Applied Geochemistry 23 (2008) 2845–2856

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Applied Geochemistry

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Stable isotope records (d18O and d13C) of Lower-Middle Jurassicbelemnites from the Western Balkan mountains (Bulgaria):Palaeoenvironmental application

Lubomir Metodiev *, Elena Koleva-RekalovaGeological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Street Bl. 24, 1113 Sofia, Bulgaria

a r t i c l e i n f o

Article history:Available online 18 April 2008

0883-2927/$ - see front matter � 2008 Published bdoi:10.1016/j.apgeochem.2008.04.010

* Corresponding author. Fax: +359 2 872 46 38.E-mail address: lubo@geology.bas.bg (L. Metodie

a b s t r a c t

Stable O and C isotope data of 110 Upper Pliensbachian-Lower Bajocian belemnites havebeen obtained and used to attempt a reconstruction of palaeotemperature and its variationin two epicontinental depositional environments from the Western Balkan mountains(Bulgaria). The samples were collected from 3 sections with high-resolution ammonitesubdivision. Initially taphonomic, cathodoluminescence and geochemical analyses wereused for evidence of diagenetic alteration. Non-luminescent parts of the belemnite rostrahave been sampled for isotope analyses and 76 samples, having d18O < �4‰ (PDB),d13C > �0.5‰ (PDB), Fe < 250 ppm, Mn < 50 ppm, Sr > 950 ppm and Sr/Mn ratio > 80 wereused for palaeotemperature interpretations. The O and C isotope data generally exhibit lit-tle stratigraphical variability with minor fluctuations, however, there are prominent posi-tive C isotope excursions and coeval negative O isotope shifts detected in the LowerToarcian Tenuicostatum, Falciferum and Bifrons Zones. The O isotope data, interpreted interms of palaeotemperature, revealed relatively high seawater temperatures during theToarcian, Aalenian and Early Bajocian, with detectable temperature rises during the EarlyToarcian (maximum value of 29.6 �C). Both C isotope maxima and O isotope minima areused as evidence of the Early Toarcian anoxic event reported from many localities of thesame age and in similar facies in Western Europe. In the study the latter is recognized as3 episodes, which are closely related with the seawater temperature maxima. This isotoperecord pattern is considered as a consequence of a global Tethyan transgression during theEarly Toarcian.

� 2008 Published by Elsevier Ltd.

1. Introduction

Oxygen and C isotope research (d18O and d13C) em-ployed for palaeoenvironmental interpretations has along history that can be traced back to the middle ofthe last century. There is agreement that O isotope val-ues of marine carbonates are a function of the O isotopecomposition and temperature of the ambient seawater(Broecker and Peng, 1982). The d13C values in carbonatesreflect the 13C/12C ratio of CO2 dissolved in seawater aswell as indicating the source of the C in the CO2. If the

y Elsevier Ltd.

v).

d18O and d13C values of marine carbonates do indeedform in equilibrium with the seawater, and if there hasbeen no later diagenetic alteration, they can be usedfor palaeotemperature reconstructions and discrimina-tion of palaeoceanic events. Empirical and experimen-tally derived equations are drawn for carbonatepalaeotemperature estimations (e.g., O’Neil et al., 1969;Hays and Grossman, 1991; S�len et al., 1996). The theo-retical basis and analytical techniques of carbonate iso-tope geochemistry are undergoing progressiverefinement enabling the recognition of anomalies (excur-sions) of the O and C isotope record, which are consid-ered to reflect the existence of palaeoclimatic changesof global significance.

2846 L. Metodiev, E. Koleva-Rekalova / Applied Geochemistry 23 (2008) 2845–2856

New knowledge of belemnite taphomomy has becomeavailable during the last decade. Belemnite rostra havebeen increasingly used as a reliable source for isotopeand geochemical investigations of Jurassic marine environ-ments. Recent studies have demonstrated that well-pre-served rostra retain primary isotope and geochemicalsignatures of the ambient seawater (S�len et al., 1996;Podlaha et al., 1998; McArthur et al., 2000; Rosales et al.,2001, 2003, 2004). Utilization of belemnites for isotopemeasurements has several advantages: (1) belemnites areamong the most widespread fossil groups of the Jurassic;(2) belemnite rostra are resistant to post-depositionalalteration by burial fluids due to their primary low-Mg cal-cite composition; (3) the rostra have a simple internalstructure and any consequent diagenetic transformationis easily recognizable (Rosales et al., 2001).

This paper represents the first stratigraphical investiga-tion on the O and C isotope composition of belemnitesfrom the Lower-Middle Jurassic in Bulgaria. No C isotopedata were available from the Jurassic in Bulgaria untilnow. A previous attempt at palaeotemperature calculationwas based on O isotope measurements of Bulgarian belem-nite rostra from the Jurassic and Cretaceous (Teis et al.,1975). This pioneering work suggested high palaeotemper-atures for the Toarcian and early Bajocian, however, thesamples used in this study came from scattered localitiesand different stratigraphic levels that do not enable thereconstruction of a palaeotemperature trend. Some of thereported Jurassic estimates have unrealistically highvalues, for example, 38.4 �C and 39.9 �C for the UpperToarcian, that suggest that they are derived from diage-netically modified samples. Here the results of isotopemeasurements of a collection of Late Pliensbachian-Early Bajocian belemnites obtained from ammonite-calibrated sections using material that has been carefullyevaluated with regard to the state of preservation arepresented.

Fig. 1. Palaeogeographic sketch of the Western part of the Tethyan Realm durinFourcade et al., 1995), showing the location of the basin in which Lower-Middledeposited.

2. Geological setting and stratigraphic framework

The southern slopes of the Western Balkan mountains(Bulgaria), between the border with the Republic of Serbiaand the Iskar Rover Gorge, comprise a thick Mesozoic sed-imentary cover upon a low-grade metamorphosed Paleo-zoic basement which is part of the Balkan orogenicsystem. These rocks have been folded and thrust duringthe Alpine tectonic inversion (Dabovski et al., 2002). In thisarea, the Lower-Middle Jurassic rocks are exposed in sev-eral narrow and parallel strips with NW–SE trends. Sedi-ments were accumulated in a highly fragmentedepicontinental basin, situated on the Southern edge ofthe Moesian Early-Middle Jurassic Platform (sensu Sapu-nov et al., 1988). According to recent palaeogeographicalreconstructions (Bassoulet et al., 1993; Fourcade et al.,1995), it was developed due to early Jurassic extensionalfaulting of the northern passive margin of the Tethysocean. In this context, the Lower-Middle Jurassic sedimen-tary successions in the Western Balkan mountains displayan evolution from an initially isolated lacustrine setting(Early Hettangian) to a rapidly expanding shallow-marinesandy and carbonate depositional setting (Late Hettan-gian-Early Bajocian). In the beginning of the Toarcian thisarea became part of a broad epeiric sea, extending north-wards and westwards from the central Tethys and coveringmuch of Western and South-eastern Europe (Fig. 1).

