The Nordvik section is one of the most comprehensively studied Jurassic sections in the Russian Arctic. It has been studied in detail by sedimentologists, geochemists, physicists and biostratigraphers (Zaharov & Judovnyj 1974). The Oxfordian–Ryazanian part of this section has been subdivided biostratigraphically by means of ammonites (Zaharov et al. 1983; Zakharov & Rogov 2008; Rogov & Wierzbowski 2009), belemnites (Dzûba 2004; Dzyuba 2012), buchiid bivalves (Zaharov et al. 1983), foraminifers and dinoflagellate cysts (Nikitenko et al. 2008, 2011). A remarkable positive δ¹³C excursion (carbonate in belemnite rostra), which is considered as a useful marker for the Panboreal and Boreal–Tethyan correlation of J/K boundary beds, has recently been recorded in the Nordvik section in the top part of the Taimyrensis Zone (Dzyuba et al. 2013).

The Nordvik Peninsula is located in the eastern part of the Yenisei–Khatanga basin (Fig. 2) which is a part of the Mesozoic depression extending from the Yenisei River mouth to the Lena River mouth along the northern margin of the Siberian platform (Saks et al. 1959). In the south, the Yenisei–Khatanga basin is bounded by the northern edge of the platform which is covered by Palaeozoic and Lower Triassic deposits. In the west, it is connected with the Ust–Yenisei basin. In the east, the Yenisei–Khatanga basin reaches the Lena–Anabar basin, being separated from it by a buried uplift. The latter continues as a projection of the Siberian platform between the Popigai and Anabar rivers towards the Anabar Gulf.

The basement of the Yenisei–Khatanga basin is composed of Palaeozoic and volcanogenic Lower Triassic rocks. The basin is primarily filled by marine Jurassic to lowermost Hauterivian deposits, and in the west overlain by continental Lower Cretaceous deposits and marine Upper Cretaceous sediments. The Triassic deposits occur in the eastern part of the basin.

In Volgian and Ryazanian times, the Nordvik Peninsula was located in the middle part of the Yenisei–Khatanga Sea Strait (Fig. 2). Due to the great distance (about 200 km) from the source areas in the north and south, fine-grained sediments such as alternating clays, bituminous clays and clayey silts accumulated there (Fig. 3). The total thickness of the studied interval from the Middle Oxfordian to basal Ryazanian is slightly more than 50 m. As judged from relative thicknesses of the ammonite zones, sedimentation rates gradually decreased during the Volgian to reach a minimum at the J/K transition and began to increase again in the earliest Cretaceous (Zakharov et al. 1993). Sedimentation rates for the Nordvik section were recently calculated by Grabowski (2011) and Bragin et al. (2013) on the basis of durations of magnetochrons presented by Ogg (2004). Grabowski (2011) concluded that quite uniform sedimentation rates characterized chrons M20n1n and M19r (ca. 11–12 m/My), while sedimentation rates decreased drastically at the Taimyrensis–Chetae transition (M19n and M18r) to 1.5–2.0 m/My. Additional studies of the Nordvik section have shown that sedimentation tends to decrease in Chron M18n (about 1.7 m/My) and especially in M17r (0.5 m/My), embracing the Volgian/Ryazanian boundary, whereas M17n reveals a major increase in the sedimentation rate up to 12 m/My (Bragin et al. 2013). Sedimentation rates recalculated on the basis of work by Ogg et al. (2012) are similar to previous rates.

