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Sonne 163 (Subduction I), 13.03. - 20.05.2002

Volatiles and fluids play a major role in the global geological material budget. At subduction zones, material from the oceanic plate (the oceanic crust and its overlying sediments) is brought to great depth. Sediments from the continental margin transported downslope by mass wasting, add to the subducted material. These processes extract considerable amounts of carbon, sulphur, water, and halogenes out of the exosphere.

Inside the subduction zone, the downgoing material is remobilized, fractionated, and forms new reservoirs with different volatile compositions. A portion of the subducted and remobilized material is partly fed back into the exosphere through different transport pathways and mechanisms, partly subducted into the lower mantle. The return flow is accomplished through three distinct pathways:

* fluid venting at the deformation front
* mud diapirism and gas hydrate dynamics at the margin
* magmatism and exhalations at the volcanic arc

The long-term development of the Earth's climate, the geochemical evolution of the hydrosphere and atmosphere, and the causes of natural disasters (earthquakes, volcanic eruptions, submarine landslides, tsunamis) are all connected with the return flow and impact of volatiles and fluids from subduction zones. The main objective of the SFB 574 is to understand and quantify these processes and relationships.

Cruise SO-163 "SUBDUCTION" of RV SONNE is set about to investigate the material input, mobilization, and return flow of volatiles in the subduction zone off Costa Rica. During the first leg from March 13 to April 20, 2002 from Balboa (Panama) to Caldera (Costa Rica) a comprehensive study of the area of investigation will be carried out, applying a whole set of different mapping tools with different resolution. In particular deep-towed side-scan sonar systems will be used. The TOBI system of the Southampton Oceanography Centre will be used for a reconnaissance survey to detect places of active mud diapirism and mass wasting. Following the TOBI survey a detailed study of selected targets with the new GEOMAR DTS-1 will be carried out. As a further step to even greater resolution and to ground-thruthing, direct observation and sampling of the seafloor in the target areas by TV-sleds and TV-grabs will complete the survey.

Figure 1: Trackplot S163 (Balboa 13.03. - 26.03.)

 

Preliminary Results

During cruise SO163 (13. March to 20. Mai 2002, Balboa to Balboa) the Pacific continental margin off Cost Rica was surveyed using a variety of tools. These investigations follow several previous cruises and are part of the new research initiative (Sonderforschungsbereich) SFB574:"Volatiles and Fluids in Subduction Zones: Climate Feedback and Trigger mechanisms for Natural Desasters". Several other cruises will follow in 2002 and 2003. The cruise was split into two legs with an intermediate port call on April 20 in Caldera., Costa Rica.

Leg 1 was set about to investigate the material input, mobilisation, and return flow of volatiles in the subduction zone off Costa Rica. During leg 1 a comprehensive study of the area of investigation was carried out, applying a whole set of different mapping tools with different resolution. The results comprise an important database for all forthcoming expeditions and will be an invaluable help in finding appropriate target areas for future sampling work. The investigations during leg 1 also aimed in quantifying the processes in the material cycle which directly affect the seafloor and its morphology, such as sediment deposition, erosion, submarine landslides as well as manifestations of fluid venting. In particular deep-towed systems were used. The TOBI system of the Southampton Oceanography Centre was used for a reconnaissance survey to detect places of active mud diapirism and mass wasting. In total, this system was used for 315 hours to map an area of about 8000 km² with a resolution on the order of around 10 metres. Together with the results of the TOBI survey during the SO-144 cruise in 1999, nearly a complete coverage of the whole continental margin from Santa Elena Peninsula in the northwest to the Cocos Ridge in the southeast was achieved. In order to image parts of the surveyed area with greater resolution, the newly acquired GEOMAR DTS-1 deep-towed sidescan sonar system was used to map five key areas. In total an area of about 650 km was mapped in 175 hours with a resolution of less than one metre. As a further step to even greater resolution and ground truthing, direct observations of the seafloor by TV-sled OFOS was carried out at 24 deployment sites. The survey was completed with sampling the seafloor by the TV-guided grab at nine positions.

During leg 2 numerous seismic and seismological experiments and a water sampling programme dominated the work carried out. The active seismic work focussed on three areas. The Megalens structure, being interpreted as a key factor for erosional processes, the decollement reflection and the BSR occurences off Nicoya. In all three areas numerous wide angle data sets were collected using ocean bottom hydrophones (OBH) and seismometers (OBS), The Megalens reflections are not easily identified in the records processed onboard. The profiles collected on the decollement reflection show a pronounced lateral variability of the interface, and surprisingly several deep penetrating faults that cut the subducting oceanic plate have been identified on preliminarily processed sections. Off Nicoya several profiles were shot to collect three component data in areas were a strong BSR reflection had been observed before. These three component data should allow to identify shear waves and be used for receiver transfer function methods to investigate the shear wave velocity of gas hydrate bearing sediment. Numerous records were collected where p-s conversions observed at about the time expected for a conversion at the BSR.

Two seismological network were operated. The first one comprising ten instruments was placed on and around an active vent site and was operational for about 40 days. Preliminary analysis of the data showed a special kind of events , termed hongo events, which could be localised at the vent site. These events consist of an approximate 1 sec long monochromatic (36 Hz) burst with increasing amplitudes, followed by a two second long monochromatic (20 Hz) arrival of lower frequency. These events are most probably related to the hydrologic system in the vent site. A second network was installed around Jaco Scar. This network comprises 28 stations (5 of them are actually centered on the near by Quepos mound) and shall be operational until October 2002. It is further augmented by an 15 element land array. In addition, two tiltmeters were deployed in this network to monitor long term slope changes.

In total, the seismic programme comprised 212 OBH/S deployments, with 184 successful recoveries and 28 instruments left on the seafloor for monitoring. More than 800 nm of seismic profiles were collected, with a total of about 25000 airgun shots fired.

Figure 2: The combined land and seafloor seismic network of the JACO area. The land stations have been installed in April 2002. The ocean bottom instruments have been deployed during RV Sonne cruise SO 163-2 in April/mai 2002. Some stations of the first deployment (14-21) have already been recovered and allow a first impression of the data quality. All other stations will be recovered in the beginning of October 2002. Traces of a subducting seamount are clearly visible as large depressions in the bathymetry of the continental margin.

 

The water column sampling program comprised a total of 24 stations and had two main objectives. (a) The investigation of the general oxygen and methane distribution off Costa Rica. Two sections with 6 CTD/rosette stations spread over 100 nm were sampled in the northern and southern part of the working area. The results will provide information about the production of methane within the upper 600m of the water column due to the geostrophical upwelling along the western coast of Middle America (Costa Rica Dome), and will define the methane background for the detection of venting-related methane-rich plumes in bottom-near waters. (b) Detailed surveys of the methane distribution at potential vent sites in order to quantify the release of CH4 to the water column and to define targets for a detailed geochemical program during expeditions later this year. Multiple CTD casts were deployed in the Jaco Scarp area as well as at Quepos Mound and Mound Culebra. The results revealed evidence for active venting of methane rich fluids in all 3 areas. The water column program was accompanied by a small sediment sampling program, which lead to first data on the geochemical composition of the non-vent-influenced upper sediment in the SFB target area.

The Simrad system of Sonne was operated continuously, and magnetic data were collected along 4 profiles extending over more than 200nm.

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