55. history of the mediterranean salinity crisis · 55. history of the mediterranean salinity...

55. HISTORY OF THE MEDITERRANEAN SALINITY CRISIS

Kenneth J. Hsü,1 Lucien Montadert,2 Daniel Bernoulli,3 Maria Bianca Cita,4 Albert Erickson,5 Robert E.Garrison,6 Robert B. Kidd,7 Frederic Mélières,8 Carla Müller,9 and Ramil Wright10

ABSTRACT

An equatorial ocean existed during the Mesozoic between Africaand Eurasia known as Tethys. The Alpine Orogeny, culminating inthe late Eocene and Oligocene, eliminated much of this ancientocean. However, the relic Tethys, combined with the newly createdpost-orogenic basins, still retained host to a Mediterranean seaway.The link was first broken in the Burdigalian with the collision ofAfrica and Eurasia along the Middle East front. This event startedthe gradual changes towards a cooler and drier climate in regionswhich were now situated on the west side of a continent.

Middle Miocene movements cut off forever the opening on theeast to the Indo-Pacific. Also the Perialpine Depression north of theAlps was raised and the communication between the Mediterra-nean and the Paratethys was severed. The considerable fresh-watersupply from Central Europe was then diverted to the Paratethys,and the reorganization of drainages placed considerable strain onthe water budget of the Mediterranean. Finally, soon after thebeginning of the Messinian stage, the last opening to the Atlantic,the Betic and Rif straits were closed. Evaporative drawdown of theisolated Mediterranean sea level, leading to desiccation, becameunavoidable, since evaporative loss considerably exceeded precipi-tation and influx from rivers.

The Mediterranean Evaporite can be divided into two units. Thedeep-sea drilling did not penetrate the Main Salt or the lower unit.The history of the onset of the salinity crisis and of the early saltdeposition has been told by studies of sections on land. The landrecord in Italy indicates a very sudden change from deep, openmarine conditions to environments of shallow-water carbonateevaporite deposition and of subaerial diagenesis. This first desicca-tion was followed by the deposition of the Main Salt unit. A thickbody of salt was built by the evaporation of seawater spilled overfrom the Atlantic into Mediterranean brine lakes of considerabledepth.

An intra-Messinian desiccation led to widespread erosion andrecycling of primary halites. The Upper Evaporite depositionfollowed a marine flooding which may have again filled theMediterranean to the brim. Evaporative drawdown of the Mediter-ranean sea level and the gradual concentration of the brines led toformation of evaporites characterized by the "bull's eye" pattern ofsaline zonation: Dolomite was the earliest mineral formed, followedby sulfate deposition. Halite was laid down on the bottom ofplayas, which are now abyssal plains, and the potash salts were

Geological Institute, ETH, Zurich, Switzerland.2Institut Français du Pétrole, Rueil-Malmaison, France.3Geologisches Institute, Universitat Basel, Basel, Switzerland.4Istituto di Geologia, Universita, Milano, Italy.5Department of Geology, University of Georgia, Athens, USA.6Earth Sciences Board, University of California, Santa Cruz, USA.^Institute of Oceanographic Sciences, Wormley, U.K.8Laboratoire de Géologie Dynamique, Université de Paris, Paris,

France.^Geologisch-Palàontologisches Institut, Johann Wolfgang Goethe Uni-

versitat, Frankfurt, W. Germany.10Department of Geology, Beloit College, Beloit, USA.

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K. J. HSU ET AL.

restricted to the deepest and most central parts of the Mediterra-nean basins. Repeated flooding and desiccation led to the accumu-lation of cyclically deposited sediments. Shallow-water diatoms andalgal stromatolites prove that the basins were covered by veryshallow waters during the flooding stage, and sedimentary struc-tures indicate subaerial exposure during the desiccation stage ofthese cycles.

The Mediterranean was underlain by a series of desert and saltlakes towards the end of the Messinian, when the eastern Mediter-ranean was inundated by brackish water, probably from a Para-tethys source. A number of lakes known collectively as the "LagoMare" came into existence. Marl, or dolomitic marl with an Ammo-nia-Cyprideis fauna was deposited in many of these lakes. Thewestern Mediterranean may have remained desiccated until the At-lantic water began to spill over the western portal. The infill beganwith the deposition of marls with restricted marine, dwarf faunas.

The final and irrevocable flooding of the Mediterranean tookplace at the beginning of the Pliocene. Normal marine circulationhas prevailed ever since that time.

INTRODUCTION

The discovery of the Mediterranean Evaporite for-mation by the drilling from Glomar Challenger in 1970proved that salt exists under deep sea floor (Ryan, Hsüet al, 1973). We could no longer avoid the issue of saltgenesis in ocean basins by assuming that diapiricstructures on seismic profiling records on the Atlanticmargins, the Mediterranean, and of the Gulf of Mexicoare mud diapirs. The fact was also brought home thatgiant salt deposits could have been formed within arelatively short geologic time: No longer could thegenesis of enormous evaporite deposits be related tolateral accretion of coastal salina deposits, consequentupon facies migration. The synchronous onset andtermination of the pan-Mediterranean salinity crisisimplies drastic changes of environmental conditions ina region over two and half million square kilometers inextent, and that these changes took place within tens orhundreds of thousand years. The discovery of theMessinian Mediterranean Evaporite was the nail intothe coffin of Lyell 's substantive uniformitarianism. It isnot surprising that we should have heard again callsfor a restrained return to catastrophism (e.g., Sylvester-Bradley, 1973).

The significance of the DSDP discovery was, how-ever, overshadowed by disagreements over the genesisof the Mediterranean Evaporite. That the Mediterra-nean Evaporite formation was Messinian in ageseemed to be just about the only consensus after theLeg 13 drilling. Even the shipboard scientific staffcould not reach an agreement on the genesis of this un-usual formation. Chapter 43 of the Leg 13 cruise reportpresenting the desiccated deep-basin model was au-thored by K. J. Hsü, Maria Cita, and W. B. F. Ryanbecause they were the only members of the shipboardstaff totally convinced of its plausibility. Alternative in-terpretations were given by other shipboard staff in theVolume 13 Initial Reports and later in other publica-tions (e.g., Nesteroff, 1973a, b; Wezel, 1974). The re-actions of the scientific community to the desiccateddeep-basin model were also divergent. Favorable com-

ment was not rare, and journalists had a field day,headlining the story in the press, radio, and television,with reports of varying accuracy. On the other hand,many published critiques in scientific journals wereskeptical, and some downright hostile. There was awidespread impression that a tall tale had been spunfrom a few core chips. This harsh judgment was part-ly justified in view of the modest amount of recov-ered core material. The porponents of the modelwere almost too shy to admit that the only drillingevidence for the presence of Messinian evaporites inthe eastern Meditteranean was afforded by severalchips of gypsum in a bucket of odorous mud scrapedoff the outer-bit after the drill string was taken out ofHole 125.

The importance of clarifying the question of theMessinian salinity-crisis did not escape the attention ofthe Joint Oceanographic Institutions for deep EarthSampling (JOIDES) organization. A cruise to theMediterranean was included in the very first scheduleproposed by the JOIDES Planning Committee as soonas the Third Phase of DSDP was ascertained. Mean-while, the Mediterranean Advisory Panel was reorga-nized to infuse new blood and to permit considerationof divergent viewpoints within the international scien-tific community.

The essence of the desiccated deep-basin model liesin the postulate that the Mediterranean basins as weknow them today were largely in existence prior to theMessinian salinity-crisis, and that these basins weredesiccated, or partially desiccated, one or more times toform the evaporite deposits that had been sampled.Leg 13 drilling had not penetrated very far beyond thetop of the Messinian. We had very little data on pre-Messinian history, and we did not have many samplesof the evaporites themselves. The emphasis in planningLeg 42A was, therefore, to penetrate stratigraphicallyas deep as possible to obtain information on thegeologic evolution of the Mediterranean prior to thesalinity crisis. Secondly, a site in the central abyssalplain was to be drilled to provide as continuous arecord as possible of this unusual event.

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HISTORY OF MEDITERRANEAN SALINITY CRISIS

Our aims were not completely fulfilled during theLeg 42A drilling because of time constraints, technicallimitations, and safety requirements. We did not drillbeyond the lower Miocene, nor did we obtain a com-plete record of the Mediterranean Evaporite formation.Nevertheless, we have cored sufficient materials andhave produced sufficient data to enable us to reach anear consensus on the previously controversial questionof the origin of the Mediterranean Evaporite. Thisarticle is not a perfunctory report requiring co-author-ship by the shipboard scientific staff. It is a unanimousopinion written by the senior author for 10 co-authors,who represent all but two of the shipboard scientificstaff of Leg 42 A.

THE MIOCENE MEDITERRANEANOne of the most often debated questions during the

March 1973 Utrecht Symposium on Messinian Eventsin the Mediterranean revolved around the basin depthsof the evaporite depositions. The participants thereseemed to have agreed that "evaporation took placeunder shallow water conditions, at least for the carbon-ates and sulfates" (Drooger, 1973). It was also agreedthat the configuration of the Alpine-Mediterraneanregion was changed drastically by the Alpine orogeny.The orogenic movement had eliminated most of theoceanic area between Eurasia and Africa by the end ofthe Eocene (Argand, 1924). The Mediterranean is,therefore, at least in part a new creation. But which, ifnot all, of the Mediterranean basins were new? Whendid these new basins form? In the early and middleMiocene prior to the Messinian salinity crisis (e.g.,Argand, 1924)? In the Pliocene shortly after the salin-ity crisis (Selli and Fabbri, 1971)? Or "did the mainphase of subsidence occur rather abruptly during thePleistocene and Holocene" (Neev et al., 1976)?

Many different tectonic models have been proposedto interpret the evolution of the Alpine Mediterraneansystem. From the classical approach of Argand (1924),

to the plate-tectonic advocates of more recent years(Smith, 1971; Dewey et al., 1973), syntheses of re-gional geological data reached a conscensus that theMediterranean had already evolved to a stage quitesimilar to the present configuration prior to the lateMiocene when the salinity crisis began. Several of ushad reviewed the tectonic data on land and under thesea, and had reached the same conclusion (Montadertin Biju-Duval et al., this volume; Hsü and Bernoulli,this volume). Of the five major Mediterranean basins(Figure 1), the Levantine and Ionian basins may berelics of Tethys and of Mesozoic age; the Balearic andTyrrhenian basins were formed during the early andmiddle Miocene, after the acme of the Alpine orogeny;only the Aegean may have owed its origin primarily toPliocene-Quaternary subsidence. We shall not repeatall the arguments here and the readers are referred tothese two synthesis chapters of this volume.

