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TethyanMediterranean organic carbon-rich sediments fromMesozoic black shales to sapropels

KAY-CHRISTIAN EMEIS* and HELMUT WEISSERT

*IfBM, University of Hamburg, Hamburg, Germany (E-mail: [email protected])Geological Institute, ETH Zurich, Zurich, Switzerland

ABSTRACT

The Jurassic to Holocene record of black shale deposition in the Tethys-Mediterranean region is unrivalled by that of any other ocean basin, either inland sections or drill cores. The term black shale is used here broadly forsediments with elevated organic carbon concentrations (> 1%), including thePliocene to Recent sapropels. Most of the black shales are devoid of benthonicorganisms, are laminated, and were deposited in distinct rhythms duringperiods when the deep waters of the ocean basins were anoxic or dysoxic. The

Tethyan black shale records have become essential in studies of the transfer oforganic carbon into the sediment record and for astronomical tuning andgeological time scales. These records have been central in understandingclimate control on ocean dynamics and biogeochemical cycles. The Mesozoicblack shales were deposited within well-defined time envelopes of around05 to 1 Myr. These black shales, which were confined to certainchronostratigraphic intervals in the Jurassic and Cretaceous, were recognizedas expressions of global Oceanic Anoxic Events in the mid-1970s andsubsequently named after prominent researchers (Bonarelli, Selli, Goguel).The black shale episodes were dated by biostratigraphic methods and by high-resolution chemostratigraphy and cyclostratigraphy. Mesozoic black shales arenow interpreted as the oceanographic expression of major perturbations of the

global carbon cycle and climate. Research into younger (Pliocene toPleistocene) sedimentary cycles (including black shales, termed sapropels)exposed in land sections, or found in pelagic and hemipelagic marinesediment cores of Late Quaternary age, started in the 1950s. Main threadspursued from this end of the record were the climatic control of oceanicprocesses that permitted the development of highly detailed and precise timescales tuned to the astronomical clock of insolation changes, palaeoclimateevolution of the circum-Mediterranean area and biogeochemical dynamics inanoxic basins. After 50 years of intensive research, the Tethyan andMediterranean black shales remain subjects of fascination in Earth Science.Tracing the origins and building upon recent progress, the current hypotheseson their formation are reviewed here. It is a panorama of complex interplays

between global and regional tectonics, climate dynamics during both IceHouse and Greenhouse states of global and regional climate, oceanographicresponses to these climate changes, and biogeochemical adaptations that wereall needed to shape an extraordinary archive of global change in the absence ofhuman activity.

Keywords Mesozoic black shale, Oceanic Anoxic Event (OAE), organiccarbon-rich sediment, sapropel

Sedimentology (2009) 56, 247266 doi: 10.1111/j.1365-3091.2008.01026.x

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INTRODUCTION AND HISTORY OFRESEARCH

Black shales have been and remain difficult todefine and they are hard to categorize. A commoncharacteristic is an enrichment of organic carbonof up to 7%, and rarely up to 15% (Bitterli, 1963;

Hallam & Bradshaw, 1979), but organic carboncontent up to 30% is known from Pliocenesapropels and black shales of Cenomanian age.These shales tend to be different from the sur-rounding host rocks: visually, chemically and infacies. In terms of sedimentology, distinction isoften much less straightforward (Spears, 1980)and occasionally they leave no visual trace whenoxidized. Black shales or sapropelites (Potonie,1904) occur in all aquatic sedimentary settingsbut the ones which are dealt with here are marinehemipelagic or pelagic muds or mudrocks, bio-

turbated or laminated, with or without carbonate,sometimes siliceous, with organic carbon > 1% to2% of marine or terrestrial origin and frequentlyenriched in trace metals. These shales are inter-bedded commonly in sediments lacking organiccarbon enrichment, and are often members ofcycles that have time periods in the range of tensto hundreds of thousands of years (Vine & Tourt-elot, 1970; Kidd et al., 1978; Hallam, 1980; Arthuret al., 1984; Jones, 1987; Arthur & Sageman, 1994).

Organic carbon-rich sediments, known as bitu-minous rocks, as sapropels or simply as blackmudstones, shales or marls have attracted

geologists since the days when Leopold von Buch(1839) introduced the term Schwarzer Jura intostratigraphic nomenclature. The dark shale suc-cessions consisting of the Posidonien-Schieferand Opalinuston were deposited during theEarly Jurassic (Lias-Dogger) in a shallow epi-continental sea covering wide parts of North-western Europe. Several authors investigated theprocess of bituminization as early as the verybeginning of the 20th Century (see reviews inBitterli, 1963; Arthur et al., 1984). After WorldWar II, the Posidonien-Schiefer became one of the

prominent research targets for the petroleumindustry in Europe. In a research project between1950 and 1960, Koninklijke Shell investigatedTethyan organic carbon-enriched sedimentaryrocks throughout Western Europe. Petroleumresearch focused on finding the appropriatestructural settings for source rocks and reservoirs,with ancillary work on the possible origin oforganic carbon-enriched sedimentary rocks. Shellresearchers analysed almost 1600 samples fortheir chemical composition, their sedimentary

petrography and their palynology. The petroleumpotential of bituminous sediments was investi-gated and their geochemical characteristics andpossible origins of organic carbon-enrichedsedimentary rocks were established (Bray &Evans, 1965; Vine & Tourtelot, 1970; Welte,1972).

These investigations clearly demonstrated thatblack shales were unevenly distributed in spaceand time but subsequent research and explorationdetermined that the vast majority of petroleumreserves and source rocks were clustered in theTethyan realm. It also became clear that stratafrom two geological periods, the Late Jurassic andthe Early to Middle Cretaceous, were responsiblefor > 50% of the known generated petroleum(Klemme & Ulmishek, 1991) (Fig. 1). After indus-trial production of oil started in Western andCentral Europe, it quickly spread offshore. Since

then, a vast literature base has accumulated onthe sedimentary, palaeontological and geo-chemical aspects of black shales as a petroleumsource and as a geological archive of past states ofthe Earth.

The second arena where work on organiccarbon-rich sediments broke ground in the Medi-terranean realm was the Neogene sapropel record.The term sapropel dates back to the turn of the20th Century and was proposed as the interna-tional term for the German word Faulschlamm(Potonie, 1904). Since then it has been used in ageneric sense to describe fine-grained and uncon-

solidated sediments rich in organic matter depos-ited in stagnant water and also to denote distinctdark layers (sapropelites) interbedded in organiccarbon-poor host sediments.

