Marie-Madeleine Blanc-Valleron

Paleoenvironmental changes at the Messinian–Pliocene boundary in the eastern Mediterranean (southern Cyprus basins): significance of the Messinian Lago-Mare

Abstract The study of the late Messinian–Early Pliocene sediments in south Cyprus (Pissouri and Polemi basins) and the comparison with the offshore coeval deposits drilled in the ODP Leg 160 boreholes, provide new data illustrating the environmental changes which occurred in the eastern Mediterranean at the end of the salinity crisis. The Messinian–Pliocene transition is marked by a very rapid environmental change indicated by sharp variations in lithology, sedimentology, microfaunal assemblages and stable isotope composition. The latest Messinian interval is dominated by the ‘Lago-Mare’ oligo-mesohaline environments. The most striking feature in the Pissouri Basin is the intercalation between the uppermost gypsum bed and the base of the Zanclean deposits of four main horizons of paleosols interbedded with carbonates and conglomerates. Paleosols indicate periods of subaerial exposure long enough to permit pedogenesis to develop. Conglomerates indicate intense erosion of the Troodos Massif and its sedimentary cover as a result of the water level drop, tectonic activity and more humid climatic conditions. Thus, several subaerial exposure phases occurred in the marginal areas like the Pissouri Basin whereas shallow oligohaline conditions persisted in the deeper parts of the basin, as indicated at ODP Site 968. Both in the Pissouri Basin and on the flank of the Erathostenes Seamount the late Messinian paleosols are directly overlain by the lowermost Pliocene sediments deposited in well-oxygenated deep marine conditions. This drastic change of water depth shows that the water level of the late Messinian saline to brackish lakes dropped far below that of the world ocean, at least more than several hundred meters. Both these base level fluctuations and the oligohaline conditions caused intensive erosion–karstification of the upper gypsum beds as also reported from many other Mediterranean regions. Moreover, the study of the Polemi sections shows that oligohaline conditions had already started during the deposition of the upper gypsum sub-unit. Typical brackish and fresh water assemblages are present in the last two gypsum layers and intergypsum beds. This indicates that freshwater dilution, i.e. the Lago-Mare conditions, resulted from a progressive change of the hydrological budget which started during deposition of the upper evaporites and climaxed after the deposition of the uppermost gypsum layer (classical Lago-Mare facies). During this time, periods of increased evaporation over precipitation led to deposition of the uppermost gypsum layers. This evolution is interpreted as being due both to the severance from the open ocean and to the increasing contribution of the runoff–precipitation versus marine inputs in the hydrological budget of the Mediterranean.

The onset of the Messinian salinity crisis in the Eastern Mediterranean (Pissouri Basin, Cyprus)

The Pissouri Basin in Cyprus contains one of the most suitable sedimentary successions with which to study the onset of the Messinian Salinity Crisis in the Eastern Mediterranean. Exposures along the new Paphos^Limassol motorway near Pissouri exhibit distinct cyclic bedding which permits the construction of a chronology based on orbital tuning. Biostratigraphic results reveal 10 planktonic foraminifera events that have been astronomically dated in other Mediterranean sections, and as such provide an excellent first-order age control. Magnetostratigraphic results are in good agreement with the biostratigraphic data and show that all magnetic chrons between C4n.1n and C3An.1n are present. The pattern of sedimentary cycles generally fits well with the insolation curve. Astronomical tuning of the succession shows that the first gypsum bed at Pissouri overlies a 40^60 kyr stromatolite-bearing transitional interval and correlates with the amplitude increase in insolation at 5.96 Ma, as in the western Mediterranean. This indicates that the onset of evaporite precipitation was synchronous right across the entire Mediterranean Basin. fl 2002 Elsevier Science B.V. All rights reserved.

