Early-Middle Frasnian (Late Devonian) carbon isotope Event in the Timan-Pechora Basin (Chernyshev Swell, Pymvashor River section, North Cis-Urals, Russia)

 I.S. Kotik, A.V. Zhuravlev, T.V. Maydl, D.A. Bushnev, I.V. Smoleva, 2021 CC BY-SA I . S K o t i k e t a l . G e o l o g i c a A c t a , 1 9 . 3 , 1 1 7 ( 2 0 2 1 ) D O I : 1 0 . 1 3 4 4 / G e o l o g i c a A c t a 2 0 2 1 . 1 9 . 3 Early-Middle Frasnian isotope Event in Timan-Pechora Basin 2 pattern (Ma et al., 2008; Racki et al., 2008; Pisarzowska et al., 2006, 2020). Manifestations of the transitanspunctata isotope Event have been well studied in West and Central Europe, South China, and North America (Bai et al., 1994; Baliński, 2006; Lash, 2019; Morrow et al., 2009; Pisarzowska et al., 2006, 2020; Yans et al., 2007). However, much less is known of this event in East Europe (Soboleva et al., 2018; Zhuravlev et al., 2006). This paper contributes to our knowledge of the regional manifestation of the transitans-punctata isotope Event in north-east Europe. We present here the results of a multidisciplinary research in the Pymvashor River section located in the north-eastern part of the TimanPechora Basin (North Cis-Urals). The chronostratigraphic framework of this study follows the Subcommission on Devonian Stratigraphy propositions (see Ziegler and Sandberg, 2001). GEOLOGICAL AND PALEOGEOGRAPHIC SETTING The study area is located in the Timan-Pechora Basin in the north-east of the East European Craton in Russia (Fig. 1A, B, C). The object of our research was the Upper Devonian (Frasnian) deposits, which are widespread in the Timan-Pechora Basin (Fig. 1D, E). Upper Devonian deposits crop out at the Timan Ridge, Chernyshev Swell and Polar and Subpolar Urals in the Timan-North Ural region (Belyaeva and Ivanov, 2000; Tsyganko, 2011; Fig. 1C). The studied Frasnian section is located in the northern area of the Chernyshev Swell, a linear structure located in the north-east of the Timan-Pechora Basin (Fig. 1B, C). The swell is interpreted to have formed as a result of disruption by an Upper Ordovician evaporite detachment fault in the final stage of the Uralian orogeny during the Jurassic (Timonin, 1998; Yudin, 1994). The western boundary of the Chernyshev Swell is a detachment fault and its eastern boundary is a back-thrust fault that forms a pop-up structure (Fig. 1C, E). Several fold-thrust structures that affected the Silurian-Triassic sequences (Belonin et al., 2004) are identified as integral structural components of the swell. The Timan-Pechora Basin (Fig. 1D, E) developed during Late Devonian time as a passive shelf margin in the northeast edge of the Laurussian continent at the front of the Uralian Strait (Timonin, 1998; Fig. 1D). The great diversity of Late Devonian facies in the basin is attributed to the presence of a complex relief and local tectonics (Belyaeva et al., 1998; Parmusina, 2007). The following facies belts can be observed from west to east: coastal plain, shallow shelf, reef, deep water outer shelf basin, and isolated carbonate platform (Gruzdev, 2017; Nikonov et al., 2000; Parmusina, 2007; Fig. 1D). The sequences accumulated in deep water environments belong to the Domanik Formation, named after the Domanik Stream in South Timan (Belyaeva and Ivanov, 2000). The Domanik Formation comprises Middle Frasnian organic-rich carbonate-clay-siliceous facies that were deposited in deep-water shelf basin (Fig. 1D). The development of anoxic bottom conditions during deposition of the Domanik Formation was proposed on the base of lithological, paleontological, and geochemical investigations, including those of Bushnev et al. (2016) and Kuzmin et al. (1997). Most studies of the Upper Devonian deposits of the Timan-Pechora Basin have relied upon core analysis. The studied section is located within the Talbey block, where Frasnian deposits are exposed on the left bank of the Pymvashor River at the left tributary of the Adzva River (Fig. 1C). The Frasnian deposits of the Pymvashor River section consist of deep-water facies (i.e. the Domanik Formation).