The Mediterranean sapropels

One motivation for sapropel research was toestablish the stratigraphy of the Neogene. Workcentred on the beautiful exposures of geologi-cally young and tectonically uplifted marinesediments in Southern Italy, Greece and other

places in the Mediterranean Sea basin (Selliet al., 1977). In the land exposures, the sedi-mentary cycles are barely recognized as contain-ing black shales (Meulenkamp et al., 1979) thatwere oxidized and preserved only as marl layers.However, it quickly became clear after theSwedish Deep-Sea Expedition in 1947 to 1948that matching organic carbon-rich sapropel inter-vals occur in recent Pleistocene sediments(Kullenberg, 1952; Olausson, 1961) and, afterdrilling by the Deep Sea Drilling Project (DSDP)

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and the Ocean Drilling Program (ODP), through-out the entire post-Miocene period (Ryan & Hsu,1973; Hsu & Montadert, 1978; Kidd et al., 1978)

and in the entire Mediterranean Sea basin(Kastens et al., 1987; Comas et al., 1996; Emeiset al., 1996).

A

B

Fig. 1. (A) Petroleum realms, realm areas and percentage of petroleum reserves per area and (B) stratigraphic age ofpetroleum source rocks (after Klemme & Ulmishek, 1991).

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The presence of anoxic layers in the geologicalrecord of the Eastern Mediterranean Sea had beenpostulated as a reaction to sea-level lowering inthe glacial Mediterranean Sea (Bradley, 1938).After these layers had been recovered in cores,Kullenberg (1952) thought that they indicatedanoxic conditions, brought about by stagnation

during pluvial conditions in glacial periods.However, in the mid-1950s, it was recognizedthat the youngest sapropel post-dated the glacialand the glacially lowered sea-level, so that theoriginal idea of Bradley and Kullenberg wasrejected. Investigations of faunal (Parker, 1958)and isotopic (Emiliani, 1955) properties sug-gested pronounced coldwarm cycles in the LatePleistocene, and Olausson (1961) established thatmost sapropels occurred after major cool periods.Excellent reviews of the status of knowledge (andignorance) in the 1970s are given in Ryan (1972)

and Ryan & Cita (1977).In combination with tephra layers (Keller et al.,1978), the characteristic sequence of sapropellayers (named S1 to S12) in sediment cores of theLate Quaternary proved to be an excellent tool forstratigraphic purposes. The observed temporalpattern of sapropel deposition was clearly pacedby climate (Cita et al., 1977; Vergnaud-Grazziniet al., 1977) and, in seminal papers by Rossignol-Strick (1983, 1985), were found to be locked to theorbital rhythms of precession and eccentricity.The temporal link is so robust that the astronom-ically tuned stratigraphy (Hilgen, 1991), estab-

lished from Mediterranean sediment cyclesexposed on land and recovered by scientificocean drilling, set the standard for Pliocene andQuaternary chronostratigraphy worldwide (Lou-rens et al., 2004). Sapropel and geochemicalrecords even have been used to reconstructchanges in tidal dissipation and dynamical ellip-ticity of the orbit of the Earth because of massload variations induced by glacialinterglacialcycles (Lourens et al., 2001).

OCEANIC ANOXIC EVENTS GLOBALAND REGIONAL

The concept of plate tectonics had a profoundimpact on research targets and research questionsin Earth history. For the first time a globalframework existed not only for tectonics but alsofor the newly developing field of palaeoceano-graphy (Hsu, 1976). With the inception of theDSDP, a new phase in black shale research wasinitiated. While its first target was testing the

theory of sea floor spreading, the DSDP alsoprovided a wealth of new geological data on theevolution of Mesozoic and Cenozoic oceans. Newresearch in the field of sedimentology shiftedfrom shallow-water to pelagic and deep-sea sed-iments (Hsu & Jenkyns, 1974). Information onMesozoic ocean history, mainly originating from

research in the Mediterranean region, was broa-dened greatly, in that data from continentalmargins could be combined with an increasingamount of data from pelagic and deep-sea envi-ronments. Bernoulli (1972) recognized close sim-ilarities between Tethyan Mesozoic deep-seasediments and sediments recovered by drillingfrom the North Atlantic Ocean. Bernoulli wasable to correlate a prominent, up to 2 m thickblack shale stratum found in Tethyan sedimentsof Middle Cretaceous age with a correspondingblack shale level at DSDP Sites 101 and 105.

Schlanger & Jenkyns (1976) proposed that dis-crete and isochronous black shale horizons werenot limited to the Atlantic and Tethys Oceans butthat they occurred globally. These authors pro-posed that the widespread occurrence of theseblack shales was related to global sea-level andproposed Oceanic Anoxic Events (OAE) of globalextension. Based on DSDP and Tethys Ocean data,Schlanger and Jenkyns defined two OAEs. TheAptianAlbian OAE covered millions of years,whereas the CenomanianTuronian OAE was ofshorter duration (< 1 Myr). Additional OAEs wereidentified later (ConiacianSantonian, Valangin-

ian, Toarcian) and OAEs have been reinterpretedas time envelopes within which organic carbon-rich sediments were formed episodically on aglobal scale (Jenkyns, 1980).

Even if the initial hypothesis on the dominatingrole of sea-level did not stand up to detailedstudy, interdisciplinary studies revealed a corre-spondence of OAEs with phases of increasedocean crust formation and the emplacement ofLarge Igneous Provinces (LIPs; Larson, 1991).Today, a clear link between volcanism and blackshale formation is established in the Aptian with

the LIP of the Ontong-Java plateau (Larson, 1991)and Weissert et al. (1998) proposed a coincidencebetween Parana volcanism and the ValanginianOAE.

Tethyan archives for the Mesozoic OAEs

Complementary to the DSDP/ODP record, Meso-zoic pelagic and hemipelagic sediments formed inthe Alpine Tethys Ocean became a prime target inblack shale research (see Fig. 2A, B and C for




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examples). The CenomanianTuronian, Aptian,Valanginian and Toarcian black shales were firstidentified in numerous pelagic sections from theAppennines and Southern Alps (Italy) and inhemipelagic successions from the VocontianTrough (Jenkyns, 1980; Arthur & Premoli-Silva,1982; Breheret, 1985; Weissert et al., 1985). Most

of these black shales were deposited in deep-waterenvironments of more than 1 km water depth.

