Palaeoenvironments and palaeoceanography changes across the Jurassic/Cretaceous boundary in the Arctic realm: case study of the Nordvik section (north Siberia, Russia)

RESEARCH/REVIEW ARTICLE Palaeoenvironments and palaeoceanography changes across the Jurassic/Cretaceous boundary in the Arctic realm: case study of the Nordvik section (north Siberia, Russia) Victor A. Zakharov, 1 Mikhail A. Rogov, 1 Oksana S. Dzyuba, 2 Karel Z ˇa ´k, 3 Martin Kos ˇt’a ´k, 4 Petr Pruner, 3 Petr Skupien, 5 Martin Chadima, 3 Martin Mazuch 4 & Boris L. Nikitenko 2 1 Geological Institute of Russian Academy of Sciences, Pyzhevski lane, 7, Moscow RU-119017, Russia 2 Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of Russian Academy of Sciences, Acad. Koptyug av., 3, Novosibirsk RU-630090, Russia 3 Institute of Geology of the Academy of Sciences of the Czech Republic, v.v.i., Rozvojova´ 269, CZ-165 00 Prague 6, Czech Republic 4 Institute of Geology and Palaeontology, Faculty of Science, Charles University in Prague, Albertov 6, CZ-128 43 Prague 2, Czech Republic 5 Institute of Geological Engineering, Technical University of Ostrava, 17 listopadu 15, CZ-708 33, Ostrava-Poruba, Czech Republic Keywords Biodiversity; stable isotopes; J/K boundary; Arctic Realm; palaeoceanography. Correspondence Mikhail A. Rogov, Geological Institute of Russian Academy of Sciences, Pyzhevski lane, 7, Moscow, RU-119017, Russia. E-mail: [email protected] Abstract The Jurassic/Cretaceous transition was accompanied by significant changes in palaeoceanography and palaeoenvironments in the Tethyan Realm, but outside the Tethys such data are very scarce. Here we present results of a study of the most complete section in the Panboreal Superrealm, the Nordvik section. Belemnite d 18 O data show an irregular decrease from values reaching up to 1.6in the Middle Oxfordian and from 0.8 to 1.7in the basal Ryazanian, indicating a prolonged warming. The biodiversity changes were strongly related to sea-level oscillations, showing a relatively low belemnite and high ammonite diversity during sea-level rise, accompanied by a decrease of the macrobenthos taxonomical richness. The most prominent sea-level rise is marked by the occurrence of open sea ammonites with Pacific affinities. Peak abundances of spores and prasinophytes correlate with a negative excursion in organic carbon d 13 C near the J/K boundary and could reflect blooms of green algae caused by disturbance of the marine ecosystem. The J/K boundary interval was characterized by remarkable palaeoceanographic and palaeoenvironmental changes in Europe (Tremolada et al. 2006), but in contrast to other system boundaries it was not connected to a global extinction event. The decrease in marine family diversity at the J/K boundary was estimated to lie between 5.1 and 6.5% (Hallam & Wignall 1997), while the decrease in generic diversity of all marine animals was less than 10% (Rogov 2013). Moreover, even this extinction rate could be overestimated due to the incomplete databases used, high evolutionary rates in Tithonian ammonites, time-averaging (Rogov 2013) and taphonomic features (i.e., a presence of numerous Lagersta ¨tten of KimmeridgianTithonian age and a scarcity of earliest Cretaceous ones), along with well-known relations of faunal diversity and the surface area of sediments surviving at outcrops (Smith & McGowan 2007). This suggests a continual biotic turnover caused partly by palaeoceanographic changes and, especially, large palaeogeographic events (Raup & Sepkoski 1982; Sepkoski 2002; Keller 2008). In the Arctic Realm, the transition between the Jurassic and Cretaceous was connected with neither extinction nor a radical reorganization in marine biota (Zakharov et al. 1993). This view is supported by faunal analysis (Dzyuba 2013; Rogov 2013) that indicated an increase in ammonite and belemnite diversity. However, on a global scale, the tectonic movements at the end of the Jurassic led to closure of many seaways and further differentiation of the Panboreal and TethysPantalassa superrealms (Gasin ´ski 1997). Following proposals by Westermann (2000), names and ranges of these units were subsequently adjusted. Polar Research 2014. #2014 V.A. Zakharov et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution-Noncommercial 3.0 Unported License (http://creativecommons.org/licenses/by-nc/3.0/), permitting all non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 1 Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 (page number not for citation purpose) In the Panboreal Superrealm, there