INTRODUCTION
The stratigraphic level near the Early-Middle Frasnian boundary, known as the Middlesex Event (Becker et al., 1993;Ma et al., 2006;Pisarzowska et al., 2020), is attributed to the Late Devonian global transgression equivalent to the initial phase of cycle IIc (Johnson et al., 1985). Tectonic formation of deep intracratonic basins in the Cis-Uralian region occurred in the Middle Frasnian as well. Anoxic sedimentation in these basins is known as the regional 'Domanik Crisis' (Kuzmin et al., 1997). The hypoxic Middlesex Event coincides with the start of a δ 13 C positive excursion in the latest Palmatolepis transitans Zone preceding a prominent drop in δ 13 C values near the Palmatolepis punctata-Early Palmatolepis hassi boundary (Ma et al., 2008;Pisarzowska et al., 2006Pisarzowska et al., , 2020Pisarzowska and Racki, 2012;Yans et al., 2007). Significant variations in the δ 13 C values in the latest transitans-punctata time interval are known as the transitans-punctata isotope Event or Early-Middle Frasnian isotopic perturbation (Pisarzowska et al., 2020;Racki et al., 2008;Yans et al., 2007), which is distinguished in distant disparate sedimentary basins throughout the world and considered to be a reliable chemostratigraphic marker of the Early-Middle Frasnian boundary (Bai et al., 1994;Baliński, 2006;Ma et al., 2008;Pisarzowska et al., 2006Pisarzowska et al., , 2020Yans et al., 2007). Detailed study of the event structure revealed a four-step positive-to-negative excursion

GEOLOGICAL AND PALEOGEOGRAPHIC SETTING
The study area is located in the Timan-Pechora Basin in the north-east of the East European Craton in Russia (Fig. 1A, B, C). The object of our research was the Upper Devonian (Frasnian) deposits, which are widespread in the Timan-Pechora Basin (Fig. 1D, E). Upper Devonian deposits crop out at the Timan Ridge, Chernyshev Swell and Polar and Subpolar Urals in the Timan-North Ural region (Belyaeva and Ivanov, 2000;Tsyganko, 2011;Fig. 1C).
The studied Frasnian section is located in the northern area of the Chernyshev Swell, a linear structure located in the north-east of the Timan-Pechora Basin (Fig. 1B, C). The swell is interpreted to have formed as a result of disruption by an Upper Ordovician evaporite detachment fault in the final stage of the Uralian orogeny during the Jurassic (Timonin, 1998;Yudin, 1994). The western boundary of the Chernyshev Swell is a detachment fault and its eastern boundary is a back-thrust fault that forms a pop-up structure (Fig. 1C, E). Several fold-thrust structures that affected the Silurian-Triassic sequences (Belonin et al., 2004) are identified as integral structural components of the swell.
The Timan-Pechora Basin (Fig. 1D, E) developed during Late Devonian time as a passive shelf margin in the northeast edge of the Laurussian continent at the front of the Uralian Strait (Timonin, 1998;Fig. 1D). The great diversity of Late Devonian facies in the basin is attributed to the presence of a complex relief and local tectonics (Belyaeva et al., 1998;Parmusina, 2007). The following facies belts can be observed from west to east: coastal plain, shallow shelf, reef, deep water outer shelf basin, and isolated carbonate platform (Gruzdev, 2017;Nikonov et al., 2000;Parmusina, 2007;Fig. 1D). The sequences accumulated in deep water environments belong to the Domanik Formation, named after the Domanik Stream in South Timan (Belyaeva and Ivanov, 2000). The Domanik Formation comprises Middle Frasnian organic-rich carbonate-clay-siliceous facies that were deposited in deep-water shelf basin (Fig. 1D).
The development of anoxic bottom conditions during deposition of the Domanik Formation was proposed on the base of lithological, paleontological, and geochemical investigations, including those of Bushnev et al. (2016) and Kuzmin et al. (1997). Most studies of the Upper Devonian deposits of the Timan-Pechora Basin have relied upon core analysis.
The studied section is located within the Talbey block, where Frasnian deposits are exposed on the left bank of the Pymvashor River at the left tributary of the Adzva River (Fig. 1C). The Frasnian deposits of the Pymvashor River section consist of deep-water facies (i.e. the Domanik Formation).

Organic carbon content (C org )
Organic carbon was determined by treating dried samples with 15% solution of HCl to remove carbonate prior to instrument analysis. C org content was measured on an AN-7529 rapid carbon analyzer. Glucose and standard steel were used as standards. Measured values are reported as percent relative to whole rock.

