Integrated stratigraphy, astronomical dating and proxy records of the Ain el Beida and Loulja sections (NW Morocco). Implications for regional climate change, glacial history and the Messinian Salinity Crisis. Geologica Ultraiectina (324)

Abstract

This Ph.D. thesis aims to provide a better understanding of the origin of sedimentary colour cycles in the Bou Regreg area in NW Morocco. Sediments were deposited during the late Miocene and early Pliocene at times that the Mediterranean was isolated from the Atlantic during the Messinian salinity crisis (MSC), of which the exact origin is still not understood. Because the Mediterranean evaporites are difficult to date, studying open marine parallel sections on the Atlantic side may provide a better understanding of the MSC. The main goal of this thesis is to discriminate between tectonic influences and global and regional climate variations in initiating and ending the MSC, and to establish relations between astronomical parameters and climate change. The colour cycles could be the equivalent of Mediterranean sapropels that are dominantly precession-controlled. First, a high-resolution integrated stratigraphy and tuned age model for the Ain el Beida (AEB) section is presented, dating the section between 6.47 and 5.52 Ma. The colour cycles are precession controlled and reddish layers characterized by 18O minima and Globigerinoides maxima, similar to Mediterranean sapropels. The 18O records reveal an obliquity controlled 41-kyr glacial cyclicity and fit perfectly in a global oxygen isotope framework reflecting glacial history during the latest Miocene. Peak glacial stages TG22 and TG20 post-date the onset of evaporite formation in the Mediterranean, indicating that the initiation of the MSC is not associated with a major eustatic sea-level drop. The AEB section does not include the end of the MSC and the Miocene-Pliocene (M-P) boundary dated at 5.33 Ma. The integrated stratigraphic framework was extended using the Loulja section which ranges from 5.59 and 5.12 Ma and, like AEB, shows precession controlled colour cycles and a dominantly obliquity controlled 18O signal. The M-P boundary does not coincide with interglacial TG5 as previously assumed, but with a minor, precession-related shift to lighter 18O values. We thus conclude that the reflooding of the Mediterranean was not controlled by a glacio-eustatic sea level rise. However, this could have played a role in the onset of the Upper Evaporites, which coincides with the major deglaciation between stages TG12 and TG11 at ~5.54 Ma. An extensive multi-proxy study of the colour cycles at AEB shows that precession-related variations are related to ‘dry-wet’ oscillations in Northwest African climate, whereby more humid (arid) conditions occurred during precession minima (maxima). This scenario is linked to the Atlantic climate system rather than to the African monsoon because of the northward location of Ain el Beida. Obliquity-controlled glacial cycles dominate the 18O (and SST) records. Obliquity-controlled variations in 13C are inversely related to 18O, pointing to large-scale glacial-controlled variations in the deep-sea carbon reservoir. A marked ~1.0‰ negative excursion in planktonic 13C at 6.0 Ma is coupled to a final step in the closure of the Rifian Corridor. In summary, both tectonic gateway closure and climate evolution played a role during the MSC. The climate factor can be subdivided in dominantly precession controlled regional climate change and dominant obliquity driven glacial cycles.