Maria Bianca Cita

Paleoclimatic record of a long deep sea core from the eastern Mediterranean

Abstract A deep-sea core over 16 m long from the crestal area of the Mediterranean Ridge has been investigated with different techniques, including quantitative micropaleontology, stable isotopes (measured on the epipelagic species Globigerinoides ruber and on the mesopelagic species Globorotalia inflata), and clay mineralogy. The resulting record of climatic fluctuations can be cross correlated to other Mediterranean cores by means of isochronous lithologies (tephra layers and sapropels). The climatic record of the Mediterranean is similar in character, phase, and chronology to the records investigated in the equatorial Pacific and in the Caribbean. Isotope stages 1 to 17 have been recognized. Cyclically repeated stagnant cycles resulting in sapropel deposition complicate both the isotopic and the faunal signal. The isotopic investigations reveal that the temperature change in the surface layers of the eastern Mediterranean was no greater than 8°C in the late “glacial” Pleistocene. The chronostratigraphic and biostratigraphic interpretation of Core KS09 indicate that the mean sedimentation rate was 2.4 cm/1000 years, a value very close to the 2.5 cm/1000 years calculated for the entire Quaternary section at DSDP Site 125, also located in the crestal area of the Mediterranean Ridge in the Ionian Basin. The base of KS09 is likely to be very close to the Brunhes/Matuyama boundary dated at 0.7 my.

The nature and distribution of Messinian erosional surfaces — Indicators of a several-kilometer-deep Mediterranean in the Miocene☆

Abstract The deep-basin desiccation model for the origin of Messinian-age Mediterranean evaporites implies two basic assumptions: 1. (1) The evaporites were in part laid down under shallow-water conditions, with intermittent subaerial episodes brought about by evaporitic drawdown. 2. (2) Some of the depressions in which the evaporites were deposited were already several kilometers deep during Messinian time, and hence, the general configuration of these basins predated, and was essentially independent of the salinity crisis. The first assumption has been more or less accepted by the scientific community as a consequence of the discovery of supratidal to intertidal features in the evaporites, whereas there has been less acceptance of the second assumption. The arguments on which the second assumption was first formulated, were multiple and included: (a) the presence in seismic reflection profiles of large pre-salinity-crisis depocenters with sediment-distribution patterns not substantially different from those of the post-salinity-crisis strata; (b) systematic changes in the evaporitic facies along basin-slope transects indicative of existing paleorelief; and (c) the occurrence of continental-rise and abyssal-plain-type deposits in both pre- and post-salinity-crisis successions. New attention is herein focused on Messinian-age erosional surfaces created by evaporitic drawdown of an isolated Mediterranean Sea. These erosional surfaces are detected as discordances in seismic reflection profiles. The discordances can be traced from the subsurface of the present coastal plains and continental shelves to the subsurface of the modern abyssal plains and show entrenchment into the underlying strata. On the passive-type continental margins, like those of the Balearic Sea and those in the southeastern corner of the Levantine Sea, the seaward gradients of the erosion surfaces, corrected for post-Messinian isostatic sediment loading and compaction and adjusted for regional subsidence, permit the calculation of a former relief between the pre-salinity-crisis shore-line and basin center of more than 2.5 km for the western Mediterranean and more than 3.0 km for the eastern Mediterranean.