The lower part of the Upper Jurassic is represented by the uppermost Middle Oxfordian, Upper Oxfordian, Lower and Upper Kimmeridgian. In contrast to the same interval located 500 km to the west, the Nordvik section contains no representatives of Aulacostephanidae, i.e., no index species of the Kimmeridgian zones at the Boyarka River (Kheta River basin; Saks 1969). Only cardioceratids Amoeboceras s.l. and oppeliids Suboxydiscites have been found in the studied section (Zaharov et al. 1983; Rogov & Wierzbowski 2009). Therefore, the Kimmeridgian ammonite zones of the Kheta River basin could only be tentatively correlated to those of the Nordvik Peninsula, but the entire Kimmeridgian belemnite succession of the Boyarka River section is recognized here (Dzûba 2004; Dzyuba et al. 2007). The Kimmeridgian/Volgian boundary in the Nordvik section is satisfactorily identified by using belemnite data. All Volgian ammonite zones are well correlated throughout the Arctic and the lower boreal regions of Europe and Canada. The Ryazanian sequences are also represented by Arctic ammonite and buchiid zones (Zakharov & Rogov 2008; Rogov & Zakharov 2009).

Comprehensive studies of the Mesozoic deposits of the Khatanga depression began in the 1930s. Oil exploration, including drilling and geophysical investigations, was carried out from 1933 to 1953 in the Nordvik and some other regions of the Yenisei–Khatanga basin. The data on the Mesozoic deposits were summarized by Saks et al. (1959).

Saks (1958) compiled palaeogeographic and palaeofacies schemes of the Yenisei–Khatanga basin and the whole Arctic separately for the Jurassic and Cretaceous. A significant contribution to the palaeobiogeographic studies of the boreal climatic belt was given by Saks et al. (1971), who established palaeozoogeographic realms and provinces for the Jurassic and Neocomian of the Arctic. In the early 1980s, the palaeogeographic and facies maps for the northern USSR for each of 11 ages of the Jurassic Period were prepared (Bogolepov 1983). According to these reconstructions, a deep-water strait connected to the North Pacific existed throughout the Jurassic Period in the eastern sector of the Arctic (Zakharov et al. 2002). In the Late Jurassic, the margin of the bay was near the New Siberian (Novosibirskie) Islands. In Volgian and Valanginian times, deep-water troughs of the bay penetrated into the eastern part of the Laptev Sea. Flyschoid successions of terrigenous rocks, such as sandstones, siltstones and mudstones—over 1200 m in total thickness—were formed in these troughs (Kuz'michev et al. 2009). Apparently, the nearby water masses affected the environments of northern East Siberia by bringing about a climatic warming and consequent changes in biota. The diverse Late Jurassic biota of the North Siberian seas were formed under the influence of the northern European seas, starting with the Central Russian Sea (Saks & Nalnjaeva 1973; Bogolepov 1983; Dzûba et al. 2006; Rogov 2012).

Macrofossils used for the present study were collected noting their position within the section to the nearest centimetre (cephalopods) or were collected separately from each member (bivalves). Data concerning the relative abundance of macrofossils were obtained from field observations using a semi-quantitative approach.

Palynological samples were collected every 0.2–1.0 m. A total of 72 rock samples, recovered from dark claystones, were processed for palynomorphs. After washing and drying, the standard processing involved chemical treatment of 10 g of the sample with HCl to remove the calcareous fraction and with HF to remove silicates, sieving through a 15-µm nylon mesh, and centrifuging to concentrate the residues. Oxidation was not used. Three microscope slides were made from each sample for palynofacies analysis and dinoflagellate cyst analysis. Whole slides of residues were investigated under a binocular transmitted light microscope to identify and count the palynomorphs and other organic materials. A total of 63 samples for foraminiferal analysis were collected within the Kimmeridgian—Lower Ryazanian, with a sample resolution of 1.2 m in the lowermost part of the section and 10–20 cm in the Upper Volgian—Ryazanian.

For the determination of the δ¹³C values of organic carbon, sample aliquots were first boiled with HCl to remove any carbonates. Samples were then repeatedly washed by distilled water, dried and the δ¹³C values determined through gas chromatography–isotope-ratio mass spectrometry (GC–IRMS), using a 1108 Elemental Analyzer (Fisons, Ipswich, UK) with a ConFlo (Thermo Finnigan, San Jose, CA, USA) interface and a MAT 251 mass spectrometer (Thermo Finnigan) at the Laboratories of the Czech Geological Survey in Prague. Overall, analytical uncertainty of the δ¹³C values of the organic matter was ±0.2‰.

