Christian Gorini
email: christian.gorini@upmc.fr

Lucien Montadert

Marina Rabineau

New imaging of the salinity crisis: Dual Messinian lowstand megasequences recorded in the deep basin of both the eastern and western Mediterranean

Keywords: Messinian salinity crisis, Deep Mediterranean basins, Seismic stratigraphy, Messinian lower megasequence (MLM), Messinian upper megasequence (MUM), dual low stand

New imaging of the salinity crisis: Dual Messinian lowstand megasequences recorded in the deep basin of both the eastern and western Mediterranean

Christian Gorini, Lucien Montadert, Marina Rabineau

Introduction

The Messinian event and its repercussions have been the subject of renewed interest both in academia and industry in recent years. MSC paleoceanography continues to be one of the most fascinating topics in its own right, but the potential existence of petroleum systems in M A N U S C R I P T

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3 the subsalt pre-Messinian successions in both eastern and western Mediterranean provinces has also piqued the interest of industry [START_REF] Belopolsky | New and emerging plays in the Eastern Mediterranean: an introduction[END_REF][START_REF] Roveri | The Messinian Salinity Crisis: past and future of a great challenge for marine sciences[END_REF]. The MSC was triggered by a combination of tectonic and climatic mechanisms and its effects on Holocene margins are still being felt in terms of deep karsts [START_REF] Audra | The effect of the Messinian Deep Stage on karst development around the French Mediterranean[END_REF], recent slope instabilities [START_REF] Savoye | The Messinian event on the margin of the Mediterranean Sea in the Nice area, southern France[END_REF], and Quaternary submarine canyons that occupy old topographies [START_REF] Tassy | Coupling cosmogenic dating and magnetostratigraphy to constrain the chronological evolution of peri-Mediterranean karsts during the Messinian and the Pliocene: Example of Ardèche Valley, Southern France[END_REF]2014). At the peak of the MSC, the depocenters shifted to the deepest basins [START_REF] Chumakov | Pliocene and Pleistocene deposits of the Nile Valley in Nubia and Upper Egypt[END_REF][START_REF] Ryan | Messinian badlands on the southeastern margin of the Mediterranean Sea[END_REF][START_REF] Barber | Messinian subaerial erosion of the proto-Nile delta[END_REF][START_REF] Clauzon | Le canyon messinien du Rhone, une preuve décisive du "desiccated deepbasin model[END_REF]Gorini et al., 2005;[START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF]. Old seismic profiles in the deep Mediterranean basins suggested the presence of evaporites on the seafloor [START_REF] Mauffret | Etude des profils sismiques obtenus au cours de la campagne « Geomede 1 » au large des Baléares et dans la mer Ligure[END_REF]Ryan and Hsu, 1970;[START_REF] Mauffret | Northwestern Mediterranean sedimentary basin from seismic reflection profile[END_REF][START_REF] Montadert | Messinian event: seismic evidence[END_REF], and these were ground-truthed in 1970 by the Deep Sea Drilling Program (DSDP) [START_REF] Ryan | Geodynamic implications of the Messinian crisis of salinity[END_REF]. Since then, three more drilling expeditions (DSDP Leg 42: Hsü et al., 1978;ODP Leg 107: Kastens, et al., 1987;ODP Leg 161: Zahn et al., 1999) were able to sample only the upper veneer of the Messinian upper unit (Lofi et al., 2011b; Fig. 1). The lower part of the Messinian salinity crisis (MSC) megasequence was only poorly seismically imaged (e.g. Hsü et al., 1973a and1973b;[START_REF] Ryan | Quantitative evaluation of the depth of the western Mediterranean before, during and after the late Miocene salinity crisis[END_REF]Hsü et al., 1978a;[START_REF] Ryan | The nature and distribution of Messinian erosion surfaces, indicators of a several kilometer-deep Mediterranean in the Miocene[END_REF][START_REF] Mart | Some depositional patterns at continental margin of southeastern Mediterranean Sea[END_REF][START_REF] Garfunkel | Active salt dome development in the Levant basin, southeast Mediterranean[END_REF][START_REF] Stampfli | Messinian paleorelief from a 3-Dseismic survey in the Tarraco concession area (Spanish Mediterranean Sea)[END_REF][START_REF] Savoye | The Messinian event on the margin of the Mediterranean Sea in the Nice area, southern France[END_REF][START_REF] Escutia | Paleoceanographic implications of the Messinian surface in the Valencia Trough, northwestern Mediterranean Sea[END_REF], until extensive hydrocarbon exploration in the offshore Mediterranean basins in the last three decades increased both available high-quality seismic lines and well log data. These advances led to new interpretations and publications (since the year 2000: [START_REF] Guennoc | Histoire géologique du golfe du Lion et cartographie du rift oligo-aquitanien et de la surface messinienne (Geological history of the gulf of Lions: Oligo-Aquitanian rift and Messinian surface maps)[END_REF][START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF][START_REF] Sage | Seismic evidence for Messinian detrital deposits at the western Sardinia margin, northwestern Mediterranean[END_REF][START_REF] Bertoni | Controls on the basinwide architecture of Messinian evaporites on the Levant margin (Eastern Mediterranean)[END_REF][START_REF] Maillard | Offshore evidence of polyphasic erosion in the Valencia Basin (Northwestern Mediterranean): scenario for the Messinian Salinity Crisis[END_REF][START_REF] Netzeband | The structural evolution of the Messinian evaporites in the Levantine Basin[END_REF][START_REF] Schattner | Oligocene-Miocene formation of the Haifa basin: Qishon-Sirhan rifting coeval with the Red Sea-Suez rift System[END_REF]Bertoni andCartwright, 2007a and2007b;Lofi and Berné;[START_REF] Bache | Evolution Oligo-Mioce`ne des Marges du Micro Océan Liguro-Provençal[END_REF][START_REF] Bache | Messinian erosional and salinity crisis: view from the Provence basin (Gulf of Lions, western Mediterranean)[END_REF][START_REF] Ghielmi | Sedimentary and tectonic evolution in the eastern Po-Plain and northern Adriatic Sea area from Messinian to Middle Pleistocene (Italy)[END_REF]Urgeles et al., 2011;[START_REF] Garcia | The Catalan margin during the Messinian Salinity Crisis: physiography, morphology and sedimentary record[END_REF]Lofi et al., 2011a and2011b;[START_REF] Maillard | Deformed Messinian markers in the Cyprus Arc: tectonic and/or Messinian Salinity Crisis indicators?[END_REF][START_REF] Obone Zue Obame | The sedimentary markers of the Messinian salinity crisis and their relation with salt tectonics on the Provençal margin (western Mediterranean): results from the "MAURESC" cruise[END_REF][START_REF] Bowman | A comprehensive review of the Msc facies and their origins in the offshore Sirt Basin, Libya[END_REF][START_REF] Geletti | The Messinian Salinity Crisis: New seismic evidence in the West-Sardinian Margin and Eastern Sardo-Provençal basin (West Mediterranean Sea)[END_REF][START_REF] Gaullier | Salt tectonics and crustal tectonics along the Eastern Sardinian margin, Western Tyrrhenian: New insights from the «METYSS 1» cruise[END_REF]. In the eastern Mediterranean Sea, the Messinian

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evaporites in the Nile cone and the Levant areas have been the subject of a number of studies focused on the depositional context of the evaporites [START_REF] Mart | Quaternary tectonic patterns along the continental margin of the southeastern Mediterranean[END_REF][START_REF] Druckman | The buried Afiq Canyon (eastern Mediterranean, Israel): a case study of a Tertiary submarine canyon exposed in Late Messinian times[END_REF][START_REF] Buchbinder | Sequence stratigraphy of Miocene-Pliocene carbonate_siliciclastic shelf deposits in the eastern Mediterranean Margin (Israel): effects of eustasy and tectonics[END_REF][START_REF] Manzi | Age refinement of the Messinian salinity crisis onset in the Mediterranean[END_REF] and the post-evaporitic structural deformation [START_REF] Garfunkel | Active salt dome development in the Levant basin, southeast Mediterranean[END_REF]Abdel Aal et al., 2000;[START_REF] Loncke | Le delta profond du Nil: structure et evolution depuis le Messinien (Miocene terminal)[END_REF][START_REF] Hübscher | Evolution of a young salt giant: the example of the Messinian evaporites in the Levantine Basin[END_REF][START_REF] Cartwright | Initiation of gravitational collapse of an evaporitic basin margin: the Messinian saline giant, Levant Basin, eastern Mediterranean[END_REF][START_REF] Hübscher | Seismic Atlas of the "Messinian Salinity Crisis" Markers in the Mediterranean and Black Seas[END_REF][START_REF] Reiche | Fault-controlled evaporite deformation in the Levant Basin, Eastern Mediterranean[END_REF]. However, there have been few attempts to make a detailed comparison of the stratigraphic architecture of Messinian deposits in the eastern and western Mediterranean (apart from the Atlas, by Lofi et al., 2011a). This paper presents a general pan-Mediterranean scenario for the Messinian erosional and salinity crisis that could alter our view of the potential presence of hydrocarbon reservoirs. In addition, it could help plan academic IODP drilling locations in both the western and eastern Mediterranean.

Brief review of knowledge of the different sub-basins

The sequestration of the Mediterranean Sea from the adjacent oceans at the end of the Miocene caused significant evaporation of seawater and increased the evaporitic concentration of the Mediterranean Sea [START_REF] Blanc | Of sills and straits: a quantitative assessment of the Messinian Salinity Crisis. Deep-Sea Research Part 1[END_REF][START_REF] Blanc | Improved modelling of the Messinian Salinity Crisis and conceptual implication[END_REF][START_REF] Meijer | A quantitative analysis of the desiccation and re-filling of the Mediterranean during the Messinian Salinity Crisis[END_REF][START_REF] Meijer | A box model of the blocked-outflow scenario for the Messinian Salinity Crisis[END_REF][START_REF] Gargani | Mediterranean sea-level variations during Messinian Salinity Crisis[END_REF][START_REF] Gargani | Evaporite accumulation during theMessinian Salinity Crisis: the Suez rift case[END_REF]. As a result, a thick (up to 2 km) sequence of evaporites was deposited in the center of its deep basins (Hsü et al., 1973;[START_REF] Cita | Mediterranean evaporite: paleontological arguments for a deep-basin desiccation model[END_REF][START_REF] Ryan | Geodynamic implications of the Messinian crisis of salinity[END_REF][START_REF] Garfunkel | Active salt dome development in the Levant basin, southeast Mediterranean[END_REF]. This deposition coincided with intense subaerial erosion of the basin margins [START_REF] Denizot | Le Pliocène dans la vallée du Rhône[END_REF][START_REF] Barr | Late Tertiary channel system in Northern Libya and its implications on Mediterranean sea level changes[END_REF][START_REF] Chumakov | Pliocene and Pleistocene deposits of the Nile Valley in Nubia and Upper Egypt[END_REF]Clauzon, 1973[START_REF] Clauzon | The Messinian Var canyon (Provence, Southern France). Paleogeographic implications[END_REF][START_REF] Clauzon | Le canyon messinien du Rhone, une preuve décisive du "desiccated deepbasin model[END_REF][START_REF] Gvirtzman | The Late Tertiary of the Coastal Plain and continental shelf of Israel and its bearing of the history of the Eastern Mediterranean[END_REF]Hsü et al., 1978a;[START_REF] Ryan | The nature and distribution of Messinian erosion surfaces, indicators of a several kilometer-deep Mediterranean in the Miocene[END_REF][START_REF] Barber | Messinian subaerial erosion of the proto-Nile delta[END_REF][START_REF] Savoye | The Messinian event on the margin of the Mediterranean Sea in the Nice area, southern France[END_REF][START_REF] Druckman | The buried Afiq Canyon (eastern Mediterranean, Israel): a case study of a Tertiary submarine canyon exposed in Late Messinian times[END_REF][START_REF] Guennoc | Histoire géologique du golfe du Lion et cartographie du rift oligo-aquitanien et de la surface messinienne (Geological history of the gulf of Lions: Oligo-Aquitanian rift and Messinian surface maps)[END_REF][START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF][START_REF] Ryan | Decoding the Mediterranean salinity crisis[END_REF][START_REF] Bache | Messinian erosional and salinity crisis: view from the Provence basin (Gulf of Lions, western Mediterranean)[END_REF]. The Messinian salinity crisis ended with catastrophic re-flooding [START_REF] Garcia-Castellanos | Catastrophic flood of the Mediterranean after the Messinian salinity crisis[END_REF]Estrada et al., 2011). However, it took a few million years for sedimentation in the Mediterranean to

