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Paleoenvironmental conditions recorded by 87Sr/86Sr, δ13C and δ18O inlate Pliensbachian–Toarcian (Jurassic) belemnites from Bulgaria
Lubomir S. Metodiev a, Ivan P. Savov b,⁎, Darren R. Gröcke c, Paul B. Wignall b, Robert J. Newton b,Polina V. Andreeva a, Elena K. Koleva-Rekalova a
a Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Str. Bl. 24, 1113 Sofia, Bulgariab University of Leeds, School of Earth and Environment, Earth Science Department, Leeds LS2-9JT, UKc Durham University, Department of Earth Sciences, Durham DH1 3LE, UK
The late Pliensbachian–Toarcian (Jurassic) sedimentological, paleontological and geochemical (belemnite87Sr/86Sr, δ13C and δ18O) record is examined in two Eastern Tethyan (Bulgarian) locations. This interval containsthe well-known early Toarcian ocean anoxic event (T-OAE) and its manifestation and temporal context isexamined in Bulgaria. Many of the features seen in south-western Europe are identified: collapse of carbonateplatform productivity at the Pliensbachian/Toarcian boundary, the T-OAE (a short pulse of euxinic depositionin the Falciferum Zone), an early Toarcian rapid warming event seen in the belemnite δ18O record that peakedaround the Falciferum/Bifrons Zone boundary. The long-recognized positive δ13C excursion in the late FalciferumZone is also seen but a precursor, sharp δ13C negative excursion seen around the Tenuicostatum/FalciferumZoneboundary in most organic carbon records is not seen in the belemnite data, a curious absence noted from otherbelemnite records. Subsequent perturbations in 87Sr/86Sr, δ13C and δ18O suggest that there may be more globalisotopic excursions in the Early Jurassic. On the other hand, belemnite Sr isotope values from Bulgaria are in ac-cord with those recorded in Western Europe and hence, demonstrating its value as a chronostratigraphic tool.
The majority of studies on the biogeochemical cycles of the EarlyJurassic have been devoted to investigations of the Pliensbachian–Toarcian time slice. During this time interval there is a wide range ofpaleontological, sedimentological and isotope evidence supporting thenotion that a marine mass extinction event is associated with promi-nent δ13C excursions, negative δ18O shifts (i.e., warmer seawater tem-peratures or changes in the isotopic composition of seawater), arecognizable shift in the seawater 87Sr/86Sr ratio, widespread anoxia,and substantial sea-level changes (e.g., Jenkyns, 1988; Jones et al.,1994; Sælen et al., 1996; Harries and Little, 1999; Hesselbo et al.,2000; McArthur et al., 2000; Jones and Jenkyns, 2001; Jenkyns et al.,2002; Bailey et al., 2003; Rosales et al., 2003, 2004; Kemp et al., 2005;Wignall et al., 2005; Gröcke et al., 2007; McArthur, 2008; Dera et al.,2009; Jenkyns, 2010; Suan et al., 2010; Dera et al., 2011). These majorbiogeochemical disturbances deeply affected both marine biota and
global carbonate production in the shallow and deep ocean (Joneset al., 1994; Cecca and Macchioni, 2004; Tremolada et al., 2005; Deraet al., 2009; Morten and Twitchett, 2009; Al-Suwaidi et al., 2010;Jenkyns, 2010; Gröcke et al., 2011; Izumi et al., 2011). A majorpaleoceanographic phenomenon at this time – the Early Toarcian oce-anic anoxic event (T-OAE) – may have been a consequence of some ofthese changes (Jenkyns, 1988; Jones et al., 1994; Jones and Jenkyns,2001). Subsequently, global environmental conditions are consideredto have remained relatively stable (Jenkyns, 1988; Jones et al., 1994;Jenkyns et al., 2002) although theupper ToarcianVariabilis Zone record-ed minor, short-term δ13C and δ18O oscillations in some locations (e.g.,Wales, Jenkyns and Clayton, 1997; Spain, Gómez et al., 2008; Bulgaria,Metodiev and Koleva-Rekalova, 2008; Morocco, Bodin et al., 2010). It isunknown if these events record further global paleoenvironmentalchanges and faunal turnover after the T-OAE and if they are discreteevents or a consequence of the post-T-OAE stabilization (Gómez et al.,2008). Notably, there is evidence for turnover and abundance-diversityvariations in late Toarcian fossil assemblages: these include the extinctionof the ammonite subfamily Phymatoceratinae, the resurgence of theammonite subfamily Harpoceratinae and the incoming in abundanceof the ammonite subfamily Grammoceratinae and the familyHammatoceratidae (Bécaud et al., 2005; Dera et al., 2010), as well asthe turnover of brachiopods and small benthic foraminifers (Alméras
et al., 1997; Ruget andNicollin, 1997;Mailliot et al., 2009; Caruthers et al.,2013).
The marine 87Sr/86Sr record is buffered against restricted and short-term fluctuations in ancient seawater due to the long residence time ofSr in the oceans (e.g., McArthur et al., 2000) and provides a record ofmajor plate-scale events, linked to variations in the marine Sr input–output fluxes (e.g., Peterman et al., 1970; Elderfield, 1986; Veizeret al., 1997; McArthur et al., 2000; Jones and Jenkyns, 2001; Walthamand Gröcke, 2006; McArthur and Wignall, 2007). Jenkyns et al. (2002),among others, have shown that the Early Jurassic Sr-isotope curve hasa well-defined shape. However, the late Toarcian portion of this curveis poorly defined and it is considered uneventful and of lesser use inevaluating paleoenvironments compared to the early Toarcian record(McArthur and Wignall, 2007). The same also holds true for the lateToarcian δ13C and δ18O records (i.e., Gröcke et al., 2007).
The history of the Early Jurassic isotopic and faunal events isthus well known, although much evidence has come from westernEuropean sections and most research effort has been focussed on theearliest Toarcian and its celebrated oceanic anoxic event. In order toassess the context of this interval both regionally and temporally wehave undertaken a study in the relatively poorly known eastern Tethyansections of Bulgaria and considerably expanded the interval to includethe entire late Pliensbachian–late Toarcian interval. We use multiplelines of evidence fromwell-defined ammonite biostratigraphy, detailedfacies analysis and have exploited belemnite rostra to decipher thevariations of seawater 87Sr/86Sr, δ13C and δ18O.
2. Geological setting
2.1. Background geology and stratigraphy
The Jurassic sediments of the Teteven region (Central Fore-Balkan,Bulgaria) have long been known for their abundant and very diversefossils. This particularly applies to the exposures of the Lower Jurassicrocks, which have attracted much attention for more than a centurynow (e.g., Toula, 1881, 1889; Zlatarski, 1908; Cohen, 1931, 1932;Sapunov, 1961, 1968, 1969; Sapunov et al., 1971; Metodiev, 2008).Locally, these rocks are considered to be an integral part of the mostelevated segments of the Teteven Arch (Bonchev, 1971), which is aprominent positive structure of the Balkan Zone of the Balkan orogenicsystem (Fig. 1a, b). Regionally, the Balkan orogenic system representsthe northernmost part of the Alpine orogenic belt in Bulgaria that wascreated during multiphase collisional and extensional tectonic eventsin the late Palaeozoic to mid-Eocene (Zagorchev et al., 2009). Accordingto Bonchev (1971), the Teteven Arch contains a basement of redPermian polymictic clastic sediments, associated with volcanoclasticrocks and acid tuffs, covered by dark-red polymictic clastic sedimentsof the Lower Triassic Petrohan Terrigenous Group. The Lower Triassicsediments are overlain by thick carbonates of the Middle Triassic IskarCarbonate Group, which grade upwards into the regressive carbonatefacies of the Upper Triassic Moesian Terrigenous-Carbonate Group.This variegated basement is covered unconformably by thick Jurassicsuccessions that continue up to the Lower Cretaceous (Fig. 1b).
