Early Darriwilian Research Articles

Ordovician conodont biodiversity analysis from the Precordillera (Argentina): a new insight in the global context of the GOBE

ABSTRACT The rapid diversification of conodonts during the Ordovician has been extensively studied by several authors, each presenting distinct diversity curves for different paleocontinents. This study analyzes trends in conodont biodiversity in the Precordillera region, spanning from the late Tremadocian to the early Sandbian ages. A conodont dataset, incorporating species previously identified in sections across Central and Eastern Precordillera, is utillized. This contribution presents the conodont diversity curve for the Lower to early Upper Ordovician in the Precordillera for the first time. The resulting conodont biodiversity pattern reveals that the initial peak of diversification occurs during the O. intermedius Zone (middle-late Floian). A sharp decline in biodiversity is observed during the Dapingian. Finally, during the early Darriwilian, there is a rise in the diversification of conodont species, reaching a peak in the L. pseudoplaunus Zone. This pattern is also compared to curves from Baltica, South China, North China, and Tarim, showing similar trends among them. The variation in Lower-Middle Ordovician conodont diversity in the Precordillera may be linked to different pulses or events within the ‘GOBE’ (Great Ordovician Biodiversification Event).

Diversity dynamics, faunal turnover and radiation pattern of the Middle Ordovician trilobites in South China

Available data on the temporal and geographical distribution of Middle–early Late Ordovician trilobites in South China were reviewed under a high-resolution biostratigraphical framework to investigate the diversity dynamics and explain the faunal turnover and radiation pattern of trilobites during the Middle Ordovician. Three macroevolutionary phases are recognized on the basis of the diversity changes at taxonomic levels. Cluster analysis shows three trilobite groups, two distinct trilobite faunas and three faunal turnovers through nine time slices. Three trilobite groups (Groups A, B and C) are coupled with three macroevolutionary phases, and characterized by different proportions of the Whiterock Fauna. Two distinct trilobite faunas (Early–Middle Ordovician and Late Ordovician Trilobite Faunas) and three faunal turnovers could be recognized at different similarity coefficients, their replacements were mainly affected by changes of sea level and substrate conditions. The trilobite radiation was manifested by the development, expansion and domination of the Whiterock Fauna (part of the Paleozoic Evolutionary Fauna) through the Middle Ordovician. The first diversity acme of trilobites occurred during the Dapingian–early Darriwilian, while the radiation centre gradually shifted from inner platform to outer platform environments. From the end Darriwilian onwards, elements of the Whiterock Fauna had further developed and dominated all environmental settings from the inner platform to the lower slope, coinciding with the second and more extensive radiation of trilobites in South China. Two phases of radiation may have been directly triggered by two major transgressions during the late Dapingian–early Darriwilian and the late Darriwilian–early Sandbian respectively. In addition, climate cooling and various substrate conditions, as well as the increased oxygen levels in the atmosphere and marine water, nutrients and tectonic movements during the Middle Ordovician may have provided essential conditions for the trilobite radiation.

Ordovician plate tectonic and palaeogeographical maps

Abstract Plate tectonic and palaeogeographical maps are presented for nine time intervals during the Ordovician: early Tremadocian (485 Ma), late Tremadocian (480 Ma), early Floian (475 Ma), earliest Dapingian (470 Ma), early Darriwilian (465 Ma), late Darriwilian (460 Ma), middle Sandbian (455 Ma), middle Katian (450 Ma) and early Hirnantian (445 Ma). A plate model was used to predict the changing age of the ocean floor and, hence, the changing volume of the ocean basins and the resulting degree of continental flooding. The maps show the locations of active plate boundaries (i.e. subduction zones, mid-ocean rifts, transform faults, island arcs, back-arc basins and collision zones) and the age of the ocean floor. During the Ordovician there were six continental plates (Gondwana, Laurentia, Baltica, Cathaysia, Avalonia and Cuyania) and six oceanic plates (Iapetus, Ran, Kipchak, and Panthallasa 1, 2 and 3), as well as several poorly constrained back-arc basins (Zealandia, Alexander and Farwell). The palaeogeographical maps also illustrate the ancient distribution of mountains, land, shallow seas and deep ocean basins. Sea level was high during the Early Ordovician, fell during the middle Ordovician and rose again during the Late Ordovician. A precipitous fall in sea level occurred at the end of the Ordovician due to the short-lived Hirnantian Ice Age.