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rebuild its slopes and shelves to the modern and pre-MSC configurations [START_REF] Lofi | Plio-Quaternary prograding clinoform wedges of the western Gulf of Lion continental margin (NW Mediterranean) after the Messinian Salinity Crisis[END_REF]Urgeles et al., 2011;[START_REF] Leroux | Stratigraphic simulation on the shelf of the gulf of lion: testing subsidence rates and sealevel curves during pliocene and quaternary[END_REF]. Contemporaneously, Messinian valleys and canyons identified on seismic lines and in drill cores, were infilled and buried [START_REF] Druckman | The buried Afiq Canyon (eastern Mediterranean, Israel): a case study of a Tertiary submarine canyon exposed in Late Messinian times[END_REF][START_REF] Buchbinder | Sequence stratigraphy of Miocene-Pliocene carbonate_siliciclastic shelf deposits in the eastern Mediterranean Margin (Israel): effects of eustasy and tectonics[END_REF][START_REF] Guennoc | Histoire géologique du golfe du Lion et cartographie du rift oligo-aquitanien et de la surface messinienne (Geological history of the gulf of Lions: Oligo-Aquitanian rift and Messinian surface maps)[END_REF]. The amplitude of the sea level rise that led to the reestablishment of marine conditions in the Mediterranean is attested by the geometries of the deltaic progradational-aggradational wedges above the MES [START_REF] Lofi | Plio-Quaternary prograding clinoform wedges of the western Gulf of Lion continental margin (NW Mediterranean) after the Messinian Salinity Crisis[END_REF][START_REF] Duvail | Correlation between onshore and offshore Pliocene-Quaternary systems tract below the Roussillon Basin (eastern Pyrenees, France)[END_REF]Gorini et al., 2005, Kertznus and[START_REF] Kertznus | Clinoform quantification for assessing the effects of external forcing on continental margin development[END_REF]Hawie et al., 2013b, Leroux et al., 2014). In addition, clinoform geometries are good paleodepth indicators (Urgeles et al., 2011;[START_REF] Rabineau | Quantifying subsidence and isostatic readjustment using sedimentary paleomarkers, example from the Gulf of Lions[END_REF] that provide an estimated sea level rise of 800-1000 m (above the last Messinian lowstand paleoshore position, [START_REF] Leroux | Stratigraphic simulation on the shelf of the gulf of lion: testing subsidence rates and sealevel curves during pliocene and quaternary[END_REF]. Since the Oligocene, the Levant margin and basin sediments have been sourced mostly from the Nile (Hawie et al., 2013b;Tibor et al, 1992;Druckman et al., 1995, Fig. 1). Sediments in the western Mediterranean basin mainly originated from the Rhone and Ebro since the Tortonian [START_REF] Gorini | The Late Messinian salinity crisis and Late Miocene tectonism: interaction and consequences on the physiography and post-rift evolution of the Gulf of Lions margin[END_REF][START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF][START_REF] Bache | Messinian erosional and salinity crisis: view from the Provence basin (Gulf of Lions, western Mediterranean)[END_REF][START_REF] Garcia | The Catalan margin during the Messinian Salinity Crisis: physiography, morphology and sedimentary record[END_REF]Urgeles et al., 2011;[START_REF] Estrada | Impact of pulsed Atlantic water inflow into the Alboran Basin at the time of the Zanclean flooding[END_REF]Fig. 1). The paroxysm of the MSC started around 5.6 Ma (CIESM Consensus Report, 2008). The debate concerning the amplitude of sea-level drop associated with this crisis continues (see for example: [START_REF] Lugli | Evidence of clastic evaporites in the canyons of the Levant Basin (Israel): implications for the Messinian Salinity Crisis[END_REF][START_REF] Roveri | The Messinian Salinity Crisis: past and future of a great challenge for marine sciences[END_REF]. Also, the amount of erosion and clastic input in the deep basin is not wholly agreed upon (see e.g., Lofi andBerné, 2008 vs. Bache et al., 2012).

Imaging the Eastern Mediterranean Messinian Deep Basin

The NW Levant Basin: A stratigraphic type section

Understanding the seismic stratigraphy of the Messinian evaporites is challenging in that it is often hindered by the ductile state of the mobile unit (MU) and its subsequent internal deformation [(e.g., in the Herodotus Basin [START_REF] Loncke | The Nile deep-sea fan: An example of interacting sedimentation, salt tectonics, and inherited subsalt paleotopographic features[END_REF] or next to the Levantine [START_REF] Netzeband | The structural evolution of the Messinian evaporites in the Levantine Basin[END_REF][START_REF] Hübscher | Seismic Atlas of the "Messinian Salinity Crisis" Markers in the Mediterranean and Black Seas[END_REF][START_REF] Reiche | Fault-controlled evaporite deformation in the Levant Basin, Eastern Mediterranean[END_REF]].

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT eastern margin
Nevertheless, most of the Levant basin remains rather undeformed. Therefore, investigating the Levant basin is crucial to unravelling the stratigraphic evolution of the Messinian evaporites.

Figure 2 shows the seismic stratigraphic subdivisions used along a seismic profile located in the NW Levant basin (Fig. 2a). Five major units can be identified within the 1,500 m thick layer of Messinian evaporites (Fig. 2b).

-Unit A is limited to the base and top by high-amplitude reflectors. The configuration of the internal seismic facies is similar to the underlying Miocene units, with presumably high detrital content. The base of the Messinian salinity crisis (MSC) depositional sequence can be identified exactly at the base of Unit A (see Figs. 2b and3b, and the discussion section).

-Unit B is transparent, with intercalations of high frequency reflectors. Deformation is visible within this unit, indicating the presence of mobile/ductile materials (i.e. halite, [START_REF] Hübscher | Seismic Atlas of the "Messinian Salinity Crisis" Markers in the Mediterranean and Black Seas[END_REF].

-Unit C is composed of a set of high amplitude reflectors separated by more transparent intervals that may represent halite. The nature of the high amplitude reflectors is uncertain (e.g. acoustic impedance contrast between evaporites of different nature or detritic levels).

Results of the inversion of seismic data collected in the northern Levantine basin suggest the presence of clastics (Fig. 2c, [START_REF] Montadert | Petroleum systems offshore Cyprus[END_REF].

-Unit D is similar to Unit B with transparent seismic facies indicating another mobile unit (halite deposits) with some intercalated high frequency reflectors that are more frequent towards the top of the unit.

-Unit E is characterized by high frequency reflectors, pointing to high frequency hydrological changes at the top of the MSC Megasequence. This unit corresponds to the uppermost stratified Messinian unit ME V previously described by other authors (Hübscher and Dümmong 2011;[START_REF] Dümmong | Seismic Atlas of the "Messinian Salinity Crisis" Markers in the Mediterranean and Black Seas[END_REF][START_REF] Gvirtzman | Intense salt deformation in the Levant Basin in themiddle of the Messinian Salinity Crisis[END_REF][START_REF] Reiche | Fault-controlled evaporite deformation in the Levant Basin, Eastern Mediterranean[END_REF].

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It shows reduced refraction velocities (3,700-3,600 m/s), suggesting intercalated gypsum and clastics. A high amplitude reflector marks the top of unit E (Top erosion surface: Hübscher and Dümmong 2011; [START_REF] Reiche | Fault-controlled evaporite deformation in the Levant Basin, Eastern Mediterranean[END_REF] and the boundary with the Pliocene sediments characterized by low seismic velocities (2,600m/s; [START_REF] Gvirtzman | Intense salt deformation in the Levant Basin in themiddle of the Messinian Salinity Crisis[END_REF].

Other authors agree with a basic subdivision of the succession into five main units [ [START_REF] Netzeband | The structural evolution of the Messinian evaporites in the Levantine Basin[END_REF] see also: [START_REF] Cartwright | Strain partitioning in gravity-driven shortening of a thick, multilayered evaporite sequence[END_REF] on the Israeli margin with their M1-M5 units], although six units have been described in the westernmost part of the basin [Bertoni and Cartwright, 2007 (T1-T4 and L1-L2); [START_REF] Hübscher | Stratigraphy, fluid dynamics and structural evolution of the Messinian evaporites in the Levantine Basin, Eastern Mediterranean[END_REF][START_REF] Cartwright | Initiation of gravitational collapse of an evaporitic basin margin: the Messinian saline giant, Levant Basin, eastern Mediterranean[END_REF][START_REF] Maillard | Deformed Messinian markers in the Cyprus Arc: tectonic and/or Messinian Salinity Crisis indicators?[END_REF][START_REF] Dümmong | Seismic Atlas of the "Messinian Salinity Crisis" Markers in the Mediterranean and Black Seas[END_REF][START_REF] Gvirtzman | Intense salt deformation in the Levant Basin in themiddle of the Messinian Salinity Crisis[END_REF].

These areas show thin-skinned salt tectonic activity.

In the northern part of the Levant margin, the onshore drainage basin is characterized by carbonate dominated rocks and restricted river systems (Hawie et al., 2013a(Hawie et al., , 2013b)), which limit siliciclastic inputs. Offshore the Lebanon margin, the Messinian clastics are limited to the lower slope and the lower half of the evaporite interval (Fig. 3b).

