Abstract
Tectonic plates form the Earth’s outer shell and move over the underlying mantle with a speed of a few centimetres per year. These plate-tectonic motions affect Earth’s geography over geological timescales (on the order of a million years) by displacing the surface in horizontal and/or vertical directions. Examples of response
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to the tectonic movements include opening or closing of oceanic gateways which are the means for the exchange of water, heat, salt, and nutrients between oceans. Opening or closing of oceanic gateways in turn may affect ocean circulation and the properties of the water residing in the basin which leaves an expression in the accumulating sediments on the seafloor. Moreover, changes in the gateways may have regional to global-scale impacts on climate. Role of oceanic gateways in the climate, ocean circulation and sedimentary environment of the basins has recently became subject of interest for paleoclimatologists, paleoceanographers and projects on oil source rock predictions. Earth’s history has recorded many of these tectonic-related changes in gateways. Around 20 million years ago (Early Miocene), Paratethys (of which at present only the Black Sea, Caspian Sea and Lake Aral remain) and the Mediterranean Sea were connected to the Indian Ocean through a seaway lain at the present-day Zagros and Persian Gulf region. The northward motion of the African plate, together with the counter-clockwise rotation of Arabian plate, ultimately closed the gateway connecting the Mediterranean/Paratethys to the Indian Ocean around 13 Million years ago (Middle Miocene). The closure had significant impact on sedimentary environment of the Mediterranean Sea, Paratethys and proto-Persian Gulf region, and affected the climate over Europe, north-eastern Africa, Arabia and Iranian basin. In this thesis, I used a hierarchy of numerical ocean models to study how the closure of gateways between the Mediterranean Sea/Paratethys and the Indian Ocean affected the circulation and water properties of these basins. For the starting point, I developed oceanic box-models and proceeded with 3-D general circulation models. This was the first time that the regional-scale ocean models were applied to the Miocene paleogeography of the region for studying the paleocirculation of the Mediterranean Sea and Paratethys prior to and after closure. The main aims were to determine the variation of (1) temperature, salinity, and residence time, (2) flows in the gateways, and (3) circulation of the basin, in response to changes in the gateways and in particular their closure. It was also of interest to show whether the changes observed in proxy data were caused by closure of the gateways. In a more general sense, this research addressed fundamental issues concerning the role of gateways, basin planform and bathymetry in controlling the circulation and the sedimentary environment of a semi-enclosed sea. The results of this thesis showed that gateways played an important role in controlling the circulation and water properties of the basins. The model analysis indicated that with open and deep gateways to the Indian Ocean, Paratethys and the Mediterranean Sea had westward surface flow and eastward deep flow. Restriction of the gateways to the Indian Ocean reversed the overturning circulation in Paratethys and the Mediterranean Sea, and increased the sensitivity of these basins to changes in climate. It was found that the response to closure was a non-linear process. The Paratethyan temperature decreased in response to closure, but the Mediterranean temperature could increase or decrease depending on the atmospheric conditions. The change in salinity and temperature of the Mediterranean and Paratethys after closure was not uniform throughout the basins and the deep layer was found to show a stronger change than the surface layer. Furthermore, the regions where the closure had the most significant effects on the water properties were identified. The future geological investigation in these areas would provide important insights in timing and effect of closure. I specifically compiled the geological data on the Paratethyan temperature as well as the state of gateways to the Indian Ocean (open/closed) during the Miocene. The temperatures computed in the model by considering the inferred gateway states are compared with the ones estimated from data to assess the role of gateways in the evolution of Paratethyan temperature. It was shown that closure of gateways between the Paratethys/ Mediterranean and the Indian Ocean underlies the hitherto mysterious mid-Burdigalian cooling evidenced by the sedimentary record of Paratethys. This study is appropriate for studying the development of Eurasian climate during Miocene. Moreover, it is applicable in exploring the distribution of sediments that are likely to be hydrocarbon source units to predict the existence of petroleum systems.
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