Primary productivity and early diagenesis in the Toarcian Tethys on

the example of the Mn-rich black shales of the Sachrang Formation,

Northern Calcareous Alps

OSKAR EBLI1, ISTVAÂ N VETOÂ Â 2*, HARALD LOBITZER3,CSANAÂ D SAJGOÂ 4, ATTILA DEMEÂ NY4 and MAGDOLNA HETEÂ NYI5

1PalaÈ ontologische Institut, D-80133, Richard-Wagner Str. 10, MuÈ nchen, Germany, 2Geological Instituteof Hungary, Stefa nia u t 14, H-1143 Budapest, Hungary, 3Geologische Bundesanstalt, Rasumofskygasse23, A-1031 Wien, Austria, 4Laboratory for Geochemical Research, Budao  rsi u t 45, H-1112 Budapest,

Hungary and 5Jo zsef Attila University, Egyetem u. 2, H-6722 Szeged, Hungary

AbstractÐOrganic, inorganic and isotope geochemical investigations have been performed on 25samples Ð 20 laminated marl samples and 5 re-sediment samples Ð taken from a 27 m thick blackshale section of early Toarcian age, cropping out in the Northern Calcareous Alps. On the basis ofMnO, Fe, organic carbon (TOC) contents and Hydrogen Index (HI) values, the laminated marls of thesection can be divided into two parts. TOC contents and HI values are below 2% and 500 mg HC/gTOC in the lower, Mn-rich (and Fe-rich) part (MnO between 2 and 16%, Fe between 3.9 and 6.4%),while they range between 5 and 9% and 600±700 mg HC/g TOC, respectively in the upper, Mn-poor(and Fe-poor) part (MnO <1%, Fe between 1.5 and 3.8%). Both original amount of organic carbon(TOCor), calculated from TOC, MnO and sulphur contents, and CaO concentration show a strongupward increase in the lower, Mn±Fe-rich part of the section. TOCor and CaO are believed to representplanktonic particles hence their simultaneous upward increases are interpreted as the result of an atleast 2.5 fold increase of productivity during the ®rst part of the ``black shale event''. Following thesame logic, in the second part of the ``black shale event'' productivity is believed to have slightlydecreased from the previously reached high level. Stratigraphic variation of the sulphur isotopic ratiosupport this scenario. Comparison of organic geochemical and d34S data of the re-sediments with thoseof the neighbouring marls suggests that increase and decrease (?) of productivity was paralleled byexpansion and withdrawal of oxygen-depleted waters. During the ``black shale event'' the extent of theoxygen-depleted bottom water was governed by changes in intensity of productivity. Changes in ratesof deposition of Mn and Fe were not related to those of the productivity but they deeply in¯uencednature and intensity of bacterial degradation of organic matter and especially the incorporation of sul-phur into kerogen. The high contribution of Mn- and Fe-minerals in the case of the lower Mn-richpart prevented by both dilution and degradation of OM due to Mn-reduction an intense incorporationof sulphur into OM. In the upper, Mn- (and iron) poor part the high initial Corg/Fe ratio led to an im-portant incorporation of sulphur into OM and an early termination of sulphate reduction. # 1998Elsevier Science Ltd. All rights reserved

Key wordsÐearly Toarcian, black shales, Mn-reduction, natural sulphurisation, productivity

INTRODUCTION

Both coupling of high rates of burial of TOC and

manganese and presence of fair to excellent oil

source rocks and ore-grade concentrations of Mn

explain the rich literature of the last 25 years deal-

ing with the Toarcian black shales of the Alpine-

Mediterranean domain. A part of the papers is

devoted to individual sections and/or to only one

aspect of the problem (e.g. source of Mn, process

leading to precipitation of Mn-carbonates, primary

productivity, O2-level of the sea-water, etc.), while

other ones are treating the relationships between

black shales and enrichment of Mn and/or cover alarge region or even the whole domain.The following non-exhaustive review shows in a

roughly chronological order the evolution of ideason certain geochemical aspects of the black shale-Mn association.

Organic matter

There is a general agreement concerning the dom-inantly marine origin of sedimentary organic matter(OM) in the Toarcian black shales of the Tethyan

domain. A signi®cant part of the OM is of bacterialorigin in some N-Italian black shale sections(Farrimond et al., 1988). Palynological studies

revealed that prasinophycean algae and/or acri-

Org. Geochem. Vol. 29, No. 5±7, pp. 1635±1647, 1998# 1998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain0146-6380/98 $ - see front matterPII: S0146-6380(98)00069-2

*To whom correspondence should be addressed. Fax:+36-1-251-0703; E-mail: vetoi@ma®.hu

1635

tarchs were signi®cant contributors to the OM ofthe Pozzale section/Umbria (Bucefalo Palliani and

Cirilli, 1993) and the Sachrang Formation/NorthernCalcareous Alps (Ebli, 1991). Contribution of dino-¯agellates to the OM is inferred from biomarker

studies (BaÈ chental section/Northern CalcareousAlps, KoÈ ster et al., 1995, several sections in N-Italy, Farrimond et al., 1988), while palynological

studies indicate their minor role in the Ionian zone(Baudin and Lachkar, 1990) and the Pozzale section(Bucefalo Palliani and Cirilli, 1993). High abun-

dance of radiolarians in the Toarcian black shalesof the Tethys suggests that these organisms weresigni®cant contributors to the OM (De Wever andBaudin, 1996; Veto  et al., 1997).

