New strati graphic and iso tope data on the Kimmeridgian–Volgian bound ary beds of the Sub po lar Urals, West ern Si be ria
Mikhail A. ROGOV and Greg ory D. PRICE
Rogov M. A. and Price G. G. (2010) – New strati graphic and iso tope data on the Kimmeridgian–Volgian bound ary beds of the Sub po lar Urals, West ern Si be ria. Geol. Quart., 54 (1): 33–40. Warszawa.
A re view and new data re gard ing the ammonite fau nas and biostratigraphy of the up per most Kimmeridgian–low er most Volgian of the Sub po lar Urals are pre sented. The com bined ranges of al most all ammonite gen era in the low er most Volgian of the Sub po lar Urals sup - ports the tra di tional point of view re gard ing the equiv a lence of the Mag num Zone with the two low er most Bolonian Zones and with the bulk of the Klimovi Zone. This strati graphic in ter pre ta tion per mits the sta ble iso tope data (car bon and ox y gen) de rived from as so ci ated bel em nites to be pre sented in con text. The iso to pic re cords from the bel em nites sug gest that the lower Volgian sed i ments of the Yatria River, Sub po lar Ural Moun tains were de pos ited in a warm ma rine en vi ron ment con sis tent with a warm high lat i tude sce nario. If re duced salinites are in voked ap par ent tem per a tures are low ered by ~5°C, but still re main well above freez ing and are rel a tively warm com pared to some re cent mod elled es ti mates of Late Ju ras sic ocean tem per a tures.
Mikhail A. Rogov, Geo log i cal In sti tute of Rus sian Acad emy of Sci ences, 7 Pyzhevskii Lane, 119017 Mos cow, Rus sia; Greg ory D. Price, School of Ge og ra phy, Earth and En vi ron men tal Sci ences, Uni ver sity of Plym outh, Drake Cir cus, Plym outh, PL4 8AA, UK (re ceived: Au - gust 19, 2008; ac cepted: De cem ber 29, 2009).
Key words: Kimmeridgian, Volgian, ammonite biostratigraphy, bel em nites, ox y gen iso topes.
INTRODUCTION
Sec tions of the Kimmeridgian–Volgian bound ary beds sit - u ated on the east ern slope of the Sub po lar Ural Moun tains have been known for more than 100 years. Over the last few de cades these de pos its have been in ten sively stud ied at lo cal i ties along the banks of the rivers Yatria, Lopsia and Tolja for the pur pose of biostratigraphy (Mesezhnikov, 1963, 1977, 1984;
Mikhailov, 1964; Zakharov and Mesezhnikov, 1974; Golbert et al., 1975; Lebedeva and Nikitenko, 1999) as well as palaeoclimate stud ies (e.g., Sahagian et al., 1996; Price and Mutterlose, 2004; Zakharov et al., 2005). Fol low ing Mesezhnikov’s in ves ti ga tions, the Kimmeridgian–Volgian bound ary was traced to the base of Mag num Zone (Zakharov and Mesezhnikov, 1974; Mesezhnikov, 1977, 1984; Zakharov et al., 1997, 2005). Its ear li est Volgian age has been dem on - strated by the oc cur rence of Gravesia (and also an ab sence of the Kimmeridgian Aulacostephanus fauna). The Mag num Zone has typ i cally been con sid ered by Rus sian au thors as the equiv a lent of the two basal zones seen in north-west Eu rope (Elegans and Scitulus), the Klimovi Zone of the Volgian Stage
of the Rus sian Plat form as well as the Hybonotum Zone of Tithonian Stage. Gravesia, fig ured by Zakharov and Mesezhnikov (1974), was re cently re vised by Hantzpergue (1989) and Schweigert (1993) and par tially as cribed to the Kimmeridgian spe cies G. lafaugiana Hantzpergue. This data, in con junc tion with re cords of ammonoids that re sem ble Eosphinctoceras mag num in the up per Kimmeridgian of Ger - many, led Schweigert (1993) to sug gest a Kimmeridgian age of the Mag num Zone. On the other hand, it has been sug gested that the Mag num Zone cor re sponds at least in part to the Wheatleyensis Zone (Callomon and Birkelund, 1982;
Callomon, 1994). Hence cor re la tive charts based upon Eosphinctoceras alone lead to some con tra dic tions. This study pres ents new fau nal de scrip tions from the clas sic sec tion of the Kimmeridgian–Volgian bound ary beds sit u ated on the east ern slope of Sub po lar Ural Moun tains. Sta ble car bon and ox y gen iso tope data de rived from as so ci ated bel em nites are also pre - sented. A num ber of stud ies (e.g., Ditchfield, 1997; Price and Mutterlose, 2004; Wierzbowski, 2004; Price et al., 2009) have dem on strated that, with ap pro pri ate con straints placed upon the iso to pic com po si tion of sea wa ter, cred i ble palaeotemperature trends can be de rived from the iso to pic anal y sis of bel em nites.
