Breadcrumb Breadcrumb
Navigation Navigation
Web Content Display Web Content Display

 

Asset Publisher Asset Publisher
« Back

SO163-2, 2. Weekly report 05.05.02

at Sea at 8°50´N/84°20´W

In the previous week we achieved high data productivity - as is typical of cruises once the starting phase is over and routine work has begun. Apart from occasional showers of rain, weather was pleasantly warm, and the sea very calm.

The hydrographical profile, consisting of 6 CTD stations in the north of the operation area, was completed at the end of the preceding week; it shows a large methane plume in the core area of the oxygen minimum zone (at a water depth of about 400 m). This plume is the predominant feature of methane distribution, whose concentration of up to 25 nmol/L can be discerned at a distance of at least 120 sm off the coast. However, the observed oversaturation of the surface of the water seems to have been caused essentially by methane production occurring within a thin layer beneath the intermixed upper layer. Analyses of sediment cores obtained by MUC at a water depth of 1400, 2400, and 3200 m show that alkalinity increases from 2,5 mmol/L at the sediment surface to reach 3-4.5 mmol/L at a sediment depth of 20 cm. The value of ammonium concentrations in pore water has continuously been <200 micromol/L along the entire core length as expected for these reference stations, which are not influenced by active venting.

In the afternoon of 28.04., we again reached the region of Jaco Scarp where we recovered the tiltmeter. Data had been recorded without any problems, however, the significance of registered values has yet to be clarified. The settling behavior of the equipment support seemed to have lasted longer than expected. We deployed the first five units of the seismological net planned for the Jaco Scarp region; RV METEOR will recover them in October. When starting CDT/KW/MUC operation at the scarp (CTD07), further samples were taken at a depth of approx. 1900 m from a site where the most significant methane plume had been discovered during expedition SO144/Paganini. Results confirm active venting in a depth range of 1650 down to 1850 m. Maximum concentrations of up to 300nmol/L were detected.

On 29.04., 17 OBH/S (43-59) were deployed on the first of the 4 profiles planned in the region of the mega lens; the profile was shot by two different air gun systems. When the units were recovered on 30.04., the newly designed OBS could not be released successfully. Since it was positioned at a depth of only 200 m we tried to retrieve it by dredging. Unfortunately, the geology cable, when being retracted, was damaged at 2000 m of cable length. Another attempt had to be interrupted due to difficult flow conditions and a large number of fishing boats. Thus, we started sampling of another two stations located at Jaco Scarp (CTD 08, 09) at water depths of 1400 and 900 m along OFOS track 2 (SO 144). However, results obtained from sampling did not indicate active venting and were not comparable to those achieved by CTD 07. We will supplement the profile in southwest direction later on this cruise.
Eventually, the third attempt of dredging was successful. We presumed that the bracket of the seismometer had slipped beneath the anchor, holding the instrument to the ocean bottom. We will modify the design and perform tests in the days following.

The two tiltmeters were positioned on the presumably calcareous and thus solid soil of the Quepos Plateau to conduct another series of testing.

During the next days samples were taken alternately from further CTD/KW/MUC stations on the Quepos Mound (stations 10 to 12) and from two transversal profiles above the mega lens.
CTD Station 10 in the top region of the Quepos Mounds showed only very slight indications - which may not be interpretable at all - of fluid venting. Unfortunately, the minicorer installed beneath the CTD did not supply any core material, whereas each of the first 6 operations had been successful. Since contradictory results obtained from water column sampling and from OFOS operations during the previous cruise (OFOS 23 and 24) had astonished us, we decided to map methane distribution in the region of Quepos Mounds. For this purpose, the unit was towed along the western and eastern flank of the mounds in north-south direction during CTD operations 11 and 12.

Today, we are going to tow a CTD above the center of the mound. Owing mainly to excellent navigation, we have been able to implement a new strategy successfully and position CTD/rosettes with an accuracy of only a few meters. Presently, we are retrieving the OBHs deployed to register the last transversal profile (OBH/S76 to 90). The seismic sections we have represented so far give reason to optimism with regard to the aims in view.

Today, precisely half the expedition is over. Enjoying glorious sunshine and an absolutely calm sea, we are sending regards to all at home.

E. Flüh

Events Events

Kieler Wissenschaftler fühlen den 'Puls der Erde'

Web Content Image
Buch zum Sonderforschungsbereich über Naturkatastrophen am GEOMAR vorgestellt

Wie funktioniert die Recyclingmaschine der Erde?

Nach elf Jahren endet der Kieler Sonderforschungsbereich 574 zu Subduktionszonen

Final colloquium of SFB 574

Web Content Image
The Lübeck Retreat, May 23-25, 2012

Teilprojekt Öffentlichkeitsarbeit

MEERESFORSCHUNG FÜR MICH UND DICH