Richard H. Benson

An important current reversal (influx) in the Rifian Corridor (Morocco) at the Tortonian‐Messinian boundary: The end of Tethys Ocean

Atlantic psychrospheric and temperate mesopelagic faunas found in the lower Messinian marls in Morocco indicate that a strong, eastward flowing, bottom current was present in the Rifian Corridor before the Salinity Crisis. This influx began just before diatomite deposition in the Paleo-Mediterranean, continued during a decrease in species diversity in coral reef formation, and diminished with the initial stages of “brine” concentration in the deep-water phase of the Crisis. The influx is most readily studied in a condensed section of marl in the Bou Regreg valley near Rabat. The beginning of this “siphon event” coincides with the Tortonian/Messinian boundary (6.4 Ma, subchron 6N1). It is identified by (1) a change in the planktonic foraminifera from dominance of warm, tropical, epipelagic Globorotalia menardii with Globigerinoides to the temperate, mesopelagic Gl. miotumida plexus with conomiozea; (2) the sudden appearance of an upper psychrospheric ostracode fauna with Agrenocythere pliocenica and Oblitacythereis ruggierii, (3) a change in nannoflora; and (4) beginning of the 6.3 Ma Global Carbon Shift. The initial strong influx stage of the siphon lasted at least 0.7 m.y., decreasing after the middle of Chron 5, ca. 5.7, to be lost ca. 5.3 Ma. Conditions for the siphon formed when the continental climate created a deficit in the water budget of the Paleo- Mediterranean Sea. The reversal took place when tectonic movement in the foredeeps of the Betic- Rif Orogene changed the thresholds of the twin straits, the Rifian Corridor and the Iberian Portal. Inflow increased rapidly in the southern Corridor to draw in waters from beneath the rising Atlantic pycnocline, while Paleo-Mediterranean Overflow Water (PMOW) continued out of the northern Iberian Portal. The invasion of “nappes” or olistoliths, first into the portal and then into the corridor, led to the end of the outflow of the PMOW terminating the need for the siphon, and then to the isolation of the Paleo-Mediterranean.

Magnetostratigraphic, biostratigraphic, and stable isotope stratigraphy of an Upper Miocene drill core from the Salé Briqueterie (northwestern Morocco): A high-resolution chronology for the Messinian stage

We report a high-resolution stable isotope, carbonate, magnetostratigraphic, and biostratigraphic record from a 175-m drill core from the Sale Briqueterie, which is part of the Bou Regreg section in northwestern Morocco. The Sale drill core spans the interval from paleomagnetic Chron C4n partim to C3r (earliest Gilbert), which represents the time leading up to and including the isolation and desiccation of the Mediterranean (i.e., the Messinian salinity crisis). During Chrons C3An and C3Ar (6.935 to 5.894 Ma) the isotope and carbonate signals display quasi-periodic variations with estimated periods of 40 and 100 kyr, respectively. We interpret the 40-kyr δ18O variations as reflecting changes in global ice volume caused by obliquity-induced changes (41 kyr) in solar insolation in polar regions. The 100-kyr carbonate variations probably represent long-term modulation of the amplitude of the precessional cycle (∼21 kyr), which is not resolved by our sampling frequency. The cyclic nature of the oxygen isotope signal permits us to extend the isotope nomenclature of Shackleton et al. (1994a) from stage TG24 in Chron C3r (earliest Gilbert) to stage C3Ar.δ18O.18 at the base of Chron C3Ar (6.935 Ma). A major change in paleoceanographic conditions is recorded across the Tortonian/Messinian boundary, which we correlate to Chron C3Bn at 7.04 Ma. Benthic foraminiferal δ18O values increased by an average of 0.4‰ in two steps at 7.17 Ma and 6.8 Ma and δ13C values decreased by 0.7–0.8‰ between 7.1 and 6.8 Ma, representing the late Miocene carbon shift. The first step in δ18O values coincides with an inferred reversal in deep water circulation through the Rifian Corridor, and the second correlates with the base of the Tripoli Formation and onset of “crisis conditions” in the Mediterranean. We suggest that the increase in δ18O values represents, at least in part, an increase in global ice volume that lowered sea level and contributed to the establishment of a negative water budget in the Mediterranean. Average δ18O values remained high throughout most of Chrons C3Ar and C3An, reaching maximum δ18O values during isotope stages TG20 and 22 in Chron C3r (earliest Gilbert). The glacio-eustatic falls associated with these events may have resulted in the complete isolation of the Mediterranean from the world ocean (Shackleton et al., 1994a). Following stage TG12 in the Sale record, there exists a trend toward progressively lower δ18O values that may represent a series of marine transgressions that eventually reflooded the Mediterranean and ended the Salinity Crisis.

