwana icecaps and are reflected in the sedimentary succession by major sea-level lowstands recognized also outside the study region. Results of our research do not support a simple, straightforward correlation between temperature trends inferred from δ18Ophos measurements and changes in the δ13C record. Comparison of δ18Ophos and δ13C curves reveal that these two variables behave quite independently: in some intervals their trends fit quite well, in others they might show completely the opposite. For this comparison we provide also a new, detailed δ13C curve from the Valga-10 core section.

The 186-m-thick Shiyanhe Formation exposed at the Sigang section, Neixiang area, Henan Province, central China has been investigated for conodonts. The collected 33 samples yielded 8858 conodont specimens. The conodonts are referred to 15 genera and 34 species, but four species cannot be positively identified and one taxon is kept in open nomenclature. The Oulodus ulrichi, the Oulodus robustus, the Aphelognathus grandis and the Aphelognathus divergens conodont biozones are introduced and the conodont fauna and biozones of the Shiyanhe Formation are referred to the Katian Stage (Stage slices Ka2–Ka4) of the Upper Ordovician. The presence of biostratigraphically important species of the genera Aphelognathus, Oulodus and Yaoxianognathus allows for the recognition of the previously defined local conodont biozones that were established from the studied section. Regionally, the Shiyanhe Formation fauna is relatively similar to the contemporaneous faunas from the Tarim terrane in western China and from the southwestern margin of the North China Platform. However, the faunal composition also shows a close affinity to the North American Midcontinent Province and correlation to North America is clear. The conodont fauna is dominated by the species of Aphelognathus, Belodina, Drepanoistodus, Oulodus, Panderodus, Phragmodus, “Plectodina” and Yaoxianognathus. The Aphelognathus–Oulodus biofacies, the “Plectodina” biofacies, the Panderodus biofacies, the Drepanoistodus biofacies and the Phragmodus biofacies have been established using multivariate statistical techniques on the conodont assemblage. As their turnovers were related to sea-level changes, the conodont biofacies are used to demonstrate the transgressive-regressive pattern in the studied section.

Carbon and oxygen isotopes of both well-preserved articulate brachiopod Paucicrura rogata and cement in mid-Cincinnatian samples (Maysvillian to early Richmondian) on the eastern North American passive margin shelf in SW Ontario show no simple relationships. The δ13C trends of the brachiopods, organic matter and cements are little altered and vary little up section. The brachiopod δ13Cbrach pattern is very close to the American mid-continent whole rock δ13CWRcarb pattern across the Fairview excursions. The δ18Obrach variations, if primary, indicate fluctuating but generally slightly cooling temperatures upwards, though the actual temperatures depend on inferred salinities of the seawater. The temperature/ salinity relationships variations are compatible with the warm deep saline water hypothesis. The paleoceanography shows a change from a warm open passive carbonate margin to a fluctuating warmer/cooler enclosed clastic marginal basin with freshwater input, heralding the cold conditions of the Hirnantian ice age. The Milankovitch cyclicity determined from δ13Ccarb patterns, is practically identical to that from determined from magnetic intensities on the contemporary Yangtse Shelf.

The Hirnantian Glaciation resulted in a perturbation in global carbon cycling, which is recognized as a positive excursion in the δ13C values of both carbonates and organic matter. To investigate the factors controlling the carbon and nitrogen cycles from the late Ordovician to the early Silurian, the C and N isotopic and geochemical compositions of marine sediments deposited on the Yangtze Platform were measured. The majority of rocks in the Wukemuchang section exhibit high δ15Nbulk values, reflecting aerobic nitrogen cycling in the inner shelf. The global spatial variation in δ15Nbulk values likely reflects the occurrence of nitrate gradients in seawater, implying that nitrate was limited in offshore environments that were characterized by oxic conditions. More eukaryotes may be restricted to the inner shelf environments, where large amounts of bioavailable nitrate were accessible. The limited availability of nitrate may have affected the fossil distributions and the complexity of animal ecosystems, thereby influencing the late Ordovician extinction and early Silurian biotic recovery. The δ13Corg profiles from the Yangtze Platform are consistent with earlier reports of Hirnantian shelf gradients in sedimentary δ13CDIC values. We suggest that the global variation in δ13Corg values during the late Ordovician was related to the amount of nitrate because productivity is limited when bioavailable N is scarce. In addition to intense carbonate weathering, the high level of photosynthesis in the inner shelf caused by abundant nitrate and nutrients may have enhanced the dissolved inorganic carbon gradient on shelves globally, driving sedimentary δ13Corg values even higher than those of sediments deposited in the outer shelf where carbon exchange with the surface ocean was less restricted