Deep-sea drilling during Leg 42A has unearthedconvincing evidence that deep Mediterranean basinswere in existence prior to the Messinian. Drilling atSite 372 indicated that the Balearic Basin was formedby rifting during the latest Oligocene or earliest Mio-cene time (Figure 2). Paleobathymetric analyses on thebasis of benthic foraminifers estimated that this part ofthe Balearic margin had a water depth of at least 900meters as early as Burdigalian time. The water depthreached in excess of 1200 meters during the lateBurdigalian and had subsided gradually to a depthmore than 1500 meters prior to the end of the Serra-vallian (Figure 3, see also Wright, this volume). Inaddition to the initial subsidence associated with therifting phase of the basin, the Balearic Basin may havecontinued to subside during the Neogene in responseto mantle cooling. Furthermore, the accumulation ofthe thick salt deposits and the eventual flooding of adesiccated deep basin must have induced considerablymore subsidence in isostatic response to these addi-tional loads. We do not question the evidence that

II 11 Northern Continental Margin

λ:<i Early Cretaceous Flysch Troughs

Ophiolites

Southern Continental Margin

Figure 1. Tectonic setting of the Mediterranean basins (from Hsü and Bernoulli, this volume).

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K. J. HSU, ET AL.

W

MENORCA RISE SITE 372

J J Plio-Quaternaryto upper Burdigalian

(4) Upper Burdigalian-Aquitanian<?)

Messinian: Evaporites

Oligocene(?)

3J Tortonian(?)/Serravallian

6) Continental basement

Figure 2. Rifted margin of the Balearic Abyssal Plain (after Hsu et al, 1975).

there has been subsidence during the Pliocene-Quater-nary. However, there is no evidence that the BalearicBasin was a shallow shelf sea prior to that subsidence.In fact seismic evidence proves the existence of a deepBalearic Basin long before the salinity crisis began. Thewedge of the sediments at the Balearic margin north ofMajorca near the pre-Messinian escarpment is about1000 meters thick (Figure 4). The same thickness wasobserved on the Catalania margin, on the " o l d "margin at the approach to the Gulf of Valencia, andprobably also in the Gulf of Lyon (Montadert et al.,this volume).

The pre-Messinian sequence at Site 372 is mainlyhemipelagic, similar to the sediments deposited theretoday (Figure 5, see also Chapter 3, this volume). Theplanktonic and benthic fauna and flora are fully openmarine. As well as benthic foraminifers, the study ofwhich formed the basis of a systematic paleobathymet-ric analysis, psychrospheric ostracodes also occur insome Burdigalian and Serravallian cores and indicatewater depths in excess of 1000 meters. Similar psychro-spheric ostracodes are present in the middle Mioceneof the Betic in Andalusia and in the DSDP cores fromthe Atlantic; these occurrences suggest an oceaniccontinuity at that time between the Mediterranean andthe Atlantic (Benson, this volume). Isotopic analysis ofthe planktonic and benthic foraminifers shows a largedifference in the values of δθ 1 8 , which has been inter-preted to mean a temperature difference between thesurface and the bottom of some 10 °C (Grazzini, this vol-ume). The difference implies the existence of a middle Mio-cene Balearic Sea of considerable depth. Assuming thatMediterranean bottom waters of that period werecontributed by the ocean with a δ around -1.2%o onthe PDB scale (Shackleton and Kennett, 1975), thebottom temperature of the lower Miocene sea shouldhave ranged between 7° to 12°C, average 8.5°C (Graz-zini, this volume). This conclusion, in our opinion,confirms the ecologic interpretation of psychrosphericostracodes by Benson (this volume).

Drilling results at Site 373 indicated that the Tyrrhe-nian Basin was also in existence prior to the Messinian.The drill string penetrated some 200 meters into thetop of the seamount there and cored tholeiitic basalts.The chemistry of the basalts is comparable to that ofthe Pacific marginal basins or that of spreading mid-ocean ridges, and is distinctly different from that ofalkali basalts on land or in rifted continental valleys(Barbieri et al., this volume; Dietrich et al., thisvolume). Pillow structures and isotopic evidence alsoindicate submarine eruption. The Tyrrhenian basaltshave been dated radio metrically to range from lateMiocene to early Pliocene, showing that the seamountvolcanism there ceased to be active during the Plio-cene, or about 3.5 m.y.B.P. However, the seamountgrew at a slow rate of about 40 m/m.y. The submarinevolcanism that was responsible for the growth of theseamount must have started considerably earlier than7.5 m.y. (or the age of the oldest core obtained). Wemight thus speculate that the rifting of the Tyrrhenianand the accompanying submarine volcanism was amiddle or early Miocene event. Furthermore, thebathymetry, the structure, and the history of volcanismsuggest that the Tyrrhenian Basin is a younger featurethan the Balearic; the submarine volcanism, and pre-sumably the processes leading to the subsidence of theTyrrhenian continued from the Miocene to the Plio-cene Quaternary and on to the present. Nevertheless,the genesis of the basins began, and the central portionof the basin most probably sank to bathyal depths,prior to the Messinian salinity crisis. That the Tyrrhe-nian was already a deep-sea basin at the very begin-ning of the Pliocene has been established by thedrilling results of Leg 13 (Ryan, Hsü, et al.,1973). Theoccurrence of basalt breccias in Hole 3 73A supportsthis interpretation. The paleotemperature for the sub-marine cementation (by calcite) of the breccias wasestimated by isotopic evidence to be 6°-8°C (Bernoulliet al., this volume). Such a low temperature for theearly Pliocene seabottom implies that the seamount

1056


150lower'PLIO

upper

PLIO

PaleodepthMinimumEstimate

(m)

1000

Benthonic ForaminiferalAssemblages

(cumulative percent)Percent

Planktonic

0

2000

100

0 50

W/////////////J

V//////////////.

V////////////Δ(barren)

V//////////////ΔV//////////////Å

W////////////ΔV////////////MV//////////////ÅV//////////////ΔW/////////////A

V/////////////AV/////////////ÅV/////////////Å

W///////////A'//////////////A

Y/////////////,

V////////////Å

(See Citaet al., thisvolume for detailsof Core 9)

1 6 5 0

I 7 0 0

PaleodepthMinimumEstimate

(m)

Benthonic ForaminiferalAssemblages

(cumulative percent)Percent

Planktonic

02000

1000

MW/////Δ

V///////////Å

W////////Δ

W////////Δ

W//////Δ

W////////Å

W/////Δ

Figure 3. Paleobathymetric analysis of Site 372 (from Wright, this volume).

was submerged to a great water depth shortly after thesalinity-crisis.

Our drilling penetrated beyond the Messinian at Site375 on Florence Rise in the Levantine Basin (Figure 6). Asin Hole 372, the pre-Messinian sequence in this easternMediterranean hole is also mainly hemipelagic. Pa-leobathymetric analyses on the basis of benthic fora-minifers were more difficult here, but the data aresufficiently compelling to indicate that the LevantineBasin was in existence during the early and middleMiocene and that the water depth reached in excess of1000 meters during the Tortonian (Wright, this vol-ume). Hole 375 penetrated only as far as the lowerMiocene. However, the pre-Miocene history might bedeciphered by correlation with land section on Cyprus,which is only 50 km east of Site 375. On the island, in

the northern part of Mesaoria and along the KyreniaRange, a sedimentary sequence described by Barozand Bizon (1974) is in part identical with the pre-evaporitic Neogene at Site 375 (Baroz et al., thisvolume). The Kyrenia sequence in turn is similar andpasses laterally into the Tertiary deposits above theTroodos Massif described by Robertson and Hudson(1974). The Troodos was a fragment of the Meso-Tethys ocean overlain by Late Cretaceous and Tertiaryoceanic sediments (Moores and Vine, 1971). Thecorrelation with Cyprus thus suggests that this part ofthe Levantine Basin is underlain by thick pelagic andhemipelagic sequences above a deformed oceanic crustof late Mesozoic age. The very old age of the basin ismanifested geothermally by the unusually low heat-flow values measured here (Erickson, this volume).

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K. J. HSU, ET AL.

m

MIOCENE SEDIMENTARY WEDGEOFF MAJORCA

*°*\ Upper Evaporites

Acoustic Basement

Berthe I VERTICAL EXAGGERATION X 5

Figure 4. Seismic profile of the Balearic margin north of Majorca. Note that thewedge of sediment butting against the pre-Messinian basement is about 1 kmthick (from Montadert, et al, this volume).

At Site 377 on the Mediterranean Ridge, we drilledinto middle Miocene sediments. The pre-Messiniansediments there are not entirely hemipelagic: the blackshales and cross-laminated siltstones were apparentlydeposited on a continental rise or in a basinal setting.The few samples yielded a poorly preserved benthicforaminifer fauna. A paleobathymetric analysis tenta-tively placed the fauna in the mesobathyal zone;greater than 1000 meters water depth. (Wright, thisvolume).

Whereas the paleobathymetric results from theeastern Mediterranean basins are less complete andless conclusive than those from the west, the tectonic

evidence is more convincing. Syntheses of data fromgeology on land from off-shore drilling, from marinegeophysics, and from deep sea drilling leave littledoubt that the eastern Mediterranean is a relic of theMesozoic Tethys (Biju-Duval et al., this volume; Hsüand Bernoulli, this volume). It has been undergoingcompressional movement since the late Mesozoic andthroughout the Tertiary. The Hellenic Arc, the Medi-terranean Ridge, and the Cyprus Arc all underwenttectonic deformatins and uplifts, which are continu-ing today. An overview of available data cannotsupport the hypothesis by Israeli geologists (Neev etal., 1976; Gvirtzman and Buchbinder, this volume)

1058


WSBSSBBBBSBBB

. ÷ :::;|:p,:.|f«li|H

-60

1-75

-80

-85

merits, Site 372, Core 9, Section 2 (fromCita et ah, this volume).

that the Levantine Basin hosted a shelf sea before itfoundered abruptly during the Pliocene-Quaternary.There exists even less evidence to support the corollaryof the hypothesis by Gvirtzman and Buchbinder (thisvolume) that the African and Arabian platforms wereelevated thousands of meters above global sea level forabout 2 million years only during the Messinian toaccount for the deep erosion of the Nile and otherrivers emptying into the desiccated eastern Mediterra-nean.