Very negative kerogen d13C values (mostly belowÿ29-) characterise every studied black shale sec-tion (Jenkyns and Clayton, 1986; Kodina et al.,

1988; Ebli, 1991; Mola k and Buchardt, 1996; Veto Â

et al., 1997)

Manganese

In the case of the Liassic Mn-ore deposits ofHungary the Mn is of continental origin (Drubina-Szabo , 1959). The laminated upper Liassic shale

horizon associated with manganese ores is devel-oped over more than 250 km distance in theNorthern Calcareous Alps; the sequence displays atransition from Mn-rich (up to 15%), partly silic-

eous limestones and marls at the base to Mn-poor(less than 1%) black shales at the top. Ca-rhodo-chrosite and Mn-rich calcite are the prevailing

manganese minerals in the Mn-rich limestones andmarls while the Mn-poor black shales are character-ised by abundant pyrite framboids. Coupling of Mn

and Fe, high Co content and the presence of theMg-rich clay mineral celadonite, supposed to rep-resent altered tu� material, suggest volcanogenic

solutions as the most probable source of the Mn-enrichment (Germann, 1973; Krainer et al., 1994).The close association of Mn and Fe, the low Mn/Co ratio and the presence of celadonite in a

Toarcian Mn-carbonate deposit of Hungary suggestthat the carbonatic Mn-ore chemically precipitatedfrom volcanogenic solutions (Kaeding et al., 1983).

Early Toarcian organic-rich black shale sequencesof the Alpine-Mediterranean domain are ofteninterbedded with Mn-rich limestones. High amounts

of dissolved manganese of continental origin weretransported through the well-developed oxygenminimum zone to the southern margins of theTethys. Development of manganoan limestones was

probably associated with impingement of the Mn-rich waters of the oxygen minimum zone (Jenkyns,1988). Jenkyns et al. (1991) continue to support the

continental derivation of Mn and the importance ofthe O2-minimum layer in its channelling and depo-sition but they do not exclude that volcanism and

local hydrothermal solutions played a role in the

genesis of ore grade deposits. The isotopic study ofPolga ri et al. (1991) proves that Mn-carbonates of

the U rku t deposit were diagenetically formed bybacterial reduction of Mn-oxyhydroxides duringvery early burial. Veto  et al. (1997) assume that the

low TOC content of the laminated Mn-carbonateore of U rku t is the result of both the dilution of thesediment by the high amount of Mn-Fe oxyhydrox-

ides and the bacterial reduction of the Mn resultingin a signi®cant loss of organic carbon.

Degree of oxygen depletion

Terms used here to characterise degree of oxygendepletion in bottom waters are taken from thepaper of Tyson and Pearson (1991). Black shales

were deposited from suboxic (i.e. oxygen contentbetween 0.2 and 0.0 ml/l) and in a part of the caseseven anoxic bottom waters (Jenkyns, 1988).

Molecular evidence of archebacterial activity in theupper part of the basinal sections in N-Italysuggests complete anoxia during the later stage ofblack shale deposition (Farrimond et al., 1988). The

bulk of the Sachrang Formation, characterised bycoincidence of lamination and benthic foraminiferaproves that bottom water was suboxic but not

anoxic, while traces of bioturbation in the top ofsome sections indicate dysoxic (i.e. oxygen contentbetween 2.0 and 0.2 ml/l) conditions at the very end

of the deposition of the formation (Ebli, 1989,1991). The bulk of the Pozzale section has beendeposited under dysoxic waters while a thin interval

in its middle part reveals suboxic bottom conditions(palynological study of Bucefalo Palliani and Cirilli,1993). In the Umbria basin the lowest O2-level co-incided with deposition of the middle part of the

black shales (palynological and nannofossil study ofBucefalo Palliani and Mattioli, 1994). Gradualintensi®cation of oxygen depletion through the

early stages of black shale deposition is inferredfrom organic geochemical study of a N-Italian sec-tion (Farrimond et al., 1994). Biomarkers indicate

that the BaÈ chental section was deposited fromanoxic water (KoÈ ster et al., 1995). Laminated blackshales and interbedding bioturbated Mn-rich lime-stones in a N-Italian section (Claps et al., 1995) in-

dicate alternation of at least suboxic and dysoxicconditions.

Productivity vs. stagnation

High productivity areas of the NW-Europeanshelf were the most probable sources of the poorlyventilated waters from which black shales of the

Alpine-Mediterranean domain were deposited(Jenkyns, 1988). Poor ventilation, introduction ofoxygen-depleted waters produced beyond the area

and even upwelling are among the factors whichwere able to lead to the development of oxygen-poor bottom conditions (Farrimond et al., 1988). In

the Tethyan area and in the Ionian Basin particu-

O. Ebli et al.1636

larly, black shales are preserved only in restrictedsub-basins favouring stagnation and oxygen de-

pletion of bottom waters (Baudin and Lachkar,1990; Baudin et al., 1990; Karakitsios, 1995).Alternation of bioturbated manganoan limestones

and laminated black shales in the Longarone sec-tion (N-Italy) is interpreted as the result of alterna-tion of periods with low and high productivity in

the surface waters. The coincidence of low pro-ductivity and limestones is suggested to re¯ect pre-dominance of calcareous forms in the planktic

community during low productivity periods (Clapset al., 1995). Toarcian black shales of the Tethyandomain were deposited beneath upwelling-relatedwaters (De Wever and Baudin, 1996). The Sachrang

Formation was deposited under an upwellingregime (Ebli, 1997). Primary productivity issuggested to have increased during deposition of

the U rku t section and to have been independent ofthe stratigraphic variations of the Mn-content (Veto Â

et al., 1997).