The analytical methods used to obtain inorganic carbon (C) and oxygen (O) isotope data from the belemnite rostra (genera Cylindroteuthis and Arctoteuthis [76% of the analysed samples] Lagonibelus [10%], Pachyteuthis [6%], Simobelus [2%] and unidentified genera [6%]) as well as detailed stable isotope data and sample chemical composition from Nordvik are described by Žák et al. (2011) and Dzyuba et al. (2013) and are not repeated here. It should be noted that Arctoteuthis is not mentioned in Žák et al. (2011) because this taxon was considered as a subgenus of the genus Cylindroteuthis. In the present work, the belemnite taxonomy used to identify taxa to the genus level follows Dzûba (2011). The sampled interval covers a period from the Middle Oxfordian to the basal Ryazanian. Two positive excursions in the lower part of the section (in the Middle Oxfordian and in the basal part of the Upper Kimmeridgian) were recorded, with carbonate δ¹³C values reaching up to of +3‰ (Vienna Pee Dee Belemnite [VPDB] scale; Fig. 6). These peaks are located at similar positions as positive δ¹³C excursions in the belemnite record of the Russian Platform described by Riboulleau et al. (1998) and Price & Rogov (2009). Similar Middle Oxfordian positive carbon isotope excursions have been observed by Wierzbowski (2004) in the Western Carpathians. A stratigraphically useful carbon isotope signal has recently been recorded in the Nordvik section slightly above the J/K boundary, where the δ¹³C values increase from 0.0–0.2 to +0.9‰ and then decrease to pre-excursion values. According to Dzyuba et al. (2013), this positive excursion is most distinctively observed in the Maurynya section of western Siberia and is less obvious in the Marievka section of central Russia and in pelagic limestones of the Tethyan Guppen–Heuberge section of Switzerland.

The whole Upper Jurassic is characterized by an irregular carbonate δ¹³C decrease upward throughout the Nordvik section. This is a trend which has been detected at many other sections, both on the Russian Platform and in the Tethyan Realm (e.g., Weissert & Channell 1989; Podlaha et al. 1998; Savary et al. 2003; Price & Rogov 2009). Such a decrease in δ¹³C could be connected with a gradual increase of the carbon dioxide contents in the atmosphere–ocean system, leading to warming, as reflected by coeval changes in the belemnite oxygen isotope record (see below). On the other hand, carbon isotopes in belemnite rostra are reported to not be in equilibrium with ambient seawater due to vital effects (Wierzbowski & Joachimski 2009). Thus, changes in carbon isotope records could be partially connected with changes of belemnite assemblages within the section, as vital effects could be different in different taxa. For example, the average carbon isotope values of the Nordvik Pachyteuthis belemnites are notably higher throughout the Upper Jurassic and lowermost Cretaceous than those in other belemnite genera. The values attain 2.29‰ in the Middle Oxfordian–Kimmeridgian and 0.99‰ in the Volgian-basal Ryazanian. The average δ¹³C values of Lagonibelus change from 1.41 to 0.14‰ between the same stratigraphic intervals, and for Cylindroteuthis they show a similar range from 1.33 to 0.32‰. However, a general trend of decreasing δ¹³ C is seen within the data derived from all belemnite genera: Volgian-basal Ryazanian average δ¹³C values recorded by different genera are lower than those observed for the Middle Oxfordian–Kimmeridgian. Here, we interpret the results of stable isotope analyses taking into account the mode of life and habitat of different belemnite taxa and their possible influence on the stable oxygen isotope values. For this purpose cylindroteuthid belemnites (those which inhabited the Siberian seas) were classified based on morphological, palaeoecological and distributional data (Gustomesov 1976; Saks & Nalnjaeva 1979; Dzyuba 2004). In accordance with this classification, they were arranged in three groups characterized by common features of the rostral morphology that reflect a similar style of life and habitat depth.

