INTERPRETATION OF TEMPERATURE-MEASUREMENTS FROM THE KAIKO-NANKAI CRUISE - MODELING OF FLUID-FLOW IN CLAM COLONIES

During the Kaiko-Nankai detailed submersible survey, numerous measurements of the temperature gradient inside the sediment were performed on the deepest active zone of fluid venting, which is situated on the anticline related to the frontal thrust, using the Ifremer T-Naut temperature probe operated from the submersible Nautile. We thus obtained the temperature structure below different types of clam colonies associated with fluid venting. We used the finite element method to model the thermal structure and fluid flow pattern of these vents and to determine the velocity of upward fluid flow through the colonies. On a biological basis, four types of clam colonies are defined. Each biological type has distinctive thermal characteristics and corresponds to a particular fluid flow pattern. Darcian flow velocity in the most active type of colony (type A) is of the order of 100 m/a. The total amount of fluid flowing through colonies in the studied area is estimated to be 200 m3 a-1 per metre width of subduction zone. Most of the flow is vented through type A colonies. This value is more than one order of magnitude too high to be compatible with the amount of water available from steady-state compaction of sediments in the whole wedge. Thermal arguments suggest that downwelling of seawater occurs around type A colonies and that seawater is then mixed with upcoming fluids at a depth of 1 or 2 metres. Furthermore, finite element modeling shows that a salinity difference of a few parts per mil between the upcoming fluids and seawater is sufficient to drive convection around the colonies. As water samples from a few vents indicate that the fluid source should actually be significantly less saline than seawater, we propose that the very high fluid flows measured are a consequence of the dilution of the fluid of deep origin with seawater by a factor of 5 to 10.