THE MEDITERRANEAN SALINITY CRISIS:ALTERNATIVE HYPOTHESES

Robert F. SchmalzDepartment of Geosciences

The Pennsylvania State UniversityUniversity Park, Pennsylvania

"I can't believe that!" said Alice. "Can't you?" the Oueen said in a pitying tone."Try again: draw a long breath and shut your eyes."

(Lewis Carrol, Alice in Wonderland)

Abstract: Many ancient salt deposits share characteristics quite different from those observed in salts deposited in Recent coastalsalinas and sabkahs: theirvast areal extent (up to 5 million km2) and great thickness of contained salt (exceeding 2,000 meters in somecases) in particular. They may exhibit features developed in littoral and supra-tidal saline environments which have been cited asevidence of a shallow origin for the saline giants: laterally-persistent, thin (annual?) laminae, poikiolitic and displacive (nodular)gypsum and anhydrite, chevron crystals, "satin-spar" veinlets, desiccation polygons. However, conditions which are sufficient maynot be necessary for the development of a sedimentary feature; to be rigorously interpreted the conditions must be both. Otherfeatures of the saline giants suggest a depositional environment unlike any known today. The salts appear to have accumulated veryrapidly (1 mm to 1 decimeter/year) in sediment-starved rift or intra-cratonal basins several hundred meters deep. Water depth, atleast during part of the depositional phase, approximated the depth of the basin, and surface water was of normal salinity or brackishduring intervals. Basin margins are typically ornamented bybioherms and thin platform sediments, stratigraphically high above time­equivalent salts of the basin center.

Parallels between these ancient giant salt deposits and the Messinian salt of the western Mediterranean Basin are striking,and suggest that the depositional environment of the Messinian may have been a deep, brine-filled basin rather than a desert salt pan2,000meters below sealevel.

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INTRODUCTION

To explain the marine salt deposits foundin sediment cores recovered from the floor of theBalearic Basin during Leg 13 of the Deep SeaDrilling Project, Hsu and his scientific colleaguesaboard Glomar Challenger proposed a startlinghypothesis: That during Messinian time (5.5 - 6.0million years B.P.) the western MediterraneanBasin had been repeatedly filled with seawater,then isolated and desiccated to form a vast sun­scorched salt pan two thousand meters or morebelow the adjoining land masses and the AtlanticOcean to the west (Hsii, et 01, 1973-a, 1973-b). Theproposal was received with interest and someskepticism by the scientific community, but withenthusiasm by a popular press fascinated by theimage of a seawaterfall or cascade plunging fromthe Strait of Gibraltar to the desert basin floornearly two miles below (eg: Cita, 1973; Matthews,1973. pp. 20-21). The hypothesis was based on fourrelatively straightforward observations: Firstly,there was abundant evidence that a deep, seawater­filled basin was present in the western Mediterra­nean region at the beginning of Messinian time(Parsons & Sclater, 1970; Selli, 1985). Secondly,marine salts recovered from the floor of theBalearic Basin appeared to represent several dis-

Carbonates and Evaporites,v, 6, no. 2, 1991, p. 121-126

tinct depositional episodes, in each of which thevolume of salt deposited corresponded to theevaporation of a large volume of seawater (Ryan &Hsii, 1973; Hsti, et 01, 1977). Thirdly, littoral orshallow water evaporite deposits of equivalent agewere exposed at or above sealevel in coastal regionsof Italy, Sicily and Spain adjacent to the BalearicBasin (Schreiber & Friedman, 1976). Finally,although marine sediments beneath, interstratifiedwith, and overlying the salt deposits of the Basinfloor bore evidence of deposition in water severalhundred, perhaps several thousand meters deep,primary depositional features observed in theevaporite salts were similar to those found inevaporites formed in modern shallow lagoonal,salina or sabkah environments (Nesteroff, 1973;Ruggieri & Sproveri, 1978; Schreiber, 1973).