Condorodus n. gen., a new Ordovician conodont genus from Argentina: origin, evolution and dispersal through the western margin of Gondwana

Ordovician conodont collections from several Argentinian basins including the Eastern Cordillera, Famatina and Precordillera allow recognition of a group of conodonts that comprise a new genus here named Condorodus n. gen. Species of this genus have an apparatus composed of six elements recovered so far: Pa, Pb, Sb1, Sb2, Sc and Sd. The differences mainly between the P elements support recognizing three species, from the older to younger: C. diablensis n. gen., n. sp., C. gracielae n. gen., n. sp. and C. chilcaensis n. gen., n. sp., that appeared in the upper Floian (Lower Ordovician) and vanished in middle Darriwilian time (Middle Ordovician). The Eastern Cordillera is here assumed as the place of origin of the Condorodus n. gen. lineage during the late Floian, and then this genus dispersed through the western margin of Gondwana, reaching the Precordillera in the early Darriwilian, from there it could have dispersed to different regions of Gondwana, Perigondwana and Laurentia during the late Darriwilian, and probably give rise to conodont apparatuses of similar morphology in the Late Ordovician.

Biostratigraphically-controlled Darriwilian (Middle Ordovician) δ13C excursions in North China: Implications for correlation and climate change

A continuous, 545 m thick carbonate succession through the Darriwilian Bei'anzhuang and Machiakou (Majiagou) formations in the Tao-112 drillcore in the Ordos Basin, North China was sampled for an integrated study of conodont biostratigraphy (48 samples) and carbon isotope chemostratigraphy (510 samples). Two internationally correlatable conodont biozones were recognized in the drillcore, namely the Histiodella holodentata-Tangshanodus tangshanensis Biozone and the Plectodina onychodonta Biozone in stratigraphically ascending order. Constrained by the conodont biostratigraphy, two carbon isotopic anomalies can be distinguished, including the short-term LDNICE (Lower Darriwilian Negative Isotopic Carbon Excursion) in the lower part of the Bei'anzhuang Formation, and the long-lasting MDICE (Middle Darriwilian Isotopic Carbon Excursion) spanning the upper part of the Bei'anzhuang Formation and much of the Machiakou Formation. Moreover, the present high-resolution carbon isotope record allows the recognition of two peaks of the MDICE, the stratigraphically lower, small peak MDICE-1 and the follow-up, long-lived peak MDICE-2. The Darriwilian carbon isotope excursions in North China can be comparable to the δ13Ccarb trends described in coeval successions in Baltoscandia, North America, South Korea, and other regions of China, suggesting their significance for global correlations and providing an integrated and high-resolution chemo-biostratigraphic framework for sedimentary records of Darriwilian age in North China. Based on widely accepted driving mechanisms of global carbon-cycle perturbations, here we put forth an inference on the climatic pattern of the Darriwilian that a short warm period causing the LDNICE in the early Darriwilian was followed in turn by a moderate global cooling event resulting in the MDICE during the middle and late Darriwilian. Carbon cycle processes of the Middle and Late Ordovician indicate that there was a clear pre-Hirnantian cooling step, with gradually enlarged alternating relative warmer and rather cooler interludes, culminating in the ‘Hirnantian’ ice age.

Conodont biostratigraphy and carbon isotope chemostratigraphy of the Middle to Upper Ordovician on the western Yangtze Platform, South China

Middle and Late Ordovician strata in the nearshore area of the western Yangtze Platform are investigated. In total, 241 δ13Ccarb samples and 108 conodont samples from the Songliang and Tuanjie sections in the northeastern Yunnan Province were collected for chemostratigraphic and biostratigraphic studies. Altogether, 21 species belonging to 16 conodont genera were recovered from the two sections, among which Ansella jemtlandica, Drepanoistodus basiovalis, Hamarodus brevirameus, and Microzarkodina hagetiana are important age indicators within the Dapingian to early Katian interval. Four carbon isotope shifts are recognized in the studied sections: a negative shift in the early Darriwilian, the middle Darriwilian carbon isotope excursion (MDICE), the early Katian Guttenberg carbon isotope excursion (GICE), and a further positive carbon isotope shift later in the early Katian which is a new record of this event in South China. Integrated carbon chemostratigraphy and conodont biostratigraphy indicate early Darriwilian and early Katian ages for the base and top, respectively, of the Huadan Formation at the Songliang section, and a Darriwilian to early Katian age for the Shihtzupu Formation at the Tuanjie section.