Group I is characterized by short robust rostra and includes the genera Simobelus, Liobelus, Acroteuthis, Microbelus and—the most robust—Pachyteuthis and Lagonibelus. These relatively less active swimmers preferred shallow near-shore water and are most common in the upper sublittoral zone deposits. It cannot be excluded that all these genera, or only the dorso-ventrally depressed taxa (Liobelus, Microbelus, and Acroteuthis), were nektobenthic organisms. Isotope ratios have been analysed from belemnites Simobelus sp. (position in profile see table 3 in Žák et al. 2011).

Belemnites in Group II have elongate to moderately elongate rostra with a well-defined ventral groove; transverse sections are circular to dorso-ventrally depressed. Arctoteuthis, Holcobeloides, Eulagonibelus, Boreioteuthis, some of Cylindroteuthis and Lagonibelus were moderately active epipelagic swimmers living not far from the shore. They are most common in the middle sublittoral zone deposits. Taxa characterized by a moderately elongate rostrum with a strongly depressed transverse section and a long and wide ventral groove (Holcobeloides, Eulagonibelus and many Boreioteuthis) probably lived in near-bottom waters. Isotope ratios have been analysed from belemnites belonging to the taxa Arctoteuthis porrectiformis, A. aff. porrectiformis, A. cf. sachsi, A. urdjukhaensis, Cylindroteuthis cf. comes, C. cf. jacutica, Lagonibelus strigatus, L. parvulus and L. sibiricus.

In Group III, rostra are elongate to moderately elongate and slender, with a weak ventral groove; transverse sections are usually laterally compressed. Most of Cylindroteuthis and Communicobelus, as well as some of Lagonibelus and Pachyteuthis, belong to this group of active epipelagic swimmers. They are most common in the middle–lower sublittoral zone deposits. Isotope ratios have been analysed from belemnites belonging to the taxa Cylindroteuthis knoxvillensis, C. lenaensis, C. obeliscoides, Lagonibelus cf. nordvikensis, Pachyteuthis cf. excentralis and P. cf. panderiana.

Several authors (Anderson et al. 1994; Price & Page 2008; Wierzbowski & Rogov 2011) consider belemnites as bottom-dwellers (with the ability of short vertical migration) on the basis of a comparison of their oxygen isotope records with those of ammonites and the similarity of oxygen isotope values to those of co-occurring bivalves. However, it is possible that the difference in δ¹⁸O palaeotemperatures recorded by different groups of cephalopods is due to the fact that belemnites are endocochleate (having an inner shell) whereas ammonites are ectocochleate (having an external shell). In any case, the morphological diversity among belemnites is too great to assume only a nektobenthic mode of life. Belemnite finds are known from black shales that lack any benthic organisms due to anoxic bottom waters (Mutterlose 1983; Seilacher et al. 1985; Oschmann et al. 1999). Rexfort & Mutterlose (2009) consider such findings as clear support for the view that belemnites (all or at least certain genera) had a nektonic mode of life.