In this palaeogeographic setting sediments near theLower-Middle Jurassic boundary in the Western Balkanmountains are represented by two main facies: (1) ironooidal limestones and ooidal ironstones (Ozirovo Forma-tion), which are exposed to the SE (Ponor-KremikovtsiJurassic Strip); (2) dark-colored marl-limestone sediments(Boukorovtsi Member of the Ozirovo Formation) and shaleswith sideritic concretions (Etropole Formation), croppingout to the NW and NE (Zaburde, Vidlich, Ponor, MilanovoRegion and Golema Planina Mountain) (Fig. 2). Three sec-

g the mid-Toarcian (Early Jurassic) (simplified after Bassoulet et al., 1993;Jurassic sediments of the Western Balkan mountains (Bulgaria) have been

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L.Metodiev,E.K

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2848 L. Metodiev, E. Koleva-Rekalova / Applied Geochemistry 23 (2008) 2845–2856

biostratigraphic calibration of the isotope data. In addition,the sections display a transition between the two faciesdescribed above. The first section is located near the villageof Beledie Han, 26 km north of Sofia (co-ordinates: 23�, 100,260 0/42�, 530, 230 0). The other two sections, Dobravitsa-1and Dobravitsa-2, are exposed in the vicinity of the villageof Breze, 60 km to the north of Sofia, (co-ordinates: 23�,140, 150 0/43�, 010, 460 0 and 23�, 130, 570 0/43�, 010, 380 0).

The sampled sections display a distinct cyclic deposi-tional pattern (Metodiev and Koleva-Rekalova, 2005; Kol-eva-Rekalova et al., 2006). A complete succession of 9Toarcian ammonite zones, from the Tenuicostatum to theAalensis Zone has been identified at the Beledie Han sec-tion (Fig. 2). This sequence has been recently described inmore detail with the main lithological types outlined byMetodiev et al. (2005). The succession (3.25 m thick) be-gins with dark-grey ferruginized marlstones, with inter-bedded thin sandy micrite limestones (wackestones).Upwards, it continues with yellowish to reddish brownbioclastic, iron-ooidal limestones (wackestones, pack-stones and packstones to grainstones). The ammonitesand belemnites are the main macrofossils present. Bivalvesand brachiopods also occur in a fragmentary state withepifaunal forms dominant. The majority of fossils are con-centrated in two distinct shell-beds (rudstones) (Fig. 2).

The sections Dobravitsa-1 and Dobravitsa-2 consist ofalternating marls, shales and limestones with fossilsagain dominated by ammonites and belemnites, and asubordinate fauna of brachiopods and bivalves. The pat-tern of fossil distribution and general depositional fea-tures within these profiles have also been describedpreviously (Metodiev and Koleva-Rekalova, 2005). TheDobravitsa-1 section comprises a 7 m thick BoukorovtsiMember of the Ozirovo Formation with two levels of con-densed strata and with the Etropole Formation exposedabove (Fig. 2). It has been subdivided into 15 ammonitezones, from the Upper Pliensbachian Spinatum Zone atthe base to the Lower Bajocian Discites Zone at the top.This section consists of black offshore ferruginous marls,grey fine-laminated ferruginous shales, and limestones ofvaried types: micritic mudstones, mudstones to wacke-stones with scarce fossils, wackestones to packstoneswith phosphatic ooids, bioclastic floatstones and rud-stones (shell-beds) (Fig. 2). Despite its dark color theserocks have very low organic C contents, between 0.19and 1.25 wt% (Metodiev and Koleva-Rekalova, 2006). Atthe Dobravitsa-2 section, the studied interval is repre-sented by a 2 m thick Boukorovtsi Member, from theupper part of the Lower Toarcian Bifrons Zone to thetop of the Upper Toarcian Fallaciosum Zone (Fig. 2). Itconsists of rusty-brown ferruginous marls, phosphatizedmicritic limestones (mudstones) and iron-ooidal lime-stones (wackestones to packstones) with abundant amm-onites and belemnites. There is no evidence ofcondensation.

3. Material and analytical methods

This work is based on the geochemical and isotopemeasurements of 110 belemnites, which are part of the

Bulgarian Geological Institute collections (Coll. NumbersF.MFSR.FSR.1.2003.3 and FSR.SR.1. 2005.1). The analyticalprocedures described by Rosales et al. (2001, 2003, 2004)were followed in this study. In order to constrain thepossible diagenetic transformation of the samples, theisotope analyses have been preceded by preliminarytaphonomic observations, cathodoluminescence and geo-chemical tests. The results of these initial investigationshave already been published (Metodiev and Koleva-Rekalova, 2003a, 2005; Koleva-Rekalova and Metodiev,2005). They are briefly described below to support theisotope data obtained.

Specimens used for this study consist of almost com-plete rostra, but without the phragmocone. Apart fromthe two condensed shell-beds at the Dobravitsa-1 section(the middle of bed 5 and the base of bed 7) and the twoshell-beds at the Beledie Han section (bed 7 and 11) wherethe rostra are highly concentrated, belemnites show scat-tered occurrence within the host rocks. Sampling densitywas 0.1–0.5 m. The pattern of distribution permitted col-lection of rostra from almost all levels. The Upper Pliensba-chian beds yielded big to medium-sized, subcylindrical andconical rostra from the genera Passaloteuthis, Catateuthis,Orthobelus and Brachybelus. Medium-sized cylindrical, sub-cylindrical and subconical to slightly fusiform specimens ofthe genera Dactyloteuthis, Gastrobelus, Acrocoelites and Sal-pingoteuthis were found in the Toarcian parts of the sec-tions. Small- to medium-sized cylindrical and conicalrostra of the genera Belemnopsis, Holcobelus, Nannobelusand Megateuthis were obtained from the Aalenian andLower Bajocian beds.

