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 Palaeoen i onmen s and palaeoceanog aphy changes ac oss he Ju assic/C e aceous bounda y in he A c ic ealm: case s udy o he No d ik sec ion (no h Sibe ia, Russia) Vic o A. Zakha o , 1 Mikhail A. Rogo , 1 Oksana S. Dzyuba, 2 Ka el Z ˇa ´k, 3 Ma in Kos ˇ ’a ´k, 4 Pe P une , 3 Pe Skupien, 5 Ma in Chadima, 3 Ma in Mazuch 4 & Bo is L. Niki enko 2 1 Geological Ins i u e o Russian Academy o Sciences, Pyzhe ski lane, 7, Moscow RU-119017, Russia 2 T o imuk Ins i u e o Pe oleum Geology and Geophysics, Sibe ian B anch o Russian Academy o Sciences, Acad. Kop yug a ., 3, No osibi sk RU-630090, Russia 3 Ins i u e o Geology o he Academy o Sciences o he Czech Republic, . .i., Roz ojo a´ 269, CZ-165 00 P ague 6, Czech Republic 4 Ins i u e o Geology and Palaeon ology, Facul y o Science, Cha les Uni e si y in P ague, Albe o 6, CZ-128 43 P ague 2, Czech Republic 5 Ins i u e o Geological Enginee ing, Technical Uni e si y o Os a a, 17 lis opadu 15, CZ-708 33, Os a a-Po uba, Czech Republic Keywo ds Biodi e si y; s able iso opes; J/K bounda y; A c ic Realm; palaeoceanog aphy. Co espondence Mikhail A. Rogo , Geological Ins i u e o Russian Academy o Sciences, Pyzhe ski lane, 7, Moscow, RU-119017, Russia. E-mail: [email p o ec ed] Abs ac The Ju assic/C e aceous ansi ion was accompanied by signi ican changes in palaeoceanog aphy and palaeoen i onmen s in he Te hyan Realm, bu ou side he Te hys such da a a e e y sca ce. He e we p esen esul s o a s udy o he mos comple e sec ion in he Panbo eal Supe ealm, he No d ik sec ion. Belemni e d 18 O da a show an i egula dec ease om alues eaching up o 1.6in he Middle Ox o dian and om 0.8 o 1.7in he basal Ryazanian, indica ing a p olonged wa ming. The biodi e si y changes we e s ongly ela ed o sea-le el oscilla ions, showing a ela i ely low belemni e and high ammoni e di e si y du ing sea-le el ise, accompanied by a dec ease o he mac oben hos axonomical ichness. The mos p ominen sea-le el ise is ma ked by he occu ence o open sea ammoni es wi h Paci ic a ini ies. Peak abundances o spo es and p asinophy es co ela e wi h a nega i e excu sion in o ganic ca bon d 13 C nea he J/K bounda y and could e lec blooms o g een algae caused by dis u bance o he ma ine ecosys em. The J/K bounda y in e al was cha ac e ized by e- ma kable palaeoceanog aphic and palaeoen i onmen al changes in Eu ope (T emolada e al. 2006), bu in con as o o he sys em bounda ies i was no connec ed o a global ex inc ion e en . The dec ease in ma ine amily di e si y a he J/K bounda y was es ima ed o lie be ween 5.1 and 6.5% (Hallam & Wignall 1997), while he de- c ease in gene ic di e si y o all ma ine animals was less han 10% (Rogo 2013). Mo eo e , e en his ex inc- ion a e could be o e es ima ed due o he incomple e da abases used, high e olu iona y a es in Ti honian ammoni es, ime-a e aging (Rogo 2013) and apho- nomic ea u es (i.e., a p esence o nume ous Lage s a ¨ en o Kimme idgianTi honian age and a sca ci y o ea lies C e aceous ones), along wi h well-known ela ions o au- nal di e si y and he su ace a ea o sedimen s su i ing a ou c ops (Smi h & McGowan 2007). This sugges s a con inual bio ic u no e caused pa ly by palaeoceano- g aphic changes and, especially, la ge palaeogeog aphic e en s (Raup & Sepkoski 1982; Sepkoski 2002; Kelle 2008). In he A c ic Realm, he ansi ion be ween he Ju assic and C e aceous was connec ed wi h nei he ex inc ion no a adical eo ganiza ion in ma ine bio a (Zakha o e al. 1993). This iew is suppo ed by aunal analysis (Dzyuba 2013; Rogo 2013) ha indica ed an inc ease in ammoni e and belemni e di e si y. Howe e , on a global scale, he ec onic mo emen s a he end o he Ju assic led o closu e o many seaways and u he di e en ia ion o he Panbo eal and Te hysPan alassa supe ealms (Gasin ´ski 1997). Following p oposals by Wes e mann (2000), names and anges o hese uni s we e subsequen ly adjus ed. Pola Resea ch 2014. #2014 V.A. Zakha o e al. This is an Open Access a icle dis ibu ed unde he e ms o he C ea i e Commons A ibu ion-Noncomme cial 3.0 Unpo ed License (h p://c ea i ecommons.o g/licenses/by-nc/3.0/), pe mi ing all non-comme cial use, dis ibu ion, and ep oduc ion in any medium, p o ided he o iginal wo k is p ope ly ci ed. 1 Ci a ion: Pola Resea ch 2014, 33, 19714, h p://dx.doi.o g/10.3402/pola . 