The article presents results of studies of the stromatolites from Middle Ordovician deposits of the Moyero River section (Siberian platform). It demonstrates that despite the growing dominance of skeletal metazoans against the backdrop of Great Ordovician Biodiversification Event (GOBE) in Siberian Platform reef systems, microbial communities remained integral components of reef formation from the Middle Ordovician onward. These communities functioned both as crucial auxiliary elements and as sole framework builders. Stromatolites are associated with bioherms dominated by skeletal metazoan reef-builders (Angarella sp., Moyeronia sp., tabulate coral, Cryptolichenaria miranda, and stromatoporoid, Priscastroma gemina). This indicates that in some parts of the platform, due to regional peculiarities, microbial communities not only retained their role in Middle Ordovician reef systems, but also formed bioherms without symbiosis with other reef builders. Microbial communities form two distinct stromatolite types: less numerous spheroidal forms (SS and SS-I) and more prevalent stromatolite bioherms (SS → LH). The persistence of microbial influence in this region is attributed to specific depositional environment conditions. Due to their morphology and diagenetic alterations, these stromatolites have a potential for hydrocarbon reservoir formation. The study highlights that while contemporary reef zones in the Ordovician of West Taimyr are prospective yet poorly understood reservoirs, the Moyero River organogenic build-ups serve as valuable geological models. Further research into their structure, texture, and genesis is essential to refine predictions for Ordovician reservoir distribution. These findings emphasize the need to consider stromatolites as significant targets in reservoir exploration.

Stromatoporoids have played a role in reef construction globally since the late Middle Ordovician. The diachronous and patchy distribution of stromatoporoids has limited our understanding of their diversification and palaeoecological involvement in early reef ecosystems during the Ordovician. This study re-examines calcimicrobe-stromatoporoid bioherms from the upper Darriwilian Moyero Formation, exposed along the Moyero River in Siberia. The bioherm is dominated by calcimicrobes and stromatolitic fabric that intermittently alternate with stromatoporoid sponges. Unlike other contemporaneous reef structures from peri-Gondwana and Laurentia, the stromatoporoids in the Moyero River locality are accessory components with a subordinate role in reef construction, represented by a single stromatoporoid species, Cystostroma insuetum. This difference of stromatoporoid occurrence from other Middle Ordovician examples is interpreted to reflect the progressive absorption of stromatoporoids into reef communities, contrasting with their more abrupt appearances in contemporary reefs in other regions of peri-Gondwana and Laurentia. This study highlights the importance of recognizing regional variability in reef development and emphasizes the role of stromatoporoids in the transition from microbial-to metazoan-dominated reef ecosystems. The re-investigation provides new insights of palaeogeographic variations and the environmental conditions that favoured the proliferation of stromatoporoids during the Middle Ordovician. (c) 2024 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

The Kulyumbe River section (NW of the Siberian Platform) is one of the most complete, best exposed, well-studied, and relatively easily accessible Cambrian-Ordovician (Cm-O) successions on the entire Siberian Platform. It is a key section for regional correlation of the Furongian-lower Ordovician interval in Siberia. However, the exact position of the Cm-O boundary as well as the boundaries of the global Ordovician stages in the section is still a matter of debate. We present new magneto-, chemo-, and biostratigraphic data, which allow us to discuss the position of the Cm-O and Tremadocian-Floian boundaries in the Kulyumbe River section. These data determine a new confident levels for correlation of transitional Siberian Cm-O layers with the International Stratigraphic Scale.

The phosphate-bearing rocks (phosphate rocks and phosphorites) were studied over a 45-meter interval of the Moyero river section, covering the upper part of the Darriwilian and the lower part of the Sandbian stages of the Ordovician. These rocks were investigated by field observations and laboratory methods, including optic and scanning electron microscopy, X-ray diffraction analyses. The accumulation of phosphatic matter is manifested in carbonates, sandstones, and aleurolites in the form of grains (ooids and peloids), intraclasts, phosphatic and phosphatized shells. In argillites, it is represented by cryptocrystalline matter. Phosphate matter consists of fluorapatite. The peaks of phosphate accumulation are associated with the formation of layers of physically reworked granular phosphorites (condensation horizons) directly above the depositional sequence boundaries. The formation of the studied phosphate-bearing rocks during the Darriwilian-Sandbian transition was influenced by a combination of global (Great Ordovician Biodiversification Event (GOBE), reduction in sea surface temperature, atmospheric CO2, high sea level, flooding of craton margins) and regional (equatorial position of Siberia, arid climate, facies) factors. Global conditions led to the enrichment of seawater with phosphorus and the effect of upwelling. Regional conditions determined the characteristics of phosphate formation. Studied phosphate-bearing rocks can be considered as a record of upwelling on the Siberian craton during the Middle-Late Ordovician transition and one of the manifestations of long-term global cooling started early in the Middle Ordovician.