Stable ocean redox during the main phase of the Great Ordovician Biodiversification Event

The Great Ordovician Biodiversification Event (GOBE) represents the greatest increase in marine animal biodiversity ever recorded. What caused this transformation is heavily debated. One hypothesis states that rising atmospheric oxygen levels drove the biodiversification based on the premise that animals require oxygen for their metabolism. Here, we present uranium isotope data from a Middle Ordovician marine carbonate succession that shows the steepest rise in generic richness occurred with global marine redox stability. Ocean oxygenation ensued later and could not have driven the biodiversification. Stable marine anoxic zones prevailed during the maximum increase in biodiversity (Dapingian–early Darriwilian) when the life expectancy of evolving genera greatly increased. Subsequently, unstable ocean redox conditions occurred together with a marine carbon cycle disturbance and a decrease in relative diversification rates. Therefore, we propose that oceanic redox stability was a factor in facilitating the establishment of more resilient ecosystems allowing marine animal life to radiate.

Biodiversity changes of the Ordovician trilobites in the middle Yangtze region of South China

The biodiversity changes of trilobites in the middle Yangtze region of South China in the Great Ordovician Biodiversification Event and the end-Ordovician mass extinction have been analysed. The Ordovician trilobites in this region originated in the early Tremadocian and radiated from the late Floian to the early Katian. Two peaks of diversity appeared in the late Dapingian to the early Darriwilian and the early Katian. The extinction occurred in the late Katian, followed by a survival period in the Hirnantian. The Whiterock fauna began to surpass the Ibex fauna during the early Katian and were dominant by the late Katian to the Hirnantian, indicating that Cambrian Evolutionary Fauna was gradually replaced. The study once again confirms the view that the effects of factors such as sea level, climate, oxygen content, nutrient supply, and tectogenesis differ temporally and to different degrees in different regions, resulting in differences in the vertical sequence and horizontal combination of ecosystems, which is the reason for the different evolution patterns of Ordovician trilobites in different regions.

Ordovician graptolite biostratigraphy of the Röstånga-2 drill core (Scania, southern Sweden)

ABSTRACT The graptolite biostratigraphy of the 116.11-m-long Röstånga-2 drill core from Scania, southern Sweden, includes the Darriwilian (Middle Ordovician) Holmograptus lentus, Nicholsonograptus fasciculatus, Pterograptus elegans, Pseudamplexograptus distichus and Jiangxigraptus vagus biozones, and the lower Sandbian (Upper Ordovician) Nemagraptus gracilis Biozone. The early Darriwilian Komstad Limestone was not reached. The succession includes numerous levels of brecciated rocks and slickensides, suggesting considerable tectonic deformation and possible gaps. The boundary between the Almelund Shale Formation (below) and the Sularp Shale Formation (above) has been identified at 19.55–19.70 m, where the Fågelsång Phosphorite Bed has been identified tentatively in the Jiangxigraptus vagus Biozone. The first specimens of Nemagraptus gracilis appear higher in the succession. The international correlation of the Darriwilian graptolite succession is discussed.

δ30Si and δ18O of multiple silica phases in chert: Implications for δ30Siseawater of Darriwilian seawater and sea surface temperatures