33.19714 (page numbe no o ci a ion pu pose) In he Panbo eal Supe ealm, he e is no sec ion whe e sedimen a ion pa e ns and bios a ig aphy o he J/K bounda y in e al a e ep esen ed mo e comple ely han in a sec ion on he No d ik Peninsula (no h-eas e n Sibe ia; Fig. 1). On he No d ik Peninsula, he Lap e Sea cli exposes a con inuous succession o sil y muds ones spanning he Middle Ox o dian o Uppe Valanginian. The bo eal ammoni e and buchiid successions a e ecog- nized in g ea de ail om he Middle Ox o dian o he op o he sec ion (Zaha o e al. 1983; Zakha o & Rogo 2008; Rogo & Wie zbowski 2009). A belemni e bios a ig- aphy is well de eloped o he Middle Ox o dianLowe Ryazanian in e al (Dzu ˆba 2004; Dzyuba 2012). Fo amini- e s and ma ine palynomo phs om he Middle Volgian o he Lowe Valanginian ha e been s udied in de ail by Niki enko e al. (2008). He e we p esen esul s o an in eg a ed s udy ha includes an analysis o changes in assemblages (including ela i e abundance) o all majo ossil g oups and an analysis o sea-le el oscilla ions. An ea lie in es iga ion o s able iso ope da a de i ed om belemni e os a (Z ˇa ´k e al. 2011; Dzyuba e al. 2013) was complemen ed by analysing he d 13 C alues o bulk o ganic ma e . Majo aims o his in eg a ed s udy include he de e mi- na ion o biodi e si y changes ac oss he J/K bounda y and he iden i ica ion o possible ela ionships be ween di e en en i onmen al ac o s, including sea-le el os- cilla ions, palaeoclima e, oxygen le el in he sea wa e and biodi e si y changes. The unique o iginal high-la i ude palaeogeog aphic posi ion (see Hous ˇa e al. 2007) o he in es iga ed sec ion was qui e a om he coas line. The comple e sedimen- a y succession wi hou majo hia ii and he p ecise s a ig aphic amewo k make his p o ile mos sui able o a palaeocological and palaeoceanog aphic in es iga- ion o he A c ic Realm. Geological se ings and palaeogeog aphic aspec s The No d ik sec ion is one o he mos comp ehensi ely s udied Ju assic sec ions in he Russian A c ic. I has been s udied in de ail by sedimen ologis s, geochemis s, phy- sicis s and bios a ig aphe s (Zaha o & Judo nyj 1974). The Ox o dianRyazanian pa o his sec ion has been subdi ided bios a ig aphically by means o ammoni es (Zaha o e al. 1983; Zakha o & Rogo 2008; Rogo & Wie zbowski 2009), belemni es (Dzu ˆba 2004; Dzyuba 2012), buchiid bi al es (Zaha o e al. 1983), o amini- e s and dino lagella e cys s (Niki enko e al. 2008, 2011). A ema kable posi i e d 13 C excu sion (ca bona e in bel- emni e os a), which is conside ed as a use ul ma ke o he Panbo eal and Bo ealTe hyan co ela ion o J/K bounda y beds, has ecen ly been eco ded in he No d ik sec ion in he op pa o he Taimy ensis Zone (Dzyuba e al. 2013). The No d ik Peninsula is loca ed in he eas e n pa o he YeniseiKha anga basin (Fig. 2) which is a pa o he Mesozoic dep ession ex ending om he Yenisei Ri e mou h o he Lena Ri e mou h along he no he n ma gin o he Sibe ian pla o m (Saks e al. 1959). In he sou h, he YeniseiKha anga basin is bounded by he no he n edge o he pla o m which is co e ed by Palaeozoic and Lowe T iassic deposi s. In he wes , i is connec ed wi h he Us Yenisei basin. In he eas , he YeniseiKha anga basin eaches he LenaAnaba Fig. 1 Palaeogeog aphic and geog aphic posi ion o he No d ik sec ion. Palaeoen i onmen s and palaeoceanog aphy changes ac oss he J/K bounda y V.A. Zakha o e al. 2 (page numbe no o ci a ion pu pose) Ci a ion: Pola Resea ch 2014, 33, 19714, h p://dx.doi.o g/10.3402/pola . 33.19714 basin, being sepa a ed om i by a bu ied upli . The la e con inues as a p ojec ion o he Sibe ian pla - o m be ween he Popigai and Anaba i e s owa ds he Anaba Gul . The basemen o he YeniseiKha anga basin is com- posed o Palaeozoic and olcanogenic Lowe T iassic ocks. The basin is p ima ily illed by ma ine Ju assic o lowe mos Hau e i ian deposi s, and in he wes o e lain Fig. 2 Palaeogeog aphy and acial succession in he YeniseiKha anga S ai du ing he Ju assic/C e aceous ansi ional ime (modi ied a e Zaha o & S ˇu ygin 1983). (a) De ailed palaeogeog aphy o he YeniseiKha anga S ai du ing he Ryazanian. Land a ea is indica ed in beige. Bionomic ma ine zones a e ma ked by Roman nume als: I, deep-wa e ; II, mode a ely deep-wa e ; III, shallow wa e . (b) Facies p o ile h ough he YeniseiKha anga S ai and changes in bi al e and o amini e al assemblages. V.A. Zakha o e al. Palaeoen i onmen s and palaeoceanog aphy changes ac oss he J/K bounda y Ci a ion: Pola Resea ch 2014, 33, 19714, h p://dx.doi.o g/10.3402/pola . 