The OAE time envelopes marking episodes ofincreased black shale formation in the Cretaceouswere redefined and dated with biostratigraphic,magnetostratigraphic and chemostratigraphicmethods (Alvarez et al., 1977; Channell et al.,1993) and were renamed after early workers in thefield of black shales. The organic-rich sedimentsof the Livello Bonarelli were deposited at thevery end of the Cenomanian within a few tens to a

few hundred kiloyears (Tsikos et al., 2004). Theblack shalelimestone/marlstone succession ofthe Livello Selli (Wezel, 1985; Coccioni et al.,1987) or Niveau Goguel (Breheret, 1988) accu-mulated within 1 Myr in the Early Aptian andcorresponds to OAE 1a. The durations of LivelloSelli and Livello Bonarelli were calculated with

cyclostratigraphic methods (Herbert, 1992;Wissler et al., 2003; Kuhnt et al., 2005). Otherblack shale intervals were recognized in theAlbian (e.g. OAE 1b or Niveau Paquier (Breheret,1985), in the Hauterivian (Faraoni Level; Baudinet al., 1999), in the Valanginian (Barrande layers;Reboulet et al., 2003; or Weissert Event; Erbaet al., 2004) and in the Toarcian (Jenkyns &Clayton, 1986). These intervals were identifiedas either of regional (Faraoni Level) or of globalextent (Valanginian, Toarcian black shales).

172.3633

A B D

C

Fig. 2. Examples of black shales and sapropels in drill cores: (A) Close-up of the lower Albian black shale in ODPHole 1049C (Blake Nose) that is time equivalent to Oceanic Anoxic Event 1b (photograph courtesy of J. Erbacher).(B) AptianAlbian shales intercalated with quartz sandstones in a hemipelagic Tethyan setting (Zone Sion-Cour-mayeur, Val Ferret, W. Switzerland). Hammer for scale is 25 cm long. (C) Black shale levels alternating with pelagic

limestones (Barremian and Aptian, S. Alps, S. Switzerland and N. Italy), the limestone bed on the right is 10 cmthick. (D) Core photograph showing black sapropels intercalated in calcareous oozes of Pliocene age (50 4 to 595 m

below sea floor) at ODP Site 969 (Eastern Mediterranean Sea). In sections 5 and 6, and in the core catcher, sapropelghosts are regularly spaced reddish brown layers. Each section is 150 cm long.




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Mediterranean archives for Late Neogenesapropels

The most complete and undisturbed record ofmarine sapropel sequences in the MediterraneanSea was recovered during ODP Legs 160 and 161that specifically set out to clarify the palaeoenvi-

ronmental history of this sea and the sequence ofPliocene to Holocene sapropel events. Many con-ventional gravity cores from the Eastern Mediter-ranean Sea and material from DSDP Legs 13 and42A were the basis for planning these drillingcampaigns(Ryan, 1972; Ryan & Hsu, 1973; McCoy,1974; Kidd et al., 1978). The ODP drilling of atransect of sites recovered several complete Plio-cene to Holocene sediment sequences that repre-sent a range of depositional environments, waterdepths and oceanographic and biological prov-inces in the modern Mediterranean (Comas et al.,

1996; Emeis et al., 1996). In the drill cores fromtheEastern Mediterranean Sea, a record containingmore than 80 individual sapropel events wasrecovered from Pliocene to Holocene age. Thesapropels at the drill sites of Leg 160 werecorrelated with the astronomical rhythms of pre-cession, tilt and eccentricity (Emeis et al., 2000),and with land exposures of the same age (Lourens,2004). In the Western Mediterranean Sea, thesapropel record was found to extend only toapproximately 2 Ma and was less complete(Cramp & OSullivan, 1999; de Kaenel et al., 1999).

The Eastern Mediterranean sapropels occur in

distinctive packets that often are separated byintervals of yellowish-brown, oxidized sediment(Fig. 2D). Many individual sapropels are extra-ordinarily rich in organic carbon (up to 30% byweight and predominantly marine in origin),many are at least in part laminated and range inthickness between a few centimetres and metres.Other sapropels are bioturbated or oxidized to aconsiderable extent and discernable only asburnt-out ghosts of reddish carbonate oozes withtell-tale chemical or magnetic properties. Duringcertain periods, diagenetic overprinting has

apparently occurred in the entire Eastern Medi-terranean basin, leading to isochronous, reddishsediment packages that may encompass severalhundreds of thousands of years (e.g. between 1and 07 Myr). Composite sapropel and isotoperecords showed that the climate influence chan-ged from obliquity and precession forcing in theperiod from 32 to 19 Myr to dominant obliquityinfluence in the period from 19 to 1 Myr; in thelast million years, all orbital cycles were clearlyexpressed (Kroon et al., 1998).

TECTONIC AND CLIMATIC SETTINGS OFBLACK SHALE DEPOSITION

Early on, the kingpins of the environmentalsignificance of black shales were put into place:Bitterli (1963) proposed that bituminous sedi-ments mark turning points in palaeogeographic

history (e.g. orogeny, eustasy) followed by prolificplankton production and by stagnation duringwarm and moist climate. The Cretaceous was agreenhouse time (Chamberlin & Salisbury, 1906),when enhanced volcanic activity increased thevolumes of mid-ocean ridges (Pitman, 1978) andraised both global atmospheric carbon dioxidelevels (Berner, 1991) and global sea-level (Haqet al., 1987). These effects resulted in high tem-peratures, high weathering rates and a peculiarocean circulation pattern described as sluggishby the early investigators. The high latitudes were

ice-free, the latitudinal temperature contrast wasweak (Barron & Peterson, 1990) and, with oceanbasins elongated in east-west rather than in thepresent north-south orientation, ocean circulationwas very different from that prevailing today(Brass et al., 1982; Barron & Peterson, 1990; Haupt& Seidov, 2001). An important difference was thatdeep water formed at low latitudes in the EasternTethys and that the rate of deep-water formationwas susceptible to variations in global tempera-ture and in monsoonal moisture transport.