Correlation with the Nile margin: Thick clastic unit at the base of the Messinian event

In the Nile deep sea fan in the Herodotus Basin, considerable amounts of sediments have been deposited along the Egyptian margin since the Oligocene (Fig. 1). The Messinian sea level drop increased the role of the Nile as a conduit for sediments. Subaerial erosion affected large areas of the former Miocene Nile cone [START_REF] Montadert | Petroleum systems offshore Cyprus[END_REF]. Seismic profiles suggest that during this period as well as during the Pliocene-Pleistocene, most of the sediments were diverted westward by the prominent Eratosthenes continental block [START_REF] Montadert | Petroleum systems offshore Cyprus[END_REF] Fig. 1). The seismic lines (Fig. 4b and4c) in the lower part of the Nile cone display intercalation of a large, thick clastic unit (about 2,000 m thick) at the base of the MSC megasequence. This unit corresponds to the Messinian lowstand Nile delta-fed clastics (gravity flow deposits). In detail, stacked channel complex systems characterize this seismic facies [START_REF] Montadert | Petroleum systems offshore Cyprus[END_REF].

The change in the lateral facies between this clastic deposits (low frequency, high amplitude, low continuity reflections) and halite (transparent seismic facies), which is clearly visible in Figs. 3b, 4b and4c, is evidence that these clastics accumulated in a restricted hypersaline Messinian basin. Messinian clastics were eroded from the shelf margins and redeposited in the deep basins during the early stages of the Upper Messinian sea level drop. They interfinger with evaporite deposits and therefore fully belong to the MSC event. Caution is therefore called for in defining the base of the MSC event on seismic profiles in the deep Mediterranean basin, e.g., choosing the base of the transparent layer (halite) as the base of the Messinian salinity crisis deposits, particularly in front of the big rivers, would lead to false/incomplete interpretations. Hence the base of the MSC event must be located at the base of these high frequency reflections.

Correlation with the Florence Ridge: the Antalya Basin

The Antalya Basin is located north of the Florence Ridge [START_REF] Biju-Duval | Structure and evolution of the Mediterranean basins[END_REF][START_REF] Biju-Duval | Variety of margins and deep basins in the Mediterranean. Geologic and geophysical investigations of continental margins[END_REF]Fig. 5a) and is part of the Cyprus arc system. This basin was formed after the emplacement of the Cyprus arc during the Late Cretaceous and later infilled by a thick Cenozoic cover with sediments sourced from Turkey (Fig. 1 and Fig. 5a). Thick Messinian evaporites are observed in this basin similar to those seen in the nearby Herodotus Basin (Fig 5b). The Antalya and Herodotus basins used to be connected, whereas isolated Messinian islands persisted along the Florence Ridge [START_REF] Biju-Duval | Structure and evolution of the Mediterranean basins[END_REF][START_REF] Biju-Duval | Variety of margins and deep basins in the Mediterranean. Geologic and geophysical investigations of continental margins[END_REF].

In the seismic profiles (Fig. 5b), the same stratigraphic stacking pattern can be seen as in the Levant Basin (Fig. 2b and Fig. 3b). Continuous reflectors (Unit C) can be observed in

Geometries and facies distribution in the western and eastern Mediterranean: a comparison

The MSC seismic stratigraphy off the Rhone River was defined by [START_REF] Ryan | Geodynamic implications of the Messinian crisis of salinity[END_REF][START_REF] Bessis | Some remarks on the study of subsidence of sedimentary basins: application to the Gulf of Lions margin (Western Mediterranean)[END_REF], [START_REF] Bache | Messinian erosional and salinity crisis: view from the Provence basin (Gulf of Lions, western Mediterranean)[END_REF], [START_REF] Ryan | Decoding the Mediterranean salinity crisis[END_REF], Lofi et al. (2011aLofi et al. ( , 2011b)). Differences in facies and geometries between the western and eastern Mediterranean were identified by these authors.

The MSC megasequence is commonly subdivided into three units: the lower unit (LU), the mobile unit (MU) and the upper unit (UU). However, we propose to subdivide the thick megasequence into two layers in both basins that are comparable in terms of thickness and seismic signature (Fig. 6a and Fig. 6b): the Messinian lower megasequence (MLM) and the Messinian upper megasequence (MUM). An improved correlation between the two deep basins is provided here.

The MSC Lower Megasequence (MLM)

In the eastern Mediterranean the A, B and C subunits are grouped in the Messinian lower megasequence (MLM). In front of the biggest Messinian rivers, like in the distal part of the Nile Messinian cone, or off the Rhone deep sea fan, the MLM unit is imaged by a chaotic seismic facies we interpret as stacked systems of mixed silicoclastic sediments and evaporites.

In the eastern basin, units A B and C are stacked in one seismic facies in front of the Nile, Levantine and Antalyan rivers (Figs.3b,4b,4c and 5b). The seismic facies and the thickness

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of this lower unit resemble those of the lower unit (LU) described by [START_REF] Bache | Messinian erosional and salinity crisis: view from the Provence basin (Gulf of Lions, western Mediterranean)[END_REF] and [START_REF] Ryan | Decoding the Mediterranean salinity crisis[END_REF] in the western basin (labelled 2 in Fig. 6a). Note that the LU described by [START_REF] Bache | Messinian erosional and salinity crisis: view from the Provence basin (Gulf of Lions, western Mediterranean)[END_REF] is much thicker than the LU described by Lofi et al., (2011a). In the western basin, this unit is up to 2.5 km thick using ESP velocity [START_REF] Leroux | Quantifications des flux sédimentaires et de la subsidence dans le bassin Provençal, Mediterranée Occidentale[END_REF]. We interpret the complex features and internal erosions in both the Rhone deep sea fan and in the Nile cone as channels (labelled 2 in Fig. 6a and Fig. 6b). On top of this MLM, chaotic facies are also visible (labelled 5 in Fig. 6a) and that we suggest are the result of instabilities triggered by the rapid sea level drop [START_REF] Gargani | Slope Destabilization during the Messinian Salinity Crisis[END_REF],which was still underway at the peak of the Messinian crisis. As stated in section 3, the change in the lateral facies from high-amplitude seismic facies to the transparent seismic facies in the eastern basin (Figs. 3 to 6) shows that the clastics were redeposited in a hypersaline environment. High P wave velocities in the Messinian lower megasequence, point to the probable alternation of evaporate sedimentation and clastics in both the western Mediterranean [START_REF] Leroux | Quantifications des flux sédimentaires et de la subsidence dans le bassin Provençal, Mediterranée Occidentale[END_REF] and eastern Mediterranean [START_REF] Montadert | Petroleum systems offshore Cyprus[END_REF]. The explanation for these alternations is that in front of the big rivers, saline concentrations were diluted by fluxes of riverine water leading to the deposition and intercalation of shale, gypsum and halite. In this interpretation, clastic deposits are synchronous with the deposition of evaporites in the deep basins.

Upslope, high angle clinoforms have been described [START_REF] Bache | Messinian erosional and salinity crisis: view from the Provence basin (Gulf of Lions, western Mediterranean)[END_REF][START_REF] Leroux | Quantifications des flux sédimentaires et de la subsidence dans le bassin Provençal, Mediterranée Occidentale[END_REF] labelled 1 in fig. 6) with an underlying surface of marine erosion (regressive surface of marine erosion, RSME). We interpret these clinoforms as forced regressive deltas related to the increasing rate of MSC sea level fall. Similar (but smaller) forced regressive systems have been described on the outer shelf and were deposited during the glacio-eustatic falling stages in the Late Quaternary [START_REF] Berné | The impact of Quaternary global changes on strata formation[END_REF][START_REF] Rabineau | Sedimentary sequences in the Gulf of Lion: A record of 100, 000 years climatic cycles[END_REF][START_REF] Rabineau | Paleo sea levels reconsidered from direct observation of paleoshoreline position during Glacial Maxima (for the last 500,000 yr)[END_REF] and were later ground-truthed by drilling [START_REF] Bassetti | The 100-ka and rapid sea level changes recorded by prograding shelf sand bodies in the Gulf of Lions (western Mediterranean Sea)[END_REF]. These forced regressive deltas are related to 100,000 year glacio-eustatic cycles with an overall sea level fall of 120 m. The observed

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Messinian prisms are located much deeper and more seaward on the slope, with steeper clinoforms, indicating a much larger drop in sea level [START_REF] Bache | Messinian erosional and salinity crisis: view from the Provence basin (Gulf of Lions, western Mediterranean)[END_REF]Urgeles et al., 2011), i.e. between 800 m and 1,500 m.

The Messinian Upper Megasequence (MUM)

We divide the MUM into two corresponding to unit D and E in the eastern Basin (Figs.3b,4b,4c and 5b) and to the mobile unit (MU, labelled 6 in Fig. 6a) and upper unit (UU, labelled 7 in Fig. 6a) of (Lofi et al., 2011a(Lofi et al., , 2011b) ) in the western basin. In the western and eastern area, the MUM is comparable in terms of thickness (0.5 TWT) and seismic facies (high amplitude reflections on top of a thick transparent unit). In detail, the top of the MUM shows lateral variations, e.g., transparent facies between high amplitude reflections [START_REF] Reiche | Fault-controlled evaporite deformation in the Levant Basin, Eastern Mediterranean[END_REF][START_REF] Geletti | The Messinian Salinity Crisis: New seismic evidence in the West-Sardinian Margin and Eastern Sardo-Provençal basin (West Mediterranean Sea)[END_REF]Fig. 5b). In the western basin, the upper part of the MUM clearly onlaps the Messinian canyons and the incised valleys (Fig. 7). This architecture has already been described in proximal settings throughout the western Mediterranean [START_REF] Montadert | Messinian event: seismic evidence[END_REF][START_REF] Sage | Seismic evidence for Messinian detrital deposits at the western Sardinia margin, northwestern Mediterranean[END_REF], Lofi et al., 2011a, 2011b;Figs. 7a, 7b, 7c) and the eastern Mediterranean [START_REF] Gargani | Mediterranean sea-level variations during Messinian Salinity Crisis[END_REF].

The top of the MSC megasequence has been strongly eroded in most parts of the eastern and western deep basins. For example, on the Florence Ridge west of the island of Cyprus, the thick Messinian evaporites are clearly eroded, resulting in a flat surface (Fig. 8). The same substantial erosion of the top of the MSC megasequence is also visible in the western basin (labelled top erosional surface, TES on Fig. 6,a").

Partial ground-truthing

A review of drilling sites and samples in the Mediterranean shows that:

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1-In medium water depths (~1,500 m; Sites 371, 372, 375 376 and GLP2, Fig. 1) the intercalation of sands, marls, gypsum and halite characterize the upper part of MUM (labelled 7 in Figs. 6a and6b). These evaporites onlap the Messinian maximum regressive erosional surfaces (MES in Fig. 7c). In the most proximal part of the margins, this sequence corresponds to aggrading fluvial conglomeratic deposits (Fig. 7c) during the late MSC lowstand related to a drainage network with tributaries, terraces and meandering rivers, i.e. the incised valley fill described by Savoye and Pipper (1991) and Urgeles et al., (2011).

2-The Mediterranean reflooding was much more erosive than often described, even at great depth. In most drill cores, the upper part of the MUM revealed reworked evaporitic deposits and sands deposited during the uppermost Messinian (5.46-5.33 Ma, Bache et al., 2012).