As the Late Ju ras sic and in par tic u lar the Kimmeridgian has
been iden ti fied as a pe riod of time when tem per a tures reached a max i mum (e.g., Frakes, 1979) it is im por tant that iso tope data and con se quent in ter pre ta tions are placed in con text. In ad di - tion to an as sess ment of the ammonite oc cur rence at the Kimmeridgian–Volgian bound ary, it is the pur pose of this con - tri bu tion to pro vide ro bust, biostratigraphically con strained, ox y gen and car bon iso to pic data from high lat i tudes. These data will con trib ute to the de bate on whether the Late Ju ras sic was at times char ac ter ized by sub-freez ing or warm po lar tem - per a tures (e.g., Frakes, 1979; Ditchfield, 1997; Price, 1999;
Dromart et al., 2003).
MATERIAL AND METHODS
The Yatria River near to the Nia-Yu River mouth was ex - am ined in this study (Fig. 1). This sec tion has been pre vi ously de scribed by Mikhailov (1964), Zakharov and Mesezhnikov (1974) and Mesezhnikov (1984). From this sec tion a num ber of key ammonite spe cies have been iden ti fied (Fig. 2). Ad di tion - ally a num ber of bel em nite ros tra (Cylindroteuthis sp.) were
also col lected. Ox y gen and car bon iso to pic com po si tions have been de ter mined on these bel em nite spec i mens. Car bon ate stain ing (Dick son, 1966) re vealed par tial re place ment by ferroan cal cite pref er en tially along the out er most con cen tric growth bands and as so ci ated with the api cal line. Ar eas such as these were ei ther re moved prior to or avoided dur ing sub-sam - pling. The re mains were frag mented (sub-mm), washed in ultrapure wa ter, and dried in a clean en vi ron ment for sub se - quent iso to pic anal y sis. Car bon and ox y gen iso topes have been de ter mined on a VG In stru ments Op tima Iso tope Ra tio Mass Spec trom e ter with a Multiprep Au to mated Car bon ate Sys tem (at the Uni ver sity of Plym outh) us ing 200 to 300 mi cro grams of car bon ate. The d13C and d18O val ues ob tained were cal i brated against NBS-19. An a lyt i cal reproducibility of the mea sure - ments is ±0.2 permil based upon rep li cate anal y ses. The d18O and d13C data are re ported in the con ven tional delta no ta tion with re spect to V-PDB. Diagenetic al ter ation of each of the bel - em nite sam ples was char ac ter ized by a com bi na tion of petrographic in ves ti ga tions and trace el e ment geo chem is try.
All sam ples were ana lysed for trace el e ment con tents (Fe, Mn) on 50–100 mg subsamples us ing a Perkin Elmer 3100 Atomic Ab sorp tion Spec trom e ter. Based upon anal y sis of du pli cate sam ples reproducibility was better than ±3% of the mea sured con cen tra tion of each el e ment.
34 Mikhail A. Rogov and Gregory D. Price
Fig. 1. Lo ca tion of the stud ied sec tion within the Sub po lar Ural Moun tains
Fig. 2. Strati graphic log with new ammonite re cords show ing ho ri zon from which bel em nites were sam pled
AMMONITE OCCURRENCE
Based upon the pres ence of Eosphinctoceras cf. mag num (Fig. 3A, B) re corded from near to the top of the sec tion, it is pos si ble to con sider the Mag num Zone as the age of the up per part of sec tion. Our de scribed sec tion would there fore in clude beds 1–2 of the lo cal ity no. 12 de scribed by Mesezhnikov (1984). Both in situ ammonoids and ammonoids from fallen blocks re sem bling the Eng lish spe cies Pectinatites (Arkellites) and P. (Virgatosphinctoides) were gath ered. P.