The late Ordovician was a critical interval in Earth's history, during which changes in the biosphere, climate and environment occurred. In this study, we analyse the nitrogen isotope composition and multiple geochemical indices of Ordovician-Silurian (O S) shales in the Yangtze Sea (South China) to elucidate the evolution of nitrogen cycling and sedimentary environments. The nitrogen isotope values of Wufeng shale samples (Upper Ordovician) range from −0.4‰ to +1.6‰, and those of Longmaxi shale samples (Lower Silurian) range from +0.6‰ to +3.1‰. According to the longitudinal distribution of nitrogen isotope, there was a positive shift from the Wufeng Formation to Longmaxi Formation. Moreover, the redox-sensitive element ratios and nitrogen isotopes were strongly related. These relationships imply that the pre- and post-Hirnantian intervals of nitrogen cycle perturbation were directly related to changes in the oceanic redox conditions in the Yangtze Sea region. During the depositional stage of the Wufeng Formation (pre-Hirnantian) the water column was deep, causing anaerobic reduction in the sedimentary environment. The nitrogen cycle was dominated by anaerobic nitrogen fixation with little isotopic fractionation; thus, the δ15N values are light. In contrast, during the depositional stage of the Longmaxi Formation (post-Hirnantian), global cooling and tectonic uplift led to sea-level change, and sedimentation was increasingly affected by the supply of terrigenous clastic materials. The oxygen content of the water column increased, and redox stratification was maintained dynamically. Denitrification and anaerobic ammonium oxidation (anammox) played important roles in the Longmaxi Formation nitrogen cycle, leading to strong isotopic fractionation. Nitrogen isotopes are a reliable indicator of palaeo-environmental changes and have great value for the restoration of palaeo-depositional conditions and the comparative analysis of global Ordovician-Silurian geological events.

Pronounced excursions of carbon and sulfur isotopes (δ13C and δ34S) spanning the Ordovician–Silurian (O–S) transition have been recognized globally, yet the causes of these isotope events and their causal relationships to environmental changes, especially during Early Silurian time, remain the topics of debate. In this study, we present organic carbon and pyrite sulfur isotope (δ13Corg and δ34Spyr) records and elemental concentrations from a drill core spanning from the Upper Ordovician (Katian) to Lower Silurian (Aeronian) succession of the Yangtze Platform, China. The newly presented δ13Corg profile records previously recognized positive excursions, including the Hirnantian isotopic carbon excursion (HICE), the middle Rhuddanian excursion, and the early Aeronian positive carbon isotope excursion (EACIE). In addition, two negative δ13Corg excursions are recognized during the Katian to early Hirnantian and late Hirnantian to middle Rhuddanian stages. The δ34Spyr profile exhibits a great amount of variation (−17.8‰ to +19.6‰) but mimics the δ13Corg record. Coupled positive δ13Corg and δ34Spyr excursions during the middle Hirnantian and late Rhuddanian to Aeronian are associated with decreased total organic carbon (TOC) content and less reducing water column conditions, whereas the negative δ13Corg and δ34Spyr excursions during the Katian to early Hirnantian and late Hirnantian to middle Rhuddanian are associated with enhanced TOC values and the establishment of more strongly anoxic to euxinic conditions. The δ13C and δ34S excursions during the O–S transition, then, cannot be explained by changes in redox conditions or the burial of organic carbon and pyrite. The heavier and less variable middle Hirnantian and late Rhuddanian to Aeronian δ34Spyr values probably reflect low marine sulfate concentration. In contrast, negative Katian to early Hirnantian and late Hirnantian to middle Rhuddanian δ34Spyr excursions may reflect increased sulfate concentration. The two negative δ13Corg and δ34Spyr excursions associated with the Katian to early Hirnantian and late Hirnantian to middle Rhuddanian correspond with episodes of volcanism. We therefore propose that volcanism was the main driver of coupled negative δ13Corg and δ34Spyr excursions. As volcanism diminished during the middle Hirnantian and late Rhuddanian to Aeronian, both δ13Corg and δ34Spyr returned to more positive values. Our results indicate that volcanism played a crucial role in triggering carbon and sulfur isotope perturbations and environmental changes during the Late Ordovician and Early Silurian.

