Volatiles and Fluids in Subduction Zones

The third week of the cruise began with a deployment of the DTS-1 deep-towed side-scan sonar system. This system operates with higher frequencies (75 kHz, 410 kHz) than TOBI (30 kHz), and is therefore capable of mapping the seafloor with a much higher resolution but also with a considerably smaller range. The two systems are complementary.

After the new USBL underwater positioning system POSIDONIA had been calibrated, DTS-1 was launched on monday evening for a 36 hours survey around the mud volcano area at 9°36' N, 85°50' W. To acquire more detailed information this survey was completed by five missions with the TV-sled OFOS.

So far, target areas for OFOS deployments concentrated on features interpreted as mud-volcanoes. These structures are small circular features, several hundred meters in diameter, some showing a central crater, which can be well detected in the bathymetry, and display high backscatter in TOBI images. During the OFOS surveys it became evident that these structures are actually related to fluid venting. At the top or the flanks of the mounds we found massive chemoherm-like carbonates, and at some of these structures, vesicomyid clams and pogonophoran tubeworms. However, we found no geological evidence for active fluid expulsion.

The next working area was located on the oceanic plate, whereas all other targets so far were situated on the continental plate. A recent cruise with RV EWING found an area with local anomalously high heat flux. This area occurs at the inception of the trench parallel flexural faults created by the bending of the oceanic lithosphere into the subduction zone. The anomalous heat flux and the faulting point towards fluid flow in the area, and we looked for venting activity and specific fauna. Due to the limited length of the deep-sea cable on RV SONNE (around 7,000 m), the success of a survey of this area with the DTS-1 is at least highly questionable. An operation of the deep-towed vehicle at a depth of 3,300 m requires a longer cable. Based on experience, a cable length 2.5 to 3.5 times the towing depth is necessary. Therefore we used the Simrad EM120 multibeam system for a detailed survey switching the beams focused in the center, and the ship's speed adjusted in order to obtain a dense coverage along-track. We processed and displayed the "sidescan" information of the system to obtain the backscattering properties. The area of high heat-flux is characterised by a gentle high and marked by higher backscatter. However, two missions with OFOS failed to find typical vent fauna.

Following this survey towards the end of the week, an area just downslope of the headwall of the prominent Nicoya Slide is surveyed with DTS-1. The TOBI images of this region, acquired during SO-144 cruise in 1999, show spots of high backscatter not justified by seafloor topography. We are currently mapping the area with DTS-1 75 kHz side-scan and subbottom profiler on five parallel lines.

The constantly warm and sunny weather with 27°C let us forget that it is Easter time, however, an easterly decorated mess-room and extraordinary meals remind us.

With best regards of all aboard
Wilhelm Weinrebe