Tsunami-induced sediment transport in the abyssal Mediterranean Sea

An unusual stratigraphic unit (nicknamed “homogenite”) fills topographic lows in the complex ridge and trough bathymetry at two survey sites on the Western Mediterranean Ridge and the Calabrian Ridge. On near-bottom 4-kHz seismic-reflection profiles, this unit us an acoustically transparent, near-surface, flat-lying layer, whereas in cores, it is a homogeneous gray marl as much as 7 m thick. Grain size decreases upcore within the unit, implying that it was deposited in a single event controlled by gravitational settling. The stratigraphic position of the homogenite relative to a firmly dated sapropel bed suggests emplacement between 4400 and 3100 yr B.P. The source of the homogenite is inferred to be the nearby basin walls. Farther east, two other sites with similar rugged topography lack homogenite entirely. A triggering mechanism is required which is capable of initiating massive sediment transport simultaneously in many separate basins at the western two survey sites, but which is not effective at the eastern sites. A large archeologically recorded earthquake of the correct age is considered and rejected because its epicenter is closer to the nonhomogenite-bearing sites than to the sites where this sediment type was observed, and because several other earthquakes of comparable magnitude have since been recorded in the area, whereas the homogenite is unique. The 3,500 yr B.P. collapse of the caldera of the volcano of Santorini caused a huge tsunami which is recorded archeologically and geologically around the eastern Mediterranean. Because of refraction of the tsunami by the bathymetry, and because the caldera collapsed in its southwest corner, a disproportionate amount of tsunami energy was directed toward the western area where homogenite is observed. In contrast, the homogenite-free sites were relatively sheltered. An order-of-magnitude calculation shows that the near-bottom oscillating currents accompanying the Santorini tsunami were at or above the threshold erosion velocity at the homogenite-bearing sites. In addition, the near-bottom pressure pulse under the tsunami at the homogenite-bearing sites was sufficient to cause liquefaction of sediments. Neither mechanism was adequate to cause sediment transport or slope failure at the homogenite-free sites.

Ignorance concerning episodes of ocean-wide stagnation

Abstract Episodes of basin-wide abyssal stagnation have occurred in the Mediterranean Sea during the “glacial” Pleistocene and on a much larger scale in the Atlantic and Indian Oceans during the Cretaceous Period. The sedimentary products of euxinification are organic-rich sapropels which have accumulated an order of magnitude more carbon during the Cretaceous than that which is present in all the known world reserves of coal and petroleum. The storage in the Cretaceous strata of excess carbon and sulfur in the form of fossilized photosynthetic substances and pyrite is thought to have led to a significant global increase in atmospheric oxygen and to an unusual sequence of calcium-rich evaporitic salts in the South Atlantic during the Aptian. Stagnant episodes in the Mediterranean and Atlantic effectively destroyed all benthic life formerly existing on substratums underlying hydrogen-sulfide bearing anoxic bottom waters. Locally thin organic-rich strata on topographic highs near the paleo-equator of the Pacific Ocean during the Cretaceous owe their origin to an intermittently expanding oxygen-minimum zone as contrasted to total euxinification within the deep basins of the Atlantic and Indian Oceans. In the context of both land and sea areas, the late Mesozoic may have conceivably been the major “carboniferous” period of the earth since the end of the Precambrian.

Geological evidence for mud diapirism on the Mediterranean Ridge accretionary complex

Abstract A mud breccia has been repeatedly cored from four mud diapir fields located on the crest of the Mediterranean Ridge. This breccia is composed of subrounded clasts supported by a clay-rich matrix and ranges in age from Cretaceous to early Miocene. Sedimentological analysis allows identification of a “primitive” intrusive and a “reworked” mud breccia, the latter being formed by mud extrusion and gravitational reworking of the former. Homogeneous mud, sorted by grain size, which occurs at the top of the diapirs has been interpreted as a lake deposit. Based on composition and carbon isotopic ratios, gases present within the diapiric material suggest a partial thermogenic origin, and gas escape structures have been identified in the mud breccia and host sediments. Carbonate crusts occurring in the host sediments are not related to bacterial oxidation of methane and consequent authigenic calcite precipitation. Their carbon and oxygen isotopic ratios reveal that they are characteristic of early deep-sea lithification in the Mediterranean Sea. The presence of a marker bed composed of Mn nodules and bacterial colonies, the mud lake deposits showing lower carbonate content than primitive mud breccia, and the gas escape structures suggest the presence of active fluid vents in the vicinity of the diapiric structures. Evidence of active mud diapirism can be found from about 300,000 yrs B.P. to the present.