(A.) cf. damoni [m, M] (Fig. 4A1, 2 and B1, 2), were re corded in situ. These ammonoids are vir tu ally the same as the typ i cal Eng lish ex am ples from the Scitulus Zone – which co-oc cur with the guide fos sil Gravesia gravesiana in both Eng land Cope (1967) and the Urals Zakharov and Mesezhnikov (1974), in clud ing a very char ac ter is tic fea ture, the horn in the ap er ture of microconchs. From the fallen blocks a sus pected macroconch of the other typ i cal Bolonian spe cies (Elegans Zone), P. (V.) elegans (Fig. 3E), was also col lected. All ammonites are housed in the geo log i cal In sti tute of RAS ex - cept Fig ure 3D (Oil In sti tute, St Pe ters burg) and Fig ure 4C.
Of note is that the Tithonian is now in ter na tion ally ac cepted as the pri mary stan dard ter mi nal Ju ras sic Stage and thus the un der ly ing Kimmeridgian Stage can only be used in its sensu gallico in ter pre ta tion (Cope, 2006, but see also Zakharov and Rogov, 2008). Cope (1993, 1995) sug gested be cause of the dif fi cul ties of cor re lat ing Tithonian ammonite fau nas with those to the north of Tethys, a se ries of Sec ond ary Stan dard Stages were re quired. Blakes’s (1881) pro posal for a Bolonian Stage, equiv a lent to the up per Kimmeridgian sensu anglico (Cope, 2006) and is used in this pa per.
STABLE ISOTOPE AND GEOCHEMICAL DATA
The iso tope data for the bel em nites are pre sented in Ta ble 1 and Fig ure 5. The bel em nites sam pled in this study were com - posed trans lu cent cal cite. Mn and Fe con cen tra tions of the bel - em nites pro vide a means to fur ther ver ify their state of pres er - va tion. The low Mn (<10 ppm) and Fe (<40 ppm) val ues, re - corded, in con junc tion with the petrographic ev i dence, are con - sis tent min i mal diagenetic al ter ation (see Mar shall, 1992; Price and Page, 2008; Price and Rogov, 2009). The con cen tra tions of Sr and Mg are also typ i cal of well-pre served bel em nites (e.g., Ditchfield, 1997; Price and Mutterlose, 2004). The ox y gen and car bon iso tope val ues of bel em nites from range from –1.86 to –0.11‰ and 0.32 to 1.69‰ re spec tively. Cal cite palaeo - temperatures were cal cu lated us ing the equa tion of Ep stein et al. (1953) mod i fied by An der son and Ar thur (1983):
T[°C] = 16.0–4.14 (dc–dw) + 0.13 (dc–dw)2
where: dc equals the ox y gen iso to pic com po si tion of the cal cite with re - spect to the PDB in ter na tional stan dard and dw equates to the ox y gen iso - to pic com po si tion of the wa ter from which the cal cite was pre cip i tated with re spect to the SMOW stan dard. In or der to cal cu late palaeotemperatures, an as sump tion re gard ing the dw of the Ju ras sic
ocean which is in part in flu enced by the pres ence or ab sence of po lar ice dur ing the Ju ras sic, must be made. Sea wa ter on Earth dur ing pe ri ods that were free from ma jor ice caps would have been iso to pi cally lighter than at pres ent and a dsea wa ter of –1.0‰ (SMOW), has been sug gested as ap pro pri - ate (Shackle ton and Ken nett, 1975).
DISCUSSION
Our in ter pre ta tions of the bel em nite iso tope data (see be - low) rely upon a sound biostratigraphic frame work. The cor re - la tion charts of the Mag num Zone with Eu ro pean suc ces sions, how ever, dif fer from each other (cf. Mesezhnikov, 1984;
Schweigert, 1993; Callomon, 1994). Ev i dence for these cor re - la tions is based upon only lim ited amount of data which are some times con tra dic tory. Each of these schemes are ex am ined step by step (Fig. 6).