It had been realized that black shales wereformed as the result of oceanographic changes

controlled by Cretaceous plate tectonics (seesummary in Weissert, 1981) and rising sea-level(Jenkyns, 1980). The influence of sea-level wasobvious in creating fertile, shallow marginal seaswith the potential to export organic matter intoyoung offshore basins, as well as for advection ofoxygen-depleted water masses from these mar-ginal seas far out into the open ocean (Hallam &Bradshaw, 1979; Jenkyns, 1980). Schlanger &Jenkyns (1976) and Thiede & Van Andel (1977)proposed that mid-water oxygen minimum zoneswere responsible for black shale formation in the

Pacific. Advection of shelf organic matter wasseen as a significant factor for black shale forma-tion in the Atlantic and Caribbean Basins, wherethe contribution of terrestrial organic matter wassignificant (Jansa et al., 1979; Degens et al., 1986).

When attempting to decipher the causes, pro-cesses and sequence of events leading to blackshale deposits, it is found that Mesozoic blackshales have disadvantages in stratigraphy, pres-ervation or comparison with recent analogues.Studies on these topics have concentrated on




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much younger and analogous sapropels, butErbacher et al. (2001) explicitly pointed out par-allels between emerging models for MesozoicBlack Shale and those used to explain PlioceneHolocene sapropel formation in the Mediterra-nean Sea. Clearly, the hypotheses on underlyingprinciples of black shale and sapropel deposition

in the respective scientific communities appear toconverge (Herrle et al., 2003). In both cases, theclimatic forcing at Milankovitch frequencies isinvoked and translates into: (i) changes in themonsoon system; (ii) increased moisture advec-tion; and (iii) changes in the water mass circula-tion of the Mediterranean Sea (and Tethys).

Sapropel periods were preceded by and coin-cided with an increase in the density contrastbetween surface and deep waters and in decreas-ing oxygen supply to the deep basins. Surfacewater density decreased because of the combined

effects of warming and freshening, so that thecold and salty deep water the old deep waterformed during preceding cold climatic stages prevented deep convection of oxygenated surfacewater. Both freshening and increasing sea surfacetemperature (SST) of surface water are indicatedby a variety of floral, faunal, isotopic and geo-chemical proxies. Deep waters in the basinsbecame anoxic after their dissolved oxygen wasdepleted several hundreds to thousands of yearsafter convection ceased.

The climatic trigger for hydrographic changesand sapropel formation is still not identified

with certainty and there is reason to believe thatseveral acted in concert. The original hypothesis(Olausson, 1961) was that in the Late Quaternaryice sheet melt water from the northern catchment(via the Black Sea) triggered stratification andstagnation. Timing discrepancies with Black Seaflushing, the very regular temporal pattern ofsapropels during the Pliocene and the absence ofice on the Northern Hemisphere are argumentsagainst this hypothesis. The northern catchmentof the Eastern Mediterranean Sea may havereceived increased precipitation during maxi-

mum insolation, leading to decreased salinity inthe deep-water formation areas (Rohling & Hil-gen, 1991). However, since approximately600 ka, some sapropels formed during glacialsor cold stadials that make runoff from thenorthern catchment unlikely and create condi-tions adverse to water-column stratification, ingeneral. Rohling (1994) suggested a link betweendeep-water stagnation in the Mediterraneanbasins with deglacial sea-level rise and surfacewater freshening in the Atlantic Ocean and

enhanced inflow of Atlantic surface water overconsiderably denser cold and saline Mediterra-nean water. Bethoux & Pierre (1999) suggestedthe same link as a primary influence responsiblefor the formation of sapropels in the WesternMediterranean Sea. The effects of rising sea-leveland embedded melt water pulses, as well as the

effect of Black Sea fresh water outflow followingconnection to the Aegean Sea, were modelled byMatthiesen & Haines (2003). These calculationssuggest that the effects of meltwater pulsesincreased stratification in the MediterraneanSea by 21% (meltwater pulse 1A around 12 ka)and 14% (meltwater pulsepulse 1B around95 ka). Gradual opening of the Black Seaincreased stratification by 13% and catastrophicopening by 43%.

Several sapropels formed when the EasternMediterranean Sea was in a glacial mode, as

suggested by cold SST estimates and pollenassemblages (e.g. S6 at around 176 ka, S8 ataround 220 ka). Sapropel formation under glacialconditions in the Late Quaternary is a strongargument for a source of fresh water in themonsoon system of Africa. A systematic correla-tion exists between the distribution of sapropelsand maxima of monsoon index which is afunction of precession and eccentricity controlledinsolation (Rossignol-Strick, 1983, 1985). Maximain the monsoon index coincide with the forma-tion of cold sapropels S6 and S8 and point to anintensified African monsoon during northern

summer. This effect enhanced precipitation intropical Ethiopia and, thus, enhanced flood dis-charge of the Blue Nile River. In this hypothesis,sapropel formation would be linked closely tonorthern tropical and even southern hemisphereclimate. Recent modelling experiments suggestthat Nile discharge alone was not sufficient toexplain the long-lasting enhanced stratification(Tuenter, 2004). Additionally, enhanced north-ward migration of the Intertropical ConvergenceZone (the meteorological equator) into the south-ern catchment of the Mediterranean Sea has been

proposed for sapropel periods at obliquity max-ima; this would have channelled tropical rainfallinto the Mediterranean Sea through what is nowthe Sahara Desert (Rohling et al., 2002).

A further possible source of fresh water is theMediterranean Sea itself, which today provides40% of precipitation in its own catchment.Almost all sapropels (except the cold S6 andS8) coincide with significant warming of surfacewaters at the transition from cold to warmclimatic periods. This warming would, on the




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one hand, enhance the stratification of watermasses and would, on the other hand, result inincreased evaporation in the entire Mediterra-nean Sea and rainfall specifically in the EasternMediterranean catchment (and further east),effectively pooling fresh and warm waters at thesea surface and impeding deep-water formation

(Rohling & Hilgen, 1991; Rohling, 1994; Emeiset al., 2003; Tuenter, 2004). This concept issupported by observations on isotopic composi-tion of connate waters in speleothems of Israelwhich suggest greatly enhanced precipitationrates on land adjacent to the Levantine Basinduring sapropel events. This rain had an isotopiccomposition that followed the MediterraneanMeteoric Water line (Bar-Matthews et al., 2003)except during periods corresponding to coldsapropel S6 and S8 deposition. The climaticcause of these sapropel events is still not

explained, although modelling experimentsimplicate glacially lowered evaporation ratesand enhanced runoff from the Nile and thusthe African (and Indian) low-latitude monsoonsystems (Masson et al., 2000).