Geophysical evidences of mud diapirism on the Mediterranean Ridge Accretionary Complex

Mud volcanoes, mud cones, and mud ridges have been identified on the inner portion of the crestal area, and possibly on the inner escarpment, of the Mediterranean Ridge accretionary complex. Four areas containing one or more mud diapirs have been investigated through bathymetric profiling, single channel seismic reflection profiling, heat flow measurements, and coring. A sequence of events is identified in the evolution of the mud diapirs: initially the expulsion on the seafloor of gasrich mud produces a seafloor depression outlined in the seismic record by downward dip of the host sediment reflectors towards the mud conduit; subsequent eruptions of fluid mud may create a flat topped mud volcano with step-like profile; finally, the intrusion of viscous mud produces a mud cone.The origin of the diapirs is deep within the Mediterranean Ridge. Although a minimum depth of about 400 m below the seafloor has been computed from the hydrostatic balance between the diapiric sediments and the host sediments, a maximum depth, suggested by geometric considerations, ranges between 5.3 and 7 km. The presence of thermogenic gas in the diapiric sediments suggests a better constrained origin depth of at least 2.2 km.The heat flow measured within the Olimpi mud diapir field and along a transect orthogonal to the diapiric field is low, ranging between 16 ± 5 and 41 ± 6 mW m−2. Due to the presence of gas, the thermal conductivity of the diapiric sediments is lower than that of the host hemipelagic oozes (0.6–0.9 and 1.0–1.15 W m−1 K−1 respectively).We consider the distribution of mud diapirs to be controlled by the presence of tectonic features such as reverse faults or thrusts (inner escarpment) that develop where the thickness of the Late Miocene evaporites appears to be minimum. An upward migration through time of the position of the décollement within the stratigraphic column from the Upper Oligocene (diapiric sediments) to the Upper Miocene (present position) is identified.

Towards a quaternary time scale

Abstract Nine first-appearance datums (FADs), twenty-three last-appearance datums (LADs), and three other micropaleontological datums are related to the magnetic-reversal, oxygen-isotope, and calcite-dissolution/coarse-fraction time scales to provide a preliminary basis for subdivision of the Quaternary in deep-sea sediments. The magnetic-reversal, oxygen-isotope, and calcite-dissolution/coarse-fraction scales have been correlated by determination on the same core materials, and absolute dates applied by 40 K 40 Ar or 14C dating of materials in known positions on one or another of these scales. FADS and LADs have been determined in cores for which either a magnetic-reversal, oxygen-isotope, or calcite-dissolution/coarse-fraction scale has also been available. Altogether 3 FADs and 5 LADs based on diatoms, 4 FADs and 5 LADs based on calcareous nannoplankton, 1 FAD and 8 LADs based on radiolarians, 1 FAD and 5 LADs based on planktonic foraminifers, 2 acme datums, and 1 ratio reversal datum have been determined, and absolute dates inferred by interpolation from known dates on the reference time scales. Some of the FADs and LADs apply or are synchronous only over limited areas of the oceans; others appear to be synchronous throughout the oceans. The base of the Quaternary is set at the top of the Olduvai event at 1.7 my. Four FADs, twelve LADs, two acme datums, and one ratio reversal datum occur above the base of the Quaternary at an average rate of about 1 per 100,000 yr. Five FADs and twelve LADs are recognized in the 0.8-my interval between the top of the Olduvai event and the Gauss/Matuyama Boundary at 2.5 my at an average incidence of about 1 per 50,000 yr.

Deep-sea tsunami deposits in the eastern Mediterranean: New evidence and depositional models

