Improved Modelling of the Messinian Salinity Crisis and Conceptual Implications

Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372 www.elsevier.com/locate/palaeo

Improved modelling of the Messinian Salinity Crisis and conceptual implications

Paul-Louis Blanc

Institut de Radioprotection et de Sûreté Nucléaire, Boîte Postale 17, 92262 Fontenay-aux-roses Cedex, France Received 17 April 2003; accepted 7 March 2006

Abstract

This paper presents the last developments of a simple oceanographic modelling of the water and salt budgets of the Mediterranean Sea during the late Miocene Salinity Crisis. The Messinian Mediterranean is treated as analogous to the present one, i.e. divided into two main basins separated by a sill at shallow or intermediate depth. When the supply of marine water from the Atlantic is progressively reduced, both basins undergo a rise in salinity, until saturation is reached: this is when the true evaporitic sedimentation begins, before the level drawdown. The partition of the Mediterranean into two mains basins causes a shift in the evaporitic sedimentation from the distal (eastern) basin to the proximal (western) basin, so that the evaporitic deposits are not fully contemporaneous in the western and the eastern basin. The drawdown is limited by the equilibrium of the evaporation and chemical activity of the brines at the surface, against the surface area and evaporation. Attempts at adjusting the model both to an accurate stratigraphic frame and to a rough budget of the evaporites shows that the Upper Evaporites and brackish-water Lago-mare series must have been deposited as secondary deposits, after the closure of the Atlantic passages was completed. The present evaporitic potentialities of the Mediterranean Sea remains quite as strong as during the Miocene, so that climatic change cannot be inferred from the MSC itself. © 2006 Elsevier B.V. All rights reserved.

Keywords: Messinian; Evaporites; Hydrology; Budget model

1. The Messinian evaporites and Salinity Crisis in its upper course, is buried below Pliocene marine and Quaternary fluvial sediments. Mayer-Eymar (1867) included Sicilian evaporites in The first cruise in the Mediterranean by the M.S. his equivocal type for the Messinian stage, the last one Glomar Challenger within the frame of the Deep Sea in the Miocene series. In a different geological domain, Drilling Project (1970) showed that the evaporitic Fontannes (1882) described, under the name fjord, the deposits actually spread over the whole deep Mediter- Pliocene ria (originally a drowned valley) of the river ranean. Such deposits, to which geophysical investiga- Rhône. This palæovalley, now also recognized by tions attribute 2000m of average thickness, must result boreholes and geophysical methods in its lower from severe restraints on the exchange of water between course, supplemented by morphological observations the Mediterranean and the Atlantic Ocean. The Mediterranean Sea had already lost any communication to the east, with the possible exception of the Paratethys Sea, ancestor to the Black Sea, and itself a hydrological E-mail address: [email protected]. dead-end.

During the 1970s, two geological schools happened the modelling process, the comprehensive duration of to be confronted. the MSC (i.e. including the time-lapse covering the Lago-mare facies) is now estimated to be 0.63Ma – For the French school of the time (1950–1975), the (Benson et al., 1995; Clauzon et al., 1996; Krijgsman et basins would have not been deeper than some tens to al., 1999). 300m at most during the Messinian Salinity Crisis The relative duration of the peculiar final phase (MSC). The Mediterranean Sea could only have within the MSC, the Lago-mare facies on top of the acquired its present oceanic characters at the evaporitic series, remained unclear. It is now accepted beginning of the Pliocene (Bourcart, 1960). that it results from the hydrological isolation of the – For the Italo–American school, the Mediterranean Mediterranean basins rather than from flooding by Sea results from the closure of an ancient ocean freshwater from the Parathetys Sea (Orszag-Sperber et called the Tethys by geologists and already encom- al., 2000). These facies are recognized as extending to passed basins several kilometres deep. The level both the eastern and the western Mediterranean drawdown could then have reached anything from domains. The continuous and contemporaneous charac- 1000 to 3000m (Hsü et al., 1973). A gigantic salt ter of the deposits in the different sites may still be desert should have extended between Africa and questioned. Europe. Nothing now opposes a global understanding of the phenomenon. Blanc (2000) synthesized the climatic Initially, the observation, done essentially on the observation that the area has been prone to xeric French Riviera, that the coastal streams were followed at conditions since the early Miocene (Suc and Bessais, sea by canyons with a very steep slope, appeared to 1990), the rough quantitative estimates of the evaporitic favour the French school. These canyons shew a sub- deposits, the present hydrology and the improved aerial erosive morphology and their slope increased chronology, into an oceanographic budget model of from the shore towards the abyssal plains: according to the Mediterranean during the MSC. This article aims at Bourcart (1962), they had been created above sea level, detailing the last developments in the model and at with a “normal” slope. Later, during the collapse of the contributing to the description of the possible evolution bottom that had marked the oceanization of the Plio- of hypersaline deep oceanic basins. Quaternary Mediterranean, they had been submitted to a tilt towards the sea. 2. The exchanges at the western straits This interpretation was torn to pieces by the morphologic study of more powerful rivers. The best- 2.1. Present water and salt balance known Messinian valley, that of the river Rhône, indeed encompasses a deep canyon, but its palæovalley extends Physical oceanographers define the Mediterranean very far upstream. It remains below the present sea level Sea as a basin with a sill and a negative budget. as far as the town of Lyon (Clauzon, 1973, 1982), much Rainfall, runoff and rivers from the surrounding further in the continental domain than any flexure which continents (including the Danube and the Russian the supposed collapse of the bottom of the Mediterra- rivers, through the Black Sea) do not balance the nean basins could have caused. The consequence is that evaporation there. this palæovalley results from in situ sub-aerial excava- The level drawdown which the evaporation should tion, without any change in the relative elevation of the cause is not observed, because a continuous influx of continents and marine basins. This could only occur North Atlantic Surface Water (NASW) through the when the Mediterranean water level was deeply Strait of Gibraltar compensates the deficit. At any time, depressed with reference to the present level. the net input (δV) is equal to the evaporation (e) less the The reason why such controversy developed between freshwater budget (f): these schools is, up to a point, the want of any present case of evaporitic sedimentation in a deep basin and ðyV ¼je−f jÞ: below a high water-column. It was also the lack of an accurate time scale to calibrate the phenomenon, and However, a continuous salt–water input at the finally the want of applying to it the basic principles of Gibraltar Strait should cause a regular increase in physical oceanography. salinity. This is not observed either, so that some During the last 15years, much improvement has been mechanism must evacuate the excess salt. No increase in obtained in the field of stratigraphy. Most important for salinity is presently observed, because the exchange of P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372 351 water between the Mediterranean Sea and the Atlantic The configuration of the Miocene Portals, especially Ocean at Gibraltar involves volumes far beyond the net towards their final closure, remains poorly documented. balance of the water budget. An outflow of saltwater This is not surprising, as thresholds are not proper places must carry away the salt. It must be noted that the for sedimentation: actually, the Miocene terminal phase exchanges at the Strait of Sicily, between the two major of the Atlantic–Mediterranean exchanges cannot be basins of the Mediterranean, have to follow the same recorded at all. rules. It is now clear that the Miocene northern strait closed Accordingly, the NASW admitted at the Gibraltar first. The Betic Straits, along the Iberian block, were Strait shows a salinity of 36.18gl− 1, while the originally a series of shallow neritic basins (100m deep Mediterranean Sea Overflow Water (MSOW), denser ?), alternating with half emergent structural or coralline due to its higher salinity (37.9gl− 1), flows back at depth sills (20m deep ?). When the evaporitic sedimentation to the Atlantic Ocean. In a stationary regime, the law is occurred, they cannot have been any longer the route by simple, the volumes of water exchanged (V) are in which Atlantic water reached the Mediterranean: an inverse ratio to the salinities (S): influx of normal seawater should have rinsed them of any excess salt, to the benefit of the Mediterranean. The Vout Sin evaporitic deposits in the marginal basins show that this ¼ or VoutSout ¼ VinSin Vin Sout was not the case. These basins can only have been dead- end diverticula of the Alboran Sea. As the salt is transferred back to the world ocean, the The Rifian Corridor appears narrower than the Betic average salinity of the Mediterranean remains constant Straits, but it must have remained deeper during the pre- at our scale of observation. It has not always been such evaporitic Messinian. No sedimentary record of the very during geological times. closure can remain, but at the easternmost end there is evidence that the Melilla-Nador Basin may still have 2.2. The present Strait of Gibraltar been marine during the beginning of the MSC (5.77Ma, Münch et al., 2001). The present currents in the Strait of The present morphology of the strait shows a trench, Gibraltar are such that there is almost no record of late five times as long as wide at the depth of 400m. The Quaternary sediments: only slumped breccias and some bottom of the main channel dips from 400m in the ahermatypic corallian crusts are found (Esteras et al., Atlantic domain, 15km west of the Tangier meridian 2000). By analogy, during the final phase of the MSC, line, to more than 900m of depth between Algeçiras and the currents through a restricted channel, though Ceuta. Two sills, the Spartel sill (350m below sea level), hydrologically inefficient due to the reduced section, north of cape Spartel, and the Camarinal sill (284m must have been fast enough to prevent deposition. Any below sea level), a few kilometres to the east of the marine record of this ultimate phase should only be late Tangier meridian line, intercept the deepest trench, so Messinian hard-ground: the odds that it might be that the Tangier Basin belongs to the Atlantic domain. preserved anywhere are low. Blanc (2002) has shown that the two sills result from a The oceanographic analysis of how water exchanges complementary process to the strait itself. The present between the Atlantic Ocean and the Mediterranean Sea deep flow is in an opposite direction to the dip: the operated appears more efficient. Prior to the MSC, the lighter NASW enters the Mediterranean at the surface, portal(s), whether double or single, operated in the same between 0 and −150m, while the denser MSOW way as the present strait, with double layered circula- escapes to the Atlantic above the hydrological sills. tion. If a crisis occurred at all, it means that during the Messinian the exchanges were being reduced until no 2.3. The Miocene Betico-Rifian Portals longer sufficient for a balanced budget. This implies constraints on the exchanges (width and depth of Both in Andalusia and in the Moroccan Rif straits), not changes in the physical laws governing Mountains, the domains of the Miocene straits are them. now fully continentalized. The term “Betico-Rifian The east–west length of the Gibraltar Strait is 50km Portals” acknowledges the fact that the present strait has from Ceuta to Cap Spartel, west of Tangier: the no common feature with them. If the Strait of Gibraltar Tortonian to early Messinian Portals were hundreds of has to bear the mark of the MSC, it can only be that of kilometres long, either on the northern shore, from the the very final phase, i.e. the Zanclian refill (Blanc, bay of Cadiz to Alicante, in east Andalusia (Müller and 2002), not of the crisis itself. Hsü, 1987), or on the southern shore, from Rabat to 352 P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372

