Abstract
Amfilochia Bay (Eastern Amvrakikos Gulf, Western Greece), a complex marine area affected by tectonism, was investigated for seabed seepage manifestations and for possible inter-relationships between shallow gas accumulations and hypoxia. For this purpose, an integrated research methodology that combined geophysical, geochemical, and hydrographic surveys was applied. Marine geophysical and bathymetric surveys led to the discovery of a gas-induced pockmark group in the study area. Oceanographic surveying confirmed that the bay is hypoxic/anoxic below a depth of 15 m (dissolved oxygen from ~4 to 60 μM). Very weak CH4 seepage was detected in correspondence to the pockmark group that seemed to slightly enhance the hypoxic environment locally and close to the seabed, with no effect in shallower waters and the oxycline. Methane isotopic analysis showed variable carbon isotopic composition (from −41‰ to −86‰) which could be either related to differential CH4 oxidation or mixing between microbial and thermogenic gas. However, the pathway of degassing is clearly related to the fault-controlled pockmark group. A protrusion mound, which has erroneously been reported as a submarine “volcano” since 1876, could be the result of mud volcanism based on the geophysical data of this study.
Highlights
Amfilochia Bay is located in the eastern-southernmost margin of the Amvrakikos Gulf on the north-western coast of Greece
The integrated oceanographic survey that took place in Amfilochia Bay showed that the bay is characterized by gas-charged sediments, while the southern part is affected by gas seepage and related erosional and piercement morphological features
This study resulted in the discovery of a pockmarks group and a possible mud volcano off the southern coast of Amfilochia Bay
Summary
Amfilochia Bay is located in the eastern-southernmost margin of the Amvrakikos Gulf on the north-western coast of Greece. The Amvrakikos Gulf is connected to the Ionian Sea through a narrow, elongated (~8 m deep, 6 km long, ~0.8–2 km wide) channel (Figure 1b). The Gulf’s water column is stratified in two layers: a surface layer with brackish water that is out-flowing, and a dense, cold, saline water bottom layer that is inflowing from the narrow channel that connects the Gulf to the open sea. The channel seasonally oxygenates the western part of the Gulf This density driven stratification of the water column prevents the vertical distribution of the dissolved oxygen and, coupled with anthropogenic pressures, leads to hypoxia and seasonal anoxia of the bottom layer, in the eastern, innermost part of the Gulf, where Amfilochia
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