Made available in DSpace on 2018-11-26T17:06:10Z (GMT). No. of bitstreams: 0 Previous issue date: 2016-10-15 Hanover College Faculty Development Committee The fossil record of conulariids (Cnidaria, Scyphozoa) extends downward into the topmost part of the Ediacaran System, but the first appearance of diverse, widespread conulariids is in siliciclastic rock units of Early Ordovician age, which collectively host at least six conulariid genera. Some of these same units also contain Sphenothallus, a probable medusozoan that frequently co-occurs with conulariids in Ordovician and younger deposits. Lower Ordovician conulariid localities are distributed among five (originally) Southern Hemisphere terranes, namely Core Gondwana (Archaeoconularia, Eoconularia and Teresconularia), Armorica (Conularia azaisi), Avalonia (Archaeoconularia, Eoconularia and Exoconularia), Perunica (Archaeoconularia, Conularia and Conulariella) and South China (Conulariella). C azaisi, currently known from the Southern Montagne Noire (France), probably represents a new genus. Sphenothallus occurs in South China, North China (Korea), Armorica (Southern Montagne Noire) and Core Gondwana (Morocco). In southeastern Morocco, Burgess Shale-type Konservat-Lagerstatten in the Fezouata Shale (Tremadocian Floian) yield Archaeoconularia sp., Eoconularia sp. and at least one species of Sphenothallus. This low-diversity conulariid assemblage is most similar to the Tremadocian assemblage of Wales (Avalonia), which likewise consists of a single species each of Archaeoconularia and Eoconularia. In the Fezouata Shale, Archaeoconularia sp. and Eoconularia sp. frequently occur in monospecific mass associations. Such associations probably represent an original clumped distribution on the seafloor. Additionally, some Eoconularia sp. occur in V-like pairs or radial clusters, and also some specimens were attached at the apical end to a phosphatic brachiopod or to a corner sulcus of a larger specimen of Eoconularia sp. Similar conulariid/brachiopod associations, consisting of Conularia trentonensis and Onniella sp., occur in the Upper Ordovician (Katian) Collingwood Shale of southern Ontario, Canada. (C) 2016 Elsevier B.V. All rights reserved. Hanover Coll, Dept Geol, Hanover, IN 47243 USA Cincinnati Museum Ctr, Dept Invertebrate Paleontol, 1301 Western Ave, Cincinnati, OH 45203 USA Amgueddfa Cymru Natl Museum Wales, Dept Geol, Cathays Pk, Cardiff CF10 3NP, S Glam, Wales Sao Paulo State Univ, Dept Zool, BR-18618000 Sao Paulo, Brazil Univ Sao Paulo, Geosci Inst, R Logo 562, BR-05508080 Sao Paulo, Brazil Univ Sao Paulo, Biosci Inst, R Matao 101, BR-05508090 Sao Paulo, Brazil Univ Sao Paulo, Ctr Marine Biol, Sao Sebastiao, Brazil Univ Southern Mississippi, Dept Marine Sci, Stennis Space Ctr, MS 39529 USA Sao Paulo State Univ, Dept Zool, BR-18618000 Sao Paulo, Brazil

During the late Sandbian and early to middle Katian, a major marine transgression flooded the intracratonic basins and platforms of eastern North America, allowing brachiopods to disperse into the newly formed epicontinental seas. Two main brachiopod faunas can be differentiated using multivariate analysis of faunal data from localities across Laurentia during this time: a relatively diverse Scoto-Appalachian Fauna confined to the eastern margin of North America and peri-Laurentian terranes, and a less diverse but more widely distributed Epicontinental Fauna that expanded into the remainder of the flooded continent. The Epicontinental Fauna can be divided into four subfaunas with subtle compositional differences that correspond to different paleoenvironmental settings of eastern North America, influenced by such paleoecological factors as nutrient-rich cool-water upwelling onto the continent through the Sebree Trough, a greater range of habitable environments on the continental margin and in the adjacent deep Taconic foreland basin, and the interactions between pericratonic and intracratonic water masses of different physical and chemical properties. The Epicontinental brachiopod faunas dominated basins and platforms of continental interior until the Hirnantian extinction event.

Here we report and describe for the first time the Foliomena Fauna fossil record from Portugal (Kralodvorian, upper Katian). This deep-water brachiopod association originates from the uppermost beds of the Porto de Santa Anna Formation (uppermost Katian), cropping out in several localities of the Portuguese Bucaco-Penacova region (Central Iberian Zone). The three studied localities have yielded different associations, composed of 15 species of brachiopods and two trilobites, including both key elements of the deep water Foliomena Fauna and taxa typically from shallower benthic biotas, e.g. the Nicolella Fauna. The mixed character of the associations indicates that they are probably representatives of the atypical Foliomena Fauna, characteristic of relatively shallow benthic assemblages. The Portuguese associations represent the highest-latitude occurrences of the Foliomena Fauna so far and one of the youngest. In the analyses carried out, the two Portuguese associations are grouped together with the other peri-Gondwanan associations (Sardinia and Carnic Alps) and are included within a major grouping including associations previously interpreted as inhabiting deep-water environments and also predominantly late Katian in age. According to these results, the association of Locality 2 may have inhabited a slightly deeper environment than the one present in Locality 3. This agrees with the presence in the former of a greater number of deep-water genera. Despite of the occurrence of a great number of deep-water taxa in the association of Locality 2, the co-occurrence of several elements related to the shallower water Nicolella Fauna, probably transported offshore as a result of the slope of the inclined shelf, suggest that this association probably inhabited slightly shallower environments than those expected for a typical Foliomena Fauna (BA5-6) with no shallow-water elements represented. This inference is also supported by the absence from the association of cyclopygid trilobites.