Soluble organic matter composition
Bitumen was extracted from 50-100g of powdered rock samples using a Soxhlet apparatus for 48h with chloroform. The extracted bitumen was treated with elemental copper during extraction of elemental sulphur. The extracted bitumen was separated into aliphatic and aromatic fractions by liquid chromatography on a silica-gel column.
Gas Chromatography (GC) and Gas Chromatography-Mass Spectrometry (GC-MS) were used to determine the abundance of selected families of molecular biomarkers (i.e. n-alkanes and acyclic isoprenoids, terpanes, and steranes) in saturated bitumen fractions from six selected samples. Aromatic fractions were analysed using GC-MS. Analyses of n-alkanes and isoprenoids were carried out using a Crystal 2000M GC System with a DB-1 column. The GC-MS investigations were performed using Shimadzu QP2010 Ultra instrument equipped with a DB-5 column. Details are described in Bushnev et al. (2016Bushnev et al. ( , 2017a Belonin et al., 2004) showing the major morphostructural features of the study area (Chernyshev Swell and related units). The location of the studied outcrop and the cross-section in E are also shown. D) Middle Frasnian (Upper Devonian) palaeogeography and palaeoenvironmental setting of the studied Pymvashor River section in the north of the Timan-Pechora Basin (based on Nikonov et al., 2000;Parmusina, 2007). See B for location. E) Geological cross-section showing the Chernyshev Swell pop-up structure and the overall stratigraphy of the region (modified from Yudin, 1994) Early-Middle Frasnian isotope Event in Timan-Pechora Basin 4

Organic carbon stable isotope composition
Organic matter was extracted from limestones by dissolution of carbonates in HCl and combusted in the presence of oxygen at 900ºC. Carbon isotope values were measured with a DELTA V Advantage mass spectrometer equipped with a Thermo Electron Continuous Flow Interface (ConFlo III) and Element Analyzer (Flash EA 1112). International standard USGS-40 (L-Glutamic acid) and internal standard Acetanilide (C 8 H 9 NO) were used. The precision of the δ 13 C org value is ±0.15‰ and δ 13 C org values are reported relative to the VPDB (Vienna Pee Dee Belemnite) standard.

Isotope composition of organic carbon of conodont elements
Preparation of samples for isotopic analysis of organic carbon from conodonts followed standard procedures using 10% acetic acid. Good conodont preservation and a moderate Conodont Alteration Index (CAI lower than 3) favor near-primary isotopic composition of organic matter (Zhuravlev and Smoleva, 2018;Zhuravlev, 2020).
Conodont elements of Mesotaxis asymmetricus were used to study the carbon isotope composition of conodont organic matter. Separated conodont elements were washed with ethanol and distilled water. The samples were analyzed for carbon isotope values on a DELTA V Advantage mass spectrometer equipped with a Thermo Electron Continuous Flow Interface (ConFlo III) and Element Analyzer (Flash EA 1112). International standard USGS-40 (L-Glutamic acid) was used. The precision of the δ 13 C org-con value is ±0.15‰, and measured values are reported relative to the VPDB standard.

Carbonate carbon and oxygen isotope composition
Samples for carbonate carbon and oxygen isotope analysis were drilled from fresh surfaces using microdrilling equipment with steel bits. The samples mainly represent the micritic part of the rock. Carbonate carbon and oxygen isotope compositions were obtained on a DELTA V Advantage mass spectrometer with sample preparation on a Gas Bench II line using standard methods. The δ 13 C carb values were reported relative to the VPDB standard, and δ 18 O carb values are relative to the SMOW (Standard Mean Ocean Water). The precision of the δ 13 C carb value is ±0.04‰, and that of the δ 18 O carb value is ±0.06‰.
Isotope analyses were performed at the CKP "Geonauka" of Institute of Geology Komi SC UrB RAS (Syktyvkar, Russia). The reliability of the isotope record was enhanced by application of several screening tests to the bulk-carbonate isotope data (Brand et al., 2011): I) Visual examination. Fresh surfaces of samples were drilled to collect fresh carbonate powder for analysis. II) Optical examination in thin section. Samples demonstrating significant re-crystallisation of the micritic component were avoided.
III) Stable isotope distribution. A synoptic screening diagram based on Huck et al. (2017), Immenhauser et al. (2003), Lohmann (1988) and Chen et al. (2016) was used (Fig. 2). IV) Total organic content. High content of organic carbon in carbonates (>1.7% that corresponds to C org / C carb ratio less than 1/7) may lead to incorporation into carbonates of 12 C derived from oxidised organic matter (Scholle and Arthur, 1980). This process potentially decreases the δ 13 C carb value. The carbonate samples demonstrating high content of organic carbon were used with care.
Samples passing at least two of tests 2, 3 and 4 were considered as providing reliable isotope signal. Early-Middle Frasnian isotope Event in Timan-Pechora Basin 5 STRATIGRAPHY The first lithological and stratigraphic description of the studied section was carried out by A.I. Pershina (1962). Pershina (1962) based on the study of brachiopods, cephalopods, bivalves, ostracods, trilobites, and foraminifers proposed a Domanik-Mendymian age (punctata Lower rhenana conodont zones) for the exposed deposits. V.V. Kachmashev led a geological investigation of the study area in 1984-1988 (unpublished technical report, 1988) that included a paleontological-biostratigraphic sampling. Two conodont samples collected from the upper part of the Pymvashor River section during Kachmashev's work confirmed the Mendymian age (ca. Lower rhenana Zone). We studied the Frasnian deposits exposed along the Pymvashor River in 2017 in order to improve the biostratigraphic and lithofacies characterisation of the section and to obtain isotope and geochemical data for the first time.

