After a successful second traverse of the Drake Passage for both the biological and geological work groups we once again approach coastal waters.
This week’s geological sampling was focused east of Cape Horn in the western Scotia Sea and in the southeast Pacific along the Chilean continental margin. The weather conditions have been turbulent with wind speeds of up to 10 Beaufort and wave heights of up to 8 meters, perfectly timed for Easter Sunday (Fig. 1).
After completing our geological work in the sub-Antarctic southeast Pacific with a sediment core of 16.5 meters in length, we steamed northward, reaching Cape Horn in the morning of the 21st of March. During our second week, we had installed a temporary geodetic station, which could now be collected. Additionally we had made a helicopter flight to a lake on Cape Horn, however, were hindered in working on the lake due to the weather conditions at the time. This time looked more promising. We decided to send a three men team (Rolf Kilian, Univ. Trier, Sascha Plewe, IOW and Marc Wengler, AWI) with lake core equipment to the island for two days. Though weather conditions were unstable, the group managed to obtain a long sediment core from the lake (Fig. 2). The most important goal of the lake coring is to reconstruct the palaeoclimate at this location. Due to the lake’s position and orientation it receives a high amount of sea spray, which alters the salt concentration of the fresh water lake. Dominik Hodgson’s group, based at the British Antarctic Survey (BAS), had been working on the neighbouring Isla Hermite taking short lake sediment cores. They had recommended the lake on Cape Horn Island to us for detailed study. The sea salt contribution to the lake influences the sweet water diatoms and also alters the trace element geochemistry of the sediment. Using micro-palaeontological and geochemical sediment analysis, past sea spray contributions can be reconstructed. The sea spray contribution to the lake is directly related to wind strength in the southeast Pacific. Therefore lakes located on the exposed south-east Pacific islands provide an archive of wind strength changes. These play a critical role in understanding the palaeo-oceanography of the southern Ocean, the carbon cycle and the global ocean circulation.
Whilst the land team spent two cosy nights on Cape Horn Island the marine geological work on Polarstern continued. On the 22nd and 23rd we focused on identifying suitable sediment deposits with our Parasound and Bathymetry suite. Similarly to our experience in the second expedition week, localising suitable coring sites was difficult. Here, at the edge of the Chilean continental margin some of the strongest deep-water currents exist. Though we obtained several “bananas” (bent cores) we managed to collect surface samples and up to 8 meter long cores at three sites. On the 23rd we interrupted our work in order to pick up the land group. Unfortunately the weather was deteriorating which meant following the first helicopter flight, the operation had to be abandoned. Though we managed to get the land team back including the lake core, Rolf Kilian’s customised coring equipment had to be left behind on the Island.
On the 23rd of March we finished our geological work in the Scotia Sea and started heading to the tip of the South American continental shelf into the south-eastern Pacific. Here we started a detailed oceanographic profile with CTD and Rosette. This transect forms the most southern part of four - aimed at understanding the dynamics of the little investigated Cape Horn Current. At the continental tip the Cape Horn Current flows into the Antarctic Circumpolar Current and hence contributes to the total water mass through flow in the Drake Passage. Eleven CTD stations were driven on the profile. Unfortunately we could not complete all the planed stations as the weather and sea conditions were rapidly deteriorating on the night from the 24th to 25th of March. That said an unexpected good weather window occurred on the morning of the 25th, which was then used for helicopter operations. Gratitude goes out to the helicopter pilots’ Lars Vaupel and Harold de Jager, supported by the mechanics Roland Richter and Mark Rothenburg, for their flexibility. In the end we managed to get back Rolf Kilian’s invaluable coring equipment we had to leave behind on Cape Horn
After abandoning the southern most CTD profile we travelled through rough seas with wind speeds of up to 10 Beaufort on the 26th of March. With decreased Polarstern velocity against wind and waves we sought shelter in Bahia Cook (Fig. 1). We remained here to see a second storm reach its climax. With view of the glaciated mountains in the Chilean fjords and localised foehn, we reached our expedition’s record temperature of balmy 19°C. Whilst exiting Bahia Cook, wind and wave activity increased rapidly. Several items fell of tables, with the one or other broken glass, however, nothing serious was broken. With slowly decreasing wind speeds we reached our first coring site on the 27th of March, a site where we had previously been (see weekly report 2). Our aim was to use our piston corer to obtain a longer record for our deep ocean-coring proposal to the International Ocean Discovery Program (IODP). Though we were expecting more, we did obtain 3 meters of sediment. At our second IODP site 40 nautical miles north we only obtained a disappointing 36 centimetres of sediment.
On East Sunday the weather situation strongly deteriorated again. Around noon and the afternoon wave heights once again reached over 8 meters. Many of us headed to the navigation bridge to ogle the amazing waves and to get spectacular shots and films of the unrolling events (Fig. 3 & 4). Our lunch, featuring a festive duck, was also slightly impacted by the sea state. None the less we enjoyed the meal and thank goes out to our chef Klaus-Peter Redmer and his team. In addition the 1st stewardess Bärbel Cryborra and colleagues put a lot of effort into the Easter decorations, which made for a lovely homely experience. Thank you very much!
Following another short geological station on the 27th of March with close to a 10-meter sediment core, we took towards west, crossing the Chile trench, ca. 80 nautical miles into the open southeast Pacific Ocean. We wanted to obtain sediment cores from the open ocean north of the Drake Passage. The only sediment cores available from this area were done by Americans in the 1960’s. Using the American military ship El Tanin a relatively tight knit of sediment cores were obtained over several years. This pioneer work has significantly contributed to our understanding of the marine geology and palaeo-oceanography of the southern Ocean. Unfortunately these cores are in a bad state and many of them are unsuitable for modern palaeo-oceanographic methods. We knew of a very nice 10-meter long “El Tanin” core in our direct area of interest. However, the geographic positioning in the 60’s was not very precise, hence locating the site would be tricky. We planed for a joint parasound and hydrosweep profile to identify a suitable core location. Shortly after crossing the Chile trench with significant turbidite deposits the character of the sediment changed. Roughly 50 nautical miles west of the Chile trench at ca. 3900 meters water depth the Geologists were getting very excited. The parasound showed nicely layered sediment sequences with wave penetrations of up to 100 meters. This meant, relatively soft sediment, perfect for coring. We quickly localised a suitable sample site and at 9 am the 20 meter piston corer was lowered into the water. Though our sediment recovery of ca. 15 meters was very good, we were wondering whether we could get more out of it. In addition we spontaneously decided to undertake our first seismic work, as this location would be very much suited for a potential IODP coring location application. Though we had some initial issues with the equipment the seismic profile was completed successfully (Fig. 5). Our first analysis of the seismic data suggests a sediment cover of ca. 500-600 meters in this area. Following the above, we also obtained surface sediment with the multi corer and for the first time during our PS97 cruise we decided to use the 25-meter long piston corer (Fig. 6). With more than 22 m core recovery, this deployment was very successful (Fig. 7).
The bad weather has led to seasickness relapse of a few. In addition the sea state is certainly not contributing to regeneration after a hard day’s work. Nonetheless the overall sentiment on board is very good. The end of our PS97 expedition is nearing and we hope that the weather will start cooperating more during the last push! There is still plenty of marine geological, oceanographic and land work to be conducted.
Frank Lamy
Chief Scientist PS97
Position: 54°35´S; 76°36´W
(120 nautical miles west of the Chilean coast)