Belemnite δ¹⁸O data show an irregular decrease from values reaching up to +1.6‰ in the Middle Oxfordian to values between +0.8 and −1.7‰ in the Upper Volgian and basal Ryazanian. This trend is similar to that of the belemnite δ¹⁸O in the Upper Jurassic of the Russian Platform (Riboulleau et al. 1998; Price & Rogov 2009) and Scotland (Nunn et al. 2009), possibly indicating a gradual warming during the Late Jurassic. Other explanations of such a trend include a gradual increase of freshwater input to the boreal seas during the Late Jurassic and a decrease in salinity. Nevertheless, there is no fossil evidence for any changes in salinity, as there are no fossils typical for brackish water from boreal marine deposits. Cephalopods, brachiopods and echinoderms are restricted to normal-salinity environments and occur in the same abundance during the end of the Jurassic and the beginning of the Cretaceous. Decreasing oxygen isotope values is a trend recognized from the Arctoteuthis, Cylindroteuthis and Pachyteuthis records. The Lagonibelus oxygen isotope data are scattered. According to the adopted classification (see above), samples analysed here belong to the cylindroteuthid belemnites of the second and third group, combining moderately to very active epipelagic swimmers that were able to live in (or migrate into) relatively deep water (down to 200 m). Exceptions are two samples of Simobelus sp. (position 28.56 and 29.16 m in the profile) from the first, most “shallow-water” group and two samples of Pachyteuthis sp. (height 38.36 and 40.26 m in profile) of an uncertain group affiliation. Suppositional nektobenthic forms among them are not established. In many cases, if the belemnite samples could not be identified to the species level, we could not determine whether the samples should be ascribed to the second or the third group. All these samples are attributed to the genera Cylindroteuthis and Arctoteuthis (Žák et al. 2011; Dzyuba et al. 2013) for which a nektobenthic mode of life is unlikely (Gustomesov 1976; Saks & Nalnjaeva 1979). The absence of bottom-dwellers probably explains why, despite the deepening of the studied area during the Late Jurassic, the belemnite δ¹⁸O data could record a climatic warming. However, many aspects of the ecology of belemnites remain unclear, complicating interpretations of isotope data.

The δ¹³C values of organic matter in sediments of the upper part of the Nordvik section range from −26.3 to −30.3‰ (VPDB; see Fig. 6). The values negatively correlate with the content of organic carbon in the sediments. Higher contents of organic carbon are accompanied by lower δ¹³C data and vice versa. The variability of organic carbon isotope data is about twice as large as the variability of the isotopic composition of inorganic carbon. The differences between δ¹³C values of the organic matter and of carbonate (belemnite rostra) Δ values (Δ¹³C = δ¹³C_carbonate – δ¹³C_{organic matter}) range from 26.76 to 30.81‰.

Oscillations of carbonate and organic matter δ¹³C values are not parallel in the studied part of the section. Positive peaks in δ¹³C values of organic matter are locally accompanied by negative peaks in δ¹³C of belemnite rostra, but the degree of this negative correlation is again weak, like in the case of the relations between organic matter contents and δ¹³C values.

During the Late Jurassic, the Yenisei–Khatanga strait experienced a gradual deepening, indicated by the change of sedimentation from sands and silts in the Oxfordian to mudstones and oil-shales in the Volgian (Fig. 3). Only after the beginning of the Ryazanian Hectoroceras kochi Chron silt deposition begin to increase and silts became dominant sediments in Nordvik during the Valanginian (Zaharov et al. 1983). These changes in grain size through the section were accompanied by an oscillation in sedimentation rates, recognized by significant differences in the thicknesses of stages and zones. The lowest sedimentation rate and thickness of ammonite zones is found close to the J/K boundary, coinciding with palaeomagnetic evidence for a slow sedimentation (Grabowski 2011). Above the J/K boundary the thickness of each ammonite zone gradually increased.

Our record of relative sea-level variations is based especially on lithological changes and the T/M index. The sea-level rise interpretation (TST in Fig. 6) towards the boreal J/K boundary is supported by micro- and macropalaeontological, geochemical, lithological and—especially—stable isotopic data. Sea-level variations were related to climatic cooling and warming phases corresponding to positive and negative excursions in δ¹⁸O values, respectively. Three positive peaks in δ¹⁸O values within the Explanata belemnite zone, Variabilis ammonite zone (Middle Volgian), and ca. 1 m below the base of the Okensis ammonite zone, are related to a relative sea-level decrease. The marked negative peak of δ¹⁸O just above the Kysuca M20n.1r. magneto-subzone is stratigraphically very important. Similar δ¹⁸O values are well known from the boreal J/K boundary and they may represent a rising sea level. The relative sea-level fall partly corresponds to negative δ¹³C values of organic matter and the T/M index (Fig. 5). However, the Okensis ammonite zone and lower part of the Taimyrensis ammonite zone lack relevant δ¹⁸O data from belemnite rostra. Several peaks of the T/M index may indicate specific trangressive/regressive pulses (probably fourth or fifth order sea-level changes; Fig. 6). The major eustatic excursions should represent third order sea-level variations during greenhouse phases (Tucker 1993). A long-term (first order) rise in sea level through the Jurassic and Cretaceous (Hallam 1988; Haq et al. 1988, Surlyk 1991) is well documented in stable isotopes in the Nordvik section and is well correlated worldwide.