It is most probable that several sources ofmoisture, each with a specific timing in relationto insolation, warming and related low-latitudeand high-latitude climate processes, succeededeach other and together created the very specialconditions in the Mediterranean Sea. Recentrecognition that Mesozoic OAEs are also rhyth-mic and composed of individual layers (Mene-

gatti et al., 1998; Herrle et al., 2003) may point toa similar external forcing.

PRODUCTIVITY OR PRESERVATION?

The publication by Schlanger & Jenkyns (1976)was the starting point of a new episode of blackshale research. In numerous publications,researchers discussed causes and consequencesof black shale formation. Did black shales formunder anoxic or dysoxic conditions related to

basin-wide stratification of water masses? Werethey the results of turbidity currents transportinglarge amounts of terrestrial organic matter fromland into the deep sea? Was productivity duringthe time of black shale formation exceptionallyhigh? Very early on, the debates concentrated onhydrological changes associated with black shaleand sapropel deposition and the roles of produc-tivity and anoxia in accumulating organic matterin sediments (see Weissert, 1981 for an earlysummary).

The Neogene sapropel and Mesozoic blackshale material recovered by drilling marinesequences has been found to be considerably lessaltered by diagenesis and weathering than landexposures and evidence from geochemical andmineralogical analyses has helped to constrainthe depositional environment. With this informa-

tion at hand, the arguments can be evaluated onwhether enhanced preservation (under anoxicconditions) or enhanced biological productivitywas the root cause for organic carbon enrichmentin black shales and sapropels (Calvert, 1987;Pedersen & Calvert, 1990). Much of the datagathered on sapropels and black shales supportthe hypothesis of low-oxygen or anoxic condi-tions during black shale formation, paired with(or accelerated and in part caused by) an increasein biological productivity (Calvert et al., 1992;Struck et al., 2001; Kuypers et al., 2002; Herrle

et al., 2003).The Mediterranean Sea was anoxic duringsapropel deposition but not as extremely andlastingly as the modern Black Sea the typeeuxinic basin is today. Because the shallowestvisible sapropels were found below a modernwater depth of 400 m, it was originally thoughtthat waters below that depth in the EasternMediterranean were anoxic. Anastasakis &Stanley (1986) suggested that time lags existedbetween the onset of sapropel conditions betweendifferent basins and water depths during sapropeldeposition, a concept that was later reformulated

by Strohle & Krom (1997) to indicate changes inthe extent and severity of oxygen depletion in amid-water oxygen minimum zone. Spatiallyresolved studies on benthonic fauna during iso-chronous sapropel events changed the view ofcompletely anoxic deep-water masses; depthtransects across northern sub-basins (Adriaticand Aegean Sea both are areas of deep-waterformation today) suggest that the deep-wateroxygen content continued to support impover-ished benthonic fauna throughout periods ofsapropel deposition (Casford et al., 2003); the

same holds for the main basins. In the S5 sapropel[transition from Marine Isotope Stage (MIS) 6 toMIS 5; around 127 ka], benthonic foraminiferaindeed vanish for some time (several hundreds tothousands of years) shortly after the onset oforganic-rich sedimentation. However, a low oxy-gen fauna often returns within the visible sapro-pel layer and several sapropels show eithercontinuous benthonic populations (Jorissen,1999; Schmiedl et al., 2003) or intermittent, inpart regional, reoxygenation events (Rohling




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et al., 1997; Casford et al., 2003; Schmiedl et al.,2003). This observation implies that convectionand deep-water formation in the northern basinscontinued and that oxygen recharge by convec-tion was not shut off completely. Benthonic faunain depth transects of isochronous sapropels sug-gest that convection reached down to around

1000 to 2000 m fairly regularly and occasionallysupplied oxygen even to the deepest sea floors.Although in low numbers, benthonic foraminiferaalso persisted in OAE 1b of the Mesozoic (Herrleet al., 2003). Bioturbation structures in many ofthe black shales deposited during the OAE 1a arefurther evidence for a benthonic fauna survivingin poorly oxygenated deep-water environments.

Molecular fossils characteristic of bacterialpigments specific to green sulphur bacteria sup-port the hypothesis of anoxic conditions in thewater column during deposition of Pliocene/

Pleistocene sapropels and black shales (Menzelet al., 2002; Pancost et al., 2004; Wagner et al.,2004). However, in the light of the distributionpatterns of benthonic foraminifera, the occur-rence of these markers also poses a problem.Nowadays, the strictly anaerobic bacteria thatproduce the pigments are known from stagnantponds and lakes and recently have been identi-fied as major primary producers in a deep chlo-rophyll maximum zone in the anoxic Black Sea.These bacteria require the combination of lightand hydrogen sulphide-containing waters. Thepresence of their biomarkers in black shales and

sapropels thus supports the concept that achemocline separates oxic and anoxic waters;however, it must have existed above the 01%light level that appears to be the lower end oftheir photosynthesis range. Until palaeo-depthpatterns of the chemocline and associated bacte-rial photosynthesis and benthonic faunas alikehave been clarified, the exact vertical segregationinto anoxic, suboxic and oxic water zones in mid-water remains open. However, with evidence forsulphidic waters from sulphur speciation studies(Passier et al., 1999) and lamination found in

many sapropels, there are four independentarguments for anoxia during at least some stagesof sapropel and black shale deposition.

Anoxia has an effect on the preservation oforganic matter in marine sediments (Demaison &Moore, 1980; Emerson & Hedges, 1988) andcauses significantly higher concentrationsof palynomorphs and individual lipids insapropels than in surrounding oxic sediments(Cheddadi & Rossignol-Strick, 1995; Bouloubassiet al., 1999). Biological productivity at the sea

surface is among the primary factors controllingconcentration and accumulation of organic car-bon in sea floor sediments (Suess, 1980), togetherwith sedimentation rate (sealing factor) andoxygen content at the sedimentwater interface(Muller & Suess, 1979; Canfield, 1994). Becausethe sedimentation rate found in most sapropel

layers of Late Pleistocene age is equal to or higherthan that in surrounding calcareous oozes, theenrichment in organic carbon in sapropels cannotbe explained by decreased dilution with othersedimentary components at equal flux rates oforganic carbon. The significant enrichment insapropels thus requires substantially higher bio-logical production than that found in theMediterranean Sea today, even under anoxicconditions that favour the preservation of organicmatter (Howell & Thunell, 1992).

