ANT XXIV/1 Weekly report No. 3

12 to 18 November 2007

We have reached the Angola Basin approximately 500 miles west of the coast of Angola and Namibia. At 6 o'clock on November 17, the large MOCNESS 10 was deployed and around noon it had descended to 5110 m. It was the deepest tow ever that Peter Wiebe, Oceanographer from the Woods Hole Oceanographic Institution in Massachusetts and constructor of the MOCNESS has done. To get to that depth the maximum amount of wire (more than 8 kilometers) on the winch drum was paid out, down to the last wrap. Deep sea tows are greatly strenuous for net and winch and often the unexpected happens: during the employment the cable connecting the net bar to the toggle release mechanism parted already at 115m instead of at 1000 m of water column. This small misfortune could be easily compensated by a longer employment of the following net. The catches were spectacular. Very large copepods of 1.5 cm body length, one of the largest known, were present in substantial numbers. A large eyeless chaetognath was caught. Species of fish rarely seen were present in several nets, including a female angler fish with an attached dwarf male. This type of reproduction is unique within the vertebrates.

While most of the scientists on board are interested in the zooplankton, the "Phyto-Optic" Group from the AWI and the GKSS research institute in Geesthacht, Germany has been continuously taking water samples since the beginning of the cruise to measure phytoplankton, the planktonic primary producers in the surface waters, which form the basis of all oceanic foodwebs. Endlessly, they filter around 200 liter sea water daily, to measure the concentration of the microscopically small (between 1µm and 1mm) phytoplankton of the upper water layers along the cruise track. The samples are preserved in liquid nitrogen for species identification back in the home labs. As the optical properties of the surface waters depend on the phytoplankton within, measurements of absorption, reflectance and fluorescence can give important information about the phytoplankton community. As expected, the phytoplankton abundance in the open ocean is relatively low, but a bloom indicated by higher fluorescence and absorption data was detected at the equator.

The hypothesis of the existence of a so called biological pump influencing the atmospheric and seawater concentrations of POPs (Persistent organic pollutants) is being investigated by the group of the University of Lancaster and the GKSS by collecting air and water samples in parallel with biological measurements, such as oceanic chlorophyll pigment concentrations and fluorescence levels. Is there a possible link between the diurnal cycling of POPs in air over parts of the open ocean, and biological processes in the sea, which may control these phenomena? Such measurements will help us to understand how the changes in size, species and community composition of phytoplankton may influence the atmospheric concentrations of pollutants.

Furthermore, the chemists from Lancaster analyse perfluorinated compounds (PFCs), a newly emerging class of chemical contaminants, whose global distribution is currently poorly known. Due to the findings of PFCs in numerous organisms even from such remote locations as the Arctic, as well as in human blood, it is of special interest to investigate their long-range transport from likely source regions in Europe to the more pristine Southern hemisphere. Air and water concentrations obtained along a latitudinal transect during the cruise will be the first spatially highly resolved data for the Atlantic Ocean.

The distribution and amount of different atmospheric trace gases in the atmosphere is measured by the Heidelberg Institute of Environmental Physics en route. The measurements are carried out with the "Differential Optical Absorption Spectroscopy" (DOAS) which measure the scattering and absorption of the sunlight in the atmosphere. A telescope set up on the Monkey-Deck collects scattered sunlight from different viewing angles. The characteristic (absorption-) cross section of each trace gas allows us to calculate the concentration of many important components such as Ozone, NO2, BrO etc in concert. The collected data will be integrated within a worldwide net of atmospheric research.

The meteorologists from the Leibniz Institute of Marine Science IFM-GEOMAR in Kiel, determine temperature, humidity, the liquid water and water vapor path from the surface up to 10 km. The atmospheric profiles will also be used to validate the satellite based profiles from the IASI (infrared atmospheric sounding interferometer) instrument on board the new European polar orbiting satellite MetOP. At the same time, the box sky imager takes a picture of the whole sky dome from horizon to the zenith every 15 seconds. These pictures are used to determine the cloud coverage, which is used to improve the parameterization of insolation at different cloud conditions in climate and weather prediction models.

On November 13, in the early morning at 5 minutes to 6 am, we crossed the geographical equator. No changes were obvious in the ocean but, on board Polarstern strange things happened. Dirty and coming from the northern hemisphere, 45 filthy subjects had to pass a ritual washing - irrespective of age and rank, accomplished by Neptun and his wife Thetys themselves - Equator baptism! A trifle disgusting happening, but all participants survived the procedure with lots of good humour. In the evening colourful certificates of baptism were handed over by the captain along with beer and bratwurst out on the open deck which was felt as a due compensation for the hardships endured.

Best regards from all of us from board Polarstern and the vast realm of the South Atlantic.

Sigrid Schiel