ANT-XXVI/1, Weekly Report No. 4

10 November - 16 November 2009

After Polarstern has finished its 23°-W section at 20°S, on November 11 we take direct route towards the Vema Channel in order to run two deep CTDs for our colleagues from the Physical Oceanography section, in addition to our continuous ocean/atmosphere observations. Since 1972 these CTD measurements are performed. They shall provide insights into a possible warming of the Antarctic bottom water. At the other day we catch the highest solar elevation of 88° on our travel. The last two weeks we got used to the situation that the sun exactly rises at port side and sinks at starboard. The surface water is of a breathtaking azure and at this rather high solar elevation we can see light rays deeply penetrating into the water (Fig. 1). These are ideal conditions for our measurements of the penetration depth and variability of the available radiation. However, one man's meat is the other man's poison. Our marine biologists go out empty handed in these clear water conditions.
During the night to the 14th of November we reach our first station at the Vema Channel and descent the CTD to about 4300 down to the bottom of the ocean. The lights on the working deck first attract flying fish followed by calamari. Indeed, we get more and more animal observations. Besides albatrosses, northern gannets, whales and dolphins also a young sea gull family has been observed (see Fig. 2). We run the second Vema CTD in the night to November 16. Again, calamari visited our station. A first glance to the bottom-near CTD-measurements shows that very similar values for potential temperature and salinity have been found compared to 2008.
With deepest regrets we received the sad announcement of the sudden and totally unexpected death of Dr. Sönke Neben from AWI. Sönke Neben was responsible for the logistics of the scientific operations on board, and many of us have enjoyed working together with him in preparing the current expedition. Our thoughts and deepest sympathy are with his family.

This week, Steffen Aßmann, Tina Baustian, Peer Fietzek and Harald Schunk report about the chemical and biological measurements during this cruise:

The focus of the chemical work on board is on the investigation of the marine carbon cycle and its interaction with the atmosphere.
Since CO₂ dissolves in water very well, there is an active exchange of carbon dioxide between the atmosphere and the seawater driven by weather. In contrast to other gases, which show a simple solution behavior, carbon dioxide reacts in seawater leading to e.g. the formation of carbonic acid. This chemical process is the reason for a decrease in the ocean’s pH value as a consequence to the anthropogenic increase of CO₂ in the atmosphere. This decline is referred to as ocean acidification and it effects the marine biota to a great extend.

The amount of dissolved CO₂ is being measured on board with several instruments (Fig. 3: the cavity ringdown spectrometer (front left), the blue underway box (back left), the permanently installed pCO₂ system as well as the FerryBox (both back right) and one of the in situ pCO₂ sensors (lying behind Peer on the desk). On the table in front of Steffen the pH-instrument can be seen).
First of all, there is a permanently installed pCO₂-underway-system in the wet laboratory, which measures the partial pressure of dissolved CO₂ in a flow-through setup. Secondly, a system of the same kind was set up in a slightly modified version and it is used on sea for the first time together with a new detector type: a cavity ringdown spectrometer. Besides measuring the absolute CO₂, this very accurate, optical instrument allows the determination of isotope resolved concentrations of ¹²CO₂ and ¹³CO₂. The isotope ratio is another significant parameter for the investigation of the carbon cycle. Additionally, an oxygen, a total gas tension as well as an underwater pCO₂ sensor are run in an underway box, which is continuously flushed with surface water. CTD measurements during this cruise leg allow for the analysis of the depth dependent response behavior of the already mentioned underwater pCO₂ sensor. Water samples are taken regularly for a later total inorganic carbon as well as total alkalinity analysis back home in the lab. These provide another reference for the conducted CO₂ measurements. Furthermore the pH value of the surface water is regularly determined by means of a highly accurate, optical sensor.

Within the framework of OCEANET a FerryBox is installed permanently on the Polarstern since this expedition. The FerryBox is a position controlled, low-maintenance measuring system, which was designed for long-term application on ships or other measuring platforms to enable a basic, but yet comprehensive, description of the water condition. Therefore oceanographic, chemical and biological parameters such as temperature, turbidity, salinity, pH, oxygen, chlorophyll a, phycocyanin and yellow matter are measured.

Next to the studies mentioned above, the Nitrogen cycle is being looked at more closely on this cruise. Nitrogen compounds provide fundamental building blocks for cells and are therefore essential nutrients for all living organisms.  In vast parts of the world’s ocean nitrogen limits primary production. In these areas prokaryotic organisms, known as diazotrophs, are particularly important as they are the only organisms capable of converting molecular Nitrogen (N₂) into Ammonia (NH₄), which is readily assimilated also by other organisms.
Global rate estimates of nitrogen fluxes between ocean and atmosphere do not balance and suggest a net loss of fixed nitrogen from oceanic systems. However, since most parts of the world’s ocean are still under-sampled with regards to marine diazotrophs, one reason for the imbalance of the marine nitrogen budged may be due to the paucity of these data. Therefore abundance, diversity and activity of diazotrophs as well as bulk primary production rates are being determined on this cruise.
For that purpose seawater samples from the surface as well as from depth profiles down to 450 m are being taken at regular times. The seawater samples are being filtered and conserved on board. Following the cruise they will be analyzed using molecular methods as well as flow cytometry and mass spectrometry. In order to get a better understanding for the results gained, samples for the determination of further environmental parameters (nutrients, particular organic carbon, particular organic nitrogen) are being taken on the cruise as well.

For the course of the weather: We leave the ITCZ and arrive into the influence of the South-Atlantic subtropical high with its centre to the south-east of us. Almost the entire reporting week the trade winds are blowing from south-easterly direction, at the end more from the east. The corresponding swell of 2 m at most and wind sea of about 0.5 – 1 m do not hamper our zodiac tasks. A low originating in South-East-Brazil disturbs the high pressure influence with occasional precipitation. The weekend is mostly cloudy. With this our passage through the southern trade wind region is much cloudier than that through the northern trade winds. This renders the hemispheric inter-comparison difficult for our aerosol team. We are slowly approaching the midlatitudes of the southern hemisphere with its clockwise rotating deep pressure systems. A view to which a northern hemisphere Meteorologist has to get used to. Quasi as a compensation for our deep pressure influence in the otherwise rather cloudless sub-tropics the westerlies welcome us with a strong high with much sunshine.

Best regards on behalf of crew and scientists,

Andreas Macke