Paleoecological and paleoceanographical study of Messinian deposits from the lower Guadalquivir basin (SW Spain)

Abstract

This PhD thesis deals with the study of Messinian marine deposits from the Montemayor-1 core that is located closed to Huelva (SW Spain) at the northwestern margin of the lower Guadalquivir Basin. The core ranges from the latest Tortonian (late Miocene) to the early Zanclean (early Pliocene in age), thus including marine sediments coetaneous to the interval before, during and after the Mediterranean Messinian salinity crisis (MSC). The accuracy of the magnetobiostratigraphic age model proposed for the core has been improved using oxygen stable isotopes that have allowed the identification of the Messinian glacial-interglacial cycles (TG¿s cycles) based on astronomical tuning. In this thesis, the paleoenvironmental evolution of these Messinian sediments, as well as changes in paleoceanographic circulation and paleoproductivity in the NE Atlantic have been studied using benthic foraminiferal assemblages and oxygen and carbon stable isotopes. Changes in the benthic foraminiferal assemblages along the Montemayor-1 core show a transgressive-regressive trend. A sharp sea-level rise from the inner-middle shelf to the middle slope took place during the latest Tortonian-earliest Messinian. Sea level then dropped from the middle slope to the inner-middle shelf during the early Messinian-early Pliocene. Other key factors controlling the distribution of the benthic foraminifera, in addition to sea-level variations, were the quantity and the quality of the organic matter that reached the sea floor. The inner-middle shelf was a eutrophic setting with very low oxygenation and high input of degraded continental organic matter derived from river run-off. The outer shelf and the shelf edge were oligotrophic environments with high oxygenation. In the outer shelf, organic matter supply was mostly due to riverine discharge. Finally, mesotrophic conditions with moderate oxygenation prevailed in the upper and middle slope settings, where upwelling currents supplied marine fresh organic matter to the sea floor. The paleogeographic position of the Montemayor-1 core close to the last Betic seaway to be closed, the Guadalhorce Corridor, is crucial to the study of the impact of the Mediterranean outflow water (MOW) on the eastern Atlantic oceanography. The improved age model suggests that the closure of the Guadalhorce Corridor occurred at 6.18 Ma. Oxygen stable isotopes indicate that the study area was bathed by the MOW up to the closure of the Guadalhorce Corridor. When the MOW was interrupted, only Atlantic upwelled water (AUW) reached the study area. The cessation of the MOW reduced the formation of North Atlantic deep water (NADW) and, consequently, weakened the Atlantic meridional overturning circulation (AMOC) and promoted cooling in the northern hemisphere. Furthermore, the AMOC was weak during glacial periods but strong during interglacials. In the earliest Pliocene, the opening of the Strait of Gibraltar increased the NADW promoting a stronger AMOC. Stable isotopic records and high-productivity target taxa have revealed that glacioeustatic fluctuations exerted a significant control on the paleoproductivity in the lower Guadalquivir Basin during the Messinian. Under glacial conditions, there was high productivity related to the supply of fresh organic matter from upwelling currents induced by strong winds. In contrast, interglacial periods are characterised by the presence of marine degraded organic matter in the upper slope after upwelling events, and supply of continental degraded organic matter in the outer shelf derived from riverine discharge. The detailed study of the paleoenvironmental and paleoceanographic changes throughout the core provides support for a new model for the onset and end of the MSC. This model suggests that glacioeustatic sea-level fall with only a minor contribution of tectonic uplift provoked initiation of the MSC. The marine reflooding of the Mediterranean, which marks the end of the MSC occurred in two-steps: 1) glacioeustatic sea-level rise during interglacial stage TG 11 (5.52 Ma) allowed relatively warm Atlantic waters to at least enter the western-central Mediterranean; and 2) the opening of the Strait of Gibraltar at the lowermost Pliocene caused the final refilling of the entire Mediterranean Sea.