33.19714 3 (page numbe no o ci a ion pu pose) Fig. 3 (Con inued) Palaeoen i onmen s and palaeoceanog aphy changes ac oss he J/K bounda y V.A. Zakha o e al. 4 (page numbe no o ci a ion pu pose) Ci a ion: Pola Resea ch 2014, 33, 19714, h p://dx.doi.o g/10.3402/pola . 33.19714 by con inen al Lowe C e aceous deposi s and ma ine Uppe C e aceous sedimen s. The T iassic deposi s occu in he eas e n pa o he basin. In Volgian and Ryazanian imes, he No d ik Peninsula was loca ed in he middle pa o he YeniseiKha anga Sea S ai (Fig. 2). Due o he g ea dis ance (abou 200 km) om he sou ce a eas in he no h and sou h, ine-g ained sedimen s such as al e na ing clays, bi umi- nous clays and clayey sil s accumula ed he e (Fig. 3). The o al hickness o he s udied in e al om he Middle Ox o dian o basal Ryazanian is sligh ly mo e han 50 m. As judged om ela i e hicknesses o he ammoni e zones, sedimen a ion a es g adually de- c eased du ing he Volgian o each a minimum a he J/K ansi ion and began o inc ease again in he ea lies C e aceous (Zakha o e al. 1993). Sedimen a ion a es o he No d ik sec ion we e ecen ly calcula ed by G abowski (2011) and B agin e al. (2013) on he basis o du a ions o magne och ons p esen ed by Ogg (2004). G abowski (2011) concluded ha qui e uni o m sedi- men a ion a es cha ac e ized ch ons M20n1n and M19 (ca. 1112 m/My), while sedimen a ion a es dec eased d as ically a he Taimy ensisChe ae ansi ion (M19n and M18 ) o 1.52.0 m/My. Addi ional s udies o he No d ik sec ion ha e shown ha sedimen a ion ends o dec ease in Ch on M18n (abou 1.7 m/My) and especially in M17 (0.5 m/My), emb acing he Volgian/ Ryazanian bounda y, whe eas M17n e eals a majo inc ease in he sedimen a ion a e up o 12 m/My (B agin e al. 2013). Sedimen a ion a es ecalcula ed on he basis o wo k by Ogg e al. (2012) a e simila o p e ious a es. The lowe pa o he Uppe Ju assic is ep esen ed by he uppe mos Middle Ox o dian, Uppe Ox o dian, Lowe and Uppe Kimme idgian. In con as o he same in e al loca ed 500 km o he wes , he No d ik sec ion con ains no ep esen a i es o Aulacos ephanidae, i.e., no index species o he Kimme idgian zones a he Boya ka Ri e (Khe a Ri e basin; Saks 1969). Only ca dioce a ids Amoeboce as s.l. and oppeliids Suboxydisci es ha e been ound in he s udied sec ion (Zaha o e al. 1983; Rogo & Wie zbowski 2009). The e o e, he Kimme idgian am- moni e zones o he Khe a Ri e basin could only be en a i ely co ela ed o hose o he No d ik Peninsula, bu he en i e Kimme idgian belemni e succession o he Boya ka Ri e sec ion is ecognized he e (Dzu ˆba 2004; Dzyuba e al. 2007). The Kimme idgian/Volgian bounda y in he No d ik sec ion is sa is ac o ily iden i ied by using belemni e da a. All Volgian ammoni e zones a e well co ela ed h oughou he A c ic and he lowe bo eal egions o Eu ope and Canada. The Ryazanian sequences a e also ep esen ed by A c ic ammoni e and buchiid zones (Zakha o & Rogo 2008; Rogo & Zakha o 2009). Comp ehensi e s udies o he Mesozoic deposi s o he Kha anga dep ession began in he 1930s. Oil explo a ion, including d illing and geophysical in es iga ions, was ca ied ou om 1933 o 1953 in he No d ik and some o he egions o he YeniseiKha anga basin. The da a on he Mesozoic deposi s we e summa ized by Saks e al. (1959). Saks (1958) compiled palaeogeog aphic and palaeo a- cies schemes o he YeniseiKha anga basin and he whole A c ic sepa a ely o he Ju assic and C e aceous. A signi ican con ibu ion o he palaeobiogeog aphic s udies o he bo eal clima ic bel was gi en by Saks e al. (1971), who es ablished palaeozoogeog aphic ealms and p o inces o he Ju assic and Neocomian o he A c ic. In he ea ly 1980s, he palaeogeog aphic and acies maps o he no he n USSR o each o 11 ages o he Ju assic Pe iod we e p epa ed (Bogolepo 1983). Acco ding o hese econs uc ions, a deep-wa e s ai connec