Lithostratigraphy and facies description
The Frasnian section of the Domanik Formation is 10.7m thick and mainly constituted by interbedded black shale and dark-grey, micro-to finely-crystalline limestone beds. Lithology changes within this unit include the occurrence of diverse proportions of shale and limestone beds. Scarce chert layers also occur (Fig. 3). Limestone layers range from a few (2-5) to tens (15-20) cm in thickness. The diverse limestone microfacies are shown in Figure 4.

Conodont biostratigraphy
Ten of the eleven samples collected for conodont analysis yielded conodonts. Each sample weighted 0.5 to 1.0kg. The processing of samples followed standard procedure using 10% acetic acid.
Conodont associations in the lower part of the section are dominated by Mesotaxis (beds 1A-2) and Polygnathus (beds 4-6) ( Table 1). In the upper part of the section, conodont associations are dominated by Palmatolepis (beds 8-9 and bed 12) and Polygnathus (beds 10-11) ( Table 1). Characteristic conodont species are illustrated in Figure 5. Conodont associations found in the productive conodont samples allowed us to distinguish two conodont zones and an unzoned interval in the lowermost part of the section.
The lowermost part of the section (bed 1A) contains Frasnian conodonts of wide stratigraphic range (Table 1). The presence of Mesotaxis bogoslovskyi ovnatanova and kuzmin suggests an Early-Middle Frasnian (transitanspunctata zones) age for this part of the section. The first occurrence of Palmatolepis punctata in bed 1 marks the base of the punctata Zone (Table 1; Fig. 3). The lower part of the zone contains representatives of Mesotaxis, which are absent from the base of bed 9. The upper part of the zone is characterized by taxonomically poor conodont associations dominated by Polygnathus. The entry of Polygnathus uchtensis in bed 6 allowed us to identify the upper part of the punctata Zone. Early-Middle Frasnian isotope Event in Timan-Pechora Basin 6 (bed 11) suggests the likelihood of a lower position of the Lower hassi Zone, probably near the base of bed 11 (Table  1; Fig. 3).
The reported conodont data suggests that the sequence studied comprises the entire Middle Frasnian punctata Zone. It appears, however, that the first occurrence of zonal index species (Palmatolepis punctata and Palmatolepis hassi) is delayed in the succession.

ORGANIC GEOCHEMISTRY RESULTS
C org content C org content of analysed samples ranges from 0.2% to 22% ( Fig. 3; Table 2) depending on sample lithology. The greatest C org values are observed in shales, varying from 4.8% to 22%. Limestone samples are characterised by low C org content (0.2% and 1.1%), and the single chert sample analysed contains 1.9% C org.

n-Alkanes and isoprenoids
Geochemical analysis conducted in a previous study (Kotik et al., 2019) of six samples from the studied succession indicate that samples of high (shale) and low (limestone, chert) C org content differ in the distribution of hydrocarbon biomarkers. The n-alkanes exhibit unimodal distribution in samples 7, 9, 12 (shales) and 13 (limestone), with a maximum distribution in the range of C 13 -C 18 (Fig.  6A). The distribution of n-alkanes in limestone (sample 8) and chert (sample 4) are characterised by a wide maximum, ranging from C 16 to C 24 (Fig. 6B). Pristane/phytane (Pr/Ph) ratios of all samples fall into a relatively narrow range from 1.10 to 1.91, and isoprenoid/n-alkane ratios range from 0.45-1.03 for Pr/n-C 17 and 0.32-0.80 for Ph/n-C 18 (Table  3).