Here certain aspects of the organic, inorganic andisotope geochemistry of a Mn-rich immature blackshale section of early Toarcian age, cropping out

near Sachrang, Bavaria will be discussed and con-clusions will be drawn about early diagenesis andevolution of productivity. Although such a localstudy is obviously unable to resolve the numerous

problems related to geochemistry of the Alpine-Mediterranean Toarcian black shales, the resultsare expected to advance our knowledge not only on

a local level.

GEOLOGICAL SETTING

With the onset of the Jurassic sedimentationdrastically changed in the Northern CalcareousAlps. Carbonate platforms collapsed and a di�eren-

tiated bottom relief was generated by tectonicmovements. On the submarine swells red nodular

limestones of the thin (up to 20 m) Adneth

Formation were deposited whereas basinal areaswere characterised by accumulation of the greylimestones and marls of the relatively thick (up to

200 m) AllgaÈ u Formation. In the Lower Toarcianthe bituminous marls of the Sachrang Formation

(SF), a representant of the early Toarcian ``blackshale event'', replace the AllgaÈ u Formation andpartly the Adneth Formation, too (Fig. 1). The SF

has a thickness varying between 70 cm and morethan 27 m. The bituminous marls of the formation

very often show an alternation of nannofossil-richand clay-rich laminae, of a thickness of 21±72 and4±29 mm, respectively. Parallel lamination is predo-

minant although wavy lamination can be alsoobserved.

On the basis of geological setting and microfos-sils, two members can be di�erentiated in the SF.The Sachrang Member is developed in the basinal

area (Fig. 1). Its microfacies comprise rocks rich inradiolaria (Radiolaria-Biomicrites, -Pelbiomicrites).

Re-sediments, consisting of densely packed Intra-Biomicrites, are rather common. Their thicknesscan reach up to 1 m. The development of the

Sachrang Member started as early as the tenuicosta-tum-zone and covered the whole early Toarcian.

The Unken Member, sandwiched between red,often condensed limestones, represents the marginalfacies of the SF (Fig. 1). The rocks are rich in

bivalves. The frequent re-sediments often exhibitgreen colour and extremely high amounts of calcar-

eous nannoplankton and foraminifera.Sedimentation of the Unken Member started in the®nal part of the early Toarcian (bifrons-zone).

The coincidence of lamination and benthic micro-fossils, typical for recent sediments accumulating

beneath suboxic waters (Tyson and Pearson, 1991)proves that the Sachrang Member was deposited insuboxic conditions. Thus the abundant pyrite con-

tent of the SF (Ebli, 1997) is of a diagenetic origin.

Fig. 1. Lithostratigraphy of the Toarcian in the Northern Calcareous Alps between rivers Isar andSaalach.

The Toarcian Tethys 1637

The presence of opportunistic foraminifera (e.g.Lingulina tenera) indicates phases of improved oxy-

genation possibly of short duration because of lackof bioturbation and occurrence of only juvenilebivalves, died shortly after colonisation.

Common occurrence of tests of calcareous nan-noplankton and radiolarians makes probable an im-portant contribution of the cell material of these

planktonic organisms to the preserved organic ma-terial.For more details of stratigraphy and sedimentol-

ogy readers are referred to the paper of Ebli (1997).

EXPERIMENTAL METHODS

Samples were ground in a Fritsch ball-mill. Si,Al, Ti, Fe, Mn, Ca, Mg, Na, K, S, P, Ba and Srmeasurements were carried out on a Jobin-Yvon JY

70 ICP-AES device after fusion with Li-metaboratefollowed by acid dissolution. Relative standard de-viation of the determinations was 1±2%. CO2 was

volumetrically measured after an acid digestion. Ina number of samples sulphur was also determinedafter fusion with a Na2CO3±NaNO3 mixture andweighing as precipitate of BaSO4.

The BaSO4 precipitate was used for sulphur-iso-tope analysis following the method introduced byHalas et al. (1982). The sulphur isotopic ratio of

the evolved SO2 was measured in a home-mademass-spectrometer (Hertelendi et al., 1986).Due to their high carbonate-bound Mn content,

samples were suspected of generating CO2 by ther-mal breakdown of rhodochrosite and Mn-rich cal-cite during pyrolysis. For this reason TOC content,

Hydrogen Index (HI) and Tmax were determined bya Delsi Oil Show Analyser after decarbonation byHCl. Organic carbon isotope analysis of the decar-bonated samples, mixed with CuO, was performed

after combustion in a quartz tube at 8508C measur-ing the carbon isotopic ratio of the evolved CO2 ina Finnigan Mat Delta S type mass-spectrometer.

Carbon and oxygen isotope compositions of car-bonates were determined using the H3PO4 digestionmethod of McCrea (1950) with slight modi®cations

(Deme ny and Fo rizs, 1991). The 13C/12C and 18O/16O ratios were measured with a Finnigan MATdelta S mass spectrometer. The reproducibilities ofthe d13C and d18O data are better than20.2-.

Kerogen was concentrated by HCl and HF treat-ment then pyrolyzed in a Quantum device (MSSVpyrolyzer directly connected to a GC, for details see

Hors®eld et al., 1989). 1±2 mg samples were sealedin glass capillary tubes then heated from 300 to5308C within 8 min and held 2 min before a single

step on-line GC-analysis.

RESULTS

Samples, representing some cm thick intervals Ð

19 laminated marls and 5 re-sedimented marls fromthe SF and one grey limestone from the AllgaÈ u

Formation, just above the SF Ð were taken froman outcrop section of the Sachrang Member (Fig. 2),

located close the German±Austrian border, on mid-

way between the rivers Isar and Saalach (sectionNo 52 of Ebli, 1997).