ed o he No h Paci ic exis ed h oughou he Ju assic Pe iod in he eas e n sec o o he A c ic (Zakha o e al. 2002). In he La e Ju assic, he ma gin o he bay was nea he New Sibe ian (No osibi skie) Islands. In Volgian and Valanginian imes, deep-wa e oughs o he bay Fig. 3 Ammoni e, belemni e and buchiid zona ions in ela ion o li ho- and magne o-s a ig aphy and biodi e si y o majo axonomic g oups. Membe 1 (sec ion 33, beds 13) is Middle Ox o dian, wi h sil y clay o sil y muds one, da k-g ey o black, and an in e band o sandy-sil a he middle pa o he bed. Membe 2 (sec ion 33, beds 45) is Uppe Ox o dian, wi h sil y hin-bedded black clay. Membe 3 (sec ion 33, bed 6) is Uppe Ox o dianLowe Kimme idgian, wi h sil y da k-g ey o g eenish clay. The con en s o he sil ac ion dec eases upwa ds and clay became well-so ed. Membe 4 (sec ion 33, bed 7) is Lowe Kimme idgianMiddle Volgian, wi h sil y da k-g ey glauconi elep ochlo i e badly so ed clay and small addi ion o sandy g ains. Membe 5 (sec ion 33, beds 89; sec ion 32, beds 12) is Middle Volgian and belonging o he Epi i ga i es a iabilis Zone. The muds one-like ac u ed da k-g ey glauconi elep ochlo i e clay wi hin his membe has a bluish in . Membe 6 (sec ion 33, bed 1014; sec ion 32, bed 37) is Middle VolgianUppe Volgian, wi h muds one-like ac u ed clay consis ing o al e na ing laye s o da k-g ey, b own and bluish-g ey colou , and a glauconi e- ich band a he base. Membe 7 (sec ion 33, beds 15, 16; sec ion 32, beds 8, 9) is Uppe Volgian. The muds one-like clay is da k-g ey, some imes sil y, bedded, wi h occasional bands o bluish-g ey ac u ed clay and wi h high con en s o o ganic ma e and py i e. Membe 8 (sec ion 33, bed 17; sec ion 32, bed 10) is Uppe Volgian and belonging o he Che ai es che ae Zone. The hin-bedded da k-g ey b ownish clay has ossil plan emains and py i e globules. Membe 9 (sec ion 33, beds 1823; sec ion 32, beds 1116) is Lowe Ryazanian, and belonging o he Che ai es sibi icus Zone. The clay is some imes muds one-like and is da k-g ey, hin-bedded, wi h in e beds o ac u ed bluish-g ey clay. The lowe pa o membe 10 (sec ion 33, bed 24; sec ion 32, beds 1718) is Lowe Ryazanian and belonging o he basal bed o he Hec o oce as kochi Zone. The clay is muds one-like, pa ially sil y, ac u ed, bluish-g ey, wi h in e cala ions o hin-bedded da k-g ey clay. 1C2A, No o le els. V.A. Zakha o e al. Palaeoen i onmen s and palaeoceanog aphy changes ac oss he J/K bounda y Ci a ion: Pola Resea ch 2014, 33, 19714, h p://dx.doi.o g/10.3402/pola . 33.19714 5 (page numbe no o ci a ion pu pose) pene a ed in o he eas e n pa o he Lap e Sea. Flyschoid successions o e igenous ocks, such as sand- s ones, sil s ones and muds ones*o e 1200 m in o al hickness*we e o med in hese oughs (Kuz’miche e al. 2009). Appa en ly, he nea by wa e masses a ec ed he en i onmen s o no he n Eas Sibe ia by b inging abou a clima ic wa ming and consequen changes in bio a. The di e se La e Ju assic bio a o he No h Sibe ian seas we e o med unde he in luence o he no he n Eu opean seas, s a ing wi h he Cen al Russian Sea (Saks & Nalnjae a 1973; Bogolepo 1983; Dzu ˆba e al. 2006; Rogo 2012). Ma e ials and me hods Mac o ossils used o he p esen s udy we e collec ed no ing hei posi ion wi hin he sec ion o he nea es cen ime e (cephalopods) o we e collec ed sepa a ely om each membe (bi al es). Da a conce ning he ela- i e abundance o mac o ossils we e ob ained om ield obse a ions using a semi-quan i a i e app oach. Palynological samples we e collec ed e e y 0.21.0 m. A o al o 72 ock samples, eco e ed om da k clays ones, we e p ocessed o palynomo phs. A e washing and d y- ing, he s anda d p ocessing in ol ed chemical ea men o 10 g o he sample wi h HCl o emo e he calca eous ac ion and wi h HF o emo e silica es, sie ing h ough a 15-mm nylon mesh, and cen i uging o concen a e he esidues. Oxida ion was no used. Th ee mic oscope slides we e made om each sample o palyno acies analysis and dino lagella e cys analysis. Whole slides o esidues we e in es iga ed unde a binocula ansmi ed ligh mic o- scope o iden i y and coun he palynomo phs and o he o ganic ma e ials. A o al o 63 samples o o amini e al analysis we e collec ed wi hin he Kimme idgian*Lowe Ryazanian, wi h