Steranes and terpanes
All samples analyzed for this study display a similar distribution of regular steranes (Fig. 7A). The relative percentages of C 27 , C 28 , and C 29 steranes (Table 3) indicate that the C 29 homologues predominate in all samples, followed by the C 27 and C 28 steranes. Concentrations of diasteranes are highest in limestone and chert samples. The C 27 diasterane/regular sterane ratio in limestone and chert is 0.4-0.7, but in shale it varies from 0.3-0.4 (Table  3). The relative amount of diasteranes compared to regular steranes depends on both lithology and thermal maturity of the deposits (Waples and Michihara, 1991). In our case, varying diasterane/regular sterane ratios reflect a dependence on lithology because all samples have a similar level of maturity (R o = 0.6-0.75%). Evaluation of the thermal maturity of the studied succession was addressed earlier (Kotik et al., 2019). Among the hopanes, C 29 and C 30 are predominant compounds in all samples. The clear predominance of C 31 hopanes among homohopanes C 31 -C 35 is obvious from the mass chromatograms (m/z 191, Fig.  7B).

Aromatic hydrocarbons
Analysis of the aromatic fraction of the bitumen from the Pymvashor River section reveals the presence of isorenieratane and paleorenieratane derivatives (French et al., 2015), mainly represented by monoaromatic derivatives of aryl carotenoids (Fig. 8). The content in the aromatic fraction of bitumen decreases in the series of samples as follows: 9>12>7>>4. The content of 2,3,6-trimethyl aryl isoprenoids exceeds that of the 3,4,5-trimethyl derivatives, as noted from analysis of samples from other sections of the Domanik Formation in the Timan-Pechora Basin (Bushnev et al., 2016(Bushnev et al., , 2017b.

Carbonate isotope composition
The original carbon and oxygen isotope compositions were screened as described earlier in this paper. Samples 1A-2, 3, 8, 10, and 16 contain traces of recrystallized micritic cement. Sample 4 appears to have experienced strong silicification. Such diagenetic processes can result in reduced δ 13 C carb values. The low organic carbon content of carbonates (less than 1.7%, that corresponds to a C org /C carb ratio less than 1/7) may suggests an insignificant contribution of 12 C derived from oxidized organic matter (Scholle and Arthur, 1980). Samples 4,5,7,9,12 l o g i c a A c t a , 1 9 . 3 , 1 -1 7 ( 2 0 2 1  Early-Middle Frasnian isotope Event in Timan-Pechora Basin 8 (Table 2) and likely demonstrate depleted δ 13 C carb values. The lack of covariance of C org and δ 13 C carb values of the Pymvashor River section sample suite (R= 0.10) suggests that pore-water conditions exerted minimal control of δ 13 C carb values. Carbon and oxygen isotope distribution on a synoptic screening diagram (Fig. 2) suggests modification of the isotope composition of samples 3,4,5,6,8,9 and 15. Thus,samples 3,4,5,8,9 and 15 show a set of features (combination any two features out of recrystallized micritic cement, high organic carbon content, position on a screening diagram) suggesting unreliable isotope signal (Fig. 3).    l o g i c a A c t a , 1 9 . 3 , 1 -1 7 ( 2 0 2 1  Early-Middle Frasnian isotope Event in Timan-Pechora Basin 9 The following trends are observed in the δ 13 C carb record (Fig. 3): an increase from -1.4‰ to 1.1‰ through the lower part of the punctata Zone, followed by a negative shift of -1.5‰ in beds 5-8 (middle part of the punctata Zone). A strong positive excursion in the upper part of the punctata Zone (bed 9) is represented by a single sample with a δ 13 C carb value of 2.3‰. Overlying deposits spanning the uppermost part of the punctata Zone and lower part of the  Early-Middle Frasnian isotope Event in Timan-Pechora Basin
Organic matter isotope composition δ 13 C org values of bulk organic matter show fluctuations around -29‰ (Fig. 3). δ 13 C org values are greatest in the lower part of the punctata Zone, with values as high as -27‰ at bed 3. This positive excursion coincides with a positive excursion in δ 13 C carb . Negative δ 13 C org excursions in samples 7 and 8 roughly correspond to the beginning of the negative excursion in the carbonate carbon isotope composition. The mean δ 13 C org value is -29‰ with a standard deviation of 0.8.

