GASCHARGED SEDIMENTS AND ASSOCIATED SEABED MORPHOLOGICAL FEATURES IN THE AEGEAN AND IONIAN SEAS, GREECE GAS CHARGED SEDIMENTS AND ASSOCIATED SEABED MORPHOLOGICAL FEATURES IN THE AEGEAN AND IONIAN SEAS, GREECE Laboratory of Marine Geology & Physical Oceanography, Department of Geology, University of Patras, Greece Contribution 10, by: Papatheodorou, G., Christodoulou, D. and Ferentinos, G Papatheodorou, G., Christodoulou, D. and Ferentinos, G.

During the last fifteen years, marine seismic surveys in the Aegean and Ionian Seas (Fig. 1) have revealed numerous acoustic anomalies, i.e. acoustic turbid zones, gas pockets, gas plumes, enhanced reflectors, columnar disturbances, wipe outs and meso- to micro-morphological features, such as pockmarks, domes, mud volcanoes and elongated depressions (Papatheodorou et al., 1993). These are attributed to the presence of gas in marine sediments interstices. The gas charged sediments are found in Pleistocene and present day fjord-like environments, Pleistocene and present day deltaic environments, lakes and pre-Quaternary open sea environments. The gas found in the Quaternary and present day fjord-like and deltaic environments is assumed to be of biogenic origin. The gas found in the pre-Quaternary open sea environments is associated with faulting and salt doming and may, therefore, be of thermogenic origin.

Fig.1. Map showing the location of the surveyed areas where gas- charged sediments have been found. (A): Corfu shelf (B): Northwestern Aegean Sea (C): Lake Trichonis (D): Amvrakikos Gulf (E): Gulf of Patras (F): West Gulf of Corinth

In the eastern Corfu shelf (Ionian Sea) the central part is covered by a thin veneer of sediments underlain by deformed and upward curving sedimentary strata. The deformation and doming strata is caused by salt diapirism. The acoustic character of the sediments in the 3.5kHz and sparker records indicates that they are gas-charged because of the acoustic turbid zones and enhanced reflectors. Side scan sonar images show that along the rims of the salt diapir there are dark patches of high reflectivity. Abbreviations: ATZ: Acoustic Turbid Zone; ER: Enhanced Reflector; GS: Gas Seepage; D: Doming; HRP: High Reflectivity Patches. OPEN SEA ENVIRONMENT Fig. 2. Block diagram showing the dome structure and related 3.5kHz, sparker profiles and side scan sonar images.

OPEN SEA ENVIRONMENT NORTHWESTERN AEGEAN SEA Fig. 3. Airgun seismic profile across the Sporades shelf showing gas charged sediments. ATZ: Acoustic Turbid Zone, CD: Columnar Disturbance, F: Fault, M: Multiple Fig. 4. Side Scan Sonar image of the Sporades shelf showing mud volcanoes (MV) and gas seeps (GS) in the water column. Fig. 5. Side Scan Sonar image of the Sporades shelf showing fault line (F) on the seafloor associated with small shallow pockmarks (PM). In the Sporades shelf, in the Northwestern Aegean Sea, gas is found in the upper 70-80m as is indicated by the acoustic turbid zones and columnar disturbances in the airgun records (Fig. 3) and water column reflections in the side scan sonar images (Fig. 4), which probably represent gas plumes (GS) rising from elongated mounds, which appear to be mud volcanoes (Fig. 4). Negative topographic features, probably pockmarks near the hanging-wall of a faultline, indicate that gas uses the fault plain as a migratory path (Fig. 5).

PRESENT DAY AND PLEISTOCENE FJORD-LIKE ENVIRONMENTS AMVRAKIKOS GULF The Pleistocene / Holocene boundary in Amvrakikos Gulf seems to be a gas accumulative horizon as is indicated by its strong reflectivity in the 3.5kHz records. Buried or “fossil” pockmarks developed on the gas accumulative horizon, range from 35 to 70m in diameter and are m deep (Fig. 6a). Gas pockets in the Holocene and gas plume in the water column indicate gas seeping through the seabed (Fig. 6b). Low relief intra-sedimentary and seabed doming (Fig. 6b) is apparently due to the expansion of the sediment-trapped gases. Fig kHz profiles of the Amvrakikos Gulf showing buried pockmarks (BPM) developed at the Pleistocene/Holocene interface; gas plumes (GPL), enhanced reflectors (ER), doming (D) and faults (F). b a

Fig kHZ and corresponding airgun seismic profiles of the Gulf of Patras showing: Pleistocene gas charged sediments indicated by the acoustic turbid zones (ATZ); Gas pockets (GP) in the Holocene sedimentary cover associated with faulting (F) and doming (D) of the seabed. Notice how the acoustic character of the gas charged sediments changes due to the difference in the frequency and resolution of the profiling systems used. Holocene / Pleistocene boundary (HOL/PL); Enhanced Reflector (ER); Gas Plume (GP); Multiple (M). PRESENT DAY AND PLEISTOCENE FJORD-LIKE ENVIRONMENTS GULF OF PATRAS GULF OF PATRAS GAS-CHARGED SEDIMENTS Seismic data collected from the Gulf of Patras has shown the presence of a gas accumulative horizon at about 15-20m under the seafloor (Fig. 7). This accumulative horizon may be the boundary between the Holocene and Pleistocene sediments The accumulation of gas in the Holocene / Pleistocene interface may be due to differences in the gas-bearing properties of these sediments (Fig. 7). This horizon can not always restrain the vertical migration of gas into the Holocene sediments, as is indicated by the gas plumes, gas pockets, enhanced reflectors, intra-sedimentary and seabed dome-like features, which have been frequently detected on the seismic records (Fig. 7, 8). Some of the pockmarks are associated with seabed displacements due to faulting, suggesting that the gas uses fault planes as a migration path (Fig. 8). 8

Fig kHz profile of the Gulf of Patras showing: Pleistocene gas charged sediments indicated by the acoustic turbid zones (ATZ) and pockmarks (PM) formed along faults (F). (GPl: gas plumes).