The Messinian desiccation model has beenwidely adopted, particularly among European ge­ologists (Berckhemer & Hsii, 1982). The evidencein support of the desiccation model is not unambigu­ous, however. Sonnenfeld, and more recently,Dietz, are among the geologists who have seriouslychallenged the hypothesis (Dietz & Woodhouse,1988; Sonnenfeld, 1985). Evaluation of the desic­cation model clearly requires consideration ofalternative hypotheses(Chamberlin, 1897).Three" work-

122 ROBERT F. SCHMAlZ

ing hypotheses" are presented below. The first two ofthese workinghypotheses presume that evaporite depositsmayform onlyin shallowwater or subaerial environmentslike those observed today. The third departs from Unifor­mitarianism and assumes that salt deposition may takeplace beneath an overlying water column as much asseveral kilometers high:

The desiccation model

As advocated by Hsu and his co-workers,this model supposes the existence of a deepbasin filled initially with seawater, butrepeatedly isolated and evaporated to dry­ness. During basin-full phases, deep-watermarine sediments accumulated on the basinfloor while evaporites were deposited insalina- or sabkah-like environments aroundthe basin margins. During intervals ofisolation and desiccation, salts were depos­ited in the deeper portions of the basinwhere concentrated brine accumulated.

The tectonic model.

Advocated by Selli and others, this hypoth­esis presumes that portions of the basinfloor were subjected to repeated verticaltectonic movements, allowing salt depos­ited under shallow or emergent conditionsin the central basin regions to be interlami­nated with deepwater sediments.

The deep-basin model.

Essentially a modified barred basin model,the deep-basin model elaborated by Schmalz(1969), proposes that both salts and deep­water facies accumulated on the floor of adeep basin ftlled to sill depth with seawateror more concentrated brine.

To choose among these working hypothesesrequires that we assess our basic ideas of marine saltdeposition, and may demand the judicious appli­cation of Occam's razor.

Modern salt deposits

I would like to suggest initially that thereare two major types of marine salt deposit: ­modem ones and BIG ones. Modem environmentsin which marine salts are known to be accumulatinginclude coastal salinas such as those of BajaCalifornia or coastal Chile; restricted lagoons likethe Laguna Madre of Texas; sabkahs of the typefound along the Persian Gulf; through-flowing

channel and shelf environments like the Suez­Bitter Lake complex or the Bahama Bank; andisolated dry basins like Lake MacLoed in WesternAustralia.

Modem evaporite deposits are not confmedto hot, arid climates, but may form whereverevaporation exceeds precipitation (plus runoff)during at least part of the year. This condition issatisfied in areas as climatically diverse as theAntarctic dry valley, the jungle coast of Luzon, orsouthern San Francisco Bay.

Despite their geographic and climatologicaldiversity, most modem environments of evaporitedeposition share several characteristics. They arevery close to sea level and if submerged, they arecovered by shallow water and commonly areperiodically exposed. The salt deposits are small(occupying just a few square kilometers in mostcases), they are thin, and they are" dirty" . Bythat I meanthat the deposits are rarely more than a few meters thickand are commonlycomposed of halopelites, salt-arenitesor interlaminated gypsiferous silts, carbonates and salt.Depositswhichare essentiallymonomineralicare rare andwhere they do occur they are usuallyvery thin.

Certain primary sedimentological featureshave been associated with salts formed in each ofthese modern depositional environments. Variousinvestigators have described " desiccation" cracks andpolygons, laminites," stromatolitic" organic layers, salthoppers or chevron crystals, enterolitic gypsum beds,displacive-," nodular" - or" chicken wire" -gypsum andanhydrite, even" ripple marks" and bird tracks (Dean,etal, 1975; Dellwig, 1955; Kinsman, 1966; Schreiber &Friedman, 1976; Borchert & Muir, 1964; Briggs, 1958).Because depositional conditions in shallow, modem saltenvironments are conducive to the development of suchfeatures, their occurrence in ancient rocks is taken asevidence that the older evaporites formed in similar envi­ronments. Although modem shallow evaporite environ­ments may provide conditions sufficient for the develop­ment of these sedimentary features, we have no evidencethat they can be formed only in such environments or thatthose particular depositional conditions are necessary fortheir development. Neither do we have evidence thatmarine salt deposits can form only in environments likethose whichoccur today. Unambiguous interpretation ofthese or any sedimentary features requires that we showwith reasonable certainty that the conditions to whichweattribute them are not only sufficient but necessary fortheir development. We most demonstrate more than asimple correlation; we must establish an exclusive causeand effect relationship between the sedimentaryfeatures and the depositional environment(s) inwhich they are found. In the ease of many ancient

MEDITERRANEAN SALINITY CRISIS 123

marine salt deposits, this has not been done.Moreover, many ancient salt deposits presentcharacteristics so unusual as to strongly suggest thatthey formed under conditions quite different fromthose which prevail in any environment wheremarine salts are known to be accumulating today.