The main controlling factors of organic matter accumulation in the Upper Ordovician Wufeng–Lower Silurian Longmaxi Formations are complex and remain highly controversial. This study investigates the vertical variation of total organic carbon (TOC) content as well as major and trace element concentrations of four Ordovician–Silurian transition sections from the Upper Yangtze Platform of South China to reconstruct the paleoenvironment of these deposits and to improve our understanding of those factors that have influenced organic matter accumulation in these deposits. The residual TOC content of the Wufeng Formation averages 3.2% and ranges from 0.12 to 6.0%. The overlying lower Longmaxi Formation displays higher TOC content (avg. 4.4%), followed upsection by consistent and lower values that average 1.6% in the upper Longmaxi Formation. The concentration and covariation of redox-sensitive trace elements (Mo, U and V) suggest that organic-rich intervals of the Wufeng Formation accumulated under predominantly anoxic conditions. Organic-rich horizons of the lower Longmaxi Formation were deposited under strongly anoxic to euxinic conditions, whereas organic-poor intervals of the upper Longmaxi Formation accumulated under suboxic conditions. Positive correlations between redox proxies and TOC contents suggest that organic matter accumulation was predominantly controlled by preservation. Barium excess (Ba xs ) values indicate high paleoproductivity throughout the entire depositional sequence, with an increase in the lower Longmaxi Formation. Increased productivity may have been induced by enhanced P recycling, as evidenced by elevated C org /P tot ratios. Mo–U covariation and Mo/TOC values reveal that the Wufeng Formation was deposited under extremely restricted conditions, whereas the Longmaxi Formation accumulated under moderately restricted conditions. During the Late Ordovician, the extremely restricted nature of ocean circulation on the Upper Yangtze Platform in tandem with enhanced stratification of the water column promoted anoxic conditions favorable for the preservation of organic matter. During Early Silurian time, organic matter accumulation was principally controlled by changes in sea level, which affected terrigenous flux, redox conditions, and the degree of nutrition recycling.

Global climate change and tectonism are principal causes of spatial and temporal differences in Ordovician-Silurian (O S) transition sedimentary environments of China's Upper Yangtze Platform. However, the relative significance of each of these drivers is not always obvious. This paper describes results of a geochemical study of a drill core section (Well X3) recovered from Xiang'e Submarine High in the central part of the Upper Yangtze Platform. This work focused on shale deposits of the Wufeng Formation and Longmaxi Formation that accumulated during the O S transition. Geochemical characteristics, including total organic carbon (TOC), pyrite S (Spy), stable carbon and sulfur isotopes, and Mo and U enrichment values, were combined with pyrite framboid size distribution data to reconstruct the O S paleoenvironmental evolution of this region of the Upper Yangtze Platform. The studied succession appears to record the transition from lower Katian-Hirnantian shale deposited under sulfate-limited conditions to Rhuddanian organic-rich shale and finally to organic-depleted Aeronian sediment. The depositional history of these deposits was controlled by changes in water depth and watermass chemistry as a result of changing sea level. The sea-level history of the studied succession was largely controlled by global climate change related to Hirnantian glaciation and Rhuddanian warming and tectonism associated with the Kwangsian Orogeny that resulted in uplift of the Xiang'e Submarine High. Similar geochemical investigation of the Pengshui and Shuanghe sections, also in the Upper Yangtze Platform, demonstrate the dominant role of global climate change and tectonism in controlling organic carbon content and thickness of potential shale gas targets. Specifically, the low concentrations of organic carbon in thin shale deposits proximal to uplifted regions of the basin pose a major risk to shale gas exploration.

Early Ordovician bivalves are rare worldwide; thus discovery of any new bivalve fauna of this age presents an exceptional event. A new abundant bivalve fauna from the Fezouata Formation of Morocco includes nine genera; Alococoncha?, Babinka, Cardiolaria?, Coxiconchia, Ekaterodonta, Glyptarca, Cienagomya?, Praenucula?, and Redonia. Modern evaluation of the recently collected bivalves as an important infaunal element substantially supplements our knowledge of the fossil assemblage of the Fezouta Formation. Bivalves from the Fezouata Formation show close affinities with assemblages of comparable age known from the Montagne Noire as well as with Middle Ordovician bivalve faunas from Perunica, Iberia, Avalonia and Australia. Some genera are in common with Early Ordovician bivalves from South American West Gondwana (Argentina and Bolivia). During the Early Ordovician, the migration of the bivalves was limited only to Gondwana and peri-Gondwanan terrains. The Montagne Noire and Morocco played an important role in the distribution of important groups of bivalves to other regions of European peri-Gondwana (Armorica, Perunica, Iberia) and probably similarly during the Middle Ordovician for the expansion of bivalves through Avalonia to other continents like Baltica and Laurentia. Quantitatively, the Early Ordovician bivalve fauna of Morocco is dominated by heteroconchs associated with protobranchs and pteriomorphs. The dominance of heteroconchs and poor evidence of pteriomorphs are conspicuous here and both of these features characterise a high latitude bivalve fauna. Exclusively infaunal burrowers (filter feeders and deposit feeders) are present in siliciclastic facies of the Fezouata Formation. The genus Redonia completely dominates the Early Ordovician bivalve fauna of Morocco. Functional morphology of muscle scars in Early Ordovician and Cambrian bivalves is compared. Two conspicuous complex muscular systems, decisive for an efficient burrowing, appear already in the Early Ordovician. Myophoric buttress in Early Ordovician protobranchs and heteroconchs seems to be an important character for serving as the attachment point for muscles, which helped to hold the visceral sac during burrowing.

