PS97 Weekly Report No. 3 | 29 February - 6 March 2016
Drake Passage Traverse
After wrapping up our work along the Chilean continental margin on the 28th of February we embarked on our first Drake Passage traverse toward the West Antarctic Peninsula. Our route was located just west of the narrowest point between South America and Antarctica in the south Pacific. We covered approximately 500 nautical miles over a widely unstudied section of the open ocean (Fig. 1). The main focus on our first Drake Passage traverse was to take water samples, sediment coring and oceanographic profiles.
Cruising southward from the Chilean margin, we finished our oceanographic profile of the Cape Horn current located in the southeast Pacific. To present little is known about the Cape Horn current even though it plays a key role in our understanding of the present day and paleoceanography in the Subantarctic Southeast Pacific as well as the exchange of water masses in the northern Drake Passage. Our oceanography group led by Wolfgang Schneider (University of Concepcion, Chile) and Harold Fenco (National Institute for Research and Development, Argentina) is being strongly supported by our Argentinean and Chilean observers on board. The main device used for the oceanographic work is the CTD-Rosette which measures water conductivity, temperature and depth in the entire water column. Additionally installed acoustic current meters provide information on ocean currents.
With the CDT-Rosette water samples from different depths can be obtained. Coordinated by Lester Lembke-Jene (AWI) and Hartmut Schulz (University of Tübingen), these water samples are used to improve, calibrate and validate microfossil proxy methods. For the Cape Horn current we had eight CTD stations and during our Drake Passage traverse we performed 16 CTD stations. The transect covers the major oceanographic currents of the southern Antarctic Ocean over a relatively short distance (from north to south: Sub-Antarctic, Polar and Southern Antarctic Circumpolar Current Front). For that reason oceanographic work during the past 25 years has extensively focussed on the Drake Passage. However, this work has mainly been located between Cape Horn on the Sub-Antarctic islands and hence not on the Pacific side.
The Geology team led by Helge Arz (IOW), Gerhard Kuhn (AWI) and Dirk Nürnberg (GEOMAR) were keeping a watchful eye on the PARASOUND sonar. They had been following large sediment packages from the Chilean deep sea trenches into the Southeast Pacific toward the Sub-Antarctic Front. Several tens of miles before the front, the ocean floor was devoid of any form of sediment. From here on they were following the ocean floor crust’s mounds and troughs. The explanation for the lack of sediment is the same as our interpretations at “Isla de los Estados”. The ocean currents at the Sub-Antarctic Front are extremely strong, which means fine sediment is not able to settle. Hence, no substantial sediment was identified until past the Polar Front. Once we reached the Antarctic-Pheonix Ridge were we able to take sediment cores again. Shortly before the Hero Fracture Zone we managed to obtain a 14 meter long sediment core (Fig. 1).
The ProIron group (Fig. 2) from the Alfred Wegener Institute, University of Geneva, University of Bremen, University of Oldenburg and ETH Zurich, conducted its first test sampling on February 28th. The goals of the study are to elucidate the impact of iron limitation and trace metal cycling (iron, zinc, cobalt, manganese and vitamin B12) in the Drake Passage and the West Antarctic Peninsula under current and future climate. The test sampling from the open ocean is necessary to ensure that the low trace metal containing seawater could be sampled without introducing any trace metal contamination from the Polarstern ship and equipment. To achieve this, a teflon membrane pump connected to a polythelene line was used to pump surface seawater from 25 meters depth (Fig. 3) directly into a trace metal clean van where the seawater could be sampled. Our first test run was carried out under very good weather conditions, the sun was shining and the sea state was calm, hence providing perfect sampling conditions. The test was successful as microscopic observations revealed that we were able to sample a diverse and healthy phytoplankton assemblage showing no sign of damage due to the pumping.
Since we were still not in waters south of the Polar Front, our target region in the Drake Passage, we had to wait to sample for our main experiment until March 1st at which point we had finally passed the front. Unfortunately this also coincided with a storm, not atypical for this region, causing wind speeds of up to 17 m s-1 and producing waves of up to 4 meters. This made it impossible to pump any seawater. Hence the ProIron group was forced to wait yet again for better weather conditions and to catch a few more hours of sleep. Finally, on March 2nd at 5:25 a.m. the polythelene line could be deployed for our first big BIO1 station. We then proceeded to pump water for the next 23h. This water contained a, for a HNLC (high nutrient low chlorophyll) area, very typical and diverse phytoplankton community (Fig. 4). We used this water to set up experiments aimed at shedding light on the role of iron limitation on Southern Ocean phytoplankton community structure and productivity, and to identify other potential co-limiting factors such as cobalt, vitamin B12, zink, manganese and light. Given that most of the iron is associated with loosely defined organic compounds, which greatly affects the chemistry and bioavailability, thus potentially limiting the growth of phytoplankton, experiments were also designed to test phytoplankton sensitivity to different iron complexes. Here we focussed on compounds that are biologically produced such as siderophore, saccharides and exopolymeric substances.
In total, we are carrying out seven different experiments, using the same seawater and its plankton community in order to study the development of its phytoplankton community structure, effects on plankton growth rates, productivity, photophysiology and trace metal uptake rates. All experiments are running for at least ten days. To elucidate the nature of the organic compounds that control iron chemistry in-situ, 1000 L filtered seawater were sampled in a tank that was then subjected to two different concentration mechanisms (ultrafiltration and solid phase extraction) at sea. Finally, phytoplankton, marine bacteria and viruses were isolated for further studies in our home laboratories as these are central actors for iron recycling. On March 3rd at 5 p.m. after having pumped more than 8000 L of seawater, using trace metal clean techniques, we successfully finished our sampling and brought the polythelene line back on board of RV Polarstern. Even though the water sampling is now finished, having obtained all the water we need, the bulk of the work starts now, since observing the development of our different incubation experiments and their consequent processing is very labor-intensive. To execute this challenging task we occupy six laboratory containers, which house all our different experimental bottles. Our first iron measurements (very low iron concentrations of 0.07 nmol L-1) confirmed that we were successfully able to sample without contamination, affirming our trace metal clean techniques. These are concentrations, which are typically observed in the offshore iron-limited waters of the Drake Passage.
After completing the biological work we continued toward the Antarctic. The wind and waves calmed down whilst air and water temperatures approached close to freezing. Yesterday evening we crossed the southernmost oceanographic front of the Antarctic Circumpolar Current at the 5000 meter deep South Shetland trench. Land was slowly nearing, however, heavy fog hindered the view of the impressive peaks of Smith Island, steeply emanating from sea level up to 2000 meters in altitude. The afternoon would bring opening in cloud cover, where we got our first glances of icebergs and the spectacularly glaciated islands and the Antarctic Peninsula. At the moment we are at our geological station with clouds slowly pulling back in. All participants are well and are looking forward to the scheduled barbeque on Tuesday.
Chief Scientist PS97
Position: 63°45,41´S; 60°26,5´W
(10 miles north of Graham Land; Antarctic Peninsula)