Independent evidence for higher productivity

comes from distribution patterns of benthonicforaminifera and chemical analyses of productiv-ity proxies. Assemblages of benthonic foramini-fera have higher numbers of individuals and achange to faunas indicative of higher benthonicfluxes of organic matter (Schmiedl et al., 2003)before and after anoxic periods; the accumulationrates of barium and Ba/Al ratios (Mercone et al.,2000; Weldeab et al., 2003) and opal accumula-tion rates were high (Kemp et al., 1999). Organicgeochemical and palynological data suggest thatthe bulk of organic carbon in sapropels and olderblack shales is dominantly from marine sources,

with land-plant material consistently present inonly subordinate amounts (Bouloubassi et al.,1999; Hochuli et al., 1999).

Views differ on the mechanisms that enhancedproductivity. In some well-preserved Mediterra-nean sapropels, detailed investigation of diatomassemblages has been possible because of un-usually good preservation of the opaline silicathat in many marine sediments is usually lost todissolution (Schrader & Matherne, 1981; Kempet al., 1999). The best preserved sequences wererich in fragile diatom mats that form a pro-

nounced set of laminations, including possibleannual bloom layers. The flora is indicative ofsloping isopycnals and stratification found inassociation with hydrographic frontal systems.The diatoms are adapted to migrate between anutrient-rich lower water body of the front andnutrient-poor upper water. Collecting near theinterface between the upper and lower watermass in the front, the disintegration of slowlygrown mats sporadically resulted in large fluxrates of opal and organic matter to the sea floor.




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The floral assemblage in the diatomaceous sapro-pel strengthens the hypothesis of fresh waterinput as a cause for black shale formation whichwould result in the development of frontal sys-tems in the sea surface. The microfacies studiesprovide evidence for anoxic conditions duringsapropel formation and suggest that diatoms, even

if they are preserved rarely, were a prominentsource for the organic matter in sapropels.

A possible explanation for the link betweenanoxia and enhanced productivity comes fromstudies of nitrogen isotopes. Both in sapropelsand most, but not all, black shales, the nitrogenisotope ratio (15N/14N, expressed as d15N) ofsedimentary nitrogen is very low (between 0 and2&). Originally thought to support the hypo-thesis of enhanced nutrient input from land anddecreasing nutrient utilization (Calvert et al.,1992), recent interpretations suggest significant

atmospheric dinitrogen fixation by diazotrophiccyanobacteria during sapropel and black shaleperiods (Struck et al., 2001; Kuypers et al., 2004;see Jenkyns et al., 2001 for exceptions). Inbiogeochemical terms, that increase in nitrogenfixation suggests a decrease in the stoichiometric

ratio of reactive nitrogen over phosphate, so thatphytoplankton is limited by nitrate. Cyanobacte-ria use the remaining phosphate (and organicallybound phosphate that is not accessible to otherplankton) and convert it to organic matter (seereview article by Karl et al., 2002 for details).Decaying cyanobacterial biomass supplies phos-

phate and nitrate that can be assimilated byother phytoplankton (Tyrrell, 1999). Thus, toincrease productivity and organic matter fluxduring sapropel times, new phosphorus (Epp-ley & Peterson, 1979) had to enter the watercolumn in the Mediterranean Sea to stimulatecyanobacterial blooms. The only plausiblesource is sediments overlain by anoxic waters(van Cappellen & Ingall, 1994; Wallmann, 2003).Under the anoxic conditions in near-bottomwaters, phosphorus leaked out from sedimentsinto the deep-water body, as indicated by the

very high ratios of organic carbon to phosphorusin sapropels (Slomp et al., 2002; see Fig. 3below). In many black shales and virtually allsapropels that ratio is significantly higher thanthe expected molar ratio of 100 of aquaticbiomass.

Fig. 3. Illustration of the feedback mechanism leading to organic carbon sequestration in the case of Mediterraneansapropels S7 to S5 (marine isotope stages 7 to 5, 103 to 201 ka). Corrected for sea surface temperature changes andglobal ice effect, the decreased d18 O of planktonic foraminiferal calcite suggests freshening of surface waters by asmuch as 12 psu at the very base of sapropel layers. Together with increases in SST (determined by alkenoneunsaturation ratios) of up to 10 C, this freshening reduced the density of surface waters and convection ceased.Enhanced organic carbon burial was most probably aided not only by enhanced preservation under oxygen-deficientconditions at the sea floor but also by the addition of phosphorus from anoxic sediments (indicated by high C:Pmolar ratios), non-Redfield N:P ratios initially in the deep-water body and after shallow convection also in theeuphotic zone and concomitant nitrogen fixation (indicated by depleted d15 N ratios of bulk nitrogen). Compiled fromEmeis et al. (2003), Struck et al. (2001) and Weldeab et al. (2003). [mbsf = metres below sea floor]




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It is yet unclear how phosphate-rich deep waterreached the euphotic zone. Conceivably, ongoingshallow convection periodically or seasonallyeroded the pycnocline and chemocline andmixed phosphate-rich and nitrate-poor sub-pycnocline waters into the biologically activesurface water layer. This effect may have caused

excess phosphate over nitrate ratios in the eupho-tic zone, triggering cyanobacterial dinitrogenfixation at the oceanatmosphere interface and aconcomitant general rise in fertility (see below).Other hypotheses argue that the productivityregime during black shale and sapropel formationremained oligotrophic, because cyanobacterialnitrogen fixation today is an indication for lowproductivity regimes (Rau et al., 1987; Sachs &Repeta, 1999). Clearly, the last words on theproductivity/preservation issue have not beenspoken.

BLACK SHALES, CARBON CYCLE ANDCLIMATE: A NEGATIVE FEEDBACK INSYSTEM EARTH?

A remarkable shift in black shale research wastriggered around 1980 by major new develop-ments in climate research. Investigations on airbubbles frozen in ice cores of Antarctica providedexciting evidence for fluctuating carbon dioxideconcentration in the atmosphere during glacial-interglacial cycles (Neftel et al., 1982) with high

carbon dioxide levels coinciding with warmtimes during the Late Pleistocene. Carbon dioxidewas appreciated newly as a climate forcing gasthat acted as an amplifier in an orbitally drivenLate Neogene climate system.