Sedimentary, stable isotope and micropaleontological records of paleoceanographic change in the Messinian Tripoli Formation (Sicily, Italy)

Abstract The Tripoli Formation (6.96–5.98 Ma) of the Central Sicilian Basin provides a good record of the paleoceanographical changes that affected the Mediterranean during the transition from slightly restricted conditions to the onset of the Messinian Salinity Crisis. The Falconara/Gibliscemi section has been selected for an integrated approach at a high resolution scale using sedimentology, stable isotopes of the carbonates and microfossils. The sedimentary succession includes 46 precession-controlled cycles resulting from the periodical increase in biosiliceous productivity (diatomites) that followed the deposition of marls and pinkish laminites, which appear as sapropel-type deposits induced by the oceanic fertilization by terrestrial nutrients during wet periods. Higher scale environmental changes are superimposed to this precession forced rhythmicity. There is a general trend of increasing basin restriction from near marine conditions at the base of the Tripoli to semi-closed settings in its uppermost part, which are the prelude of the salinity crisis. This pattern reflects the hydrological response of the Mediterranean to the progressive decrease of the Atlantic inputs and an enhanced influence of the climate on depositional conditions. However, this evolution is not linear and shows successive phases of different duration. During the first period (until 6.71 Ma), open Atlantic–Mediterranean exchanges maintained relatively stable marine conditions. The second period (6.71–6.29 Ma), marks an important step in the basin restriction with a wider range of salinity fluctuations and an increased bottom stagnation. The 6.71-Ma event, which is correlated at a Mediterranean scale, may have resulted from shallowing of the Mediterranean gateway under a tectonic control. This shallowing reduced the oceanic inputs resulting in an increased climatic constraint of the Mediterranean hydrology. During the third period (6.29–6.03 Ma) an increase of the surface water salinity resulted in stressful conditions for the marine microfauna. The 6.29-Ma change is a major step in the restriction that may be correlated with the intensification of the glaciation recorded in the Atlantic, which could have enhanced the effects of the tectonic closure. The last two cycles (48 and 49), that underlie the ‘Calcare di Base’, witnessed the rapid transition to a semi-closed Mediterranean setting characterized by large variations of salinity from diluted to hypersaline conditions, under a dominant climatic control, and by the nearly complete disappearance of the marine organisms. Long-trend environmental changes recognized within the Tripoli Formation resulted from a complex set of interfering factors controlling the water fluxes exchanged between the Mediterranean and the world ocean. Most of the rapid changes identified in Falconara/Gibliscemi at 7.16, 6.71 and 6.29 Ma, that occurred simultaneously in the western and eastern Mediterranean, were mainly controlled by the stepwise tectonic closure of the Atlantic connections, although a glacio-eustatic overprint cannot be completely excluded.

Sedimentary and diagenetic markers of the restriction in a marine basin: the Lorca Basin (SE Spain) during the Messinian

Apparatus for automatically measuring and plotting the force-deflection curve of springs, especially small springs on which forces are to be applied in order of 0.1 to 100 grams.

Late Messinian to Early Pliocene paleoenvironmental changes in the Melilla Basin (NE Morocco) and their relation to Mediterranean evolution

Abstract Three major paleoenvironmental changes have been recognized during the late Miocene to Early Pliocene in the Melilla Basin (Northeastern Morocco) and compared with the regional events that affected the Mediterranean hydrology during this crucial period. The first change was the definitive conversion of the restricted marine conditions that prevailed since the end of the reef carbonate complex into lacustrine environments; this occurred around 5.8 Ma which is earlier than in the rest of the Mediterranean where the Lower Evaporites were still forming. These lacustrine settings display great similarities with the Lago-Mare environments that started in the Mediterranean during the deposition of the Upper Evaporites and climaxed during the latest Messinian. The second change corresponds to a long period of subaerial exposure that caused a widespread erosional surface; this resulted in a deep paleovalley, which truncates the Messinian sedimentary succession of the deeper southwestern part of the basin. This event postdates a lava flow dated at 5.77 Ma and confirms the importance of the latest Messinian erosional event. The third change was the infilling of this topography by marine sediments when the basin was rapidly reflooded in Early Pliocene. The lack of evaporites indicates that the Melilla Basin did not experience the Messinian Salinity Crisis. This could be explained either by the drainage of the brines towards the deeper South Alboran Basin or rather by an early dilution of the basin due to local excess of continental water inputs while most of the other Mediterranean basins evolved at the same time into evaporitic settings. These data confirm that the Rifian gateway was completely closed, thus, preventing Atlantic water inputs in the Mediterranean during the whole evaporitic deposition and the Lago-Mare episode. Such an evolution fits well with the history of depositional environments in the Alboran domain where evaporites are poorly developed and the Messinian is mainly represented by an erosional surface.