Isotope composition of conodont organic matter
Variations in isotope composition of conodont organic matter (δ 13 C org-con ) of the conodont species Mesotaxis asymmetricus of the lower part of the succession that spans stratigraphic range of this species were evaluated.
Low δ 13 C org-con values (-26‰ to -27‰) are characteristic of the lower part of the punctata Zone, excluding the positive excursion corresponding to the upper part of the main positive shift in δ 13 C carb (Fig. 3). The mean value of δ 13 C org-con, -25.9‰, is comparable with that documented from the Late Devonian-Mississippian conodonts (-25.3‰ with a standard deviation of 2.56; Zhuravlev et al., 2020). Differences between δ 13 C org-con values and δ 13 C org values range from 1.9‰ up to 4.1‰ and display a negative correlation with C org content (R= -0.82). At the same time δ 13 C org-con and C org values exhibit a clear negative correlation (R= -0.91). These correlations may be attributed to trophic relations of conodonts (Zhuravlev, 2020). The carbon isotope composition of conodont organic matter probably was controlled by the isotope composition of conodont diet, represented by phyto-and zooplankton (Zhuravlev, 2020). Changes in the nutrition caused variations in the δ 13 C org-con : an increase in the phytoplankton portion in the diet led to a decrease of δ 13 C org-con and vice versa. Also, phytoplankton bloom and subsequent eutrophication led to C org increase in sediment. The eutrophication probably provoked shortening   of the food chains (Ward and McCann, 2017) and following decreasing of differences between δ 13 C org-con and δ 13 C org .

Sedimentary model and paleoenvironments
The studied Frasnian (Upper Devonian) Pymvashor River section is composed mainly of thin dark-grey mudstone, wackestone, and packstone beds, bearing tentaculites and/ or radiolarians, and black shales with high organic carbon content (C org as much as 22%, average 4.33%). Rock composition displays a rhythmical alternation of marlcarbonate-dominated beds, with minor occurrence of chert beds (Einsele, 1982;Einsele and Ricken, 1991). These rhythms may reflect variations in productivity, dilution of one component by another (e.g. carbonate/siliciclastic mud/organic matter/silica), and/or dissolution (Einsele, 1982). The studied succession is thought to be the result of cyclical changes in the accumulation rates of carbonate and chert, which do not coincide in phase because of changing supplies of clay and organic matter. Thus, the Pymvashor River succession preserves very probably a record of productivity cycles, that might be linked to periodic paleoclimate and/or paleoceanographic changes (Einsele     o l o g i c a A c t a , 1 9 . 3 , 1 -1 7 ( 2 0 2 1  Early-Middle Frasnian isotope Event in Timan-Pechora Basin 12 and Ricken, 1991). Compositional aspects of the succession described earlier suggest that the main components of productivity were biogenic and biochemogenic carbonates (mainly tentaculites and green algae) and biogenic silica (radiolarians and sponge spicules). Settling of suspended particles, including dead plankton, pellets, and mineral particles from the photic zone, provided the main volume of carbonate mud (micrite), biogenic silica (chert), and accumulations of shells and detritus deposited.
The finely-crystalline matrix, the laminar texture, and dominance of pelagic fossils (tentaculitids and radiolarians) in the studied facies indicate a deep water, low energy depositional regime, below the storm wave base. The presence of tentaculites in the limestones (packstone-grainstone; Fig. 4B) and the gradational character of their distribution in the sediment suggest of deposition from lateral suspension flows. Such deposits in the deep water basin could be related to erosion and re-deposition of shallow-water sediments during storm surges that resulted in granular flows (tempestites). Similar deposits are described from Middle Pleistocene-Holocene sediments on the northern slope of the Little Bahama Bank (Lantzsch et al., 2007). Thus, the studied Early-Middle Frasnian interval consists of rhythmically intercalated thinly bedded limestone and shale deposited in deep water by episodic carbonate influx from the shallow-water platform induced by storms. Periodicity of the storms could be mediated by short-term climate changes accompanying the Middle Frasnian cooling (Pisarzowska et al., 2020). Increasing latitudinal temperature gradients probably provoked increasing in tropical storm frequency and vice versa. Beds 3, 7 and 10 dominated by organic-rich laminated shales are considered to represent episodes of low climatic gradient. Predominance of pelagic cherts and shales in the bed 3 suggests correspondence of this bed to the radiolarian bloom associated with the Middlesex Event.