In the vicinity of the town of Teteven, the Jurassic strata form a spec-tacular landscape on the northern slope of the Beli Vit River valley andprovide a continuous depositional record of the Jurassic (e.g., Sapunov,1961, 1968, 1969; Shopov, 1970; Sapunov et al., 1971; Sapunovand Tchoumatchenco, 1989 and references therein) (Fig. 1c). Mixedshallow- to medium-depth transgressive carbonates and siliciclasticsediments represent the Lower–Middle Jurassic rocks of this area.
Fig. 1. Location of the sections used for this study: (a) Simplified tectonic sketch showing positioin Bulgaria; (b) Geological sketch map of the Teteven Arch (Central Fore-Balkan, Bulgaria) witharound the town of Teteven; (d) Generalized lithostratigraphic scheme of the Lower and the Maverage depositional rates of the Early–Middle Jurassic ages calculated on the time-scale of Graand the corresponding depositional environments.
These deposits largely correspond to the Ozirovo and the EtropoleFormations that span the Early Sinemurian to the Early Bajocian(Fig. 1d) (Sapunov and Tchoumatchenco, 1989).
TheOzirovo Formation is subdivided into threemembers, in ascend-ing order: the Teteven, Dolni Loukovit and Boukorovtsi Members. TheTeteven Member is a regionally extensive shallow-marine sequence ofEarly Sinemurian to early Pliensbachian age, composed of a 10–30 mthick succession of alternating sandy bioclastic limestones, calcareoussandstones and silty marls with abundant bivalves, common brachio-pods and scarce belemnites. The Dolni Loukovit Member is a 30–80 mthick succession of ferruginized sandy bioclastic limestones, of EarlySinemurian to late Pliensbachian age. Above this the BoukorovtsiMember is a 20–40 m thick hemipelagic, irregular shale–marl–limestone alternation of late Pliensbachian to late Aalenian age. The up-permost Pliensbachian and the Toarcian segments of the BoukorovtsiMember are themost fossiliferous (mainly ammonites and belemnites)and notably ooid-bearing. The rest of the Boukorovtsi Member is a mo-notonous Aalenian sequence with scarce fossils but common Zoophycosburrows.
The Ozirovo Formation is sharply overlain by 150 m thick poorlyfossiliferous, deeper-water shales and siltstones of the Etropole Forma-tion that span the late Aalenian to the middle Bajocian (Sapunov andTchoumatchenco, 1989). The Lower–Middle Jurassic sedimentary suc-cession of the Teteven area displays uneven depositional rates thatwere highest in two intervals: the Sinemurian to Pliensbachian andthe Aalenian to middle Bajocian, with a markedly condensed Toarcianportion (Fig. 1d), reflecting an often interrupted sedimentary influx(Metodiev, 2008). The scarcity of Toarcian fossils prevents a high-resolution biostratigraphic subdivision and thus correlation with othercoeval strata from elsewhere. In this study, we adopt the recently pro-posed Toarcian ammonite zonation for Bulgaria (Metodiev, 2008) thatcan be correlated with the NW European chart of Elmi et al. (1997)(Fig. 2).
2.2. Paleogeography
The Lower–Middle Jurassic rocks in the Teteven area represent innershelf sediments deposited into an epicontinental basin of strait-likeconfiguration (Zagorchev et al., 2009). It developed on the MoesianPlatform due to an Early Jurassic extension and normal faulting, proxi-mal to the southern Eurasian passive continental margin (Bassouletet al., 1993; Fourcade et al., 1995). This basin was part of the wide NWPeritethyan epicontinental sea, at a paleolatitude between 33°N and38°N (Dera et al., 2009 and references therein).
3. Materials and methods
This work is based on the study of petrographic samples, belemniterostra, and ammonite specimens, which are part of the Bulgarian Geo-logical Institute collections (Coll. No. F. FSR.SR.2012.1). Twenty-threesamples of the host rocks were taken for facies analysis and 48 belem-nites (mostly Dactyloteuthis and Acrocoelites, and less commonlyPassaloteuthis and Gastrobelus) were chosen for isotopic measurements.Thin sections were studied using conventional microscopy and repre-sent each rock type identified in the field. In general, the sampling den-sity of the belemnites was in the range of a few vertical centimeters,depending on the amount and the density of occurrence of their rostra.For the purposes of our study, we also collected 230 ammonites in orderto achieve the best possible biostratigraphic subdivision and to supple-ment the available biochronostratigraphic database (Sapunov, 1968;
n of the TetevenArchwithin the framework of the BalkanOrogenic Systemand its forelandthe area containing sections Varbanchovets and Babintsi; (c) Geological map of the areaiddle Jurassic deposits on the geological map with positions of sections sampled, and thedstein et al. (2004), relative distribution of major fossil groups (after Sapunov et al., 1971),
Fig. 2. Approximate correlation between the ammonite biostratigraphy for the Toarcian in Bulgaria and that of north-western Europe (for comparison see also Page, 2003). The gray areaon the table indicates the zones and subzones determinedby the ammonite occurrence of the studied sections. The substage subdivision of the Toarcian in Bulgaria follows that of Howarth(1992) and references therein. The Pliensbachian is not shown because of lack of ammonite evidence.
Sapunov et al., 1971; Metodiev, 2008). Before the isotope measure-ments, belemnite rostra were carefully screened under plane polarizedlight and cathodoluminescence for evidence of preservation, recrystalli-zation, and luminescence characteristics. From each belemnite, apolished thick-section was prepared for a cathodoluminescence studyand microsampling. After the assessment from cathodoluminescence,only the non-luminescent areas of the rostra interior were chosen andthe sampling was carried out by using a dentist drill, avoiding rostraperiphery, apical lines, portions of non-homogeneous pattern, smallveins and fractures filled with secondary calcite and borings.