Early in the course of research into MesozoicOAEs and black shales it became clear that thesesequences recorded global perturbations of thecarbon cycle and climate. Ryan & Cita (1977)pointed out that the deposition of vast black shaledeposits must have had consequences for theglobal carbon cycle and estimated that 80 1012 g

carbon per year were extracted to sedimentsduring these episodes. Weissert et al. (1979)suggested that elevated carbon dioxide levels inthe atmosphere could have triggered black shaledeposition. Arthur et al. (1984, 1985) developedthe first palaeoclimate models of OAE times.These authors proposed that changes in oceano-graphy were triggered by episodic increase involcanic carbon dioxide emission that raisedpCO2 levels of the atmosphere. In a self-regulatedEarth System, climate regulation and regulation

of the carbon cycle were envisioned as a combi-nation of biotic and abiotic processes. Elevatedcarbon dioxide concentrations were balanced byincreased weathering rates (Berner et al., 1983)and by an intensification of the biological carbonpump (Arthur et al., 1988).

The global footprints of past carbon cycling

emerged from studies with a new tool: stableisotope analyses of sedimentary organic andcarbonate carbon. Years after the Craig (1953)fundamental investigation of carbon isotope geo-chemistry in natural environments, carbon iso-tope geochemistry was introduced as a newtechnique in palaeoceanography (Berger et al.,1978). The carbon isotope composition of bio-genic carbonate could be used as a proxy for thepast sea water carbonate system and short-termfluctuations (< 100 kyr) could be related to fluc-tuations within the carbon reservoirs and to

changes in surface-water/deep-water gradientscaused by changes in ocean circulation. Studieson the Early Mesozoic focused on either long-term trends in carbon isotope records acrossOAEs, which reflected transfers of massiveamounts of carbon between carbon reservoirsand depicted effects of altered flows betweenreservoirs on the isotopic balance of the globalcarbon cycle (Scholle & Arthur, 1980), or onshorter term and regional fluctuations duringblack shalelimestone cycles which were relatedto changes in oceanography (Weissert et al.,1979).

Many of the early reconstructions of the Meso-zoic carbon isotope stratigraphy were done inTethyan pelagic successions and the carbon iso-tope records showed considerable fluctuations inboth inorganic and organic carbon fluxes. Therewere time intervals of up to a million years whenbiogenic carbonate was enriched in 13C. Thesepositive carbon isotope anomalies in carbonatewere interpreted to reflect burial of isotopicallydepleted organic carbon in sediments and, in-deed, the positive carbon isotope excursions inthe biogenic carbonate minerals of limestones and

marls were found to coincide with black shales ofOAEs (Fig. 4). Because all carbon reservoirs arelinked, burial of isotopically depleted organiccarbon had to be reflected by all other reservoirsin the sea (dissolved inorganic carbon), theatmosphere and on land (land vegetation, soils).When terrestrial carbon isotope curves had beenestablished for specific time intervals (Grockeet al., 1999), they confirmed that carbon isotopeanomalies recorded in both marine and terrestrialorganic matter were synchronous and of global




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extent, with positive carbon isotope anomalies asthe response signature of the biosphere to alteredcarbon dioxide levels in the atmosphere due tomassive organic carbon sequestration in OAEs.Over the last decades, a complete carbon isotopestratigraphy of organic and inorganic carbon hasbeen established (Weissert et al., 1998; Veizeret al., 1999; Voigt, 2000; Jenkyns et al., 2002;Weissert & Erba, 2004) (see Fig. 4).

A clear relationship between volcanism, carbonisotope excursions and black shale formation hasbeen established in the Aptian and to some extentin the Valanginian (Larson & Erba, 1999; Weissert& Erba, 2004). The main pulse of volcanic activityin the large igneous province of the Ontong-JavaPlateau between 123 and 120 Ma coincided withthe beginning of the major positive carbon isotopeanomaly corresponding to OAE 1a. Volcanic

Fig. 4. Compilation of Late JurassicEarly Cretaceous carbon isotope stratigraphy, major volcanic events, black shaleepisodes and marine biocalcification crises. Black shales: JA, Niveau Jacob; LS, Selli Level; BA, Barremian blackshales; FA, Faraoni Level; WE, Weissert Event or Barrande Layers (modified from Weissert & Erba, 2004).




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carbon dioxide that was added to the Aptianatmosphere triggered the carbon cycle perturba-tion recorded in the carbon isotope record. Yet,surprising results were obtained by high-resolu-tion carbon isotope studies across the OAE 1a inthe Aptian, where Menegatti et al. (1998) couldshow that the time of black shale formation did

not coincide with the most positive carbonisotope values. The OAE 1a coincided only withthe beginning of the main positive carbon isotopeexcursion. In the Valanginian, four minor blackshale levels identified in the successions of theSouthern Alps and of the Vocontian Trough alsocoincide with the very beginning of the carbonisotope excursion (Lini et al., 1992; Rebouletet al., 2003).

Carbon isotopes and methane bursts: anexample from the Early Cretaceous

Increasingly detailed studies revealed other puz-zling evidence: in several records, the expectedpositive d13C excursions of carbonates werepunctuated by negative isotope anomalies ofshort duration (Fig. 5). Originally, these anoma-lies were attributed to diagenesis and lithifica-tion, but when carbon isotope investigations ofplanktonic and benthonic foraminifera across thePalaeocene to Eocene thermal maximum hadconfirmed the negative carbon isotope pulses,Dickens et al. (1995) proposed, based on earlier

work by Kvenvolden (1988), that the release ofisotopically depleted methane (d13C = )50&)stored in clathrates could bring enormousamounts of12C into the oceanic carbon reservoir.Presently, these negative spikes are generallyaccepted as expressions of additions of 12C fromthe geosphere into the atmosphere/ocean system,

as a consequence either of rapid methane releaseat times of warming climate (Grocke et al., 1999;Hesselbo et al., 2000), or by volcanic intrusionsinto continental margin deposits and associatedheat flow anomalies (Jenkyns, 2003; Svensenet al., 2004).

