ALBERT

Description: Between 30.10.2007 and 28.11.2007, bathymetric data was acquired on the continental margin of Pakistan in the Makran subduction zone during R/V METEOR cruise M74/3. This expedition builds on the findings of the previous cruise M74/2, which was dedicated to a systematic seep search in the working area, where several bubble flares, backscatter anomalies and distinct seismic records were detected by using sidescan sonar, multi-channel seismics and a sediment echosounder. These findings provided the basis for detailed surveying during the cruise M74/3. The multibeam echosounders (MBES) KONGSBERG SIMRAD EM120 and EM710 were utilized for extensive mapping of the working area, which is essential for most investigations. Furthermore, bathymetric mapping during M74/3 significantly increased the resolution of previous bathymetry grids and provided information for the deployment of the remotely operated vehicle (ROV) MARUM-QUEST. The ROV was utilized to map the seafloor, take samples at potential seep sites and deploy and recover autonomous tools, such as a bubble-meter and in-situ pore water sampler. CI Citation: Paul Wintersteller (seafloor-imaging@marum.de) as responsible party for bathymetry raw data ingest and approval. Description of the data source: During the M74/3 cruise, the hull-mounted KONGSBERG SIMRAD EM120 multibeam ecosounder (MBES) was utilized to perform bathymetric mapping. The system covers full ocean depth and transmits a nominal sounding frequency of 12 kHz. It generates 191 beams with a 1°(Tx)/2°(Rx) and a maximum opening angle of 140°. For further information consult: https://epic.awi.de/26725/1/Kon2007a.pdf The acquisition mode was set to obtain equally spaced soundings on the sea floor. Yaw movements of the ship were compensated automatically by transmitting the swath perpendicular to the track rather than to the ship's axis. The opening angle was limited by either the maximum angle possible, a maximum angle set or a maximum coverage on the sea floor. Those values were adjusted to the requirements of the special surveys in a range of 5.5 to 7 km. Where no data were available at all, the full opening angle of 140° was set. Ship speed varied between 2.5 kn and 12 kn. A sound velocity profile for the cruise was delivered during the first CTD station of the previous cruise M74/2. The depth of the water column is estimated through the two-way-travel time, beam angle and ray bending due to refraction in the water column by sound speed variations. Systematically biased outer beams produced problems in areas with large overlap of parallel profiles. The applied sound velocity profile and a roll bias were tested as possible error sources, but no significant error was found. As the effect seems to be strongest on steep slopes, it might be a problem in yaw, which was not corrected for so far. Responsible person during this cruise / PI: Markus Brüning Chief Scientist: Gerhard Bohrmann (gbohrmann@marum.de) CR: https://www.tib.eu/en/search/id/awi%3Adoi~10.2312%252Fcr_m74/ CSR: https://www2.bsh.de/aktdat/dod/fahrtergebnis/2007/20070155.htm

Description: Following the discovery of asphalt volcanism in the Campeche Knolls a research cruise was carried out in 2006 to unravel the nature of the asphalt deposits at Chapopote. The novel results support the concept that the asphalt deposits at the seafloor in 3000 m of water depth originate from the seepage of heavy petroleum with a density slightly greater than water. The released petroleum forms characteristic flow structures at the seafloor with surfaces that are 'ropy' or 'rough' similar to magmatic lava flows. The surface structures indicate that the viscosity of the heavy petroleum rapidly increases after extrusion due to loss of volatiles. Consequently, the heavy petroleum forms the observed asphalt deposit and solidifies. Detailed survey with a remotely operated vehicle revealed that the asphalts are subject to sequential alterations: e.g. volume reduction leading to the formation of visible cracks in the asphalt surface, followed by fragmentation of the entire deposit. While relatively fresh asphalt samples were gooey and sticky, older, fragmented pieces were found to be brittle without residual stickiness. Furthermore, there is evidence for petroleum seepage from below the asphalt deposits, leading to local up-doming and, sometimes, to whip-shaped extrusions. Extensive mapping by TV-guided tools of Chapopote Asphalt Volcano indicates that the main asphalt deposits occur at the south-western rim that borders a central, crater-like depression. The most recent asphalt deposit at Chapopote is the main asphalt field covering an area of ~2000 m**2. Asphalt volcanism is distinct from oil and gas seepage previously described in the Gulf of Mexico and elsewhere because it is characterized by episodic intrusions of semi-solid hydrocarbons that spread laterally over a substantial area and produce structures with significant vertical relief. As Chapopote occurs at the crest of a salt structure it is inferred that asphalt volcanism is a secondary result of salt tectonism.