The late Miocene – early Pliocene offshore onshore sedimentary records in the vicinity of Gibraltar

  1. Francesca Bulian
Supervised by:
  1. Francisco Javier Sierro Sánchez Director

Defence university: Universidad de Salamanca

Year of defence: 2022

Committee:
  1. Paul Meijer Chair
  2. Montserrat Alonso García Secretary
  3. Francisco José Jiménez Espejo Committee member

Type: Thesis

Abstract

During the late Miocene, tectonic processes led to the closure of the Mediterranean – Atlantic gateways, progressively isolating the Mediterranean Basin from the Global Ocean. This change in gateway configuration modified radically the circulation patterns, water residence time and salinity of the Mediterranean waters leading to the extraordinary paleoenvironmental change known as the Messinian Salinity Crisis (MSC). This event lasted between 5.97 and 5.33 Ma and led to the deposition of huge evaporite accumulations both in the marginal and deep Mediterranean basins. Now, more than 50 years after the Glomar challenger ventured Mediterranean waters, and the evaporites in deep basins were discovered, the debate regarding the conditions and timing of the deposition of the Mediterranean salt giant is still ongoing as many theories regarding the dynamics and chronology of the Gibraltar arc gateway/s closure and reopening are waiting to be validated. In this optic, the study of cores and outcrops in the proximity of the current Strait of Gibraltar is essential to better understand the evolution of the Mediterranean – Atlantic gateways. In this thesis we perform a detailed planktic and benthic foraminifer, geochemical (XRF and stable isotopes) and sedimentological analyses of Alboran Basin ODP Site 976, DSDP Site 121, industrial boreholes Andalucia-G1, Alboran-A1, land-based sections from southern Spanish basins including Nijar, Sorbas and Malaga and Montemayor-1 core from the Guadalquivir Basin. The obtained results, paired with the interpretation of seismic profiles acquired in the Alboran Basin gave some new insights and results towards the better understanding of the Late Miocene to early Pliocene evolution of the Mediterranean – Atlantic gateways and the effects of the restriction on the Mediterranean environments before and after the MSC. The main outcomes of this thesis are outlined in the next paragraphs, as follows:  A high-resolution planktonic foraminifer stratigraphy performed on Sites 976 and Montemayor-1 in combination with the analyses of the astronomically driven cyclical changes in the geochemical record enabled the astronomical tuning of the two locations. Having a firm age model allowed to pinpoint the moment when the uplift of the Gibraltar arc gateway/s started affecting the Mediterranean Basin and Betic corridor.  The first sign of the Mediterranean – Atlantic gateway restriction is visible in the Mediterranean basin from 7.17 Ma, when active tectonism at the Gibraltar arc started uplifting the Betic and Rifian corridors. At ODP Site 976, the uplift is visible from the increase in terrigenous input arriving to the Alboran basin and parallel higher sedimentation rates related with an increased river erosion. On the other hand, the shift from benthic foraminifer open-marine high oxygen fauna to shallow infaunal taxa, tolerant to a wide range of conditions and suboptimal oxygen levels, paired with a significant drop in benthic δ13C values suggests that the gateway restriction led to the decrease in bottom water oxygen levels and increase in its residence time much earlier than the onset of the MSC.  A correlation between data from ODP Site 976 and other Mediterranean records confirmed that the 7.17 Ma gateway restriction, affected at the same time different locations all over the Mediterranean, inferring a Mediterranean-scale change in thermohaline circulation. From these data we concluded that the West Alboran Basin (WAB) and the East Alboran Basin (EAB) were not separated by a sill at that time but were both part of the Mediterranean realm. Furthermore, it was possible to create a refined Mediterranean circulation model for before and after the 7.17 Ma event.  The gateway restriction registered in the Mediterranean record since 7.17 Ma, is visible also from the geochemical data of Montemayor-1 core in the Guadalquivir Basin. Because the geochemical data from Montemyor-1 reveals that after 7.15-7.17 Ma, the Guadalquivir Basin was bathed by only one water mass, probably Atlantic, we believe that the connection between the Mediterranean and Atlantic through the Betic corridor was restricted at that time. Consequently, we suggest that the restriction of the last Betic gateway, the Guadalhorce Basin, could have happened at 7.15-7.17 Ma and caused the above-mentioned changes in the Mediterranean paleoenvironment.  Because the gateway restriction was contemporaneous with the global Late Miocene Carbon Isotope Shift (LMCIS) it was important to discern between global and local effects and compare the Mediterranean and global records. Given the synchronism of the global and local Mediterranean change in the δ13C record, a global effect certainly affected the Mediterranean Basin. However, opposite phase relations of the global and local δ13C signals with orbital parameters, paired with a higher magnitude change identified in our WAB isotope record suggests that the local imprint overruled the global one. A similar effect can be seen in the Montemayor-1 record, where apart from the changes related to the uplift of the Gibraltar arc, a global signal cannot be overruled.  Finally, through the development of this thesis it is shown how the dark layer often enriched in organic matter, present at the Miocene – Pliocene boundary in several Mediterranean marginal and deep basins, suggests that the Zanclean reflooding created water column stratification, and reduced bottom water oxygen levels. Such stratification could be the result of a sinking of more saline Atlantic water mass entering into a less saline Mediterranean Basin still under the influence of the Paratethys. The benthic foraminifer repopulation sequence identified at the base of the Pliocene shows similarities with more recent events of repopulation of hostile environments or following low-oxic episodes during sapropel deposition. However, Atlantic values of the benthic δ13C registered in the Alboran Basin suggest that bottom water renewal and circulation were efficient during the early Zanclean, preventing the reduction of δ13C at the seafloor seen after 7.17 Ma. Furthermore, the slight discrepancies in the benthic foraminifer repopulation sequences of the marginal basins at the Miocene – Pliocene boundary, and the much lighter benthic δ13C values in the Malaga Basin can suggest a diachronous reflooding of the shallower marginal basins.