Mellila (Gentil, 1911a,b). The shallow and narrow Table 2 channels lessened the efficiency of double-layered Hydrological data on the Mediterranean Sea circulation, while, under the negative budget and with Height Volume the help of west winds and Atlantic tides, the Evaporation, −1.40m year−1 1.186·1012m3 year−1 configuration allowed for the influx of water from the western basin −1 12 3 −1 Atlantic Ocean to the Mediterranean Sea. Evaporation, −1.61m year 2.793·10 m year eastern basin Freshwater to +0.60m year− 1 0.498·1012m3 year−1 3. Budget of the evaporitic sedimentation western basin Freshwater to eastern +0.47m year− 1 0.799·1012m3 year−1 The degree to which the Mediterranean Sea has basin (less Black Sea) − 1a 12 3 −1 changed since the late Miocene cannot yet be accurately Black Sea outflow to +0.12m year 0.204·10 m year eastern basin assessed: the present configuration represents the best − − Freshwater to eastern +0.59m year 1 1.003·1012m3 year 1 analogue allowing to establish a budget of the water and basin (total) salt supply during the MSC. Estimates of the NASW Western basin water deficit −0.80m year−1 0.664·1012m3 year−1 supply at the Atlantic Strait are derived from the present Eastern basin water deficit −1.02m year−1 1.734·1012m3 year−1 12 3 −1 external factors and may be adjusted to the budget of the Mediterranean deficit 2.398·10 m year or net inflow of NASW Messinian evaporites, taking the chronology of the MSC − Gross inflow of NASW 5.284·1013m3 year 1 as an estimate of the duration of application of these through Gibraltar factors. Outflow of MSOW 5.044·1013m3 year−1 through Gibraltar − 1 3.1. Present hydrological budget of the Mediterranean Salinity of NASW 36.18g l Salinity of MSOW 37.90g l− 1 Sea − Salinity of surface water 37.05g l 1 at the Strait of Sicily The present physical data on the Mediterranean Salinity of East 38.74g l− 1 basins (Table 1) are rather well known. Most of the Mediterranean differences from one author to the other result from outflow water simplified calculations or display, rather than from a With reference to eastern surface only. independent measurements. A more complete table is given in Blanc (2000). The hydrological data for the present Mediterra- By comparison, the data on the hydrology are more nean used in the new version of the model appear in diverse, because they imply the interpretation of a wide Table 2. They originate from a publication by array of methods, from direct measurements in the Bethoux (1980), and they may show maximum straits to climatic budget estimates. Physical oceano- values for the total fluxes (Gross NASW inflow and graphers can be grouped into two schools: one Gross MSOW outflow): a recent estimate published favouring a larger rate of exchange between the Atlantic by Tsimplis and Bryden (2000) is 0.66±0.17Sverdrup Ocean and the Mediterranean Sea, the other favouring a (1Sverdrup or Sv=1km3 s−1) or 2.08·1013 m3 year− 1 smaller rate, though both can provide a set of data for the input of NASW at the Strait of Gibraltar and consistent with the water and salt conservation state- 0.57±0.04·106 Sv or 1.80·1013 m3 year− 1 for the ment for the Mediterranean. In Blanc (2000),a output of MSOW, and this is about 2.8 times smaller composite data set, derived from estimates by different than the estimates from Bethoux (1980). However, authors, was used: however, this vain attempt at the net NASW inflow (difference between the gross reconciling two oceanographic schools of thought by inflow and gross outflow), compensating for the using intermediate estimates did not rely on new Mediterranean deficit, appears higher in the latest original measurements. estimate. We have to consider this data set (Table 2) as the best Table 1 available, because it provides evaporation and precip- Physical data on the Mediterranean Sea itation estimates consistent with the seawater fluxes, and Surface (less Black Sea) 2.53·1012m2 thus with the differences in the climatic regime between Average depth 1430m the western and eastern basin. The model itself does not Volume 3.6148·1015m3 depend on the data set: and the touchstone is as much the Surface of western basin 0.83·1012m2 internal consistency of the set of data as their factual 12 2 Surface of eastern basin 1.70·10 m integrity. Any discrepancy might increase to prohibiting P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372 353

Table 3 also implies that the present covered area may just be Budget of the eastern Mediterranean evaporates smaller than it was when the (un-duplicated) series Surface (half of basin) 0.85·1012 m2 were deposited. Our estimate of the salt mass in the Thickness of evaporates 3500m eastern basin (Table 3) is thus based on this average Density of halite (rock-salt) 2165kg m− 3 18 thickness, admitted to cover about one half of the Mass of evaporates 6.44·10 kg present zero-level surface: originally, the thickness was less, but this is up to a point compensated because the surface of deposition must have been levels when a time-factor of half a million years is more. introduced. After the major drawdown of the western basin below the level of the sill of Sicily, neither seawater nor 3.2. Erosion phases in the western Mediterranean and brines could be transferred any longer to the eastern evaporites in the eastern basin Mediterranean. This implies that the chemical constitu- ents of the deep evaporites entered the eastern basin The profile of the Messinian valley of the Rhône before the basins were separated, i.e. beginning at the river, below the Pliocene and Quaternary deposits, same time as the marginal evaporites were formed and shows several sections according to its slope (Clauzon, during the first phase of erosion of the valleys of the 1982). In its upper course, above Lyon, the slope is rivers tributary to the western basin. about 3‰. The mid-section from Lyon to the Pierrelatte Basin is closer to 1.8‰. The lower section 3.3. Budget of the evaporites in the western basin dips down to the oceanic margin with a 9‰ slope. This composite profile originates from differences in A similar calculation can be done for the western the resistance of various rocks to erosion and from two deep evaporites, based upon the assumption of salt phases of rejuvenation of the valley (Beaudoin et al., deposits 1600m thick (Montadert et al., 1978), over half 1995). The upper sections, with a slope from 3‰ to the surface of the basin (Table 4). 1.8‰, belong to an early stage of the rejuvenation The deposition of these evaporites is accepted to have process, which resulted in a profile much closer to taken place after the level of the Mediterranean has equilibrium than reached during the second, deeper, receded down below the level of the sill of Sicily. erosion phase. The relative equilibrium profile obtained during the early phase of erosion implies 3.4. Accuracy of the mass budget that its base level remained steady for some time. However, it was already hundreds of meters below the Whatever the consensus over their acceptance, mass Ocean level of the time and Messinian pre-crisis sea estimates can only be considered as orders of magni- level. tude. Though public, these thickness estimates for the The only possible explanation for this temporary deep evaporites were maximal figures deduced long ago equilibrium is that, at the time, the supply of Atlantic from interpreted geophysical tracks, which have never water happened to be less than the evaporation over been fully calibrated. The surface over which the the whole Mediterranean Sea surface but more than the deposits actually extend is not known more accurately. evaporation on the western basin alone (Blanc, 2000). The figures adopted here are definitely maximum During this phase, the western basin only transferred estimates of the total mass of salt deposits, particularly above the sill of Sicily the small amount of water in in the eastern basin, but the modelling remains excess to the volume necessary to the preservation of consistent because the hydrological budget used in the its own level. This overspill was no longer enough to modelling is also rather high. This will be further make up for the evaporation over the eastern basin. discussed below (Section 5.5). Thus, the surface of this basin dropped at a rate controlled by the decrease of the supply, to levels only governed by the balance between the surface of Table 4 evaporation and the activity of the brines. Budget of the western Mediterranean evaporates A thickness estimate of about 3500m has been Surface (half of basin) 0.415·1012m2 suggested for the eastern basin deep evaporites Thickness of evaporates 1600m − (Montadert et al., 1978): such values probably Density of halite (rock-salt) 2165kg m 3 18 involve tectonically duplicated series. However, this Mass of evaporates 1.44·10 kg 354 P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372

4. Principles of the budget model 4.2. Calculation of water levels

The model was built under Microsoft Excel (from The transfer of water from one basin to the other XL 5toXL 2003), and followed both the improvements above the Sicily sill is solved by conditional in the power of the PC hardware and the evolution of calculations. the software. It deals with the budgets, not the dynamics of the phenomenon. The geographical, hydrological – If the total water volume after evaporation and supply and climatic data used in the modelling are shown in remains less than the volume at the zero level, but Tables 1 and 2. more than the volume at the level fixed for the internal Sicily sill, the model equalizes the level in 4.1. Geometry of the basins the two basins (Fig. 3) and calculates the volume of water transferred from the western to the eastern The western and the eastern Mediterranean basins are Mediterranean. It then recalculates the parameters treated separately. Both basins are considered as conical and proceeds to the next time-step. from the surface to −4290m, i.e. three times the average – When the western level after supply is still above the depth of 1430m (Fig. 1). internal sill, but when the total volume no longer This simple geometry accounts for the decrease in reaches the sum of the basins at sill level, only the free surface when the level drops down. The Strait of water volume above the sill is transferred to the Gibraltar is characterized by its initial input and a law of eastern basin (which then remains below sill level). evolution. The Strait of Sicily is characterized by the The western basin remains at the level of the internal depth of the hydrological sill, either the present one or sill (Fig. 4). adjusted to scenario. – When the level of the western basin after evaporation Present conditions: Totalling the inwards flux, the and supply no longer reaches the level of the internal freshwater contribution (rainfall, runoff and river sill, each volume is calculated separately (Fig. 5), discharge, considered as invariable in the course of until no marine supply remains at all. each experiment) and subtracting the evaporation, this – In the process of evaporation calculation, the water model, with a 100-year time-step: activity is calculated according to data from Robin- son and Stokes (1959, fide Gonfiantini, 1986). – calculates the western volume after evaporation and water supply and that of the eastern basin after In order to take into account the precipitation of evaporation (Fig. 2); evaporitic minerals, the calculation fixes to the salinity – calculates the water volume transferred to the eastern an upper limit at 10.25 times the normal oceanic basin: as modelling the Mediterranean water budget value. This brine concentration (371.56g l− 1) corre- implies simulating the transfer of water back to the sponds to sodium chloride saturation at 43°C. This Atlantic Ocean, it also calculates the amount of prevents unrealistic salinity and activity calculations, dissolved salt transferred back to the western basin; but the salt remains in the cumulative budget of each – recalculates separately for each basin the salinity and basin. the water activity. 4.3. Need for surface salinity calculations