The Aegean is obviously the youngest of the Medi-terranean basins. The Aegean back-arc basins mayhave originated in the middle Miocene (Biju-Duval etal., this volume). Vertical movements continued duringthe Tortonian when the marine waters invading theAegean severed the connection of Crete from continen-tal Europe (Meulenkamp and Zachariasse, 1973). TheCretan Arc was technically active during the Pliocene-Quaternary and is still active today (Hsü and Ryan,1974; Papazachos, 1975) and there has been consider-able Aegean subsidence during the Quaternary (SeeSite 378 report this volume). Hole 378a penetrated thetop of the Messinian, and sampled selenitic gypsum,which was deposited in an evaporite basin. However,we have little information on which to judge if this wasa deep- or a shallow-water depression prior to thesalinity crisis.

EVENTS LEADING TO THE SALINITY CRISISAn equatorial ocean separated Europe from Africa

during the Jurassic and Cretaceous known as Tethys(Neumayr, 1885; Suess, 1893; Hsü and Bernoulli, thisvolume). With the approach of these two continents,much of the Tethys was eliminated by the AlpineOrogeny (Figure 7). With the creation of the newMediterranean basins after the orogeny, the connectionbetween the Atlantic and Indo-Pacific was maintaineduntil the early Miocene. This connection was severedwith the joining of the two continents along the MiddleEast front during the Burdigalian (see Table 1). Thiswas one of the major paleobiogeographical events ofthe Tertiary; it was during this period that "the presentpattern of land and sea was established" (Adams,1973, p. 453).11 The immediate effect of this Burdiga-lian event on the Mediterranean sedimentation wasrather obscure. We noted changes in lithology andsome minor change in faunas from the early Burdiga-lian sediments of Unit IV to the late Burdigalian sedi-ments of Unit III at Site 372 in the Balearic Basin (seeSite 372 report, this volume). The long-term influencewas, however, global: the joining of Europe and Asiapermitted the exchange of land faunas between thesecontinents, and started the differentiation of the Indo-Pacific and Atlantic-Mediterranean marine faunas (see

Figure 5. Hemipelagic sediments overlain un-conformably by Messinian varve-like sedi-

πThe Atlantic and the Pacific did not lose their Central Americanconnection until much later, but the effect of that event is beyond thescope of this paper.

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K. J. HSU, ET AL.

Florence Rise

FLORENCE RISE W. Cyprus SITE 375-376

(T) Plio-Quaternary

(2} Messinian: dolomitic marls with turbidites

0

Messinian: Evaporites

[4) Tortonian-Serravallian: marlstones with turbidites

.5) Langhian-Burdigalian

10 km

Figure 6. Structural cross-section across the Florence Rise. Note that the sequence has been subjected to compressionaldeformation during the Tertiary.

Suture of Paleotethys

Suture of Tethys

Discontinuous trend

Figure 7. Tethys suture zones. Much of Mesotethys is believed to^ have disappeared bysubduction along the Alpine-Himalaya suture zone (from Hsu and Bernoulli, thisvolume).

papers in Adams and Ager, 1967; Hallam, 1973).Whereas the evidence from vertebrate paleontologyindicates the establishment of an intercontinental con-nection between Eurasia and Africa during the Burdi-galian (Savage, 1967), the micropaleontological datasuggest that there might have been repeated transgres-sions into the Mediterranean from the Indopacific untilmiddle Miocene time (Rögl et al., this volume). Thefinal severance of the Mediterranean link with theIndian Ocean probably took place at about 14m.y.B.P., during the Serravallian (Buchbinder andGvirtzman, 1976). A late Serravallian (14-13 m.y.)Indopacific transgression reached Paratethys, but didnot enter the Mediterranean province.

The pattern of faunal distribution as we know ittoday was thus largely established during the middleMiocene; the Mediterranean was cut off from theIndian Ocean and the Indopacific species that had notalready gained entrance were unable to do so. The

joining of the continents also began a gradual changein the Mediterranean climate, which became increas-ingly more arid as is typical of regions in low latitudeson the west side of a continent. We might thus assumethat the first seed of the salinity crisis was implanted inthe Burdigalian and became viable in the Serravallian. Ittook some 15 million years of incubation before thecrisis was born!

The orogenic movements which fused Eurasia andAfrica also raised new mountains in the Taurides, theHellenides, the Dinarides, and eventually, somewhatlater in the middle or early late Miocene, the HelveticAlps. This series of movements led to a separation ofthe Mediterranean from Paratethys. Land geologyprovides excellent information with which to recon-struct the paleogeography. The distribution of land andsea during the middle Miocene was as shown in Figure8, a figure modified from the text of Gignoux (1950),through the addition of deep-sea drilling information.

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TABLE 1Outline of the History of Messinian Salinity Crisis

Age Event Consequence to Mediterranean

Burdigalian

Serravallian

After beginning ofMessinian

MessinianLower Evaporite

Intra-Messinian

Intra-Messinian

MessinianUpper EvaporiteNear end ofMessinianEnd of Messinian

Beginning ofPliocene

Joining of Africa and Eurasia

Rise of Perialpine DepressionTectonic movement in northernItalyClosure of the Betic and Rifstraits

Continuous influx of Atlanticwater into the Mediterranean,without refluxingCutoff of seawater influx

Influx of Atlantic water

Alternating closure and influxfrom AtlanticInflux of water from Paratethys

Influx of water from Atlantic

Opening of the Strait of GibraltarFlooding by Atlantic water

Loss of the connection to the Indian Ocean; Mediter-ranean now on west side of a continent, with climaticchanges towards increasing aridityLoss of the connection to Paratethys; cutoff of con-siderable fresh-water supply, straining the Mediter-ranean water budgetLoss of connection to Atlantic Ocean Evaporativedrawdown of Mediterranean sea level. Formation ofthe Calcare di baseFormation of the lower part of the Main Salt

Intra-Messinian unconformity, erosion, and deposi-tion of recycled salts (upper Main Salt)Marine deposition at the base of Upper Evaporite;transgression over the intra-Messinian unconformityCyclic sedimentation of the Upper Evaporite

"Lago Mare" deposition in the eastern MediterraneanbasinsRestricted marine deposition in the western Mediter-ranean basinsTrubi TransgressionEnd of Messinian salinity crisis

The Paratethys was separated from the Mediterraneanby the mountain chain which stretched from the Alpsthrough the Dinarides and the Hellenides to Anatolia.Four possible connections between the two waterbodies have been suggested; the link may have ex-tended across (1) the peri-alpine depression (2) north-western Yugoslavia, (3) the Bosporus, or (4) theCaucasus (Senes, 1973). At the beginning of theMiocene, the Bosporus certainly did not exist (seediscussion later). Nor was there any evidence of earlyMiocene marine connections linking Paratethys and theMediterranean across the mountains of Anatolia or theBalkans. Stratigraphic and paleobiogeographic dataindicate that the Burdigalian connection between thetwo seas went through the Peri-Alpine Depressionnorth of the Alps (Gignoux, 1950; Rögl et al., thisvolume). This seaway was eliminated early in middleMiocene, when the Helvetic Alps rose and marinewaters withdrew from the Molasse trough; but a lastthread of contact was maintained between the Mediter-ranean (Langhian) and the Paratethys (early Bade-nian) through an opening in northern Italy and north-western Yugoslavia (Rögl et al., this volume). Thecomplete separation of the two inland seas took placesome 14 or 15 million years ago during the earlySerravallian (Gheorghian and Popescu, 1975; Rögl etal., this volume). As mentioned above, the later andlast Indopacific flooding invaded the Paratethys butdid not reach the Mediterranean.

The middle Miocene separation had two conse-quences. The Paratethys could no longer receive ma-rine waters from the Mediterranean nor could theMediterranean receive fresh waters from those of theEurasian rivers which now emptied themselves into theParatethys. Furthermore when the connection wasopen, some of the surplus ions resulting from evapora-

tive excess in the Mediterranean found their way to,and were deposited as lower middle Miocene salts inthe Pannonian and Transylvanian basins (see thediscussion of the Badenian Desiccation in Rögl et al.,this volume). After the loss of communication, theMediterranean was deprived of one of its means ofelimination of evaporative waste; exchange with theAtlantic became its only possibility.

Now that the Paratethyan basins received such alarge share of the fresh water from rivers, the Para-tethyan waters evolved with a progressive freshening(Gheorghian and Popescu, 1975; Rögl et al., thisvolume). The Paratethys supported brackish and/orfreshwater faunas even during Messinian time whenthe Mediterranean was evaporated dry (Senes, 1973;Rögl et al., this volume). Paratethys' gain was theMediterranean^ loss. The reduced influx of fresh waterplaced even greater stress on the water budget of theMediterranean. We might thus blame the middleMiocene event for having nourished the trend toward aMediterranean salinity crisis.

The Mediterranean^ last links with the world oceanwere the Betic and the Rif straits (Benson, this volume;Cita et al., this volume; Biju-Duval et al., this volume).With the relentless northward march of Africa, theirfate was inevitable. The deep Betic Strait shoaled afterthe end of the middle Miocene; no Tortonian psychro-spheric ostracodes have been found inside the Mediter-ranean (Benson, this volume). Nevertheless, weaklinks, similar to the Gibraltar Strait of today, seem tohave been competent enough to keep the Mediterra-nean waters normal marine. Open marine planktonicand benthic fossils are found in lower Messiniansediments directly beneath the evaporites of landsections in northern Italy and in Sicily (see Cita et al.,this volume).

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Figure 8. Relationship between the Mediterranean and Paratethys and also the AtlanticOcean during early to middle Miocene. The shaded areas are marine (modified afterGignoux, 1950). Possible communications between Paratethys and the Mediterraneanare (1) Peri-Alpine Depression, (2) Istria, (3) Bosporus, (4) Caucasus.

The change from normal marine to an evaporite -forming condition was a sudden event! Yet the paleo-botanical record gives no evidence of an abrupt changein climate. Benda (1973) summarized the evidencefrom Turkey, Greece, and Italy and stated that 'themost remarkable climatic changes to a drier and coolerclimate must have taken place before the Tortonian orin the transitional interval between Serravalian andTortonian.' In fact, even this change must have been acontinuation of the general trend that had been inevidence ever since the Burdigalian, when the Mediter-ranean was first cut off from the Indian Ocean. Bendafurther remarked that his pollen evidence showed littleclimatic variation 'since early Tortonian and during thewhole Messinian.' This conclusion is confirmed byBertolani-Marchetti and Accorsi's (this volume) studiesof Leg 42A core material. They found a cooler anddrier pollen assemblage, similar to that of the Messin-ian, in the latest Tortonian. Thus we cannot interpretthe change from normal marine to evaporative condi-tions as a response to a drastic change in climate; thechange had to be related to a final closure of theopenings at the west sometime during the Messinian12.With the isolation and its hydrographic deficit thesalinity crisis of the Mediterranean became finallyinescapable.

