| Περίληψη: | The present thesis is an integrated geological, geochemical, and oceanographic study of the environmental impact of hydrocarbon rich fluid seepage occurring in two areas located in Western Greece, Katakolo Bay and Amvrakikos Gulf. Western Greece is particularly affected by hydrocarbon seepage processes related to the presence of underground oil and gas reservoirs. The seepage is just the natural, upward migration of fluids from these reservoirs. Environmental impacts of hydrocarbon rich fluid seepage include:(a)the emission of greenhouse gases to the atmosphere, (b)hypoxia in aquatic systems(c)degradation of geotechnical properties of ground, soil, and sediments and,(d)geohazard due to explosiveness and toxicity of gas.This work is focused on points (a), (b) and (c).(a)Methane (CH4), the primary component of natural gas, is a powerful greenhouse gas and its seepage to the surface is an important natural source of greenhouse gas for the atmosphere. The seepage may occur on land (on shore) or in the marine environment (offshore), always associated to the presence of tectonic faults, as documented in this work. Measuring the flux of methane (generally expressed in mg m-2 per day, or tonnes per day) is the key task for addressing its atmospheric impact.(b)Hypoxia (i.e., oxygen depletion) in aquatic environments is an important environmental phenomenon with strong implications for life and its biodiversity, and as a result, aquatic ecosystem goods and services. Well oxygenated water bodies result from a fragile balance between production, transport and consumption. Hypoxia can be induced by natural and anthropogenic factors, such as water circulation, high water temperature, stratification of the water column, and anthropogenic pressures such as pollution. Hydrocarbon seepage is an additional natural factor that is yet poorly studied; it is known in theory that oxygen dissolved in water is consumed to oxidize methane released from the seabed. Knowing the conditions that regulate this process is still elusive. (c)Pockmarks are cone shaped deep depressions that are the result of erosion/degradation of the marine sediments caused by escaping fluids from below the seafloor (Amfilochia Bay in this thesis). Pockmarks constitute evidence that may illustrate and justify submarine fluid flow. On land gas seepage may be the cause of degradation of ground conditions, such as asphalt deformation and cracking. This occurs because the asphalt and concrete pavement over a seepage site (Katakolo Harbour in this thesis) acts as a cap for gas that gradually accumulates below. When the gas pressure increases the asphalt and concrete may swell and crack. In this study gas emission and hypoxia are investigated through geochemical and oceanographic methods, also using modern instrumentation such as benthic observatories. Katakolo Bay and Amvrakikos Gulf represent two different environments, hosting different type of seepage, geology, and seawater current circulation: Katakolo Bay is an open sea environment, but it includes a semi-enclosed harbor area where water circulation is limited. Amvrakikos Gulf is a naturally semi-enclosed basin receiving limited input from the open sea. The specific objectives of this study were: •Assessment of seepage distribution in the two studied areas•Estimate overall emission of methane into the atmosphere•Monitor over time oxygen concentration in seawater in relation to seepage•Determine the mechanism of hypoxia evolution due to seepageThe thesis is divided in two parts: -theoretical evaluation, describing general theory of hydrocarbon seepage and its environmental impact, with specific reference to hypoxia in seawater;-experimental study reporting original, experimental data, some of them acquired in the framework of an EU funded project, HYPOX. The data include marine geophysical data, methane flux from the ground and from the seawater/seabed, methane and oxygen concentration in seawater from areal surveys and from a temporal monitoring in a selected site within Katakolo Port. A descriptive model of the seepage/hypoxia relationship is provided for the first time and can be used to study hypoxia in other marine environments.
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