Trypanites is the most common macroboring with a global occurrence throughout the Phanerozoic and is the eponym of the Trypanites ichnofacies. It has great value for the recognition of discontinuity and hiatal surfaces on top of carbonate hardgrounds. Four ichnospecies of this ichnogenus are currently regarded as valid, while a characteristic fifth form from the early Palaeozoic is frequently reported in open nomenclature. The latter commonly occurs in lithic hardgrounds and skeletal hard substrates (such as stromatoporoids, bryozoans and corals) in Ordovician and Silurian carbonates of the Baltic region. A review of the old literature found that this ichnospecies has previously been introduced twice, as Trypanites sozialis Eisenack from a Silurian erratic boulder found on the Sambia Peninsula near Kaliningrad (now Russia), and as T. keilaensis Orviku from the Ordovician of Estonia. Both ichnospecies became forgotten names for unknown reasons. A morphometric analysis of the type material and topotype specimens indicates gradational transitions between both ichnospecies, rendering T. sozialis as the senior synonym of T. keilaensis . A lectotype from the type series of both ichnospecies is defined and additional material from Ordovician and Silurian carbonates of Estonia and Russia is described and illustrated to show the morphological and morphometric variation of this important bioerosion trace fossil. To enable interspecies comparison, a lectotype of the type ichnospecies of Trypanites , T. weisei Mägdefrau, is selected. Trypanites sozialis is a common boring in Cambrian to Devonian hardgrounds and occurs on the palaeocontinents of Baltica, Laurentia and Gondwana.

Paleomagnetic studies of the Ordovician rocks of the Mishina Gora explosive structure (Gdovsky district, Pskov region) have been carried out. Based on the results of thermal magnetic cleaning, two stable magnetization components S and P have been identified. These components have similar unblocking temperature ranges (~200–600°C), but their mean directions are statistically different the difference of their mean directions are statistically significant. Refional fold–test indicates the postfolding age of the component S and prefolding age of the component P. Prefolding age of the latter is supported by the positive result of the local fold-test which has been carried out in the scale of the studied section. Comparison of the paleomagnetic pole calculated from the S component with the Apparent Polar Wander Path for the East European Platform definitely indicates the Late Permian-Early Triassic age of this component, which, in combination with the available geological observations, determines the time range of the formation of the Mishina Gora structure as ~370-250 million years ago. The ancient component of the magnetization P has the same direction as the components, which have been identified earlier in a number of coeval sections of the North-West of Russia, Estonia, and Scandinavia and which have been considered to be primary. The corresponding paleomagnetic poles are widely used in Paleozoic geodynamic and tectonic paleoreconstructions with the participation of the Baltic paleocontinent. A positive fold test result for the P component validates the use of these poles for paleoreconstructions. For the stratigraphic interval, which includes the upper Floian, Dapinian, and lower 2/3 of the Darrivilian stages, a magnetostratigraphic section has been constructed containing a single magnetozone corresponding to the reverse polarity of the geomagnetic field. Thus, the magnetostratigraphic data do not support the hypothesis of the existence of intervals of normal polarity within the Ordovician geomagnetic superchron of reverse polarity (Moyero superchron).

The C–O–Sr isotope composition of carbonates and the S–Sr isotope composition of gypsum were studied in the Ordovician section and the Upper Cambrian–Lower Silurian boundary horizons of the Vilyui structural-facies zone located in the central Siberian Platform. The upsection decrease of the 87 Sr/ 86 Sr ratio within 0.70920‒0.70795 and the value of δ 34 S ~ 25‒28‰, typical for the reference sections of this stratigraphic interval, indicates a good connection between the basin and the World Ocean in the Ordovician and Early Silurian, but its isolation in the Late Cambrian (Kholomolokh Formation) when the 87 Sr/ 86 Sr ratio (0.7085) was lower than the oceanic one. The sharp 87 Sr/ 86 Sr ratio decrease from ~0.7087 to 0.7080 in the Khar’yalakh Formation (~45 m thick) can be correlated with the interval from the upper Darriwilian-to-upper Sandbian stages of the ICC (Middle Ordovician Volgian horizon–Upper Ordovician Baksanian or Dolborian horizon of the Russian GSS). Based on this benchmark and lithological features of the section, we discuss the causes of wide variations in δ 13 С (‒7.2 to 1.6‰) and δ 18 О (16.9‒29.8‰) in the carbonates, as well as a possible correlation of carbon isotope excursions in the Vilyui section with the global carbon isotope events.