We have, unfortunately, no record of the onset ofthe salinity crisis from the deep-sea cores. The contactbetween the restricted marine Messinian and the open

12One of the authors (CM) believes that the Betic Strait wasnever completely closed, citing "the evidence of the apparentlycontinuous upper Miocene sequence there described by Montenat(1973). The question was debated heatedly during the UtrechtSymposium. Several of the co-authors (e.g., KJH, MBC) questionedwhether Montenat was able to discern obscure disconformities of theBetic Pliocene-Miocene sequence.

marine Serravallian sediments in Hole 372 is a discon-formity, representing a stratigraphic hiatus of 6 millionyears (Figure 5). That we failed to obtain such arecord is accountable by the fact that the planningstrategy required that drill sites be placed where thethick lower, salt-bearing lower evaporites are absent,so as to enable the drill string from Glomar Challengerto penetrate sediments older than the MediterraneanEvaporite. For this reason, we have to rely on landrecords for our knowledge, and for our future investi-gations, of the critical time interval when the salinitycrisis first began (See Sturani, in press; Cita et al., thisvolume).

THE GENESIS OF UPPER EVAPORITE

Division of the Mediterranean Evaporite Formation

Seismic records suggested a twofold division of theMediterranean Evaporite (See Figure 9 and Montadertet al., this volume).

1) An Upper Evaporite sequence, with numerousreflectors, up to several hundred meters thick, andconsisting of dolomite, gypsum, anhydrite, and somesalts.

2) The Main Salt sequence, seismically homoge-neous, up to a thousand meters thick or more, togetherwith a lower sequence of evaporites with severalreflectors.

The distribution of the Main Salt unit is restricted tothe more central part of each of the Mediterraneanbasins. The distribution of the Upper Evaporite unit is,however, much more extensive. The top of the unit,represented by the M-reflector, has been recognized onthe seismic record throughout the Mediterranean(Ryan et al., 1971; Wong and Zarudsky, 1969). Therelief of this reflector closely simulates the relief of thepresent submarine topography, indicating that the

1062

10 km

0 m570 600 750 700 750 800 840

0 m

1000 m 1000 m

2000 mWATER DEPTH IN MIDDLE OF BASIN=2450 m

5000 m £

2000 m

r 5000 m

Plio-QuaternarySequence

UpperEvaporiteSeries

MainSalt

Infra-saltSeries

AcousticBasement

Figure 9. Twofold subdivision of the Mediterranean Evaporite. Note that the Upper Evaporite extends further toward the basin edge than the Lower Evaporite,which includes the main salt deposit, (a) Ligurian Sea near the northwestern corner of the Balearic Basin. The lower evaporite pinches out and the Pliocene/Quaternary sediments thicken toward the Rhone submarine fan area to the northwest, (b) South Balearic Abyssal Plain near Site 371 (location of which isindicated by vertical arrow). Note that both the upper and lower evaporite pinch out toward the top of the submarine high. The lower evaporite (Main Salt)is restricted to the more basinal position under the abyssal plain.

1 - Plio-Quaternary Sequence2 - Upper Evaporite Series3 - Main Salt \ Lower Evaporite4 - Infra-salt Series ) Series5 - Volcanic Basement

Vertical Scale: 5 cm/sec, t. d.

o Figure 9. Continued

K. J. HSU, ET AL.

BALEARIC TYRRHENIAN IONIAN

100 km

Figure 10. M-reflector, top of the Mediterranean evaporite (from Hsu et al, 1973). Vertical scale is in seconds (two-waytime); vertical exaggeration is 55:1.

Messinian bottom configuration was not much differentfrom that of today (Figure 10)13. The parallelism ofthe M-reflector to the modern sea floor is particularlyimpressive in the Tyrrhenian Basin with its numerousseamounts and other rugged relief. Where the lowerunit is absent, the Upper Evaporite overlies the pre-Messinian sediments unconformably, as we proved bydrilling Site 372 on the Balearic margin (Figure 2) andSite 375 on the Florence Rise (Figure 6).

A similar twofold division has been recognized inthe Messinian salt basins of Sicily (Figure 11). Decimaand Wezel (1973) suggested intra-Messinian tectonicdeformation. Field observations by some of us duringthe 1975 Erice Seminar at Eraclea Minoa near Agri-gento showed that the intra-Messinian deformationwas, at least in part, gravitational in response towidespread collapse associated with extensive saltdissolution.

The Sicily salt basins during the Miocene wereprobably some of the deeper Mediterranean basins,they had no drainage outlet so that halite and potashsalts were deposited from the last bitterns. Other peri-continental evaporite basins such as those in Piedmont,in the peri-Adriatic and in Africa are not saliferous;they were relatively shallow and probably had an exitto deeper basins. The evaporite beds in those basins

13The statement that the Messinian Mediterranean "was notmuch different from that of today" seems to be a source of misun-derstanding. We are quite aware of the many significant paleogeo-graphic changes since the Messinian. Marginal areas or peri-conti-nental basins around the Mediterranean have been severely de-formed. The Tyrrhenian and the Aegean basins subsided in responseto back-arc tectonic activities. Finally, there was isostatic subsidenceof the basin floor everywhere as a consequence of the flooding ofpreviously desiccated basins. It should be pointed out that we do notadvocate a suspension of tectonic activities since the end of theMessinian. We do, however, believe the evidence that the oceaniccrust under the Mediterranean was largely in existence long beforethe salinity crisis. The Balearic, the Tyrrhenian, the Ionian, and theLevantine basins were deep basins for open marine hemipelagicsedimentation prior to their desiccation and their resubmergence.

are mainly gypsum (Drooger et al., 1973; also Messin-ian Seminar, 1975, see Kidd, 1976), and the Messinianevaporites there cannot be subjected to a twofolddivision. Instead the sections are characterized by cyclicdeposition, with the cycles repeated from several to 13or 14 times (Vai and Ricci Lucchi, 1976). The correla-tion of sequences in peri-continental basins, with theSicilian or the submarine sections is not certain as willbe discussed later.

The Mediterranean evaporites sampled by the DeepSea Drilling Project during its two cruises belongexclusively to the Upper Evaporite unit. Contributionsto the understanding of the environment of evaporite-deposition during the Messinian from drilling are thuslimited to interpretations of the Upper Evaporite. Itsgenesis will be discussed first, followed later by somecomments on the origin of the Main Salt and of thecyclically deposited Messinian sections on land.

Subdivision of the Upper Evaporite Unit

We have penetrated the edges of the Upper Eva-porite at Site 372 and at Site 375, where its thickness is50 meters or less. Under the central abyssal plains ofthe Mediterranean basins, the Upper Evaporitereached maximum thicknesses of several hundredmeters. Under the Messinian Abyssal Plain (Site 374)and on the edge of the Antalya Basin (Site 376) weexplored the central, saliferous facies, but we pene-trated only to some 80 and 160 meters, respectively,below the base of the Pliocene, and thus were unableto obtain a complete record. The Upper Evaporite hasalso been sampled at DSDP Sites 121, 122, 124, 125,129, 132, 134, 371, and 378. The cores from thesestrategically placed sites provided considerable infor-mation on the sedimentary environment and faciesdistribution of the Upper Evaporite unit.

If we define the Upper Evaporite unit as the sedi-ments below the base of Pliocene and above the MainSalt, this upper unit can again be given a twofoldsubdivision (Figure 12).

1064


Pasquasia

jMussomeii

N E

1 2

S. Antonio

1 sw

Upper10 Evaporite

Series

98d

8c

8b

Lower8a Evaporite-, Series

Figure 11. Twofold division of the Mediterranean Evaporite in Sicily (from Decima and Wezel, 1973). NE-SW stratigraphicsection of Solfifera Series along the line approximately correspondent to the Cattolica Basin axis. The lower group of theSolfifera Series (No. 1-8): (1) lower Messinian marly clay, (2) whitish marls, (3) Tripoli Formation, (4) basal limestone,(5) Cattolica gypsum beds, (6) gypsum turbidites, (7) marly anhydritic basal breccia, (8a-8d) halite ("Zone A") and potash("Zone B") beds with intercalations of potash salts ("Zone C") and basal dark anhydritic marl ("Zone D"). The uppergroup of the Solfifera Series (No. 9-11): (9) transgressive gypsum-arentite, (10) Pasquasia gypsum beds, (11) Arenazzolo.The Trubi Formation: No. 12.

FORAM Q.SANDY SILTSTO NANNOFORAM. MUDS

• . . • . . . ; . ,

NANNOFORAM.OOZE AND MARLSWITH SAPROPELS

• • B • r

0 m

-100

-200

-300

•400

1 PLIOCENE-QUATERNARY2 UPPER EVAPORITE SERIES3 MAIN SALT4 PRE-EVAPORITE SEQUENCE

Figure 12. Upper Evaporite from Site 374. Note that we did not reach the base of the evaporitic subdivision.

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K. J. HSU, ET AL.

1) "Lago Mare," or nonevaporitic, upper subdivi-sion

2) Evaporitic subdivision including cyclically de-posited gypsum, marls, and salts.

A complete record of the "Lago Mare" depositswas obtained in Holes 374 and 376; its significance willbe discussed in the next section. We shall limit ourdiscussions here mainly to the genesis of the evaporiticsubdivision of the Upper Evaporite. Please note thatwe failed to penetrate to the bottom of this subdivi-sion; the portion of the Upper Evaporite not reachedby drilling probably also consists of cyclically depos-ited sulfates and carbonates plus salts as describedhere.

Saline ZonationDrilling confirmed the "bulls eye" pattern of saline

zonation of this evaporitic subdivision which wassuspected from seismic evidence (Figure 13). The foursaline zones and their distribution are:

Dolomite only at Site 121Dolomite and gypsum (and/or anhydrite) at Sites

124, 125, 132, 134, 371, 372, 375, 378.Dolomite and gypsum (and/or anhydrite) and hal-

ite at Sites 134 and 376.Dolomite and gypsum (and/or anhydrite) and hal-

ite and potash salts at Site 374.Through drilling sites near topographic promi-

nences, namely Sites 122, 133, and 371, Messinian, terrigenous elastics have been cored in place of, or in

addition to, evaporite minerals. Messinian sedintentsare absent on or near seamounts or tilted basementhighs bounding abyssal plains, such as at Sites 123,129, 134, and 373.