An estimate of the average surface salinity is of use for two purposes: the first one, for both basins, is to calculate the activity of the water and the evaporation; the second one, for the western basin only, is to estimate the amount of salt transferred to the eastern basin. The early version of the model (Blanc, 2000) introduced a double bias in the evolution of the free surface of the basins and in the salinity values, because it did not differentiate in each basin the surface water from the deep water. The evaporation was then calculated Fig. 1. Geometry of the basins: surface of western basin, 0.83·1012m2; from the activity value of the deep water, which has the surface of eastern basin, 1.7·1012 m2. highest density and salinity, and also shows a low P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372 355

Fig. 2. Present state of the Mediterranean, the input being enough to maintain full level: left, water input to the western basin; right, transfer to the eastern basin. activity. The calculated evaporation was underesti- be equivalent to a mixture between their surface input mated: the most extreme the salinity, the stronger this water and their deep resident water. effect. We thus consider a fictitious Western Mediterranean Central Surface Water to be a mixture of 49.42% NASW 4.4. Surface salinities, steady state (s=36.18g l− 1) and 50.58% MSOW (s=37.9g l− 1). This ratio is derived from the salinity value of the In the first instance, let us consider that the global present water passing the Sicily sill at s=37.05g l− 1. ocean salinity did not change for the duration of the M. This salinity value is then used both for the calculation S.C: whether true or not, it did not introduce any relative of the evaporation on the western basin and for the change between the basins involved. calculation of the amount of salt transferred to the In the present state of the Atlantic Ocean–Mediter- eastern basin. ranean Sea system, the western basin receives NASW In a similar way, we consider that the evaporation on with an average salinity of 36.18g l− 1. The water the eastern basin operates on a fictitious Eastern transferred to the eastern basin above the sill of Sicily Mediterranean Central Surface Water defined from the comes from the western basin as NASW modified by salinity of the surface water entering the eastern basin at evaporation and mixing and reaching a salinity of the Strait of Sicily (s=37.05g l− 1) and from the deep 37.05g l− 1. The deep to intermediate return current to water overflowing the Sicily sill in the opposite the western basin at the sill Sicily shows a salinity of direction at s=38.74g l− 1. For want of a better law, 38.74g l− 1. The MSOW, when crossing the Strait of we consider the mixing rate to be the same. This yields Gibraltar to the Atlantic at intermediate depth, shows an average surface salinity value over the eastern basin an average salinity of 37.9g l− 1.Thesalinities of 37.905g l− 1. discussed here originate from Bethoux (1980, Diagram In both cases, it must remain clear that this is not the 1). They are consistent with the hydrology data shown true description of the physical processes by which in Table 2. By comparison to the first version of the actually described water masses are constituted. It is model (Blanc, 2000), adopting these values implies an only a rather empirical way to obtain surface salinity acceptance of the evaporation/precipitations/water def- values to be applied to the calculation of the evaporation icit estimates from Bethoux (1980), no longer mitigated at the surface of each basin and to the supply of salt– by any other consideration, as the “Standard-Case” water to the eastern basin. The values obtained do not hydrology. introduce any bias in the calculations when the present In order to make up for the lack of a description of the hydrology data are used. The model describes correctly vertical structure of the hydrology in the two basins, we the present Mediterranean budget when the present now consider the central surface water in each basin to constant flux is kept. With other scenarios, inasmuch as

Fig. 3. First stage in the evolution of the MSC level drawdown: the input is no longer sufficient to maintain full level; the level remains the same in both basins (above the depth of internal sill: left, water input to the western basin; right, transfer to the eastern basin). 356 P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372

Fig. 4. Second stage in the evolution of the MSC level drawdown: the input being little more than needed to maintain the western basin at Sicily sill level; only the excess water is transferred to the eastern basin, the level of which begins to recede. these calculations yield surface salinity values which fall deficit, so that the rejection of salt to the Atlantic ocean necessarily in the range of what is physically possible, is impossible. During the evaporitic phase itself, the they are rather better, or at least less bad, than to apply in reduction in the supply proceeds from the net inflow to each basin an average salinity value dominated by the zero and the basins reach a balance between the surface deep waters. Still, this is little or no more than a “better of evaporation and the activity of the evaporated water. than nothing” approach. It has not been tried to estimate The model now uses a yearly NASW supply any such thing as a volume or a height of a water column reduction law in inverse to the third power of the 3 for these surface waters. They only appear in the budget relative time elapsed: Vt =V0(T−t/T) where (Vt) is the through an increased water activity and evaporation rate. supply at time t, V0 is the initial supply, T is the total duration modelled and t is the time-step reached since 4.5. Evolution of the surface salinities during the the beginning of the run. reduction in Atlantic supply This is the ratio which a channel with a triangular section would follow. The first power shows the A finite salt budget and duration should ideally have regional uplift of the Atlantic portals, the second characterized each sedimentary phase, but no series of power shows the reduction of the open section when discrete, adjusted phases can account for the observed the depth decreases and the third power accounts for a pattern: the development of the Messinian evaporitic reduction in the velocity of the flow when the section sedimentation phases shows that it was not just a series decreases. of stationary regimes (Blanc, 2000). We thus consider the MSC to result from a transitory period from normal 4.6. Surface salinities with a decreased NASW input oceanic to lacustrine endoreic conditions. The present balance of salinity between the Atlantic A most hazardous assumption would be to suppose ocean and the Mediterranean Sea is only maintained that, for the whole duration of the evaporative process, through salt and water exchanges involving a gross the surface salinity values evolved in such a way that inflow of NASW about 22 times higher (and a gross they could be assimilated to mixtures, at a constant rate, outflow of MSOW about 21 times higher) than the of changing components (see above). This cannot apply deficit or net inflow. During a long Pre-Evaporitic alone when the rate of supply of marine water (either Messinian period, the supply was progressively re- constant salinity NASW at the Atlantic Portals, or duced, but it was still enough to preserve a normal level. fictitious Western Mediterranean Central Surface Water The evaporitic sedimentation phase of the Salinity Crisis at the Strait of Sicily) is regularly decreasing. begins when the NASW supply at the Atlantic portals To take this into account, this model now uses, as a equals the net inflow, just compensating the water reduction factor to the mixing rate defined above, the