Concentrations of chemical components and HI

values are listed in Table 1. The formation can bedivided into two parts on the basis of MnO and

TOC contents and HI values (Figs 3 and 4). MnOcontent of the marls strongly varies in a range

between 2 and 16% in the lower part (below of

about 16 m) of the section while it is below 1% inthe upper part. Mn-rich marls, moderately rich in

TOC (1±2%) are characterised by HI values varyingbetween 300 and 500 while Mn-poor ones show dis-

tinctly higher TOC contents and HI values (5±9%

and 600±700 mg HC/g TOC, respectively). CaOcontent displays an upward increase in the lower,

Mn-rich part from 11 to 19%, then, starting fromhigher values, rapidly decreases in the Mn-poor

part (Fig. 5). S content varies in a broad range in

the Mn-rich marls with highest values in the middleof the section, while the Mn-poor marls are charac-

terised by intermediate values (Table 1).Microscopical observations of abundant pyrite

suggest that a great part of the S is obviouslybound to pyrite. Combined X-ray and thermoanaly-

tical study of 3 samples (2 Mn-poor and 1 Mn-rich

marls) has found pyrite to range between 4 to 10%.It is noteworthy that pyrite never shows traces of

limonitization under the microscope. The Mn-richmarls are more carbonatic than the Mn-poor marls

(Table 1, average concentrations of CO2 are 21 and

15%, respectively). A great part of the CO2 in theMn-rich marls is obviously bound to Mn-rich car-

bonates. On the basis of combined X-ray and ther-moanalytical study, 20±30% Mn-rich carbonate Ð

rhodochrosite and kutnahorite Ð is present in 2

Mn-rich marl samples.The lower, Mn-rich part of the formation is

characterised by higher Fe and MgO contents, rela-

tive to the upper, Mn-poor part (Table 1), averagevalues of the Mn-rich and Mn-poor parts are

4.84% Fe, 2.26% MgO and 2.31% Fe, 1.11%MgO, respectively. Consequently, abundances of

TiO2, K2O, Al2O3, CaO and SiO2 are lower in the

Mn-rich marls than in the Mn-poor ones (Table 1).These ®ndings ®t well the upward transition from

Mn-rich shales to Mn-poor shales, the coupling ofMn and Fe and indirectly the presence of Mg-rich

clay minerals, features characterising the upperLiassic shale horizon of the Northern Calcareous

Alps as described by Germann (1973).

MnO content of the ®ve re-sediment samples stu-

died ranges from 1.7 to 5% (Table 1, Fig. 3). The

O. Ebli et al.1638

two samples taken from the lower, Mn-rich part of

the section are characterised by distinctly lower

TOC contents and HI values than the neighbouring

laminated marls. TOC content and HI of the re-

sediment taken from 15.2 m are very similar to

those of the marl taken from 14 m. TOC content

and HI of the re-sediment taken from 26.2 m are

close to those of the neighbouring laminated marl

while those of the re-sediment taken from 27.1 m

are distinctly lower (Table 1, Fig. 4). Sulphur con-

tents of the re-sediments are similar to those of the

neighbouring marls (Table 1).

Tmax varying between 409 and 4218C indicates

that the kerogen is immature.

The pyrolysates of samples, dominated by alk-1-

enes and n-alkanes in the range of C2±C27 indicate

that the overwhelming majority of pyrolysable or-

ganic matter is of planktonic origin (Larter, 1984).

This assumption agrees well with the high HI

values. The other, regularly detectable compounds

were the C0±C3 alkylbenzenes, alkylthiophenes, phe-

nol, naphthalene and prist-1-ene. (The more

detailed study of the pyrolysates will be published

later.) Here the sulphur richness of the kerogen was

expressed by the ratio of 2-methylthiophene over

toluene (Table 2). Pyrolysis-gas-chromatography

traces of a Mn-rich marl sample (14.3 m above

datum) and of a Mn-poor marl sample (17.4 m

above datum) are shown in Fig. 6. Mn-poor marl

Fig. 2. Lithological column of the studied section of the Sachrang Formation. The lowermost onemeter of the section, close to the Sachrang River and very exposed to weathering, has not beensampled. Microfacies (1) pelbiomicrite with radiolaria; wacke- to packstone; (2) radiolaria biomicrite;mud- to wackestone; (3a) crossbedded radiolaria-spicula biomicrite; wacke-to packstone; (3b) massive

radiolaria-spicula biomicrite; wacke-to packstone.

The Toarcian Tethys 1639

pyrolysates contain 3±4 times more organic sulphur than Mn-rich marl pyrolysates. The 2-methylthio-

Table 1. Stratigraphic variations of the elemental composition (%) and hydrogen index

Datum (m) TOC (%) HI SiO2 TiO2 Al2O3 MnO CaO MgO Na2O K2O CO2 Fe S

27.6a 0.23 372 6.94 0.08 7.43 0.64 48.40 0.77 0.22 0.44 37.98 0.73 0.47

Mn-poor part of the SF27.1b 1.97 550 30.00 0.25 5.53 1.74 26.90 1.16 0.32 1.08 21.33 2.36 1.9926.2b 5.54 605 32.50 0.31 6.18 2.82 17.90 1.17 0.34 1.25 13.71 3.75 2.2724.9 8.13 664 33.30 0.41 7.71 0.47 17.30 1.27 0.35 1.35 12.61 2.65 2.6423.2 5.29 628 35.80 0.39 7.53 0.41 17.70 1.44 0.36 1.31 12.46 3.40 3.2821.7 6.96 675 28.70 0.30 5.94 0.71 23.80 1.06 0.34 1.00 17.92 1.93 2.0020.3 8.96 688 22.60 0.23 4.83 0.69 26.80 0.87 0.29 0.76 19.90 1.53 2.0318.7 6.73 689 30.40 0.26 5.18 0.83 23.50 0.96 0.31 0.95 17.50 1.85 2.1117.4 8.39 667 34.70 0.38 6.96 0.70 17.40 1.05 0.35 1.18 12.26 2.49 1.46