The Tarim Basin, located in NW-China, represents one of China's major palaeoplates. Its palaeogeographic position in the Ordovician, however, is debated. In this study we present biostratigraphically controlled stable carbon isotope data from some 600 samples spanning almost the entire Ordovician in order to provide a tool that can be used for future chemostratigraphic correlations. The data correlate well with results from South China, and several major global δ13C excursions such as the MDICE (Dw2/3), SAICE (Sa2), GICE (Ka1), and Whitewater excursion (Ka4) are documented for the first time from the Tarim Basin. Major sedimentary gaps are detected in the lower Floian between the Penglaiba and Yinshan formations, in the upper Darriwilian to lower Sandbian between the Dawangou and Saergan (or Kanling) formations, and in the latest Sandbian between the Kanling and Qilang formations as well as between the Tumuxiuke and Lianglitag formations. Three scenarios for the depositional history of the late Darriwilian to early Sandbian Saergan Formation, which represents an important hydrocarbon source rock in the Tarim Basin, are discussed with respect to their bio- and chemostratigraphic implications. The mid-upper Ordovician rocks recording high δ13C values have been deposited during transgressive or highstand intervals, whereas the regressive intervals are mostly missing. This results in a stacking of several positive excursions, and thus chemostratigraphic correlation without biostratigraphic control might produce misleading results, especially for shallow-water carbonates.

The Guttenberg Isotope Carbon Excursion (GICE), a positive carbon isotope excursion that occurs near the base of the Katian Stage, is thought to be a global event possibly related to Late Ordovician cooling. Documenting how much regional and global variability exists in carbon isotopic trends prior to and during the GICE is a critical aspect in understanding the implications of the excursion for interpreting changes in the global carbon cycle, paleoclimate, and chemostratigraphic correlation during the Ordovician. To investigate carbon isotopic trends along the southern margin of the Laurentian carbonate platform during the Late Ordovician, we measured bulk carbonate δ 13 C carb and δ 18 O carb as well as organic carbon δ 13 C org values from four locations in Alabama. These sections are excellent study sites because they are well exposed, contain the regionally well-correlated Deicke and Millbrig K-bentonites, and are in a region where δ 13 C trends have not been studied. Carbonate δ 13 C carb results from one section (i.e. Fort Payne) in northeastern Alabama record a 1.5‰ positive excursion above the Millbrig K-bentonite. We interpret the 1.5‰ positive excursion as a primary feature and identify it as the GICE. The characteristic GICE excursion is absent in the other study sections and we attribute this absence to the presence of unconformities (either non-depositional or erosional) in these sections. We find neither evidence for sea level control on δ 13 C trends nor conclusive evidence for δ 13 C DIC gradients along the southeast margin of Laurentia's epeiric sea. Combined with the proximity to the Iapetus Ocean, these results suggest that carbon isotopic trends in Alabama are a reliable proxy for open ocean conditions. Therefore, despite the absence of GICE excursion in all but one of our study sections, increasing δ 13 C values in Alabama prior to the GICE provides evidence for a steady 13 C enrichment of the global surficial carbon reservoir consistent with a drawdown of atmospheric CO 2 during the early Late Ordovician

Analysis of 161 nodular and bedded cherts shows that shallow-water biosiliceous facies, which were common in the Lower Ordovician, had mostly retreated from peritidal–lagoonal facies to deeper settings by the Middle Ordovician. For decades, this retreat was thought to have spanned the Cambrian and Ordovician Periods. Our data suggest that the basinward shift may have taken approximately 2–14 m.y. in the late Lower to early Middle Ordovician. A likely cause for the departure of these often siliceous-sponge cherts from shallow waters was depletion of shallow-water dissolved silica (DSi). Increased use of DSi by radiolarians may have commenced in the Middle Ordovician when radiolarians increased just as a pulse of radiolarite deposition began. Siliceous sponges may have been forced to relocate to deeper waters because radiolarians depleted shallow-water DSi. Apparent low relative sea level in the Middle Ordovician may have hampered the formation and/or preservation of shallow-water cherts by reducing available shelf area to host such deposits. However, the resurgence of shelf chert in the Upper Ordovician while peritidal cherts remained virtually absent suggests that biology was critical to driving the basinward shift of cherty facies. Formation and preservation of abundant latest Darriwilian through Sandbian cherts may have been favored in deep-water settings during a possible warming interval marked by sea-level rise and extensive anoxia and euxinia that coincided with an ongoing decline in 87Sr/86Sr ratios and increased sea-floor volcanic activity that probably enriched the DSi content of deep-ocean waters. Ocean circulation associated with the onset of cooling in the Katian may have optimized chert accumulation by stimulating surface water biosiliceous productivity before DSi-rich chert-preserving anoxic basins were ventilated. As Late Ordovician glaciation intensified, deep-ocean ventilation by thermohaline sinking of oxygen-rich waters probably decreased preservation potential of biosiliceous deposits while increasing oceanic recycling of silica.