Belemnites were carefully examined and documentedfor evidence of post-mortal biogenic and abiogenicreworking. They were cut for thin and polished thick sec-tions, usually at 0.5 cm below the alveolar chambers.Samples were then examined under plane/polarized lightand cathodoluminescence. The cathodoluminescenceobservations have been carried out with a Cold CathodeLuminescence instrument ‘‘Technosyn” (Model 8200MkII). After careful microscopic observations, and selec-tion of the target areas, the altered parts of belemnitespecimens were removed with diamond cutting tools,and the samples were cleaned with alcohol and distilledwater and dried at room temperature. The non-lumines-cent areas were sampled (about 50–60 mg powder) forelemental and isotope analyzes with a dental drill. Ele-mental measurements of the belemnite calcite were per-formed with an ICP-OES instrument (Perkin Elmer,Optima 3000), following microwave digestion (MLS Ethos1600). The analytical precision was better than ±5%. Sta-ble isotope data were collected after phosphoric acid dis-solution in an online carbonate preparation connected toa FINNIGAN MAT 251/252 Thermal Ionization Mass Spec-trometer (TIMS). Reproducibility of replicate analyses ofstandards and samples was generally better than 0.1‰

for both O and C isotope ratios. The results are reportedusing the conventional d notation in ‰ relative to theVienna Pee Dee Belemnite standard – (PDB) (Tables 1–3, Fig. 4). All analyzes have been carried out at the Fac-ulty of Geosciences of the University of Bremen(Germany).

Table 1Concentration of major and trace elements, calculated Mg/Ca, Sr/Ca and Sr/Mn ratios, C and O isotope data and palaeotemperature values, obtained from the belemnites of section Beledie Han

Substage Ammonite zone Sample CaO(wt%)

Mg(ppm)

Sr (ppm) Mg/Ca(mmol/mol)

Sr/Ca(mmol/mol)

Fe(ppm)

Mn(ppm)

Sr/Mn d13C(PDB) ‰

d18O(PDB) ‰

T (�C)(dw = �1 ‰ SMOW)

Upper Toarcian Aalensis Be-26 39.30 2130 1350 8.93 1.57 79 4.4 306.8 +1.07 �1.65 18.6Be-25 39.33 2440 1426 10.22 1.65 68 5.9 241.6 +1.68 �0.81 14.9

Pseudoradiosa Be-24 38.85 2700 1382 11.45 1.62 199 13.5 102.3 +1.04 �1.35 17.2Be-23a* 38.10 2100 1152 9.09 1.38 321 39.7 29.0 +0.64 �1.48 17.8

Fallaciosum Be-22 39.46 2560 1315 10.70 1.52 141 11.8 111.4 +1.17 �1.66 18.6Be-21 39.05 2430 1164 10.25 1.35 121 9.4 123.8 +0.20 �1.70 18.8Be-20 38.76 2110 1075 8.98 1.26 195 20.4 52.6 -0.46 �1.77 19.1

Thouarsense Be-19a* 38.43 2510 1348 10.76 1.60 598 51.4 26.2 +1.71 �1.34 17.2Be-19 38.95 2350 1248 9.94 1.46 275 21.0 59.4 +1.33 �1.42 17.6Be-18 38.67 2720 1248 11.60 1.47 64 6.0 208 +0.38 �2.10 20.6Be-18a 38.67 2660 1352 11.34 1.60 200 27.3 49.5 +1.12 �1.31 17.1Be-17 39.30 2600 1501 10.90 1.74 161 9.3 161.3 +1.05 �1.17 16.5Be-16b 38.72 2670 1501 11.37 1.77 118 13.4 112 +1.82 �1.15 16.4Be-16a0* 38.66 2670 1331 11.38 1.57 357 35.3 37.7 +1.00 �1.47 17.8Be-16a* 38.58 2280 1267 9.75 1.50 730 37.1 34.2 +1.74 �1.43 17.6

Variabilis Be-16* 38.95 2760 1221 11.68 1.43 1037 91.7 13.3 +0.47 �1.53 18.0Be-15 38.88 2550 1380 10.81 1.62 149 11.3 122.1 +1.60 �0.96 15.5Be-14 39.12 3040 1373 12.65 1.60 101 8.5 161.5 +1.83 �1.50 17.9Be-13a 38.96 2890 1222 12.23 1.43 266 18.7 65.3 +1.80 �1.31 17.1

Lower Toarcian Bifrons Be-13 38.97 3050 1399 12.90 1.64 128 9.2 152 +1.57 �1.05 15.9Be-12 39.05 2950 1300 12.45 1.52 154 12.1 107.4 +1.78 �1.32 17.1Be-11* 38.78 2520 1314 10.71 1.54 536 33.6 39.1 +1.02 �1.32 17.1Be-10a 38.38 3290 1344 14.13 1.60 115 10.1 133 +2.11 �1.46 17.7Be-10 39.17 2750 1266 11.57 1.47 93 9.7 130.5 +1.14 �1.40 17.5

Falciferum Be-9 38.56 2870 1361 12.26 1.61 128 12.8 106.3 +1.30 �1.66 18.6Be-8 38.96 3410 1307 14.43 1.53 99 10.4 125.6 +1.97 �1.43 17.6Be-7a 38.84 2570 1259 10.91 1.48 113 13.7 91.8 +1.89 �1.39 17.4Be-7 39.27 2700 1317 11.33 1.53 120 9.3 141.6 +1.35 �1.60 18.4Be-6 39.04 3200 1370 13.51 1.60 212 19.2 71.4 +1.59 �1.36 17.3Be-5 39.22 3440 1295 14.46 1.50 177 13.4 96.6 +2.13 �1.62 18.5

Tenuicostatum Be-4* 38.87 3170 1351 13.43 1.59 445 33.2 40.7 +2.65 �1.38 17.4Be-3 39.10 3880 1312 16.08 1.53 101 9.1 144.1 +1.68 �1.43 17.6Be-2 39.24 3750 1245 15.75 1.45 167 15.9 78.3 +2.45 �1.71 18.9Be-1 39.06 3850 1205 16.25 1.40 278 15.5 77.7 +3.75 �2.09 20.6Be-0 38.95 3660 1204 15.49 1.41 185 18.9 63.7 +2.69 �1.73 18.9

* Samples, which are not used for reconstruction of the palaeotemperature trend for this section.