The tsunami wave induced by the collapse of the Santorini caldera after the Bronze age (Minoan) eruption (3500 BP) produced turbidites and large volume mega-turbidites in the abyssal plains of the Ionian Sea as well as on the floor of small basins of the Mediterranean and Calabrian Ridges, characterized by the so-called ‘Cobblestone Topography’. Since the first discovery in 1978, a Holocene mud layer which has been termed ‘homogenite’ and which typically shows a graded interval at its base, has been identified and correlated in over 50 deep-sea cores recovered in the eastern Mediterranean. Four types of ‘homogenite’ can be distinguished, each related to a particular depositional setting: 1. (a) ‘Closed Cobblestone’: these are from a few decimetres to several metres thick pelagic turbidites of local provenance, exclusively found at the bottom of small-sized ponded basins of the Mediterranean and Calabrian ridges. A debris flow may be present at the base of the turbidite where the vertical relief of the basins is over 200 m. 2. (b) ‘Abyssal Plain’: a 10 to 20-m thick megaturbidite recorded in the Ionian and Sirte abyssal plains. The source area is the African continental margin, possibly the continental shelf; it can be easily identified as a transparent acoustic layer that shows recent deformation across the deformation front of the Mediterranean Ridge accretionary prism. The volume of the Minoan ‘homogenite’ in the Ionian Abyssal Plain has been calculated at a minimum of 11 km3. 3. (c) ‘Open Cobblestone’: this type is exclusively found on the outer slope of the Mediterranean Ridge, near the deformation front. Unlike types (a) and (b), the base of the turbidite here is erosional instead of depositional. This ‘homogenite’ has been deposited in small basins, and occasionally on topographic highs of the Mediterranean Ridge by the up-slope flow of turbidity currents of African provenance that formed the abyssal plain deposit (type b). 4. (d) A fourth depositional model has been identified: homogenites present in deep anoxic basins of the Mediterranean Ridge. This setting is substantially similar to type (a), but the vertical relief of the basin is much higher (up to 800 m) and the deposition occurs in high-density anoxic brines which modify the settling rate and hence the resulting sedimentological characters.

The Mediterranean Ridge and related mud diapirism: a background

Abstract The Mediterranean Ridge, stretching from the Calabrian Rise to the Florence Rise, is the largest structural unit of the Eastern Mediterranean Sea. It is directly related to ongoing convergence and collision of the African and Eurasian plates, starting in the Oligocene, and is considered to be a giant accretionary complex consisting of intensively folded and faulted rocks of the African margin. Since its origin in the late Miocene, the Ridge continued to grow up and outward, experiencing more deformation because of the developing collision. The mud diapirism and mud volcanism are usual and wide-spread phenomena for the Mediterranean Ridge that developed as a result of an intensive tectonic overburden due to stacking of rock units by thrusting and strong lateral compressional stress pressing up and squeezing plastic sedimentary series out onto the seafloor.

Geological evidence concerning compressional tectonics in the eastern mediterranean

Abstract Data from the 1978 “Cobblestone Project” Deep Tow surveys and associated intensive sampling programs have been combined with published data to address the question: “Are the Mediterranean- and Calabrian Ridge convergent plate margin accretionary prisms?”. A dome-shaped feature on the crest of the Mediterranean Ridge was found to be made of sticky and watery mud breccia of Early Cretaceous age. Its circular plan view, concentric surface deformation, old but uniform-aged clasts and matrix, diachronous Quaternary veneer, brecciated texture and plastic matrix suggest a piercement structure, but the total lack of evaporitic or dolomitic lithologies, the thinness of both Messinian evaporites and Plio-Quaternary cover in the observation area and the great age and presumably great depth of the Cretaceous mother bed, rule out simple salt diapirism. We suggest instead that the diapir rose from the overpressured zone at the base of a deep seated overthrust, where grinding and injection of water shattered a low permeability shale layer and reduced its density and effective viscosity. The perimeters of the Calabrian- and Mediterranean Ridge are marked by an abrupt transition (“deformation front”) from flat abyssal plains to ridge-and-trough morphology. The deformation front apparently migrates outward with time and is the site of uplift and folding of abyssal plain sediments: A search for analogous “deformation fronts” in better understood convergent plate margins revealed a very close analogy with the toe of the Curacao- and Barbados Ridge which are interpreted as classic accretionary wedges. The analogy is further supported in the Calabrian Ridge case by the observation of an arcward-dipping reflector, which may be the detachment surface, beneath a thickening wedge of Messinian evaporites. Extension of the analogy requires that the outcrop of the subduction zone or the site of initiation of decollement be located external to the Mediterranean Ridge and not in the Hellenic Trough. Our surveys showed that (except possibly along the recently deformed outermost perimeter of the deformation front) the surface of both ridges is extensively modified by the secondary processes of diapirism, dissolution, mass-wasting and downhill gliding. Thus the fine-scale surface morphology alone, as revealed by Seabeam, Deep-Tow or GLORIA, may be a poor or misleading indicator of deep crustal processes in this region.