Eustatic implications of late Miocene depositional sequences in the Melilla Basin, northeastern Morocco

Abstract The age (∼5.78 Ma or lower chron C3r) of the major drawdown of the Paleo-Mediterranean Sea during the Messinian Salinity Crisis has been established by combining results from stratigraphy, paleontology, magnetostratigraphy, and argon dating for a late Miocene sedimentary succession in the Melilla Basin, NE Morocco. This event is inferred from a marine-to-continental series of carbonate and siliciclastic rocks that record the end of Messinian marine deposition in the Melilla Basin and presumably marks the final isolation of the Paleo-Mediterranean Sea. The evidence from the Melilla Basin is approximately coeval with an increase in benthic foraminiferal δ 18 O values from a deep-marine section in the Bou Regreg valley, NW Morocco (Hodell et al., 1994). This increase suggests that a glacio-eustatic lowering of sea level, at least, contributed to the final closure of the Mediterranean during the Messinian Salinity Crisis. The marine-to-continental succession onlaps a carbonate complex that contains evidence for multiple relative sea-level changes leading up to the main drawdown. From bottom to top, the carbonate complex is composed of: (1) an onlapping ramp; (2) a prograding bioclastic platform; (3) a prograding and, locally, downstepping Porites -reef complex; and (4) a topography-draping sequence composed of grainstones, Porites reefs, and stromatolites (terminal carbonate complex of Esteban, 1979). The transgressive ramp correlates to relatively low values of benthic foraminiferal δ 18 O values from a Tortonian-to-lower Messinian section at Bou Regreg (Hodell et al., 1994). This correlation indicates, at least in part, a link between rising sea level and a reduction in global ice volume during deposition of the ramp. A major fall in relative sea level (∼60 m) occurred near the demise of the reef complex during chron C3n.1n at 5.95 ± 0.10 Ma. This signals the initiation of drawdown and changing environmental conditions in the Melilla Basin (a marginal basin), and perhaps the entire Paleo-Mediterranean Sea. A megabreccia interpreted as forming by solution collapse of evaporites on the basin margin of the reef complex occurs at the base of the terminal carbonate complex. Updip, a major subaerial unconformity separates the reef complex and terminal carbonate complex. Evaporite deposition likely occurred during this exposure event and has been dated at 5.82 ± 0.02 Ma near the base of chron C3r. We contend that these evaporites, restricted to the shallow Melilla Basin, are related to the continuation of the initial stage of the major drawdown of the Paleo-Mediterranean Sea.

Changes in the ostracodes of the Mediterranean with the Messinian salinity crisis

Abstract The likelihood of an environmental “crisis” in the evolution of the Tethys Ocean-Sea to the Mediterranean Ocean-Sea during Late Miocene is examined and tested against the present knowledge of the ostracode fossil record in Italy, Sicily, Crete, northern Tunisia, southern Spain and from some of the DSDP cores of the deep regions of the Mediterranean, especially in the Balearic Basin. It is concluded that there has been a substantial evolutionary and ecological change in the ostracode fauna that occurred at the time of the crisis. At the present time the best explanation available is that of isolation of deep basins from the World Ocean accompanied by extreme environmental change. The sudden oceanic “normalization” th that followed the “crisis” was perhaps just as extreme, but because of the buffering effects of the re-establishment of marine conditions in the newly formed Mediterranean, the shallower ostracode faunas coming from the Atlantic were not greatly different from those of westernmost Tethys.