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1700700 900 1100 1300 15000

10

20

30

40

50

60

Mn

(ppm

)

Sr (ppm)

A

1000 2000 3000 4000 5000 6000500

700

900

1100

1300

1500

1700

Sr (p

pm)

Mg (ppm)

B

10 20 30 40 500

100

200

300

400

500

600

Mn (ppm)

Fe (p

pm)

C rostra from Beledie Hansection

rostra from Dobravitsa-1section

rostra from Dobravitsa-2 section

concentrations of the modern low-Mgmarine calcite shells

Fig. 3. Plots of the concentrations of Mn, Sr, Mg and Fe, obtained from belemnite rostra, compared with the data for modern marine low-Mg calcite shells(grey areas): A: Mn vs. Sr, B: Sr vs. Mg, C: Fe vs. Mn.

2850 L. Metodiev, E. Koleva-Rekalova / Applied Geochemistry 23 (2008) 2845–2856

4. Results and discussion

4.1. Taphonomic study

It was evident that some of the belemnite specimensexamined in this study have quite complicated post-mortalhistory (Metodiev and Koleva-Rekalova, 2005). It was clearthat many of the rostra from the Dobravitsa-1 section haveundergone considerable taphonomic reworking. The evi-dence for that appeared from the presence of frequentbioerosion and encrustation traces on the belemnite skele-tons with random location. Some of the rostra displayedprimary corrosion suggesting passage through the diges-tive system of a large predator. The comparative investiga-tion of the burrows and alveolar infillings of the rostra andtheir host rocks has shown a remarkable contrast betweenthe amount of borings, alveolar chambers and surroundingsediments. For example, some of the host shales and marlswhich are rich in clay minerals, stained by Fe oxides andhydroxides, and containing terrigenous components, fre-quently include belemnites having alveolar chambers filledwith fine-bioclastic limestones (mudstones and wacke-stones without terrigenous components) and iron ooidallimestones (packstones). This suggested that these rostra

have initially been fossilized in a different environmentand then exhumed, displaced and reburied. The degree oftaphonomic elaboration of belemnites from section Dobra-vitsa-2 was found to be less and probably indicates simpletaphonomic resedimentation. In contrast, the depositionalenvironment at the Beledie Han section appeared to be suf-ficiently stable to preserve the rostra from change and theyhave been fossilized in situ. Regardless of the post mortemchanges identified it was found that almost all specimenshad fresh and homogeneous interiors with enough mate-rial for geochemical and isotope measurements.

4.2. Cathodoluminescence and geochemical tests

Cathodoluminescence investigations revealed the pres-ence of narrow altered (luminescent) parts of the exam-ined rostra, usually detected on their exteriors, along theapical lines and alveolar chambers, as well as along somegrowth rings and borings, and they contain wide pristine(non-luminescent) areas (Metodiev and Koleva-Rekalova,2003a; Koleva-Rekalova and Metodiev, 2005). Further evi-dence for the state of preservation of the material has beenobtained by ICP-measurements of Ca, Mg, Fe, Mn and Srconcentrations of the non-luminescent areas. According

toR

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Fig.Plien

4.

Tempo

sbachian

Section Dobravitsa-1

Ammonitezones

Discites

Concavum

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Aale

nian

Toar

cian

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chia

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Murchisonae

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Isotope record

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Isotope recordSection Dobravitsa-2

Ammonitezones

Upp

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Low

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Pseudoradiosa

Dispansum

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Variabilis

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Section Beledie HanIsotope record

+20-2 +4

+20-2 +4

-4 -2 0 10 13 16

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amol)

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2852 L. Metodiev, E. Koleva-Rekalova / Applied Geochemistry 23 (2008) 2845–2856

investigation are listed in Tables 1–3. The samples yieldedCaO contents between 38.1 and 39.5 wt% and Mg contentsfrom 1510 to 4300 ppm that confirms its low-Mg charac-ter. The contents of Sr range from 1020 to 1650 ppm, Fe-contents vary from 18 to 250 ppm (except for a few sam-ples), Mn values are mainly less than 50 ppm, and the Sr/Mn ratio is almost always over 80. A comparison of theMn, Sr, Mg and Fe concentrations measured in the rostrawith the values of these elements encountered in the mod-ern marine low-Mg shells (Milliman, 1974; Morrison andBrand, 1986; Podlaha et al., 1998) demonstrated that thebulk of the samples overlap their modern counterparts(Fig. 3). Such clustering of data suggests that most of thebelemnites collected for this study have retained their ori-ginal chemical composition. The samples providing ele-mental concentrations and ratios outside the adoptedlimits for good chemical preservation were eliminatedfrom the dataset for the isotope and palaeotemperaturetrends.

4.3. Oxygen and carbon isotope record andpalaeotemperature trends

The O and C isotope record derived from the non-lumi-nescent parts of the belemnites is shown against theammonite zonal subdivisions in Tables 1–3 and Fig. 4.The bulk data clearly reveal once more the good preserva-tion of specimens used in this study, having d18O < �4(PDB)‰ and d13C > �0.5 (PDB)‰ as established previously(Rosales et al., 2003, 2004). The Toarcian rostra from theBeledie Han section yielded d18O values from �0.81 to�2.09 (PDB)‰ and d13C values from �0.46 to +3.75(PDB)‰ (Table 1). These comprise temporal trends with aclear coeval minimum of d18O and maximum of d13C inthe Lower Toarcian, at the base of the Tenuicostatum Zone(Fig. 4). Similar O and C isotope values were obtained fromthe Upper Pliensbachian, Aalenian and the Lower Bajocianbelemnites of the Dobravitsa-1 section (Tables 2 and 3).The data derived from these intervals displayed slightlybigger stratigraphic variations, but no distinct oscillationsare superimposed onto the overall isotopic trend (Fig. 4).The Upper Pliensbachian specimens gave d18O values rang-ing between �0.47 and �2.16 (PDB)‰ and d13C values thatvary from +0.74 to +2.73 (PDB)‰. The Aalenian and LowerBajocian samples produced d18O values from �1.0 to �2.86(PDB)‰ and d13C values between +0.02 and +1.47 (PDB)‰.Toarcian rostra from the Dobravitsa-1 and Dobravitsa-2sections yielded lighter d18O and moderately heavier d13Cvalues in comparison with the rest of the dataset (Tables2 and 3). The d18O range between�1.01 and �3.94 (PDB)‰and d13C are from�0.03 to +3.21 (PDB)‰. The extreme val-ues of O and C isotope records were obtained from coevalsamples of the Lower Toarcian as well. They were mea-sured from the samples coming from the lowermost partof the Tenuicostatum Zone (close to the Pliensbachian-Toarcian boundary) as well as from the upper part of theFalciferum Zone and from the lower part of the BifronsZone (Fig. 4).

