Volatile und Fluide in Subduktionszonen

Balboa

On 20 May, at 11:00 h, SONNE cruise SO163 ended in Balboa. We got through an intensive work schedule in the previous week. 20 OBH/S were deployed along each of two long profiles and were shot in transversal and strike direction. In general, data is of very good quality. Particularly, the S-waves converted at the BSR are well discernible so that the main purpose of these profiles was fulfilled. Besides, we are able to derive velocity models, using previously obtained reflection profiles too, for the continental margin. The northern profile reaches far into the flat shelf between Nicoya and Sta.
Elena peninsula. One of the largest fluid venting regions discovered, Culebra Mound, was investigated by means of a short seismic profile and 2 CTD stations (22 and 23). This mound is almost 1000 m wide and 100 m high. The BSR reflection bends upwards at the edges and is not discernible in the center.

CTD 22 at the top of the mound indicated a methane distribution typical of fluid venting. Concentrations at the bottom (1500 m) exceeded 20 nmol/L; this value lessened considerably with distance to the floor increasing. After SSBL-data had been interpretated to evaluate the direction of current, another CTD was operated at the foot of the mound in northwest direction to search for indications of fluid venting both at the slope and at the bottom of the mound. Analyses show a CH4 plume with a maximum at approx. 1570 m. However, both stations may have observed the same plume, for isopycnics seem to run almost in parallel.

Unfortunately, two attempts made to sample material from Culebra Mound by using the chain sack dredge failed. This geological structure should be investigated during future cruises.
Towards the end of the week, work continued in the region of Jaco Scar. Here, we launched another 20 OBS/H - including 2 tiltmeters - to make seismologic long-term observations. Thus, 28 units have been deployed in this region; METEOR will recover them in autumn.

In total, cruise SO163 included 212 positions - clearly more than managed on previous cruises. A minimal number of failures occurred, and only wide band seismometers failed to supply useful data. Consequently, we have not deployed them. They have either to be further tested in Kiel or their operability has to be ensured.

After having taken readings from the last CDT (24) at Jaco Scar, we departed for Panama on Saturday noon to complete our scientific work schedule by measuring a hydroacoustic profile including a magnetic survey of Quepos Plateau across the Cocos Ridge to 83°W.

At the end of this very labour-intensive and - above all - successful cruise it is a pleasure to thank Capt. H. Andresen and his crew for their excellent co-operation, continuous assistance, and the pleasant atmosphere aboard. We are looking forward to seeing our families and friends at home.

E. Flüh