Middle–Late Ordovician sequences from the Appalachian Basin and Arbuckle Mountain regions of North America were analyzed for carbonate-associated sulfate (δ 34 S CAS ) and pyrite (δ 34 S pyr ) paired with carbonate (δ 13 C carb ) and organic matter (δ 13 C org ) chemostratigraphy. Two major negative drops in δ 34 S CAS (12‰ excursions) are recognized: the older decline in δ 34 S CAS occurs within the Histiodella holodentata–Phragmodus polonicus Conodont Zones and the younger drop is within the Cahabagnathus sweeti–Amorphognathus tvaerensis ( Baltoniodus gerdae subzone) Zones. These overall these negative shifts in δ 34 S CAS have an antithetical relationship with positive shifts in δ 34 S pyr (~+ 10‰) and δ 13 C carb (~+ 2‰) recorded in the same successions. The older negative δ 34 S CAS shift is coincident with the widely documented mid-Darriwilian δ 13 C excursion (MDICE), and the younger negative δ 34 S CAS shift is coincident with another positive δ 13 C carb shift in the early Sandbian. Geochemical modeling of these sulfur isotope shifts suggests that a decrease in the global rate of pyrite burial or isotope fractionation between seawater sulfate and sedimentary pyrite could account for these negative δ 34 S CAS trends. Additionally, a substantial increase in the weathering flux of pyrite to the global oceans could also explain these secular sulfur isotope trends. While increased crustal weathering is broadly consistent with a sea-level lowstand, and the seawater 87 Sr/ 86 Sr isotope record of change in continental weathering in the late Darriwilian Stage of the Ordovician, geologic and geochemical proxy evidence do not support distinct pulses of continental weathering required to generate two separate negative shifts in δ 34 S CAS . These antithetical isotope trends may be best explained by changes in the marine redox state that significantly reduced microbially mediated pyrite burial and organic matter remineralization rates. Pulses of oceanic ventilation would have expanded habitable environments for marine organisms, and thus is broadly consistent with major increases in biodiversification during this period of the Ordovician.

Patterns of change in the isotope composition of sulfur-bearing minerals play a key role in reconstructing the marine sulfur cycle. Determining marine sulfate concentration, in particular, is critical to understanding the linkages between oxygenation of the Earth's atmosphere and oceans, and the history of life. Much of our current understanding of marine oxygenation relies on a traditional single reservoir model, wherein the isotopic composition of marine sulfate is controlled primarily by burial of sedimentary pyrite. Utility of this model is limited, however, during times of persistent marine euxinia, which marks a fundamental decoupling between oxic and anoxic marine sulfur reservoirs. At these times, short-term fluxes that act between the two reservoirs (e.g., sulfate reduction and sulfide oxidation processes) often dominate over the long term fluxes (e.g., weathering and pyrite burial) that control the single reservoir ocean model. Ordovician strata from Argentina, western Newfoundland, and South China illustrate how the marine sulfur cycle is affected by the presence of a persistent euxinic reservoir. Regional euxinic reservoirs remain generally stable from the Floian through the Dapingian, despite evidence for periodic short-term oxygenation events. Dramatic reorganization of the marine sulfur cycle in the early Darriwilian, however, reflects dynamic disequilibrium between oxic and anoxic (euxinic) marine reservoirs, driven by ocean ventilation. Ventilation resulted in a rapid change in the isotopic composition of marine sulfate in surface oceans, and ultimately led to the near depletion of hydrogen sulfide within the anoxic reservoir, as marked by formation of superheavy pyrite. Ventilation occurred coincident with a decline in sea surface temperatures and may represent the onset of climatic change that ultimately led to late Ordovician glaciation.

A new reef-building consortium from a Middle Ordovician succession of the western North China Platform is described, consisting mainly of bivalves, siliceous sponges, and microbial carbonates ( Epiphyton -like, Renalcis -like and Girvanella calcified microbes, and microcrystalline microstromatolites), in addition to minor brachiopods, Amsassia and Rhabdotetradium . The bivalves are thin-walled and mostly articulated, indicating in situ preservation. The siliceous sponges are characterized by regularly spaced spicule networks embedded within micrite, which partly grade into peloidal textures. Three main types of bivalve–sponge associations are found: (1) larger bivalves (2–13 mm) encrusted by sponges, (2) sponges occupying internal spaces of larger bivalves, and (3) smaller (0.2–0.4 mm) bivalves embedded within sponge spicule networks. Microbial carbonates either cover the upper surfaces of siliceous sponges and bivalves, or occur independently as centimeter-scale patches. The reefal boundstones were constructed mainly by bivalves, siliceous sponges and microbes, which were subsequently encrusted and stabilized by additional sponges and microbes. Extensive early marine cementation forming fibrous cement helped stabilize the reef framework. The co-occurrence of bivalves and siliceous sponges is possibly analogous to modern-day counterparts in which sponges encrust bivalves or bivalves are living within sponges, suggesting a symbiotic relationship. The bivalve–siliceous sponge–microbial reefs of this study, together with other Ordovician reefs, represent the changeover from microbial- to skeletal-dominated reefs during the Middle Ordovician. The current example may represent an ancestral association of bivalve–siliceous sponge–microbe reefs, similar to those in the Mesozoic and Cenozoic, thus shedding light on the roots of such associations.

