Marco Roveri

The Mediterranean Messinian salinity crisis: an Apennine foredeep perspective

Abstract Owing to its expanded stratigraphic sections, the Apennine thrust belt offers the opportunity to better understand the evaporitic and post-evaporitic Messinian events. A physical stratigraphic framework of Messinian deposits, based on facies analysis and basin-wide correlation of key surfaces and sedimentary cycles, is presented. It is shown that the Messinian Apennine foredeep had marginal basins with shallow-water primary evaporites and deeper basins where resedimented evaporites accumulated under relatively deep-water conditions. Like many other Mediterranean examples, primary shallow-water evaporites of Apenninic marginal basins show evidence for subaerial exposure and erosion. However, the development of such an erosional surface does not correspond to the deposition of primary evaporites in the deepest part of the basin(s); here, the unconformity can be traced towards the base of resedimented evaporites or to a level within them, implying that the deeper basins of the Apennine foredeep never underwent desiccation during the Messinian salinity crisis, but rather received the eroded marginal evaporites. This fact, usually overlooked, raises important questions about the deep desiccation model of the Mediterranean.

Styles of Failure in Late Holocene Highstand Prodelta Wedges on the Adriatic Shelf

On the continental shelves of the northern Mediterranean basin, the late Holocene highstand systems tract (HST) prograded un- der the influence of major rivers, after the attainment of the present sea-level highstand (about 5.5 cal kyr BP). On the Adriatic shelf, the thickness distribution of the late Holocene HST reflects the location of major deltas on the western side of the basin and the geostrophic cir- culation, which prevents a more uniform sediment dispersal toward the center of the basin. Very high sediment accumulation rates (1 to 10 cm/year) resulted in the construction of a HST depocenter up to 35 m thick. This shore-parallel depocenter is affected by failure of limited displacement over as much as 40% of its extent. Gas impregnation is common in the topset region and occurs at very shallow levels (a few meters) below the sea floor. Five areas are characterized by a variety of sea-floor and subsurface crenulations. Although locally some of these crenulations have an intriguingly regular geometry, sediment failure is the most plausible mechanism for their formation. Sediment failure better explains the large variety of geometries that characterizes the coastal mud prism of the late Holocene HST. Furthermore, we observe that these crenulations occur only where the downlap surface at the base of the HST is disrupted and affected by geometries that are con- sistent with fluid escape processes. This relationship suggests that the basal surface acts as a weak layer for sediment failure. Failure occurred in variable water depths from the northern slope of the modern Po prodelta (10-20 m water depth) to the narrow shelf offshore Bari (40-110 m water depth). In all these areas the proximal part of the HST prodelta wedge is intensely gas-charged. The thickness and age of the sediment sections affected by failure are slightly differ- ent from place to place but appear everywhere younger than 5.5 cal kyr BP. Where failure affects the entire HST the detachment occurs on the downlap surface at its base. Failure geometries characterize the head region whereas compressional features, such as pressure ridges and mud diapirs, dominate in the toe region, ranging in depth between 70 and 110 m. Where failure is limited to the upper few meters of the HST, there is a clear lithologic change (decrease in carbonate fraction and grain size) across the basal surface. This lithological change reflects a switch in sediment supply from local Apennine rivers (below) to Po- derived mud; this change occurred at the onset of the Little Ice Age, documenting the indirect control of short-term climate change and hu- man impact on sediment architecture. The deformations affecting the late Holocene HST in the various areas show differences in internal geometry, but appear everywhere to be characterized by limited downward displacement and can be attri- buted to shear-dominated retrogressive failure. It is suggested that some degree of consolidation occurred immediately after mobilization, possibly induced by the escape of fluids. Nowhere has failure evolved into disintegration and flow, likely because the type of cyclic loading that triggered it was not prolonged over a long enough interval. Short-lived radionuclides in the uppermost stratigraphic layers, which postdate the failure in the area offshore Ortona, allowed us to quantify systematic changes in sediment accumulation rates as a func- tion of the underlying deformed sea floor. In areas of wavy sea floor, troughs show sediment accumulation rates of greater than 16 mm/yr, a figure that is fourfold the rate measured on the flanks of the troughs.