In Tables 1–3 seawater palaeotemperatures calculatedon the basis of d18O values are given. The main factor inthese palaeotemperature estimations was the O isotope

composition of the Jurassic seawater. It is suggested thatthe Jurassic was a ‘‘greenhouse” period and a dwater valueof -1‰ SMOW is reasonable for temperature calculations(e.g., Price and Sellwood, 1997; Rosales et al., 2003). Theequation of Hays and Grossman (1991) was used:

Tð�CÞ ¼ 15:7� 4:36ðdcal � dwÞ þ 0:12ðdcal � dwÞ2

where T is temperature, dcal is d18O composition measuredfrom the belemnite calcite with respect to the PDB stan-dard and dw is d18O ratio of the seawater from which thecalcite precipitated with respect to the SMOW standard.A background dwater value was adopted for the ‘‘non-gla-cial” seawater of �1‰ SMOW, as applied above for theJurassic.

According to the equation, the belemnite calcite sam-ples from the Toarcian of the Beledie Han section indicaterelatively warm seawater and give a mean temperature of17.7 �C. The highest values are from the rostra of the LowerToarcian Tenuicostatum Zone (20.6 �C). The palaeotemper-atures that were calculated from the rostra from sectionsDobravitsa-1 and Dobravitsa-2 are generally higher (mean23.1 �C). The values obtained from the Upper Pliensbachiansamples range between 13.4 �C and 20.9 �C (mean 16.9 �C).The Toarcian estimates vary from 17.9 �C to 29.6 �C(Dobravitsa-1 section) and from 19.6 �C to 26.8 �C (Dobra-vitsa-2 section). The Aalenian and the Lower Bajocian spec-imens from the Dobravitsa-1 section gave a variationbetween 15.8 �C and 23.1 �C (mean 17.9 �C). A temperaturerise is again detected into the Lower Toarcian, which ishowever divided into two peaks: close to the Pliensba-chian-Toarcian boundary in the Tenuicostatum Zone(25.2 �C) and in the Falciferum Zone (29.6 �C). Smaller tem-perature rise occurs also into the Lower Toarcian BifronsZone (24.8 �C).

Many authors have suggested that the O-isotope com-position of the skeletal calcite depends not only on temper-ature, but also on salinity of the ambient seawater (e.g.,S�len et al., 1996; Rosales et al., 2003, 2004). It has beensuggested that the Mg/Ca and Sr/Ca ratios of biogenic car-bonates are well correlated with palaeotemperature(McArthur et al., 2000; Bailey et al., 2003; Rosales et al.,2003, 2004). Comparison of Mg/Ca and Sr/Ca ratios andd18O data derived from the present samples (Tables 1–3)revealed an apparent coincidence of high Mg/Ca ratiosand lower d18O values (high seawater temperatures)(Fig. 4). Conversely, low Mg/Ca ratios correlate with higherd18O values (i.e., lower temperatures). Within the BeledieHan section, the highest Mg/Ca value (16.25 mmol/mol)concurs with the lowest d18O value �2.09 (PDB)‰ (Tenui-costatum Zone, Table 1). In section Dobravitsa-1, the high-est Mg/Ca ratios are associated again with the lowest Oisotope values, for example specimen Do-10 yielded Mg/Ca of 17.30 mmol/mol and a d18O of �3.94 (PDB)‰(Fig. 4). The Sr/Ca ratios, which range between 1.19 and1.91 mmol/mol, showed a weak correlation with d18O (Ta-bles 1–3).

The observed variability both in d18O and Mg/Ca sug-gests that seawater temperature estimates based on d18Odata are suitable for reconstruction of a palaeotemperaturetrend of the studied Lower-Middle Jurassic successions. It

Table 2Concentration of major and trace elements, calculated Mg/Ca, Sr/Ca and Sr/Mn ratios, C and O isotope data and palaeotemperature values, obtained from the belemnites of section Dobravitsa-1

Stage Ammonite zone Sample CaO(wt%)

Mg(ppm)

Sr(ppm)

Mg/Ca(mmol/mol)

Sr/Ca(mmol/mol)

Fe(ppm)

Mn(ppm)

Sr/Mn d13C(PDB) ‰

d18O(PDB) ‰

T (�C)(dw = �1 ‰ SMOW)

Aalenianand Bajocian

Discites Do-20a 38.85 1930 1170 8.19 1.37 222 5 234 +1.06 �1.01 15.8Do-20 38.85 3310 1550 14.04 1.82 116 12 129.2 +1.42 �2.62 23.1

Concavum Do-19s 38.80 2290 1200 9.73 1.40 90 11 109 +0.78 �1.52 18.0Murchisonae Do-18a 38.70 2840 1150 12.10 1.36 163 7 164.3 +0.22 �1.89 19.7Opalinum Do-18 38.75 2230 1340 9.48 1.57 120 6 223.3 +0.97 �1.07 16.0

Do-17a 39.00 2380 1440 10.06 1.69 29 2 720 +1.15 �1.17 16.5Do-16 39.05 2260 1310 9.54 1.52 56 3 436.6 +1.05 �1.24 16.8

Toarcian Aalensis Do-15b 38.90 2260 1490 9.57 1.75 24 2 745 +1.82 �1.51 17.9Pseudoradiosa Do-14 39.10 2490 1480 10.50 1.72 67 5 296 +1.00 �1.55 18.1Dispansum Do-14a 39.25 2580 1350 10.83 1.57 24 2 675 +0.14 �2.81 23.9Fallaciosum – Bifrons Do-13c 39.25 2810 1350 11.80 1.57 30 6 225 +1.49 �2.73 23.6

Do-12 39.35 3040 1650 12.73 1.91 44 3 550 +2.27 �2.98 24.8Do-12a 39.40 2300 1340 9.62 1.55 75 4 335 +0.51 �2.02 20.3

Falciferum Tenuicostatum Do-11 39.20 3390 1320 14.28 1.53 415 13 101.5 +0.96 �2.89 24.4Do-10 39.05 4100 1310 17.30 1.53 142 9 145.5 +3.21 �3.94 29.6Do-9 39.20 4300 1300 18.09 1.51 170 11 118.2 +2.32 �3.50 27.4Do-7 39.20 2990 1390 12.60 1.62 229 12 115.8 +1.49 �3.25 26.1Do-6b 39.20 2120 1310 8.93 1.52 59 5 262 +2.59 �1.21 16.6Do-6a 39.20 2560 1160 10.77 1.35 217 12 96.7 +1.12 �2.33 21.7Do-6 39.30 2340 1240 9.81 1.44 114 6 206.7 +2.55 �3.06 25.2