Mn-rich part of the SF15.2b 1.78 486 21.40 0.22 4.50 4.92 26.20 1.46 0.22 0.81 24.75 3.83 4.1314.3 1.74 493 18.90 0.16 3.52 13.07 19.40 2.17 0.28 0.55 24.11 4.93 5.0012.9 1.54 368 26.60 0.27 5.54 7.13 19.30 2.09 0.31 0.88 20.20 4.73 3.6311.4 1.64 477 28.70 0.24 5.00 12.00 15.00 2.33 0.30 0.84 21.33 3.80 1.599.8 1.47 348 29.80 0.27 5.71 8.97 16.50 3.00 0.35 0.96 18.87 4.77 3.918.1 1.56 491 22.60 0.25 5.12 15.40 13.50 3.21 0.33 0.84 24.40 4.25 0.677.2 1.54 502 26.80 0.24 4.75 14.20 11.40 2.82 0.31 0.88 21.94 5.54 0.386.0 1.60 417 30.80 0.23 4.58 11.50 11.50 2.52 0.28 0.89 21.30 5.64 0.57c

5.8 1.87 426 34.90 0.29 5.81 8.76 11.60 2.30 0.30 0.89 18.10 4.79 1.08c

5.7 1.88 421 34.60 0.31 5.98 8.38 11.20 2.24 0.30 1.09 17.30 4.90 1.80c

5.6 2.33 469 34.50 0.31 6.08 7.84 11.80 2.13 0.32 1.07 16.30 4.69 1.61c

5.2b 0.39 283 29.81 0.14 3.17 2.15 30.20 1.01 0.33 0.54 19.73 2.14 0.785.0b 0.51 231 31.70 0.21 4.72 2.65 25.40 1.52 0.27 0.90 22.20 2.38 1.73c

4.5 1.29 342 41.00 0.35 7.02 2.17 14.80 1.96 0.27 1.03 13.20 4.57 2.79c

3.8 1.48 320 36.80 0.34 6.54 7.32 12.80 2.15 0.25 1.03 16.10 3.93 1.43c

3.2 1.54 378 34.90 0.32 6.48 6.40 10.80 2.70 0.38 1.01 16.78 6.36 1.49

Datum is the base of the Sachrang Formation.The sum of the components listed is always less than 100%: water content and amounts of organically bound O, H have not been

measured; a part of the Fe is not bounded to pyrite but the corresponding amount of O has not been taken into consideration.aLimestone above the Sachrang Formation.bRe-sediment.cSulphur content measured by ICP.

Fig. 3. Stratigraphic variation of the MnO content.

O. Ebli et al.1640

phene/toluene ratio of the re-sediment taken from

26.2 m is close to that of the neighbouring lami-

nated marl while that of the re-sediment taken from

27.1 m is distinctly lower (Table 2).

d13Corg and d34S values are listed in Table 2 and

the latter are displayed in Fig. 5. d13Corg ranges

between ÿ27.7 and ÿ31.6-, its average for the

laminated marls is ÿ29.76-. This ®nding supports

Fig. 4. Stratigraphic variation of the TOC content and Hydrogen Index

Fig. 5. Stratigraphic variation of the CaO content, TOCor and d34S.

The Toarcian Tethys 1641

Table 2. Stratigraphic variations of d13Ccarb, d13Corg, d34S and 2-methylthiophene/toluene ratio

Datum (m) d13Ccarb (-) d13Corg (-) d34S (-) 2-methylthiophene/toluene ratio

27.6a 2.12 ÿ28.50 ÿ11.44 nd

Mn-poor part of the SF27.1b ÿ1.42 ÿ30.20 ÿ11.50 0.4026.2b ÿ1.46 ÿ30.90 ÿ4.09 0.8024.9 nd ÿ31.60 ÿ1.83 0.9823.2 ÿ1.82 ÿ28.60 3.48 0.7921.7 nd ÿ27.70 1.11 nd20.3 nd ÿ28.90 3.34 0.9918.7 ÿ2.69 ÿ28.90 2.58 1.0517.4 ÿ2.47 ÿ31.60 5.79 1.06

Mn-rich part of the SF15.2b ÿ2.39 nd 12.12 nd14.3 ÿ7.92 ÿ30.00 15.23 0.1812.9 ÿ7.90 ÿ29.40 6.87 nd11.4 ÿ8.41 ÿ30.60 14.06 nd9.8 ÿ5.38 ÿ28.30 1.53 0.318.1 ÿ4.41 ÿ31.40 6.06 nd7.2 ÿ4.73 ÿ29.10 ÿ3.37 0.235.2b ÿ3.08 ÿ29.70 ÿ13.49 nd3.2 ÿ5.40 ÿ30.90 12.28 0.37

Datum is the base of the formation.aLimestone above the Sachrang Formation.bRe-sediment.nd, not determined.

Fig. 6. Gas chromatograms of kerogen pyrolysates.

O. Ebli et al.1642

well the planktonic origin of the OM. The lowestMn-rich marl sample disregarded, variation of d34Swith depth follows a V-shaped curve (Fig. 5). There-deposited samples taken from the bottom andtop of the section contain the isotopically lightest

sulphur while it is the isotopically heaviest in theuppermost Mn-rich samples, both in the marl andthe re-sediment.