Ancient salt deposits

If modern salt deposits are usually small,thin and dirty, many ancient deposits are verylarge, very thick and in some cases virtually free ofimpurities. One of the largest ancient deposits (theZechstein), for example, extends over an area ofnearly 2.5 million square kilometers (roughly one­third the area of the United States), and itsthickness exceeds six hundred meters (Borchert &Muir, 1964). Many ancient salt deposits arehundreds of meters thick and often include verythick sections which are essentially monomineralic.Some appear to have formed in young rift areasassociated with spreading centers, others accumu­lated in intracratonic basins. Examples of suchancient" saline giants" include the European Zechstein,the evaporites of the Permian Basin of west Texas, theSilurian age Salina Formation of the Michigan Basin, theDevonian PrairieEvaporites ofA1berta,and the MacArthurRiver deposit of northern Australia. In each of theseexamples we find strong or compelling evidence that thesalt accumulated on the floor of a deep sediment-starvedbasin which was filled to the approximate level of theadjoining ocean. Stratigraphicrelief, pinnacle reefs, anoxicbottom conditions, turbidite and fan deposits beneath orintercalated with salt, and micro-laminations which can betraced continuously for tens of kilometers are commonlyobserved and suggest deep-water deposition (Anderson,etal; 1m; Briggs, 1958; Wardlaw & Schwerdtner, 1966).

Salt deposition in deep water

Although there are no known modernexamples, many investigators studying ancient saltdeposits have concluded that the salts accumulatedbeneath water depths of several hundred meters(Borchert & Muir, 1964; Wardlaw & Schwerdtner,1966; Dean et al, 1m). In many cases, the initialdepth of the basin appears to have been comparableto the thickness of salts which they now accommo­date (Schmalz, 1969). Hypothetical deep-basinmodels of evaporite deposition are usually based onthe barred basin model first presented by Ochsenius(1877). Krull (1917) modified the barred basinmodel by postulating an escaping current ofpartially concentrated brine to explain the anoma­lously high gypsum-anhydrite:halite ratio typicalof most evaporite deposits. Subsequent investiga­tors have refined the modified barred basin model

(eg. Sonnenfeld, 1985) but none has specificallyaddressed the problems of precipitating and pre­serving soluble salts in a deep water-filled basin.

Prompted by a discussion with R.S. Dietzmore than twenty years ago, this author undertookto develop an oceanographic model of salt deposi­tion and accumulation in a deep brine-filled basinwhich could explain many of the features peculiarto the ancient saline giants. The resulting model,published in two earlier papers (Schmalz, 1966;1969), is summarized briefly below. The modelpresumes a barred or silled basin of any depth,within which evaporation exceeds precipitationplus run-off during at least part of the year. If thewater budget deficit is relatively small, anti­estuarine circulation will result in a well-venti­lated, normal marine environment like that of themodem Mediterranean Sea. An increased deficitcaused by seasonal variation or longer-term cli­matic change will produce a volume of high­density brine too great to be flushed from the basinby ordinary processes of oceanographic mixing,and the deeper parts of the basin will fill eventuallywith stagnant, oxygen-deficient brine. Stagnationand inadequate ventilation will persist throughoutsubsequent stages of basin development unless theheavy brine is displaced by tectonism, by theaccumulating sediments or is flushed out of thebasin by an altered oceanographic regimen. So longas stagnation persists, bottom waters will be anaero­bic, and sapropels may accumulate on the basinfloor whenever sufficient organic matter settlesfrom the water column above. The deposition ofsuch sapropels, some of which may be enriched inbase-metal sulfides or may resemble petroleumsource beds, would be especially likely at the closeof normal marine sedimentation early in basindevelopment, but might also occur at any later stageif short-lived climatic fluctuations favored thedevelopment of depositional conditions like thoseobserved in the modern Black Sea. Continuedincrease in the water budget deficit might causesalts to precipitate from surface waters, particular­ly in restricted lagoons and along the distal marginof the basin. Salt crystals which sink or aretransported into the deeper parts of the basin willredissolve as they settle to the bottom, however,until the deeper water becomes saturated. Thiscould be regarded as an" ephemeral evaporite" phase inbasin evolution. As soon as an appreciable layer of salt­saturated brine has accumulated behind the basin sill,however, precipitating salts will be preserved in the sedi­ment column, and a" permanent evaporite" phase begins.