A promising quantitative method is the use of fluctuations of seawater silicon isotope compositions (δ30Siseawater) to estimate the silicon cycle and silicon isotopic budgets of both the marine and terrestrial systems in a pre-diatom ocean. However, a thorough global calibration of Phanerozoic δ30Siseawater, which is linked to continental tectonics and silicate weathering, has not been established. The silicon and oxygen isotope compositions of Darriwilian cherts (δ30Sichert and δ18Ochert, respectively) have considerable potential as proxies for δ30Siseawater and sea surface temperatures (SSTs), which can expand our understanding of the influence of continental tectonics and temperature changes on the Great Ordovician Biodiversification Event (GOBE). In this study, primary depositional and early diagenetic cherts were sampled from bedded chert, lump-shaped chert, nodular chert, and banded chert from five Darriwilian outcrops in the Kalpin and Yijianfang areas of the Tarim Basin in China. The preservation conditions of these cherts were assessed based on petrographic criteria such as the absence of calcite veins and fractures under macroscopic observation and the absence of aggregate chalcedony (CHA), microquartz (MQ), and cryptocrystalline quartz (MQC) using optical microscopy and scanning electronic microscopy (SEM). The SSTs and δ30Siseawater of Darriwilian seawater were reconstructed using the δ18Ochert and δ30Sichert of well-preserved cherts. Our results show that the SSTs were approximately 32 ± 5 °C in the early Darriwilian and approximately 30 ± 5 °C in the late Darriwilian. The δ30Siseawater was approximately +3.1‰ in the Kalpin and Yijianfang areas during the Darriwilian Stage. This is the first estimation of δ30Siseawater in the Darriwilian Stage, which provides a parameter for the global calibration of the δ30Siseawater of the pre-diatom ocean in the Phanerozoic Eon and validates the hypothesis that the pre-diatom ocean waters had higher δ30Si values than modern seawater.

Feeding traces related to shells from the Prague Basin, Czech Republic (Tremadocian to early Darriwilian, Ordovician)

Post-mortem feeding activities along or within the interiors of shallowly buried shells can be preserved by sediment infilling and be visible on internal moulds of shelly fossils. Almost no such activities were recorded in three lowermost Ordovician formations (Třenice, Mílina and Klabava; Tremadocian to Dapingian) of the Prague Basin in the Barrandian area (Czech Republic). In contrast, a distinct increase occurs in the Šárka Formation (early and middle Darriwilian). Šárka feeding traces are common in shells preserved within siliceous nodules. They are associated with almost all taxonomic groups composing a rich and diversified fossil assemblage within that formation; but there is no evidence of similar traces produced by feeding on soft-bodied fauna. Arachnostega, Pilichnus and Palaeophycus are the most abundant ichnogera observed. Their trace makers selectively oriented on decaying soft tissues. Palaeophycus producers apparently preferred an easily accessible and nourishing food that was easily consumed. These traces typically intersect a digestive tract, especially the gut. The Arachnostega and Pilichnus traces are oriented in a manner suggesting systematic feeding. These tracemakers spent more time in a carcass, reaching lower nutrient tissues, including various membranes, that degraded more slowly and were more difficult to access. The preferential distribution of feeding traces in each shell reflects the topology of the soft tissues serving as a food. All studied feeding traces were found in nodules, never in shales of the Šárka Formation. This reflects a taphonomical bias influencing occurrences of these traces. Therefore, a lack of the feeding traces in the units underlying the Šárka Formation could be the result of taphonomic differences related to depositional conditions and diagenetic processes. Nevertheless, the abundance of those traces in the Šárka Formation illustrates the prosperity of this feeding behavior during the Darriwilian. An increase in the abundance, diversity and complexity of communities that fed within the internal ecospace of shells while utilizing feeding strategies developed during previous periods is considered to be a primary effect related to increased ecosystem complexity during the Great Ordovician Biodiversification Event.

New conodont records of the Los Sombreros Formation (Cambrian–Ordovician) from the Western Precordillera, Argentina: biostratigraphic and palaeoenvironmental implications

AbstractThe presence of a carbonate platform that interfingers towards the west with slope facies allows for the identification of an ancient lower Palaeozoic continental margin in the Western Precordillera of Argentina. The Los Sombreros Formation is essential for the interpretation of the continental slope of the Precordillera, which accreted to Gondwana as part of the Cuyania Terrane in the early Palaeozoic. The age of these slope deposits is controversial; therefore, a precise biostratigraphic scheme is critical to reveal the evolution of the South American continental margin of Gondwana. The study of lithic deposits of two sections of the Los Sombreros Formation, the El Salto and Los Túneles sections, provides important information for further understanding the depositional history of the slope. At El Salto section, the conodonts recovered from an allochthonous block refer to theCordylodus proavusZone (upper Furongian). The conodonts recovered from the matrix of a calclithite bed of the Los Sombreros Formation in the Los Túneles section are assigned to theLenodus variabilisZone (early Darriwilian), providing a minimum age for this stratigraphic unit. In addition, clasts from this sample yielded conodonts from thePaltodus deltifer−Macerodus dianaezones (upper Tremadocian). The contrasting conodont colour alterations and preservation states from the elements of two latter records, coming from the same sample, argue the reworked clasts originated in the carbonate platform and later transported to the slope during the accretion process of the Precordilleran Terrane to the South American Gondwanan margin during the Middle–Late Ordovician.