is no section where sedimentation patterns and biostratigraphy of the J/K boundary interval are represented more completely than in a section on the Nordvik Peninsula (north-eastern Siberia; Fig. 1). On the Nordvik Peninsula, the Laptev Sea cliff exposes a continuous succession of silty mudstones spanning the Middle Oxfordian to Upper Valanginian. The boreal ammonite and buchiid successions are recognized in great detail from the Middle Oxfordian to the top of the section (Zaharov et al. 1983; Zakharov & Rogov 2008; Rogov & Wierzbowski 2009). A belemnite biostratigraphy is well developed for the Middle OxfordianLower Ryazanian interval (Dzu ˆba 2004; Dzyuba 2012). Foraminifers and marine palynomorphs from the Middle Volgian to the Lower Valanginian have been studied in detail by Nikitenko et al. (2008). Here we present results of an integrated study that includes an analysis of changes in assemblages (including relative abundance) of all major fossil groups and an analysis of sea-level oscillations. An earlier investigation of stable isotope data derived from belemnite rostra (Z ˇa ´k et al. 2011; Dzyuba et al. 2013) was complemented by analysing the d 13 C values of bulk organic matter. Major aims of this integrated study include the determination of biodiversity changes across the J/K boundary and the identification of possible relationships between different environmental factors, including sea-level oscillations, palaeoclimate, oxygen level in the sea water and biodiversity changes. The unique original high-latitude palaeogeographic position (see Hous ˇa et al. 2007) of the investigated section was quite far from the coastline. The complete sedimentary succession without major hiatii and the precise stratigraphic framework make this profile most suitable for a palaeocological and palaeoceanographic investigation of the Arctic Realm. Geological settings and palaeogeographic aspects 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 (Dzu ˆba 2004; Dzyuba 2012), buchiid bivalves (Zaharov et al. 1983), foraminifers and dinoflagellate cysts (Nikitenko et al. 2008, 2011). A remarkable positive d 13 C 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 Fig. 1 Palaeogeographic and geographic position of the Nordvik section. Palaeoenvironments and palaeoceanography changes across the J/K boundary V.A. Zakharov et al. 2 (page number not for citation purpose) Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 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 Fig. 2 Palaeogeography and facial succession in the YeniseiKhatanga Strait during the Jurassic/Cretaceous transitional time (modified after Zaharov & S ˇurygin 1983). (a) Detailed palaeogeography of the YeniseiKhatanga Strait during the Ryazanian. Land area is indicated in beige. Bionomic marine zones are marked by Roman numerals: I, deep-water; II, moderately deep-water; III, shallow water. (b) Facies profile through the YeniseiKhatanga Strait and changes in bivalve and foraminiferal assemblages. V.A. Zakharov et al. Palaeoenvironments and palaeoceanography changes across the J/K boundary Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 3 (page number not for citation purpose) Fig. 3 (Continued) Palaeoenvironments and palaeoceanography changes across the J/K boundary V.A. Zakharov et al. 4 (page number not for citation purpose) Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 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 (Dzu ˆba 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 Fig. 3 Ammonite, belemnite and buchiid zonations in relation to lithoand magneto-stratigraphy and biodiversity of major taxonomic groups. Member 1 (section 33, beds 13) is Middle Oxfordian, with silty clay to silty mudstone, dark-grey to black, and an interband of sandy-silt at the middle part of the bed. Member 2 (section 33, beds 45) is Upper Oxfordian, with silty thin-bedded black clay. Member 3 (section 33, bed 6) is Upper OxfordianLower Kimmeridgian, with silty dark-grey to greenish clay. The contents of the silt fraction decreases upwards and clay became well-sorted. Member 4 (section 33, bed 7) is Lower KimmeridgianMiddle Volgian, with silty dark-grey glauconiteleptochlorite badly sorted clay and small addition of sandy grains. Member 5 (section 33, beds 89; section 32, beds 12) is Middle Volgian and belonging to the Epivirgatites variabilis Zone. The mudstone-like fractured dark-grey glauconiteleptochlorite clay within this member has a bluish tint. Member 6 (section 33, bed 1014; section 32, bed 37) is Middle