Composition of organic matter and depositional environments
The predominance of n-alkanes in the range below n-C 20 and Pr/n-C 17 less than 1 reflects a dominantly marine algal source of organic matter (Tissot and Welte, 1984). Pristane (Pr) to phytane (Ph) ratios of the studied sample are generally greater than 1.0 (1.10-1.91, Table 3), typical of the range generally considered to describe a normal marine environment (Didyk et al., 1978). This, relatively high Pr/Ph values are accompanying with isorenieratene derivatives presents. The same situation was early reported for the Domanik Formation of the Timan-Pechora Basin (Bushnev et al., 2017b). Pr/Ph values ranging from 1 to 2 in combination with paleorenieratene and isorenieratene derivatives were recently reported (Connock et al., 2018), where some possible causes were discussed: thermal maturity, heterogeneous pristane and phytane source, pristane coelution, phytane structure binding into sulphur compounds as well as relatively short euxinic events during deposition. Thus, we assume other pristane source in Domanik Formation of the Timan-Pechora Basin and short time episodic euxinic events.
The sterane distribution of the Pymvashor River sample suite (C 27 -C 29 , Table 3) is typical of planktonic and algal sources of organic matter. Whereas the main source of C 27 steranes is marine phytoplankton, C 29 steranes are typically associated with land plants (Huang and Meinschein, 1979;Waples and Michihara, 1991). However comparatively higher contribution of C 29 to C 27 steranes has been observed in marine Paleozoic sediments (Moldowan et al., 1985;Romero and Philp, 2012;Volkman, 1986) of different locations and also in previous studies of this basin (Bazhenova et al., 2008;Bushnev et al., 2017a, b) making unnecessary the terrestrial input to explain sterane distribution. This is compatible with the deposition in a relatively deep shelfal water environment as reported by Belyaeva et al. (1998).
The C 29 /C 30 and C 35 S/C 34 S hopane ratios are used in tandem to elucidate depositional environments and redox conditions (Peters et al., 2005). The obtained C 29 /C 30 values of 0.64-0.75 and C 35 S/C 34 S values of 0.63-0.83 (Table 3) are typical of oxygen-depleted depositional environments. The sterane/17α(H)-hopane ratio is commonly used as an indicator of the proportion of bacterial biomass (prokaryotes) relative to that of eukaryotes (Peters and Moldowan, 1991;Waples and Michihara, 1991). The lowest values of this ratio were observed in shale samples (Table  3), suggesting a greater contribution of prokaryotes to the organic matter of these samples.
The presence of isorenieratene derivatives is confirmed in the aromatic fraction of the analysed bitumen extracts.
Isorenieratene is a biomarker of green sulphur bacteria (Chlorobiacea), indicating photic zone anoxia (Hartgers et al., 1994;Koopmans et al., 1996). Previous studies of the Domanik Formation in the Chut' and Shar'yu river sections (Bushnev et al., 2016) have revealed that aryl carotenoids derivatives are characterised by the presence of carbon enriched with the heavy isotope, a reliable sign of the connection of these compounds with organic matter of sulphur bacteria of the genus Chlorobium (van der Meer et al., 1998). Therefore, the accumulation and preservation of organic matter in sediments of the studied section was probably favored by anoxic conditions. The presence of isorenieratene derivatives, an indicator of photic zone anoxia, has been described from Late Devonian strata in other Laurussian carbonate shelf regions (Hartgers et al., 1994;Marynowski et al., 2008;Poludetkina et al., 2017;Racka et al., 2010). Poludetkina et al. (2017) noted that variable concentrations of isorenieratene derivatives tell of the changing intensity of anoxia. Differences of the aryl carotenoid derivatives in bitumen among samples of the Pymvashor River section likely reflect changes in the degree of anoxia in the paleobasin. Bushnev et al. (2016) suggested that the prevalence of anoxia in deep water shelf depressions of the Timan-Pechora Basin during Middle Frasnian time resulted from the establishment of a thermocline, the formation of which has been attributed to upwelling near the shelf margin. Similar paleohydrographic conditions of weak vertical circulation have been reported from the North American cratonic basin, where black shale of Middle Devonian to Early Carboniferous age accumulated (Algeo et al., 2007). Oxygen depletion in the water column of a semi-isolated deep-water depression may be triggered by local phytoplankton blooms induced by increased nutrient delivery from the continent and/or ocean.