Ordovician conodonts and the Tornquist Lineament

The Holy Cross Mts. in southern Poland are generally believed to be split by a tectonic dislocation into two separate parts, a NE one being a part of the Baltic Craton and a SW part belonging to the Małopolska Terrane of a complex geotectonic history connected with the Trans-European Suture Zone (Tornquist Lineament). Unexpectedly, conodont assemblages of earliest Middle Ordovician (early Darriwilian) age from Pobroszyn in the northeastern Łysogóry region and from Szumsko in the southwestern Kielce region show virtually identical species composition. One of the dominant species both in Pobroszyn and Szumsko, Trapezognathus pectinatus sp. n., characterized by denticulated M elements, occurs elsewhere only on the northern margin of Gondwana. Separation of the Małopolska microcontinent from Baltica continued after the disappearance of Trapezognathus and an apparently allopatric speciation process was initiated by a population of Baltoniodus. Also in this case, denticulation developed in the M elements of the apparatus but the process of speciation of B. norrlandicus denticulatus ssp. n. was truncated by re-appearance of the Baltic lineage of Baltoniodus. Later conodont faunas from the region are of Baltic affinities, but remain distinct in showing a relatively high contribution from exotic species of Sagittodontina, Phragmodus, and Complexodus. The periodic North American Midcontinent tropical influences observable in the Baltic region did not reach the Holy Cross Mts. The Ordovician conodont faunas from the Holy Cross Mts. can be fit into a transect extending from Baltica to Gondwana, with several intermediate terranes. This new evidence disproves the long-held concept that the Łysogóry and Kielce regions had separate early Palaeozoic fates and that the Małopolska Terrane was unified with Baltica already in the earliest Ordovician. The Trans-European Suture Zone truly marks the presence of a wide Tornquist Sea in the early Palaeozoic.

Early diversification of Ordovician chitinozoans on Baltica: New data from the Jägala waterfall section, northern Estonia

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 Jägala 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 Jägala 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 Jägala 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 Jägala includes Rhabdochitina sp. A with up to 2.7 mm long vesicle, making it the largest chitinozoan ever reported.

Lower and Middle Ordovician chitinozoans from Honghuayuan, South China: Biodiversity patterns and response to environmental changes

A complete chitinozoan succession through the uppermost Tremadocian to lower Darriwilian (Lower and Middle Ordovician) at Honghuayuan, northern Guizhou Province in South China, was systematically studied and 47 species of 14 genera were identified. Two chitinozoan biodiversity acmes were recognized, which occur in the lower part of the Acrograptus filiformis (uppermost Hunghuayuan Formation) and middle part of the Azygograptus suecicus (middle member of the Meitan Formation) graptolite biozones. In the upper Dapingian and lower Darriwilian (uppermost part of the Meitan Formation), chitinozoan diversity was relatively high after a less conspicuous peak in the upper Dapingian. Comparisons between diversity trends of chitinozoans, acritarchs and brachiopods, and the environmental changes revealed both similarities and differences. In the Floian, the chitinozoan biodiversity curve at Honghuayuan shows good correlation with sea-level changes. The group temporarily disappeared when the sea level was at the highest, then reappeared and reached a biodiversity acme when sea level was low. Furthermore, a large organic carbon isotope excursion is coincident with the absence of chitinozoans and a highstand period after the sea-level rise in the lower and middle Floian. Diversity curves show fewer connections with the sea-level and organic C-isotope undulations during the Dapingian and early Darriwilian. New data suggest that the environmental changes played a significant role in the early stage of Ordovician chitinozoan radiation, roughly corresponding to the Floian at the Honghuayuan section. In the Dapingian and early Darriwilian, the intrinsic factors seem to have played a more important role in shaping biodiversity patterns of chitinozoans in South China.