VolgianUpper Volgian, with mudstone-like fractured clay consisting of alternating layers of dark-grey, brown and bluish-grey colour, and a glauconite-rich band at the base. Member 7 (section 33, beds 15, 16; section 32, beds 8, 9) is Upper Volgian. The mudstone-like clay is dark-grey, sometimes silty, bedded, with occasional bands of bluish-grey fractured clay and with high contents of organic matter and pyrite. Member 8 (section 33, bed 17; section 32, bed 10) is Upper Volgian and belonging to the Chetaites chetae Zone. The thin-bedded dark-grey brownish clay has fossil plant remains and pyrite globules. Member 9 (section 33, beds 1823; section 32, beds 1116) is Lower Ryazanian, and belonging to the Chetaites sibiricus Zone. The clay is sometimes mudstone-like and is dark-grey, thin-bedded, with interbeds of fractured bluish-grey clay. The lower part of member 10 (section 33, bed 24; section 32, beds 1718) is Lower Ryazanian and belonging to the basal bed of the Hectoroceras kochi Zone. The clay is mudstone-like, partially silty, fractured, bluish-grey, with intercalations of thin-bedded dark-grey clay. 1C2A, No of levels. V.A. Zakharov et al. Palaeoenvironments and palaeoceanography changes across the J/K boundary Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 5 (page number not for citation purpose) 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; Dzu ˆba et al. 2006; Rogov 2012). Materials and methods 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-mm 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 d 13 C 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 d 13 C 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 d 13 C values of the organic matter was 90.2. Palaeoecological changes: available data and environmental interpretation Changes in bivalve communities Analysis of benthic communities can provide detailed information on the environment and the character of sedimentation. The most promising approach is a trophic structural study by analysing the levels of feeding (Turpaeva 1953). Benthic molluscs can be divided into two large groups on the basis of the feeding method: deposit-feeders (detritophagous) and suspension-feeders (sestonophagous), both subdivided in two subgroups: high or low feeders. The first subgroup of deposit-feeder bivalves feeds at the sediment surface. The second group gains its food below the sediment surface. Suspension-feeder bivalves feed near the sediment surface, or above it. The presence of detritophagous bivalves in the community always indicates quiet conditions with stable rates of sedimentation. If species feeding at the surface are dominant in the community, this shows that there is poor oxygenation at the bottom. The well-recognized ‘‘taxonomic stairs’’ are a good indicator of the stability of all environmental factors, showing the presence of few dominant, characteristic species and numerous rare species in the community (Odum 1971). A ‘‘mature community’’ is characterized by the presence of all groups in the ‘‘taxonomic stairs’’; such biocoenoses are formed in stable environments. The absence of one or more ‘‘stairs’’ indicates an unstable environment and the influence of some kind of abnormality. On this basis, we conclude that mature communities are absent (Fig. 3). Only the first (Middle Oxfordian) and tenth (Kochi Chron) communities are characterized by the absence of strongly dominated taxa and can be considered to be rather uniform. The most noticeable characteristic in all cases is the absence of an obvious dominant species. The predominance of Aequipecten in taphococenoses is not significant since this genus with thin-walled equilateral and equivalved shells seems to be pseudoplanktonic. Therefore, we can reasonably conclude that the influence of one or more environmental factors was anomalous during the Late Jurassic and in the very beginning of the Early Cretaceous. Among the most probable factors is an oxygen deficit of near-bottom water, with possible periodic anoxic events at the sediment surface. The trophic structure of the communities confirms this idea because epifaunal deposit-feeders represent a considerable fraction in most communities, sometimes accounting for more than half of all the bivalves (Fig. 3). Palaeoenvironments and palaeoceanography changes across the J/K boundary V.A. Zakharov et al. 6 (page number not for citation purpose) Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 Another possible factor disturbing the development of benthic communities is a low temperature of near-bottom