THE ROLE OF GAS BUBBLE EMISSIONS AT DEEP-WATER COLD SEEP SYSTEMS: AN EXAMPLE FROM THE MAKRAN CONTINENTAL MARGIN, OFFSHORE PAKISTAN Miriam Römer*, Heiko Sahling, Volkhard Spieß, Gerhard Bohrmann MARUM – Center for Marine Environmental Sciences and Faculty of Geosciences, University of Bremen Klagenfurter Str., 28359 Bremen GERMANY ABSTRACT A systematic search for fluid seeps along an entire continental margin segment offshore Pakistan was carried out during R/V Meteor cruise M74/2 and 3. The entirely mapped area was 50 km wide and ranged from the upper shelf at about 400 m to the abyssal plain at about 3000 m depth. Seeps were detected by using a combination of exploration techniques including multibeam mapping, sidescan sonar imagery, high-resolution seismics, PARASOUND sediment echosounder, TV-sled and remotely operated vehicle (ROV) QUEST seafloor observations. During this systematic search 18 seep sites where identified including twelve sites with evidence for gas emission. Gas emissions were detected as flares, which are acoustic anomalies recorded in the 18 kHz PARASOUND echosounder or in the unprocessed sidescan sonar data. Typical chemosynthetic communities and authigenic carbonates were found at all of the eight seep sites that were investigated by ROV Quest within this area. Bubble emissions were studied mainly by using a high definition ROV mounted camera providing sufficient resolution for the estimation of bubble sizes, rising speeds and the amount of rising bubbles, which resulted in a rough estimate of total fluxes of four different flare sites. Assuming that methane constitutes more than 95 vol-% of the free gas contained in the bubbles, flux rates are in the range of at least 0.25 (± 0.13) mol CH 4 min -1 to 10.7 (± 8.0) mol CH 4 min -1 for individual seep sites. In combination with observations of the bubble streams at the seafloor it became apparent that low bubble fluxes caused small flares in the 18 kHz PARASOUND, whereas strong bubble emissions caused intensive and very high flares. This allows a rough calculation of the total methane flux by bubble ebullition in the study area of approximately 4700 km 2 . Temporal variations could not be specified in this region, but assuming a steady discharge the amount of emitted methane from the seafloor can be calculated to be about 50 ×10 6 mol CH 4 yr -1 . Keywords: Fluid seepage, gas bubbles, Makran accretionary wedge NOMENCLATURE a The major axis length of a bubble [cm] b The minor axis length of a bubble [cm] r e The equivalent spherical bubble radius [cm] INTRODUCTION The Makran continental margin (MCM) is located along the coasts of Iran and Pakistan and has formed due the subduction of the Arabian Plate under the Eurasian Plate. It is one of the most extensive accretionary wedges worldwide. At MCM about 7 km thick sediments enter the subduction Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011 ), Edinburgh, Scotland, United Kingdom, July 17-21, 2011. * Corresponding author: Phone +49(0)421 218 65059 Fax +49(0)421 218 659099 E-mail: mroemer@marum.de