Volatiles and Fluids in Subduction Zones

In the second week, our investigations focused on a systematic bathymetric and magnetic mapping of the continental margin off Guatemala. The surveys were terminated by Sunday evening. The thorough planning of profiles enabled us to record almost completely a region of approx. 300 km x 160 km. In spite of the fact that data has as yet not been smoothed and has only been processed preliminary, we have already obtained a comprehensive image of subduction zone morphology. After the recording of only a few profiles, the pronounced pattern of fracture formation appeared as the predominant feature within a plate area of about 50 km before the deep-sea trench, where the bending oceanic plate is dipping into the subduction zone. A very high resolution of the bathymetric data display has revealed some small "pinnacle"-shaped structures, most of which are related with the graben structures. However, the equipment available on this cruise does not allow for a direct sampling at this depth; we hope that a detailed interpretation of swath-mapping data will provide further information. We discovered several "mound"-shaped structures on the continental slope at depths in the range of 2,000 to 3,500 meters. Two dredges produced samples from the largest one. The first material dredged from the foot of this structure consisted of fine-grained sandstones and of a multitude of igneous rocks showing various degrees of disintegration. A preliminary comparison to the results obtained during Leg 84 from the nearby DSDP drilling site suggests that these rocks date from the eocene epoch. The second dredger from an area situated nearer to the top edge of the structure produced mainly more recent, predominantly dendritic rocks, whose veins and brecciation indicate wide-ranging deformation processes.

Four trawls were performed for the biological projects during the 2nd week. We fished in depth between 900 and 400m in areas off the Guatemalan coast. Two catches came up during the day and two more at night. The results were consistent with the observations of the first week. On a regular basis, we found lanternfish, melamphaeids, bathylagids, anglerfish, deep-sea eels, and a single stomiid, all of them being relevant to our experiments. On the other hand, viperfish, dragonfish, and hatchetfish were absent in the trawls. Since we fished systematically at various depths we can exclude that this is due to a sampling error occurring when we used our net gear. Rather, we must conclude from our trawls that these otherwise circumglobal species are either very rare or that they are presently absent off the coast of Guatemala.

In view of the limited number of species, we had to choose alternative species for the study of biological rhythms of deep-sea fish. Instead of the hatchetfish, which had been planned as a migratory model species, and on which previous experiments had already been performed, we had to work on Bathylagus longirostris, which was found in sufficient number, and also belongs to the fish with vertical migrations. We collected pineal organs of these fish and preserved them (shock frozen) to be analyzed for their melatonin content when we are back in the laboratory in Tübingen. Comparison between day and night values will yield clues as to the existence of an internal clock. This could control the migratory cycle independently of the ambient light intensities of the water column. As a non-migratory species that might serve as a control, we chose the melamphaeid (Scopeloberyx robustus.). Our working hypothesis would predict that in this latter species the difference between night and day values of melatonin content should be smaller than in B. longirostris. In order to confirm the endogenous nature of the predicted oscillator, we plan to perform long-term culture experiments of isolated pineals under continual darkness conditions and to study the amount and patterns of melatonin released into the medium.

On Sunday afternoon, having terminated the investigations in the economic region off Guatemala, we said goodbye to the Navy's observer, who had been accompanying us for 12 days, at a place of rendez-vous with a coast-guard boat off the coast near Puerto Quetzal.

Apart from scientific activities, there were animated discussions last week of the future of German research vessels and their crews. Medium-sized research vessels will probably be operated by the shipping company Briese - an outlook that causes scientists' concern and non-understanding, while the crew fears to lose the jobs on SONNE. Scientists working on SONNE are of the opinion that such a measure would jeopardize the trouble-free and successful process of experimental work on deck and thus the success of future cruises too, if - as obviously planned - crews from low-price countries are employed. Scientists put great emphasis on the fact that the success of work on research vessels depends to a large extent and decisively upon the deck crews' and navigators' qualification and upon the scientists being able to discuss with them in detail work to be done. The changes planned do not seem to take this precondition for proper scientific work into account at all. For this reason, the scientists participating in cruise SO-173/2 fear that the implementation of such changes will bring into peril the future of the entire marine research done on German research vessels. The scientists share the crew's concern and indignation regarding the social effects of measures such change of the shipping company would involve.

At sea, 13° 18' N, 9°39' W, 24th August 2003
W. Weinrebe, J. Wagner