The MnO content of the clayey limestone takenfrom the immediate cover of the formation is unu-sually high for a common marine carbonate.

DISCUSSION

Compositional dichotomy of the formation Ð pre-liminary remarks

The marked stratigraphic changes in concen-trations of most of the studied chemical com-ponents are interpreted as follows. Duringdeposition of the formation a mixture of planktonic

(OM, tests of radiolarians and calcareous nanno-plankton) and terrigenous particles reached the bot-tom. The variation of the CaO/SiO2 ratios by a

factor of 4 suggests that relative weights of theplanktonic and terrigenous particles, especiallythose of radiolarians and calcareous nannoplank-

ton, strongly varied during deposition. In the caseof the Mn-rich marls the mixture of the planktonicand terrigenous particles was diluted by a strong

input of Mn, Fe and Mg-minerals of unknownorigin.The particularly strong di�erence in TOC con-

tents between the two parts of the formation can

not be explained solely by dilution of the lower partwith Mn±Fe±Mg minerals; it also re¯ects di�er-ences in bacterial degradation of OM. While bac-

terial oxidation of OM in the Mn-poor marls wasprimarily the result of sulphate reduction, in theMn-rich marls both Mn-reduction and sulphate re-

duction were powerful factors of it. The di�erencein HI values between Mn-rich and Mn-poor marlsalso re¯ects the above discussed di�erence in bac-

terial degradation. The carbon isotopic compositionof the carbonate in the lower, Mn-rich part of thesection is distinctly lighter than that in the upper,Mn-poor part (Table 2), indicating that the contri-

bution of the organic carbon to the carbonate wassigni®cantly higher in the case of the Mn-richmarls. In non-bioturbated sediments rich in reduced

manganese compounds, the amounts of organic car-bon lost by Mn reduction and sulphate reduction(TOCMn and TOCsr) can be expressed as follows

(Veto  et al., 1997):

TOCsr � S� 0:75� 1:33 �1�

TOCMn � divalent Mn� 0:11 �2�where 0.75 and 0.11 are factors determined by stoi-

chiometry and atomic weights and 1.33 takes intoaccount the about 25% loss of H2S from the sedi-

ment during early diagenesis (for details see Veto  etal., 1995, 1997). Average Mn and S contents are7.37% and 2.00% for the lower part of the section

while the same values are 0.50% and 2.25% for theupper part. Replacing these values in equations (1)and (2) 0.81% and 2.00% are obtained as average

TOCMn and TOCsr for the lower part while thesame average values are 0.06% and 2.25% for theupper part. Thus the Mn-reduction played an insig-

ni®cant role in the bacterial consumption of OM inthe case of the upper, Mn-poor part of the sectionwhile it was responsible for about 30% of the bac-terial oxidation of organic carbon in the lower, Mn-

rich part.The sharp di�erence in 2-methylthiophene-to-

toluene ratios between Mn-rich and Mn-poor marls

obviously re¯ects di�erences in sulphur contentbetween the corresponding OM: the relatively highpyrolytical yield of thiophenes suggests that kerogen

of the Mn-poor marls is rich in sulphur (Boussa®rand Lallier-VergeÁ s, 1997).Sulphate reduction in the Mn-poor marls ceased

when they contained yet a high amount of OM veryrich in hydrogen (present-day average TOC contentand HI value are 7.13% and 669, respectively)while in the Mn-rich marls it continued until the

sediments contained only a relatively low amountof OM, moderately rich in hydrogen (present-dayaverage TOC content and HI value are 1.88% and

419, respectively). Recently, from oceanic sediments,lying below the zone of active sulphate reductionand containing abundant, hydrogen-rich OM, sig-

ni®cant amounts of dissolved sulphate have beenreported by Schulz et al. (1994), LuÈ ckge et al.(1996) and LuÈ ckge (1997). These ®ndings indicatethat bacterial sulphate reduction ceased in these

sediments when both of the reactants needed Ðrelatively non-degraded OM and dissolved sulphateÐ were yet present in large amounts. LuÈ ckge et al.

(1996) assume that this surprisingly early termin-ation of the sulphate reduction is the result of anintense incorporation of sulphur into the algal OM,

making it non-metabolizable for sulphate reducingbacteria. The coincidence of very high HI valuesand sulphur-rich OM in the Sachrang section

suggests that such a ``vulcanisation'' was respon-sible for the early termination of sulphate reductionin the Mn-poor marls, too.

Re-sediments as records of sedimentation on submar-

ine swells

Organic and isotope geochemistry of the re-sedi-

ments are expected to re¯ect the oxygenation ofbottom water on submarine swells, even if duringthe transport to basinal areas some separation of

the sediment particles occurred and it can not be

The Toarcian Tethys 1643

excluded that bacterial processes recommenced after

re-deposition.

TOC contents and HI values of the two re-sedi-

ment samples studied from the lower part of the

section, signi®cantly smaller than those of the

neighbouring marls (Fig. 3) suggest that at the

beginning of the suboxic sedimentation in the basi-

nal areas bottom water on the swells was yet nor-

mally oxygenated. This conclusion ®ts well with the

di�erence in d34S between the re-deposited sample

and the neighbouring marl (Fig. 5). This fact

re¯ects the di�erence in early diagenetic sulphate

transport between the two environments. On the

normally oxygenated swells bioturbation constantly

furnished new sulphate, non-depleted in 32S, for the

sulphate reducing bacteria. Hence isotopic compo-

sition of the reduced sulphur, ®xed in the sediments,

was principally determined by the intense isotopic

fractionation accompanying bacterial sulphate re-

duction. On the contrary, in the basinal area, where

no bioturbation occurred, sulphate entered the sedi-

ments only via downward di�usion. As this process

is very slow, bacteria were forced to consume sul-

phate already depleted in 32S. Thus the sulphur iso-

topic fractionation was much smaller than that

resulting from the bacterial reduction alone.

