Revised astrochronology for the Ain el Beida section (Atlantic Morocco): No glacio-eustatic control for the onset of the Messinian Salinity Crisis

Abstract

Glacio-eustatic sea level lowering has often been proposed as a key mechanism for explaining the onset of the Mediterranean evaporites during the Messinian Salinity Crisis (MSC). To examine the role of glacio-eustacy during the progressive isolation of the Mediterranean in more detail, we reinvestigated the Ain el Beida quarry section that is located at the Atlantic margin of Morocco and therefore free of destructive MSC-related complications. The section consists of deep marine, cyclically bedded (reddish/beige), silty marls of late Messinian age. The reddish layers reveal the same characteristic signals as found in the Mediterranean sapropels (Globigerinoides spp. maxima and δ¹⁸O minima), suggesting that they correspond to precession minima and, hence, (boreal) summer insolation maxima.We have established an integrated stratigraphy (calcareous plankton biostratigraphy, magnetostratigraphy, stable isotope stratigraphy and cyclostratigraphy) and an astronomical tuning of the Ain el Beida section by using conventional techniques and calibration methods. The astronomical ages for the biostratigraphic events and paleomagnetic reversals are in good agreement with the Mediterranean chronology, and confirm our cyclostratigraphic correlations. Stable isotope analyses reveal that the peak glacial stages TG22 and TG20 clearly post-date the initiation of the Messinian evaporites in the Mediterranean and that there is no evidence for a glacio-eustatic control for the onset of the MSC.Only the TG12-11 transition seems to correspond to a major intra-MSC event, i.e. the beginning of the Upper Evaporites. The typical Lago Mare facies of the Upper Evaporites was thus deposited during warmer climate conditions and associated higher global sealevel stands.