Fig. 5. Third and fourth stages of level drop: the input is too weak to maintain the western basin at Sicily sill level; the eastern basin is deprived of marine supply: at the end of the crisis, the NASW input is nil and the western basin also reaches its level of equilibrium with evaporation. P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372 357 ratio of the calculated marine input (at each time step) to applied to the admission of NASW at the Betico-Rifian the water deficit, over both basins for the salinity of the Portals or Strait of Gibraltar, except for the available surface water passing the Strait of Sicily, or over the entry parameters. In the case of the Miocene Atlantic eastern basin alone for the surface salinity to be used in portals, the initial water flux is constrained by a the calculation of evaporation over the eastern basin. specified decrease law, though the depth remains When the supply of surface saltwater is nil (at the sill of unknown. For the internal sill, the water flux is not an Sicily, when the level of the western basin is no longer entry parameter, but it is calculated from the depth of the higher than the sill, or at the Atlantic straits, when a sill (eventually modified according to the scenario continuous Gibraltar Arc tectonic belt is fully emer- considered) and the western Mediterranean water level. gent), the value of this factor is zero, so that the salinity of the surface waters also falls to zero. This fits with the 4.9. Transfer of salt from the eastern back to the increased importance of the freshwaters (rainfall, runoff, western basin rivers) in the supply of the basins, but it does not mean that the total amount of these has increased. It also fits The present salinity values in the Mediterranean with reduced mixing properties when the salinity show that marine basins cannot remain isolated from gradient between the brines and the surface waters each other when the sill which separates them is increases. Orszag-Sperber et al. (2000) reached a similar immersed below several hundred metres of water. Any interpretation of the origin of the brackish Lago-mare model must include a calculation of the transfer of salt facies through different methods. from the eastern back to the western basin, as long as the water level in the eastern Mediterranean remains above 4.7. Transfer of salt from the Mediterranean Sea back the level of the internal sill of Sicily. to the Atlantic Ocean The salt mass returned from the eastern to the western basin remains proportional to the amount which the The transfer of salt from the Mediterranean Sea back eastern basin received with the transfer of fictitious to the Atlantic Ocean is calculated. In the steady state, it Western Mediterranean Central Surface Water, limited is based on the present regime, according to which the by the same condition on salt saturation as the rejection salinity of the MSOW going out at Gibraltar is such that of salt back to the Atlantic. the salt mass transferred back is equal to the mass admitted with the NASW influx. When the NASW 5. Concurrence of sedimentary and hydrological supply decreases, this return has to be limited to the phases amount, which the excess water can carry out at the salinity calculated for the western basin deep water at 5.1. Definition of a Standard-Case scenario: assump- the previous time-step. The salinity of the outflow water tions and consequences is limited below saturation, in order to prevent the model from returning back to the ocean over-saturated brines The initial modelling exercises (Blanc, 2000) were with unrealistic salinity values. When the water budget only conducted on 0.5Ma, which we initially accepted is so negative that the possibility of back-transfer of as the duration of the MSC (Benson et al., 1995; water no longer applies, there is no back-transfer of salt Clauzon et al., 1996). Contradictory assumptions biased either. For want of any reliable estimates of the section this version of the model. An initial Mediterranean Sea and depth of the Miocene Atlantic Portals, it is not with present salinity values is inconsistent with an initial possible to calculate a more progressive law to curtail NASW supply just compensating for the water deficit. the exchanges of salt with the Atlantic Ocean when they The early, short version of the model was wrong because closed. an initial phase of calculation, planned to be part of the MSC, was in fact used to catch up with the pre- 4.8. Transfer of Western Mediterranean Central Sur- evaporitic evolution of the salinities. The description of face Water to the eastern basin this phase was not reliable either. The present updated version of the model calculates, In the last version of the model, the transfer of from the water deficit of the Mediterranean and from the Western Mediterranean Central Surface Water to the present NASW salinity value, an initial NASW supply eastern basin decreases in inverse ratio to the third to the Mediterranean, eventually higher than the present power of the remaining relative water depth above the inflow (which is about 53 1012 m3 year− 1; Bethoux, Sicily sill. Actually, this is the same treatment as the law 1980), and regulates it for the specified decrease law and 358 P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372 duration of time. The model assumes permanent 16.34% vs. 83.66%. The modelled budget could be hyperventilation of the Mediterranean basins, so that, balanced by adjusting the depth of the sill of Sicily and when assuming a higher flux of NASW than at present, this should ideally provide an estimate of the level of the salinities (both deep and surface) are lower than the this sill in Messinian times, but this is not trustworthy present ones, i.e. closer to the original NASW from because of the high uncertainty in the evaporitic budget which they derive. of the basins. The Standard-Case scenario has been extended to With the present climatic parameters, a 0.5Ma 15,000 calculation steps, i.e. a total lapse of time of Salinity Crisis (as defined by a flow decrease from 1.5Ma. This allows for a 1Ma pre-evaporitic evolution Vin =∣e−f∣ to Vin =0) would yield about 125% of the of the salinities, beginning from a water supply 27-fold Messinian evaporitic budget considered in Tables 3 and (33) the water deficit of the Mediterranean area, and for 4. The model would then require either reduced an estimated duration of the MSC of 0.5Ma (following evaporation over both basins, or increased freshwater the stratigraphy by Clauzon et al., 1996, when the model supply to both basins, or any combination of both, but was first developed). The inception of the Salinity Crisis this would be a rather paradoxical implication in the proper is considered to correspond to the time when the investigation of the conditions of evaporitic sedimen- supply of marine water just compensates for the water tation: a balance in accordance with our compiled deficit (Vin =∣e−f∣). budget could be obtained by reducing the climatic The values defining the initial parameters for the parameters–both evaporation and precipitation–by a Standard-Case are discussed with Tables 1–4: this 0.736 factor. scenario can be considered as a test of the evaporitic potentialities of the present Mediterranean rather than a 5.2. Pre-evaporitic phase and rise in salinity reconstruction of the MSC itself. Under the Standard-Case conditions, this model has During the pre-evaporitic modelled duration of 1Ma, shown a disequilibrium in the distribution, in favour of the salinity increases at the same rate in both basins (Fig. deposition in the eastern basin: the compiled budget 6), because the whole Mediterranean behaves as a single being 18.25% vs. 81.75% in the western and eastern basin, due to the transfer of saltwater or brine from the basin, respectively, the model yields a distribution of eastern basin back to the western basin at intermediate

Fig. 6. Standard-Case evolution of the levels and of the deep water/brines salinity in the Mediterranean basins, for a 1Ma pre-crisis Messinian period (6.82 to 5.82Ma BP) plus a 0.5Ma Salinity Crisis proper (5.82 to 5.32Ma BP) (the oscillatory pattern of the curves results from discretization artifices). P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372 359 levels. This progressive salinity increase has no effect the Mediterranean basins, while sodium chloride was whatsoever on the level: despite the restricted circula- still re-exported to the Atlantic Ocean (Copin-Montégut, tion, it remains the same in both basins, at the initial 1996, pp. 29–30). oceanic level. During this period, the Mediterranean Sea Some tens of thousand years later, before any level also transfers some deep water and salt back to the change occurs, both basins reach sodium chloride Atlantic. A detailed description of the progressive saturation (about 10 times normal) at the same time, restriction, geochemical changes and reduction in due to the transfer of salt from the eastern basin back to abundance and specific diversity of the microfauna the western basin. However, the western basin does not and microflora during the deposition of the Tripoli reach a continuous saturation episode (Fig. 7) because it diatomaceous facies can be found in Blanc-Valleron et transfers to the eastern basin an evolved mixture of al. (2002), showing that the biological environment in evaporated NASW and deep to intermediate western the Mediterranean was less and less that of the open Mediterranean water: but gypsum precipitation must ocean, and that marine conditions were less affected at have taken place locally in marginal basins. the surface than in the deep environment. As the original level of the Betic marginal basins is generally estimated to be between 0 and −150m, we 5.3. Inception of the MSC and marginal deposition agree with Clauzon et al. (1996), who denied the synchronism between the marginal and abyssal evapo- The conventional inception of the crisis (as defined ritic deposits. The initial high salinity episode is the only by Vin =∣e−f∣ and Vout =0) appears to postdate calcium favourable period for evaporitic sedimentation in sulphate saturation (Figs. 6 and 7), but this originates epicontinental basins, because later the salinity in the from the use of sodium-equivalent salinity data, and western Mediterranean undergoes a drastic reduction. evaporation parameters defined for marine water: As a Actually, the marginal basins and the eastern basin are in matter of fact, gypsum deposition actually begins at a the same position of distal basins with respect to the sodium-equivalent salinity of about 170g l− 1 and, when proximal western basin. this concentration is reached, rejection of sodium The same period is also favourable for evaporitic chloride still takes place at the Atlantic strait. This is sedimentation in marginal basins depending on the the main reason why calcium sulphate appears much eastern Mediterranean, as later the level drops, so that more abundant in the sediments that the normal marine they could no longer be fed: the evaporitic sediments in ratio could explain: the calcium sulphate was trapped in marginal basins tributary to the eastern Mediterranean

Fig. 7. Same as Fig. 6, but showing only the 0.5Ma Salinity Crisis proper (5.82 to 5.32Ma BP). 360 P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372 could only be contemporaneous to the lower evaporites part of its dissolved salt content, and the average salinity deposited in the deep basin. decreases, at the beginning of this period, to less than half the saturation ratio. The deposition of evaporites, both of 5.4. Second phase: joint and moderate fall in level halite and of gypsum, is interrupted there. The evolutions of the salinity in the two basins are As the NASW inflow at the Atlantic Strait steadily shown in Fig. 9. The separate two phases of evaporitic decreases, the levels of the basins begin to fall, until the sedimentation in the proximal basin appears as intrinsic Sicily sill is reached. When the section of the Strait of to the two-basin system. The model may overestimate Sicily is reduced, the eastbound surface current is the salinity of the surface waters at the beginning of the maintained by the negative budget of the eastern basin, MSC, but there can be no doubt as to the surface salinity while the westwards return current decreases. In the decrease in the western basin during the phase of level eastern basin, 45% of the evaporitic load accumulates stabilization. The undersaturation of the bottom waters before the levels in the two basins are separated from in the western basin depends very much on the degree of each other (Fig. 8). This is the beginning of the first isolation of the brines from the surface waters. It phase of erosion of the river valleys tributary to the remains one of the problems, which a simple budget western basin, now preserved as their upper buried rias. model cannot resolve with a high degree of certainty, The initial sedimentation in the deep western basin is because the vertical hydrological structure remains dominated by the so-called lower evaporites, most oversimplified. probably a mixture of calcium sulphates (gypsum, This undersaturation episode in the western basin anhydrite ?), detritals provided by erosion and slumped may have been responsible for the observations of materials from the emergent upper margins of the basin. intercalated clayey sediments with sparse foraminiferal or other planktonic fauna, interpreted up to now as 5.5. Third phase: drawdown of the eastern basin indicators of an intra-Messinian re-inundation (Müller and Hsü, 1987). During extreme salinity episodes, the During the following time-lapse, the spillage of water planktonic elements, imported to the Mediterranean above the Sicily sill controls the level of the western with Atlantic surface water, become diluted in the basin, as well as the base level of the erosion of their evaporitic sediment. During the period of decreased valleys by the tributary rivers to the western basin. The deep salinity in the western basin (still twice above western basin transfers to the eastern basin an evolved normal), these elements may appear less dispersed, mixture of evaporated NASW and deep to intermediate because the sedimentation rates are much lower. western Mediterranean water, with a salinity above When the transfers of saltwater to the eastern basin normal, while it only receives normal salinity NASW are reduced, the salinity increases again in the western from the Atlantic. basin while it remains level with the sill: this is when the The return of salt back from the eastern basin is now deposition of evaporitic sediment begins in this basin. made impossible by the one-way circulation over the sill The interpretation that the deep evaporites cannot be of Sicily. As a consequence, the western basin is rinsed of coeval in the two main Mediterranean basins is sustained

Fig. 8. Standard-Case evolution of the distribution of salt deposits in the Mediterranean basins, during a 0.5Ma Salinity Crisis proper (5.82 to 5.32Ma BP). P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372 361