Approximately 50 μg calcite powder was collected for 87Sr/86Srmeasurements and a minimum of 150 μg was used for δ18O and δ13Canalyses. The 87Sr/86Sr measurements were performed at the TIMSLaboratory of the School of Earth and Environment at the University ofLeeds (UK). Each calcite powder underwent a leaching procedure asrecommended by McArthur et al. (2000). Briefly, this procedure
included the submergence of sample powders in 0.9 ml 18 MΩ water,addition of 0.2 ml of 0.4 M acetic acid and centrifuging for ~5 min.,followed by removal of up to 1 ml of the leached solution, in order forsome of the insoluble residue to remain in the vial. To the insoluble res-idue, we added 1 ml of 1.7 M acetic acid until total dissolution wasachieved. The solution was then evaporated to dryness at 80 °Cfor ~1 h. The white carbonate residues were re-dissolved in 1.5 ml of2.5 M HCl solution and centrifuged again prior to the column separa-tions. Strontium was separated via standard chromatography methodusing Eichrom Sr-resin and the purified solution was subsequentlydried at 80 °C. The evaporated Sr-extractswere re-dissolved in ultrapureweak HCl acid and added onto tungsten wire with a previously appliedand gently dried TaCl5 ionization cocktail. The 87Sr/86Sr ratios weremea-sured on Thermo-Finnigan Triton-series thermal ionization mass spec-trometer. To achieve maximum precision and accuracy (see McArthuret al., 2000), the 88Sr signal was bracketed between 5 and 8 V and a
minimum of 200 isotope ratios were collected. The internal precisionwasmaintained between 1.3 × 10−6 and 7.1 × 10−6. External precisionwasmonitored by repeated analysis of the standard SRM (NIST). Duringanalysis of the samples, the mean measured value obtained forSRM (NIST) 987 was 0.710250 ± 0.000008 (2σ, n = 33). All measured87Sr/86Sr data has been normalized to SRM (NIST) literature value of0.710248 (McArthur et al., 2000). Total blankswere b2 ng Sr. Concentra-tions of Rb were too low to require correction for radiogenic 87Sr.
The δ18O and δ13C analyses were performed in the Total Laboratoryfor Source Rock Geochronology and Geochemistry at the Department ofEarth Sciences, Durham University (UK). Approximately 200–250 μg ofcalcite powder was placed in a glass vial. The sample vials were purgedwith He and subsequently injected with 103% phosphoric acid to pro-duce gaseous CO2, while being maintained at 50 °C. Isotopic analysiswas conducted using a Thermo-Finnigan MAT 253 isotope-ratio massspectrometer coupled with a Gas Bench II. Samples were calibratedagainst NBS-19 and LSVEC and isotopic data are reported againstVPDB, with a 1σ precision error of 0.06‰ for C and 0.08‰ for O.
4. Description of the sections
4.1. Varbanchovets
The studied interval, located near the northern end of the town ofTeteven (42°55′11″N; 24°16′16″E), is part of a thick section (N80 m)that spans the entire Lower Jurassic (Fig. 1c, d), and has a well-established biostratigraphy (Sapunov, 1961, 1968; Sapunov et al.,1971; Metodiev, 2008). Here, a 3.3 m thick Boukorovtsi Member ofthe Ozirovo Formation yielded 26 well-preserved belemnites and 150ammonite specimens that enabled us to stratigraphically place it fromthe lower Toarcian Tenuicostatum Zone (Semicelatum Subzone) tothe upper Toarcian Fallaciosum Zone (Figs. 2, 3, 4). The ammonitesuccession of this section was previously reported to extend from theFallaciosum Zone onwards (Sapunov, 1968; Sapunov et al., 1971), but
Fig. 3. Stratigraphic log of the Toarcian sediments of section Varbanchovets. The ammonite biogiven as column, composition, and distribution of grains and allochems of the rocks. The patammonite associations are shown, linkedwith a chart of ranges of the ammonite taxa determinonbelemnites. The small diagrambelow the fossil range-chart represents the fossil-empirical bacollected and belemnites analyzed from each unit. Zones/Subzones abbreviations: SEMC—SemVariabilis, THOU—Thouarsense, FALLA—Fallaciosum.
this was not confirmed by our study. Due to no exposures and/or lackof both ammonites and belemnites, the beds below the TenuicostatumZone and above the Fallaciosum Zone were not sampled. The summa-rized bed-by-bed description of the section is provided in Appendix A.
4.2. Babintsi
This is a newly discovered section located 2 km SW of Babintsi Vil-lage and 6 km NW to the town of Teteven (42°56′46″N; 24°15′25″E)(Fig. 1c, d). It is a 3 m thick succession of the topmost beds of theDolni Loukovit Member and the lower parts of the BoukorovtsiMemberof the Ozirovo Formation (Figs. 5, 6). The age of the sampled intervalwas determined as extending from the late Pliensbachian to thelate Toarcian, with no recorded lower Toarcian Tenuicostatum andFalciferum Zones, and the lack of ammonites at the top made it impos-sible to determine surely if there are any strata younger than the upperToarcian Fallaciosum Zone. About 80 ammonites were collected from a2 m thick sequence, and 22 well-preserved belemnite rostra wereselected for isotopemeasurements. A summarized bed-by-bed descrip-tion of the section is provided in Appendix B.
5. Results and discussion
5.1. Facies and fauna
5.1.1. Sedimentary recordAlthough thin, the clayey–carbonate successions of Varbanchovets
and Babintsi sections represent examples of Toarcian hemipelagicdeposits. These sections record a carbonate crisis that iswidely recordedin the late Pliensbachian–middle Bajocian interval in both Bulgariaand elsewhere in the north-western Tethyan domain of Europe(e.g., Mattioli et al., 2009). The two sequences comprise two faciesassociations: marls, and to a lesser extent, finely-laminated, carbonate-free shales that alternate with thin bioclastic limestones (Figs. 3–6, and
stratigraphic subdivision is represented against the fossil-bearing lithological succession,tern of distribution of belemnite rostra through the section and types of preservation ofed, as well as the 87Sr/86Sr, the δ13C, and the δ18O curves obtained after themeasurementssement of the study of this section: thickness of the zones/subzones, number of ammonitesicelatum, FALC—Falciferum, LUSI—Lusitanicum, BIFR—Bifrons, SEMP—Semipolitum, VARI—
Fig. 4.Microphotographs of the rocks from section Varbanchovets. (a) Marl with carbonate–clayey matrix containing poorly sorted and randomly dispersed broken skeletal grains, rareclastic grains (quartz, feldspars) and pyrite; Bed No. 5, PPL. (b) Marls composed of bioclasts, clastic non-carbonate grains and deformed phosphatized ferruginous ooids (white arrows)dispersed within carbonate–clayey matrix; Bed No. 4, PPL. (c), and (d) Plane- and cross-polarized light microphotographs of phosphatized and partly carbonatized ferruginous ooids;Bed No. 3c. (e) Bioclastic wackestone containing recrystallized biodetritus and sporadic foraminifers (white arrow); Bed No. 3b, PPL. (f) Marl with phosphate nodule (white arrow)anddeformedphosphatizedooids (black arrows); BedNo. 3a, PPL. (g) Ferruginous shale composedof ferruginous clayeymatrix and bioclasts— echinoid spines (black arrow), foraminifers(white arrow), ostracods, and crinoids. Clastic non-carbonate grains also occur; BedNo. 2, PPL; (h)Marl containing poorly sorted biodetritus (crinoids, brachiopods, echinoids, thin-shelledostracods and foraminifers) and clastic quartz and feldspar grains (white arrows) presented within calcareous–clayey matrix; Bed No. 1c, PPL.