Sedimentary and tectonic evolution of the Vena del Gesso basin (Northern Apennines, Italy): Implications for the onset of the Messinian salinity crisis

The integration of field and subsurface data permits a substantial revision of the sedimentary evolution of the Vena del Gesso basin, a thrust-top basin in the Northern Apennines where shallow-water primary gypsum deposits related to the Messinian salinity crisis were well developed and preserved. As inferred from lateral and vertical facies changes within the underlying deep-marine turbidites of the Marnoso-arenacea Formation, evaporite precipitation occurred in a basin bounded to the north and to the east by a thrust-related anticline actively growing since the late Tortonian. Both gypsum deposition and subsequent deformation were strongly controlled by evolving paleobathymetry driven by tectonics. Primary gypsum precipitated in a shallow, silled basin, while in the adjacent deeper and larger foredeep basin, organic-rich shales were deposited. Gypsum deposits underwent severe postdepositional deformation related to large-scale gravitational collapse, as a result of a regional uplift event, also coincident with the end of the evaporitic phase. Along the inner, shallower-dipping limb of the anticline bounding the basin, large-scale, poorly deformed gypsum slabs moved downslope along a detachment surface developed at the contact with the underlying euxinic shales, forming both extensional and compressional features and showing an overall southwestern vergence. The identification of a south-southwest–dipping paleoslope, here pointed out for the first time, suggests that the deformational features affecting the gypsum unit were probably driven by gravity and not by active thrusting, as thought up to now. The steeper frontal limb of the anticline promoted the transformation of gypsum slides into debris flows and turbidite currents that deposited their load in the adjacent deeper basin. This gravitational deformation was sealed by postevaporitic upper Messinian Lagomare deposits. The sedimentary history of the Vena del Gesso basin suggests that the Messinian salinity crisis in the Apennine foredeep, as well as in the Balearic, Tyrrhenian, Sicily, and Eastern Mediterranean Basins, was tightly linked to tectonic processes. The large-scale, postdepositional collapse of primary evaporitic deposits is a widespread feature in the Mediterranean basins, and it may have altered the original stratigraphic relationships in some places. This finding has potentially important implications for a correct paleoenvironmental reconstruction of the onset of the Messinian salinity crisis.

Sediment drifts and erosional surfaces in the central Mediterranean: seismic evidence of bottom-current activity

Abstract We present evidence, based on seismic reflection profiles, of sediment drift accumulation and deep submarine erosion driven by bottom currents in different physiographic settings of the central Mediterranean. The study shows that significant bottom-current activity is enhanced by the focussing effects of particular sea-floor morphologies where these interact with the established deep circulation. We cite occurrences from the Corsica Channel, the eastern Tyrrhenian slope basins, the Sicily Channel, the eastern flank of the Apulian swell and the deep Ionian Basin at the foot of the Malta Escarpment. Examples range from asymmetric, convex upwards sediment drift accumulation associated with a lateral moat, as in the Tyrrhenian slope basins or the Sicily Channel (600/800 m depth), to the large-scale bedforms at 2000 m depth in the Ionian Basin. A more complex upslope-migrating sediment drift sequence, bounded by diverse unconformities occurs in the Corsica Channel (300 m depth). The identified features occur in the uppermost portion of the sediment column and commonly control the present-day sea-bottom morphology. Bottom-current-related sedimentary processes seem to be more widespread than previously thought in the Mediterranean. These processes make a significant contribution to sediment accumulation of Quaternary age in marginal basin settings and their potential for generating unconformities of various magnitudes should be taken into account when inferring depositional histories.