The Upper Ordovician and Silurian dark to black shales and claystones in the northern Holy Cross Mountains (Poland) are transgressive facies developed in the early Sandbian, early Katian and early Sheinwoodian times, respectively. They were deposited under dysoxic to anoxic bottom waters interrupted, however, by short-term periods of oxygen increase. They contain lenticular and oval (and subordinately circular) structures composed of angular and well sorted micron-size quartz grains bound by cryptocrystalline silica cement. These structures are interpreted as remnants of agglutinated benthic foraminifera which have been members of opportunistic community inhibiting the subsurface sediment. They were able to survive of temporal anaerobic conditions at the bottom of sedimentary basin. Their high frequency in the Upper Ordovician black/dark claystones was facilitated by absence of grazing pressure from predators due to shift of the oxygen gradient above the sediment/water interface. However, in the case of the Sheinwoodian black shales, a high activity of predators and destructive impact of storm currents appear to be an important agents responsible for much lower frequency of foraminifera specimens.

The decimeter-scale 3-D network structure of the Upper Ordovician Pagoda Formation in South China has been an enigmatic sedimentary feature and a case of non-uniformitarian carbonate depositional system, without modern analog. The network bands are developed pervasively throughout the entire Pagoda Formation, superficially resembling desiccation or syneresis cracks, especially in bedding-plane view, with decimeter-scale sub-polygonal to irregularly sinuous patterns. Detailed petrographic analysis, however, shows that the crack-like network bands have the same carbonate matrix, skeletal grains, and overall fabrics as the rock they surround. In this study, we demonstrate that the network structure is not generated by shrinkage cracks but is rather a time-specific lithofacies. It originated from prolonged growth of nodular carbonate, controlled by the convergence of several geological and paleoclimatic factors, including a) a protracted sea-level highstand lasting for about 5 Myr, b) an interval of tectonic quiescence favoring the dominance of carbonate production, with minimal siliciclastic input, and c) location of the South China paleoplate in the hurricane-free paleoequatorial zone devoid of severe storm disturbances between the fairweather and maximum storm wave bases (normally 15–120 m depths), favoring long-term, uninterrupted growth of CaCO3 meganodules on a mid-shelf substrate. This study corroborates recent paleogeographic reconstructions that position the South China paleoplate on the Equator during the Late Ordovician.

The Late Ordovician was a critical interval in geologic history, during which both the biosphere and marine environments underwent severe perturbations, including one of the ‘Big Five’ Phanerozoic mass extinctions and the massive but short-term (~ 0.5-Myr) Hirnantian glaciation. The onset and termination of the Hirnantian glaciation have been widely accepted as the triggers for the two extinction pulses that comprise the Late Ordovician biocrisis, but the mechanisms that caused the Hirnantian glaciation itself remain poorly known. Here, we analyze the nitrogen isotope composition (δ 15 N) of two sections in South China (Wangjiawan and Nanbazi) in order to better understand nitrogen cycle perturbations in the Late Ordovician ocean and their relationship to contemporaneous climatic and biogeochemical changes. Low δ 15 N (~ 1‰) in the upper Katian and lower Rhuddanian of both sections suggests intensive (i.e., near-quantitative) denitrification and, thus, nitrogen fixation as the main source of biologically available nitrogen for primary producers. A positive δ 15 N excursion in both sections during the Hirnantian indicates weaker (i.e., non-quantitative) denitrification, possibly as a result of more vigorous thermohaline circulation and improved ocean ventilation. Weaker denitrification would have reduced the flux of N 2 O, an intermediate product of denitrification, to the atmosphere. N 2 O is a potent greenhouse gas, and a major decline in its production would have led to cooler climatic conditions and, ultimately, the Hirnantian glaciation. A global survey of published nitrogen isotope records suggests that similar processes operated broadly within the Late Ordovician global ocean.