The magnetostratigraphic study of the geomagnetic reversal frequency just before the Ordovician Moyero superchron can be very useful for testing various geodynamic conceptions and different geodynamo models describing the geomagnetic field evolution. In terms of completeness of the geological record, the Ordovician carbonate-terrigenous key section of the Moyero River valley, located in the northern part of the Siberian Platform, is considered to be one of the best Ordovician sections of Northern Eurasia and seems to be very promising to carry out such a study. However, the Tremadocian part of the section, which could record the pre-superchron reversal history, until now remained poorly studied and exact positions of its boundaries with underlying Upper Cambrian and overlying Arenigian (Floian) strata are still not well determined. In this study we present magnetostratigraphic data, which taken together with obtained biostratigraphic data, allow constraining the position of the Tremadocian–Arenigian (Floian) boundary within the key Moyero River section.

A paleomagnetic study of the Chertovskii Horizon of the Moyero River valley was carried out. Based on published and newly obtained data, the sequence of seven reliable determinations of the paleomagnetic pole position is obtained for Siberia within the time interval of ~505–450 Ma. This sequence makes it possible to determine with high precision the paleogeographic position of the Siberian platform over a period of ~50 Ma from the Middle Cambrian until the middle of the Upper Ordovician.

The Ordovician Period has emerged as a highly dynamic time in Earth history. Comprehensive work on chrono-, chemo- and biostratigraphy has resulted in an overall well-constrained systemic framework, but several local successions around the globe still await detailed analysis in many respects. Herein we perform a high-resolution analysis of abiotic and biotic signals in the Lynna River section, a key locality in northwestern Russia. As this section has been pivotal in documenting the temporal evolution of the Great Ordovician Biodiversification Event on Baltica, the macroscopic and microscopic characteristics of the local succession reveal important paleoenvironmental information that ties into the global development during the Middle Ordovician. The results add particularly to the understanding of the characteristics and large-scale sedimentary 'behavior' of the Baltoscandian paleobasin. Microfacies vary consistently with the macroscopic appearance of the rocks, with intervals characterized by competent limestone being associated with coarser carbonate textures and intervals dominated by marly beds associated with finer textures. Along with carbonate textures, fossil grain assemblages vary in a rhythmic (similar to cyclic) manner. The local rocks are commonly partly dolomitized, with the proportion of dolomitization increasing up-section. Regional comparisons suggest that the changes in overall macro- and microfacies were strongly related to variations in sea level. New high-resolution conodont biostratigraphic data largely confirm previous regional correlations based on lithostratigraphy and trilobite faunas, and enable more robust correlations worldwide.

The Middle Darriwilian Isotopic Carbon Excursion (MDICE) is a global isotopic event described in sections from different palaeocontinents. Here we present new stable carbon isotopic data from carbonates of ten sections in different parts of the Baltoscandian Palaeobasin (Estonia, Latvia, Lithuania, Sweden, NW Russia). The definition of the MDICE as a chemostratigraphic unit is discussed, as well as the subdivision of its peak into two distinct peaks. The MDICE is one of the longest carbon isotopic events in the Palaeozoic. It was preceded by the L-chondritic cosmic dust flow event, which may have been responsible for cooling through the Darriwilian and the initiation of the Great Ordovician Biodiversification Event. High-resolution chemostratigraphic analyses show that the time interval between these environmental events and the base of the MDICE is up to one million years. Due to the long duration of the MDICE the modelling of this excursion should address more complex scenarios than a simple response of the carbon cycle to rapid climatic perturbations.

The Tremadocian (Early Ordovician) is currently considered a time span of greenhouse conditions with tropical water surface temperature estimates, interpolated from oxygen isotopes, approaching 40 °C. In the mid-latitude Baltoscandian Basin, conodonts displaying low δ18O values, which suggest high temperatures (>40 °C) in the water column, are in contrast with the discovery of contemporaneous glendonite clusters, a pseudomorph of ikaite (CaCO3·6H2O) traditionally considered as indicator of near-freezing bottom-water conditions. The massive precipitation of this temperature sensitive mineral is associated with transgressive conditions and high organic productivity. As a result, the lower Tremadocian sediments of Baltoscandia apparently contain both “greenhouse” pelagic signals and near-freezing substrate indicators. This paradox points to other primary controlling mechanisms for ikaite precipitation in kerogenous substrates, such as carbonate alkalinity, pH and Mg/Ca ratios, as recently constrained by laboratory experiments. Preservation of “hot” conodonts embedded in kerogenous shales rich in δ18O-depleted glendonites suggests both the onset of sharp thermal stratification patterns in a semi-closed basin and the assumed influence of isotopically depleted freshwater yielded by fluvial systems.

