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. 0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.03.033 350 P.-L. Blanc / Palaeogeography, Palaeoclimatology, Palaeoecology 238 (2006) 349–372 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.

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