These observations are consistent with an early slower apparent transgression inducing wave ravinement reworking (with sands and evaporites) followed by the rapid and erosive reflooding event [START_REF] Garcia-Castellanos | Catastrophic flood of the Mediterranean after the Messinian salinity crisis[END_REF][START_REF] Estrada | Impact of pulsed Atlantic water inflow into the Alboran Basin at the time of the Zanclean flooding[END_REF].

Proposed conceptual model

Based on our observations and on our interpretation of the new seismic lines, we assume that global ocean during the MSC and characterized by a giant salt deposit. We have divided the MSC depositional megasequence associated with a complete relative sea level (RSL) cycle into two MSC sub-units of the same RSL cycle in both the eastern and western deep basins, and we have refined the earlier observations (Lofi et al., 2011a).

The Messinian Lower Megasequence (MLM)

The onset of the MSC relative sea level fall is marked by a regressive surface of marine erosion (RSME, Fig. 10). Mass transport complexes (MTCs) and forced regressive prisms (labelled 1 on fig 10) are visible at the transition between the slope and the deep Messinian basin. Clastics were deposited at the mouth of the first Messinian incised valleys. The RSME is an erosional truncation we interpret as scars of mega slides triggered by the high rate of the sea level fall at that time. The thick chaotic seismic facies at the transition between the Miocene slope and the deep basin (labelled 1 in Fig. 6) is evidence for the occurrence of instabilities during the uncommonly high rate and amplitude of sea level variation during the MSC megacycle. In the deep basin, the continuation of this surface is a correlative conformity (MES-CC*, Fig. 10) in the sense of [START_REF] Posamentier | Siliciclastic sequence stratigraphy: concepts and applications[END_REF] and defines the base of the MLM in the deep basin. In the distal part of the big deltas, the MLM is characterized by seismic facies we interpret as deep-water clastics deposited in a hypersaline environment. The major drop in sea level occurred when the connections were restricted, at 5.6 Ma according to a now widely accepted time model, CIESM (2008). Observations from paleoshelves indicate high rates of erosion of at least 1 km during the event [START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF]Rubino et al 2005[START_REF] Bache | Evolution Oligo-Mioce`ne des Marges du Micro Océan Liguro-Provençal[END_REF][START_REF] Bache | Messinian erosional and salinity crisis: view from the Provence basin (Gulf of Lions, western Mediterranean)[END_REF]; more recently quantified at 1.3 km [START_REF] Rabineau | Quantifying subsidence and isostatic readjustment using sedimentary paleomarkers, example from the Gulf of Lions[END_REF].

The high rate of the sea level fall, coupled with a high sediment flux in front of major rivers, led to a strongly decreasing accommodation/sediment supply ratio. This paper shows that in the deep eastern Mediterranean basin, the falling stage is clearly associated with the beginning of evaporite/halite precipitation, the subsidence at that time allowed the accumulation of more

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than 2,000 m of mixed silicoclastic/evaporitic sediments (early low stand). We envisage deposition occurring below relatively deep-water conditions (600-1,000 m below wave base) with a coeval Mediterranean-wide halite deposition. In this scenario, halite accumulated in a subaqueous environment during the sea level drop and its deposition was ended by a quasiinstantaneous salt precipitation on top of the Lower Messinian megasequence. We hypothesize that the strong net evaporation, which concentrated Mediterranean seawater prior to drawdown, resulted in this rapid precipitation of halite at the lowest sea level. The strong acoustic impedance contrasts within eastern halite deposits (Units A, B and C) were interpreted as clastic intercalations (Bertoni and Cartwright 2007b, Figs. 2b and2c).

Tributaries located around the margins were the source of clastic and meteoric waters (Figs. 4 and5). These facies were previously interpreted as turbidite deposits due to their distribution at the mouth of Israeli canyons [START_REF] Lugli | Evidence of clastic evaporites in the canyons of the Levant Basin (Israel): implications for the Messinian Salinity Crisis[END_REF] and Lebanon canyons (Hawie et al., 2013b). On the Levant margin, canyon incisions were superimposed on preexisting canyons [START_REF] Druckman | The buried Afiq Canyon (eastern Mediterranean, Israel): a case study of a Tertiary submarine canyon exposed in Late Messinian times[END_REF][START_REF] Gardosh | The Levant Basin offshoreIsrael: stratigraphy, structure, tectonic evolution and implications for hydrocarbon exploration -revised edition[END_REF]Hawie et al., 2013b). Israeli canyons are filled by MTD deposited interpreted by others as subaqueous gravity flows (reworked evaporites and clastic sampled in Mavqi'im and Be'eri evaporites, [START_REF] Lugli | Evidence of clastic evaporites in the canyons of the Levant Basin (Israel): implications for the Messinian Salinity Crisis[END_REF]. We hypothesize that these "gravity flows" (mass transport deposits) originated during the initial falling stage and are coeval with the MLM depositional sequence. This is not incompatible with the formation of subaerial incisions upslope (valleys) and the later development of a widespread erosion surface characterizing the Mediterranean fluvial network on platforms at the continental margins (Rubino et al., 2010). Large-scale mass wasting processes along the shelf margins and slopes following or coeval with the Messinian drawdown are reported all around the Mediterranean [START_REF] Gargani | Slope Destabilization during the Messinian Salinity Crisis[END_REF][START_REF] Bowman | A comprehensive review of the Msc facies and their origins in the offshore Sirt Basin, Libya[END_REF].

A combination of several sea level drawdown events in the Mediterranean, associated with limited seawater inflow and continuous river discharge, is needed to explain the quantity of

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evaporites deposited in a very short time and could explain the alternation between clastics and evaporites suggested by the indentation between Messinian Nile lowstand and salt (Figs. 4b and4c).

In the saline eastern basin, the top of the MLM corresponds to unit C. This group of reflectors at the top of the MLM constitutes a sharp regional contrast between the MLM and the MUM (Figs. 2 b and 2c, 3 to 6). Based on seismic velocities, other authors have interpreted this stratified unit as embedded clastics, or a more diluted facies, such as limestone, anhydrite, or high salinity deliquescent facies (Sylvinite, Carnallite; Vp 3-3.9 Km/s eg. [START_REF] Bertoni | Controls on the basinwide architecture of Messinian evaporites on the Levant margin (Eastern Mediterranean)[END_REF][START_REF] Hübscher | Stratigraphy, fluid dynamics and structural evolution of the Messinian evaporites in the Levantine Basin, Eastern Mediterranean[END_REF]Lofi et al., 2011a). Several hypotheses can be proposed to explain the nature of the transition between the MLM and the MUM throughout the Mediterranean (Figs. 2, 3 and5). A layered seismic facies is widely observed in evaporitic systems and has generally been linked to changes in lithology, i.e. to the alternation of bitterns, dolomite, anhydrite, halite and/or siliciclastic sediments (e.g. Upper Permian basin in the North Sea: [START_REF] Birrel | Structural analysis of 3D seismic data, using the correlation attribute: a case study -Carboniferous of the Southern North Sea (UK)[END_REF]; Aptian evaporites of the Santos basin: [START_REF] Gamboa | The South Atlantic Aptian evaporitic sequence in the deep water areas of the Santos Basin. 32nd[END_REF]. A definitive interpretation of the seismic facies is not possible at this stage, and will require direct well calibration. Nevertheless, we propose a theory to explain the nature of the transition between the two seismic megasequences. The sharp reflectors at the top of the MLM are either detrital or diluted sediments. In either case they correspond to the increase in the influx from the rivers around the Mediterranean in a period when the connections between the deep basins and the world oceans were at least seriously limited. When the sea level was at its lowest, the salinity concentration and the flux of clastics and meteoric water in the deep basins were at their highest. Moreover, the influx from North African rivers to the basin may have been higher at that stage due to a major change in monsoon intensity after 6.2 My [START_REF] Colin | Late Miocene to early Pliocene climate variability off NW Africa (ODP Site 659)[END_REF].

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Our observations (Figs. 2,3,4,5,6) suggest that the top of the MLM corresponds to the maximum spread of clastics and meteoric water in the deep MSC basins. This "turning point" is equivalent to the correlative conformity in the sense of [START_REF] Hunt | Stranded parasequences and the forced regressive wedge systems tract: deposition during base-level fall[END_REF], which we interpret as the peak of the "salinity" crisis. The clastics fans in the deep basin (2 in Fig. 10) and the evaporites (3 in Fig. 10) are comparable to the early low stand systems tract described in the Exxon models [START_REF] Catuneanu | Sequence stratigraphy: methodology and nomenclature[END_REF]. The MLM depositional sequence generated during the MSC falling stage thus corresponds to an early low stand deposited during a high rate of MSC relative sea level fall. MLM clastic fans and evaporates are the equivalent of a falling stage systems tract (FSST in Fig. 11, [START_REF] Catuneanu | Sequence stratigraphy: methodology and nomenclature[END_REF] at the scale of the MSC depositional megasequence (more than 2,000 m of sediments).

The Messinian Upper Megasequence:

Significant changes in the paleogeography of the Mediterranean Sea at the end of the MSC led to the reorganization of the atmospheric circulation and drier conditions over North Africa [START_REF] Colin | Late Miocene to early Pliocene climate variability off NW Africa (ODP Site 659)[END_REF]. At that point, salt started to precipitate, probably very rapidly (lower part of the MUM).

The MUM has already been described by other authors [START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF][START_REF] Bache | Messinian erosional and salinity crisis: view from the Provence basin (Gulf of Lions, western Mediterranean)[END_REF][START_REF] Lofi | Giant solution-subsidence structure in the Western Mediterranean related to deep substratum dissolution[END_REF] and has been partially drilled at its top (Fig. 9). It consists of a transparent unit (halite, e.g. a mobile unit, Lofi et al., 2011a) and a more diluted facies at the top (Unit E, alternations between gypsum and fine marine clastics, upper unit described by Lofi et al., 2011a).

The characteristic transparent seismic signature of the MUM is due to the prevailing conditions when the river flux into the basin was minimal. This points to a major change in the depositional environment, e.g. (1) detrital material that was trapped upslope due to a rapid increase in accommodation coeval with the rapid precipitation of salt (relatively reflectionfree seismic facies resulting from a very high rate of halite precipitation) infilling canyons in (3) rivers reached their base level. Even if the onset of the MSC was initially affected by the tectonically-driven reduction in the hydrological exchange with the Atlantic Ocean, some authors point out that it was finally triggered by glacial conditions in the northern Hemisphere and by arid conditions in North Africa around 5.55 Ma [START_REF] Manzi | Age refinement of the Messinian salinity crisis onset in the Mediterranean[END_REF][START_REF] Roveri | The Messinian Salinity Crisis: past and future of a great challenge for marine sciences[END_REF]. At that stage, the MUM (and its deposits labelled 5 and 6 and 7 in Fig. 10) started to be precipitated/deposited on a flat Messinian "erosional or bypass surface" on top the intermediate basins seafloor (of which the bottom was located near the Messinian base level) e.g. the Valence basin [START_REF] Maillard | Offshore evidence of polyphasic erosion in the Valencia Basin (Northwestern Mediterranean): scenario for the Messinian Salinity Crisis[END_REF][START_REF] Garcia | The Catalan margin during the Messinian Salinity Crisis: physiography, morphology and sedimentary record[END_REF], the Algerian basin (see Fig. 3 in Lofi et al., 2011a, and Fig. 10), and the Sicilian basin (Bowan, 2012;[START_REF] Roveri | A high-resolution stratigraphic framework for the latest Messinian events in the Mediterranean area[END_REF][START_REF] Roveri | The Messinian Salinity Crisis: past and future of a great challenge for marine sciences[END_REF].