appendices). The non-winnowed limestone textures (see Fig. 6) andcommonmarl beds suggest deposition in a relatively low-energymarinesetting located below effective wave base. Marls dominate theVarbanchovets section suggesting that it was deposited in a more
basinal setting than the Babintsi section, where carbonates are morecommon. The abundance of ammonites and belemnites, as well as cri-noids, ostracods and brachiopods, indicates an open-marine settingwith normal salinity and normal water circulation. The iron ooids are
Fig. 5. Stratigraphic log of theUppermost Pliensbachian and the Toarcian sediments of section Babintsi. The ammonite biostratigraphic subdivision is represented against the fossil-bearinglithological succession, given as column, composition, and distribution of grains and allochems of the rocks. Belemnite occurrence through the section, the types of preservation ofammonite associations, range-chart of the bivalve and ammonite taxa determined, and the 87Sr/86Sr, the δ13C, and the δ18O curves obtained after the measurements on belemnitescollected are given as well. The aim of the diagram below the fossil range-chart and abbreviations used, as well as symbols for belemnite distribution and ammonite taphocoenoses,are the same as in Fig. 3.
very common inmost of the studied limestones andmarl beds, and theyare considered to be allochthonous grains transported from a shallowmarine environment. The genesis of ooidal ironstones in marineenvironments is favored by clastic sediment starvation and reworkingof an iron-rich hinterland (Hallam and Bradshaw, 1979; van Houtenand Purucker, 1984; Young, 1989; van Buchem and Knox, 1998). Aninterpretation in accordance with that was proposed by Nachev(1960) for the Bulgarian Lower Jurassic ooidal ironstones. Inaddition, in Bulgaria there is a lack of evidence that the shallowmarine basins experienced input from volcanic ash sourced from anynearby arc.
The thin Toarcian record from both sections (Figs. 3, 5), withcommon sharp surfaces between the different rocks types observed,suggests a sediment starved-shelf deposition, presumably during atransgressive episode. The failure of carbonate productivity to keeppace with base-level rise at this time could either reflect the rapidityof the rise and/or the occurrence of stressful conditions suppressingcarbonate productivity. The stress could include oxygen-restriction al-though evidence for this condition does not appear until the FalciferumZone (laminated shales in the Varbanchovets section). In the Bifrons toThouarsense Zones enrichment of organic matter and pyrite aggregatessuggests dysoxia. The Babintsi section lacks evidence for the oldest twoToarcian ammonite zones and the younger strata show no evidence fordysoxia in these shallower water sediments.
5.1.2. Ammonite biostratigraphy and taphonomyThe studied sections yielded characteristic Toarcian ammonite taxa
that are common throughout NW Europe, thus allowing cross correla-tions (Fig. 2). The Late Pliensbachian age of the basal beds of Babintsisection was determined by the presence of large bivalves of the genusPseudopecten. Most of the Toarcian ammonites identified belong to theHildoceratidae (eight species of genera Hildoceras and Harpoceras,accompanied by occasional Hildaites, Orthildaites, Pseudolioceras andPolyplectus), followed by abundance by Grammoceratinae (eight spe-cies of genera Podagrosites, Pseudogrammoceras and Grammoceras),Dactylioceratidae (six species of genera Dactylioceras, Zugodactylites,Catacoeloceras and Peronoceras), and Phymatoceratidae (four species
of the genus Haugia) (see range-charts in Figs. 3 and 5). The best-recorded ammonite assemblage is that of the Bifrons Zone which canbe divided into subzones in both sections. The Thouarsense and theFallaciosum Zones, and the Semicelatum Subzone of the TenuicostatumZone are clearly defined aswell (Figs. 3 and 5). TheVariabilis Zone at thesection Babintsi has the best-preserved Haugia specimens known fromBulgaria, whereas the record of this zone in the Varbanchovets sectionis poor. The Falciferum Zone of the Varbanchovets section yielded onlya few ammonites. The maximum thickness of the zones and subzonesdoes not exceed 0.9 m (for the Semicelatum Subzone and the BifronsZone), while the rest of the recognized units cover thicknesses rangingbetween 0.2 and 0.5 m. It is interesting to note that from the lowerto the upper Toarcian there is a decrease in the thickness of zones/subzones and ammonite abundance (Figs. 3, 5).
In accord with the condensed nature of deposition, the state of am-monite preservation often indicates prolonged biostratinomic processesthat affected their shells prior to final burial. Here we have adapted theapproach of Fernández-López (1991, 1997) and Fernández-López et al.(2000) to evaluate the taphonomy of the ammonite fauna. Usually,the ammonites have only partly preserved body chambers and consistof phosphatized internal molds of partial or whole phragmoconesthat are commonly concentrated in clusters. Other ammonites mayhave the same filling as the host rock or lack sedimentary infilling. Thelatter are rare and show much less deformation and damage and arepresumably not reworked. More than 90% of the ammonites from theVarbanchovets section seem to have undergone reworking, suggestingan increased degree of taphonomic removal and thus indicating lowrates of sedimentation in this depositional setting, as postulated byFernández-López et al. (2000). However, the proportion of reworkedammonites decreases upwards by a factor of three in the Varbanchovetssection: from the Semicelatum Subzone to the Falciferum Zone, fromthe Bifrons to the Variabilis Zone, and upwards from the ThouarsenseZone (Fig. 3). This decline in reworking also coincides with a transitionfrom fossiliferous beds to levels where the ammonites are particularlyrare, and where only compacted ammonites were found, suggestingan increase of sedimentation rate. It is interesting to note that most ofthe reworked elements of the ammonite associations collected from
Fig. 6.Microphotographs of the rocks from section Babintsi. (a) Bioclastic wackestone composed of poorly sorted and randomly dispersed skeletal grains (mostly crinoids and rare shelldebris), partly carbonatized ferruginous ooids (white arrow), and clay-rich micritic matrix. Pack No. 3a, PPL. (b) Ferruginous ooids with normal cortices and well-preserved concentriclayering. Some ooid nuclei are presented by other broken ooid individuals (white arrow). Pack No. 3a, PPL. (c) Bioclastic wackestone containing crinoidal fragments, gastropod shells(white arrow) and ferruginous ooids. Pack No. 2c, PPL. (d) Completely carbonatized ferruginous ooids (white arrow) associating with bioclasts. Pack No. 2c, PPL. (e) Bioclastic wackestonewith ferruginous ooids. External parts of some ooid cortices are built up of carbonate layers with radial microfabric (white arrow). Pack No. 2b, PPL. (f) Carbonatized in various degreeferruginous ooids. Pack No. 2b, PPL. (g) Distorted ooid individuals. Pack No. 2b, PPL. (h) Bioclastic wackestone/packstone containing poorly sorted bioclasts of crinoids (white arrow),shell debris and foraminifers (black arrow). Pack No. 2a, PPL.
Varbanchovets section are immature or microconchs, especially in theLusitanicum and the Bifrons Subzones, whereas complete adults andmacroconchs are very rare. In contrast, the state of preservation of theammonites from Babintsi section is more variable. It appears that the
reworked ammonites here are mainly associated with the marl inter-vals, whereas in the limestones reworked ammonites are rare. The am-monites of this sequence record all types of growth stages. The juvenilespecimens were commonly observed in the limestones. The degree of
post-mortem reworking in the upper Toarcian ammonites appears to behigher compared with those from the lower Toarcian. Despite the prev-alence of reworking in the Bulgarian sections, there is no observeddifference in the zonal assignments between the reworked and notreworked ammonites, indicating that while reworking involvedprolonged exposure on the seabed, at no point were ammonites re-exhumed from older strata.