Pliensbachian Spinatum Do-5 39.40 2200 1300 9.21 1.51 111 8 162.5 +1.57 �1.37 17.3Do-4a 39.15 1850 1170 7.79 1.34 51 4 292.5 +0.74 �0.92 15.4Do-4 39.10 1940 1020 8.18 1.19 159 14 72.9 +1.77 �2.16 20.9Do-2a 39.40 1680 1140 7.03 1.32 33 3 380 +1.18 �1.65 18.6Do-2 39.15 1910 1170 8.34 1.36 113 8 146.3 +1.18 �1.62 18.5Do-1c 39.15 1520 1020 6.40 1.19 47 6 170 +1.36 �0.53 13.7Do-1b 39.20 1510 1100 6.34 1.28 18 2 550 +1.74 �0.47 13.4Do-1 39.05 1930 1320 8.15 1.54 46 9 146.7 +2.73 �1.29 16.9Do-1ds 39.00 2620 1380 10.96 1.60 42 3 460 +1.09 �1.54 18.1Do-0 39.05 1820 1200 7.68 1.59 53 5 240 +3.08 �1.18 16.5

L.Metodiev,E.K

oleva-Rekalova

/Applied

Geochem

istry23

(2008)2845–

28562853

Tab

le3

Conc

entr

atio

nof

maj

oran

dtr

ace

elem

ents

,cal

cula

ted

Mg/

Ca,S

r/Ca

and

Sr/M

nra

tios

,Can

dO

isot

ope

data

and

pala

eote

mpe

ratu

reva

lues

,obt

aine

dfr

omth

ebe

lem

nite

sof

sect

ion

Dob

ravi

tsa-

2

Subs

tage

Am

mon

ite

zon

eSa

mpl

eC

aO(w

t%)

Mg

(ppm

)Sr (p

pm)

Mg/

Ca

(mm

ol/m

ol)

Sr/C

a(m

mol

/mol

)Fe (p

pm)

Mn

(ppm

)Sr

/Mn

d13C

(PD

B)

‰

d18O

(PD

B)

‰

T(�C

)(d

w=�

1‰

SMO

W)

Upp

erTo

arci

anFa

llac

iosu

mD

2-B

13D

2-B

12a*

38.8

523

8012

1010

.10

1.43

597

172.

9�

0.97

�1.

8719

.638

.95

3150

1480

13.3

41.

7343

535

42.3

+1.3

3�

2.80

23.9

Thou

arse

nse

D2-

B12

D2-

B11

s38

.85

2590

1420

10.9

91.

6719

614

101.

4+1

.51

�2.

9424

.638

.95

2980

1410

12.6

21.

6519

716

88.1

+1.1

9�

2.54

22.7

D2-

B10

38.9

530

7015

9013

.00

1.86

637

227.

1+2

.79

�2.

1420

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aria

bili

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938

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2550

1400

10.8

91.

6515

914

100

+1.6

7�

2.86

24.2

D2-

B7s

39.3

535

7014

8014

.95

1.71

133

1212

3.3

+0.8

6�

2.99

24.9

D2-

B6

39.2

029

5014

3012

.40

1.67

415

286

+1.5

7�

2.53

22.7

Bif

ron

sD

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5*39

.15

3810

1270

16.0

41.

4793

2257

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.61

�2.

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2700

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8424

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138

.80

3220

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13.6

41.

5377

914

5.5

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6�

3.41

26.8

*Sa

mpl

es,w

hic

har

en

otu

sed

for

reco

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ion

ofth

epa

laeo

tem

pera

ture

tren

dfo

rth

isse

ctio

n.

2854 L. Metodiev, E. Koleva-Rekalova / Applied Geochemistry 23 (2008) 2845–2856

is noteworthy however that calculated absolute tempera-tures from the Toarcian in the Beledie Han section andthose from sections Dobravitsa-1 and Dobravitsa-2 differfrom each other (Tables 1–3, Fig. 4). This is attributed todifferent depositional settings of the sections sampled.The iron ooidal limestones from the Beledie Han sectionare accumulated in a shallow, well oxygenated deposi-tional environment, whereas the shale-marlstone-lime-stone sequences from the other two sections aredeposited in deeper (offshore) parts of the basin (Metodievand Koleva-Rekalova, 2005; Koleva-Rekalova et al., 2006).However, evidence for rapid warming during the EarlyToarcian is supported by the isotope data. It is character-ized at first by a period of mean seawater temperaturesof 16.9oC (Spinatum Zone, Late Pliensbachian), followedby warmer conditions early in the Tenuicostatum Zone(25.2 �C) with a maximum in the Falciferum Zone(29.6 �C) and then decreasing during the Bifrons Zone(24.8 �C). It is suggested that this warming was of a gradualnature beginning earlier in the Western Balkan mountainsbasin, than in the other Peri-Tethyan epicontinental basins(Jenkyns and Clayton, 1986; S�len et al., 1996; Schmid-Röhl et al., 2002; Rosales et al., 2003, 2004). Post BifronsZone temperatures slowly decreased during the Late Toar-cian, Aalenian and Early Bajocian (mean 17.9 �C), and re-turned to the values of the Late Pliensbachian.