Three different models have been proposed to ex-plain zonation of saline minerals.

1) Brine-refluxing models: Several variations havebeen proposed on the theme of brine refluxing. Thebasin may either be shallow (King, 1947; Scruton,1953) or deep (Schmalz, 1970). The main suppositionshere are that an opening to the world ocean existed,and the brine level within the basin was equal to sealevel so as to receive marine waters from, and to returnbrines back to the open sea. The saline zonationresulting from such a process would give a tear-droppattern (Figure 14, see also discussions in Schmalz,1970: Hsü, 1972), but not the observed pattern.

2) Separation-basins model: The model assumesthe existence of a series of evaporite-depositing basinseach, except the last draining into a lower one (Bran-son, 1915). This originally purely hypothetical ideawas probably inspired by the processes of makingartificial salt. In each basin, the evaporite deposit couldbe monomineralic, with the evaporative carbonatedeposited in the highest, sulfate in the next, halite inthe ensuing, and potash in the lowest basin.

3) Progressive-desiccation model: This model pos-tulates gradual lowering of brine level in a completelyisolated basin. With the gradual concentration of thebrine, minerals of increasingly greater solubility wouldbe successively precipitated. This model was originallyelaborated to explain the origin of saline zonation of

playa lakes (Hunt, 1960; Jones, 1961). The distributionof saline minerals in any isolated basin being progres-sively desiccated should have a "bull's eye" pattern(Schmalz, 1970; Hsü, 1972).

The lack of a tear-drop pattern for the salinezonation of the Messinian Upper Evaporite Unit, aswas previously discussed by Hsü et al. (1973a, b),argues against a model of brine-refluxing. Also recallthat the reflux model was proposed originally to ex-plain the occurrence of monomineralic evaporites suchas thick sulfate deposits without halite, or thick halitedeposits with potash salts. The problem of segregationof the Messinian evaporite minerals can be explainedby the separate-basin model. Paratethys in easternEurope, peri-continental basins on the shelf margin ofthe Mediterranean, and the deep Mediterranean basinsmay constitute a sequence of separate basins as envi-sioned by Branson (1915). For example, Messinianevaporitic carbonates were deposited in the Black Sea(Hsü, 1977), which was one of the Paratethyan basinsdraining into the Mediterranean. Gypsum was beingdeposited in the Periadriatic Basin and in several otherperi-continental basins. Halite and potash salts weredeposited only in those deep basins which had nooutlet for the last bitterns! Furthermore, there is theadditional complication of detrital-evaporites and ofrecycling! Hardie and Eugster (1971), for example,found extensive detrital gypsum deposits in Sicily.Decima (1975) and Kuehn and Hsu (this volume)noted that continuous sequences of halite withoutpotash salts are recycled deposits, whereas the halitesderived from evaporation of seawater are interbeddedwith more soluble salts. Thus we do not have todepend upon the brine-refluxing hypothesis to come upwith a rational explanation for the segregation or theassociation of the Messinian evaporite minerals.

The pattern of saline zonation was already apparentafter the Leg 13 drilling. We were thus able to predictthe occurrence of potash salts under the central Messin-ian Abyssal Plain. Leg 42A drilling results elsewhereare also consistent with the postulate of a "bull's eye"distribution of evaporites within each of the Mediterra-nean deep basins. This fact is considered one of thestrongest arguments for the isolation and progressivedesiccation of the various basins (Hsü et al., 1973b;Drooger, 1973).

The "bull's eye" pattern of facies distribution givesno indication of basin depth; the evaporative residuemay have been rimming a deep saucer or a shallowsalt pan. The evidence that the Messinian Mediterra-nean salt basins were deep depressions has beenprovided by considerations such as those discussed inthe preceding section. The pattern of facies distributionalso cannot indicate brine depth. Brine depths inisolated deep basins can be considerable. The presentDead Sea, for example, has a water level of 400 metersbelow the sea level, but its brine depth is still another400 meters; it is not exactly a shallow salt pan! Thewater depth in the Mediterranean basins undergoingdesiccation should have been quite variable. Theoreti-cally gypsum deposition in a basin 2000 meters deepcould start when half of the original seawater was

1066

o

rO•

Algero-provencal

basin

(1600 m)

Sicily-caltanisetta

basin

(800 m )

abyssalplain

(1200 m]

Herodotusabyssalplain

(3500 m)

1 —

3 —

- 4 -

Nile deepsea fan

(1500 m)

South Florencearea

(1500 m)

Levantineabyssalplain

(1300 m)

Basin north ofMediterranean

Ridge

(3600 m)

"Upper evaporites""Salt layer"

•"Lower evaporites?'

7 -

Antalyabasin

(2000 m)

Adanabasin

(800 m) Karatas,

- 6 -

•8

Figure

(...m) Mean thickness of total evaporitic sequence in the central part of each basin

13. Fades distribution of the Mediterranean Evaporite in the Balearic Basin.

7 -

-8-1

K. J. HSU, ET AL.

( I I HUH!

Carbonates Gypsum U = ü = Halite

Figure 14. Tear-drop pattern (from Schmalz, 1970). (a)cross-section, (b) plan view.

removed by evaporation, or when the water depth wasstill 1000 meters. At more advanced stages of desicca-tion, the depth may have been only a few meters, orcompletely dry. Thus the evidence of water depth ofevaporite deposition must be sought from the eva-porites themselves.

Cyclic Deposition

Those who have been shipboard scientists onGlomar Challenger are well aware of the problem ofobtaining a continuous core section, especially if thesection consists of alternating hard and soft layers.Where coring is random and recovery poor, one rarelynotices rhythmic changes. It was, therefore, not surpris-ing that Leg 13 scientists recognized cyclic sedimenta-tion only when a 3-meter-plus continuous section ofevaporite core from Site 124 was patched together longafter the cruise. The four members of a cycle are frombottom to top: (1) laminated carbonates; (2) interlami-nated dolomite and sulfate; (3) stromatolitic carbonateand sulfate; and (4) nodular anhydrite (Hsü et al.,1973a).

A similar pattern of cyclic sedimentation of theUpper Evaporite was recognized in Hole. 374. Anidealized cycle is portrayed in Figure 15. The descrip-tions and interpretations of the three members from Aat bottom to C at top are based upon detailed sedi-mentological studies by Garrison et al. (this volume).

A Member: The major lithology is rich in organicmatter, dolomitic mudstone, homogeneous, or finelylaminated (comparable to Member I of a Site 124cycle). Gypsum crystals are commonly scattered

MAIN LITHOLOGIES

LAMINATED. COARSE GRAINED GYPSUMWITH WAVY BEDDING THAT LOCALLYGRADES INTO NODULAR STRUCTURE.

VERY EVENLYLAMINATEDBROWN TOWHITE GYPSUM.

DOLOMITIC TO DIATOMACEOUS MUDSTONE.COMMONLY LAMINATED (ALGAL STROMA-TOLITES') IN UPPER PART. LOWER PARTCOMMONLY CONTAINS DISPLACIVE NOD-ULES AND CRYSTALS OF SECONDARYGYPSUM.

SHALLOW /SUBAQUEOUS/

Figure 15. Cyclic sedimentation at Site 374 (from Garri-son et al, this volume).

throughout the mudstones and are seen to disruptlaminations. The mudstones were interpreted to be theproduct of shallow, subaqueous deposition, and thegypsum crystals were secondary growths that occurredduring sub aerial diagenesis.

B Member: The major lithology is a brown gypsumwith blotches and with even and generally silty lamina-tions (comparable to Members II and III of a Site 124cycle). This is the type of gypsum called "balatino" byworkers in Sicily. The laminae are defined by very thinseams of organic matter (possibly algal coatings). Thegypsum was interpreted as shallow subaqueous sedi-ment, deposited probably within the photic zone,before undergoing extensive recrystallization.

C Member: The dominant lithology is nodulargypsum (comparable to Member IV of a Site 124cycle). Member C sediments include small seleniticcrystals and carbonate muds deposited in shallowsubaqueous conditions. The sediments were displacedand replaced by nodular anhydrite, which in turn waschanged to nodular gypsum in a later diagenesis.

In a 20-meter section from Hole 374 (Cores 16-20),some five cycles were recognized. They resembleMessinian cycles described from Sicily (Heimann andMascle, 1974; Schreiber et al., 1976).

Garrison et al. (this volume) interpreted a cycle asindicative of a general lowering of water level, ofincreasing salinity, and of increasing degree of subaer-ial exposure. The overall pattern, therefore, is one ofsubmergence-desiccation, as a result of flooding andsubsequent evaporation. The range of features in thesecycles resembles very closely those found in Holocenetidal flat cycles of arid regions such as those beneaththe sabkhas of the Trucial Coast (Butler, 1970). In thetideless sea of a desiccating Mediterranean, the cyclesshould not represent tidal fluctuations, but climaticfluctuations which raise or lower water level overrather short time spans. The possibility that such slightfluctuations of water level could produce these drasticsedimentological changes is further evidence that thewater depth was very slight and the basin floor wasextremely flat.

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The fact that nodular anhydrite (or gypsum) hasbeen found in the basinal fades of evaporitic depositsand the unwillingness to entertain the working hypoth-esis that a basin could be occasionally desiccated lednumerous workers to question the sedimentologicalsignificance of this feature (e.g., Kendall, 1974; Deanet al., 1975). However, the interpretation of shallow-water or subaerial genesis of the Upper Evaporite hasnot been based upon comparative sedimentology. Themore critical evidence is paleoecological: shallow-waterdiatoms, and algal stromatolites, both of which aremost reliable environmental indicators, have beenfound in Leg 13 cores (Hajos, 1973; Schreiber, 1973).Diatoms, which lived in water depths less than 20meters, and algal filaments indicate growth in thephotic zone were again found in Leg 42A cores.(Schrader and Gersonde, this volume; Awramik, thisvolume). Their presence in the cyclically depositedsequence of Site 374 is an unquestionable confirmationof the environmental interpretations of Garrison et al.,this volume, who based their conclusions upon princi-ples of comparative sedimentology. The Upper Eva-porite cores from other Leg 42A sites are less wellsampled. However, sedimentological studies of thosesamples available have led to the same conclusion; theevaporitic sediments were deposited in shallow waters

and were diagenetically changed in subaerial environ-ments (see Site Reports, Chapters 2, 3, 6, 8).