Mesezhnikov (1974, 1984) pro posed the ear li est cor re la - tion chart. Fol low ing Mesezhnikov (1984), the base of the Mag num Zone was con sid ered to cor re spond to the base of the Volgian Stage of the cen tral ar eas of Rus sian Plat form, and the Mag num Zone equiv a lent of the Klimovi Zone and Bolonian Scitulus and Elegans zones. These cor re la tions are dem on - strated us ing the fol low ing rea son ing:
1. Typ i cal Kimmeridgian Aulacostephanus are en tirely ab - sent within the Mag num Zone and are un com mon in the un der - ly ing Dividuum Subzone of the Autissiodorensis Zone;
2. Gravesia en coun tered in the Mag num Zone are char ac - ter is tic of the low er most Tithonian, low er most Bolonian and of the Klimovi Zone;
3. Ilowaiskya aff. sokolovi re sem bling the in dex spe cies of the Sokolovi Zone of the Volgian Stage, oc curs in the Subcrassum Zone.
A more re cent re vi sion of the oc cur rence of Gravesia from the Sub po lar Urals has been made by Hantzpergue (1989) and Schweigert (1993). One of the fig ured spec i mens, gath ered from the iso lated ex ca va tion near to Tolja vil lage (Zakharov and Mesezhnikov, 1974, pl. 1, fig. 1), was as signed to the Kimmeridgian spe cies Gravesia lafauriana (Hantzpergue, 1989; Schweigert, 1993), and an other spec i men, from the lower part of Mag num Zone of the lo cal ity no. 28, Tolja River (Zakharov and Mesezhnikov, 1974, pl. 1, fig. 2), was re-de ter - mined as G. gravesiana (Hantzpergue, 1989). Im por tantly, those Gravesia as signed to the up per Kimmeridgian were col - lected from a small ex ca va tion, from where other beds (and even other ammonites) are un known. Gravesia from the Mag - num Zone, fig ured by Mesezhnikov (1984, pl. XIV, fig. 2) per - haps be long to the Tithonian/Bolonian spe cies Tolvericeras (Pseudogravesia) hahni (Schweigert pers. comm., Jan u ary 2003). Ammonites from the Subcrassum Zone (all fig ured spec i mens were col lected from the low er most bed of the zone in lo cal ity no. 12, Yatria River), con sid ered by Mesezhnikov (1984) as Ilowaiskya (Zakharov and Mesezhnikov, 1974, p. 86, pl. 4, figs. 2, 3, text-fig. 28), re sem ble microconchate Pectinatites (see Fig. 3D). Ammonites, fig ured as plate 4, fig. 3 by Zakharov and Mesezhnikov (1974), re sem ble Pectinatites (Arkellites) hudlestoni (Cope, 1967, pl. 2, fig. 3; pl. 5, fig. 2;
Wignall, 1993, fig. 7f), whereas the spec i men of fig ure 2 (of the same plate), is closer to P. (Virgatosphinctoides) cf. donovani
(Cope, 1967; Wignall, 1993, fig. 8a). Both spe cies are char ac - ter is tic of the Hudlestoni Zone.
Cor re la tion of the Mag num Zone with the Wheatleyensis Zone (Callomon and Birkelund, 1982, fauna 25) is based on the re cords of Eosphinctoceras cf./aff. mag num Mesezhn. within the sup posed Wheatleyensis Zone, as de ter mined by Cope (1967, p. 73). Eng lish rep re sen ta tives of Sphinctoceras and Eosphinctoceras were de scribed by Neaverson (1925) from the
same zone, but his Wheatleyensis Zone cor re sponds to Scitulus Zone and basal beds of Wheatleyensis Zone sensu Cope (1967).
