The Drake Passage – Second Traverse

The week was dominated by geological work with one day being allocated for the third biological water pump station.

During the traverse the geologists had a second chance to obtain deep and surficial sediment cores across the different oceanographic fronts in the Drake Passage. The coring work is one of the key components of the palaeo-oceanographic objectives on the PS97 expedition. In particular we are aiming to recover cores on a straight line between the Antarctic Peninsula and South America in order to reconstruct temporal and spatial oceanographic changes in the Drake Passage. During our previous more western lying transect (see weekly report No. 3) we were only able to obtain long sediment cores south of the polar front (Figs. 1 & 2). The ocean floor between the sub-Antarctic and polar front were close to devoid of any sediment accumulations. The likely reason for the lack of sediment in that area is due to strong bottom currents hindering deposition of silt and clay particles.

After finishing the second biological station about 35 nautical miles north of Elephant Island, we began the Drake Passage traverse along the Shackelton fracture zone. This zone stretches from Elephant Island up to the South American continental margin near Cape Horn (Fig. 1) and also forms the narrowest section of the Drake Passage. The Shackleton fracture zone is a major tectonic structure where the Antarctic and Scotia plates move along each other. The plate boundary is highly active and frequented by earthquakes. In addition, uplift at the Shackleton fracture zone is creating a submarine mountain chain with a relief of 2,500 meters above the surrounding deep ocean floor. Especially in the southeastern portion the submarine topography exerts a strong control on the ocean circulation, in particular the south Antarctic circumpolar current (SACC, Fig. 2). The position of the polar front is mainly influenced by the location of the Antarctic-Phoenix and the West-Scotia ridge (Fig. 2). The third important front, the sub-Antarctic current follows the Patagonian continental margin.

In the ProIron team we have now sampled our last station and we are busy with numerous incubation experiments to investigate how iron chemistry and other trace elements control phytoplankton in this part of the ocean. As organic matter strongly affects the chemistry of iron, it is also important to better delineate its role as it has cascading effect for phytoplankton. Here, we are isolating these organic compounds to study their interaction with iron back at home. To do so we need to concentrate these compounds as they are present in minute quantities in seawater using two approaches simultaneously. The first one is a separation based on size using ultrafiltration of 1,000 L of seawater down to 1 L  (Fig. 5, left panels) and the second one is a separation based on chemical properties using solid phase extraction by slowing get 50 L of seawater through a cartridge to allow chemical retention followed by recovery in methanol (Fig. 5, right panel). Samples will be sent to 5 different universities to explore the nature of the organic compounds isolated.

During our second Drake Passage traverse the weather and sea state were astonishingly good. We were starting to doubt the regions infamous reputation of being one of the most difficult to navigate. It looks like we have been extremely lucky. The forecast for the coming week is far from good. For the next couple of days we plan further oceanographic and geological work at the continental margin east of Cap Horn where we are slightly shielded from incoming low pressure cells. We will then start moving northwards along the Chilean continental slope toward the Magellan Strait. Originally we wanted to work here at the beginning of the expedition, however the weather conditions hindered exiting the Magellan Strait toward west. We hope to have more luck this time! All participants are well and station work is running efficiently and successfully, especially due to the exceptional collaboration with the ships crew.

Frank Lamy

Chief Scientist PS97

Position: 57°09´S; 70°45´W

(140 nautical miles southwest of Cape Horn)