PS111 - Weekly Report No. 7 | 3 - 9 March 2018

Across the Weddell Sea

[12. March 2018] Pteropods spend their life cycle in the water column. The organisms actively “fly” through the water, by using their feet, which transform into two wings. The Pteropods calcium shell is made of Aragonite, which makes them especially vulnerable to ocean acidification. The increasing amount of carbon dioxide in the atmosphere leads to an increased absorption of CO2 in the cold waters of the polar oceans and enhances the acidification there.

Three biologists from GEOMAR and the University of Kiel are investigating the abundance of Pteropods in the Weddell Sea and the impact of ocean acidification on their shells. The organisms were caught along the cruise track with a Bongo net and will subsequently be analyzed under the microscope back at the home laboratory. The CO2 content of the water was determined from water samples. Considering the Pteropods’ sensitivity to ocean acidification, they can serve as an early warning system for further CO2 increases in the Weddell Sea.

Fig. 1: Extracting the catch of the day from the Bongo net. (Photo: Alfred-Wegener-Institut)

Fig. 2: Group photo: Polarstern and James Clark Ross. (Photo: Alfred-Wegener-Institut)

Fig. 3: Shadow of Polarstern on an iceberg in the evening sun. (Photo: Alfred-Wegener-Institut)

Among the many different activities, PS111 carried out surface trawls to investigate whether microplastics, sadly abundant in much of the world oceans, have already reached the Antarctic waters of the Weddell Sea. A total of seven trawls provided preliminary indications of colored artificial particles, although it remains to be tested whether these are in fact microplastics or perhaps paint particles from Polarstern or other vessels. Additionally, underway water samples were pumped on board to be analyzed back at home alongside sediment cores from the seafloor.

Bathymetric work will continue during the remaining cruise track toward Punta Arenas to survey the topography of the seafloor with a Multibeam echo-sounder. The beam consists of several hundred rays and resolves a swath several times wider than the water depth. The majority of the Weddell Sea is not yet mapped, and therefore every new cruise track provides additional bathymetric information that steadily improves the IBCSO-map (International Bathymetric Chart of the Southern Ocean), which is maintained at AWI. The area that was mapped during this cruise is equivalent to the size of Schleswig-Holstein.

The sediment echo-sounder measures the thickness of the sediments on the seafloor. In contrast to the Multibeam, this echo-sounder has only a single beam, which concentrates its energy to penetrate several meters into the seafloor. The track across the Weddell Sea confirmed that the continental slope region is incised by several submarine canyons. These are the major communication and transport pathways of material from the continent to the deep sea. The echo-sounder mapped thick piles of sediment that has settled at the canyon sides. Both bathymetric systems are monitored and data are quality controlled around the clock by the bathymetry group.

Following the last deep CTD station in the northwestern corner of the Weddell Sea, we leave the region through the Antarctic Sound en route to Bransfield Strait in order to prepare the ship for the crossing of Drake Passage. No more ice means rockin’ and rollin’ so everything on the ship needs to be strapped down again. Coincidentally, the James Clark Ross, flagship of the British Antarctic Survey, is in the area, and Polarstern took the opportunity to meet and greet the British colleagues.

The scientific work of PS111 is now finalized, and overall completes another very successful expedition with Polarstern. Even after 35 years, the ship is as powerful as ever and operates without problems largely down to the professional maintenance and handling of her crew. The cruise participants are looking forward to being home again soon and would like to express their sincere gratitude to Captain Stefan Schwarze and his crew, whose knowledge and seamanship paved the way to a successful science program.

This is the last report before the end of the cruise.

Dr. Michael Schröder

Chief scientist