In the critical J/K boundary interval, the two major sequence boundaries are clearly distinguished. The base of Member 6 (15–20 cm) shows a marked shallowing event as indicated by high glauconite concentration, marked ichnofabric, bottom communities and lithologic changes. Over- and underlying beds contain rich Chondrites ichnofabrics, while the base of Member 6 includes typical Planolites assemblages. Deposit-feeders are absent in the bottom communities and only high-level suspension-feeders could exist there. This indicates a shallowing event and/or a better oxidation of the sea bottom. The conclusion on a low sea level during the Variabilis zone is also supported by macropalaeontological data including bivalve assemblages and a rich belemnite fauna (Fig. 3). Another shallowing event, probably connected to a regression, has been identified at the top of the Exoticus zone. This event, clearly recorded by a positive excursion of δ¹⁸O (see Žák et al. 2011), corresponds to the regression observed on the Russian Platform (Price & Rogov 2009). In contrast, the base of the Okensis zone is marked by the appearance of ammonites with a Pacific affinity (Euphylloceras), reflecting a sea-level rise. The sea level apparently increased during the Okensis and Taimyrensis Chrons, which caused the influx of marine waters with new ammonite assemblages and a sharply decreasing belemnite faunal diversity. High-level detritophagous bivalves appear once again in the bottom communities, supporting the existence of dysoxic bottom environments.

The base of Member 9 (“Iridium bed”) originated during the time of maximum eustatic sea level and represents a highstand systems tract in relation to the maximum flooded surface. The occurrence of anomalous levels of iridium and other platinum group elements has been reported and explained by the presence of diagenetically sulphidized iron particles of cosmic dust deposited under conditions of a drastically reduced clastic sedimentation rate (Zakharov et al. 1993; Mizera et al. 2010; Dypvik & Zakharov 2012). However, limited basaltic components from a source in the Siberian Traps (Permian–Triassic) containing also rare elements cannot be excluded (Dypvik & Zakharov 2012). The sea-level rise at the Sibiricus and Kochi zones boundary was also accompanied by the immigration of open sea ammonite taxa.

The Nordvik section demonstrates a clear trend of a strongly decreasing depositional rate upwards from the Okensis Zone to the base of the Ryazanian, with estimated changes up to 7–10 times, from ca. 11–12 m/My to 1.5–2 to 11–12 m/My (Zakharov et al. 1993; Grabowski 2011). This decrease of terrestrial matter input was quickly reversed to higher rates of sedimentation after the deposition of the phosphate-rich band in the base of Ryazanian (Zakharov et al. 1993; Bragin et al. 2013). A strongly reduced sedimentation at the beginning of the Cretaceous led to the deposition of the iridium-rich band. The same trend of decreasing sedimentary rates, succeeded by its rise, has been recently recognized in many Tethyan sections around the J/K boundary (Grabowski et al. 2010; Lukeneder et al. 2010; Pruner et al. 2010). Such a phenomenon could be explained by climatic changes, especially by a progressive aridization (Abbink et al. 2001) and a decrease of rainfall and weathering at the J/K boundary.