TOC contents, HI- and d34S values of the marl±

re-sediment sample pair from the top of the Mn-

rich part (14.3 and 15.2 m above datum) are very

close (Figs 4 and 5). These ®ndings suggest that

during deposition of the corresponding sediments

both submarine swells and basinal areas were cov-

ered by O2-depleted waters resulting in very similar

conditions of OM degradation and sulphate re-

duction/sulphur ®xation.

The uppermost two metres of the section are not

represented by studied laminated samples. TOC

content, HI value and sulphur isotopic ratio of the

re-sediment sample taken from 26.2 m are very

similar to those characterising the laminated marls

of the upper, Mn-poor part of the section (Figs 4

and 5). This coincidence suggests that close to the

end of the ``black shale event'' bottom water on the

swells was yet O2-depleted. The re-sediment sample,

taken from the top of the section (27.1 m) and

characterised by lower HI and d34S values and

much lower TOC content than the re-deposited

sample of 26.2 m, indicate that at the very end of

the ``anoxic event'' the re-oxygenation of the swells

started already.

The relatively high Mn-content of the re-depos-

ited sediments suggests that the ¯ux of Mn-minerals

was not restricted to the basinal area what is corro-

borated by the high MnO content (19.2%) of the

only Unken Member marl sample, studied for el-

emental composition (unpublished result of Veto  ).

On the basis of the MnO contents of the re-sedi-

ments studied from the top of the section the ¯ux

of Mn-minerals had been renewed at the end of de-

position of the SF. The relatively high Mn-contentof the grey limestone covering the formation

(Table 1, Fig. 3) indicates that the ¯ux of Mn-min-erals continued after the end of the ``black shaleevent''. It is worthy to mention that in U rku t/

Hungary the upper ore bed of low quality, separ-ated from the main ore bed by laminated marls,records a similar renewal of the Mn (and Fe) ¯ux

at the end of the ``black shale event'' (Veto  et al.,1997).

Planktonic productivity Ð its magnitude and evol-

ution

The formula developed by Suess (1980), describ-

ing the relationship between measured productivity(Cprod) and ¯ux of planktonic organic carbon reach-ing the bottom (C¯ux) Ð both expressed in t Corg/m2/My Ð and water depth (z)

Cprod � Cflux � �0:0238z� 0:212� if z > 50 m �3�theoretically can be used for assessing productivity

prevailing during deposition of non-bioturbatedsediments of the geological past, as discussed indetail by Veto  et al. (1997).C¯ux can be described as follows:

Cflux � TOCor � sedimentation rate� r, �4�where TOCor is the original organic carbon contentof the sediment and r is the dry sediment density.

TOCor has to be assessed by summarising TOC,TOCMn and TOCsr, calculated by equations (1) and(2).

Figure 5 clearly shows that TOCor rapidly butgradually increases upward in the Mn-rich part.Above, it varies on a somewhat higher level in the

Mn-poor part. The mean TOCor of the marls is6.12%. The whole TOCor is considered as of plank-tonic origin. It should be noted that with non-bio-

turbated sediments having TOC contents and/or HIvalues above 6% and 600, respectively, Ð as is thecase with the Mn-poor marls Ð the loss of H2Sduring early diagenesis can be higher than 45%

(Veto  et al., 1995). Since the average S content ofthe Mn-poor marls is 2.25%, the average TOCsr is2.25% if 25% H2S loss is envisaged. If the loss is

45% then the average TOCsr is 3.07%. Hence forMn-poor marls the actual value of average TOCor

could be at least about 0.8% higher than the calcu-

lated one.Following Westermann (1984), recent estimates

of the duration of the Toarcian stage vary between5 and 11 My. On the other hand, the views of Ebli

(1997) and Jenkyns (1988) are very di�erent: theformer assumes that the Sachrang Formation rep-resents the whole Lower Toarcian, in other words 3

ammonite zones while the latter believes that theblack shale event took place during only one to oneand a half ammonite zones. The combinations of

11 My and 3 ammonite zones and that of 5 My and

O. Ebli et al.1644

one ammonite zone would result in 5.5 and 0.8 My

as duration of the deposition of the Sachrang

Formation, respectively. In view of such a great

uncertainty, instead of calculating the past pro-

ductivity it is better to assess its relative changes

during deposition of the formation.

It is very probable that the greater the water

depth, the smaller was the percentage of the net

planktonic production which reached the bottom.

Since water depth is considered as remaining

roughly constant during deposition of the SF the

problem of assessing relative changes of Cprod can

be reduced to that of assessing relative changes of

C¯ux. Following equation 4 changes of C¯ux are

determined by those of TOCor, rate of sedimen-

tation and r. While changes of TOCor are well

known (Fig. 5) and those of r are of minor import-

ance, the changes of the rate of sedimentation can

only be qualitatively described.

First, two problems have to be addressed: the

nature of the co-variance between CaO (it practi-

cally corresponds to CaCO3) and TOCor in the bulk

of the formation (Fig. 5) and the di�erences caused

by the ¯ux of Mn, Fe and Mg-minerals in rate of

sedimentation and r between the Mn-rich and Mn-

poor parts of the formation.