5.1.3. Belemnite occurrence and preservationGenerally, the distribution patterns and the density of the belem-
nites found at both of the studied sections follow the characteristics ofthe ammonites (Figs. 3, 5). Our belemnite generic identification isbased on the study of Stoyanova-Vergilova (1993). The levels with ab-sent or rare belemnites are associated with the beds of little reworkingof the ammonites, and usually the marl intervals are characterized byaccumulations of abundant rostra that form two types of “belemnitebattlefields” (sensu Doyle and Macdonald, 1993, see also McArthuret al., 2007). The first type consists of monospecific assemblages witha predominance of adults and a lack of orientation. In the Varbanchovetssection this battlefield type was seen at the base and at the top of theLusitanicum Subzone, and in the Bifrons Subzone, where it is composedof medium-sized Passaloteuthis. This battlefield type in the Babintsisection was identified in each marl bed from the top of the BifronsSubzone to the middle of the Thouarsense Zone, where it consists ofmedium-sized Acrocoelites. The second type of belemnite battlefieldrecords a more heterogeneous population structure and occasionallyoriented rostra that are subordinate to the abundant ammonites.The Varbanchovets section is composed of small- to medium-sizedDactyloteuthis and Acrocoelites (Semicelatum Subzone), medium-sizedPassaloteuthis and Acrocoelites (mid-Lusitanicum and SemipolitumSubzones), and medium-sized to large Acrocoelites and Dactyloteuthisbelemnites (Thouarsense Zone). In the Upper Pliensbachian and lowerToarcian from the Babintsi section, this battlefield type is made up ofAcrocoelites and Dactyloteuthis of various sizes, occasional Gastrobelus,as well as by medium-sized Dactyloteuthis and Acrocoelites belemnites
Table 1Isotope data for belemnites collected from the Toarcian of Varbanchovets section (Central Fore
a For clarity the 87Sr/86Sr ratios of samples V-4 andV-12 are not shown on the curve in Fig. 3,used for calculation of numerical ageswere constructed as follows: SemicelatumSubzone (all dazone (all data minus sample V-12, r2 = 0.47), and Variabilis ammonite zone–base of Fallacios
in the upper Toarcian. Belemnite-poor strata from both sections containbelemnites from the genera Acrocoelites and Dactyloteuthis. The belem-nites from the Varbanchovets section appear to be better preservedthan those fromBabintsi. The rostra fromBabintsi section are frequentlybored and corroded, scavenged and broken andmost of them, especiallythe large individuals, showing intensive bioerosion — all indicatingreworking. Although some of the O and C isotope fluctuationswe reportmay be due to species-specific effects (e.g. Sælen et al., 1996), the over-all sampling density of our dataset and the nature of the deposits do notallow us to thoroughly evaluate this issue.
5.2. The isotope record
5.2.1. 87Sr/86Sr isotopic trendsThe Sr isotope ratios measured on belemnites from Varbanchovets
section show a general increase of 87Sr/86Sr ratios through the Toarcian(Table 1). The 87Sr/86Sr curve is composed of two distinct segments: alower portion of less radiogenic values and upper portion ofmore radio-genic values separated by a section lacking well-preserved belemnitessuitable for Sr isotope stratigraphy (Fig. 3). Twopeaks are superimposedon the overall smooth shape of the Sr isotope curve at this section: onearound the boundary between the Tenuicostatum and FalciferumZonesand theother at theBifrons/SemipolitumSubzoneboundary, suggestingthat the sequence might be highly condensed at these levels. Throughthe Tenuicostatum Zone (Semicelatum Subzone), the measured87Sr/86Sr ratios range between 0.707088 and 0.707154, bordered atthe bottomand at the topwith values of 0.707177 and 0.707164 respec-tively. There is an exceptionally radiogenic 87Sr/86Sr ratio of 0.707371recorded froma sample in themiddle of the zone (Table 1). The samplesfrom the base of the Falciferum Zone yield 87Sr/86Sr values that increasefrom 0.707101 to 0.707153. The interval assigned to the Bifrons Zonecontains more radiogenic values with little variability, and 87Sr/86Srratios range between 0.707217 and 0.707244. Another unusually radio-genic ratio of 0.707388 was recorded from the base of this zone(Table 1). Up section, the samples from the Variabilis Zone continue
because of their very radiogenic values that plot outside the general trend. Linear segmentstaminus sample V-4, r2 = 0.064), FalciferumZone (all data, r2 = 0.98), Bifrons ammoniteum Zone (all data, r2 = 0.49).
Table 2Isotope data for belemnites collected from the Pliensbachian and the Toarcian of Babintsi section (Central Fore-Balkan), Bulgaria.
Linear segments used for calculation of numerical ages were constructed as follows: Upper Pliensbachian (all data, r2 = 0.67), Bifrons ammonite zone (all data, r2 = 0.35), and Variabilisammonite zone–base of Fallaciosum ammonite zone (all data minus sample Ba-16, r2 = 0.21).
with the same overall trend that is seen in the Bifrons Zone, which istrending towards more radiogenic Sr isotope ratios with values be-tween 0.707236 and 0.707245. The 87Sr/86Sr ratios of belemnites fromthe Thouarsense Zone and the base of the FallaciosumZone are distinct-ly more radiogenic in respect to those from the underlying strata, andrange between 0.707270 and 0.707312 (Fig. 3; Table 1).
Overall, the Sr isotope ratios measured on belemnites from theToarcian portion of the Babintsi section are similar to those of theVarbanchovets section (Fig. 5; Table 2). However, a part of the upperPliensbachian was available and yielded 87Sr/86Sr values that declineup section from 0.707134 to 0.707087. Above this there is a sharp in-crease in 87Sr/86Sr ratios (0.707230) across the hiatus that records theabsence of the Tenuicostatum and Falciferum Zones as discussedabove. The next interval of the Bifrons Zone containsminor fluctuationsin 87Sr/86Sr ratios and a slight increase from0.707230 to 0.707256 in theLusitanicum Subzone, and weak decrease to 0.707214 in the BifronsSubzone. The bottom of the Variabilis Zone yielded 87Sr/86Sr ratiosbetween 0.707236 and 0.707219 that subsequently rise sharply to0.707311 in the middle of the Variabilis Zone and then sharply fall to0.707220 (Fig. 5; Table 2). The uppermost portion of 87Sr/86Sr curvewithin the Thouarsense Zone and the bottom of the Fallaciosum Zoneis composed by apparently more radiogenic values, ranging between0.707291 and 0.707349. The topmost beds of the Babintsi section lackbelemnites and hence no 87Sr/86Sr record is produced above the baseof the Fallaciosum Zone.
5.2.2. Belemnite δ13C trendsBoth of the studied sections reveal the same broad trends: a marked
Semicelatum Subzone–Falciferum Zone positive δ13C excursion (with amaximum near the boundary of these zones), followed by a gradualdecrease of δ13C within the Bifrons Zone, slightly higher δ13C and rathervariable values in the Variabilis Zone and a gradual negative δ13Cshift towards the base of the Fallaciosum Zone. The δ13C values ofthe Varbanchovets belemnites range between +0.8‰ to +3.3‰(average = +1.8‰) (Fig. 3; Table 1). Initially, the δ13C values displaya rise from +0.9‰ at the base of the Semicelatum Subzone to +3.2‰,near the lower boundary of the Falciferum Zone. The latter representsthemaximum δ13C values from this section. This is immediately follow-ed by an abrupt fall of δ13C reaching a low value of +1.1‰ in the
Falciferum Zone. The δ13C ratios then rise again before showing along-term decline in the upper Toarcian to values between +1‰and +2‰ (Fig. 3). There is possibly a small positive excursion at thebase of the Bifrons Subzone. The carbon isotope data from Babintsishow similar overall values to the Varbanchovets section with theexception of lower values in the Thouarsense Zone (Fig. 5; Table 2).The Variabilis Zone, sampled in more detail at Babintsi, also showsseveral oscillations following a broad trough in the Bifrons Zone.
