PS97 Weekly Report No. 2 | 22 - 28 February 2016
Tierra del Fuego
The search for a geological station in the night from 21st to 22nd February, led us from the SW corner of Banco Namuncurá (also known as Burdwood Bank) into the deep sea in the northernmost part of the Drake Passage, along the NW continental margin and up again offshore the “Isla de los Estados”.
Looking for proper marine sediment layers for surface and long coring is one of the most important - and often challenging - tasks of our PS97 expedition program. And this was true for our first working area! The search of a proper station involves hydro acoustic instruments, especially the so-called HYDROSWEEP and PARASOUND systems. Whereas the HYDROSWEEP system yields surface information from the seafloor, the PARASOUND system allows us to evaluate the thickness and geometry of the upper sediment layers. These hydro acoustic data are gathered continuously during pre-defined transects and during transit between stations. Thus, our six HYDROSWEEP/PARASOUND operators are busy around the clock to accurately record and evaluate the data and compile this information in the forms of maps. These data are a crucial prerequisite for a successful core recovery and other geological samples.
In the case of soft sediments, the PARASOUND resolves layering to a depth of about 100 m. Such sediments are a dream of every paleoceanographer. Our overall goal is to sample sediment layers at different water depths, to reconstruct changes in ocean circulation and water masses in the geological past. Unfortunately, we did not find suitable coring localities in the working area along the Argentinean continental margin. This is due to the extremely strong ocean currents along the northern rim of the Drake Passage. These reach the ocean floor and carry away the majority of the fine-grained sediment particles. Although we were aware of this complex setting, we had hoped to find small sediment basins sheltered from the strong bottom currents that may act as sediment traps.
During the night and in the morning of the 22nd February, the wind abated and we lowered the gravity corer at 2500 m water depth. Aside from few remains of deep-water corals and coarse sand, we did not succeed in recovering any sediment. We left the area immediately. Due to wind intensities of 11 Bft, we decided to get closer to land to avoid waves of 6 m in height forecasted for the open Drake Passage. We navigated sheltered by the “Isla de los Estados” and Tierra del Fuego first to the west and then to the south and crossed the border between Argentinean and Chilean waters. Using the PARASOUND system, we finally located a small area with young sediments between 600 and 750 m water depth about 45 miles west of Cape Hoorn. At these water depths, our oceanographers identified Antarctic Intermediate Water at a nearby station. Compared to the water masses above and below, this water mass is characterized by lower current velocities, which favors sedimentation (“sediment drifts”). This area is ideally suited for our paleoceanographic studies to track changes in the Antarctic Intermediate Water (which is formed in the SE Pacific). This water mass is of global relevance for the transfer of Sub-Antarctic water properties and nutrients into the global ocean. Hence, we focused on recovery of piston cores and, on the 23rd of February, retrieved three 7-9 m long cores in the area of the sediment drift.
In parallel to the marine geological work we started land operations. A team of three was flown to the Cape Hoorn Island by helicopter. Previously, our colleague Dominic Hodgson from the “British Antarctic Survey” (BAS) had kindly provided information on suitable locations for lake coring on the island. The islands offshore southern Chile are difficult to approach and the lakes can only be reached by helicopter. Unfortunately, the low cloud coverage impeded the landing of the helicopter close to the lakes and walking to the site carrying all the equipment would have been too time consuming. Thus, the first attempt to core the lake was canceled. However, our researchers Lutz Eberlein and Peter Busch successfully installed a geodesic measuring station on the island.
The following night, Polarstern proceeded and reached Cape Hoorn and thus the SE Pacific on the 24th of February at 4 o’clock. After a transit of 120 nautical miles we finally reached our second working area (“Southern Chilean Continental Margin 3”). This area is a key site for our paleoceanography team. Already in the 90s, an Italian expedition with the RV OGS Explora examined seismic profiles to study the deeper geological structures of the earth’s crust at the Chilean continental margin. They found a sediment basin with extraordinary thick - up to several kilometers - sediment layers. Such sediment basins along continental margins provide a unique opportunity to reconstruct the ocean and climate history of the past million years at a very high temporal resolution. That’s why we considered these sites in a deep drilling proposal for the “International Ocean Discovery Program” (IODP). The missing information on the upper sediment layers can now be provided by us and our coring program. Against our expectations based on seismics and PARASOUND data, only surface sediments and very short cores could be recovered with the multicorer and gravity corer. The reason for this low recovery was the stiff clay with pebbles at the base of the gravity cores. This material was very likely transported by ice derived from the Patagonian Ice Sheet during glacial times. We did not expect such severe conditions since nine years ago we had succeeded in retrieving a 30 m sediment core somewhat northwest of this site during a French expedition with the RV Marion Dufresne.
We relocated our work further to the northwest where ice-derived material was reduced permitting the recovery of piston cores. The successful recovery of these cores supports our working hypothesis of a drastic retreat of the Patagonian Ice Sheet to the west. Further to the NW the ice influence seems to be reduced. Despite the low recovery of sediment using the Polarstern gravity and piston corers, the proposed IODP drilling sites remain as promising for the potential paleo reconstruction of the Patagonian Ice Sheet. Indeed this ice sheet was the third largest one in the world during the last ice age.
In the meantime and due to good weather conditions, we could continue with our work on land. A team of four researchers led by Rolf Kilian (University of Trier) successfully retrieved sediment cores from a lake on “Isla Noir”. These cores reach down to the rocky basement of the lake and probably serve as archives for the climate history of this area since the last glacial. Due to the offshore position of this island far from land, it remains to be proven whether the ice sheet reached “Isla Noir” during the last glaciation (which ended at about 17,000 years ago) or if the lake provides an even longer climate record. We hope to answer this question by combining the information pertinent to the lake and marine sediment cores in order to better understand the hitherto unknown extension of the Patagonian Ice Sheet on the Pacific side.
At the same time, two more scientists were flown to the island to collect rock samples for further geological and glacio-geological investigations. Due to worsened weather conditions, helicopters could not operate and the lake coring team had to camp another night on the island under the rain. Finally, on the 27th of February, all temporary islanders ̶ healthy but hungry ̶ were brought back to Polarstern.
Today, on the 28th of February, we started our way to the south on the western side of the Drake Passage. The main focus will be on oceanographic work, and hopefully, coring depending on sedimentary conditions. On the 1st of March we will reach the first biological station at 59°30´S; 67°45´W, thus ending the long preparation period for the biologists. More information on their work will be given in the next weekly report.
After a stormy beginning of our track around Tierra del Fuego we were blessed with relatively good weather conditions. Today we look at the deep blue SE Pacific under light wind conditions of 4 Bft and sunny skies. Who knows how long these good conditions will last? All participants are fine. Single cases of sea sickness recovered.
Chief scientist PS97
Position: 56°52,15´S; 71°09,3´W (ca. 140 nautical miles SE of Cape Hoorn)