Whatever the reason for this change in the depositional environment, salt/evaporites continued to precipitate in the deep basins until the sea started to transgress widely. This is illustrated by the seismic facies and geometries at the top of the MUM (Figs. 7 and9). At that stage, evaporites (gypsum and salt (Geletti et al., 2014, Fig. 9)) altering with marls/shales (Fig. 9) started to be deposited in the "deep basin" passing upslope to late lowstand aggrading fluvial conglomerates infilling the Messinian valleys (Fig. 7c, [START_REF] Maillard | Offshore evidence of polyphasic erosion in the Valencia Basin (Northwestern Mediterranean): scenario for the Messinian Salinity Crisis[END_REF]Urgeles et al., 2011;[START_REF] Sage | Seismic evidence for Messinian detrital deposits at the western Sardinia margin, northwestern Mediterranean[END_REF][START_REF] Obone Zue Obame | The sedimentary markers of the Messinian salinity crisis and their relation with salt tectonics on the Provençal margin (western Mediterranean): results from the "MAURESC" cruise[END_REF][START_REF] Sage | Seismic Atlas of the "Messinian Salinity Crisis" Markers in the Mediterranean and Black Seas[END_REF]. The MES-CC** (correlative conformity of the MES, in the sense of [START_REF] Hunt | Stranded parasequences and the forced regressive wedge systems tract: deposition during base-level fall[END_REF] in Catuneanu et al., 2011, Fig. 10) is the base of the MUM. The mobile unit (labelled 5 in Fig. 10), the upper unit (labelled 6 in Fig. 10) and the fluvial incised valleys fill (7a on Fig. 10) onlapping the MES surface were coeval with an MSC RSL rise (low rate). During this event, the deep basin was still restricted and the accommodation space was rapidly filled by the evaporites. Thus the evaporites and fine clastics (shales) of the MUM correspond to the MSC late lowstand systems tract (LST in Fig. 10). At the top of the Messinian upper megasequence (MUM) transgressive marine sands (7b) and material reworked by the giant reflooding (8)

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18 were deposited during an early transgression. The marine ravinement surface eroded and flattened the irregular morphology of the MES [START_REF] Garcia | The Catalan margin during the Messinian Salinity Crisis: physiography, morphology and sedimentary record[END_REF]Bache et al., 2011). In the eastern basin, the Messinian upper megasequence was associated with sequences D and E of the type section of the NW Levant Basin (M-IV, ME-5, ME-VI of Dümmong and Hübsher, 2011). This late lowstand onlapping (aggrading) on the margin progressively hindered the arrival of clastic material in the basin (apparent transgression). The late stage of this lowstand was characterized by an increase in base level with onlap onto the MES of evaporites/or clastic materials [Fig 7;Lofi et al., 2011a(Fig. 7), Luigli et al., 2013 (Fig. 7); Geletti et al., 2014 (Fig.12)]. The dilution and shift towards an evaporite series mainly composed of marls, anhydrite and salt occurred in the later stages in both the western and eastern basins, although it is more obvious in the western basin. During the deposition of fluvio-deltaic clastics of the uppermost Messinian upslope in the incised valleys (Fig 7c, labelled 7a on Fig. 10), a minimum water level most likely persisted downslope in the deep basins, meaning that complete desiccation did not take place [START_REF] Hardie | Did the Mediterranean Sea dry out during the Miocene? A reassessment of the evaporite evidence from DSDP Legs 13 and 42A cores[END_REF]. During the MUM deposition in the deep basin, Messinian continental facies were full lacustrine when perched lakes were disconnected from the deep basin. The variability in depositional environments during the MSC depositional megasequence explains the variety of Messinian seismic facies [e.g. the bedded unit (BU)], [START_REF] Maillard | Offshore evidence of polyphasic erosion in the Valencia Basin (Northwestern Mediterranean): scenario for the Messinian Salinity Crisis[END_REF][START_REF] Guennoc | Seismic Atlas of the "Messinian Salinity Crisis" Markers in the Mediterranean and Black Seas[END_REF]Thinon et al., 2011).

At the peak of the MSC, the currently visible erosion of the upper part of these facies interpreted as lago-mare facies [START_REF] Bowman | A comprehensive review of the Msc facies and their origins in the offshore Sirt Basin, Libya[END_REF], could have been caused by the rapid draining of the perched lakes (Thinon et al., 2011). A recent model establishes an interesting scenario that attempts to correlate these intermediate settings with a deep setting [START_REF] Roveri | The Messinian Salinity Crisis: past and future of a great challenge for marine sciences[END_REF].

However these authors based their models on incomplete seismic records of the MSC (Lofi et al., 2011a and b).

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As our first conclusion, we propose that the Messinian sedimentary evolution of both western and eastern basins was similar. Therefore, we propose a regional model that unites the western and eastern Mediterranean (Fig. 10). This model is based on observations and previous works throughout the Mediterranean and the margins outlined above: Strong net evaporation concentrated Mediterranean seawater was coeval with drawdown and resulted in the rapid precipitation of halite during the sea level fall, with an optimum marking the transition between the top of the Messinian lower megasequence (MLM) and the Messinian upper megasequence (MUM). We assume that the rapid infilling of the Messinian accommodation by clastics and evaporites in the deep basin resulted from a marked reduction in the seaway connections at the Gibraltar arc region at 5.6 Ma. This ended with the reconnection/reopening of the world's oceans via the Gibraltar Straights at around 5.46 Ma. However, without a proven age model, it is risky to give ages to this sequential interpretation.

The variability of facies throughout the Mediterranean basin, including the variations in the thickness of the mobile unit, can be explained by: 1) the paleogeographic position of the basins and their margins during the MSC;

2) by the presence or absence of large clastic and meteoric water pathways flowing into the saline basins.

From 5.97 to 5.6 evaporites precipitated in shallow sub-basins [START_REF] Manzi | Age refinement of the Messinian salinity crisis onset in the Mediterranean[END_REF]; the MSC peaked when evaporite precipitation shifted at 5.6 to the deepest depocenters and the peripheral basin started to be eroded (purple area in Fig. 10; Topped by the MES);

The two major seismic megasequences in the two Mediterranean deep basins correspond to five major stages: (1) an initial stage of progressive increase in salinity; (2) a falling stage with detrital and evaporite deposition; (3) a late lowstand with rapid precipitation of salt, (4) the final stages of the MSC with a decrease in salinity due to the beginning of dilution, (5) an

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early transgressive stage with the deposition of marine sands. This Messinian depositional megasequence ended with a reflooding coeval with a worldwide rise in sea level (5.46-5.33) The Messinian lower megasequence (MLM) corresponds to a rapid relative sea level fall in a hyper saline deep basin. The water column still represented by 500-1,000 m of dense water.

The systems tract associated with this seismic megasequence is characterized by: (1) forced regressive deltas and instabilities (upslope scours and MTD in the basin and canyon flanks),

(2) widening of canyons and the deposition of turbidites in front of the big deltas. River profiles tended to readjust with time leading to the continuous generation of Messinian subaerial erosional surfaces (MES in Fig. 10) on top of peripheral basins (PB in Fig. 10).

Clastic sedimentation was not distributed evenly over time but was localized in the MLM (eastern basin and northwestern basin) and the deposition of deep sea fans located in front of major deltas/canyons in the deep basins (2 in Fig. 10) was synchronous with the first evaporite deposition in the eastern basin (seismic facies) and probably synchronous with the evaporite deposition in the western basin (based on seismic velocities and detailed seismic facies). All the Messinian depressions shallower than 1,500 m became subaerially exposed after the maximum drop in the base level, and underwent erosion and/or eventual bypass. The basinward extension of the MES, i.e. its correlative continuity, in front of the big Messinian river canyons will likely be traced to a much lower position than previously defined. The deposition of MLM massive evaporites and detritals resulted in significant loads and predicts differential acceleration of basement subsidence between the young western basins (Oligocene-Aquitanian) and the eastern deep basin located on Tethysian remnants. The differential subsidence between deep basins and their platform as well as the sea level drop, caused major instabilities. The sharp transition between MLM and MUM is coeval with the lowest sea level that occurred in the Messinian megacycle. The MUM corresponds to an apparent base level rise with onlaps on the MSC erosional surfaces. This can be linked to a

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21 constant marine water flux leading to the deposition of thick halite in both basins and reducing influxes from rivers. The MUM constitutes an upper succession of shallow to moderately deep-water deposits (5, 6, 7, 8b, 8a on Fig. 10). Geometrical features and seismic facies in the dataset are evidence for an increasing accommodation/sediment supply ratio during the deposition of these "upper evaporites". This aggradation was commonly coupled with maximum regression. From these observations, we interpret this complex MUM megasequence as a late lowstand. This stage could have begun with a period characterized by large-scale environmental fluctuations in a Mediterranean transformed in some places to brackish water (suspended lakes in an intermediate position between the peripherals basins and the deep basins) and the rapid precipitation of massive halite in the deep settings, controlled by a drier climate. MUM could correspond to the uppermost units of Sicily and Cyprus (5.5 Ma to 5.46 Ma) described previously. This scenario ends with catastrophic flooding due to the collapse of the Straights of Gibraltar. Polyphasic Messinian erosional surfaces (MES, RS and TES) are fossilized by the Zanclean hemi-pelagic marls (labelled 9a in Fig. 10). Reworked evaporites and deep water sands are widely observed in ODP drilling lithology (labelled 8 in Fig. 10) attesting to the permanency of high energy and erosive sea bottom currents during the Zanclean. In the Alboran Basin, the huge seawater flow significantly eroded the Messinian subcropping units [START_REF] Estrada | Impact of pulsed Atlantic water inflow into the Alboran Basin at the time of the Zanclean flooding[END_REF]. The missing material could correspond to thick "chaotic / channelized" units on top of the upper megasequence of the Algero-Baleares basin.