a sample esolu ion o 1.2 m in he lowe - mos pa o he sec ion and 1020 cm in he Uppe Volgian*Ryazanian. Fo he de e mina ion o he d 13 C alues o o ganic ca bon, sample aliquo s we e i s boiled wi h HCl o emo e any ca bona es. Samples we e hen epea edly washed by dis illed wa e , d ied and he d 13 C alues de e mined h ough gas ch oma og aphyiso ope- a io mass spec ome y (GCIRMS), using a 1108 Elemen al Analyze (Fisons, Ipswich, UK) wi h a ConFlo (The mo Finnigan, San Jose, CA, USA) in e ace and a MAT 251 mass spec ome e (The mo Finnigan) a he Labo a o ies o he Czech Geological Su ey in P ague. O e all, ana- ly ical unce ain y o he d 13 C alues o he o ganic ma e was 90.2. Palaeoecological changes: a ailable da a and en i onmen al in e p e a ion Changes in bi al e communi ies Analysis o ben hic communi ies can p o ide de ailed in o ma ion on he en i onmen and he cha ac e o sedimen a ion. The mos p omising app oach is a o- phic s uc u al s udy by analysing he le els o eeding (Tu pae a 1953). Ben hic molluscs can be di ided in o wo la ge g oups on he basis o he eeding me hod: deposi - eede s (de i ophagous) and suspension- eede s (ses onophagous), bo h subdi ided in wo subg oups: high o low eede s. The i s subg oup o deposi - eede bi al es eeds a he sedimen su ace. The second g oup gains i s ood below he sedimen su ace. Suspension- eede bi al es eed nea he sedimen su ace, o abo e i . The p esence o de i ophagous bi al es in he communi y always indica es quie condi ions wi h s able a es o sedimen a- ion. I species eeding a he su ace a e dominan in he communi y, his shows ha he e is poo oxygena ion a he bo om. The well- ecognized ‘‘ axonomic s ai s’’ a e a good indica o o he s abili y o all en i onmen al ac o s, showing he p esence o ew dominan , cha ac e is ic species and nume ous a e species in he communi y (Odum 1971). A ‘‘ma u e communi y’’ is cha ac e ized by he p esence o all g oups in he ‘‘ axonomic s ai s’’; such biocoenoses a e o med in s able en i onmen s. The absence o one o mo e ‘‘s ai s’’ indica es an uns able en i onmen and he in luence o some kind o abno m- ali y. On his basis, we conclude ha ma u e communi ies a e absen (Fig. 3). Only he i s (Middle Ox o dian) and en h (Kochi Ch on) communi ies a e cha ac e ized by he absence o s ongly domina ed axa and can be con- side ed o be a he uni o m. The mos no iceable cha - ac e is ic in all cases is he absence o an ob ious dominan species. The p edominance o Aequipec en in aphococenoses is no signi ican since his genus wi h hin-walled equila - e al and equi al ed shells seems o be pseudoplank onic. The e o e, we can easonably conclude ha he in luence o one o mo e en i onmen al ac o s was anomalous du ing he La e Ju assic and in he e y beginning o he Ea ly C e aceous. Among he mos p obable ac o s is an oxygen de ici o nea -bo om wa e , wi h possible pe i- odic anoxic e en s a he sedimen su ace. The ophic s uc u e o he communi ies con i ms his idea because epi aunal deposi - eede s ep esen a conside able ac- ion in mos communi ies, some imes accoun ing o mo e han hal o all he bi al es (Fig. 3). Palaeoen i onmen s and palaeoceanog aphy changes ac oss he J/K bounda y V.A. Zakha o e al. 6 (page numbe no o ci a ion pu pose) Ci a ion: Pola Resea ch 2014, 33, 19714, h p://dx.doi.o g/10.3402/pola . 33.19714 Ano he possible ac o dis u bing he de elopmen o ben hic communi ies is a low empe a u e o nea -bo om wa e s. Small size, hin, some imes anspa en shells in Kimme idgian and Volgian bi al es can be a s ong a gumen o his sugges ion. Nea -bo om wa e s could cool by he in low o cold wa e s om he eas whe e deep o edeeps we e loca ed. The p oximi y o an open ocean o he eas is indica ed by he p esence o phyllo- ce a ids in he No d ik sec ion in Volgian and Ryazanian s a a. These ammoni es a e absen in he Ox o dian and Kimme idgian o his a ea. Howe e , as inhabi an s o open seas, phylloce a ids a e qui e common in he Uppe Ju assic o no h-eas Russia (Pa akeco & Pa akeco a 1989). Possible pa hs o a phylloce a id in asion in o he YeniseiKha anga Sea a e deep-wa e oughs: lysh sedi- men s, usually illing such oughs, we e ecen ly ound a he New Sibe ian Islands (Kuz’miche e al. 2009). Changes in ammoni