In the Sub po lar Urals the last Eosphinctoceras (in clud ing those with a high rib co ef fi cient) oc cur in the Subcrassum Zone (Fig. 6), ac com pa nied by some char ac ter is tic Pectinatites spe - cies. Ad di tional (men tioned as Subdichotomoceras in Zakharov et al., 2005), ten ta tively as cribed to P. cf.
smedmorensis and P. groenlandicus (Fig. 4C) were de ter mined
36 Mikhail A. Rogov and Gregory D. Price
Fig. 3. Lower Volgian ammonoids from the Nia-Yu sec tion
All ammonites are coated with am mo nium chlo ride; A, B – Eosphinctoceras mag num Mesezhn.; A – GIN MK1835, pre - sum able microconch, not in situ, Mag num Zone; B – GIN MK653, 330 cm above the base of the sec tion, Mag num Zone;
C – Pectinatites (Arkellites) aff. arkelli Cope, GIN MK620; microconch with horn pre served, not in situ, ?Mag num Zone; D – Pectinatites sp., VNIGRI 1640a-634, sam ple 131, lo cal ity 12, col lected by Mesezhnikov (1963), Subcrassum Zone; E – Pectinatites (Virgatosphinctoides) cf. elegans Cope, GIN MK619, not in situ; scale bar = 1 cm
from the Subcrassum Zone of the Lopsia River. This al lows us to sup pose, that the Mag num Zone should be cor re lated with the low er most Bolonian, whereas Wheatleyensis Zone re cords of Eosphinctoceras and Subdichotomoceras cor re spond to the same from the Subcrassum Zone. This point of view was also sup ported by re cently col lected early Pectinatites jointly with Eosphinctoceras mag num.
A com par i son of the zonal suc ces sions of the Sub po lar Urals and the Mid dle Volga area was un der taken by Zakharov and Mesezhnikov (1974) who noted Gravesia within both the Mag -
num and Klimovi zones and the dis ap pear ance of Aulacostephanids in the un der ly ing strata (an isochronous event, traced through out in the Panboreal Super realm, from Eng land to Si be ria). Un for tu nately, Gravesia from the Mid dle Volga area have never been fig ured or de scribed, and our care ful search did not pro vide any Gravesia. It is prob a ble, that these ammonites could be mis iden ti fied and con fused with coarsely ribbed Eosphinctoceras and Subdichotomoceras, known from the Klimovi Zone (Rogov, 2005). Some coarse-ribbed Eosphinctoceras have been con fused with Gravesia by one of us
Fig. 4. Lower Volgian ammonites from Sub po lar Ural
All ammonites are coated with am mo nium chlo ride; A1, A2 – Pectinatites (Arkellites) damoni Cope, MK 721, sup - posed macroconch, 200 cm above the base of the sec tion Nia-Yu; B1, B2 – Pectinatites (Arkellites) cf. damoni Cope, MK 622, sup posed macroconch, 280 cm above the base of the sec tion Nia-Yu; C – Pectinatites groenlandicus Spath, INGG, macroconch, Lopsia, lo cal ity 41, bed 10, Subcrassum Zone, col lected by Bogomolov, 1997 (see Zakharov et al., 2005); scale bar = 1 cm
38 Mikhail A. Rogov and Gregory D. Price
(Rogov, 2004). Pres er va tion of the bulk of the Eosphinctoceras and Subdichotomoceras gath ered in the Mid dle Volga area chiefly is un sat is fac tory and pre cise spe cies de ter mi na tion dif fi - cult. Nev er the less the suc ces sion of Eosphinctoceras and Subdichotomoceras re cords in the two lower zones of the Volgian as a whole sup port the tra di tional cor re la tion of the Volgian of the Sub po lar Urals and Cen tral Rus sia with the ex - cep tion of the fact that the Mag num/Subcrassum bound ary pos - si bly lies within Klimovi Zone (Fig. 6).
Schweigert (1993, 1994) pro posed a new cor re la tion chart for the Kimmeridgian–Volgian bound ary beds of the Sub po lar Urals with Kimmeridgian–Tithonian of south-west Ger many.
This cor re la tion was based on re cords of ammonoids of sup - posed Bo real af fin i ties. The Mag num Zone was con sid ered by Schweigert (1993, 1994) as equiv a lent of the up per most Kimmeridgian based on the fol low ing points:
1. The Autissiodorensis Zone of south-west Ger many yields Gravesia lafauriana and Eosphinctoceras mag num;
2. Ammonites re sem bling Sarmatisphinctes dividuum are pres ent within the un der ly ing strata of south-west Ger many;
3. Geyer (1962) notes the pres ence of Sphinctoceras crassum in the Gigas Subzone of the Hybonotum Zone.
This cor re la tion chart con tra dicts the sug gested po si tion of the Mag num Zone within the base of the Volgian, noted above.