5.2.3. Belemnite δ18O trendsThe overall δ18O evolution from the studied sections reveals relative-
ly large variability of about 3‰, with an average value of −2‰ (Figs. 3,5; Tables 1, 2) and generally inverse correlation with the δ13C isotoperecord for the same belemnite specimens. This is best seen in theVarbanchovets data where there is a clear Semicelatum Subzone toLusitanicum Subzone negative δ18O excursion that attains a maximumnegative shift from −0.73‰ to −3.75‰ near the base of theLusitanicum Subzone (Fig. 3; Table 1). This is followed by a trend tomore enriched δ18O values (ranging between −3.0‰ and −1.5‰),though they do not retain the values seen in the Tenuicostatum Zone(from −0.8‰ to −2.6‰). In the Babintsi section a significant part ofthe δ18O record is missing at the major Pliensbachian/Toarcian bound-ary hiatus. Above this level, the δ18O values are more stable until theVariabilis Zone when they fall to lighter values in the later part of thezone before returning to somewhat variable but heavier values (Fig. 5).
5.2.4. Paleotemperature variations derived from δ18OPaleotemperatures shown in Tables 1 and 2 were calculated assum-
ing that the belemnite calcites collected in the Toarcian deposits of thetwo studied sections are diagenetically unaltered andwere precipitatedin equilibrium with ambient seawater whose oxygen isotope value andsalinity remained unchanged. To calculate temperatures from thebelemnite rostra we used the equation of Anderson and Arthur(1983), which represents a modified equation of that of Craig (1965):
T ∘C� � ¼ 16:0–4:14 δc–δwð Þ þ 0:13 δc–δwð Þ2
where δc is δ18O (‰ VPDB) of the sample, and δw equals the oxygenisotopic composition of the seawater. In this study a value of −1‰
(SMOW) has been adopted for an ice-free world (cf. Shackleton andKennett, 1975).
The calculations from the δ18O dataset of the Varbanchovets sectionyielded a succession of warming and cooling episodes during theToarcian. For the lowest Toarcian Semicelatum Subzone, the obtainedpaleotemperatures were not unusual for a paleolatitude of ~35°N.Higher in the section, warming at the base of the Bifrons Zone reachesa paleotemperature peak of 28.4 °C (Table 1). Following this warmpeak, temperatures rapidly decrease in the Variabilis Zone to ~18.6 °Cand remain only slightly warmer than this for the remainder of theToarcian, with the exception of a warming pulse in the SemipolitumSubzone which reached 24 °C.
Surprisingly, the Toarcian paleotemperature calculations fromBabintsi belemnites produced different values with a prolonged phaseof stability around 21 °C followed by warmer values, especially in theVariabilis Zone when paleotemperatures reached 26 °C (Table 2).The Babintsi section also yielded paleotemperatures for the latestPliensbachian, which produce very low values and a minimum of9.8 °C. Since the lithology, depositional history and the paleogeographyare quite similar for theVarbanchovets and Babintsi sections, we believethat the reported differences in paleotemperatures for the same inter-vals (ammonite zones) in these sections may be due to orographiceffects. Variability of freshwater fluxes have been proposed to have animpact on the basin salinity and thus on the overall temperature calcu-lations based on O isotope data, as shown by Sælen et al. (1996). How-ever, more than two sections need to be described in order to evaluatethis possibility.
Fig. 7. Belemnite 87Sr/86Sr isotope ratios versus calculated numerical age in millions of years (sections of the Yorkshire coast, UK (values from McArthur et al., 2000 and Jenkyns et al., 2002the UK Jurassic sections have been normalized to the same NIST 987 87Sr/86Sr values of 0.710following the methods outlined in detail in McArthur et al. (2000). Overall it appears that theExceptions are the Fallaciosumand the Semicelatumammonite Subzones,which in theBulgariaof theBulgarian section is better (note that thefilled vertical bar is for theUKdata and thewhitethe Bulgarian sections is much lower and that there are a few samples containing elevated 87Srclarity we have not shown unusually radiogenic (altered???) 87Sr/86Sr ratios.
5.2.5. The relative duration of ammonite zones and absolute age assessmentMcArthur et al. (2000) studied the Sr isotope variations in the upper
Pliensbachian and Toarcian sediments from the Yorkshire coast ofEngland. They demonstrated that if the rate-of-change of marine87Sr/86Sr and the sedimentation rate remain constant for a givenstratigraphic interval, then the change of 87Sr/86Sr with time is veryclose to being linear. This linear relationship can be utilized toestimate the relative durations of geological events preserved by thesedimentological record and the slope of the regression line enablesthe calculation of absolute ages. In the studied sections we foundsedimentological discontinuities (missing ammonite record in lowerlevels of Babintsi section, between the upper Pliensbachian and thefirst occurrence of the Toarcian ammonites from the Bifrons Zone)and sharp lithological surfaces that are the product of particularly lowand unstable sedimentation rates, and that unfortunately could not beconsidered constant with time. Therefore, themeasured 87Sr/86Sr ratiosfrom the Varbanchovets and Babintsi sections were grouped into fivelinear segments, where the rate-of-change of 87Sr/86Sr is assumed to re-main constantwith stratigraphic level. A portion of these segments wasconstructed as follows: upper Pliensbachian beds, Semicelatum Sub-zone interval, Falciferum Zone bed, Bifrons Zone succession andVariabilis Zone–base Fallaciosum Zone intervals. Each segment wasmodeled by linear regression analysis, excluding samples that deviatefrom the main 87Sr/86Sr trend by N10−5. Absolute ages have beenassigned to each belemnite specimen (see Tables 1, 2; Fig. 7) using183.6 + 1.7/–1.1 Myr for the Pliensbachian/Toarcian boundary, basedon the U–Pb dating of volcanic ash layers from that boundary and also
Myr) for the Bulgarian sections (Varbanchovets and Babintsi, this study), and the Jurassic). To make comparisons easier, all of the reported Sr isotope ratios in the Bulgarian and in248. The duration of the various ammonite biozones (shown on top) has been calculatedduration of the ammonite biozones in Bulgaria confirms the results from the UK sections.n sections appear to be longer and shorter, respectively. Although the analytical uncertaintyvertical bar is for theBulgarian section), please note that the belemnite sampling/density in/86Sr ratios, although those were not included in the calculation of the numerical ages. For
based on ammonoid dating from the North American Cordillera (Pálfyand Smith, 2000). In Fig. 7, the Bulgarian results are compared withthe well-studied Sr isotope fluctuations reported from England(McArthur et al., 2000; Jenkyns et al., 2002). The comparison betweenthe Bulgarian and the British sections reveals several importantinsights:
1) Overall the duration of the British Toarcian ammonite zones basedon Sr isotope stratigraphy (McArthur et al., 2000) appears to be ingood agreement with the results obtained from the Lower Jurassicsections of Central Fore-Balkan Mountains of Bulgaria (this study).