e di e si y As a o he high-la i ude si es (Rogo 2012), he ammo- ni e assemblages o he No d ik sec ion a e mainly cha ac e ized by e y low axonomic di e si y, usually anging be ween one and h ee a bo h genus and species le el. All le els ha a e cha ac e ized by inc eased di e si y coincide wi h immig a ion e en s o ocean- associa ed ammoni es, wi h he excep ion o he Kimme - idgian, when he high di e si y in e al coincides wi h he appea ance o Suboxydisci es (Fig. 4d), and he las ca di- oce a id di e si ica ion. One such high di e si y le el can be obse ed a he MiddleLa e Volgian ansi ion when, in addi ion o bo eal ammoni es, Euphylloce as occu s (Fig. 4e). The appea ance o hese phylloce a ids in he s udied a ea and in S alba d (Rogo 2010) indica es he pene a ion o ypical open sea ammoni es om he no he n Paci ic. In con as o he Middle Volgian he beginning o he La e Volgian is also cha ac e ized by he dec ease o p o inci- alism o bo eal ammoni es. Only he wes e n pa o he Panbo eal Supe ealm, wes wa d o he G eenland No wegian Seaway, was inhabi ed by an endemic c aspe- di id lineage (Swinne oniaSubc aspedi esVolgidiscus). Bo h a educ ion in ammoni e endemism and he occu ence o phylloce a ids could be explained by a sho - e m sea-le el ise in he A c ic. Such a sea-le el ise is also e lec ed in he Russian Pla o m succession (Sahagian e al. 1996). A e wa ds, he ammoni e di e si y quickly dec eased, albei ammoni e aunas o he di e en pa s o he A c ic became e y simila in his ime. Nea o he VolgianRyazanian bounda y, he ammoni e di e si y inc eased again. The Che ae Ch on is cha ac e ized by he co-occu ence o bo eal Che ai es and P aeche ai es as well as oceanic phylloce a ids (Zaha o e al. 1983). An inc ease in ammoni e di e si y a he beginning o he Ryazanian is connec ed wi h a high di e si y o bo eal axa belonging o h ee lineages: las C aspedi es (Taimy oce as); i s P ae ollia de i ed om Volgidiscus; and he las Do soplani id ammoni es, he Che ai es. The mos ema kable di e si y inc ease a he beginning o he Kochi Ch on again ela es o he in asion o oceanic ammoni es. These ammoni es include a leas one en- demic A c ic axon Bo eophylloce as, while he ypical oceanic Biasaloce as (Fig. 4b) and Bochiani es c . glennensis (Fig. 4c) esemble Cali o nian occu ences which immi- g a ed om he Paci ic (Rogo & Igol’niko 2009). Mo e- o e , such open sea ammoni es e en pene a ed he much mo e shallow seas o he Kha anga dep ession, and hei eco ds a e also known om he basal Kochi Zone o he Boja ka sec ion (Fig. 4a). This e en also closely co esponds wi h a sugges ed sea-le el ise leading o apid sp eading o he i s Hec o oce as h oughou he A c ic. Changes in belemni e di e si y Belemni es (Cylind o eu hididae) in he No d ik sec ion as a whole a e mo e abundan and highe in axonomic di e si y han ammoni es. As has been obse ed (e.g., Zakha o e al. 2005; Rogo e al. 2006), he e is an in e se ela ionship in di e si y o hese wo cephalopod g oups (see Fig. 3). In pa , his is caused by compe i- ion be ween ammoni es and belemni es and hei di - e en p e e ences conce ning en i onmen al condi ions. Consequen ly, he highes le els o ammoni e di e si y a e eached when he deepening o he No h Sibe ian basin made he in asion o open sea ammoni es possible. This is obse ed a he MiddleLa e Volgian ansi ion and a he beginning o he Ryazanian Kochi Ch on. Belemni e di e si y is no ably educed a hese imes, sugges ing ha cylind o eu hids we e poo ly adap ed o deep-wa e en i onmen s. Low di e si y belemni e assemblages a e eco ded in he wide in e al o he Uppe Volgian and lowes Ryazanian beds. Only a e ep esen a i es o he gene a A c o eu his,Cylind o eu his and Lagonibelus wi h elonga e os a a e dis ibu ed he e ( om one o i e species). Taxa wi h sho e obus os a (Pachy eu hidinae and Simo- belinae) a e absen ; his indica es a deepening and b oad- ening o he basin, especially du ing he Okensis Ch on and mos o he Taimy ensis Ch on, which is con i med by mic opalaeon ological and palynological e idence (Niki enko e al. 2008). A sligh di e si y inc ease can be ecognized a he op o he Taimy ensis Zone nea V.A. Zakha o e al. Palaeoen i onmen s and palaeoceanog aphy changes ac oss he J/K bounda y Ci a ion: Pola Resea ch 2014, 33, 19714, h p://dx.doi.o g/10.3402/pola . 