waters. Small size, thin, sometimes transparent shells in Kimmeridgian and Volgian bivalves can be a strong argument for this suggestion. Near-bottom waters could cool by the inflow of cold waters from the east where deep foredeeps were located. The proximity of an open ocean to the east is indicated by the presence of phylloceratids in the Nordvik section in Volgian and Ryazanian strata. These ammonites are absent in the Oxfordian and Kimmeridgian of this area. However, as inhabitants of open seas, phylloceratids are quite common in the Upper Jurassic of north-east Russia (Parakecov & Parakecova 1989). Possible paths of a phylloceratid invasion into the YeniseiKhatanga Sea are deep-water troughs: flysh sediments, usually filling such troughs, were recently found at the New Siberian Islands (Kuz’michev et al. 2009). Changes in ammonite diversity As at other high-latitude sites (Rogov 2012), the ammonite assemblages of the Nordvik section are mainly characterized by very low taxonomic diversity, usually ranging between one and three at both genus and species level. All levels that are characterized by increased diversity coincide with immigration events of oceanassociated ammonites, with the exception of the Kimmeridgian, when the high diversity interval coincides with the appearance of Suboxydiscites (Fig. 4d), and the last cardioceratid diversification. One such high diversity level can be observed at the MiddleLate Volgian transition when, in addition to boreal ammonites, Euphylloceras occurs (Fig. 4e). The appearance of these phylloceratids in the studied area and in Svalbard (Rogov 2010) indicates the penetration of typical open sea ammonites from the northern Pacific. In contrast to the Middle Volgian the beginning of the Late Volgian is also characterized by the decrease of provincialism of boreal ammonites. Only the western part of the Panboreal Superrealm, westward of the Greenland Norwegian Seaway, was inhabited by an endemic craspeditid lineage (SwinnertoniaSubcraspeditesVolgidiscus). Both a reduction in ammonite endemism and the occurrence of phylloceratids could be explained by a short-term sea-level rise in the Arctic. Such a sea-level rise is also reflected in the Russian Platform succession (Sahagian et al. 1996). Afterwards, the ammonite diversity quickly decreased, albeit ammonite faunas of the different parts of the Arctic became very similar in this time. Near to the VolgianRyazanian boundary, the ammonite diversity increased again. The Chetae Chron is characterized by the co-occurrence of boreal Chetaites and Praechetaites as well as oceanic phylloceratids (Zaharov et al. 1983). An increase in ammonite diversity at the beginning of the Ryazanian is connected with a high diversity of boreal taxa belonging to three lineages: last Craspedites (Taimyroceras); first Praetollia derived from Volgidiscus; and the last Dorsoplanitid ammonites, the Chetaites. The most remarkable diversity increase at the beginning of the Kochi Chron again relates to the invasion of oceanic ammonites. These ammonites include at least one endemic Arctic taxon Boreophylloceras, while the typical oceanic Biasaloceras (Fig. 4b) and Bochianites cf. glennensis (Fig. 4c) resemble Californian occurrences which immigrated from the Pacific (Rogov & Igol’nikov 2009). Moreover, such open sea ammonites even penetrated the much more shallow seas of the Khatanga depression, and their records are also known from the basal Kochi Zone of the Bojarka section (Fig. 4a). This event also closely corresponds with a suggested sea-level rise leading to rapid spreading of the first Hectoroceras throughout the Arctic. Changes in belemnite diversity Belemnites (Cylindroteuthididae) in the Nordvik section as a whole are more abundant and higher in taxonomic diversity than ammonites. As has been observed (e.g., Zakharov et al. 2005; Rogov et al. 2006), there is an inverse relationship in diversity of these two cephalopod groups (see Fig. 3). In part, this is caused by competition between ammonites and belemnites and their different preferences concerning environmental conditions. Consequently, the highest levels of ammonite diversity are reached when the deepening of the North Siberian basin made the invasion of open sea ammonites possible. This is observed at the MiddleLate Volgian transition and at the beginning of the Ryazanian Kochi Chron. Belemnite diversity is notably reduced at these times, suggesting that cylindroteuthids were poorly adapted to deep-water environments. Low diversity belemnite assemblages are recorded