Ordovician conodont studies have revealed a depositional hiatus extending from the late Floian to early Darriwilian on the North China Platform. Recognition of this widespread gap entails revision of the original concept of the Huaiyuan Epeirogeny, with definition of two distinct regional tectonic events: Event 1, which initiated this hiatus, and a subsequent Event 2 that was responsible for terminating Early Palaeozoic sedimentation in the region from the late Katian. The timing of these two events partly coincides with widely recognized eustatic sea-level falls, and separates Ordovician sedimentation into two episodes that can be broadly correlated with eustatic sea-level rises. In combination with these sea-level changes, the Huaiyuan Epeirogeny played a decisive role in shaping and controlling Ordovician sedimentation and sequence stratigraphic architecture on the North China Platform. Lower Ordovician carbonates were deposited during an apparent regression (decreasing accommodation space), resulting from rapid sediment accumulation exceeding the overall rate of basement subsidence and eustatic sea-level rise. Sedimentation ceased in the middle to late Floian when basement uplift commenced in the south and extended northward to affect the entire platform. The diachronous top surface of the Lower Ordovician succession reflects extensive erosion that is most pronounced in the south and southwest parts of the platform where the disconformity surface cut down into Tremadocian (or even upper Cambrian) strata. Deposition of the younger sequence (Darriwilian to Katian) was the result of the interplay between rejuvenated basement subsidence and the late Middle Ordovician eustatic sea-level rise. Event 2, which was initially coupled with eustatic sea-level fall induced by the end-Ordovician glaciations, terminated Ordovician deposition in the region with the top of the Ordovician marked by an unconformity, representing a hiatus of some 122 Ma extending from latest Ordovician to latest Mississippian.

Body size is an important ecological parameter that can be understood in both evolutionary and environmental terms. We present a database of changes in overall body size in brachiopods and trilobites through the Ordovician and earliest Silurian of the Oslo Region, Norway. In contrast with global studies, the limited geographical extent of our data allows correlation with environmental parameters such as climate and water depth, and simplifies taxonomical standardization and stratigraphical correlation. In our data set there is a clear indication of increasing size both for brachiopods and trilobites during most of the Ordovician, although this could result from an unbiased random walk. The size increase is followed by a reduction in size during the latest Ordovician. Trilobites show a strong increase followed by a decrease in body size during the Middle Ordovician, while brachiopod body size peaks later, in the Late Ordovician. These changes are partly correlated with the changes in species diversity in the Oslo Region. We discuss the results in terms of two models for change in body size — Cope's rule and the Lilliput effect. Cope's rule is the theory of an increasing body size through geological time, while the Lilliput effect describes a decrease in body size in the aftermath of a mass extinction or a severe environmental disturbance. We find no clear correlations between body size and sea level.

Redox variation during the Late Ordovician were an important driver of palaeoceanographic and ecologic changes that may have eventually facilitated biotic extinction in the end of the Ordovician. Previously, the Late Ordovician paleoredox studies have focused mostly on the last 1–2 million years prior to the end of the Ordovician; redox conditions during the Sandbian–Katian time (early–middle Late Ordovician) are poorly understood. In order to fully evaluate the effects of palaeoceanographic variations on biotic evolutionary trends through the entire Late Ordovician, it is important to fully understand redox variations following the Great Ordovician Biodiversification Event (GOBE) and prior to the end-Ordovician mass extinction. The present study reconstructs paleoredox conditions from the Sandbian–Katian successions of the Dawangou section, northwestern Tarim Basin, northwest China and the Puxihe section of the Middle Yangtze region, South China using pyrite framboidal and redox-sensitive element analyses. The distribution of volcanic microspherules, fossil fragment content and elemental analysis were also applied to reveal possible volcanisms and chemical weathering conditions, and to evoluate their effects on palaeoxygenation and biotic variations. The new results indicate three distinct periods of fluctuating paleoredox conditions during the Late Ordovician in the northwest Tarim and the Middle Yangtze regions, China. Overall, redox conditions varied from mainly oxic but punctuated by short-lived anoxic–dysoxic episodes to rapid fluctuations between reducing and oxidizing conditions, to persistent euxinic conditions. The varition of oxygen minimum zone (OMZ) that developed in the midwater column might be the main cause of anoxic events during the Sandbian–early Katian. Moreover, abundant volcanic microspheres, enhanced chemical weathering, decreasing fossil fragments of benthos, and positive europium anomaly from the study sections collectively indicate that intermittent volcanism during the early–middle Katian may have triggered anoxic-euxinic events during this interval. If integrating the newly obtained redox data with previously published data for the Katian to Hirnantian from the Middle Yangtze area, the long-term, stepwisely accelerated oxyen-poor conditions during the Late Ordovician may have facilitated biotic extinctions in the end of the Ordovician.

Rocks of Darriwilian and Sandbian age (Middle–Late Ordovician) record the extraordinary peak of the Great Ordovician Biodiversification Event (GOBE) and significant global environmental changes. To track paleo-environmental changes through this critical time, we have analyzed iron speciation, trace elements and total organic carbon in samples of the Miaopo Formation from an intra-shelf basin in South China. The results reveal moderate primary productivity and possible fluctuations in benthic redox. Upwelling during marine transgression is thought to have increased the availability of nutrients in surface waters, sustaining the biological pump. Enhanced export of organic matter would have consumed dissolved oxygen at depth, driving the system toward anoxia. However, it is also possible for deep-water renewal events to transfer more dissolved oxygen into the bottom water. Those proxies suggests high primary productivity and varying sea level (global and local), not benthic anoxia, as the driver of accumulated “black shale”. Thus, the local marine ecosystem thrived in this oxic intra-shelf basin, challenging the preexisting paradox that the high-diversity community flourished in an oxygen-depleted environment.