Towards the Strait of Magellan

Each storm reminded us again that the southern most part of Chile is located in a zone of intense pressure gradients with maximum southern westerly wind speeds. In the second half of the week we managed to take advantage of a short calm weather window to run our seismic transects.

After a successful run of our first seismic transect covering two promising core sites in the south-east Pacific (80 nautical miles west of the Chile trench), we returned to the Chilean continental slope on the 30th of March. Prior to crossing the Chile trench we had started the third CTD and LADCP current meter transect. This profile aims at providing key observations of the Cape Horn current with specific emphasis on its oceanographic and climatic variability. The transect started at about 4000 meter water depth, consisted of 12 sample sites and ended on the Chilean continental shelf at ca. 120 meters water depth. At ca. 2500 meters water depth the transect was temporarily stopped to accommodate a coring location where over 10 meters of sediment could be obtained. The oceanographic work at this location finished on the 31st of March.

Whilst the winds were accelerating we undertook an extensive Hydrosweep and Parasound survey of the continental slope to identify suitable locations for coring. In the night of the 1st of April we localised two sites where multicorer, gravity and piston corers were deployed successfully. We managed to cover water depths of ca. 1800 – 3000 meters. On the 2nd of April we reached our final working area at the continental margin west of the Strait of Magellan. Sediment cores had been obtained around this area during the 2007 expedition by the French research vessel Marion Dufresne. For some of us, (ie. Helge Arz, IOW; Rolf Kilian, Univ. Trier & Frank Lamy, AWI) this would be the second visit to this part of the southeast Pacific. Back then, as similar to this time, we had poor weather and a tight schedule. Nevertheless, during the Marion Dufresne expedition a 30 meter long sediment core from ca. 1000 meter water depth was obtained using 6 tons of weights. During the PS97 coring at the same site we obtained ca. 8 meters of sediment using close to 2 tons of weights, hence, not as successful as we had hoped. However, we managed to obtain sediment cores from other sites close by up to relatively shallow waters of 600-850 meters depth.

One of our palaeoceanographic goals was to obtain sediment archives from a range of water depths across the Chilean continental slope. The reasoning for this sampling strategy is that past changes in the water masses can be reconstructed. Particularly in the south-east Pacific at the entrance to the Drake Passage these changes can have a major influence on global ocean circulation patterns and climate. The marine geology research group based at the AWI have previously obtained such a depth profile during a FS Polarstern (PS75) and PS Sonne (SO13) expedition at the New Zealand continental slope. The main findings of this research were accepted for publication by the scientific journal “Nature Communication” last week, where Thomas Ronge (AWI), a geologist on board, is lead author. Many congratulations for this accomplishment! When one collates all our cores along the Chilean continental slope in the Pacific, our sediment cores cover a respectable range of water depths from 600-4000 meters. Preliminary estimates of the age range covered in the sediment cores suggest extension prior to the last glacial maximum. The cores therefore cover an interesting timescale of the transition from glacial to interglacial with critical changes in the ocean circulation, climate and the carbon cycle.

We undertook a detailed Hydrosweep and Parasound survey in the vicinity of the Marion Dufresne coring site to get a clearer picture of the sediment cover and structures at depth. We discovered a massive sediment deposit at ca. 1000 meters water depth. We are assuming that this is a sediment drift formed by low velocity currents preferentially causing sedimentation. These types of deposits are very interesting for palaeoceanographic work as they are typically associated with high sedimentation rates with high temporal resolution. As the Marion Dufresne core was obtained from this sediment drift with very good palaeoclimate time series, we decided to revisit this site during the PS97 expedition to get a seismic profile. A relatively nice and calm day meant that our Geophysicists Jürgen Gossler (AWI), Henrik Grob (Univ. Hamburg) und Sjard Stratmann (Univ. Hamburg) could deploy the geophysics-streamer together with crew and fellow scientists. Preliminary processing of the data suggests that the sediment drift is 700-900 meter thick.

Following the seismic work we started with the northern most oceanographic profile close to the Chilean continent. Due to increasing wind speeds this transect had to be halted half way through. Wave heights of up to 7 meters coming from north to north-westerly direction made our transit to the sheltered Strait of Magellan rather uncomfortable. Luckily we found shelter in the evening of the 4th of April. On the 5th of April our meteorologist forecast wind speeds of 12 Bft with wave heights of 10 meters outside the sheltered areas in the open ocean. Hence we remained in shelter until the 10pm on the 5th of April when the situation had calmed down. We then steamed toward the Pacific to finish the oceanic profile we had to abandon earlier in the week. We managed to successfully finish the profile today, the 6th of April at 4pm, forming our last and 151st scientific station. Today we will be celebrating with a lovely barbeque in evening. If the weather is favourable we are still planning some helicopter flights for tomorrow before we head toward Punta Arenas. We look forward to seeing the impressive landscape of the central Strait of Magellan tomorrow. As the harbour is currently occupied we will have to wait close to Punta Arenas – it is unclear when we will be able to disembark as another storm is forecast – however it is likely that we will manage to get solid ground under our feet at some point on Friday. Luckily the majority of the scientists have their flights home booked for Saturday.

As a final note here a few statistics of our expedition. When we arrive in Punta Arenas, Polarstern will have covered 6400 nautical miles (ca. 11500 km). During this, 5586 nautical miles were covered by hydrosweep and parasound surveys, and 120 nautical miles by seismic profiles. In total 151 biological, geological, geophysical and oceanographic work-stations were covered. Four biological water pumping stations, 66 CTD-LADCP, 53 Mulicorer and 73 piston/gravity corer stations with a total of 499,58 meters in sediment recovery were obtained.

Frank Lamy

Chief Scientist PS97

Position: 52°37´S; 74°40´W

(at the Pacifc entrance, Strait of Magellan)