in the wide interval of the Upper Volgian and lowest Ryazanian beds. Only rare representatives of the genera Arctoteuthis,Cylindroteuthis and Lagonibelus with elongate rostra are distributed here (from one to five species). Taxa with shorter robust rostra (Pachyteuthidinae and Simobelinae) are absent; this indicates a deepening and broadening of the basin, especially during the Okensis Chron and most of the Taimyrensis Chron, which is confirmed by micropalaeontological and palynological evidence (Nikitenko et al. 2008). A slight diversity increase can be recognized at the top of the Taimyrensis Zone near V.A. Zakharov et al. Palaeoenvironments and palaeoceanography changes across the J/K boundary Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 7 (page number not for citation purpose) the J/K boundary (as recently confirmed by magnetostratigraphy; Hous ˇa et al. 2007), and just above the ‘‘Iridium bed’’ at the base of Member 9. At the same levels, the majority of the belemnites common for the Arctic and Northern Proto-Pacific (California) remains steady: Cylindroteuthis knoxvillensis,C. cf. newvillensis,Arctoteuthis tehamaensis and A. porrectiformis. In deposits from the Middle Oxfordian to the Kimmeridgian, among the belemnites many cosmopolitan taxa were observed, but all are common taxa for Arctic and subboreal European seas: Cylindroteuthis puzosiana,C.obeliscoides,Pachyteuthis excentralis,P.panderiana,Boreioteuthis absoluta,Lagonibelus ingens,L.kostromensis, Simobelus breviaxis. The interval between Kimmeridgian and Late Volgian time was characterized by an increase in belemnite endemism culminating in the Middle Volgian Variabilis Chron (up to 70% Arctic endemic belemnite assemblages). The Kimmeridgian and Middle Volgian Variabilis chrons show both the highest taxonomic diversity (916 species from six to seven genera) and an abundance of belemnites. Both episodes of increasing belemnite diversity are typical features for the North Siberian seas and attributed to oceanographic (sea-level changes, expansion of sea connections) and probably climatic (warming) events (Saks & Nalnjaeva 1979; Dzu ˆba et al. 2006; Dzyuba 2013). Six major belemnite events, characterized by mass occurrences of belemnite rostra, have been recognized in the Upper Kimmeridgian through the lowest Ryazanian interval. The earliest one has been located at the top of the Septentrionalis belemnite Subzone (stratigraphic Fig. 4 Selected Upper JurassicLower Cretaceous ammonites from the Nordvik and Bojarka sections. (a) Biasaloceras sp., no number, Bojarka river, Ryazanian, Kochi Zone; (b) Biasaloceras sp., MK1056, Nordvik, Ryazanian, Kochi Zone; 0.05 m above 1C; (c) Bochianites cf. glennensis And., MK 1094, Nordvik, Ryazanian, Kochi Zone; 0.05 m above 1C; (d) Suboxydiscites sp. [m]; MVI767; 0.3 m above 2G, Lower Kimmeridgian, Kitchini Zone, subkitchini horizon; (e) Euphylloceras cf. knoxvillense (Stanton); MK1082, 9.3 m below the base of the Ryazanian, Upper Volgian, Okensis Zone. Specimens were collected by M. Rogov during fieldwork in 2003 except (a), which was collected by M. Rogov in 2008. Specimens (a), (b), (c) and (e) are stored in the Trofimuk Institute of Petroleum Geology and Geophysics of the Siberian Branch of Russian Academy of Sciences, Novosibirsk, and specimen (d) is at the Geological Institute of Russian Academy of Sciences, Moscow. Palaeoenvironments and palaeoceanography changes across the J/K boundary V.A. Zakharov et al. 8 (page number not for citation purpose) Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 equivalent of the uppermost Kimmeridgian Suboxydiscites taimyrensis ammonite Zone in Siberia). While the high sea level reflected in the middle part of the Explanata belemnite Zone is not connected with a mass occurrence of belemnites, the sea-level variations between the upper part of the Explanata and lower part of the Russiensis belemnite zones are characterized by four, clearly distinguished belemnite events. Three of these are located in the Middle Volgian Variabilis ammonite Zone. The development of favourable conditions for a belemnite expansion (see above) and mass occurrences are predominantly associated with a shallowing. The belemnite dependence on changes in sea level and nutrient delivery are well recorded in the Upper Cretaceous taxa (Mitchell 2005; Wiese et al. 2009). However, the Cretaceous taxa are typical of shallowwater platform seas. The same mode of belemnite behaviour is reported here also from the BorealArctic Basin. The depth of the sea bottom may represent