Conclusion

We propose that during the salinity crisis, the Messinian evolution of both the eastern and western deep basins is recorded in two seismic megasequences corresponding to a dual lowstand depositional model: The Messinian lower megasequence (MLM) corresponds to the Thinon, I., Réhault, J.P., Guennoc, P., 2011. East-Corsica basin. In: Lofi, J., Déverchère, J., Gaullier, V., Gillet, H., Gorini, C., Guennoc, P., Loncke, L., Maillard, A., Sage, F., Thinon, I. (Eds.), Seismic Atlas of the "Messinian Salinity Crisis" Markers in the Mediterranean and Black Seas. Commission for the Geological Map of the World and Memoires de la Société Géologique de France, Nouvelle Série, p. 45-47. Tibor, G., Ben-Avraham, Z., Steckler, M., Fligelman, H., 1992. Late Tertiary subsidence history of the southern Levant margin, eastern Mediterranean Sea, and its implications to the understanding of the Messinian event, J. Geophys. Res. 97(B12), 17593-17614. Urgeles, R., Camerlenghi, A., Garcia-Castellanos, D., De Mol, B., Garcés, M., Vergés, J., Haslam, I., Hardman, M, 2011 westward from the prominent Eratosthenes Continental Block (Montadert et al., 2014 modified). "Deep" IODP sites are located both in the eastern and western deep basins. This map is not palinspastically restored in its Messinian configuration, and some contours of the Messinian depocenters are consequently impacted by more recent tectonic episodes. The current locations of the main thrust fronts (Apennines, Calabrian wedge and Mediterranean Ridge) are indicated, as well as a tentative restoration of the active Messinian fronts (Apennines, Hellenides adapted from [START_REF] Jolivet | Lithospheric-scale geodynamic context of the Messinian salinity crisis[END_REF]. The eastern and western sub-basins were (and still are) located in distinct geodynamic environments: foreland flexure of a thick and cold lithosphere in the case of the eastern basin, back-arc basins impacted by thermal subsidence on a hot thin lithosphere in the case of the western Mediterranean and Tyrrhenian Basins. Early Messinian connections with the world ocean are in yellow (adapted from [START_REF] Esteban | Western Mediterranean reef complexes. In: Models for Carbonate Stratigraphy from Miocene Reef Complexes of Mediterranean Regions[END_REF]Martin et al., 2001). in the text. The position of the seismic line is shown in Fig. 2a). The nature of the high amplitude reflectors in the middle of the Messinian evaporites type section (Unit C) is debatable. Indeed few exploration wells in the southern Levant Basin crossed evaporites in the deep basin setting, and results have not been published. Nevertheless in offshore Lebanon (Chbat et al., 2014, Fig. 2b) stratigraphic inversion of seismic data and facies analysis is strong evidence that unit C consists of clastics, locally porous sands or claystones. et al., 2009). The mobile unit (unit 6, halite) ascribed to an increase in accommodation and aggradation (on-laps on the MES as it appears in the dataset) does not necessarily mean transgression but is linked with maximum regression (decrease in the rate of sea level fall). Fig. 7: Line drawings (Figs. 7a and 7b, after Montadert et al., 1978) and seismic line (Figs. 7c, after Sage et al., 2011) showing the geometric relationships between the upper part of the MUM (7) and late lowstand fluvial conglomerates/ early transgressive marine sands (7'). Fig. 7c shows the seismic facies of the upper part of the MUM that onlaps the MES (1) all around the western Mediterranean. This unit was sampled by Savoye and Pipper (1991) across the Cirque Marcel and clearly infills the Messinien erosion surface (MES). Red conglomerates described by [START_REF] Savoye | The Messinian event on the margin of the Mediterranean Sea in the Nice area, southern France[END_REF] at the base of these units are topped by Messinien marine sands. More recent seismic lines [START_REF] Sage | Seismic Atlas of the "Messinian Salinity Crisis" Markers in the Mediterranean and Black Seas[END_REF] show clear lateral continuity with

  facies (Units B and D, we interpret as halite). Fig. 5b depicts a large clastic body that laterally corresponds to Units A, B and C. It is consequently located within the lower half of the MSC depositional megasequence. The top of sequence C marks the transition from the lower part of the MSC megasequence, where clastic inputs dominate, to the upper part of the MSC megasequence, where evaporites dominate.

  the complete sedimentary record of the MSC is preserved only below the present-day Mediterranean seafloor of the deep central basins. The deep central Messinian basins (Fig. 1) match the present-day Mediterranean slopes and bathyal plains around the Ligurian-Provencal Basin (30 My old basement) and the eastern basin (Tethysian basement). Peripheral and intermediate basins accessible in the field and described in the literature refer to their position with respect to these deep basins. A set of terms was taken from the sequential stratigraphy (Catuneanu et al., 2011) to facilitate the description and the proposed interpretation, but the Messinian depositional megasequence requires the definition of a model-independent methodology for the deep Messinian Mediterranean basin, which was disconnected from the M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 13

  ;(2) a change in the climate to more arid conditions;

  clastics depocenters to the deeper parts of the hypersaline waters of the basins. The top of the MLM is coeval with the lowest MSC sea level and with the sharp transition to the Messinian upper megasequence (MUM). The MUM records the rapid precipitation of massive halite that filled the accommodation space. We interpret the MUM as a late lowstand related to a decreasing rate in the RSL fall accompanied by a progressive increase in the accommodation (relative sea level) and sea water supply. Our seismic interpretation of the Messinian series on the Mediterranean shows that it is important (but challenging) to distinguish between geometries due to three processes: (1) erosional truncations that occurred during the Messinian falling stage; and (2) marine ravinement surfaces caused by an early transgression at the end of the MSC; (3) severe erosion at the top of the Messinian Alboran Sea floor and the basinal evaporites caused by the strong currents during the reflooding period. The previous partial record of the MSC was due to the peripheral/intermediate position of the basins. In the deep offshore, deposits related to the MSC are a major component of the petroleum system with thick extensive sandstones or mass transport complexes with evaporites, sealed by Messinian evaporites and sourced by Messinian or older source rocks. seismic stratigraphic model, offshore Provence (SE France):Implications for the hydrographic network reconstruction,Mar. Petrol. Geol. 57,

  Figure captions

Fig. 2 :

 2 Fig. 2: in Fig. 2b, Stratigraphic subdivisions of the MSC depositional Megasequence. Units

Fig. 3

 3 Fig. 3 Stratigraphic subdivisions of the MSC Messinian seismic megasequence. Compare

Fig. 4 :

 4 Fig. 4: Seismic lines (Figs. 4band 4c) located along the lower part of the Nile cone (Fig.4a)

Fig. 5 :

 5 Fig. 5: Seismic section of the Antalya Basin: This seismic profile shows the same

Fig. 6 :

 6 Fig. 6: This figure is a comparison between the geometries and the seismic facies in the

  of Lofi et al., 2011 in the deep basin. These fluvial and coastal sediments illustrate the transition between a late Messinian lowstand onlapping the MES and an early transgressive systems tract (Early TST). In this area the forced regressive prisms are poorly imaged. (See explanations and comments in section 4.2, 4.3, and 5 in the text).

Fig. 8 :

 8 Fig. 8: Seismic line crossing Florence Ridge, west of the island of Cyprus. The thick

Fig 9 :

 9 Fig 9: Partial ground-truthing of the MUM: Site 372 on the eastern Menorca Rise (Fig. 9 a,

Fig. 9 c

 9 Fig. 9 c: The GLP2 borehole on the Gulf of Lion Margin (1,246 m water depth) revealed that

Acknowledgements

We thank Petroceltic International PLC, Petroleum Geoservices PGS and Spectrum which approved our use of the seismic line, TGS as the original acquirers and processors of the processed migration of the seismic line in fig. 6a, Françoise Sage, Sebastien Migeon and Jean Xavier Dessa and the INSU CNRS for the acquisition and processing of the seismic line in Fig. 7c (UPMC marine geology training). Two reviewers, F. Roure and M. Rossi, and the editor J.P. Suc are greatly acknowledged for their examination of our manuscript and their comments and recommendations which helped us to significantly improve it. We also thank Daphne Goodfellow for her language corrections and comments on the manuscript, Alexandre Lethier who helped us for the figures, and Jean-Loup Rubino for the fruitful discussions. This work is a contribution to the French program Action Marge and INSU Mistral-Termex program. The processed seismic data were interpreted thanks to the Kingdom Suite© software.

-New industrial seismic lines reveal the first full image of Messinian deposits; -Efficient guideline to interpret the Messinian salinity crisis (MSC) megasequence.

-Major improvement in the understanding of the MSC.

-Critical west-east similarities led to a comprehensive MSC depositional model


X Navarro
email: xavier.navarro@upmc.com

F Porée

A Beuchée

G Carrault

Denoising preterm EEG by signal decomposition and adaptive filtering: A comparative study

Keywords: Electroencephalography, Empirical mode decomposition, Mode mixing, Adaptive filter, Preterm infants

Denoising preterm EEG by signal decomposition and adaptive filtering: A comparative study

X Navarro, F Porée, A Beuchée, G Carrault

Introduction

Brain monitoring, based on a simplified electroencephalographic (EEG) montage, adds complementary and crucial information to the cardio-respiratory monitoring in the Neonatal Intensive Care Unit (NICU). Composed by one (two electrodes) or two channels (four electrodes), this EEG setup provides simplicity, reasonable cost and direct information about the neurological status of the infant and is usually time-compressed and visualized as amplitude-integrated EEG [START_REF] Maynard | Device for continuous monitoring of cerebral activity in resuscitated patients[END_REF]. As multiple sources of noise contaminate the EEG and a clean signal is mandatory for clinical interpretation [START_REF] Hagmann | Artifacts on Electroencephalograms May Influence the Amplitude-Integrated EEG Classification: A Qualitative Analysis in Neonatal Encephalopathy[END_REF], the design of a strategy to remove correctly the artifacts becomes challenging.

Particularly, the elimination of cardiac artifacts requires adapted solutions since their power spectral density overlaps θ, α and β EEG bands . When interferences and the electrocardiogram (ECG) have similar waveforms, a robust and simple solution can be found by adaptive filters (AF) using the recorded ECG as a reference [START_REF] Celka | Preprocessing and time-frequency analysis of newborn EEG seizures[END_REF]. However, as the effectiveness of AF may decrease under non-stationary conditions, sub-band or wavelet-based filtering emerged as well as alternatives to improve filters performance [START_REF] Erdol | Wavelet transform based adaptive filters: analysis and new results[END_REF][START_REF] Hosur | Wavelet transform domain adaptive FIR filtering[END_REF].

More sophisticated EEG denoising techniques based on signal decomposition were proposed later on. For instance, independent component analysis (ICA), performs decompositions in a prescribed number of components, one of them containing, presumably, the unwanted ECG artifact [START_REF] Jung | Removing electroencephalographic artifacts by blind source separation[END_REF][START_REF] Porée | Blind source separation for ambulatory sleep recording[END_REF]. However, the efficiency of the denoising process depends on the availability of several derivations and on the accomplishment of certain assumptions on the signals. Constrained by our one/twochannel EEG, these approaches have not been exploited in the present work.