At any stage in the evolution of the basinchanges in climate may stop or reverse the process,

124 ROBERT F. SCHMALZ

causingsedimentary inversions, cyclicsequences or excep­tionally thick accumulations of one particular salt. Evenmajor climatic change will not cause re-solution of theaccumulating salts, however, once a protective layer ofsaturated brine has formed on the basin floor. Duringperiods (or seasons) when the water budget does not showa deficit, water above sill depth may be fresh, brackish ornormal marine and such changes will not affect theaccumulating salts except to the extent that they maypermit the addition of foreign elements (traces of marine,estuarine or fluvial fauna and flora, or clastic detritus) tothe sediment.

Salt deposition in the basin will cease whenone of three possible terminal conditions is satis­fied: when the basin is completely filled with theaccumulating salt and associated sediment, whenthe supply of seawater is interrupted, or whenclimatic changes permanently eliminate the waterbudget deficit.

As summarized above, it is implicit in thedeep-basin model that the principal process re­sponsible for salt precipitation is evaporativeconcentration of seawater at the surface, and thatthe basin is occupied to a substantial depth by salt­saturated brine. This is not necessarily the case,however. Raup showed several years ago that halitehopper crystals will be precipitated at any depth inthe water column where concentrated but un­saturated solutions of sodium chloride and magne­sium chloride are allowed to mix (Raup, 1970). Wedo not fully understand the effects of pressure ona mixed electrolyte solution, and cannot predictwith confidence what might happen in response toincreased pressure as a concentrated brine settledto the floor of a basin several hundred meters deep.Any of these, alone or in conjunction with depth­related temperature changes, might cause non­evaporative precipitation of salts, particularly ashot, dense brines seeping out of shallow coastallagoons cooled and sank into the deeper parts of thebasin. For such non-evaporative salt deposits, theterm" precipitites," proposed by R.S. Dietz seems mostappropriate. When dealing with ancient rocks, however, itmay prove impossible to distinguish between" precipitites"and true evaporites.

These possible mechanisms for salt precipitationtake on added significance when considered in light of theprobability that salt deposition in many (most?) ancientevaporite basins was bimodal. Normal seawater appearsto have been concentrated by evaporation at the surface inrestricted shallow coastal lagoons and salinas, where earlyevaporites (predominantly carbonates and sulfates) weredeposited. Dense concentratedbrine, heated by insolationand depleted in calcium and sulfate, probably escaped and

flowed down-slope to occupy the deepest parts of the basinfloor. In the process, halite might have precipitated inresponse to temperature or pressure change, brine mixing,or some combination of these. Slumps and turbidity flowsassociated with the downslope movement of the refluxingbrine could have carried carbonate and gypsum from thelittoral zone into the deep basin. The resulting deposit onthe deep basin floor might comprise a thick and areallyextensive accumulation of precipitated salt with interca­lated turbidite or slump deposits of shallow-water evapor­ite minerals (mainly carbonates and sulfates) and clasticsediments. Such fan and turbidite deposits have beendescribed in the Midland Basin of Texas and in thesediments of the Mediterranean Messinian. Time-equiva­lent littoral and lagoonal deposits around the basin mar­gins would be dominated by shallow water carbonate andgypsum deposits with, perhaps, minor amounts of halite.

It is of interest that a volumetric analysis ofthe deep basin model predicts that potash-rich andbittern salts will be restricted to thin depositsaround the basin margins and the very late stagesof basin filling (Schmalz, 1969). This pattern iscommonly observed in ancient marine evaporitedeposits.