The occurrence of shallow-water sediments andproducts of subaerial diagenesis in basins which hadheld thousands of meters of marine waters implieseither uplift or desiccation. There is neither evidencenor a suitable mechanism for the postulate of a basin-wide uplift during the Messinian to change suddenly aseries of deep basins into a broad, flat shelf sea.Calculations, however, have shown that the waters in aMediterranean about as deep as today could be evapo-rated dry within 1000 years or so, if and when it wascompletely isolated (Hsü et al., 1973a). It is this line ofreasoning which led us to adopt the deep-desiccatedbasin model to explain the genesis of the Mediterra-nean Evaporite.

Origin of Salts in the Upper Evaporite Member

The deep-sea drilling holes failed to penetrate intothe Main Salt unit. The halite and potash salts encoun-tered in Holes 134, 374, and 376 all belong to theUpper Evaporite (Figure 16).

Sedimentological evidence indicated that the halitefrom Site 134 was precipitated from a shallow salinepool (Hsü et al., 1973a). This conclusion was subse-quently confirmed by the "bromine profile" of halite;

Figure 16. Halite and potash salts, Site 374. (from Kuehn and Hsu, this volume).

1069

K. J. HSU, ET AL.

the bromine composition changed greatly from onehorizon to another, as one would expect from saltdeposited in a shallow pool (Kuehn and Hsü, 1973).

The bromine content of the Site 134 halite rangesfrom 140 to 254 ppm, while that of Site 374, which isassociated with potash salts, ranges from 150 to 840ppm (Kuehn and Hsü, this volume). The high brominecontent suggests that the salts were deposited from abrine derived from the evaporation of seawater. Incontrast the halite of Site 376 has a very low brominecontent of only 20 to 30 ppm (Kuehn and Hsü, thisvolume). It is very likely that this halite has beenrecycled, because secondary halite, deposited from abrine which has derived its NaCl from the solution ofprimary halite, has little bromine.

It is interesting to note that the last salt has not beenrecycled in the Balearic and Ionian basins, whereasthat of the Antalya Basin has been recycled. Appar-ently the margin of the latter was exposed, and theprimary halite on the lower flank of the Florence Risewas dissolved by streams and redeposited in a brinepool occupying only the more central part of theAntalya Basin.

Marine Incursions

Marine fossil assemblages are present in the marlsoverlying, or interbedded with, evaporite layers in thewestern Mediterranean holes (124, 132, 134, and 372).The foraminifer faunas in these marls are, as a rule,dwarfed, indicating that marine organisms lived in theMediterranean during the Messinian, at times when tfredesiccated basin was again completely, or partially,filled with seawater. The ecologic conditions were,however, unfavorable. The faunal abundance, thefaunal diversity, and the degree of dwarfness aredifferent at different levels, suggesting variable degreesof stress (Cita et al., this volume). In Hole 372, forexample, a rich and diversified Messinian fauna ispresent in Sample 372-9-1, 130 cm, where keeledGlorobotalias are present. Similar dwarfed foraminiferfaunas, made up of planktonic species only and lessdiversified, are present in gypsiferous marls interca-lated between finely laminated gypsum (Sections 372-6-1 and 372-5-1). The uppermost Messinian sediments(Core 4, Section 1) in this hole contain again a ratherdiverse fauna of planktonic and benthic foraminifers.The conditions were inhospitable even then as sug-gested by the unusually high proportion of Bolivinaand Cassidulina in the benthic population. The pres-ence of deep-water species of Cibicides indicates thatthe water depth of this latest Messinian sea was at leastas deep as lower epibathyal (Cita et al., this volume).

To summarize we conclude that the western Medi-terranean basins were covered by marine waters repre-sented repeatedly during the time span by the UpperEvaporite deposition. Probably the marine incursionsmanaged to spill over the Sicilian Strait, a few times atleast, as sediments with marine fossils are also presentin the largely "Lago Mare" sequence of Sites 374 and376. However, the possibility cannot be ruled out that

the marine incursions to the eastern Mediterraneanoriginated from the Indian Ocean.

That there should have been periodic seawaterinflux alternating with episodes of evaporite-depositionis not surprising. The volume of the Mediterraneansalts necessitates repeated incursions to transportenough ions across for salt-deposition (HSü et al.,1973a, p. 1215; Hsü et al., 1973b). The total waterinflux of each marine incursion into a desiccatedMediterranean had to be greater than the basin vol-ume, because the basin could not have been filledinstantly; an excess was needed to offset evaporativelosses during the transient stage of basin infill. If ittook a few thousand years before the Mediterraneancould be filled up, the evaporative losses during thoseyears would amount to one or two times the basinvolume. Using this line of reasoning, Hsü et al. (1973a,p. 1216) suggested that 11 interludes of infill wouldhave been sufficient to account for the transport of ionsfrom the Atlantic and thus for the Messinian saltdeposition in the Mediterranean. If the rate of infillwas much slower, or if a transient state of Gibraltarsaltwater-fall should persist for 50 or a 100,000 years,much of the Messinian salt would be brought in by onesingle episode of steady seawater influx. Vai and RicciLucchi recognized some 13 to 14 cycles of marineflooding from their studies of the Apennine evaporites,but it was questionable if seawater was filled to thebrim for each or any of those episodes. The few marineepisodes registered by the Hole 372 cores may repre-sent the times when the Mediterranean water level wasup to about the same level as that of the worldwide sealevel; otherwise the salinity of the Mediterraneanwould have been intolerably high for marine organ-isms, when there was no exchange between the Medi-terranean and the open ocean to compensate forevaporitic excesses. On the other hand, the submer-gence phase of the Ionian flooding cycles (as recordedby Hole 374 cores) was neither fully marine nordemonstrably very deep. To conclude, it seems thatthere have been a few or several marine floodingsduring the time of Upper Evaporite deposition; therelatively thin salt deposits in the Upper Membermight well be residues of those floods. On the otherhand, as will be discussed later, the origin of the MainSalt body in the Lower Evaporite may be different; therecord of an apparently uninterrupted sedimentation inSicily (Decima, 1975) seems to favor an alternativeorigin for the main salt deposition such as one periodof steady seawater influx.

THE MESSINIAN SALINITY CRISISSince drill cores yielded information only on the

Upper Evaporite, we depend on land sections for anoverview of the Messinian salinity crisis. Many papersdealing with the question of the Messinian crisis onland were presented during a 1973 seminar at Utrecht(Drooger et al., 1973) and a 1975 seminar at Erice,Sicily (Kidd, 1976). The onset of the salinity crisis iscommonly indicated by the contact of Calcare di base

1070


on "Tripoli," signifying a very rapid change from abathyal environment of hemipelagic sedimentation, toa shallow, or subaerial environment of carbonatedeposition and diagenesis (see summary by Cita et al,this volume).

The history of the early evaporite deposition can bedeciphered by reference to the section of the Cattolicabasin, Sicily (see Decima and Wezel, 1973). TheLower Evaporite sequence there lies above the TripoliFormation and includes (1) Calcare di base (2) Catol-ica Gypsum, (3) Clastic Gypsum beds, (4) halite andpotash salts. The Lower Evaporite is overlain uncon-formably by an Upper Evaporite of gypsum and marls.The stratigraphy is shown diagramatically in Figure 11.

The halite and potash salts are on the average 400to 500 meters thick. Decima (1975) divided a 330-meter section of salts at Porto Empedocle into fourunits. Unit A is a halite with laminae of anhydrite andis 108 meters thick. The bromine content is in therange of 400 ppm. Unit B is well laminated halite,kainite, polyhalite, and anhydrite, and is 115 metersthick. The bromine content ranges from 200 to 250ppm except for the uppermost pure halite which hasabout 100 ppm Br. Units C and D overlie the lowersalts unconfbrmably. They are 107 meters thick andconsist of halite, with anhydrite and anhydritic shales.The halite in these upper units has only traces ofbromine (10 ppm or less).

The lower salts with their high Br were precipitatedfrom brines derived from evaporated seawater. Incontrast the upper salts were recycled. Sedimentologi-cal studies by Schreiber et al. (1976) suggested that thehalite hopper crystals in the lower part of Unit A andin the upper part of Unit D were deposited in shallowsalina environments. On the other hand, the well-laminated Unit B, with sequences of cyclic laminationthat can be correlated for long distances, were precipi-tated in a relatively deep subaqueous environment.Schreiber et al. (1976) estimated a brine-depth ofseveral hundred meters.

The peri-Adriatic Messinian sections of the northernApennines do not lend themselves to a twofold subdi-vision into lower and upper members. Above theCalcare di base are massive gypsum beds constitutingthe Gessoso-solfifera Formation. Still higher is a post-evaporitic, clastic Colombacci Formation, which isequivalent to the post-evaporitic "Lago Mare" depos-its of the eastern Mediterranean holes.

Sedimentological investigations by Vai and RicciLucchi (1977) on the Messinian of the Vena del GessoBasin revealed that the Gessoso-solfifera is character-ized by cyclic sedimentation. These cycles of floodingand exposure are different in detail, but similar in originto those at Hole 374 described by Garrison et al. (thisvolume). Each cycle was initiated with a marineflooding, followed by subaerial exposure, a conse-quence of evaporative-drawdown of the water levelwithin the basin. Counting of the cycles indicates thatthe Vena del Gesso Basin was inundated some 13 or 14times during the Messinian.

The Sicilian and the Apennine sections seem toprovide somewhat conflicting evidence with respect tothe supply of brines for salt deposition. The stratigra-phy of the Cattolica basin, Sicily, suggests that themain halite body was deposited during the earlyMessinian, and that much of the salt was precipitatedfrom a deep brine pool. The supply of brines woulddepend upon a steady influx of seawater. The waterlevel of the Mediterranean basins containing halite-precipitating brines was probably lower than that ofthe Atlantic to permit a continuous inflow of saltwater,but sufficiently high to permit the brines to spill overfrom one basin to another. The stratigraphy of theApennines, on the other hand, emphasizes cyclic sedi-mentation. There, the brines were brought in by peri-odic seawater influx, namely by repeated inundations.

We do not know which, if any, cycle of gypsumdeposition in Vena del Gesso was coeval with the mainhalite deposition (Unit B) at Porto Empedocle. In factif the sill of the former was higher than the brine levelof Mediterranean salt basins, the time of main halitedeposition might be represented by an unconformableinterval of subaerial exposure in this peri-continentalbasin.