Synchronous diversification of Laurentian and Baltic rhynchonelliform brachiopods: Implications for regional versus global triggers of the Great Ordovician Biodiversification Event

Abstract The profound global impact of marine radiations during the Great Ordovician Biodiversification Event (GOBE) is widely appreciated; however, diversification varied among paleocontinents and these individual trajectories are less understood. Here we present a new species-level diversity curve for rhynchonelliform brachiopods from midcontinental Laurentia based on bed-by-bed analysis of the Simpson Group of Oklahoma (USA). Diversity and abundance data span the Dapingian through Sandbian Stages, which encompass the interval of maximum global diversification. A rapid, statistically significant increase in brachiopod diversity was observed in the early Darriwilian Histiodella holodentata Biozone. We interpret this as a biological signal because the increase cannot be explained by sampling intensity, facies types, or position along depositional gradient. Diversifications on Laurentia and Baltica were temporally synchronous at the biozone level, and cumulative diversity curves for the regions show similar patterns, suggesting a global driver for the radiations. The taxonomic composition of the brachiopod faunas, however, differs substantially, highlighting the importance of regional controls on diversification. Thus both global and local factors controlled diversity increase during the GOBE.

Refining 2 km of Ordovician chronostratigraphy beneath Anticosti Island utilizing integrated chemostratigraphy

New high-resolution chemostratigraphy, in combination with updated biostratigraphy, refines the chronostratigraphic resolution of the nearly 2 km thick Ordovician section below Anticosti Island. A total of 1414 horizons, spaced at 1.0–1.5 m intervals, were sampled from the New Associated Consolidated Paper (NACP) drill core and analyzed for major and trace elemental composition by portable X-ray fluorescence analyzer (pXRF). Select micrite and calcareous shale powders were then analyzed for δ13Ccarb (number of samples, N = 364) and 87Sr/86Sr (N = 25). Our results indicate a Floian to early Darriwilian (F3–Dw1) age for the Romaine Formation, a middle Darriwilian to Sandbian (Dw2–Sa1) age for the Mingan Formation, a lower to mid-Katian (Ka1–Ka2) age for the Macasty Formation, an upper Katian (Ka3) age for the lower Vauréal Formation, an uppermost Katian (Ka4) age for the upper Vauréal, and a Hirnantian (H1) age for the Ellis Bay Formation. This integrated chemostratigraphic and biostratigraphic synthesis establishes the position of numerous unconformities, the duration of the intervening depositional sequences, and rates of sedimentation.

Behavior of marine sulfur in 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.

Huaiyuan Epeirogeny—Shaping Ordovician stratigraphy and sedimentation on the North China Platform

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 122Ma extending from latest Ordovician to latest Mississippian.

Middle Ordovician cephalopod biofacies and palaeoenvironments of Baltoscandia

During the Middle Ordovician cephalopods became an important part of the macrofauna of the Baltoscandian carbonate platform. The earliest cephalopod abundance peak was reached during the early Darriwilian, within the Kunda Stage Yangtzeplacognathus crassus and Lenodus pseudoplanus conodont zones. In sediments of this time interval large orthoconic cephalopods often occur in masses with more than one specimen per square-meter on bedding surfaces. The assemblages are characterized by the strong dominance of often large endocerids. In proximal depositional settings coiled tarphycerids and other cephalopod groups are an important additional component. In the most distal settings orthocerids are the most important secondary component. Correspondence Analysis of assemblages throughout Baltoscandia revealed three distinct biofacies, which here are termed Orthocerid, Proterovaginoceras and Anthoceras Biofacies, respectively. The biofacies reflect differences in depth and proximity to the shoreline and are consistent with the Baltoscandian Confacies Belts. Spatial changes in absolute abundance and taxonomic composition indicate increased original cephalopod population densities and habitat expansion within the Y. crassus and L. pseudoplanus conodont zones. A nearly coeval abundance peak in a similar facies in South China indicates supraregional causes of the mass occurrence, probably reflecting a globally increased nutrient availability in the water column during the Darriwilian.