The breakup of the L-chondrite parent body in the asteroid belt 466 million years (Ma) ago still delivers almost a third of all meteorites falling on Earth. Our new extraterrestrial chromite and 3He data for Ordovician sediments show that the breakup took place just at the onset of a major, eustatic sea level fall previously attributed to an Ordovician ice age. Shortly after the breakup, the flux to Earth of the most fine-grained, extraterrestrial material increased by three to four orders of magnitude. In the present stratosphere, extraterrestrial dust represents 1% of all the dust and has no climatic significance. Extraordinary amounts of dust in the entire inner solar system during >2 Ma following the L-chondrite breakup cooled Earth and triggered Ordovician icehouse conditions, sea level fall, and major faunal turnovers related to the Great Ordovician Biodiversification Event.

Отношения 87Sr/86Sr в гипсах и известняках ордовикского разреза р. Мойеро уменьшаются снизу вверх по разрезу от 0.7091-0.7095 в ирбуклинской свите (няйский горизонт~тремадокский ярус нижнего ордовика) до 0.7080 в верхней части джеромской свиты (долборский горизонт~катийский ярус верхнего ордовика), что находится в хорошем соответствии с биостратиграфическим расчленением разреза и существующим представлением об эволюции изотопного состава стронция в Мировом океане. Наиболее характерной особенностью изотопно-углеродной кривой является уменьшение значений 13С в карбонатах от слабо положительных (0.5…1.1‰) в ирбуклинской свите (няйском горизонте) до резко отрицательных (-5.4…-5.8‰) в средней части кочаканской свиты (кровля кимайского горизонта~кровля дапинского - основание дарривильского яруса). Увеличение в этом интервале разреза значений 18О от 20-22‰ до 26-28‰ и отрицательная корреляция между 13С и 18О, а также уменьшение значений 34S в гипсах от 30-32‰ до 22-24‰, показывающее, что обеднение карбонатов 13С не связано с сульфат-редукцией и окислением органического вещества в диагенезе, позволяют считать отрицательный экскурс 13С первичным; наличие на близком стратиграфическом уровне отрицательных аномалий 13С в ордовикских разрезах Южной и Северной Америки [Buggish et al., 2003; Edwards, Saltzman, 2014; McLaughlin et al., 2016] указывает на глобальное или субглобальное распространение этого события, возможно, связанного с появлением древнейшей наземной растительности. На фоне общего падения величин 13С в нижней части разреза выделяются 3 положительных экскурса малой амплитуды (1-2‰), положение которых в целом подтверждает существующую схему корреляции разреза р. Мойеро с международной шкалой. Верхняя часть разреза характеризуется чередованием интервалов с пониженными значениями 13С (до -2…-3‰) и короткими положительными экскурсами с амплитудой 0.5-1.3‰. Положительный экскурс 13С, которым заканчивается ордовикский разрез р. Мойеро, сопоставляется с экскурсом 13С в середине катийского яруса, а экскурс 13С в нижней части баксанского горизонта - с экскурсом, маркирующим границу катийского и сандбийского ярусов.

The 87Sr/86Sr ratio in gypsum and limestones of the Ordovician section of the Moyero River decreases from the bottom upward from 0.7091‒0.7095 in the Irbukli Formation (Nyaian Regional Stage, ~Lower Ordovician Tremadocian Stage) to 0.7080 in the upper part of the Dzherom Formation (Dolborian Regional Stage, ~Upper Ordovician Katian Stage), which is well consistent with biostratigraphic subdivision of the section and existing concept concerning the strontium isotope evolution of the World Ocean. The most characteristic feature of the carbon isotope curve is decrease of δ13С values in carbonates from weakly positive values (0.5…1.1‰) in the Irbukli Formation (Nyaian Regional Stage) to sharply negative values (–5.4...–5.8‰) in the middle part of the Kochakan Formation (top of the Kimaian Regional Stage, ~end of the Dapingian–base of the Darriwilian Stage). Increase of δ18О from 20‒22‰ to 26‒28‰, the negative correlation of δ13С and δ18О, and decrease of δ34S in gypsum from 30‒32‰ to 22‒24‰ in this interval indicate that the 13С depletion of carbonates was not related to the sulfate reduction and oxidation of organic matter during diagenesis and that the negative δ13С excursion was of primary nature. The presence of negative δ13С anomalies at this stratigraphic level in Ordovician sections of the South and North America (Buggish et al., 2003; Edwards and Saltzman, 2014; McLaughlin et al., 2016) indicates the global or subglobal distribution of this event, which was possibly related to the emergence of the oldest ground vegetation. Against the general decrease of δ13С, the lower part of the section reveals three low-amplitude (1‒2‰) positive excursions, the position of which in general confirms the existing correlation scheme of the Moyero River section with the international scale. The upper part of the section is characterized by the alternation of low-δ13С intervals (from–2 to–3‰) and brief positive excursions with amplitude of 0.5‒1.3‰. The positive δ13С excursion terminating the Ordovician section of the Moyero River correlates with the δ13С excursion in the middle Katian Stage, while the δ13С excursion in the lower part of the Baksian Regional Stage correlates with the excursion marking the Katian–Sandbian boundary.