The simultaneous increases of OM and CaCO3

concentrations in marine sediments may re¯ect two

fundamentally di�erent scenarios: either the rate of

deposition of terrigenous material decreased and

that of OM and CaCO3 remained constant or the

rate of deposition of terrigenous material remained

constant while the deposition of OM and CaCO3

really accelerated. Obviously, transitions between

these two ``pure'' scenarios can be envisaged. The

fact that the increase of TOCor during deposition of

the Mn-rich marls was accompanied by invasion of

swells by O2-depleted waters is consistent with, but

does not prove, the increase of rate of OM depo-

sition with decreasing age. Sulphur isotopic ratios

o�er an independent line of evidence for decipher-

ing the real meaning of the co-variance between

CaO and TOCor. One sample disregarded, d34Svalues of the Mn-rich marls display a net increase

with decreasing age (Fig. 5). An increase of d34Svalues could re¯ect a decrease in contribution of

syngenetic pyrite, precipitating in the euxinic water

column and of a notoriously light sulphur isotopic

ratio. On the basis of the common presence of

benthic foraminifera in the SF, the bottom water

did not contain H2S thus the above scenario can

not be invoked to explain the strong upward

increase of d34S observed. Hence the upward

increase of d34S means a decrease of isotopic frac-

tionation between sedimentary sulphur and dis-

solved sulphate during deposition of the lower, Mn-

rich part of the formation. Such a decrease can be

explained by both the increase of rate of sulphate

reduction and that of rate of sedimentation (for

details see Zaback et al., 1993). On the basis of theparallel upward increase of TOCor and CaO, a

combination of the two causes, in other words theparallel increases of C¯ux and rate of sedimentationis the most plausible explanation for the observed

sulphur isotopic trend. Considering the more thantwofold upward increase of TOCor and the sup-posed considerable parallel increase of the rate sedi-

mentations estimating a 2.5 fold increase ofproductivity would be conservative.Since during deposition of the lower part of the

formation a signi®cant amount of Mn, Fe and Mg-minerals was added to the ``background'' sedimentsconsisting of terrigenous and planktonic particles,average values of rate of sedimentation and r of the

Mn-rich marls were considerably higher than thosecharacterising the Mn-poor marls. Thus in the caseof the Mn-poor marls the transformation of

changes of TOCor to those of C¯ux (and Cprod) hasto be done with caution: while the upward increaseof TOCor in the Mn-rich marl, as discussed above,

may correspond to a real increase of C¯ux (andCprod), on the basis of the lower values of rate ofsedimentation and r, C¯ux (and Cprod) probably ex-

perienced some decrease after the ¯ux of Mn, Fe,Mg-minerals ceased. The slow upward decrease ofd34S values of the Mn-poor marls and the decreaseof CaO content displayed by the uppermost two

Mn-poor marl samples (Fig. 5) support this con-clusion.

CONCLUSIONS

On the basis of a synoptic view of the geochem-

ical data discussed above, 3 stages can be distin-guished in the early Toarcian ``black shale event'' inthe studied part of the Tethys (Fig. 7).

During the ®rst stage, corresponding to the depo-sition of the sediments between 0 and 16 m pro-ductivity and ¯ux of planktonic OM progressivelyincreased. As a result of this evolution the bottom

area where the oxygen demand surpassed the oxy-gen supply progressively increased and ®nally eventhe margins and the submarine swells were invaded

by oxygen-depleted waters. This period of increas-ing productivity was characterised by an intense¯ux of Mn, Fe and Mg-minerals of unknown origin

both in basinal and marginal areas. The variationof intensity of this ¯ux largely surpassed that of theproductivity.During the second stage, corresponding to the de-

position of the sediments between 16 and 26.5 mthe productivity slowly decreased but the dysoxic±suboxic conditions were persisting on the margins.

Shortly before the end of the second stage the Mn-¯ux renewed.Finally, during the short third stage, correspond-

ing to the deposition of the uppermost some deci-

The Toarcian Tethys 1645

metres oxic conditions were restored on the mar-

gins.

Further work will be necessary for integrating

this scenario into the stratigraphic framework estab-

lished by Ebli (1997).

Available data are not su�cient to conclude

about the cause(s) of starting and termination of

the ``black shale event'', in other words, they do

not allow us to take a stand on the ``preservation

versus production controversy'' (Parrish, 1995), but

we think that during the event itself advance and

withdrawal of O2-depleted waters were at least

partly governed by the increase and decrease of

productivity.

The rough coincidence of the long-lasting increase

of productivity and the main period of Mn-¯ux is

probably not of a causal nature. On the other hand,

conditions of early diagenesis were deeply in¯uenced

by the available amounts of MnO2 and ferric iron

minerals. High concentrations of these electron

acceptors in the lower part of the SF resulted in an

advanced degradation of OM, re¯ected by relatively

low TOC contents, HI values and organic sulphur

richness. The low amounts of MnO2 and ferric iron

minerals and the resulting high OM/ferric iron ratio

in the upper part of the SF lead to an intense incor-

poration of the sulphur into OM and an early ter-

mination of bacterial sulphate reduction, re¯ected

by the relatively high TOC contents, HI values and

organic sulphur richness.

AcknowledgementsÐThis work was supported by theHungarian Science Foundation (OTKA) under Grant No.T 020207 (V. I.) and No. A 226 (D. A.), respectively. Helpof La szlo Korpa s during sampling is gratefully acknowl-edged. Sulphur isotope measurements were carried out inthe laboratory of Ede Hertelendi. Thermoanalytical andX-ray di�raction studies were done by Ma ria FoÈ ldva riand Pe ter Kova cs-Pa l�y in the Geological Institute ofHungary. Remarks and criticism of two anonymous refer-ees improved the paper.

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