A comparison of the submarine Messinian with theland sections suggests a two-stage evolution for theMessinian salinity crisis. After an initial desiccation orpartial desiccation, which led to the formation of thewidespread Calcare di base the Lower Evaporite wasprecipitated, when the supply of the brines was suffi-ciently continuous, and lead to the accumulation ofthick halite and potash salt deposits in the deepest ofthe Mediterranean basins. Towards the end of theMain Halite deposition, the supply of brine was prob-ably stopped by complete isolation from the Atlantic.The brine level was relentlessly drawn down becauseof evaporative loss. Large areas on the periphery of thesalt basins were exposed. Waters from the continent orfrom local precipitation formed salt creeks, erodingprimary salt (e.g., Units A and B , Porto Empedocle),and depositing secondary salts (e.g., Units C and D) inthe center of the salt basins.

This period of desiccation was represented by thewidespread unconformity separating the Lower and theUpper Evaporite units. Rivers cut canyons on themargins of the continents and some of these canyonswere eventually floored by the Upper Evaporite depos-its (Montadert et al., this volume).

The erosional event was terminated by a new inun-dation of the Mediterranean, when it was probablyagain filled to the brim. The lowest marine marls ofMessinian age at Site 372 (Core 9, Section 2) are thefirst sediments above the unconformity and they mayhave been deposited during this intra-Messinian flood-ing. The subsequent history of the Upper Evaporitesedimentation was probably characterized by periodicinflux. Each marine inundation may have permitted theaccumulation of dozens of meters of salt accumulationin the central parts of the Mediterranean basins, as aresult of their partial or total desiccation. The upper

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Messinian Mediterranean was floored by a series ofdesert basins, some with salt lakes, prior to inundationby the"Lago Mare."

The history of the peripheral basins was different:Except for a few (such as the Volterra Basin in Tus-cany) the bitterns of these basins were drained into theMediterranean depressions. No salts were deposited,and the Messinian record shows multiple cycles offlooding and desiccation, but no two-stage evolution ofthe crisis.

LAGO MARE

The geological record of the eastern Mediterraneansites revealed a drastic change in depositional environ-ment during the latest Messinian before the Pliocenemarine inundation. After the last salt deposition of theUpper Evaporite, the Ionian and Antalya basins werealmost totally dried up. Slight changes in the waterbudget led to conditions of cyclic sedimentation. Theisotopic evidence suggests a considerable supply ofcontinental waters to these already desiccated basins(Pierre and Fontes, this volume; Ricchiuto andMcKenzie, this volume; Cita et al., this volume).Probably little or no marine waters ever got into theeastern Mediterranean during this stage, for onlygypsum, but no salts was deposited at the end of eachdesiccating cycle.

The very last Messinian sediments at Sites 374 and376 are marls, deposited in a standing body of water.Both the Antalya and the Ionian basins remainedunder subaqueous conditions while the marls werebeing deposited. The depth may have been considera-ble, because both basins seemed to have remainedunder subaqueous conditions. If the water bodies hadbeen very shallow, unavoidable variations of hydro-graphic budget however slight, would lead to cyclicsedimentation. Isotopic evidence indicates that thewater was derived largely from a continental source,but had undergone considerable evaporative historyresulting in differential enrichment of heavier isotopes(Pierre and Fontes, this volume; McKenzie and Ricchi-uto, this volume; Cita et al., this volume). Site 376samples contain an Ammonia becarrii-Cyprideis pan-nonica fauna, indicative of a euryhaline environment(see Site 376 report, this volume; also Cita et al., thisvolume). This fauna is not found in the Messiniandolomitic marls of Site 374, where the bottom mayhave been too deep or too stagnant to permit popula-tion by this benthic fauna. The Cyprideis pannonicaassemblage is a typical fauna of the Pannonian sedi-ments of the Paratethys, which have been dated radio-metrically to be 8 to 11 m.y. (Vass et al., 1975). TheMessinian and Pannonian faunas are not coeval, butare facies equivalent. Shallow-water diatoms arepresent in Core 17 of Site 374. The floras were de-scribed as "limnic" or brackish (Schrader and Ger-sonde, this volume).

Some marine fossils are present in the marls. Someof us consider these occurrences allochthonous, havingbeen reworked from older deposits (Cita et al., thisvolume). Others consider the occurrences as indicative

of occasional spillover of marine waters into the east-ern basins (see site reports 374 and 375/376, thisvolume). This disagreement does not affect our mainconclusion that most of the marls were deposited in asubaqueous, brackish environment.

The Cyprideis and other euryhaline faunas arepresent in numerous uppermost Messinian sections onland both east and west. Interpretations of these faunasled Ruggieri (1967) to conclude that the Mediterra-nean became gradually desalinified as in the present-day Caspian. These lakes of reduced salinity werereferred to collectively as the "Lago Mare," an Italiantranslation of the French expression "lac mer" whichrefers to the system of the Neogene brackish lakes ofthe Paratethys region (see Figure 8, also Gignoux,1950). We propose, however, that the term "LagoMare" be used to designate the latest Messinianoligohaline environment, postdating evaporite deposi-tion and predating Pliocene marine sedimentation (seeSite 375/376 report, this volume and Cita et al., thisvolume), in order to distinguish it from "lac mer"which, strictly speaking, was a Paratethyan environ-ment.

The change from salt precipitation in shallow saunasto deposition of marls in "Lago Mare" occurred withina very short period of the latest Messinian, probably in100,000 years or less. The change implies a radicalalteration of the hydrographic budget. The desiccationand salt-deposition required that the annual evapora-tive loss was considerably more than the annual supplyof water from the rivers. The existence of brackishwater lakes, on the other hand, implied the influx offresh or brackish water in excess of evaporative loss.

Sudden changes from dry to wet climate are notuncommon, especially during the change from glacialto interglacial stages. Quaternary sediments of Africanlakes, for example, include interbedded sequences ofplaya-evaporite and subaqueously deposited muds(Stoffers, 1975). However, a sudden climatic changeaffecting the whole circum-Mediterranean region wasan unlikely event, and we know of no paleobotanicalevidence for any such drastic changes during theMessinian. We prefer, therefore, to entertain the alter-native hypothesis that the environmental changes werethe consequence of a reorganization of the drainagesystem. The sudden appearance of so much fresh orbrackish water, together with a typical Paratethysfauna, in fact, indicates a sudden inundation of thedesiccated Mediterranean by waters from Paratethys.

That such an invasion did take place has beenspeculated by biogeographers on the basis of theoccurrence of relic Paratethys faunas (freshwater fish,mollusks, birds, etc.) in isolated areas of circum-Mediterranean land areas today (Stankovic, 1960;Vosus, personal communication). The evidence fromParatethys also indicated that it suddenly lost much ofits waters to the Mediterranean in late Neogene. Thewidespread regression of the Paratethys was desig-nated as "middle Pliocene" but estimated to have anabsolute age of about 5 to 6 m.y. Jiricek (1975, p. 16)concluded that the regression was "caused by the

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HISTORY OF THE MEDITERRANEAN SALINITY CRISIS

migration of (Paratethys) waters into the dried-up bedof the Mediterranean." The Black Sea probably alsowas drained during that time. Partial emptying bydrainage, and a subsequent evaporative excess result-ing from the change in water-budget led to a consider-able desiccation of this deep Paratethyan Basin and tothe formation of evaporative carbonates (Hsü, inpress).

We know little about the exact cause of the Para-tethyan invasion. However, it is not difficult to visualizethat headwater erosion of rivers draining into theMediterranean may have acquired great erosive powerduring its low water stand so that they breached thedivide and captured the Paratethyan drainage.

The route of the Paratethyan invasion is also uncer-tain. The most direct route through the Bosporus wasprobably not open. Nor do we have any evidence or apassage through the peri-Alpine depression or throughAnatolia. Biogeographical analyses suggested that the

exit of the Paratethyan drainage probably lay on theAdriatic coast. Interesting clues are provided by studiesof the present-day relic faunas of the western Dinaricregion, particularly those of Lake Ohrid on the borderbetween Yugoslavia and Albania. The faunas arebelieved to be the living fossils of a Neogene thermo-philic fauna of the circum-Mediterranean domain(Stankovic, 1960). That a Neogene connection fromParatethys to the Dinarides existed was indicated bythe presence of a diatom flora in Lake Ohrid; the florais typical of the Neogene of the Pannonian Basin. Theconnection may have threaded through from the Pan-nonian to the Valley of Nis to Metohia and Kosovobasins to Lake Ohrid (Figure 17). The Ohrid is drain-ing into the Adriatic today and may have done soduring the Messinian.

A Cyprideis fauna is present in the Messinian depos-its of Thessalonika and on the Island of Crete (Sis-singh, 1976). The fauna is believed to have been

MediterraneanBasins

ParatethysBasins

DinaricProvence

AegeanProvince

Probable drainagefrom Paratethys tothe Mediterranean

Figure 17. Messinian paleo geography ofParatethys (modified from Stankovic, 1960).

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K. J. HSU,ETAL.

derived from the Pannonian Basin via Nis and Skorpic(Stankovic, 1960). However, the present Aegean fau-nas, unlike the Dinaric, do not contain relic Tethys orNeoqene circum-Mediterranean thermophilic elements.Stankovic (1960) emphasized that the distinct differ-ence between the present Balkan faunas on the Adri-atic and the Aegean coasts cannot be attributed to aclimatic difference between two halves of the penin-sula. The Aegean faunas today are more recent immi-grants from the Black Sea province. The Black Seadrilling results indicate that the Bosporus did not comeinto existence until the Quaternary. By then, the cli-mate of Paratethys had been chilled sufficiently to killoff the Neogene thermophilic elements. They werereplaced by a central European fauna, which eventu-ally went through the Bosporus to populate the riversand lakes on the Aegean coast of today.

The postulate that the Bosporus was not in existenceduring the Messinian may explain the fact that "LagoMare" deposits were not found at our Aegean Site378; we penetrated the top of the Messinian there, butsampled only a dolomite veneer and selenitic gyp-sum

14

We have no clearcut answer to the question ofwhether the "Lago Mare" extended to the westernMediterranean. The Cyprideis fauna is present in landsections in Algeria, Spain, Tuscany, and in Sicily, but isabsent, or very doubtfully represented, in our deep-seabore holes (Cita et al., this volume); the few individu-als in the uppermost Messinian cores of Sites 124 and372 may be reworked specimens. At both of these sites,and at Site 132, the youngest Messinian sedimentsabove the highest gypsum or anhydrite bed, are ma-rine, with dwarfed faunas. They might have beendeposited during the initial infilling phase leading tothe Pliocene deluge (see discussion by Hsü et al,1973a, p. 1217-1219).

A brackish-marine sediment was present in Hole124 (124-13,2) below (not above) some gypsum layersand at about 40 meters below the base of Pliocene(Schrader and Gersonde, this volume). Could thisdeposit be equivalent to the lower part of the "LagoMare" sequence of the eastern Mediterranean?