La transition Messinien–Pliocène en Méditerranée orientale (Chypre) : la période du Lago-Mare et sa signification

Abstract The transition between the Messinian evaporites and the Lowermost marine Pliocene in the Mediterranean records a major environmental change (‘Lago-Mare’). In Cyprus, this period is characterised by fresh-water to brackish fauna, palaeosols and the dissolution/erosion of the Upper Gypsum Unit, which can be correlated with a general erosional episode in the Mediterranean. These conditions had already begun during the deposition of Upper Gypsum Unit. Thus, during this period, very shallow and episodically desiccated saline lakes were located in the Mediterranean area, below the world sea-level, in response to a major change of the hydrological budget forced by a climate variation and tectonics.

Environmental controls on perennial and ephemeral carbonate lakes: the central palaeo-Andean Basin of Bolivia during Late Cretaceous to early Tertiary times

Abstract In the central palaeo-Andean Basin (Potosi Basin) of Bolivia, the up to 450-m-thick El Molino Formation (Campanian-Maastrichtian to early Tertiary) includes two main carbonate lacustrine episodes each of different extent and duration. These episodes are separated by an extensive development of alluvial facies (floodplain deposits) and limestones deposited in playa lakes or isolated ponds. The El Molino Formation has been investigated in six different areas of the Potosi Basin and affords the opportunity to document climatic and tectonic controls on the development of perennial and ephemeral carbonate lacustrine systems. The first lacustrine episode originated predominantly in perennial lakes with wave-dominated low-gradient ‘ramp’-type margins. Outer marginal lacustrine facies include winnowed oolitic grainstones, ostracod packstones and microbialite bioherms. More agitated areas were characterized by nearshore oolitic bars with associated thrombolite mounds. The fossil content, and the lack of significant precipitation of evaporites, indicate that the lake waters were of low salinity (probably oligohaline) during most of this period. After the first lacustrine episode, the widespread development of terrigenous alluvial facies (floodplain deposits), with interspersed playa lakes or isolated ponds in depressions on the floodplain, may indicate a change in the net water budget. The second lacustrine episode is dominated by dark micritic limestones and, to a lesser extent, microbialite bioherms which developed under semiarid climatic conditions. This episode includes the deposits of ephemeral saline, locally alkaline, shallow lakes which were characterized by low-energy, low-gradient ‘ramp’-type margins. The stable isotopic analysis of 163 carbonate samples covering the different facies and depositional settings displays a wide range of values ( −14.2 18 O and −12.9 13 C ) that is typical of nonmarine environments. The distribution of the values indicates that, during the deposition of the El Molino Formation, the basin was hydrologically closed and experienced no strong hydrogeographical changes, except for variations in the palaeolake level related to fluctuations in the regional P/E ratio. These are recorded by two main isotopic trends related to changes between perennial and ephemeral lacustrine conditions. Lacustrine sedimentation was controlled predominantly by climatically driven hydrological changes with repeated oscillations of the water level (expansion and contraction) and subsequent fluctuations in the width of lacustrine-facies belts.