33.19714 7 (page numbe no o ci a ion pu pose) he J/K bounda y (as ecen ly con i med by magne o- s a ig aphy; Hous ˇa e al. 2007), and jus abo e he ‘‘I idium bed’’ a he base o Membe 9. A he same le els, he majo i y o he belemni es common o he A c ic and No he n P o o-Paci ic (Cali o nia) emains s eady: Cylind o eu his knox illensis,C. c . new illensis,A c o eu his ehamaensis and A. po ec i o mis. In deposi s om he Middle Ox o dian o he Kimme idgian, among he bel- emni es many cosmopoli an axa we e obse ed, bu all a e common axa o A c ic and subbo eal Eu opean seas: Cylind o eu his puzosiana,C.obeliscoides,Pachy eu his excen alis,P.pande iana,Bo eio eu his absolu a,Lagonibelus ingens,L.kos omensis, Simobelus b e iaxis. The in e al be ween Kimme idgian and La e Volgian ime was cha - ac e ized by an inc ease in belemni e endemism culmi- na ing in he Middle Volgian Va iabilis Ch on (up o 70% A c ic endemic belemni e assemblages). The Kimme idgian and Middle Volgian Va iabilis ch ons show bo h he highes axonomic di e si y (916 species om six o se en gene a) and an abundance o belem- ni es. Bo h episodes o inc easing belemni e di e si y a e ypical ea u es o he No h Sibe ian seas and a ibu- ed o oceanog aphic (sea-le el changes, expansion o sea connec ions) and p obably clima ic (wa ming) e en s (Saks & Nalnjae a 1979; Dzu ˆba e al. 2006; Dzyuba 2013). Six majo belemni e e en s, cha ac e ized by mass occu ences o belemni e os a, ha e been ecognized in he Uppe Kimme idgian h ough he lowes Ryazanian in e al. The ea lies one has been loca ed a he op o he Sep en ionalis belemni e Subzone (s a ig aphic Fig. 4 Selec ed Uppe Ju assicLowe C e aceous ammoni es om he No d ik and Boja ka sec ions. (a) Biasaloce as sp., no numbe , Boja ka i e , Ryazanian, Kochi Zone; (b) Biasaloce as sp., MK1056, No d ik, Ryazanian, Kochi Zone; 0.05 m abo e 1C; (c) Bochiani es c . glennensis And., MK 1094, No d ik, Ryazanian, Kochi Zone; 0.05 m abo e 1C; (d) Suboxydisci es sp. [m]; MVI767; 0.3 m abo e 2G, Lowe Kimme idgian, Ki chini Zone, subki chini ho izon; (e) Euphylloce as c . knox illense (S an on); MK1082, 9.3 m below he base o he Ryazanian, Uppe Volgian, Okensis Zone. Specimens we e collec ed by M. Rogo du ing ieldwo k in 2003 excep (a), which was collec ed by M. Rogo in 2008. Specimens (a), (b), (c) and (e) a e s o ed in he T o imuk Ins i u e o Pe oleum Geology and Geophysics o he Sibe ian B anch o Russian Academy o Sciences, No osibi sk, and specimen (d) is a he Geological Ins i u e o Russian Academy o Sciences, Moscow. Palaeoen i onmen s and palaeoceanog aphy changes ac oss he J/K bounda y V.A. Zakha o e al. 8 (page numbe no o ci a ion pu pose) Ci a ion: Pola Resea ch 2014, 33, 19714, h p://dx.doi.o g/10.3402/pola . 33.19714 equi alen o he uppe mos Kimme idgian Suboxydis- ci es aimy ensis ammoni e Zone in Sibe ia). While he high sea le el e lec ed in he middle pa o he Explana a belemni e Zone is no connec ed wi h a mass occu ence o belemni es, he sea-le el a ia ions be ween he uppe pa o he Explana a and lowe pa o he Russiensis belemni e zones a e cha ac e ized by ou , clea ly dis inguished belemni e e en s. Th ee o hese a e loca ed in he Middle Volgian Va iabilis ammoni e Zone. The de elopmen o a ou able condi- ions o a belemni e expansion (see abo e) and mass occu ences a e p edominan ly associa ed wi h a shal- lowing. The belemni e dependence on changes in sea le el and nu ien deli e y a e well eco ded in he Uppe C e aceous axa (Mi chell 2005; Wiese e al. 2009). Howe e , he C e aceous axa a e ypical o shallow- wa e pla o m seas. The same mode o belemni e be- ha iou is epo ed he e also om he Bo ealA c ic Basin. The dep h o he sea bo om may ep esen an ecological limi o a mass expansion. A six h belemni e e en is eco ded a he base o he Sibi icus ammoni e Zone. I s posi ion shows a shallowing end wi h an inc easing inpu o e igenous ma e ial. E en ually, he belemni e di e si y shows a pa ial dec ease in he J/K bounda y in e al. This co esponds o a ime o sea-le el ise and basin deepening. Changes in palynological spec a Ou palyno acies analysis (coun ing up o 400 pa icles in each sample) was