an ecological limit for a mass expansion. A sixth belemnite event is recorded at the base of the Sibiricus ammonite Zone. Its position shows a shallowing trend with an increasing input of terrigenous material. Eventually, the belemnite diversity shows a partial decrease in the J/K boundary interval. This corresponds to a time of sea-level rise and basin deepening. Changes in palynological spectra Our palynofacies analysis (counting up to 400 particles in each sample) was based on phytoclasts distinguished by Batten (1996): structural organic matter represented by black particles, brown woody particles and yellow material (cuticles); palynomorphs represented by bisaccate pollen, non-bisaccate pollen, spores, green algae (prasinophytes), dinoflagellate cysts and foraminiferal test linings. The quantitative study of the dinoflagellates and the analysis of their distribution has been published (Nikitenko et al. 2008). Three data sets were used to interpret palynological assemblage fluctuations in terms of environmental changes: (1) the ratio of terrestrial (pollen and spores) to marine palynomorphs (acritarchs, dinoflagellate cysts, foraminiferal linings, algae), known as the terrestrial/marine index (T/M index); (2) the changes in relative abundance of selected dinocyst genera; and (3) the diversity expressed as the number of genera or species found in each sample. All samples contain rich palynological material. Overall, the studied samples display palynofacies variations through the section. Most residues are characterized by abundant structural organic matter, dominated by phytoclasts (brown woody particles and yellow particles). This material dominates the section at the intervals of 3439 and 45 49.5 m. Poorly preserved dinoflagellate cysts can be found in the same intervals (3638.5 and 4647.4 m). These are usually difficult to identify and carry marks of decay caused by crystallization of pyrite. This palynofacies is typical for anoxic conditions where deposition is slow. The studied parts of the section are also characterized by a low percentage of dinoflagellate cysts (B10%) and a very low percentage of spores and pollen grains (B15%). Certain palynomorph assemblages contain a higher proportion of dinoflagellate cysts than found in the rest of the studied section. The increase in relative abundance of dinoflagellate cysts (Middle Volgian, Okensis ammonite zone of the Upper Volgian, Sibiricus and Kochi ammonite zones of the Ryazanian) is coinciding with an increased abundance of foraminiferal linings (typical of marine coastal and neritic environments) and green algae (Fig. 5). Foraminiferal test linings occur at 30 and 44 m. In the intervals from 42 to 45 m (Upper Volgian) and from 50 to 54 m (Lower Ryazanian), a distinct bloom of green algae (prasinophytes) is found (Fig. 5). Here dinoflagellate cysts comprise around 2040% of the assemblage. Prasinophytes are known to appear in a wide range of environments, from marine to brackish, being most frequent in nearshore marine and anaerobic environments (e.g., Tappan 1980; Batten 1996). According to Prauss & Riegel (1989), prasinophycean algae might have favoured cold-water environments. Smelror et al. (2001) suggested that the algal bloom was possibly induced by the large amounts of nutrients released into the water column. High productivity and limited water circulation at the bottom with a marked eutrophication cause anoxic conditions of sedimentation (Below & Kirsch 1997). Prasinophytes are abundant in this type of facies (Skupien & Vas ˇı ´c ˇek 2002; Nikitenko et al. 2008). The changes in T/M index and the composition of palynofacies may aid in the interpretation of the depositional environments in terms of water depth, sea-level oscillations and terrigenous influx (Lister & Batten 1988; Smelror & Leereveld 1989; Batten 1996). A high fraction of terrestrial organic matter points to an intense supply of land-derived organic particles and/or shallowing marine conditions. An increased supply of continental material probably reflects higher precipitation on land, resulting in an enhanced nutrient supply to coastal waters and enhanced organic particle transport to the shelf margin. The T/M ratio decreases basin-ward and increases in a shallowing succession. The low amounts of amorphous organic matter in all samples may result from a high accumulation rate. The diversity of dinoflagellates varies from eight to 25 genera and eight to 31 species in each sample. V.A. Zakharov et al. 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