Ribosomal RNA gene fragments from fossilized cyanobacteria identified in primary gypsum from the late Miocene, Italy

Earth scientists have searched for signs of microscopic life in ancient samples of permafrost, ice, deep-sea sediments, amber, salt and chert. Until now, evidence of cyanobacteria has not been reported in any studies of ancient DNA older than a few thousand years. Here, we investigate morphologically, biochemically and genetically primary evaporites deposited in situ during the late Miocene (Messinian) Salinity Crisis from the north-eastern Apennines of Italy. The evaporites contain fossilized bacterial structures having identical morphological forms as modern microbes. We successfully extracted and amplified genetic material belonging to ancient cyanobacteria from gypsum crystals dating back to 5.910-5.816 Ma, when the Mediterranean became a giant hypersaline brine pool. This finding represents the oldest ancient cyanobacterial DNA to date. Our clone library and its phylogenetic comparison with present cyanobacterial populations point to a marine origin for the depositional basin. This investigation opens the possibility of including fossil cyanobacterial DNA into the palaeo-reconstruction of various environments and could also be used to quantify the ecological importance of cyanobacteria through geological time. These genetic markers serve as biosignatures providing important clues about ancient life and begin a new discussion concerning the debate on the origin of late Miocene evaporites in the Mediterranean.

Dense shelf water cascading and Messinian Canyons: A new scenario for the Mediterranean salinity crisis

The salt giant beneath the deep Mediterranean seafloor is the impressive record of the “Messinian salinity crisis,” a dramatic event that occurred about 6 Ma ago following the reduction of the connections with the Atlantic Ocean. According to the shallow-water deep-basin model, developed for these deposits (Hsü and others, 1973a, 1973b, 1978a, 1978b), the Messinian evaporites formed in a deep but desiccated Mediterranean, while shelves and slopes underwent subaerial erosion due to fluvial rejuvenation triggered by a 1500 m sea level drawdown. Deeply incised Messinian canyons in the continental slopes surrounding the Mediterranean are the main argument supporting this scenario. Using a state of the art model and idealized but realistic numerical simulations, here we demonstrate that the activation of downslope flows of hypersaline, dense waters, in a process similar to present-day “dense shelf water cascading,” but much more energetic, may account for both slope erosion and progressive salinity rise leading to the formation of deep-seated supersaturated brines. Our findings support a deep-water deep-basin model (Schmalz, 1969, 1991; De Benedetti, 1976, 1982; Dietz and Woodhouse, 1988), thus implying that evaporite deposition may have occurred in a non-desiccated basin with strongly reduced ocean connections.

High-Frequency Cyclicity in the Mediterranean Messinian Evaporites: Evidence for Solar–Lunar Climate Forcing

Abstract The deposition of varved sedimentary sequences is usually controlled by climate conditions. The study of two late Miocene evaporite successions (one halite and the other gypsum) consisting of annual varves has been carried out to reconstruct the paleoclimatic and paleoenvironmental conditions existing during the acme of the Messinian salinity crisis, ~ 6 Ma, when thick evaporite deposits accumulated on the floor of the Mediterranean basin. Spectral analyses of these varved evaporitic successions reveal significant periodicity peaks at around 3–5, 9, 11–13, 20–27 and 50–100 yr. A comparison with modern precipitation data in the western Mediterranean shows that during the acme of the Messinian salinity crisis the climate was not in a permanent evaporitic stage, but in a dynamic situation where evaporite deposition was controlled by quasi-periodic climate oscillations with similarity to modern analogs including Quasi-Biennial Oscillation, El Nino Southern Oscillation, and decadal to secular lunar- and solar-induced cycles. Particularly we found a significant quasi-decadal oscillation with a prominent 9-year peak that is commonly also found in modern temperature records and is present in the contemporary Atlantic Multidecadal Oscillation (AMO) index and Pacific Decadal Oscillation (PDO) index. These cyclicities are common to both ancient and modern climate records because they can be associated with solar and solar-lunar tidal cycles. During the Messinian the Mediterranean basin as well as the global ocean were characterized by different configurations than at present, in terms of continent distribution, ocean size, geography, hydrological connections, and ice-sheet volumes. The recognition of modern-style climate oscillations during the Messinian suggests that, although local geographic factors acted as pre-conditioning factors turning the Mediterranean Sea into a giant brine pool, external climate forcings, regulated by solar–lunar cycles and largely independent from local geographic factors, modulated the deposition of the evaporites.

