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M76 Cruise Legs 1a/b
Short Cruise Report M76/1ab
The major objective of the first Leg of METEOR-Cruise 76 was the investigation of the deep biosphere, an area which has come into the focus of geoscientific and microbiological research only ten years ago. Previous studies on long sediment cores have demonstrated the existence of a marine deep biosphere. Due to the lack of cultured representatives our knowledge about the physiology and ecology of deep biosphere microorganism is still limited. Biomarker studies and the detection of certain marker genes in deep sediments indicated the presence of potentially chemolithotrophic bacteria and archaea, gaining their energy by metabolizing inorganic compounds.
This cruise should provide us with sediment material from the deep subsea floor at the upper continental margin off Namibia. Specific biogeochemical processes, involved microbial communities, and their interrelations with the local oceanography and sedimentation regime should be investigated with a variety of chemical and microbiological methods and approaches.
This was the first cruise where the new wire-line drilling technique of the sea floor drill rig (MeBo) should be tested in unconsolidated marine sediments. Based on the necessary support for this technique and the whole device by a commercial enterprise (Prakla Bohrtechnik) which already has been involved in the development of the MeBo system, the cruise had to be split into two legs. Therefore, the cruise was interrupted on April 25 to exchange 3 scientists.
According to plan, RV METEOR left Cape Town only with a very short delay of several hours at 5 pm on Saturday, April 12, 2008. Figure 1 shows the whole cruise track.
Due to good weather and current conditions METEOR arrived at the first station at 28°S on April 14. Unfortunately, but typical for the season, strong low-pressure areas around the Southern Ocean caused high swell up to 6m in the whole working area during most of the time. Because the dimension of the MeBo system is close to the limit which can be handled on board of a ship like RV METEOR and therefore for reasons of safety, MeBo was deployed only when the sea was below about 3 m. Despite the rough sea, several deployments of the gravity corer and the multicorer were successful.
Fig. 1 Track line of RV Meteor during cruise M76/1
Fig. 2 EM120-survey at 27,8°S, 14,2°E
First preliminary investigations show the expected low microbial activities south of the known organic carbon depocenter on the upper continental margin between 24.5-26.5°S. The waiting time for better weather conditions was used two times for intensive hydroacoustic surveys of a relatively small area (about 360 km²) were very conspicuous pock mark structures could be identified (Fig. 2).
In total more than 68 depressions could be detected between 1000 m and 1400 m water depth. With diameters between 200 m and 800 m and very steep edges, some of them are more than 140 m deep. Their orientation along lines and the comparison with recent, quite similar observations on the Guinean slope let assume that faults and/or other tectonic structures in the very deep might be causally connected. Beggiatoa-like bacterial mats were found on the surface of some multicorer sediments. A peak in dissolved sulphide exists at 5 cm and some cores had a gassy appearance, but no significant quantities of methane were measured. The overlying water was fully oxygenated, and first chemical analyses also suggest that we are not dealing with a brine intrusion. Therefore, indicated by first geochemical and microbiological results on sediments from the bottom of these depressions, they look more like sink holes than highly influenced by venting processes, at least during younger history.
Based on previously existing results, a transect across the upper slope between 25,2°S and 24,6°S has been chosen as the main working area during this expedition. Here, at about 800m water depth, the MeBo system could be deployed for the first time. A first hole with wire-line coring was drilled down to 14 mbsf. Nearly each of the 6 segments retrieved sediment, but a continuous coring failed.
Again, deteriorating wave and wind conditions prevented further MeBo operations. The meantime was used to take valuable samples from the well-known mudbelt off Namibia as well as from the continental rise. Even in the mudbelt, two gravity cores could be obtained successfully. As expected, a series of multicorer cores contained filaments of the world’s largest bacteria – Thio-margarita namibiensis – on the surface. A hugh number of samples could been taken for incubations and other special experiments to decipher microbially mediated element cycles. With good hope on improving weather we started looking on a site with more consolidated deposits. Such sediments, an outcropping strata could be found in about 300 m wd at the shelfbreak. Unfortunately several technical and electrical problems the MeBo system could not deployed successfully.
Considering the scientific goal of this leg, we decided to continue sampling down the slope. In about 3800 m water depth an excellent 5.6 m long core was retrieved. The whitish foraminiferal nannofossil ooze represents just the opposite of the organic rich sediments of the depocenter on the upper slope. Among other things these sediments reveal a big contrast in benthic life and microbial activity. However, pore water geochemistry indicates further ongoing microbial activity in the deeper, dark layers of these sediments which let surmise the magnitude of this deep sub-seafloor microbial habitat.
On account of persistent difficulties with MeBo, we decided to sample further sites in the mudbelt on the continental shelf. Two additional sites were intensely sampled with Multicorer and gravity corer.
Despite all the various difficulties with the new wire-line system on Mebo, this first test was not a waste of time. The development of such a highly complex system takes time. With this cruise we have gained a lot of valuable information which will lead to important improvements for the new challenges in the near future. After thirty days on sea the cruise ended on May 13 where is has started, in Walvis Bay. All scientists could send home a lot of new exciting data and samples for ongoing investigations in the home labs. So in total, the work on cruise science objectives was relatively successful.
A detailed listing and documentation of the scientific program achieved during M76/1 can be found in the regular final report which should be available in summer 2009 (http://www.dfg-ozean.de/de/berichte/fs_meteor).
M. Zabel & T. Ferdelman (chief scientists)