Gas hydrate dissociation in the Lorca Basin (SE Spain) during the Mediterranean Messinian salinity crisis

Abstract The Tortonian marls which were deposited in the Lorca Basin (SE Spain) prior to the Messinian Tripoli Formation contain discontinuous levels of indurated nodules and massive layers, up to a few meters thick, of diagenetic dolomite. The oxygen and carbon isotopic compositions of these dolomites indicate that the carbonate diagenesis occurred mostly in 13C-rich and 18O-rich to 18O-poor solutions. The vertical distribution of the δ values of these dolomites is interpreted as the result of the past occurrence of gas hydrate reservoirs where the methane source probably originated from the organic-rich marls. We postulate that the gas hydrates contained in the Tortonian marls were destabilized during the water level drops related to the Messinian salinity crisis. Associated sedimentary structures include numerous syn-sedimentary deformations and brecciation which are the expression of important sedimentary instabilities, which could have been generated during the gas hydrates decomposition.

Authigenic carbonates related to active seepage of methane-rich hot brines at the Cheops mud volcano, Menes caldera (Nile deep-sea fan, eastern Mediterranean Sea)

On the passive margin of the Nile deep-sea fan, the active Cheops mud volcano (MV; ca. 1,500 m diameter, ~20–30 m above seafloor, 3,010–3,020 m water depth) comprises a crater lake with hot (up to ca. 42 °C) methane-rich muddy brines in places overflowing down the MV flanks. During the Medeco2 cruise in fall 2007, ROV dives enabled detailed sampling of the brine fluid, bottom lake sediments at ca. 450 m lake depth, sub-surface sediments from the MV flanks, and carbonate crusts at the MV foot. Based on mineralogical, elemental and stable isotope analyses, this study aims at exploring the origin of the brine fluid and the key biogeochemical processes controlling the formation of these deep-sea authigenic carbonates. In addition to their patchy occurrence in crusts outcropping at the seafloor, authigenic carbonates occur as small concretions disseminated within sub-seafloor sediments, as well as in the bottom sediments and muddy brine of the crater lake. Aragonite and Mg-calcite dominate in the carbonate crusts and in sub-seafloor concretions at the MV foot, whereas Mg-calcite, dolomite and ankerite dominate in the muddy brine lake and in sub-seafloor concretions near the crater rim. The carbonate crusts and sub-seafloor concretions at the MV foot precipitated in isotopic equilibrium with bottom seawater temperature; their low δ13C values (–42.6 to –24.5‰) indicate that anaerobic oxidation of methane was the main driver of carbonate precipitation. By contrast, carbonates from the muddy lake brine, bottom lake concretions and crater rim concretions display much higher δ13C (up to –5.2‰) and low δ18O values (down to –2.8‰); this is consistent with their formation in warm fluids of deep origin characterized by 13C-rich CO2 and, as confirmed by independent evidence, slightly higher heavy rare earth element signatures, the main driver of carbonate precipitation being methanogenesis. Moreover, the benthic activity within the seafloor sediment enhances aerobic oxidation of methane and of sulphide that promotes carbonate dissolution and gypsum precipitation. These findings imply that the coupling of carbon and sulphur microbial reactions represents the major link for the transfer of elements and for carbon isotope fractionation between fluids and authigenic minerals. A new challenge awaiting future studies in cold seep environments is to expand this work to oxidized and reduced sulphur authigenic minerals.

Gas hydrate transect across Northern Cascadia Margin

Gas hydrate is a solid compound mainly comprised of methane and water that is stable under low temperature and high pressure conditions. Usually found in offshore environments with water depths exceeding about 500 meters and in arctic regions associated with permafrost, gas hydrates form an efficient storage system for natural gas. Hence, they may represent an important future energy resource [e.g., Kvenvolden, 1988]. Gas hydrates also form a natural geo-hazard, and may play a significant role in global climate change [e.g., Dillon et al., 2001].