Fig. 9. Evolution of the salinities of the incoming waters and of the surface and bottom waters in the Mediterranean basins during a Standard-Case, 1Ma pre-crisis Messinian period (6.82 to 5.82Ma BP) plus a 0.5Ma Salinity Crisis proper (5.82 to 5.32Ma BP). by the modelling. The presence of thick evaporites, lasted through the whole phase of western basin thousands of kilometres from the origin of the salt, i.e. stability. Cunningham et al. (1997) already questioned the open ocean, is explained by the shift in evaporitic the appropriate character of correlations between sedimentation from one basin to the other. However, shallow, marginal evaporites and the Sicilian series, when they later happened to be exundated, the halites which they termed “deep-basinal”. may have been reworked within their basin of deposition from shallow, sometimes emergent, zones to deeper 5.6. Inception of the “Lago-mare” in the eastern basin areas. As the supply to the eastern basin no longer makes up Notwithstanding the possibility of brackish to for the evaporation, the surface level undergoes a retreat freshwater sedimentation in emergent suspended lakes, down to −1718m, until the basin becomes stagnant. At the de-salination of the surface waters (in accordance this time, the remaining brine column may still have with the surface salinity calculation introduced in been about 2.5km high in the deepest parts of the basins. Section 4.3) may have begun in the eastern basin as The Messinian type of evaporitic sedimentation remains soon as the supply of saltwater no longer made up for without any present time analogue. the deficit: however, there must have been so much salt The Sicilian marginal basins may not have been available for dissolution that the inception of the Lago- truly marginal, as they belong to the intermediate belt mare facies must have been delayed. Locally however, between the western and the eastern Mediterranean the surface salinity may have been reduced below basins: as such, there is no doubt that they were fed by normal before the drawdown was completed and before the western basin under the same conditions as the total isolation was achieved. eastern basin itself during the two first phases (inception of the evaporitic crisis and joint drawdown 5.7. Fourth phase: drawdown of the western basin to the level of the sill of Sicily). Admitting that later they were not the main passage from the western to the As stated previously, as soon as the transfers of eastern basin, deposition in these basins could have saltwater to the eastern basin are reduced, the salinity 362 P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372 increases again in the western basin while it remains ecological indicators are poor stratigraphic indicators, level with the sill. This is when the deposition of the and because reworking of deposits may be a rule rather halites begins in this basin, as it reaches again sodium than an occasional occurrence in these facies. chloride saturation and keeps all of the incoming salt through the Atlantic strait. 5.9. The Zanclian re-opening of an Atlantic passage When the water supply at the Atlantic Strait is no longer enough to preserve the level of the western basin, The Zanclian opening of the Strait of Gibraltar was a the drawdown in the western basin is governed by the built-in process: The narrow Gibraltar Isthmus was uplift of the Atlantic passages. It reaches a value of eroded by an eastbound stream, with the bottom of the −1509m, its level of equilibrium between rainfall/ Alboran sea as a base level. The breakthrough of the runoff/river discharge vs. evaporation. This caused the valley-head of this stream into the Atlantic domain, and second phase of erosion of the river valleys (over- an Ocean-level rise of 10 to 20m, allowed the overspill deepened lower courses) and margins. However, the of Atlantic waters. The water reservoir was of planetary erosion in the upper course of the valleys went on, scale, the Atlantic flow increased exponentially and keeping the knickpoint with the lower course as a base eroded the present strait in the short time-span of about level. During the last third of this phase, deposition of 11years (Blanc, 2002). evaporites was negligible, but for the reworking of exposed emergent salt deposits. 5.10. Hydrological structure of the basins during the MSC 5.8. Fifth phase: sedimentation in the closed basin The major difficulty with the first version of this When the water input at the Atlantic Strait is model was that it did not take into account the suppressed, the surface water in both basins is only hydrological structure. Each basin was treated separate- supplied by precipitation, runoff and river discharge. ly, but they were considered as homogeneous at each The surface of each basin is then very low, −1718 (east) time step, so that no water masses could be distin- and −1509 (west) m below the present level. In most guished. In the real ocean, the water masses are stratified cases, the emergent evaporites must have been washed according to density and they can travel very far from down towards the low areas of the basins, so that the their origin with only a very slow mixing. This new evaporitic sedimentation went on, under the control of version now mitigates this bias. the precession driven precipitation supply, resulting in The modelling indicates that during the time span the bedded “upper evaporites” alternation of gypsum when the Mediterranean remains level with the global and marl. ocean, the salinity of the surface waters increases at a Some suspended basins, above the level of the lower rate than that of the bottom waters, so that the residual brines, may have retained fresh to brackish gradient reaches enormous values by comparison to lakes. At the estuaries of the smaller rivers (where the anything observed today (Fig. 9). Then, the surface detrital load may not have been too important), on the salinities begin to decrease as soon as the deep waters edges of the evaporated basins, the mixing of the runoff reach sodium chloride saturation, because this also and river discharge with the brines was minimal, coincides with the decrease in NASW input. because of the very strong density gradient. These When the freshwater supply becomes dominant, the areas remained very restrained, as the evaporation rate salinity contrast between the surface waters and the deep remained the same over the Mediterranean as during the brines is such that mixing is close to impossible: previous phases. Still, fresh to brackish water ecosys- discontinuous surface lenses of almost freshwater may tems were able to develop in these protected areas, now remain, floating on the brines. The volume of these described as the Lago-mare sedimentary facies: it Mediterranean surface freshwater lenses must have been gained a wider extent at the end of the period, when so small that we have to assume that they were the exposures of soluble salts were reduced by the discontinuous and subject to seasonality, but this is reworking process itself, as well as under the possible also a reason why, as local occurrences, they can have occurrence of pluvial episodes. The inception of these been important enough to be recorded at all in brackish facies in the basins must have been diachronous, sedimentary facies. According to the modelling, the because the supply to the western basin was terminated time-span during which a Lago-mare environment may later than in the eastern basin. However, this cannot be have been developed is longer than the time during assessed by geological methods, because most fossil which primary evaporitic sedimentation occurred, but P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372 363 this is masked by the occurrence of redeposition of simulation of the exchanges between the basins now eroded (dissolved) salts. balances the influence of the sill depth. According to Tables 3 and 4, the distribution of the deposits is 18.25% 5.11. Depth of the Strait of Sicily during the MSC on the western side and 81.75% on the eastern side. In the Standard-Case scenario, keeping the present climatic The Strait of Sicily controls the depth of the first parameters and volume or depth of the basins, this phase of erosion by the tributary rivers to the western distribution is now obtained by adjusting the depth of basin. It also controls the balance of evaporites through the internal sill to 335m, i.e. 95m higher (shallower) the relative duration of the time-span when the eastern than at present. The total evaporate deposition remain basin still receives a supply vs. the phase when only the 25.5% above the compiled budget. The discrepancy in western basin benefits from the inflow of NASW. The sill depth figures is slightly increased when we adjust higher the sill, the earlier the basins are separated in the the ocean level to a reconstructed Messinian pattern. evaporative process, and thus the higher the proportion The other available indicators of the depth of the of evaporites retained in the western basin. Strait of Sicily are the ecology of Pliocene ostracodal All parameters being standard, a difference in the palæofauna (Benson, 1973), which point to a depth of level of the sill of Sicily from −100m to −1000m, about 1000m, and the erosion profile of the upper would postpone the split between the two basins from section of the Rhône palæovalley, said to tend towards a 37,400 to 167,700years after the inception of the crisis base level of −400m below the present coast (Beaudoin (Figs. 10 and 11). The salt deposition in the western et al., 1999). basin would be reduced from 23.34% to 6.84%. It has to The presence of ostracodal palæofaunal indicators be noticed that the duration of the deeply evaporated of depths greater than 1000m in the eastern basin acme phase is very much increased in the eastern basin during the Pliocene may originate from a suction when the internal sill is high, while no difference is effect of Atlantic intermediate waters during the introduced in the western basin. Zanclian re-inundation. These figures differ notably from those obtained by Blanc (2002) has shown that the depth of the the first version of the model, because a better western reaches to the main corridor of the strait must

Fig. 10. Evolution of the levels and distribution of salt deposits in the Mediterranean basins, for a depth of the Sicily sill of 100m (otherwise, Standard-Case; 0.5Ma shown). 364 P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372

Fig. 11. Evolution of the levels and distribution of salt deposits in the Mediterranean basins, for a depth of the Sicily sill of 1000m (otherwise, Standard-Case; 0.5Ma shown). have been more than 400m at the end of the refill, of both the budget of the evaporites and the knowledge before the present Spartel and Camarinal sills were of the true surface of the Mediterranean basins in the formed, with a head loss close to 500m towards the Miocene times. As previously stated, this is only Mediterranean. The normal stratification of the water analogous modelling. masses was not immediately restored, so that Atlantic Nevertheless, the morphological evidences from the intermediate waters might have reached the eastern Rhône river valley are better reconciled by the new basin by floating above mixed brines across the version of the model than they were with the originally western basin. published one (Blanc, 2000). Last, estimating the original base level of a palæoriver from the profile of its valley is a naturalist 6. Conceptualisation of the Messinian Crisis process, not quite rigorous when successive new phases of erosion have occurred in the course of the river: this is The modelling of the salt and water budgets of the precisely the case of the river Rhône, though its profile Mediterranean Sea during the sedimentation of the remains the best documented. evaporites makes it possible to point out to some The present depth of the Strait of Sicily is 430m and conceptual implications. it is possible to find arguments both for a deep or a shallow Miocene sill. It might have been deeper, as the 6.1. Accuracy of the model Sicilian evaporitic basins have obviously been uplifted since this period. However, as the water flowing to the One must not be disconcerted by the apparent eastern Mediterranean during the Zanclian flooding accuracy of the modelling. As far as the mathematical might also have eroded the strait (at some time the flow formalism is involved, there is no doubt that a PC can above the sill of Sicily must have been equal to the flow handle 12 significant digits through the whole process of through the eroding Strait of Gibraltar), the sill of Sicily calculation, whatever the display: a happy circumstance, might just have been shallower prior to the flooding. as discrepancies introduced by oversimplified calcula- The main reason why modelling cannot yield a sure tions, if repeated by the 15,000 time-steps of the model estimate for the depth of the sill separating the western (and the number of calculations at each step), might run from the eastern Mediterranean is the want of accuracy wild. P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372 365