A relatively new decomposition technique, the empirical mode decomposition (EMD) [START_REF] Huang | The empirical mode decomposition and the hilbert spectrum for nonlinear and non-stationary time series analysis[END_REF], has been found to be very advantageous to decompose nonstationary signals in intrinsic mode functions (IMFs). The quality of the separation in noisy signals has even been improved with the latest versions, first the ensemble EMD (EEMD) [START_REF] Wu | Ensemble Empirical Mode Decomposition: A noise-assisted data analysis method[END_REF] and then the complete EEMD with adaptive noise (CEEMDAN) [START_REF] Torres | A complete ensemble empirical mode decomposition with adaptive noise[END_REF]. Their application to remove artifacts has been widely applied, as for instance, performing EMD on noisy ECG signals and then removing the IMFs dominated by noise [START_REF] Pal | Empirical mode decomposition based ECG enhancement and QRS detection[END_REF], or even combining EMD with ICA [START_REF] Lindsen | Correction of blink artifacts using independent component analysis and empirical mode decomposition[END_REF][START_REF] Mijović | Source Separation From Single-Channel Recordings by Combining Empirical-Mode Decomposition and Independent Component Analysis[END_REF]. In a preliminary study, we proposed the combination of EMD and adaptive filtering to remove cardiac noise in preterm electroencephalogram [START_REF] Navarro | ECG removal in preterm EEG combining empirical mode decomposition and adaptive filtering[END_REF]. In the present paper, we provide more insights about this approach, analyzing in detail the parameters leading to the best performances and how these are affected by mode mixing, a well-known limitation of EMD [START_REF] Wu | Ensemble Empirical Mode Decomposition: A noise-assisted data analysis method[END_REF]. Additionally, we compare the performance of EMD-based techniques to clean neonatal EEG with the use of discrete wavelet transform (DWT) as a decomposition alternative.

Description of the denoising framework

The proposed framework consists on the six-step straightforward method described in Figure 1. Given an input signal, x(n), n = 1 . . . N , corresponding to the noisy EEG:

1. Obtain M independent subcomponents x k (n), k = 1 . . . M by performing decomposition.

They correspond to sub-bands or to IMFs if DWT or EMD/CEEMDAN are chosen, respectively.

2. Estimate the main average frequency of each subcomponent (f k ) from its power spectral density (PSD).

3. Find f e from the ECG reference, z(n). This frequency splits the ECG PSD in 5% on the left and 95% on the right, so that the range [0 -f e ] Hz is considered to have a non significant portion of the ECG energy.

4. Obtain x H (n), a signal formed by the addition of subcomponents having f k ≥ f e and x L (n), the component adding those subcomponents carrying δ-waves, i.e 0.5 ≤ f k < f e . The limit imposed by 0.5 Hz responds to both clinical and technical criteria, that is, a trade-off between the inclusion of very slow EEG patterns and the minimization of low-frequency noise (LFN). Consequently, the remaining subcomponents (f k < 0.5 Hz) are rejected. 5. Remove ECG noise in x H (n) using the recorded ECG as a reference and obtain the cleaned component, x H (n). To this end, adaptive filtering can be performed because the cardiac artifacts appearing in newborns' EEG are similar to the ECG originated in the chest [START_REF] Stern | Atlas of EEG Patterns[END_REF].

x k (n) x(n) x H (n) Decomposition (DWT, EMD or CEEMDAN) Recomposition ECG cancellation (AF) x L (n) z(n) + f k < 0.5 Hz x H (n) x LFN (n) f k ≥ f e 0.5 Hz ≤ f k < f e Frequency estimation M x(n) f k , f e
6. Reconstruct the clean EEG, x(n), by the addition of x L (n) and x H (n).

3 Methods

EEG decomposition by the discrete wavelet transform

The wavelet transform is a very useful and popular analysis tool for time-scale representation and decomposition of non-stationary signals (for a complete survey about the theory of wavelets, the interested reader can refer to [START_REF] Mallat | A wavelet tour of signal processing Academic press[END_REF]). By means of the DWT, the decomposition can be performed so that the frequencies are in a power of two scale, a procedure also known as multi resolution analysis (MRA).

EEG decomposition by empirical mode decomposition

The application of empirical mode decomposition yields the so-called intrinsic modal functions [START_REF] Huang | The empirical mode decomposition and the hilbert spectrum for nonlinear and non-stationary time series analysis[END_REF], a number of subcomponents that must satisfy two conditions: 1) the number of extrema and zerocrossing must be equal or differ at most by one, and 2) the mean value of the upper and lower envelopes must be zero. Then, the unmixing process, also called sifting, can be performed:

1. Find local minima and maxima of the input signal, x(n).

2. Form upper, e u (n), and lower, e l (n), envelopes by cubic splines interpolation.

3 3. Find the mean, m(n) = eu(n)+e l (n) 2 . 4. Compute h 1 (n) = x(n) -m(n).
If h 1 is not an IMF, the procedure is repeated from step 1 using this signal instead of x(n). Else, h 1 (n) = IM F 1 (n).

5. If the residue, res 1 (n) = x(n) -IM F 1 (n) has more than a zero cross, go to step 1 and find next IMF.

The procedure continues until the last residue, res M (n) has no zero crossings or some stopping criteria is fulfilled. This residual component has the lowest frequency content of the time-series.

Once IM F k (n) are extracted, the signal can be expressed as:

x(n) = M k=1 IM F k (n) + res M (n). ( 1
)

Complete ensemble EMD with adaptive noise

The decomposition of noisy signals using EMD may result in mode mixing or corruption of modes, i.e. the presence of one oscillation in different IMFs. The ensemble EMD [START_REF] Wu | Ensemble Empirical Mode Decomposition: A noise-assisted data analysis method[END_REF] was introduced to overcome this problem by repeating decompositions R times and adding white noise to the signal:

x r (n) = x(n) + w r (n), r = 1, .., R (2)
where w r (n) are different realizations of white noise with variance ε, producing IM F r k (n). Then, the final oscillatory modes are obtained by averaging:

IM F k (n) = 1 R R r=1 IM F r k (n) (3)
This procedure improves the quality of the separation, but at the expenses of a high computational cost, and without the warranty of a perfect reconstruction of the signal because a different number of IMFs may be obtained for each iteration. The complete EEMD with adaptive noise (CEEMDAN) was proposed to ameliorate the spectral separation of modes and to reduce computational time [START_REF] Torres | A complete ensemble empirical mode decomposition with adaptive noise[END_REF]. In EEMD each x r (n) is decomposed independently and R residuals are obtained. The CEEMDAN computes the first decomposition by averaging the R IMFs and a unique first residue is retained:

res 1 (n) = x(n) -IM F 1 (n), (4)
where IM F 1 (n) is the same as the first IMF obtained by EEMD Then, EMD is performed over a set of signals obtained by adding different noise realizations to res 1 (n), so the next IMF -noted here IM F 2 (n) -is found by averaging the corresponding set of IMFs. The corresponding residue is

res 2 (n) = res 1 (n) -IM F 2 (n)
, and so on, until the stopping criterion is achieved. Once IM F k (n) and a final residue are obtained, x(n) can be reassembled by their addition similarly than in Equation 1.

Adaptive filtering

Adaptive filtering has been extensively applied to cancel cardiac artifacts in biomedical signals using the recorded ECG as a reference [START_REF] Haykin | Adaptive Filter Theory[END_REF]. In our denoising framework, we implemented the recursive least square algorithm (RLS) as it provides faster convergences than the classic least mean squares [START_REF] Eleftheriou | Tracking properties and steady-state performance of RLS adaptive filter algorithms[END_REF].

Validation strategy

To validate the efficacy of the different denoising options, contaminated signals were generated from real recordings. Data came from 30 preterm infants (36 to 39 weeks post-menstrual age) born at the University Hospital of Rennes (France). ECG and double-channel EEG (Fp1-T3, Fp2-T4) were acquired originally at 512 Hz sampling frequency, low-pass filtered (40 Hz cut-off) and subsampled to 128 Hz.

Since EEG signals acquired NICUs can also be contaminated with low frequency noise, we constructed two groups to simulate realistic conditions:

1. Group 1: Thirty EEG excerpts with added ECG noise (filtering recorded EEG by a 21thorder FIR filter with random coefficients) with signal-to-noise ratios (SNRs) ranging from -5 to 10 dB in steps of 5.

2. Group 2: Thirty EEG excerpts with added ECG at 5dB SNR plus LFN at variable SNR. To emulate breathing and other slow movements, that may appear spontaneously and discontinuously, LFN noise consisted in one cycle of a sinusoidal wave, very close to the limit of the delta band, fixed randomly between 0.3 and 0.5 Hz. The power of the wave was set to obtain SNRs (relative to the original signal) ranging from -10 to 10 dB in steps of 5 dB.

Excerpts were selected by an experienced clinician, ensuring the absence of artifacts and including different EEG neonatal patterns. The amplitude of the noisy components was properly modified to obtain SNRs according to the following equation:

SN R = 10 log P EEG P noise (5)
where P EEG and P noise are the power of an original EEG excerpt and the power of the added artifacts, respectively. Some examples of artificially contaminated EEG signals are shown in Figure 2.

After processing separately each noisy EEG excerpt, x(n), by the denoising framework, the quality of the different decomposition options described in Figure 1 is measured in terms of root mean squared error (RMSE):

RMSE = 1 N N n=1 [x o (n) -x(n)] 2 (6)
where x o (n) is the original, uncorrupted EEG, x(n) is the cleaned signal and N is the number of samples in each excerpt. 5 Tuning of parameters

Filtering and decomposition

To find the appropriate parameters for the different techniques, some preliminary tests were carried out. Firstly, we found the appropriate order and algorithm of the adaptive filter in our particular signals. A filter order L=16 and a forgetting factor λ = 1 -1 10L (as suggested by Eleftheriou and Falconer [START_REF] Eleftheriou | Tracking properties and steady-state performance of RLS adaptive filter algorithms[END_REF]) yielded the best results.

The DWT was performed using 6th order Daubechies wavelet [START_REF] Daubechies | Ten Lectures on Wavelets[END_REF], a choice with satisfactory results in these signals [START_REF] Turnbull | Automated detection of tracé alternant during sleep in healthy full-term neonates using discrete wavelet transform[END_REF]. The MRA framework was configured to obtain seven levels of successive details and the approximation interval, yielding eight subcomponents with their frequency intervals between 32-64, 16-32, 8-16, 4-8, 2-4, 1-2, 0.5-1 and <0.5 Hz, that corresponded to LFN.

The EMD was computed by an efficient algorithm [START_REF] Rilling | On empirical mode decomposition and its algorithms[END_REF] that introduces two thresholds (θ 1 and θ 2 ) in the second criterion to consider an IMF, and a tolerance factor, α ∈ [0, 1]. They aim at guaranteeing globally small fluctuations in the mean while taking into account locally large excursions. Finally, the CEEMDAN was performed by a recent algorithm improvement that optimizes speed and enhances quality with respect to its previous version [START_REF] Colominas | Torres Improved complete ensemble EMD: A suitable tool for biomedical signal processing[END_REF].

Study of mode mixing

As mentioned before, subcomponents obtained by EMD-based techniques can suffer mode mixing, however, this phenomenon is not a major concern as far as it mixes frequencies from only one side of the separation determined by f e . In this case, the sum of the IMFs yielding x L (n) and x H (n) should be the same as in the absence of mode mixing. More attention needs to be paid to those IMFs having frequency tones on both sides of the f e boundary, in particular:

1. Modes with f k ≥ f e -summed in x H (n)-containing oscillations below f e . Since x H (n) is processed by the AF, these low frequencies may lead to underperform the ECG cancellation.

2. Modes with 0.5 ≤ f k < f e containing frequencies above f e . The ECG noise could be partially installed in x L (n) so it would be unprocessed by the AF and incompletely removed in the final reassembled signal.