Did Late Miocene (Messinian) gypsum precipitate from evaporated marine brines? Insights from the Piedmont Basin (Italy)

During the first stage of the Late Miocene Messinian salinity crisis (5.97–5.60 Ma), deposition of sulfates (the Primary Lower Gypsum) occurred in shallow silled peripheral subbasins of the Mediterranean undergoing restricted water exchange with the Atlantic Ocean. Fluid inclusions in Messinian selenite crystals from the Piedmont Basin (northwest Italy) have surprisingly low salinities (average of 1.6 wt% NaCl equivalent), suggesting that parent waters were depleted in Na + and Cl – compared to modern seawater. Modern gypsum from a Mediterranean salt work, in contrast, contains fluid inclusions with elevated salinities that match the normal evaporation trend expected for seawater. The salinity data indicate that the Messinian sulfate deposits from the Piedmont Basin formed from hybrid parent waters: seawater mixed with Ca 2+ and SO 4 2– enriched freshwaters that dissolved coeval marginal marine gypsum. Such mixed parent waters and complex recycling processes should be taken into account when explaining the genesis of other Messinian gypsum deposits across the Mediterranean Basin.

The Messinian ‘Vena del Gesso’ evaporites revisited: characterization of isotopic composition and organic matter

Abstract The ‘Vena del Gesso’ (Gessoso-Solfifera Fm, Messinian) is a 227 m-thick ridge along the western Romagna Apennines (Italy) consisting of up to 16 selenite cycles separated by shales and minor carbonate. The total organic carbon values of these deposits range between 0.087–0.016% (gypsum) and 3% (shales). Organic matter is dominated by black debris associated with continental debris. Algae and dynocysts are rare (<1%). The amount of amorphous organic matter is low but it may reach up to c. 40%. The 87Sr/86Sr of gypsum and carbonate vary from 0.708890 to 0.709024, yielding non-oceanic values with several exceptions that plot within error of coeval oceanic values only in the upper part of the section (from the 6° bed). The sulphur isotope composition of gypsum range between δ34S =+21.8 and +23.7‰ and may represent precipitation of δ34S-enriched gypsum due to the fractionation effect or recycling of coeval gypsum with contributions of brine-sediment redox variations. The isotope values of carbonates show a large variability (−6.4<δ18O<+6.05‰; −14.68<δ13C<+2.5‰), suggesting a complex origin by mixing of marine and non-marine waters with a significant contribution of reduced organic matter. These data point to an evaporite basin dominated by continental waters which received significant phases of marine recharge in the upper part together with a marked facies change. Because seawater recharges and a similar facies change are present in other Messinian sections, it follows that we have new possible geochemical and facies markers to correlate the Lower Evaporites across the Mediterranean.

The Messinian salinity crisis: open problems and possible implications for Mediterranean petroleum systems

A general agreement on what actually happened during the Messinian salinity crisis (MSC) has been reached in the minds of most geologists but, in the deepest settings of the Mediterranean Basin, the picture is still far from being finalized and several different scenarios for the crisis have been proposed, with different significant implications for hydrocarbon exploration. The currently accepted MSC paradigm of the ‘shallow-water deep-basin’ model, which implies high-amplitude sea-level oscillations (> 1500 m) of the Mediterranean up to its desiccation, is usually considered as fact. As a consequence, it is on this model that the implications of the MSC events on the Mediterranean petroleum systems are commonly based. In fact, an alternative, deep-water, non-desiccated scenario of the MSC is possible: it (i) implies the permanence of a large water body in the Mediterranean throughout the entire Messinian salinity crisis, but with strongly reduced Atlantic connections; and (ii) envisages a genetic link between Messinian erosion of the Mediterranean margins and deep brine development. In this work, we focus on the strong implications of an assessment of the petroleum systems of the Mediterranean and adjoining areas (e.g. the Black Sea Basin) that can be based on such a non-desiccated MSC scenario. In particular, the near-full basin model delivers a more realistic definition of Messinian source-rock generation and distribution, as well as of the magnitude of water-unloading processes and their effects on hydrocarbon accumulation.