We had to stress the approximate character of the 6.2. Origin of the MSC hydrological data on the Mediterranean. A satisfactory point in the budget reported in Table 2 is its internal The MSC was caused by the closure of the Miocene consistency. This does not mean that the data are “true”, Portals, which in turn results from the uplift of the Betic whatever true could mean for a set obtained from a wide and Rifian Mountains, and from their banking against array of methods, but that they are recalculated, so that the Iberian Meseta to the north and the Moroccan no contradictory assumption may remain in the para- Meseta to the south. The thermo-xeric conditions over meters entered in the model. the Mediterranean area were established earlier in A major drawback is that this model does not treat Miocene times (Suc and Bessais, 1990). These condi- separately the various electrolytes found in marine tions result from the latitude of the area itself, which did waters, but the aim of the modelling cannot be an not undergo large changes since the middle to late assessment of the real nature or level of segregation of Miocene. the deposited minerals. The only clear improvement that The cause for the negative budget of the Mediterra- a more complex chemical model could bring may be a nean Sea itself is the reduction in surface and the loss of better assessment of the time of beginning of sulphate communications with the northern basins of the Tethys- deposition by comparison to halite. Paratethys Sea. The budget of the evaporites is less satisfactory than At present, the freshwater budget of the eastern basin the hydrology budget. No complete series of the truly is close to that of the western basin (+0.59m year− 1 vs. abyssal evaporites has ever been sampled. The thickness +0.60m year− 1; Table 2).This is due to the input of estimates rely mainly on seismic lines and on accepted Black Sea surface water to the eastern basin (+0.12m (not demonstrated) similarities with the evaporites of year− 1). If there was complete isolation from the Black presently emergent areas, mainly in Sicily. Unfortunate- Sea, the freshwater input to the eastern basin would be ly, these estimates have not been re-assessed recently at much less (+0.47m year− 1): the evaporitic potentialities basin scale. would be increased. Conversely, if the passage between Within each main marine basin, there should be no the Mediterranean and the Black Sea (which can be theoretical reason to introduce any difference in the considered as a northeastern temperate basin of the thickness of chemical deposits, as the elements are Tethys Sea) was easier and, if this basin was more evenly spread as long as the basin remains submersed extensive, the rate of mixing would be more: the under saturated brines, before a complete change in the evaporitic potentialities of the Mediterranean basins hydrological budget occurs, or re-inundation takes place. would be less. There is no possible doubt that the When drawdown proceeds, the emergent evaporites may isolation of the northeastern Tethys basins from the be reworked down to the deepest areas within the same south-Tethysian, now eastern Mediterranean, basins, basin and this should be the only theoretical reason for was caused by the Alpine orogen. discrepancies in the thickness of the evaporitic deposits Once the isolation from the mid-latitude basins is (Tables 3 and 4). The 1600-m thickness figure established, the onset of evaporitic conditions only (Montadert et al., 1978) was generally accepted for the requires a reduction of the supply of NASW to the abyssal evaporites in the western basin, while a Mediterranean Sea through the Atlantic straits. This maximum estimate for the thickness of deep evaporites remains unquestionable. This reduction in marine in the eastern basin (even less reliable, if possible) was supply results from the closure of the Betico-Rifian more than 3000m: but as this may have been caused by Portals, as any marine link to the Indian Ocean was tectonically duplicated series, our compiled estimate of closed much earlier (Buchbinder and Gvirtzman, 1976; the amount of evaporitic deposits in the basins Rögl et al., 1978). (particularly in the eastern basin) is definitely high. The analogous modelling shows that the late- The simplified description of the hydrological Miocene climatic conditions need not have been any vertical structure of the basins has little influence on drier than at present. the modelled global budget of the deposits, because the duration of the event is long enough for the whole of 6.3. The role of the sill of Sicily the available salts to be deposited. However, as the evaporation depends on the salinity and activity of the Most of the large tributary rivers to the Mediterra- surface water, the degree of mixing has an influence on nean show fossil channels excavated deep into the the relative distribution of the deposits in the two main continental margin: those without such a channel have basins. been shown to have formed later than the Miocene. 366 P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372

The Messinian valley of the river Rhône, the best about 22 times the Mediterranean deficit. Our model known tributary to the western basin, originates from now takes into account a long delay between the two phases of erosion. The earliest phase appears to tend beginning of the salinity increase (when did it become a towards a base level of −350 to −400m below sea level crisis ?) and the time when the NASW input just (Beaudoin et al., 1999). The second phase reaches balanced the water deficit. The proper onset of the −1300m or more below the present Rhône delta sedimentary crisis should be the time when the salinity (Clauzon, 1982). Only one mechanism can provide an reaches saturation, in calcium sulphate rather than in explanation for the regulation of the base level of this sodium chloride. river during the first erosion phase: this is the overspill According to our model, when the water levels in the to the eastern basin of the water in excess to the volume basins became separated, the salinity in the western needed for the preservation of the western basin at the basin decreased below saturation (both in sodium level of the Sicily sill. chloride and calcium sulphate) until the bottom salinity At the end of the Miocene, the physiography was reached only about four times the normal salinity value. similar to the present on a major point: a continuous This was caused by the rinsing out of the brines from the series of tectonic belts, constituted in succession of the western basin to the benefit of evaporitic deposition in Betic, Rifian, Kabilian-Maghrebian, Calabrian-Apenni- the eastern basin. The Mediterranean followed the nic chains, already divided the Mediterranean Sea into behaviour of endoreic, successive-tiers lakes, usually two basins. The only submersed parts of this continuous observed in high-plateau areas when salts are available belt are at present northeast and southwest of Sicily. The for dissolution. Bottom brines may still have occupied evidence is not unequivocal that areas in the Apennines the deeper parts of the western basin, but halite may still have been below global sea level before the deposition could not take place because the supply MSC. was not continued during this phase. The undersatura- The division into two major basins explains best the tion event may even have induced an interruption in the sequence of phases of the evaporitic sedimentation. evaporate sedimentation in the western basin, previous- Initially, the water deficiency is essentially felt on the ly interpreted as a questionable intra-Messinian re- sedimentation in the most removed basin. The closest inundation of the western Mediterranean. As the salinity basin ensures the pre-concentration of dissolved chemi- can only tend towards normal marine salinity and the cals in the water, which is transferred through it. In the western basin remained at the level of the sill of Sicily, following phases, the restricted water supply is felt on this episode cannot be related to the inception of the the sedimentation in the western basin. The series of Lago-mare. evaporitic phases only result from the enduring This undersaturation event provides us with another tendency to the closure of the western strait: it does piece of evidence to place the phase of marginal not evidence any change in the tectonic evolution. evaporitic sedimentation in the Betic basins at the very According to the reconstruction of the water levels, beginning of the MSC. These basins could only reach the deposits were formed in basins, the deepest of which saturation as dead-end diverticula of the western basin, were still immersed under more than 2000m. The model while standing at full ocean level. If they had been the now roughly takes into account a salinity difference path of a one-way influx to the Mediterranean, they between the surface water and the heavy deep brines, should have been rinsed of any excess salt in a more and this eliminates the initial bias in evaporation and sea effective way than the western basin was during the level drawdown. modelled undersaturated phase.

6.4. Undersaturation episode in the western basin 7. Adjustment to a reconstructed MSC

The MSC actually encompasses four crises: a salinity The Mediterranean can only be treated as an crisis in seawater (could not be recorded as such), a analogue to what it was in Messinian times, but it is biological crisis (marine organisms in seawater must possible to adjust a few parameters to reproduce the change or disappear when it turns into brines), a evaporitic budget, as compiled before modelling. sedimentary crisis (deposition of evaporites) and a sea level crisis. 7.1. Miocene global sea level The salinities in the Mediterranean basins result from an equilibrium between the evaporation and the marine The difference is best established for the global sea input. In the present stage, the NASW inflow amounts to level (Haq et al., 1987). If spread over the World Ocean, P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372 367 the present continental ice volume would yield a water Taking into account a lower salinity of the surface layer layer about 90m thick, of which 80m are stored in with reference to the deep brines now eliminates this Antarctica, and 10m in Greenland. In the late Miocene, bias and we abandon the idea of an increased freshwater east Antarctica is said to have already been fully supply to the eastern basin. glaciated. West Antarctica may still have experienced ice volume oscillations. Assuming that at least half of 7.3. Adjusting the mass-balance between the basins the Antarctic ice was stable, the level of the ocean cannot have been less than 50m above the present level: If the whole lapse of time (0.5Ma) from the inception 10m because no Greenland ice-cap is recorded at the of evaporitic conditions to the Zanclian re-inundation is time, plus 40m assuming a non-perennial west Antarctic allocated to the MSC, the evaporitic potentialities of the ice-cap. It cannot have been much higher either, as the Mediterranean appear stronger at present than during the late Miocene is a period of low marine stand by Miocene. The calculated budget reaches 125% to 132% comparison to the transgressive Pliocene. An oceanic of the compiled budget (Fig. 12). The model would then level 60m above the present zero level (Haq et al., 1987) require either reduced evaporation over both basins, or is thus applied to this reconstruction (Blanc, 2000). increased freshwater supply to both basins, or any combination of both, to account for the compiled 7.2. Water deficit on the eastern basin budget: a rather paradoxical implication in the investigation of the conditions of evaporitic sedimentation. In the initial version of the model (Blanc, 2000), the The geological significance appears to be quite eastern basin received an excess of salt, because its different. Although the occurrence of the MSC is supply came as averaged western basin brines instead of bracketed to 0.5Ma, from the beginning of evaporitic western basin surface water, derived from NASW. Thus, deposition (5.82Ma BP) to the final Zanclean flooding the balance of the salt budget appeared to require that (5.32Ma BP), the closure of the Miocene Portals either the Nile River or the Paratethys brackish sea proceeded at a faster rate than initially postulated. A increased the supply of freshwater to the eastern basin, perfect adjustment of the budgets is obtained by by comparison to what the Black Sea supplies at present. considering that the decrease of the Atlantic supply,