We are now getting into the realm of pure specula-tion without much solid foundation. It is probable thatthe marine waters may have spilled over the westernbasins during the latest Messinian before they reachedthe eastern Mediterranean. Perhaps the restricted ma-rine facies of the uppermost Messinian in the Balearicand the Tyrrhenian are time equivalent in part to theupper "Lago Mare" deposits. These marine sedimentswere deposited when the Atlantic water began to refill

14One of us (MBC) wished to emphasize th*e speculative nature ofour interpretation of the Messinian connection between the Para-tethys and "Lago Mare." We have to wait for the release ofinformation of drilling in the Sea of Marmara before we can finallydetermine if the Bosporus was indeed a land barrier and not a strait,during the late Miocene. She also felt that the information from Site378 is too scanty to permit a conclusion that the "Lago Mare" didnot extend to the Aegean.

the desiccated Mediterranean. The eastern basins be-yond the Sicilian Channel were not submerged undermarine waters, except for some occasional spillovers.Meanwhile, the peri-continental basins of the west mayhave remained brackish because of a surplus of fresh-water supply, or because they were totally or partiallyisolated from the deeper Mediterranean basins.

IMPLICATIONS OF THE MESSINIANSALINITY CRISIS

The consequences of the Messinian salinity crisiswere discussed thoroughly after the Leg 13 cruise(Hsü, et al., 1973a, b; Ryan, 1973; Cita, 1973; Hsü,1974), and we shall not renumerate these in detail.

The lowering of the base-level of erosion had direconsequences on the Mediterranean drainage system.Valleys carved by the deep cutting of the Rhone, of theNile, and of other rivers have been traced from farupstream to the edge of Mediterranean abyssal plains(see papers in Ryan, Hsü, et al., 1973; also Gvirtzmanand Buchbinder, this volume). New seismic evidencehas confirmed that some of these valleys were un-doubtedly cut during the Messinian (Montadert et al.,this volume) although we do not dispute the fact thatsubmarine canyons of Pliocene-Quaternary age arealso common in the Mediterranean province.

The lowering of the base-level and drainage ofParatethys waters to the Mediterranean may have beenresponsible for the late Miocene development of karsttopography in Yugoslavia. Some of us postulatedearlier that the so-called "cobble-stone terrains" on theMediterranean Ridge are buried and modified karst(Hsü et al., 1973a). Drilling at Leg 42A Sites 374, 375,and 376 was interrupted by "lost-circulation" andother anomalous drilling behavior, which could berelated to the presence of submarine caverns (seeIntroduction and Site 375/376 site reports, Chapters 1,6, this volume).

The climatic consequence of the salinity crisis wasprobably a continued change towards a cool and aridclimate, and an expansion of the montane coniferousforest vegetation in the circum-Mediterranean regions(Bertolani-Marchetti and Cita, 1975). The global effectcan only be speculated upon. We can envision theresult of a considerable albedo effect when a clear bluesea was replaced by a shiny white salt desert. Thisincreased albedo effect and expansion of sea ice be-cause of the loss of salts to the Mediterranean mayhave caused the expansion of polar glaciers (Ryan,1973). There is indeed some evidence of such anexpansion of the Antarctic ice-sheet during the Messin-ian (Hayes, Frakes, et al., 1974). Arctic ice sheets mayalso have begun to form at that time (van Hinte,personal communication). It seems probable therefore,that the Messinian salinity crisis may have triggered aglacial period. Conversely, eustatic changes in connec-tion with the build-up or the melting of glaciers mayhave been responsible for the periodic flooding of theMediterranean when the Upper Evaporite was depos-ited (Ryan, 1973; Hsü, 1974; Van Couvering et al., inpress).

1074


The cooling trend initiated during the late Miocenecontinued in Europe throughout the late Neogene.Thermophilic elements disappeared successively duringthe late Miocene and Pliocene, before they becameextinct altogether with the Quaternary advances of thecontinental glaciers (Van der Hammen, 1975; Berto-lani-Marchetti and Accorsi, this volume; Traverse, inpress).

The biogeographical implications of the Messiniancrisis have only begun to be explored. The desiccationand the subsequent inundation by the "Lago Mare"should be the key for the understanding of the relicfauna of the circum-Mediterranean regions today (seepapers cited in Hsü, 1974).

PLIOCENE FLOODING AND PLIOCENE-QUATERNARY SUBSIDENCE

Leg 42A drilling confirmed the Leg 13 drillingresults that the first Pliocene sediments above theMessinian are deep and open marine hemipelagicsediments (Figure 18). The Messinian salinity crisiswas ended by the Pliocene flooding. This event isdistinguished from Messinian marine incursions by tbefact that an open communication was firmly estab-lished through to the present day; there has been nosalinity crisis during the last 5.2 million years! Incontrast to the periodic spillovers which might berelated to eustatic changes (or to intra-Messinianmovements), the Pliocene event was evidently a majortectonic event. It seems logical to assume that move-ment along Azoras-Gibraltar fracture zone opened adeep gash between the Atlantic and the Mediterranean(see Hsü et al., 1973a; Hsü, 1972). The "dam" atGibraltar was damaged and broken. A deep connec-tion to the Atlantic was established. Even in theQuaternary, when world wide sea level dropped al-most 100 meters, the Mediterranean did not becomeisolated.

Paleobathymetric analyses indicated that the Plio-cene sediments immediately above the Miocene-Plio-cene contact invariably were deposited in water depthsin excess of 1000 or 1500 meters (Figure 3; also sitereports, this volume; Wright, this volume; Benson, thisvolume). The evidence clearly indicates that deepMediterranean depressions which had existed prior tothe Messinian salinity crisis were still in existence at theend of the crisis. The faunal data can rarely provideevidence to distinguish depth diiferences in excess of1500 meters. Seismic evidence demonstrates considera-ble Pliocene-Quaternary subsidence (Morelli, 1975;Biju-Duval et al., this volume). The subsidence was atleast in part induced by the isostatic load of the waterflooding the basins (Hsü et al., 1973a, p. 1216-1217;Ryan, 1976). Additional subsidence could be related tocooling of the mantle under the basins, or to continuedtectonic activity deepening back arc-basins (Montadertet al., this volume).

There has been considerable discussion as to whythe sediments directly overlying the Messinian are noteverywhere the oldest Pliocene. The diachronous na-ture of the basal Pliocene led some to postulate slow

Figure 18. Pliocene/Messinian contact, Site 372(from Cita et al, this volume). The laminatedsediments are Pliocene.

transgression over a gradually subsiding sea bottom(e.g., Wezel, 1974). We wish to emphasize, however,that the Pliocene sediments are entirely different fromthose one would expect of shallow marine elasticstransgressing a shelf sea. The deep-sea drilling hasuncovered considerable evidence of submarine uncon-formity, where bottom currents were active. We preferto interpret the local post-Messinian hiatus as aninterval of non-deposition, when the bottom circulationof the Mediterranean, now with a deep opening to theAtlantic, was very active. Similar regions of nondeposi-tion are present in the eastern Mediterranean todayalong the paths of its bottom circulation (Fischer andGarrison, 1967).

There has been considerable Pliocene-Quaternarytectonic activity in the circum-Mediterranean regions.The evaporite basins of Sicily, Calabria, the Apennines,the Tellian Atlas, the Ionian Islands, Crete, Cyprus, etc.were uplifted and emerged. Meanwhile continentalmargins (e.g., the Nile and Rhone deltas, the Israeli

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shelf, etc.) locally underwent subsidence. Such activityhas appreciably modified the post-Messinian paleoge-ography. However, we emphasize that the Mediterra-nean basins owe their origin to earlier Neogene move-ments, even though their floors may have been uplifted(as in the Mediterranean Ridge region) or subsidedsomewhat during the last five million years.

CONCLUSIONS

The Messinian salinity crisis was an unusual event.This analysis should however serve to show that theevent was unavoidable as a consequence of globaltectonic development. Except for Hsü and Cita, the Leg42A shipboard scientific staff boarded Glomar Chal-lenger in Malaga without having formulated conclu-sions as to the genesis of the Mediterranean Evaporite.Some favored and some opposed the view of a deepbasin desiccation model but all arrived with openminds. We had many discussions and debates onboardGlomar Challenger; they became at times acrimonious.We are pleased, however, that the great majority (10out of 12) of us could finally reach a consensus duringour final post-cruise meeting at Granada. Our conclu-sions can be briefly summarized as follows (see Table1): Most of the Mediterranean basins of today were al-ready in existence and deep (in excess of 1000 m or1500 m) prior to the salinity crisis. The basins werepartially or totally desiccated more than once duringthe Messinian. The eastern, and probably some west-ern Mediterranean basins were inundated in the latestMessinian, by continental waters, presumably drainedfrom Paratethys. Near the very end of the Messinian,the Balearic and Tyrrhenian basins began to receivemarine waters from the Atlantic, but the environmentwas not completely open marine. With the opening ofthe Strait of Gibraltar at the beginning of the Pliocene,all the Mediterranean basins were again submergedunder deep and open marine waters. The basins werefurther deepened by Pliocene-Quaternary subsidence.

ACKNOWLEDGMENTSFor a major undertaking such as this: a drilling cruise to

investigate the Messinian salinity crisis; the efforts of manypersons and organizations were indispensable. We cannotacknowledge all our indebtness individually. We would liketo thank the U.S. National Science Foundation, the interna-tional JOIDES institutions, the various JOIDES Committeesand Panels, the Deep Sea Drilling Project, Captain Clarkeand his crew, and many others who made our cruise possible.We are indebted to our shore-based investigators; theirresults helped us in no small way towards formulation of ourconclusions.

We would like to mention that it has been a particularpleasure to work with Bill Hay, former chairman of theJOIDES Planning Committee, Hollis Hedberg, Chairman ofthe JOIDES Safety Panel, Terry Edgar, former Chief Scien-tist of DSDP, and Mike Pennock, Operations Manager, Leg42A, during the various stages of planning and execution ofthis research. Finally, we would like to make a specialacknowledgment of our indebtedness to Bill Ryan. He was amost active member of the Mediterranean Advisory Panel,both before and after its reorganization. He was most helpfulin the selection of sites and his interest in the Mediterranean

in general and in the salinity crisis in particular, neverabated. He voluntarily refrained from offering his services forLeg 42A, because he sincerely wished that the Leg 42Ashipboard scientific staff should not be dominated by theholdovers from Leg 13. We appreciated his spirit of self-sacrifice. He is thus not a de jure, but a de facto, co-author ofthis article.

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