based on phy oclas s dis inguished by Ba en (1996): s uc u al o ganic ma e ep esen ed by black pa icles, b own woody pa icles and yellow ma e ial (cu icles); palynomo phs ep esen ed by bisac- ca e pollen, non-bisacca e pollen, spo es, g een algae (p asinophy es), dino lagella e cys s and o amini e al es linings. The quan i a i e s udy o he dino lagella es and he analysis o hei dis ibu ion has been published (Niki enko e al. 2008). Th ee da a se s we e used o in e p e palynological assemblage luc ua ions in e ms o en i onmen al changes: (1) he a io o e es ial (pollen and spo es) o ma ine palynomo phs (ac i a chs, dino lagella e cys s, o amini e al linings, algae), known as he e es ial/ma ine index (T/M index); (2) he changes in ela i e abundance o selec ed dinocys gene a; and (3) he di e si y exp essed as he numbe o gene a o species ound in each sample. All samples con ain ich palynological ma e ial. O e all, he s udied samples display palyno acies a ia ions h ough he sec ion. Mos esidues a e cha ac e ized by abundan s uc u al o ganic ma e , domina ed by phy oclas s (b own woody pa icles and yellow pa icles). This ma e ial domina es he sec ion a he in e als o 3439 and 45 49.5 m. Poo ly p ese ed dino lagella e cys s can be ound in he same in e als (3638.5 and 4647.4 m). These a e usually di icul o iden i y and ca y ma ks o decay caused by c ys alliza ion o py i e. This palyno acies is ypical o anoxic condi ions whe e deposi ion is slow. The s udied pa s o he sec ion a e also cha ac e ized by a low pe cen age o dino lagella e cys s (B10%) and a e y low pe cen age o spo es and pollen g ains (B15%). Ce ain palynomo ph assemblages con ain a highe p opo ion o dino lagella e cys s han ound in he es o he s udied sec ion. The inc ease in ela i e abundance o dino lagella e cys s (Middle Volgian, Okensis ammoni e zone o he Uppe Volgian, Sibi icus and Kochi ammoni e zones o he Ryazanian) is coinciding wi h an inc eased abundance o o amini e al linings ( ypical o ma ine coas al and ne i ic en i onmen s) and g een algae (Fig. 5). Fo amini e al es linings occu a 30 and 44 m. In he in e als om 42 o 45 m (Uppe Volgian) and om 50 o 54 m (Lowe Ryazanian), a dis inc bloom o g een algae (p asinophy es) is ound (Fig. 5). He e dino lagella e cys s comp ise a ound 2040% o he assemblage. P asinophy- es a e known o appea in a wide ange o en i onmen s, om ma ine o b ackish, being mos equen in nea - sho e ma ine and anae obic en i onmen s (e.g., Tappan 1980; Ba en 1996). Acco ding o P auss & Riegel (1989), p asinophycean algae migh ha e a ou ed cold-wa e en i onmen s. Smel o e al. (2001) sugges ed ha he algal bloom was possibly induced by he la ge amoun s o nu ien s eleased in o he wa e column. High p oduc- i i y and limi ed wa e ci cula ion a he bo om wi h a ma ked eu ophica ion cause anoxic condi ions o sedi- men a ion (Below & Ki sch 1997). P asinophy es a e abundan in his ype o acies (Skupien & Vas ˇı ´c ˇek 2002; Niki enko e al. 2008). The changes in T/M index and he composi ion o palyno acies may aid in he in e p e a ion o he deposi- ional en i onmen s in e ms o wa e dep h, sea-le el oscilla ions and e igenous in lux (Lis e & Ba en 1988; Smel o & Lee e eld 1989; Ba en 1996). A high ac ion o e es ial o ganic ma e poin s o an in ense supply o land-de i ed o ganic pa icles and/o shallowing ma ine condi ions. An inc eased supply o con inen al ma e ial p obably e lec s highe p ecipi a ion on land, esul ing in an enhanced nu ien supply o coas al wa e s and enhanced o ganic pa icle anspo o he shel ma gin. The T/M a io dec eases basin-wa d and inc eases in a shallowing succession. The low amoun s o amo phous o ganic ma e in all samples may esul om a high accumula ion a e. The di e si y o dino lagella es a ies om eigh o 25 gene a and eigh o 31 species in each sample. V.A. Zakha o e al. Palaeoen i onmen s and palaeoceanog aphy changes ac oss he J/K bounda y Ci a ion: Pola Resea ch 2014, 33, 19714, h p://dx.doi.o g/10.3402/pola . 33.19714 9 (page numbe no o ci a ion pu pose) hank Dean Lomax o Manches e Uni e si y o p oo - eading his manusc ip . Re e ences Abbink O., Ta ga ona J., B inkhuis H. & Vissche H. 2001. 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