Fig. 12. Comparison of the cumulated budget of salt mass in the basins (and sum of) in the Standard-Case and “Reconstructed Messinian” runs of the model: a 0.5Ma Salinity Crisis with the present oceanographic parameters would yield 20% to 25% more salts than recorded. 368 P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372 from the quantity of NASW just making up for the 0.5Ma: the isolated acme period should have lasted for deficit (Vin =δV=∣e−f∣) to zero, was achieved in less 132,000years (Figs. 12 and 13): we have to accept the than 0.5Ma. idea that the surface of the western basin remained at a In other words, the Lago-mare brackish crisis is steady low level for a significant time-span. lengthened by an hydrological stagnant acme period Under such an assumption, each phase of evaporitic when no Atlantic strait was extant at all and thus no sedimentation is of course shortened, with reference to marine supply (Figs. 12 and 13). The stagnation only the Standard-Case, to the benefit of the duration of the applies to the marine supply, as during this period the acme episode. The duration of the phase of initial water erosion and detritic sedimentation reached the deep level retreat, common to both basins, is also influenced basins. The corresponding time-lapse has to be taken by the adjustment of the depth of the Sicily sill. into account in totalling the 0.5Ma elapsed from the A strong argument in favour of this assumption is onset of evaporitic sedimentation to the Zanclian that there is no reason why the opening of the Strait of refill. Gibraltar should have occurred the very next day after If this assumption is accepted, a balanced distribution the closure of the Betico-Rifian Portals was achieved. of the evaporites between the basins (Table 5)is The Pliocene transgression could not have overreached obtained for the same depth of the Sicily sill as when the crest of the Gibraltar isthmus if there had not been a the adjustments bear on the evaporative budget (345m phase of retrogressive erosion from the Alboran basin below the assumed Messinian ocean level or 285m upwards to the crest, allowing a Messinian Mediterra- below present sea level), without any change in the nean stream to cut a deep canyon through the isthmus climatic parameters with reference to the present. (Blanc, 2002). The duration assigned to the MSC proper (time for Table 6 compares the sedimentary phases according the decrease of the Atlantic supply from the net inflow to to the Standard-Case (i.e. present physical parameters, zero) is then 0.368Ma, very close to the estimated no definite acme period) and “Reconstructed Messi- duration of 0.4Ma, advocated by Gautier et al. (1994). nian” scenarios. However, we admit that the total duration (MSC plus The “Reconstructed Messinian/0.368Ma” scenario is Lago-mare encompassing a final stagnant period) is the best-fit of the model, because the only assumptions

Fig. 13. Comparison of the duration of the retreat of the levels in the Mediterranean basins for a 0.500Ma Salinity Crisis as against a 0.368Ma Salinity Crisis proper (5.82 to 5.452Ma BP) and 0.132Ma of fully isolated, non-marine, regime. The dot on the western basin, reconstructed Messinian curve shows the time when the Miocene portals are definitely closed. P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372 369

Table 5 Adjustment of parameters to compiled budget (% figures refer to present or maximum Standard case Adjusted Adjusted ocean level, Adjusted ocean level, value of corresponding parameter) ocean level Sicily sill, freshwater input Sicily sill, duration Duration less acme (Ma) 0.5 0.368 (73.6%) Duration of acme episode (Ma) – 0.132 (26.3%) Maximum depth of basins (m) 4290 4350 [+60 vs. present] Immersion of Sicily sill (m) 430 490 [430+60] 345 [285+60] Surface of western basin (m2) 8.30·1011 8.53·1011 Surface of eastern basin (m2) 1.70·1012 1.75·1012 Evaporation on western basin (m year−1) 1.40 1.40 1.03 (73.6%) 1.40 Freshwater to western basin (m year−1) 0.60 0.58 0.43 (73.6%) 0.58 Evaporation on eastern basin (m year−1) 1.61 1.61 1.19 (73.6%) 1.61 Freshwater to eastern basin (m year−1) 0.59 0.57 0.42 (73.6%) 0.57 Atlantic net initial supply (m3 year−1) 2.40 1012 2.51·1012 1.85·1012 2.51·1012 Western free level (vs. surface) −1509 −1569 [−1509 vs. present level] Eastern free level (vs. surface) −1718 −1778 [−1718 vs. present level] Western salt deposits (% compiled) 112.4 111.6 99.95 99.96 Eastern salt deposits (% compiled) 128.4 136.3 99.81 99.98 Sum of salt deposits (% compiled) 125.5 131.8 99.83 99.98 bear on generalized parameters (budgets, global sea through the Sicily channel and through the Strait of level, duration of MSC proper vs. duration of Lago- Gibraltar strait. mare and stagnant acme brackish/freshwater crisis), It is a much more believable explanation to suppose hopefully liable to further geological investigations, thus than the transition from [Vin =δV]to[Vin =0] was faster reducing the risk of wild speculation. If this timing than 0.5 or 0.370Ma, than to suppose that evaporation happened to be reconfirmed, it may be the first was less during the MSC than at present. But despite the contribution of the modelling to the chronology of a continuum of possibilities, between inaccurate sets of sedimentary event. data, insufficiently investigated differences between the Nevertheless, this best-fit scenario has to be present and Miocene climate and the possible influence considered with all the reservations mentioned above of the physiography of the basins, there can be no doubt on the lack of accuracy of the present hydrology budget that the evaporitic potentialities of the Mediterranean and the Messinian sedimentary budget. If it were to be remain very strong and that our model and scenarios demonstrated that the evaporitic sedimentary budget account for the salinity crisis. used in our modelling is definitely too high, the explanations would be to accept that the Zanclian refill 8. Conclusions removed significant amounts of evaporitic salts deposits from the low margins of the basins, and from the Investigating the conditions of the MSC led us Tunisia–Sicily–Malta–Lybia southeast extensions of beyond the accepted theories of evaporitic sedimenta- the internal strait, and maybe further in the Ionian basin: tion. The idea of evaporitic sedimentation in deep but we have to assume that these salts were not basins, and below a high water column, which we now redeposited, but exported back to the global ocean by have to accept, appeared preposterous 30 to 20years the re-established double layered circulation—both ago, so much so that Van Couvering et al. (1976),ina

Table 6 Tentative modelled timing of oceanographic phases of the MSC Phase Characteristic events Date of beginning of phases (Ma BP) “Standard case, 0.5Ma”“Reconstructed Messinian, 0.368Ma” Initial Φ Beginning of evaporitic crisis 5,820,000 5,820,000 Second Φ Beginning of level drawdown 5,790,200 5,798,400 Third Φ Split between basins at Sicily sill and drawdown of eastern basin 5,741,400 5,770,500 Fourth Φ Stagnant depressed eastern basin and drawdown of western basin 5,600,200 5,665,300 Final Φ Acme episode: depressed basins, no marine supply – 5,452,000 Zanclian Beginning of refill 5,320,000 5,320,000 370 P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372

“note added in proof” (not unlike the retraction of The improvements in the modelling of the budgets Galileo), had to abandon the hypothesis, which they had of the two main Mediterranean basins during the MSC clearly expressed in their article. J.-M. Rouchy and F. make it possible to assess most of the conditions of Orszag-Sperber also pointed out to us how similar our this uncommon geological event. Though they have model appears to the one defended by Debenedetti no present analogue, these conditions are not (1982). It was a mistake to overlook this valuable article extraordinary. in Blanc (2000). The physiography of the Mediterranean basins may At the time (1976–1982), a demonstration of the not have differed much from the present one: soundness of these conceptual models was not yet possible by mathematical modelling: the chronology of – the Global Ocean Level was somewhat higher, 60 to the MSC was not accurate enough. The improvements 80m, but only marginal areas have been abandoned have been such during the last decade, that nothing now by the sea since the Miocene, due to tectonic uplift or opposes the global understanding of the phenomenon: to silting up of shallow basins; its date and, most important for the modelling process, – the late Miocene Tethys was already divided into two its comprehensive duration are bracketed between main basins by some kind of a tectonic sill of Sicily: 5.82Ma and 5.32Ma, or 0.5Ma with a better accuracy our attempts at deducing the depth of such a sill from than 50,000years (Benson et al., 1995; Clauzon et al., the distribution of the evaporites remain vain, 1996). because of the possible reworking of deposits during Admitting that salt saturation was reached before the Lago-mare phase and redissolution when the drawdown occurred is not enough: only a progressively Zanclian re-fill took place; reduced flux rate can explain both the distribution of the – the evaporitic potentialities of the Mediterranean evaporites and the temporary stationary level of the remain quite as strong as they were in Miocene times, western basin during the first erosion phase: the MSC but for the excavation at the beginning of the results from a long transitory period from normal Zanclian of a deeper Strait of Gibraltar. oceanic to endoreic lacustrine conditions. The hydrological numerical and intimately linked A modelled budget equivalent to the compiled conceptual models imply a single and simple explana- budget of the MSC can be obtained either by tion for: supposing a smaller deficit than at present over both Mediterranean basins or by making the isolation of the – the rise in salinity, and the beginning of the evaporitic basins proceed at a faster pace: the comprehensive sedimentation, before any drawdown of the Medi- duration of the MSC must then encompass an acme terranean level occurred, phase when the Atlantic passages are totally closed. – the precocious isolation and desiccation of the Other speculative adjustments of parameters are not marginal basins, needed. – the two phases of erosion in the valleys of the The MSC does not appear to have been controlled tributary rivers to the western basin and the base level by climatic oscillations. It does not mean that these did of the first phase, not happen, as they are indeed recorded in the series – the interrupted evaporitic sedimentation in the studied for sequence stratigraphy, but they oscillated, western basin, when the spilling of water over the with a smaller amplitude in comparison to the Sicily sill controls its level, and the Mediterranean Quaternary, about a late Miocene average climate behaves as a two-tier endoreic system, which may have been close to the present Quaternary – the distribution of the evaporites in two basins interglacial climate (which is not the average Quater- separated by the sill of Sicily, nary climate). – the reworking and distribution as concentric These conclusions remain subject to validation when “dishes”, in each basin, according to solubility, of further studies will have re-assessed the budgets of the the chemical families of evaporitic minerals (carbo- evaporites over the basins and provided accurate values nates, sulphates, chlorides), for the climatic parameters as well. – the diachronous final inception of the Lago-mare Nevertheless, this study reconfirms the position of facies, distributed over the two Mediterranean basins, the Mediterranean Sea as a preferred laboratory for – the geologically instantaneous re-opening of an developing new conceptual models in oceanography, Atlantic passage, now the Strait of Gibraltar (Blanc, allowing us to investigate and assess the conditions of a 2002). phenomenon without a present analogue. P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372 371