Biodynamics, saline giants and late miocene catastrophism

The Messinian Salinity Crisis is several crises that fit the context of catastrophic modeling of the history of a Mediterranean “saline giant.” Of the explanations available, we prefer the deep-basin/deep-water precipitation model for the early, deep-basin, massive gypsums and halites, contemporaneous with marginal, lagoonal gypsums; and the deep-basin/shallow-water desiccation theory to explain the later, shallow cyclic gypsums with associatedlago mare faunas. The western basins before and after the Crisiswere deep enough to maintain intermediate oceanic water-masses, which is not possible today because of shallow thresholds. Early Pliocene mean paleodepths of the western basins were at least in the order of than 1500 m, which is the mean depth of the present Mediterranean. Tectonic changes in the threshold to the Atlantic were responsible for the isolation of the Paleo-Mediterranean during a eustatic high stand, followed by regression consequent to the evaporite precipitation phases.The context of Messinian stratigraphy has changed to include more events leading up to the Crisis. It began with evaporite deposition in the eastern basins during the Tortonian, and served to concentrate saline bottom-waters that later overflowed across the Pelagian Sea toward the west. In the western basins, it started at 6.4 Ma with a general water-budget deficit that altered the inflow over the threshold in the Rifian Corridor. A time of unusually high organic productivity and water-mass stratification followed, contemporaneous with evaporite deposition in some of the marginal seas and lagoons.Massive gypsum and halite deposition took place in the deep western basin in Chron 5 between 5.4 and 5.3 Ma. Our studies in Morocco and Spain indicate that passages to the Atlantic were closed during the cyclic gypsum and “lago mare” deposition, which took place during the long negative polarity reversal of Chron Gilbert from 5.3 to 4.9 Ma. We doubt that there were multiple marine inundations from the Atlantic into the Mediterranean after the first drawdown. The Rifian Corridor was closed by olistostromes at about 5.5 Ma. The last “leak” through the Gibraltar Barrier probably occurred in a passage through Oued Laou in northern Morocco at 5.4 Ma. The Oued Laou valley, which now forms a gorge through the Rif Mountains, could have furnished the influx of enough marine water in 27,000 years to furnish the nearly kilometer thick salt deposits of the deep Mediterranen basins.Gypsum deposits associated with shallow Paratethyan caspi-brackish (less than 8 o/oo salinity) ostracode and mollusk faunas, in the lower parts of both the Tyrrhenian and Algero-Provencal Basins, show that water depths in the deep basins changed from more than 1000 m to in the order of 100 m or less. However, faunal alternations typical of playa deposition are missing. The chemical composition of both gypsum and fossil shells indicate significant meteoric water, signaling a “fresh-water crisis.” It is probable that phreatic, artesian, saline waters nourished a great sea-lake in the west. The Salinity Crisis is ended by the catastrophic flooding of all of the Mediterranean basins by deep-sea waters from the Atlantic.

Estimating greater paleodepths with ostracodes, especially in past thermospheric oceans

Abstract Modern ostracodes show significant morphologic and taxonomic changes with increasing depth. Deep psychrospheric faunas are less diverse and more cosmopolitan than shallow faunas. Individuals are larger, rarer, and blind. Their carapaces seem better designed relative to the mass of construction material used. Speciation is slower. Taxa are older, longer ranging and for the most part now confined to abyssal and bathyal environments. Deep thermospheric ostracodes found in pre-psychrospheric oceans or isolated basins are different. Their deep origin seems related to the ability of shelf species to submerge past what is a formidable thermal barrier. To test morphologic changes that reflect faunal submergence in thermospheric oceans such as existed in the Atlantic before about 40 m.y. B.P., the basis for an adaptive structural theory is explored. This leads to a search for limits expressive of rapid change both in the structure of morphology and in water-mass structure. Examination of the ostracodes from core samples of Cretaceous age from nine D.S.D.P. Sites in the South Atlantic suggests that the Rio Grande Rise-Walvis Ridge aseismic system has both been a water-mass barrier and yet remained deep (at least mid-bathyal) over the last 100 m.y. No evidence of either shallow ostracodes from the shelves of Africa, South America or those of islands were found indicating that models proposed previously that there would have been an archipelago or shallow submerged causeway across the Rio Grande Rise-Walvis Ridge during the Late Cretaceous are probably in error.