4.4. The evidence for the Early Toarcian anoxic event from thesections examined

The detection of an Early Toarcian seawater tempera-ture rise has previously been used as supporting evidenceof the Early Toarcian anoxic event, following a global Teth-yan transgression (e.g., Hallam and Bradshaw, 1979; Jen-kyns, 1988; Hallam, 2001; Rosales et al., 2001; Jenkynset al., 2002). Besides elevated temperatures (correspondingto lower d18O values and O2 deficiency), this event is char-acterized by higher d13Ccarb values. The positive C isotopeexcursions are interpreted as a response to abnormallyhigh storage of 12C organic C. Under normal conditions, or-ganic C is rapidly oxidized, but if significant quantitieswere suddenly buried, water would be left relatively en-riched in 13C (Jenkyns and Clayton, 1986). McArthur et al.(2000) proposed that the same positive peak is ascribedto the removal of a large amount of isotopically light C asorganic matter from the ocean into black shales, thus leav-ing oceanic C isotopically heavy. This positive C isotopesignature can be rapidly transmitted to the inorganicallyprecipitated carbonates and skeletal calcite (Jenkyns andClayton, 1997). The latter is of particular importance tothe present study, because typical organic-rich black shalesdo not occur within the sections examined (Metodiev andKoleva-Rekalova, 2003b, 2006). Three Early Toarcian epi-sodes were recorded based on d13C and d18O belemnitedata which are manifested by the coexistence of the heavi-est d13C and lightest d18O values (Tables 1–3, Fig. 4). Thefirst is recognized close to the Pliensbachian-Toarcianboundary (basal part of the Tenuicostatum Zone), whered13C and d18O values of +3.75 (PDB)‰ and �2.09 (PDB)‰,respectively, (sample Be-1, section Beledie Han) were mea-sured, and +2.55 (PDB) ‰ for d13C and �3.06 (PDB) ‰ for

L. Metodiev, E. Koleva-Rekalova / Applied Geochemistry 23 (2008) 2845–2856 2855

d18O (sample Do-6, section Dobravitsa-1) were received. Itis proposed that the first episode is associated with a localtransgression, further supported by sedimentological evi-dence (Koleva-Rekalova et al., 2006). The first episode sug-gests that a low O2 environment existed within theTenuicostatum Zone as previously suggested by Jenkynset al. (1991). The second episode is located in the upperpart of the Falciferum Zone of the Dobravitsa-1 section(Fig. 4). Sample Do-10, derived from this interval, dis-played +3.21 (PDB)‰ for d13C and �3.94 (PDB)‰ ford18O. This may reflects the impact of the Early Toarcian an-oxic event, well documented in many Toarcian localities inWestern Europe (e.g. Jenkyns, 1988; Jenkyns and Clayton,1997; S�len et al., 1996; Rosales et al., 2001; Schmid-Röhl et al., 2002). The third episode is recorded in the low-er part of the Bifrons Zone of section Dobravitsa-1, but is ofa less distinct character compared with other two exam-ples. It is documented by values of +2.27 (PDB)‰ for d13Cand �2.98 (PDB) ‰ for d18O (sample Do-12a). It is likelythat this episode reflects the existence of anoxic conditionsduring the Bifrons Zone, as described from SouthwesternGermany (Oschmann, 1995).

5. Conclusions

This study presents new O and C isotope data from bel-emnite rostra collected from 3 ammonite-calibrated sec-tions of the Western Balkan mountains, Bulgaria. Thesurrounding sediments of the belemnites are depositedinto a basin, which is probably a part of a broad epiconti-nental sea that covered the northern margin of centralTethys during the Early-Middle Jurassic. The specimenswere found to be sufficiently well preserved for isotopestudy. The O and C isotope curves displayed coeval positiveC isotope excursions and negative O isotope shifts into theLower Toarcian. They were interpreted to be evidence for apalaeotemperature rise at the beginning of the Toarcian(within the Tenuicostatum Zone) with a maximum in theupper part of the Falciferum Zone and finishing in the low-er part of the Bifrons Zone. Reported isotope excursionswere also accepted to be a manifestation of the impact ofthe Early Toarcian anoxic event, following a global Tethyantransgression during the Early Toarcian.

Acknowledgements

This work was made possible by the EU-Project Paleo-studies (Contract No. HPRI-CT-2001-00124). Analythicalstudy has been carried out at the University of Bremen,Germany. Field work and sample collection was supportedfinancially by the National Science Fund, Bulgaria (ProjectNZ-1005/00). We would like to express our warmestthanks to Dr. Holger Kuhlmann and Dr. Frank Lamy (Paleo-studies Project Managers), Dr. Martin Kölling, Dr. HenrikHecht and Dr. Monika Segl for ICP analyses and isotopemeasurements. Special thanks to Mr. Jens Wendler andDr. Hartmut Mai for the access to the cathodoluminoscopeas well as to Mrs. Angelika Freesemann for help with thesample preparation. Mr. Robert Chandler and Dr. Ivan Sa-

vov are acknowledged for improving the language con-struction of the manuscript.

References

Bailey, T.K., Rosenthal, Y., McArthur, J.M., van de Schootburge, B.,Thirlwall, M.F., 2003. Paleoceanographic changes of the latePliensbachian-early Toarcian interval: a possible link to thegenesis of Oceanic Anoxic Event. Earth Planet. Sci. Lett. 212, 307–320.

Bassoulet, J.P., Elmi, S., Poisson, A., Ricou, L.E., Cecca, F., Bellion, Y.,Guiraud, R., Baudin, F., 1993. Mid Toarcian (184–182 Ma). In:Dercourt, J., Ricou, L.E., Vrielynck, B. (Eds.), Atlas TethysPaleoenvironmental Maps, Explanatory Notes. Gauthier-Villars,Paris, pp. 63–80.

Broecker, W.S., Peng, T.H., 1982. Tracers in the Sea. Palisades. EldigioPress, New York.

Dabovski, C., Boyanov, I., Khrischev, Kh., Nikolov, T., Sapunov, I., Yanev, Y.,Zagorchev, I., 2002. Structure and Alpine evolution of Bulgaria. Geol.Balc. 32, 9–15.

Fourcade, E., Azéma, J., Bassoulet, J.P., Cecca, F., Dercourt, J., 1995.Paleogeography and paleoenvironments of the Tethyan Realmduring the Jurassic breakup of Pangea. In: Nairn, A.E.M., Ricou, L.-E.,Vrielynck, B., Dercourt, J. (Eds.), The Ocean Basins and Margins, vol. 8.Plenum Press, New York, pp. 191–214.

Hallam, A., 2001. A review of the broad pattern of Jurassic sea-levelchanges and their possible causes in the light of current knowledge.Palaeogeog. Palaeoclimatol. Palaeoecol. 167, 23–37.

Hallam, A., Bradshaw, M.J., 1979. Bituminous shales and oolitic ironstonesas indicators of transgressions and regressions. J. Geol. Soc. (London)136, 157–164.

Hays, P.D., Grossman, E.L., 1991. Oxygen isotopes in meteoric calcitesediments as indicators of continental palaeoclimate. Geology 19,441–444.

Jenkyns, H.C., 1988. The Early Toarcian (Jurassic) anoxic event:stratigraphic, sedimentary and geochemical evidence. Am. J. Sci.288, 101–151.