Methane release probably was triggered bysudden global warming caused by the greenhouseeffect of accelerated volcanic carbon dioxidedegassing (Fig. 6). Methane release/oxidationfrom clathrate dissociation would have furtherenhanced atmospheric carbon dioxide levels and,

thus, global temperature. Warming intensifies thehydrological cycle and, in concert with higherpCO2 levels, chemical weathering, which is apowerful sink for carbon dioxide. Higher atmo-spheric pCO2 levels decreased the pH and car-bonate supersaturation of the ocean to the pointthat nannofossils could not or did not need tocalcify (nannoconid crises; Erba & Tremolada,2004). The ratio of inorganic carbon to organiccarbon of sinking biogenic material and sedi-ments decreased. Along the coasts, biocalcifica-tion crises related to decreased carbonate

Fig. 5. Representation of negatived13C excursion in pelagic carbonates

associated with OAE 1a (Aptian) inNW Sicily. The negative spike at120 Ma is in temporal agreementwith the end of volcanism on theOntong-Java Plateau in the WesternPacific Ocean and is interpretedas the isotopic expression of gashydrate release (Jenkyns, 2003).




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saturation resulted in widespread drowning ofcarbonate platforms (Wissler et al., 2003). Higherfertility and intensification of organic carbonsequestration in sediments resulted from en-hanced nutrient delivery from weathering, fromthe addition of new phosphate from anoxic seafloors and through bacterial nitrogen fixation.Enhanced organic carbon preservation underanoxic conditions and diminishing dilution oforganic matter by carbonate, together with weath-ering, constitute a powerful negative feedbackthat apparently repeatedly has saved the Earth

from runaway greenhouse conditions in thecourse of the Mesozoic (Weissert, 2000).

Sapropels: an illustration of thebiogeochemical feedback

The sapropels are of limited temporal (severalthousands of years) and spatial extent (restrictedto the modern Mediterranean Sea) and themasses of carbon involved are too small toregister on the scale of the global carbon cycle,but the detail hidden in sediment cores is

impressive and it may serve to trace very basiccausal chains that outline the negative feedbackleading to black shales. Even if the physicalenvironment prior to sapropels was not un-balanced by additions of carbon from clathrates,and instead was dictated by insolation cycles orglacialinterglacial transitions, the system doesnot differentiate between causes: warming wasrapid and initiated the kick-start of a negativefeedback (organic carbon sequestration in sedi-ments) that keeps carbon dioxide in check

(Fig. 3). Warming surface waters (by as much as12 C as estimated from the Alkenone Unsatura-tion Index; SST in the graph) lead to an inten-sified hydrological cycle and precipitation, andto freshening of surface waters because ofchanged evaporationprecipitation/runoff bal-ances (indicated by d18O-decreases of surface-

dwelling Globigerinoides ruber calcite). Bothwarming and freshening of surface waters en-hanced stratification, inhibited or weakeneddeep-water formation and oxygen advection todeep-water masses and led to anoxia in deeperwater bodies isolated from the atmosphere.

Under the anoxic conditions in near-bottomwaters, phosphorus leaked out into the deep-water body, as illustrated by the soaring ratios(up to 600) of organic carbon to phosphorus justbelow and in the sapropel layers (Fig. 3). Thisobservation suggests phosphate-enriched deeper

water masses, as they occur today in the BlackSea (Fonselius, 1974). Differing from the BlackSea, convection may have diminished but con-tinued to transfer surface waters (and oxygen)into the intermediate water masses and evendeep basins. Benthonic faunas suggest that sea-sonal or more episodic convection must havecontinued and it must have seasonally erodedthe density boundary separating upper andlower water masses, entraining waters withexcess phosphate over reactive nitrogen intothe euphotic zone. This condition triggeredcyanobacterial dinitrogen fixation from the

atmosphere (as indicated by low d15N values inthe sapropel layers) and added reactive nitrogenfrom mineralization of cyanobacteria to the waterbody to adjust nutrient ratios back to Redfieldratios. That added reactive nitrogen enhancedthe overall nutrient levels and trophic status initself a positive feedback mechanism that raisesthe fertility and productivity in surface waters(Tyrrell, 1999).

The sequence of events outlined by changes intemperature, salinity, phosphate content of sedi-ments and nitrogen isotope ratios in a sapropel

sequence of Late Quaternary age may reflect ingreat detail a basic negative feedback mechanismof carbon sequestration in black shales that hasprevented runaway greenhouse conditions on theEarth throughout its history since the Palaeozoic.Pliocene to Holocene sapropels of the Mediterra-nean Sea are, thus, excellent showcases for theexternal forcing mechanisms, internal bio-geochemical cycles, trophic relationships, diver-sity and post-depositional alterations of blackshales in a broad sense.

Fig. 6. Components of a negative feedback mechanismthat causes increased organic carbon sequestration insediments; this mechanism may have operated in blackshale and sapropel formation alike. Note that bothcarbon dioxide-induced warming (elevated pCO2 of theatmosphere because of methane pulses and its oxida-tion) and insolation (in the case of the MediterraneanSea) may trigger the feedback.




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SUMMARY AND CONCLUSIONS

Mesozoic black shales indicate massive perturba-tions in the global (and specifically marine)environment and in the global carbon cycle,driven by large-scale tectonic and climaticchanges. Typical process time scales that can be

resolved by scientific study are in the order of amillion to hundreds of thousands of years. Plio-cene to sub-recent black shales (sapropels) do notregister on a global scale but they do reflect global(mainly climatic) processes. Typical process timescales are from a million years to hundreds ofyears; thus, they permit the detailed analysis ofprocesses involved in black shale formation.

All black shales global or regional areproduced by chains of events that translateclimate change to oceanographic change, whichcause reactions in chemistry and subsequent

adaptations of biology and result in enhancedcarbon sequestration in the lithosphere. Theauthors tend to view these shales as a productof a basic negative feedback mechanism that actsagainst enhanced carbon dioxide concentrations(and concomitant global temperature rise) in theatmosphere.

Future Mesozoic black shale research will focuson the improved documentation of the globalextent of these events. The impact of a suddenrelease of carbon dioxide into the atmosphere andoceans on the marine and terrestrial biosphere willbe studied in detail across the OAEs and inter-

pretation of causes and consequences of changesin carbon reservoirs and flow rates between reser-voirs on the Earth will be aided by increasinglydetailed numerical Earth system models. Obviousdifferences between the various Mesozoic OAEsare recognized but they are far from being under-stood. A further task at hand for black shaleresearchers is the identification of a regionallyvariable response of ocean systems to global short-term (


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Manuscript received 1 February 2008; revisionaccepted 26 August 2008



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