Carbon isotope record
The screened carbonate carbon isotope record of the Pymvashor River section shows a major positive excursion (major event III of Pisarzowska et al., 2006) separated by a negative shift near the boundary between the MN 5 and MN 6 conodont zones (Figs. 3; 9). The similarity of the δ 13 C profiles of the studied succession with the isotope signatures documented from south and central Laurussia and China suggests that perturbations of the carbon cycle during the punctata Zone time were global (Ma et al., 2008;Morrow et al., 2009;Pisarzowska et al., 2006Pisarzowska et al., , 2020Yans et al., 2007). The close similarity of the Cis-Uralian and Chinese δ 3 C carb oscillations, including the two-step pattern of major positive excursions (Ma et al., 2008), suggest a robust correlation potential of the entire punctata Event as well as lesser intra-event excursions. Similar δ 13 C variations have been reported from the Frasnian sequence Early-Middle Frasnian isotope Event in Timan-Pechora Basin 14 of the Main Devonian Field (biogenic calcite of brachiopod shells; shallow-water facies) of the East European Platform (Zhuravlev et al., 2006), from the Kostomłoty-Mogiłki section (hemipelagic facies; bulk carbonates) in the Holy Cross Mountains (see Pisarzowska et al., 2006: fig. 10), and from the sections of Miette Carbonate Platform, Western Canada basin (Śliwiński et al., 2011).
Lower amplitudes of δ 13 C carb shifts in the punctata Zone in the Cis-Uralian realm (<3‰) contrast with prominent excursions in south Laurussian and Chinese successions (>4‰) (Fig. 9). Such magnitude differences may reflect diagenetic depletion of δ 13 C carb values due to high C org content and/or specific local environmental conditions (e.g. decreased burial rate of marine organic matter, variable freshwater delivery to the basin). Screening of the raw isotope data used in this study suggests low-significance effects of diagenetic alterations.
The organic carbon isotope pattern of the Pymvashor River section shows three positive-to-negative excursions spanning the punctata conodont Zone (Figs. 3; 9). The amplitude of δ 13 C org variations and maximum values decrease upward from 2.5‰ to 1.4‰ and from -27‰ to -28.9‰, respectively. Magnitude of variations and values of δ 13 C org in the Pymvashor River section are less than those observed in the North American, Polish, and Chinese sections (Lash, 2019;Ma et al., 2008;Morrow et al., 2009), perhaps a function of differences in pCO 2 in sea water of these basins (Rau et al., 1989) and/or local variations in plankton structure (e.g. predominance of phytoplankton in eutrophic conditions can elevate the proportion of 12 C in C org ). For example, regional elevation of pCO 2 in sea water of the Timan-Pechora Basin at Middle-Frasnian may be caused by synchronous alkaline ultramafic magmatism in the Kola large igneous province (Wu et al., 2013).
The Cis-Uralian δ 13 C org data from conodont elements (δ 13 C org-con ) shows a minor negative excursion near the base of the punctata conodont Zone and a positive shift in the lower part of the zone (Fig. 9). The δ 13 C org-con pattern probably reflects changes in the pelagic trophic web and variations of δ 13 C org at basal trophic levels. It is likely that a Stage Zone F R A S N I A N p u n c t a t a L . h a s s i t r a n s i t a n s δ С org , / 13 0 00 FIGURE 9. Inorganic and organic carbon isotope record of the Early-Middle Frasnian transition in the Cis-Urals (this study), Poland (Pisarowska et al., 2006) and China (Ma et al., 2008). Labels I, II, III, IIIa, and IV designate the carbon isotopic events distinguished by Pisarzowska et al. (2006) and this study. Ad. Zone -conodont zonation based on Ancyrodella according to Pisarzowska et al. (2020).
negative shift of δ 13 C org-con corresponds to an increase of the 13 C-depleted phytoplankton portion in the conodont diet, and a positive shift relates to an increase of the 13 C-enriched zooplankton portion in the conodont diet.

CONCLUSIONS
The Early-Middle Frasnian succession in the Pymvashor River section in the north-eastern of the Timan-Pechora Basin (North Cis-Urals), consist of rhythmically interbedded thin limestone and shale beds (and a minor chert bed) deposited in a deep water environment, where the sedimentation of siliciclastic mud and organic matter was occasionally interrupted by storm sediment contribution from the adjacent shallower carbonate platform. The absence of bioturbation and benthic organisms in the organic-rich black shales indicates an oxygen-depleted depositional environment. Moreover, the detection of isorenieratene derivatives in the organic matter suggests anoxic conditions throughout the water column. The sedimentological characteristics and inferred redox conditions that accompanied the accumulation of organic matter are similar to that of the coeval deposits of the Holy Cross Mountains in Poland (Marynowski et al., 2008;Pisarzowska et al., 2006).
The Pymvashor River section preserves a robust record of the Early-Middle Frasnian carbon isotope Event in the carbonates and organic matter. The close similarity of the Cis-Uralian and Chinese δ 13 C carb records, including the twostep pattern of a major positive excursion, suggests a strong potential for long distance stratigraphic correlation of the punctata Event and the minor intra-event positive excursions. A similar event pattern has also been documented from the Kostomłoty-Mogiłki section in the Holy Cross Mountains. Magnitude of variations and values of δ 13 C org and δ 13 C carb in the punctata Zone in the Pymvashor River section are less than those observed in the North American, Polish, and Chinese successions. Such differences may reflect local/ regional paleoenvironmental variability.

ACKNOWLEDGMENTS
The research was supported by projects AAAA-A17-117121270033-6 and AAAA-A17-117121140081-7 of the Institute of Geology Komi SC UB RAS. Much appreciation is due to Lluís Cabrera, Gary Lash, Angeles Borrego and the anonymous reviewers for their efforts to improve the manuscript.