These two cases will be subsequently referred as to critical mixing I and II, respectively. Figure 3 shows frequencies contained in IMFs performing EMD in a real EEG. The two first IMFs clearly exhibit mode mixing, but since it occurs beyond the limit of f e , it has no consequences on the denoising process. The energies carried by frequencies in the lower whiskers of IM F 1 , IM F 2 and IM F 3 crossing the horizontal dashed line constitute critical mixing I. Likewise, the energies in the frequencies crossing f e in IM F 4 determine critical mixing II. To quantify critical mixing we took advantage of the IMFs to characterize the frequency and amplitude of the components. To illustrate this idea, let us consider an IMF with a single zerocrossing. Since the oscillation of this signal completes an entire period, its frequency can be obtained by simply computing the inverse of the cycle time. Even if decompositions of experimental signals do not yield pure tones, this operation can approximate the frequencies describing cycles within an IMF.

Therefore, if a k-th IMF is composed by C k cycles, with frequencies f l k (l = 1 . . . C k ), the total energy of the cycles into the critical mixing I region, CM I , can be expressed as: where the first summation includes those IMFs having their main frequency below f e and IM F l k (n) is the norm of the l-th cycle in the k-th IMF. Equivalently, for critical mixing II:

CM I = k f k ≥fe C k l=1 f l k ≥fe IM F l k (n) 2 (7)
CM II = k 0.5≤f k <fe C k l=1 f l k <fe IM F l k (n) 2 (8)
CM I and CM II were then divided by the sum the energies contained in all IMFs to obtain the relative portion of critical mixing in each signal. Several simulations were performed to find the decomposition parameters minimizing critical mixing at different noise levels. In a first test, ten randomly selected signals from Group 1 were contaminated with -5 dB and 5 dB SNR of cardiac noise and decomposed by both types of EMD. This test was then repeated using ten signals from Group 2, but contaminated with -5 dB and 5 dB SNR LFN. In order to prevent the sifting process from over-iteration and avoid over-decomposition of the signal, it is suggested by Rilling et al. [START_REF] Rilling | On empirical mode decomposition and its algorithms[END_REF] to use the default values θ 1 = 0.05, θ 2 = 10θ 1 and α = 0.05. We therefore modified θ 1 and α from 0.01 to 0.1 in steps of 0.005 to evaluate their effect on critical mixing.

In CEEMDAN, ε was modified from 0.05 to 1 in steps of 0.05 and R was fixed to 50, a choice providing an optimal computational time with negligible reconstruction error [START_REF] Colominas | Torres Improved complete ensemble EMD: A suitable tool for biomedical signal processing[END_REF].

The values of the parameters leading to minimal ratios of critical mixing have been depicted in Table 1. From these tests, it can be stated that CEEMDAN reduces significantly CM II , thus more effective to remove cardiac artifacts. We stated as well that the decomposition parameters (especially ε [START_REF] Wu | Ensemble Empirical Mode Decomposition: A noise-assisted data analysis method[END_REF]) depend on the noise levels of contaminated signals and, hence, they should be adjusted increasingly as the EEG signals become more noisy to reduce critical mixing.

Results and discussion

ECG removal

The results of denoising Group 1 are summarized in Table 2. Indeed, the application of signal decomposition techniques before the adaptive filter resulted in a general decrease of the RMSEs compared to denoising without EEG decomposition. However, this gain of quality was unequally distributed and statistically significant only for certain SNRs. CEEMDAN provided in average a 30% reduction of RMSEs whereas EMD and DWT resulted in 7.3% and 3.4% respective reductions.

It is noteworthy to mention that high SNR levels are then more favorable to perform EEG signal decomposition, probably due to smaller distortion introduced to the final reconstructed signal when only the noisy part of the EEG is processed by the AF. On the contrary, in low SNRs the denoising performances of all methods converge because useful information is masked by noise and decomposition become less effective.

ECG and low frequency noise removal

The scenario in which signals also contain low frequency noise has been evaluated using data from Group 2. To compare the improvements of introducing EEG decompositions, we processed the raw signals using a finite impulsional response (FIR) filter with a cutoff frequency of 0.5 Hz before applying the cancellation of ECG noise. This high-pass filter (HPF) can prevent phase distortion, but employing a high order (382) that introduced delay (>1.5 s).

As it can be stated in Table 2, to remove simultaneously LFN and ECG noise only EMD-based decompositions are advantageous in all noise levels, with a 6.7% average RMSE reduction for EMD and 13% for CEEMDAN.

Denoising real signals

Finally, we performed some tests on real signals, contaminated both with ECG and LFN. To evaluate quantitatively the noise elimination, we employed the artifact rejection ratio (ARR), defined as the ratio of the power of the removed artifacts to the cleaned EEG [START_REF] Puthusserypady | Robust adaptive techniques for minimization of EOG artefacts from EEG signals[END_REF]:

ARR = N n=1 [x(n) -x(n)] 2 N n=1
x 2 (n) [START_REF] Wu | Ensemble Empirical Mode Decomposition: A noise-assisted data analysis method[END_REF] where x(n) and x(n) are the real EEG before and after the denoising process, respectively.

Since ARR gives information of the whole denoising process (comprising low frequency and cardiac noise), we computed this ratio in small windows surrounding the QRS-peak instants (50 ms) on the noisy excerpt and on the cleaned one. This parameter, ARR qrs , is conceived to give a quantitative idea of how attenuated are the heartbeat artifacts.

The metrics ARR and ARR qrs were computed on the selected excerpts (see Table 2), contaminated with ECG artifacts and low frequency noise. As it can be noticed, the overall noise removal given by ARR and ARR qrs is greater using CEEMDAN. Of note, decomposition techniques are not always advantageous to suppress QRS artifacts, as is the case of DWT and EMD. They provided the lowest ARR qrs values probably for an incomplete separation of x H (n) before the ECG cancellation takes place.

Conclusion

The results presented in this paper show that, in general, EMD-based techniques performed better than DWT decomposition in the EEG signals studied here. This is probably due to the use Table 2: Results from denoising simulated and real signals. For simulated EEG (both G1 and G2), the efficacy of the different denoising options is expressed as RMSEs (mean ± std. dev.). Best values (lower RMSEs) are in bold. In brackets, the improvement percentages compared to the simple usage of the adaptive filter ("NO DEC" columns) or adaptive filter plus high-pass filter ("NO DEC + HPF"). A Wilcoxon signed rank test compared "NO DEC (+HPF)" with the three decomposition solutions ( * denote statistically significant differences at p <0.05). The last two rows show the noise removal estimation by the ARR metrics. Best values (greatest ARR) are highlighted in bold. of power of two in the scale factors in DWT, an imposition that yields only four subcomponents between 0.5 and 8 Hz. But as the energy distribution within this range of frequencies is variable, an optimal separation of the EEG bands may require a different number of subcomponents. The adaptive manner of decomposing signals by the EMD and CEEMDAN is, therefore, more convenient to separate and reassemble the subcomponents as a function of f e , the frequency containing the ECG artifacts. Other wavelet-based methods not explored here, such as the wavelet packet transform, could be employed alternatively to obtain more subcomponents in the aforementioned range. However, finding the optimal basis in this solution is not straightforward. The major issue of EMD, mode mixing, has not been found to be a major concern in the application presented herewith. The main problem was rather to have IMFs with oscillations crossing the threshold established by f e , critical mixing, a fact that is not necessarily related to mode mixing. By quantifying critical mixing, we corroborated that the CEEMDAN, thanks to its superior performance, reduces this effect and provides the best denoising results.

However, the use of decomposition techniques introduce some limitations that should be taken into account. Firstly, distortion in low frequency bands cannot be completely avoided since the rejection of subcomponents below 0.5 Hz implies the removal of certain content over this frequency. Although this limitation is partially solved by the enhanced spectral separation of CEEMDAN, the degree of rejected true slow-delta waves needs to be studied in future works because not only technical, but also physiological and maturational considerations appear on the scene [START_REF] Lamblin | Electroencephalography of the premature and term newborn. Maturational aspects and glossary[END_REF]. But we also would like to note that in less noisy environments the low frequency limit could be decreased below 0.5 Hz to include the slowest delta content. Secondly, the choice of CEEMDAN implies an increased computational cost, but even if a recent algorithm has reduced its complexity, it is still may be considered a drawback for current real-time systems. In the near future, the application of the family of EMD methods to denoise EEG is more plausible for the NICU as the algorithm continues to be improved in terms of speed and bedside equipment is becoming more powerful.

Figure 1 :

 1 Figure 1: Block diagram of the proposed methodology with different options to decompose the EEG.

FilteredFigure 2 :

 2 Figure 2: Example of the generation of artificially contaminated EEGs by the filtered ECG and LFN. Using the first signal (clean EEG), two examples of Group 1 (second and third signals) have been formed. The fourth signal (a Group 2 example) is formed by the addition of the third signal and LFN at 0 dB. The lower plot shows the originally recorded ECG, z(n), and the ECG noise obtained by applying the 21st-order FIR filter.

Figure 3 :

 3 Figure3: Frequencies contained in all IMFs performing EMD in a real EEG. IM F1 and IM F2 contain almost identical frequencies, evidencing mode mixing. The small frequency portions below fe of IM F1, IM F2 and IM F3 can be considered as critical mixing I, and conversely, the portion of IM F4 exceeding fe can be accounted as critical mixing II.

Table 1 :

 1 Decomposition parameters for EMD (θ1, α) and CEEMDAN ( ε) minimizing critical mixing in ten examples from Group 1 (G1) and Group 2 (G2). The average number of IMFs (M ) is also shown.

	SNRs for EMD	θ 1	α	M	CM I (%)	CM II (%)
	G1 (-5 dB)	0.03 0.03 9.80 ± 0.42 1.62 ± 0.56 0.63 ± 0.24
	G1 (5 dB)	0.02 0.01 10.3 ± 0.48 1.44 ± 1.18 1.57 ± 1.12
	G2 (-5 dB)	0.03 0.02 10.1 ± 0.31 0.63 ± 0.52 0.64 ± 0.51
	G2 (5 dB)	0.03 0.02 10.3 ± 0.58 0.97 ± 0.62 0.32 ± 0.03
	SNRs for CEEMDAN		ε
	G1 (-5 dB)		0.25 9.62 ± 0.52 0.94 ± 0.60 0.06 ± 0.02
	G1 (5 dB)		0.90 10.1 ± 0.55 1.46 ± 1.68 0.03 ± 0.02
	G2 (-5 dB)		0.35 10.2 ± 0.05 0.24 ± 0.11 0.01 ± 0.01
	G2 (5 dB)		0.95 9.80 ± 0.44 0.67 ± 0.57 0.04 ± 0.02

Acknowledgements

The research for this paper was financially supported by the project INTEM between Rennes University Hospital and the LTSI-INSERM U1099. X. Navarro is currently funded by Air Liquide Medical Systems France through the COVEM Project.

Conflict of interest

There are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome. The database involved in this study received the ethic approval by the Regional Ethics Committee (Comité de Protection des Personnes, CPP Ouest 6-598).