The biodiversity changes recorded at Nordvik are strongly related to sea-level oscillations. Macro-faunal data show a relatively high ammonite and low belemnite diversity when the sea level was high. The most prominent sea-level rise is marked by the occurrence of sea ammonites with Pacific affinities (Euphylloceras, Bochianites, etc.). Sea-level changes from the Oxfordian to the J/K boundary were also accompanied by significant turnovers in the bottom community structure. The relatively high taxonomical richness of bivalves during the Oxfordian and Kimmeridgian (up to 18 genera in the Oxfordian) was substituted by a low diversity during the Volgian, especially the Late Volgian (four genera). These changes in bivalve diversity were accompanied by a simplification of the trophical structure of bivalve communities. Late Kimmeridgian bivalve communities hold up to six detritophagous genera, including four high-level deposit-feeders, while in the Late Volgian only two detritophagous genera were encountered, and both were uncommon. Sestonophagous bivalves are represented by a single genus (Buchia) only, which is sometimes found in abundance. These data suggest an anomaly in some environmental factors. Following sedimentological, mineralogical and geochemical analyses, we conclude that these changes in benthic communities were caused by low oxygen contents in the near-bottom water layer and an oxygen deficit in the sediment due to the deepening of the sea during sea-level rise.

The most striking feature of the Nordvik succession is a synchronous peak abundance in spores, corresponding to the δ¹³C negative excursion in organic matter, and in prasinophytes near the J/K boundary (Nikitenko et al. 2008). Götz et al. (2009) reported a high abundance of prasinophytes from near the Triassic/Jurassic boundary in the Tethyan Realm, corresponding to the interval of the positive peak and an initial negative carbon isotope excursion, which they interpret as representing a bloom of green algae that flourished as a result of a disturbance in the marine ecosystem. A bloom of organic walled green algae is also known from the Triassic/Jurassic boundary interval of the St. Audrie's Bay section in Somerset, UK (van de Schootbrugge et al. 2007), which led to the suggestion that the proliferation of green algal phytoplankton may have been triggered by elevated carbon dioxide levels in the atmosphere.

During the Late Jurassic and Early Cretaceous, the Nordvik area was also affected by significant influences of cold water masses. The deep South Anyuy ocean developed at that time, forming a long but narrow bay from the northern Pacific. Oscillations in the relative diversity of different groups of molluscs could also be connected to fluctuations caused by local tectonics and sea-level changes rather than to changes in palaeotemperature. The Nordvik section is also important because information and data can be obtained time-equivalent to those of the oil-bearing Bazhenovo Formation of western Siberia. Our study makes it possible to recognize depositional conditions of the Bazhenovo Formation more precisely.

Two patterns are observed in the relationship between the Nordvik faunal diversity and the general trend of decreasing δ¹⁸O: an inverse correlation for the Oxfordian–Middle Volgian and a direct correlation for the Late Volgian–earliest Ryazanian. The mid-Oxfordian to Middle Volgian decrease of δ¹⁸O values corresponds to an irregular but consistent increase in the taxonomic diversity in all faunal groups. The subsequent warming events in the Yenisei–Khatanga Sea (Žák et al. 2011) apparently did not favour Nordvik belemnites, bivalves and foraminifers. Their diversity was considerably reduced. In contrast, the ammonite diversity increased twice during the Late Jurassic to earliest Cretaceous, at the Middle–Late Volgian transition and at the beginning of the Ryazanian Kochi Chron. The highest levels of ammonite diversity were reached when a deepening of the North Siberian basin enabled the invasion of open sea ammonites. The water depths in the Nordvik area of the Late Volgian–earliest Ryazanian Yenisei-Khatanga Sea were evidently too great not only for benthos, but also for belemnites.

The belemnite δ¹⁸O data show an irregular decrease from values reaching up to +1.6‰ in the Middle Oxfordian to values between +0.8 and −1.7‰ in the Upper Volgian and basal Ryazanian. They could be interpreted as reflecting a gradual warming (Fig. 6). This trend is similar to that of belemnite δ¹⁸O in the Upper Jurassic of the Russian Platform (Riboulleau et al. 1998; Price & Rogov 2009) and Scotland (Nunn et al. 2009). These oxygen isotopic data indicate a prolonged episode of gradual warming in boreal and subboreal areas. In contrast to this Late Jurassic warming, Tethyan data suggests a cooling in the latest Jurassic (Tremolada et al. 2006; Dera et al. 2011). Such a dichotomy may point to a weaker latitudinal temperature gradient during this time.