Ammonites thought by Schweigert (1993, 1994) as be long ing to E. mag num, have some im por tance. Trifurcate and polygyrate ribs, which are very char ac ter is tic for Eosphincto - ceras, are en tirely ab sent in the Ger man ammonites and its de - ter mi na tion even as the ge nus Eosphinctoceras ap pears to be un proven. At that time re cords of Sphinctoceras and Sarmatis - phinctes do not con flict with a lower Volgian age of Mag num Zone and cor re sponds to the re cent data ob tained from the Klimovi Zone. The Ger man ex am ples of Sphinctoceras could pos si bly be com pa ra ble with the Rus sian ammonites de rived from the Klimovi Zone (Fig. 6).
Hence the prop o si tion that the Mag num Zone cor re sponds to the base of the Volgian per mits cli mate proxy data to be placed in con text. The bel em nite ox y gen iso topes show only mod est vari abil ity rang ing from –1.86 to –0.11‰. In creas ingly neg a tive d18Ocarb val ues can be re lated to el e vated tem per a tures in en vi - ron men tal set tings where con ti nen tal ice vol ume is at a min i mum
and evap o ra tion or fresh wa ter in put are mi nor fac tors. As sum ing that these con di tions ap ply, palaeotemperatures range from ~12 to 20°C. Hence the iso to pic re cords from the bel em nites sug gest that the lower Volgian sed i ments of the Yatria River, Sub po lar Ural Moun tains were de pos ited in a warm ma rine en vi ron ment.
These Late Ju ras sic tem per a tures agree with the ac cepted warm high lat i tude sce nario (see Frakes, 1979). In ci den tally, such tem - per a ture in ter pre ta tions ri val high lat i tude tem per a tures as so ci - ated with the mid-Cre ta ceous ther mal max i mum (e.g., Huber, 1998). Price and Mutterlose (2004) and Zakharov et al. (2005) also pres ent bel em nite de rived sta ble iso tope data from the Kimmeridgian and Volgian of the Sub po lar Urals. The data of Zakharov et al. (2005) are con sis tent with the data pre sented here for the Mag num Zone (Fig. 5). The iso tope data are also com pa - ra ble with Late Ju ras sic bel em nite iso tope data from Submediterranean prov inces of Cen tral Po land and South ern Ger many (Wierzbowski, 2004) and the Mos cow Ba sin (Riboulleau et al., 1998). Zakharov et al. (2005) noted that palaeotemperatures cal cu lated from the iso tope com po si tion of bel em nite ros tra de rived from the Yatria River Ba sin may have been over es ti mated due to fresh wa ter run off. A re duced sa lin ity of 30 PSU (but still above the tol er ance of mod ern cut tle fish) re - duces ap par ent tem per a tures by ~5°C (see Price and Mutterlose, 2004). Al though these lat ter tem per a tures are cer tainly cooler – they still re main well above freez ing and are rel a tively warm com pared to some re cent mod elled es ti mates of Late Ju ras sic ocean tem per a tures (e.g., Sellwood and Valdes, 2008). A co in ci - dent sea level highstand in the mid-Bolonian (Wheatleyensis to Pectinatus zones) (e.g., Hal lam, 1992; Sahagian et al., 1996;
Tay lor et al., 2001) dur ing this pos tu lated warm ep i sode may re - flect a lack of po lar ice. Fur ther more, dur ing this ep i sode prac ti - cally the same ammonite as sem blages in hab ited the Bo real seas from Eng land to North ern Si be ria. Such a wide spread dis tri bu - tion of fauna across Bo real seas may re flect high sea lev els re - sult ing in a more open sea ways and re duced bar ri ers to mi gra - tion. Our new Pectinatites re cords as well as re vi sion of the pre - vi ously col lected fau nas sug gest wide fau nal ex change within