In the Southeastern part of the Gulf of Patras, an active pockmark field was found (Fig. 9). The pockmark field has an aerial extent of 1.7km² and was formed in soft layered Holocene silts. The larger pockmarks were up to 250m in diameter and deep up to 15m. Detailed bathymetric and seismic surveys have shown that the field consists of single and composite pockmarks. The composite pockmarks are formed by the amalgamation of single ones (Fig. 10). Fig. 9Fig. 10Fig. 9Fig. 10 Recent surveys over this pockmark field using an ROV and a methane sensor (METS) have revealed the presence of high-concentration dissolved methane ( nmol/lt) in the water column just above the pockmark, indicating methane seepage from the sediments (Fig. 11,12,) and small holes on the seafloor, which may be associated with gas escaping vents (Fig 13). 11,12Fig 1311,12Fig 13 POCKMARK ACTIVATION BY EARTHQUAKE On July 14th, 1993, a major earthquake of 5.4R, whose epicentre was located near the pockmark field, appears that has activated the pockmarks. In a temperature recording station in the pockmark field, which was located 10m above the seabed, the temperature increased anomalously on three occasions prior to the earthquake (Fig. 14). Furthermore, it was noted that few pockmarks three to four days after the earthquake were still venting gas bubbles as is indicated by the extremely high reflectivity plumes shown on the sonographs (Fig. 15). Fig. 14(Fig. 15)Fig. 14(Fig. 15) The three sudden temperature increases were probably due to bubbles of hot gas rising from the pockmark field. Similar events related to bubbling in the seawater and increased temperature have been described in the past in the nearby area of Aegion, 40km to the east, in 1817 and in Messolonghi lagoon in POCKMARK FIELD OFF PATRAS HARBOUR

Fig. 9. Investigated area in the Gulf of Patras. Fig D view of the seabed in part of the pockmark field.  Fault line

PATRAIKOS POK 4 Fig kHz profile showing a composite pockmark (POK 4) of the Gulf of Patras pockmark field. Methane concentration was measured at the centre of the pockmark using a methane sensor (METS). Fig. 12. Methane concentration variation versus water depth in the POK 4 pockmark. Maximum concentration ( nmol/l) was observed at the water – sediment interface. Methane Concentration (μmol/lt) 0 40 Depth (m)

Fig. 13. ROV photo of the seafloor in the centre of pockmark POK 4. The red arrows show small holes probably related to gas venting.

5.4R EARTHQUAKE Fig. 14. Temperature variation versus time in the hydrographic station, 10m above the seabed, from July 13th (~30h before the earthquake) to July 17th (60h after the earthquake). The vertical line after the three temperature peaks indicates the time of the earthquake.

Fig. 15. Side Scan Sonar mosaic of the pockmark field of the Gulf of Patras, showing gas plumes (white arrows) rising in the water column from the pockmarks.

Seismic data collected in the southern coastline of the gulf of Corinth has shown the presence of a small pockmark field in Eleona Bay (Fig. 16). These pockmarks are circular to subcircular in plan view and are less than 100m in diameter (Fig. 17). They appear to be flat floored, about 10m deep and are developed in silty sands (Fig. 18). During a preliminary survey, using an ROV, a methane sensor (METS) and a CTD, it was found that fresh groundwater was seeping from the pockmarks (Fig. 19, 20, 21, 22). However, the methane probe revealed low methane concentration in the seawater Another pockmark field was discovered in the same area north of Eleona Bay (Fig. 23) (Soter, 1999). A detailed survey is at present going on within the framework of the ASSEM program (Array of Sensors for long term SEabed Monitoring of geohazards) financed by the European Union. The aim of the project is the long term monitoring of temperature, salinity, methane, hydrosulfide and carbon dioxide in association with the earthquake activity in the region. 23 PRESENT DAY DELTAIC ENVIRONMENTS WEST GULF OF CORINTH

Fig. 16. Investigated area in the Western Gulf of Corinth.

Fig. 17. Side Scan Sonar mosaic showing the pockmark field in Eleona Bay.

Fig kHz profile showing a flat-floored pockmark (POK 4) in Eleona Bay.

ELEONAS POK 4 23/09/02 Fig. 19. Methane concentration (μmol/l), water temperature (ºC), salinity (‰) and DO (%) versus time just above the seabed in Eleonas pockmark “POK 4”. The yellow dotted lines define the measurements just above the freshwater seepage.

Fig. 20. ROV photos in the Eleonas pockmark POK 4 showing the sidewalls of the pockmark. Fig. 21. ROV photo in the Eleonas pockmark POK 4 showing the freshwater seepage.

Fig. 22. ROV photos in the center of the Eleonas pockmark POK 4 showing flat-floored area densely populated with small polychates.

Fig. 23. Side Scan Sonar mosaic of the pockmark field north of Valimitika. The east-west chain of pockmarks is the trace of the Aegion fault (Soter, 1999).

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