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Título
The late Miocene – early Pliocene offshore onshore sedimentary records in the vicinity of Gibraltar
Autor(es)
Director(es)
Assunto
Tesis y disertaciones académicas
Universidad de Salamanca (España)
Tesis Doctoral
Academic dissertations
Registros sedimentarios
Mioceno tardío
Plioceno temprano
Sondeo marino
Gibraltar
Clasificación UNESCO
2506 Geología
2506.18 Sedimentología
2416 Paleontología
Fecha de publicación
2022
Resumen
[EN] 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, landbased 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 abovementioned 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.
URI
DOI
10.14201/gredos.149577
Aparece en las colecciones
- GGO. Tesis [18]
- TD. Ciencias experimentales [401]
- PDG. Geología [16]
- DGL. Tesis del Departamento de Geología [45]