2) The non-deposition recorded at the Pliensbachian/Toarcian bound-ary in Babintsi section is estimated to have lasted in access of2 Myr (from 184.27 to 181.80 Ma).
3) The Upper Pliensbachian part of Babintsi section has been assignedto a time span from 184.66 to 184.27 Ma, corresponding to themid-Apyrenum Subzone.
4) The sampled interval of the Semicelatum Subzone of Varbanchovetssection is found to be incomplete and representing only the last 0.06Ma of the Semicelatum Subzone.
5) Although only a few belemnite specimenswere discovered from theFalciferum Zone of the Varbanchovets section, they allowed us to at-tribute it to the lower Serpentinum Subzone (cf. the Exaratum Sub-zone in Yorkshire). Considering the absolute ages calculated fromthe uppermost and the lowermost specimens of the SemicelatumSubzone and the Bifrons Zone, the duration of the Falciferum Zonein Varbanchovets section is found to be 1.49 Ma, and consequentlythe sedimentation rate appears to have been extremely low, around4 cm/Myr. It is worth noting that our calculated duration (1.49 Ma)for the Falciferum Zone is in good agreement with the latest resultsfrom astronomical calibrations for the same zone based on high-resolution (∼2 kyr) magnetic susceptibility (MS) measurements(2.17 Ma, Boulila et al., 2014).
6) The calculations based on the Sr isotope data from both Babintsi andVarbanchovets sections revealed that the Bifrons Zone in Bulgarialasted only about 0.47 Myr. Thus, the subzonal division used inBulgaria for this particular zone does not correspond to that inYorkshire (and probably elsewhere), and future correlations atsubzonal level involving the Bifrons Zone require caution. Interest-ingly, the reported durations for the Bifrons Zone from the UKsections (McArthur et al., 2000) are in disagreement (also lower)when compared with the durations based on the astronomicalcalibrations for the same zone (2.15 Ma; Boulila et al., 2014).
7) The Variabilis Zone in the Bulgarian sections appears to have lastedfrom 181.43 to 181.22 Ma, i.e. only 0.02 Ma longer when comparedto the same zone in Yorkshire (McArthur et al., 2000).
8) The Thouarsense/Fallaciosum Zone boundary of the Bulgarian sec-tions can be roughly placed at 180.99 Myr. The data density aroundthis interval is too low in the Bulgarian sections, but the limited datapoints indicate quite a good agreement with the general trendtowards more radiogenic 87Sr/86Sr recorded in the Yorkshire coastsections in the UK.
6. Overview
The Bulgarian isotopic data provides us information on the latePliensbachian–Toarcian sedimentary history of this eastern-centralTethyan region, which can be compared with better-known earlyToarcian records from theWestern Tethyan, thereby providing a bettertemporal coverage of the Toarcian in Europe.
6.1. Late Pliensbachian
Although this interval was only sampled at Babintsi, the belemnitesfrom this section yield low paleotemperatures that concord with theidea of a severe cool episode in the late Pliensbachian (e.g., Bailey
et al., 2003; Rosales et al., 2004; Gómez et al., 2008; Suan et al., 2010,2011). Widespread evidence for a Pliensbachian/Toarcian sequenceboundary suggests that the cooling culminated in glacio-eustatic regres-sion (e.g., Guex et al., 2001; Suan et al., 2010).
6.2. Early Toarcian
The Bulgarian sections record many of the same features seen else-where in Tethys. A base-level rise coincidedwith a crisis in platform car-bonate deposition with the result that a hiatus is developed in shallowplatform location (Babintsi) while condensed, marl sediments withFe-ooids developed in the deeper-water Varbanchovets section. In thelatter location, the presence of finely laminated shales in the bottomof the Falciferum Zone (Fig. 3) is a clear a manifestation of the T-OAEand it provides the most easterly Tethyan record of this event.
The T-OAE also coincided with the onset of a rapid rise of 87Sr/86Srratios and carbon isotope shifts that include a controversial negativeδ13C excursion near the Tenuicostatum/Falciferum zonal boundaryfollowed by a return to heavier values in the upper Falciferum andlower Bifrons Zones (Fig. 8; Jenkyns, 1988; Sælen et al., 1996;Hesselbo et al., 2000; McArthur et al., 2000; Jenkyns et al., 2002;Bailey et al., 2003; Rosales et al., 2004; Kemp et al., 2005; van deSchootbrugge et al., 2005; Wignall et al., 2006; Gröcke et al., 2007;Hesselbo et al., 2007; Dera et al., 2009; Suan et al., 2010; Dera et al.,2011; Gröcke et al., 2011; Izumi et al., 2011).
With the exception of some more radiogenic values, mostly in theSemicelatum Zone, the Sr isotope curve from the Bulgarian sectionsconfirms that produced from other European records. However, notethat the absence of suitable belemnites in the upper FalciferumZone makes it difficult to precisely locate the inflection point in the87Sr/86Sr curve (e.g., McArthur et al., 2000; Gröcke et al., 2007).
The main feature of the belemnite δ13C record is a decreasing trendin the Tenuicostatum and Falciferum Zones followed by a return toheavier values in the Bifrons Zone, and the amplitude of both oscilla-tions is ~2‰. The subsequent δ13C trend sees a further negativeshift to values around 0‰ followed by a gradual increase again in thetopmost part of the section (Fig. 8). The sharp, negative excursionrecorded in sedimentary organic carbon and carbonate from varioussections in Europe around the Tenuicostatum/Falciferum Zoneboundary (e.g., Jones et al., 1994; Hesselbo et al., 2000, 2007;Schouten et al., 2000; Kemp et al., 2005; Suan et al., 2010, 2011), andelsewhere (e.g., Al-Suwaidi et al., 2010; Caruthers et al., 2011; Gröckeet al., 2011; Izumi et al., 2011) is not seen in our data. The failure ofthe belemnite calcite record to reveal this excursion has been notedpreviously and widely debated (e.g. van de Schootbrugge et al., 2005;McArthur, 2007; see also McArthur et al., 2008), although the precisecase is still unknown. The data presented indicate that the lack of anegative δ13C excursion in belemnites is not a regional or taxon-specific signal but rather a consistent feature of this group. However,for the Bulgarian data the absence of the excursion could reflect the rel-atively low temporal resolution available from our condensed sections.