Jenkyns, H.C., Clayton, C.J., 1986. Black shales and carbon isotopes inpelagic sediments from Tethyan lower Jurassic. Sedimentology 33,87–106.

Jenkyns, H.C., Clayton, C.J., 1997. Lower Jurassic epicontinental carbonatesand mudstones from England and Wales: chemostratigraphic signalsand the early Toarcian anoxic event. Sedimentology 44, 687–706.

Jenkyns, H.C., Géczy, B., Marshall, J.D., 1991. Jurassic manganesecarbonates of central Europe and the Early Toarcian anoxic event. J.Geol. 99, 137–149.

Jenkyns, H.C., Jones, C.E., Gröcke, D.R., Hesselbo, S.P., Parkinson, D.N.,2002. Chemostratigraphy of the Jurassic system: applications,limitations and implications for palaeooceanography. J. Geol. Soc.(London) 159, 351–378.

Koleva-Rekalova, E., Metodiev, L., 2005. Preservational features of theUpper Pliensbachian-Lower Bajocian belemnite rostra from twoclayey-limestone successions of the West Balkan Mts (Bulgaria)Cathodoluminescence and geochemical tests. C. R. Acad. Bulg. Sci. 58,789–794.

Koleva-Rekalova, E., Metodiev, L., Mladenova, V., 2006. Cyclic model ofthe Toarcian iron ooidal limestone succession at the Beledie Hansection, Western Balkan Mts, Bulgaria. C. R. Acad. Bulg. Sci. 59, 51–56.

McArthur, J.M., Donovan, D.T., Thirlwall, M.F., Fouke, B.W., Mattey, D.,2000. Strontium isotope profile of the early Toarcian (Jurassic)oceanic anoxic event, the duration of ammonite biozones, andbelemnite palaeotemperatures. Earth Planet. Sci. Lett. 179, 269–285.

Metodiev, L., Koleva-Rekalova, E., 2003a. Cathodoluminescence test ofbelemnite rostra - first condition for precise isotope and chemicalanalyses: an example from the Toarcian at the section near the villageof Beledie Han (Western Balkan Mts), Bulgaria. C. R. Acad. Bulg. Sci.56, 61–66.

Metodiev, L., Koleva-Rekalova, E., 2003b. TOC-analysis of the Toarcianfrom the section at the village of Beledie Han (Western Balkan Mts,Bulgaria). C. R. Acad. Bulg. Sci. 56, 79–84.

Metodiev, L., Koleva-Rekalova, E., 2005. Preservational features of theUpper Pliensbachian-Lower Bajocian belemnite rostra from twoclayey-limestone successions of the west Balkan Mts (Bulgaria).Taphonomic and sedimentological evidence. C. R. Acad. Bulg. Sci. 58,705–710.

2856 L. Metodiev, E. Koleva-Rekalova / Applied Geochemistry 23 (2008) 2845–2856

Metodiev, L., Koleva-Rekalova, E., 2006. Carbon and oxygen isotope recordof lower-middle Jurassic belemnites and rocks from the WesternBalkan Mts. (Bulgaria). In: Dembicz, K., Prasziker, T., Wierzbowski, A.(Eds.), Volumina Jurassica 4, 188–190, 7th Internationall CongressJurassic system, session 4: integrated Stratigraphy (Kraków, 2006).Institute of Geology, Warsaw University, Warsaw.

Metodiev, L., Ivanova, D., Koleva-Rekalova, E., 2005. Biostratigraphy of theToarcian in the section at the village of Beledie Han (Western BalkanMts), Bulgaria. C. R. Acad. Bulg. Sci. 58, 39–46.

Milliman, J.D., 1974. Recent Sedimentary Carbonates. Part I. Springer,Berlin.

Morrison, J.O., Brand, U., 1986. Geochemistry of recent marineinvertebrates. Geosci. Canada 13, 237–254.

O’Neil, J.R., Clayton, R.N., Mayeda, T.K., 1969. Oxygen isotope fractionationin divalent metal carbonates. J. Chem. Phys. 51, 5547–5558.

Oschmann, W., 1995. Posidonia Shales (Toarcian, Lower Jurassic) in SWGermany. Europal 8, 44–53.

Podlaha, O.G., Mutterlose, J., Veizer, J., 1998. Preservation of d18 O and d13

C in belemnite rostra from the Jurassic/Early Cretaceous successions.Am. J. Sci. 298, 324–347.

Price, G.D., Sellwood, B.W., 1997. ‘‘Warm” palaeotemperatures from highLate Jurassic palaeolatitudes (Falkland Plateau): Ecological,environmental or diagenetic controls? Palaeogeog. Palaeoclimatol.Palaeoecol. 129, 315–327.

Rosales, I., Quesada, S., Robles, S., 2001. Primary and diagenetic isotopesignals in fossils and hemipelagic carbonates: the Lower Jurassic ofnorthern Spain. Sedimentol. 48, 1149–1169.

Rosales, I., Quesada, S., Robles, S., 2003. Paleotemperature variations ofEarly Jurassic seawater recorded in geochemical trends of belemnitesfrom the Basque-Cantabrian basin, northern Spain. Palaeogeog.Palaeoclimatol. Palaeoecol. 203, 253–275.

Rosales, I., Robles, S., Quesada, S., 2004. Elemental and oxygen isotopecomposition of Early Jurassic belemnites: salinity vs. temperaturesignals. J. Sed. Res. 74, 342–354.

S�len, G., Doyle, P., Talbot, M., 1996. Stable-isotope analyses of belemniterostra from the Whitby Mudstone Fm., England: Surface waterconditions during deposition of a marine black shale. Palaios 11,97–117.

Sapunov, I.G., Tchoumatchenco, P.V., Mitov, P.A., 1988. Jurassicdevelopment of Northwest Bulgaria. Geol. Balc. 18, 3–82 (in Russian).

Schmid-Röhl, A., Röhl, H.J., Oschmann, W., Frimmel, A., Schwark, L., 2002.Palaeoenvironmental reconstruction of Lower Toarcianepicontinental black shales (Posidonia Shale, SW Germany): globalversus regional control. Géobios 35, 13–20.

Teis, R.V., Naidin, D.P., Stoyanova-Vergilova, M., 1975. Palaeotemperaturesof the Jurassic and Early Cretaceous of Bulgaria according to theIsotope Oxygen Composition of Belemnite Guards. Geol. Balc. 5,65–80.