Climate and Atlantic sea-level recorded in Southwestern Spain from 6.3 to 5.2 Ma. Inferences on the Messinian Crisis in the Mediterranean.

The Mio-Pliocene succession of Andalusia on the Atlantic coast (Guadalquivir Basin) is known as one of the former stratotype candidate for the Andalusian Stage, proposed during the seventies as the last stage of the Miocene. Its type section is located in Carmona, east of Seville. Our investigation includes the drilling of three cored boreholes, which provide bio- and magnetic-stratigraphic data in complement to pre-existing industrial information, and a high-resolution palynological analysis (pollen grains, spores and dinoflagellate cysts). The pollen flora and its climatic quantification provide the mean to correlate the section to the oxygen isotope curve from the Montemayor-1 borehole, located about 80 km to the West of Carmona. The variations in the ratio between dinoflagellate cysts and pollen grains are used to identify high and low oceanic levels, consistently with a recent paleobathymetric reconstruction based on foraminifera: the two lowest levels being successively marked by the deposit of a littoral calcarenite (the Calizza Tosca Formation) then by a subaerial erosive episode. Based on the correlation with the Montemayor-1 regional drilling, the two major lowerings in oceanic level observed at Carmona are linked with the two episodes of the Messinian Crisis. The interlocking position of the high-energy sandstone deposits inscribed in the Messinian valley leads to an assessment of a drop in the global oceanic level of about 114 m at the beginning of the paroxysm of the Messinian Crisis, amplitude to be moderated with respect to the potential effect of isostatic readjustements due to the Messinian Crisis. Comparisons are discussed with the amplitude of the Messinian Erosional Surface in the West-Alboran Basin which potentially remained suspended and fed with Atlantic waters during the height of the crisis and isolated from the rest of the almost totally dried Mediterranean Basin. The quantified climate constructed from the pollen records confirms that dry conditions existed before the Messinian Crisis in Southern Mediterranean latitudes including the Atlantic side, making the Mediterranean Sea climatically predisposed to desiccation. Atlantic sea-level variations observed in the Guadalquivir region and measured at Carmona suggest that global glacio-eustatism somewhat facilitated the onset and completion of the Messinian Crisis in the Mediterranean Basin. At last, this work allows to discriminate two regional erosive events: the first one, dated at 5.60 Ma, of fluvial origin in relation with global eustasy; the second one, submarine, occurred just before 5.33 Ma, and referred to the strain exerted by the Guadalquivir olistostrome. La succession mio-pliocene d'Andalousie atlantique (bassin du Guadalquivir) est connue comme ancienne candidate pour le stratotype de l'Andalousien, propose comme dernier etage du Miocene dans les annees 70. Sa coupe-type est situee a Carmona, a l'est de Seville. Nos travaux comprennent notamment la realisation de trois forages carottes ou furent acquises des donnees bio- et magneto-stratigraphiques complementaires aux informations connues des recherches industrielles et, surtout, une analyse palynologique (grains de pollen et kystes de dinoflagelles) a haute resolution. La flore pollinique et la quantification climatique autorisent des correlations climatostratigraphiques avec la courbe delta 18O du forage Montemayor-1 implante pres de Huelva. Les variations du rapport entre kystes de dinoflagelles et grains de pollen permettent d'identifier les hauts et bas niveaux oceaniques en accord avec une recente estimation paleobathymetrique basee sur les foraminiferes, les deux plus bas niveaux etant successivement marques par le depot d'une calcarenite littorale (Formation de la Caliza Tosca) puis par un episode erosif subaerien. Apres correlation avec le forage Montemayor-1, ces deux baisses du niveau oceanique sont respectivement mises en correspondance avec les deux temps de la Crise messinienne. L'emboitement des depots greseux a haute energie inscrits dans la vallee messinienne conduit a evaluer une chute du niveau atlantique d'environ 114 m lors du paroxysme de la Crise messinienne, amplitude a moderer toutefois compte tenu du possible effet des rebonds isostatiques lies a la Crise messinienne. Des comparaisons sont discutees avec l'ampleur de l'erosion messinienne dans le bassin ouest-Alboran potentiellement reste suspendu et alimente en eaux atlantiques pendant le paroxysme de la crise et isole du reste du bassin mediterraneen quasi-totalement asseche. Les restitutions quantifiees du climat comparees entre la province atlantique et le domaine mediterraneen confirment les conditions xeriques des latitudes sud-mediterraneennes s.l. et ainsi la predisposition climatique de la Mediterranee a la dessiccation. Les variations du niveau atlantique observees dans la region du Guadalquivir et mesurees a Carmona suggerent que le glacio-eustatisme global a quelque peu contribue au declenchement et a l'achevement de la Crise messinienne en Mediterranee. Ce travail permet enfin de discriminer deux evenements erosifs regionaux : le premier a 5,60 Ma, d'origine fluviatile en relation avec l'eustatisme global, le second intervenu peu avant 5,33 Ma, sous-marin, lie aux contraintes exercees par l'olistostrome du Guadalquivir.

Automated summary

This study investigates the Mio-Pliocene succession of Andalusia on the Atlantic coast, focusing on the bio- and magnetic-stratigraphic data from three cored boreholes. The results suggest a correlation between the section and the oxygen isotope curve from a nearby borehole, indicating changes in oceanic levels. The study also highlights the potential role of global glacio-eustatism in the initiation and completion of the Messinian Crisis in the Mediterranean Basin.