Belemnites have also provided a paleotemperature record for theToarcian interval, notably from Germany, Spain and the UK, that sug-gests a rapid temperature rise in the Tenuicostatum–Falciferum Zones(Fig. 9; Bailey et al., 2003; Gómez et al., 2008; Gómez and Goy, 2011).The Bulgarian belemnite data produce comparable paleotemperaturesand the trends to those recorded in theWestern Tethys (Fig. 9). The cul-mination of this trend occurred around the Falciferum/Bifrons Zoneboundary. Similar paleotemperatures are also recorded in Panthalassa(Gröcke et al., 2007). In detail, our data suggests that there werehigher-order paleotemperature oscillations in the Tenuicostatum Zonesuperimposed on the overall warming trend (Fig. 9), a pattern alsoproduced in other δ18O records from belemnites (Gómez et al., 2008),brachiopods (Suan et al., 2010), as well as fish teeth (Dera et al.,2009). A recent study from northern Siberia notes the abundance ofthe thermophyllic pollen genus Classopollis in the early Falciferum to
Fig. 8. Correlation of the carbon isotope record from the sections of the Central Fore-Balkan, Bulgaria (this study), the sections from theWest BalkanMountains (Bulgaria; Metodiev andKoleva-Rekalova, 2008), the Ammelago section (High Atlas, Morocco; Bodin et al., 2010), sections Tomar and Peniche (Lusitanian Basin, Portugal; Suan et al., 2010), Asturias (NorthernSpain; Gómez et al., 2008), and Mochras and Winterborne Kingston boreholes (Wales and Dorset, UK; Jenkyns and Clayton, 1997; Jenkyns et al., 2001). The gray bands on each sectionindicate intervals of prominent δ13C excursions. The upper left corner represents paleogeographical sketch of the Western part of the Tethyan Realm during the Toarcian (simplifiedand modified after Metodiev and Koleva-Rekalova (2008), based upon the references cited therein) with the approximate locations of the sections compared.
early Bifrons Biochrons as evidence of a severe warming event (Suanet al., 2011), suggesting that this early Toarcian trend was a globalphenomenon.
6.3. Late Toarcian
The re-establishment of platform carbonate productivity ensured amore complete Toarcian record in Bulgarian shallow-water section,such as at Babintsi, while in deeper-water section an increase insedimentation rates reduced the degree of seafloor reworking of ammo-nites. Nonetheless iron ooids considered to be the product of prolongedexposure on the seafloor remain common in both sections.
In Western Europe this interval sees the continued rise of 87Sr/86Sr(McArthur and Wignall, 2007), although the carbon isotope curve ismore complex: it is stable in bulk samples fromMochras FarmBorehole,Wales (Jenkyns et al., 2001), but show a decreasing trend in belemnitedata from Rodiles-Santa Mera in Spain (Gómez et al., 2008), andsuppress Yorkshire and Dorset belemnites, UK (Jones et al., 1994). Inaddition, the observed 87Sr/86Sr long term increase is also reflected inthe values derived from sections representing the MediterraneanRealm (Woodfine et al., 2008) and the Panthalassa Ocean (Gröckeet al., 2007).
In general the Bulgarian recordmatches these trends although thereis the suggestion of discrete events within the interval of the Variabilis–Thouarsense Zones. With the resolution of our data it is difficult to
distinguish a clear pattern, thus δ13C values showa possible negative ex-cursion in the Thouarsense Zone of the Babintsi section, δ18O valuesshow a warming peak in the Variabilis Zone of the same locality, andsubstantial oscillations of the 87Sr/86Sr record implies sedimentary con-densation and/or diagenetic alteration. All these observations requireverification with more detailed studies of preferably more expandedsections, but the available data suggests that similar trends are alsopresent in western European sections (Figs. 8, 9).
7. Conclusions
We studied the Early Jurassic (late Pliensbachian–Toarcian) sedi-mentological, paleontological and isotope (belemnite 87Sr/86Sr, δ13Cand δ18O) record in two Eastern Tethyan hemipelagic successions inBulgaria. We found that in the Central Balkan Mountains, this intervalcontains the well-known Early Toarcian ocean anoxic event (T-OAE).We have studied its manifestation and temporal context via study ofits fossil and sedimentological record combined with the isotopesystematics (C, O and Sr) measured in belemnite rostra. Many of thefeatures of this event seen in other European locationswere recognized:
1) A crisis in platform carbonate deposition at the Pliensbachian/Toarcian boundary, recorded by a 2 Ma hiatus in the shallow watersedimentary succession (missing are uppermost Pliensbachian andthe Tenuicostatum and the Falciferum Zones).
Fig. 9. Correlation of the oxygen isotope record from the sections of the Central Fore-Balkan, Bulgaria (this study), the sections from theWest BalkanMountains (Bulgaria; Metodiev andKoleva-Rekalova, 2008), the Ammelago section (High Atlas, Morocco; Bodin et al., 2010), sections Tomar and Peniche (Lusitanian Basin, Portugal; Suan et al., 2010), Asturias (NorthernSpain; Gómez et al., 2008), and Mochras and Winterborne Kingston boreholes (Wales and Dorset, UK; Jenkyns and Clayton, 1997; Jenkyns et al., 2001). The gray bands of each sectionindicate intervals of prominent δ18O excursions. The upper left corner represents paleogeographical sketch of the Western part of the Tethyan Realm during the Toarcian (simplifiedand modified after Metodiev and Koleva-Rekalova (2008), based upon the references cited therein) with the approximate locations of the sections compared.
2) The presence of short pulse of oxygen-depletion during the lowerFalciferum Zone in finely laminated shales.
3) In Bulgaria the T-OAE coincided with the onset of rapid rise of87Sr/86Sr ratios and δ13C shifts but the controversial negative δ13Cexcursion near the Tenuicostatum/Falciferum zonal boundary isnot observed. The δ13C values show a gradual decrease throughthe entire Falciferum Zone followed by a return to heavier δ13Cvalues in the lower Bifrons Subzone.
4) An Early Toarcian rapid warming event was recorded in the belem-nite δ18O record. This warming appears to have peaked around theFalciferum/Bifrons zonal boundary. Bulgarian belemnite data pro-vide paleotemperature trends akin to those from Western Tethyansections. Our new data from the Tenuicostatum Zone suggests thatsuperimposed on the overall warming trends, there were higher-order paleotemperature oscillations.
5) The good quality of our Sr isotope measurements enabled us to esti-mate the relative durations of geological events preserved by thesedimentological record of our sections. The Sr isotope systematicsof the Bulgarian sections appears to match the well known smoothrise of 87Sr/86Sr ratios from the Pliensbachian/Toarcian boundaryupwards. However, there is a suggestion of several discrete eventswithin the upper Toarcian interval covering the Variabilis–Thouarsense Zones. With the resolution of our data it is difficult todistinguish a clear pattern, thus δ13C values show a possible negative
excursion in the Thouarsense Zone, δ18O values show a warmingpeak in the Variabilis Zone and substantial oscillations of the87Sr/86Sr record imply possible sedimentary condensation and/ordiagenetic alteration. All these observations require confirmationfrom additional studies of preferably more expanded sections,but the data available suggests that similar trends are also presentin several well-studied western European sections. Using the87Sr/86Sr isotope ratios we found that the duration of the Toarcianammonite zones from the studied sections of Central Fore-BalkanMountains appears to be in good agreementwith the results obtain-ed from the Yorkshire coast in the UK.
Acknowledgments
We thank Bob Cliff for the help and advice during the Sr isotopesample preparations and TIMS measurements. Iliya Dimitrov and VasilSirakov helped with the sample collection during fieldwork inBulgaria. The stable isotope portion of this research was made possibleby a NERC (NE/H021868/1) grant to DRG. Technical support at DurhamUniversity was provided by Joanne Peterkin. The early version of themanuscript benefited from comments by G. Suan, E. Mattioli andS. Hesselbo. The clarity and impact of the final manuscript benefitedfrom official journal reviews by two anonymous referees and the jour-nal editor F. Surlyk.
Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.palaeo.2014.04.025.
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