T a b l e 1 Iso to pic and el e men tal com po si tions of bel em nite ros tra
from the study site
Sam ple d13C d18O T
[°C] Fe
[ppm] Sr
[ppm] Mn
[ppm] Mg
[ppm]
NY 1-28 1.43 –1.09 16.4 5 2017 8 802
NY 1-67 1.29 –0.11 12.4 33 2015 2 650
NY 1-80 0.81 –1.00 16.0 6 1657 2 740
NY 1-83 1.69 –0.63 14.5 2 1737 2 588
NY 1-105 1.00 –0.44 13.7 21 1904 6 1143
NY 1-105 2 0.45 –1.14 16.6 17 1824 5 742
NY 1-110 1.42 –0.11 12.4 5 1558 2 615
NY 1-115 1.04 –0.83 15.3 5 1883 2 910
NY 1-200 0.32 –1.86 19.7 5 2015 2 1403
NY 1-250 0.68 –0.69 14.7 2 1502 2 624
Fig. 5. Cross-plot of d18O and d13C val ues de rived from bel em nite sam ples of the Yatria River near to the Nia-Yu River mouth
(this study, grey cir cles), with ad di tional data from Zakharov et al. (2005)
the Bo real ba sin dur ing the early Volgian. Pos si bly, some of the Ilowaiskya of Zakharov and Mesezhnikov (1974, pl. 4, figs. 2, 3) also be long to Pectinatites. Un fig ured Ilowaiskya from the Lena Ba sin, Yakutia (Djinoridze and Meledina, 1965) may also be the most east erly re cords of Pectinatites (Rogov, 2004).
CONCLUSIONS
The com bined ranges of al most all ammonite gen era in the low er most Volgian of the Sub po lar Urals sup ports the tra di tional point of view (Mesezhnikov, 1984) re gard ing the equiv a lence of the Mag num Zone with the two low er most Bolonian Zones and with the bulk of the Klimovi Zone. New Pectinatites re cords tes - tify that con nec tions be tween ammonite fau nas of Eng land and the Sub po lar Urals ex isted per ma nently dur ing the early Volgian. The Mag num Chron is char ac ter ized by the iso lated eastwards stray ing (Raw son, 1973) of Pectinatites, whilst mu - tual pen e tra tions of the typ i cal Urali an ammonoids are un known.
The Wheatleyensis to Pectinatus zones in ter val sees a rise in sea level, co in ci dent with warm palaeotemperatures as de rived from the iso to pic com po si tion of the bel em nites (this study and Zakharov et al., 2005). Fur ther more, prac ti cally the same
ammonite as sem blages in hab ited the Bo real seas from Eng land to North ern Si be ria. The eudemic Eosphinctoceras- Subdichoto - moceras lin eage of the Sub po lar Urals be came ex tinct near to Subcrassum/Pectinatum bound ary. The iso to pic re cords from the bel em nites fur ther sug gest a warm high lat i tude sce nario com pa ra ble with warmth en vis aged for times dur ing the mid dle Cre ta ceous and in com pat i ble with the ex ten sive de vel op ment of ice sheets dur ing this time. Re duced sa lini ties in the area need to be in voked to lower ap par ent tem per a tures.
Ac knowl edge ments. We thank Dr. J. C. W. Cope for valu - able com ments re gard ing Si be rian Pectinatites and Dr. S. V.
Meledina and post grad u ate stu dent A. E. Igolnikov (In sti tute of Oil and Gas Ge ol ogy, Novosibirsk) as well as Dr. A. I.
Kirichkova (Oil In sti tute, Saint-Pe ters burg) for their valu able help with ac cess to ammonite col lec tions. This work was funded by RFBR grant no.06-05-64284 to MR and the Sci ence Sup port Foun da tion and a Grant of the Pres i dent of Rus sian Fed er a tion MK 856.2008.5. Ammonite spec i mens are stored at the Geo log i cal In sti tute of RAS, Mos cow (GIN), In sti tute of Oil Ge ol ogy and Geo phys ics, Novosibirsk (INGG) and All-Rus sian Pe tro leum Re search Ex plo ra tion In sti tute, Saint-Pe ters burg (VNIGRI). This pa per bene fited from the re - views of J. C. W. Cope and an anon y mous re viewer.
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