Finally, it is appropriate to consider whethertime constraints favor one working hypothesis overthe others. Although there are no known modernenvironments in which marine salts are accumulat­ing in either a deep desiccated basin or a deepbrine-filled basin, the modern Mediterranean Seaaffords an example of an extensive (2.9 x 1()6 squarekilometer), deep (average 1,430 meters), restrictedbasin (sill depth 320 meters) comparable to manyof the postulated ancient evaporite basins. Themodern Nediterranean is also characterized by awater-budget deficit evaporation exceeds fresh waterinflux (surface runoffplus direct precipitation) byapproxi­mately 76,500 m3/second (Sverdrup, et aJ, 1942). Underprevailing climatic conditions, if the basin were isolatedfrom the Atlantic while fresh or brackish water continuedto flow into the Mediterranean from the Black Sea at thepresent rate, the Mediterranean would evaporate to dry­ness in slightly less than 2,000 years. The evaporationwould deposit approximately 6.3 x 1().l3 m3of salts which, ifconfined to the deeper bathymetric basins, would form adeposit nearly 42 meters thick, corresponding to a sedi­mentation rate of 21 mm/year. This rate is in satisfactoryagreement with evaporite deposition rates observed today,and inferred in many ancient depositits. Alternatively,under the same climatic conditions and with the sameinflux from the Black Sea, we might assume that a cascadeof Atlantic seawater flowed into the basin at a rate justsufficient to replace the volume of water lost by evapora­tion and to maintain the water level in the MediterraneanSea at (Gibraltar) sill depth. Under these conditions, the

MEDITERRANEAN SAliNITY CRISIS 125

entire basin will be saturated with respect to gypsum afterapproximately6,000years. After 18,000years thebasinwillbe filled to sill depth with halite-saturated brine, and theonly limits to halite accumulation thereafter will be thedepth of the basin itself or the duration of favorableclimatic conditions. Clearly neither the deep desiccatedbasin model nor the deep brine-filled basin model can beeliminated because of time constraints in light of the500,000 year duration of the Upper Messinian.

The Mediterranean Messinian

Three working hypotheses were presentedearlier to explain the Messinian salts of the westernMediterranean basin. The Desiccated Basin Hy­pothesis has been discussed in detail by Hsu and hiscolleagues (Cita, 1973; Hsu et al, 1977; Ryan & Hsu,1973). The Tectonic Hypothesis has been summa­rized by SeUi and more recently by Friedman (Selli,1985; Friedman, 1989). In the foregoing discussionI have tried to summarize the essential features ofthe Deep Basin Hypothesis, emphasizing thosecharacteristics of the model which appear espe­cially apposite in a discussion of the Balearic Basin.Clearly, all three hypotheses have merit; the choiceamong them must ultimately depend upon a real­istic assessment of their geologic probability andthe simplicity with which they explain the historyand characteristics of the Mediterranean Messinianas they are known at the present time. The choiceis, to some degree, subjective. For me, theDesiccated Basin model is a Deep Basin withnothing in it (I think of it as a .. Hsu-do" DeepBasin!). The Tectonic Hypothesis depends uponrepeated and very rapid (although geologicallyacceptable) vertical movements of large crustalmasses. (A crustal Polyo-yo comes to mind.)Because I am familiar with no parallel for such amodel, I am reluctant to accept it without seriousreservation, The concept of bimodal salt depositionin a deep brine-filled basin appears to offer thesimplest and most satisfactory genetic model toexplain the Mediterranean Messinian as well asmanyofthe ancient" saline giants". Indeed, the WesternMediterranean may provide as good a modem example ofsalt deposition in a deep basin as we will ever have theopportunity to study.

Fritz Kreisler once commented upon the wide­spread appeal of his music by describing it as .. MelodiusSchmalz". Perhaps I have been seduced by such a SirenSong, but in our effort to better understand Messinianevents in the Western Mediterranean, I hope that we maybe guided byT.C. Chamberlin, rather than by the Oueen sadvice to Alice..

" ..draw a long breath and shut your eyes?'

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Manuscript receivedJuly 16,1990Manuscript accepted September 4, 1991