Extending the climatic precession curve back into the Late Miocene by signature template comparison

The rhythm of sedimentary cycles reflecting the climatic precession signal (19 and 23 kyr components) from the upper Miocene of the Bou Regreg section at Ain el Beida near Rabat, Morocco, had been analyzed to determine the age of the geomagnetic polarity reversals of Subchron 5N1 (C3An.1n). A new method of analysis of cycle “signatures” uses adjusted data series as represented by a signal obtained from grey-level traces through image-enhanced photographs of the sediment cycles. The measured, “timeless” signal is reduced and geometrically transformed to a signature template in order to compare its pattern with similar time constrained segments of the target precession signal (retrodicted for 35° N). Conventional statistical and spectral analysis tests are used to regress to the best possible fit. A date estimate of 5.94 Ma for the Chron 5/Gilbert boundary (C3An.1n/C3r), which occurs near the times of the closure of the Rifian Corridor and the beginning of the “drawdown” phase of the Messinian Salinity Crisis, is found to be in close agreement with independent approximations derived from extrapolation of sea floor spreading, radiometric dating, and younger Earth-orbital tuning ages.

The El Cuervo section (Andalusia, Spain): Micropaleontologic anatomy of an early Late Miocene lower bathyal deposit

Abstract An examination of its calcareous and siliceous microfauna and microflora indicates that the El Cuervo section (Andalusia, Spain) is a lower bathyal deposit (formed between 1000 and 1500 m in a zone of near-coastal upwelling) of early Late Miocene age, penecontemporaneous with the lower part of the stratotype Andalusian Stage and equivalent to, at least, a part of Magnetic Epochs 10 and/or 11. The bottom depths where the benthonic fauna (foraminifera and ostracodes) lived at the time of deposition were well beneath the boundary between the upper warm mixing zone (thermosphere) of the Atlantic and Tethys and the lower permanently cold layers (psychrosphere) originating only in the Atlantic. Mixing of the deep faunas of Tethys and the Atlantic is indicated. The El Cuervo fauna is part of an important sequence of sections in which the benthonic fauna indicates a deep passage (Iberian Portal) from the Atlantic into Tethys in southern Spain. The significance of this section on the Late Miocene closure of the Iberian Portal is discussed.

Miocene deep-sea ostracodes of the Iberian Portal and the Balearic Basin

Abstract Discovery of deep-sea (psychrospheric) ostracodes in the Middle Miocene (Serravallian and Tortonian) of Andalusia, Spain, and in the cores of Site 372 of DSDP Leg 42a in the Balearic Basin east of Menorca indicates the presence of an opening (the Iberian Portal) between the Atlantic and the western Tethys basin complex at a depth estimated at more than 2000 m. Because of the concurrence of the ostracodes along a structural line marking the Prebetic and Subbetic boundary with the Meseta Massif, it is inferred and the rapidly shallowing ostracode faunas suggest, that this opening represents a structural trough closed by Alpine-type lateral movements during the Late Miocene (Messinian). During the Messinian “salinity crisis” the ostracodes to the east were nonmarine, while those on the western end of the line previously occupied by the trough remained marine.

Testing the messinian salinity crisis biodynamically: An introduction

Abstract The discovery of thousands of cubic kilometers of evaporites of Messinian age in the Mediterranean has posed the question of the effects of their formation on the evolution of the biologic systems that were present. Did the Tethyan faunas survive the “salinity crisis” whole or in part; or did new allopatric faunas repopulate the Mediterranean? Were there noticeable changes in rates of evolution, or in the developments of new adaptive forms — as compared to “normal” rates outside in the normal oceans? Even more fundamentally, how do faunas react to crises and the subsequent normalization in events of such magnitude, and yet short duration? To begin, to answer some of these questions of how and why biologic change occurred during the crisis, if indeed it did, the problem of demonstrating faunal continuity or discontinuity was proposed to several micropaleontologist colleagues. They have responded in the papers of this volume and their answers form the first approximations of a biodynamic model of the crisis. If short of the goal of a complete model, their answers will help others view and attack the problem better or perhaps differently.