We report a high‐resolution stable isotope, carbonate, magnetostratigraphic, and biostratigraphic record from a 175‐m drill core from the Salé Briqueterie, which is part of the Bou Regreg section in northwestern Morocco. The Salé 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 Salé 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.

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 Copyright 1991 by the American Geophysical Union. Paper number 90PA00756. 0883-8305/91/90PA-00756510.00 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. INTRODUCTION The Atlantic connections of the last vestiges of the Tethyan Ocean, or the Paleo-Mediterranean Sea, were through the Rifian Corridor in Morocco and the Iberian Portal in Spain (Figure 1). Much larger than the present Strait of Gibraltar, the twin thresholds allowed the inflow of Atlantic waters into the Paleo-Mediterranean to replenish most of its evaporation loss: a loss that with the successive closure of these straits ultimately led to the Messinian Salinity Crisis. They also controlled the outflow of warm, saline waters, the Paleo-Mediterranean Overflow Water (PMOW), into the Atlantic. Changes in its rate of outflow could have altered the water mass structure in the eastern North Atlantic and influenced the climate of Europe. This report is primarily concerned with (1) faunal evidence for a sudden strong influx into the Rifian Corridor occurring at 6.4 Ma and lasting to between 5.7 and 5.3 Ma; and (2) a paleoceanographic model ...

Oxygen and carbon isotopic ratios of foraminifers were measured from outcrop and drill core sequences from the Bou Regreg Section, northwest Morocco. This composite section was located at the western end of the Rifian Corridor during the late Miocene and thus potentially contains a record of water exchange between the Atlantic and Mediterranean during the late Tortonian and Messinian stages. Here we correlate isotopic and sedimentologic events in the Bou Regreg Section with Mediterranean and deep‐sea sequences during the time leading up to and including the deposition of the Messinian evaporites. The late Miocene chron 6 carbon shift was identified in two Moroccan sequences, providing a valuable 6.3‐Ma datum level. In both sections, the carbon shift coincides with the first occurrence of Globorotalia conomiozea and the Tortonian/Messinian boundary. Near this boundary, a major faunal turnover occurred in ostracod, planktonic foraminiferal, and nannofossil assemblages that indicates a cooling of surface and deep water in the Rifian Corridor. At 6.1 Ma, just above the Tortonian/Messinian boundary, mean ∂18O values of benthic foraminifers increased by 0.4‐0.5‰ suggesting decreased temperature and/or increased continental ice volume. The faunal and isotopic changes are interpreted as reflecting a reversal in the direction of deep water flow through the Rifian Corridor that occurred between 6.3 and 6.1 Ma. At this time, cold, nutrient‐rich waters filled the Mediterranean basins from intermediate depths of the Atlantic, and the production of Mediterranean Outflow Water ceased. The upwelling of nutrient‐rich Atlantic intermediate water stimulated productivity in the Mediterranean, which led to the deposition of organic‐rich strata such as those found in the Tripoli Formation. At ∼5.5 Ma, coincident with a coiling shift in Neogloboquadrina acostaensis, the variability of the benthic ∂18O signal increased markedly, and strong color variations appeared in the sediments (alternating between red and blue marls). These isotopic and sedimentologic changes at Bou Regreg represented markedly fluctuating oceanographic conditions in the Rifian Corridor between 5.5 and 4.8 Ma. We speculate that these cycles were related to sea level variations that controlled the periodic influx of marine water into the Mediterranean during the time of evaporite deposition. The onset of these cycles occurred at 5.5 Ma and is correlated to a eustatic fall in sea level at the base of the Caliza Tosca Formation in Carmona, Spain, and to the base of the lowermost evaporite unit in the Mediterranean (e.g., Calcare di Base in Sicily). In this proposed scenario, the duration of the Messinian salinity crisis was 700,000 years and lasted between 5.5 and 4.8 Ma.

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