Acknowledgments movements and desiccation events. Abstracts, 1st Congress on Pacific Neog. Stratigr., Tokyo, pp. 32–35. Clauzon, G., 1973. The eustatic hypothesis and the pre-Pliocene The author is indebted to Jean-Pierre Suc, Georges cutting of the Rhone valley. In: Ryan, W.B.F., et al. (Ed.), Init. Clauzon, Jean-Marie Rouchy and all the participants in Repts D.S.D.P. XIII, vol. 2. U.S. Gov. Print. Office, Washington, the programme “La crise de salinité messinienne: pp. 1251–1256. modalités, conséquences régionales et globales, quanti- Clauzon, G., 1982. Le canyon messinien du Rhône: Une preuve “ ” fications” within the project “Eclipse” of the CNRS décisive du dessicated deep basin model (Hsü, Cita and Ryan, 1973). Bull. Soc. Geol. Fr. XXIV-3, 597–609. (France), to which this article is a contribution, for Clauzon, G., Suc, J.-P., Gautier, F., Berger, A., Loutre, M.-F., 1996. accepting him as an associate member and allowing him Alternate interpretation of the Messinian Salinity Crisis: contro- to benefit of numerous meetings and discussions. His versy resolved? Geology 24 (4), 363–366. thanks also go to J.-M. Rouchy, J.-J. Cornée and, last but Copin-Montégut, G., 1996. Chimie de l'eau de mer, éd. Institut not least, F. Orszag-Sperber for their reviews of the Océanographique. 319 pp. Cunningham, K.J., Benson, R.H., Rakic-El Bied, K., McKenna, L.W., article and/or discussions on the MSC. 1997. Eustatic implications of late Miocene depositional sequences in the Melilla Basin, north-eastern Morocco. Sediment. Geol. 107, References 147–165. Debenedetti, A., 1982. The problem of the origin of the salt deposits in Beaudoin, B., Accarie, H., Berger, E., Clauzon, G., Cojan, I., Haccard, the Mediterranean and of their relations to the other salt D., Mercier, D., Mouroux, B., 1995. Un exemple d'analyse occurrences in the Neogene formations of the contiguous regions. géoprospective-La crise messinienne: expressions, âge et mod- Mar. Geol. 49, 91–114. alités. Géoprospective, actes du colloque Andra-BRGM-EMP- Esteras, M., Izquierdo, I., Sandoval, N.G., Bahmad, A., 2000. Unesco, Paris, pp. 169–188. Evolución morfológica y estratigrafía Plio-Cuaternaria del Umbral Beaudoin, B., Accarie, H., Berger, E., Brulhet, J., Cojan, I., de Camarinal (Estrecho de Gibraltar) basada en sondeos marinos. Haccard, D., Mercier, D., Mouroux, B., 1999. Les enseigne- Rev. Soc. Geol. Esp. 13 (3–4), 539–550. ments de la crise “fini-messinienne”. Etude du Gard Rhodanien, Fontannes, F., 1882. Note sur l'extension et la faune de la mer pliocène Géologie, Géochimie, Hydrogéologie, Géomécanique et Thermi- dans le Sud-Est de la France. Bull. Soc. Geol. Fr. III-11, 103–141. que, Actes des journées Scientifiques CNRS/Andra, Bagnols-sur- Gautier, F., Clauzon, G., Suc, J.-P., Cravatte, J., Violanti, D., 1994. Cèze, 20 and 21 Octobre 1997. CNRS-Andra éd., EDP-Sciences. Age et durée de la crise de salinité Messinienne. C. R. Acad. Sci., 115–135. Paris 318 (II), 1103–1109. Benson, R.H., 1973. Psychrospheric and continental Ostracoda from Gentil, L., 1911a. Sur les dépôts du détroit Sud-rifain. C. R. Acad. Sci., ancient sediments in the floor of the Mediterranean. In: Ryan, W.B. Paris 152, 293–296. F., et al. (Ed.), Init. Repts D.S.D.P. XIII, 1. U.S. Gov. Print. Office, Gentil, L., 1911b. Sur la formation du détroit Sud-rifain. C. R. Acad. Washington, pp. 1002–1008. Sci., Paris 152, 415–418. Benson, R.H., Hayek, L.-A., Hodell, D.A., Rakic-el Bied, K., 1995. Gonfiantini, R., 1986. Environmental isotopes in lake studies. In: Fritz, Extending the climatic pre-cession curve back into the late P., Fontes, J.-C. (Eds.), Handbook of Environmental Isotope Miocene by signature template comparison. Paleoceanography Geochemistry, vol. 2. Elsevier, pp. 113–168. 10-2, 5–20. Haq, B.U., Hardenbol, J., Vail, P.R., 1987. Chronology of fluctuating Bethoux, J.-P., 1980. Mean water fluxes across sections in the sea-levels since the Triassic. Science 235, 1156–1167. Mediterranean Sea, evaluated on the basis of water and salt Hsü, K.J., Cita, M.B., Ryan, W.B.F., 1973. The origin of the budgets and of observed salinities. Oceanol. Acta 3 (1), 79–88. Mediterranean evaporites. Init. Repts D.S.D.P. XIII, vol. 2. U.S. Blanc, P.-L., 2000. Of sills and straits: a quantitative assessment of the Government Printing Office, Washington, pp. 1203–1231. Messinian Salinity Crisis. Deep-Sea Res., Part 1, Oceanogr. Res. Krijgsman, W., Langereis, C.G., Zachariasse, W.J., Boccaletti, M., Pap. 47-8, 1429–1460. Moratti, G., Gelati, R., Iaccarino, S., Papani, G., Villa, G., 1999. Blanc, P.-L., 2002. The opening of the Plio-Quaternary Gibraltar Late Neogene evolution of the Taza-Guercif Basin (Rifian Strait: assessing the size of a cataclysm. Geodin. Acta 15, Corridor, Morocco) and implications for the Messinian salinity 303–317. crisis. Marine Geology 153 (1−4), 147–160. Blanc-Valleron, M.-M., Pierre, C., Caulet, J.-P., Caruso, A., Rouchy, Mayer-Eymar, C., 1867. Catalogue systématique et descriptif des J.-M., Cespuglio, G., Sprovieri, R., Pestrea, S., Di Stefano, E., fossiles des terrains tertiaires qui se trouvent au Musée Fédéral de 2002. Sedimentary, stable isotope and micropaleontological Zurich. -1, 37 p., -2, 65 p. records of paleoceanographic change in the Messinian Tripoli Montadert, L., Letouzey, J., Mauffret, A., 1978. Messinian event: Formation (Sicily, Italy). Palæogeography, Palæoclimatology, seismic evidence. In: Hsü, K.J., Montadert, L., et al. (Eds.), Init. Palæœcology 185, 255–286. ReptsD.S.D.P.XLIIA,vol.1.U.S.Gov.Print.Office, Bourcart, J., 1960. Carte topographique du fond de la Méditerranée Washington, pp. 1037–1051. occidentale. Bull. Inst. Océanogr. 57 (1163) (20 pp.). Müller, D.W., Hsü, K.J., 1987. Event stratigraphy and paleocea- Bourcart, J., 1962. La Méditerranée et la révolution du Pliocène. nography in the Fortuna Basin (southern Spain): a scenario Livre mém. Prof. Paul-Fallot. Mém. h. série Soc. Géol. Fr., vol. for the Messinian Salinity Crisis. Paleoceanography 2–6, I, pp. 103–116. 679–696. Buchbinder, B., Gvirtzman, G., 1976. The break-up of the Tethys Münch, P., Roger, S., Cornée, J.J., Saint Martin, J.P., Féraud, G., Ben ocean into the Mediterranean Sea, the Red Sea, and the Moussa, A., 2001. Restriction des communications entre l'Atlan- Mesopotamian Basin during the Miocene: a sequence of fault tique et la Méditerranée au Messinien: apport de la 372 P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372

téphrochronologie dans la plate-forme carbonatée et le bassin de Europe). Init. Repts D.S.D.P. XLII A. U.S. Gov. Print. Office, Mellila-Nador (Rif nord-oriental, Maroc). C. R. Acad. Sci., Ser. 2, Washington, pp. 985–990. Sci. Terre Planetes 332, 569–576. Suc, J.-P., Bessais, E., 1990. Pérennité d'un climat thermo-xérique en Orszag-Sperber, F., Rouchy, J.M., Blanc-Valleron, M.-M., 2000. La Sicile avant, pendant, après la crise de salinité messinienne. C. R. transition Messinien-Pliocène en Méditerranée Orientale (Chypre): Acad. Sci., Paris 310 (II), 1701–1707. La période du Lago-mare et sa signification. C. R. Acad. Sci., Ser. Tsimplis, M.N., Bryden, H.L., 2000. Estimation of the transports 2, Sci. Terre Planetes 331, 483–490. through the Strait of Gibraltar. Deep-Sea Res., Part 1, Oceanogr. Robinson, R.A., Stokes, R.H., 1959. Electrolyte Solutions. Butter- Res. Pap. 47-12, 2219–2242. worths, London. 559 pp. Van Couvering, J.A., Berggren, W.A., Drake, R.E., Aguirre, E., Curtis, Rögl, F., Steiniger, F.F., Muller, C., 1978. Middle Miocene Salinity G.H., 1976. The Miocene terminal event. Mar. Micropaleontol. 1, Crisis and palaeogeography of the paratethys (middle and eastern 263–286.