ANT-XXVI/4, Weekly Report No. 5

4 May - 10 May 2010

At eight o’clock in the morning of May 5th the anchor of R/V Polarstern is dropped in the wide bay of Mindelo of the Cape Verdean island of São Vicente. The reason for our short visit to the Republic of Cape Verde – the first ever in the 28-year history of the vessel – are scientific studies at the Cape Verdean ocean time-series station, which are carried by the IFM-GEOMAR in Kiel. This station has been operated for several years now in cooperation with INDP, the national fishery research institute of Cape Verde. The stopover was not only used for embarkation of two more scientists and delivery of scientific equipment but also for a small reception for the cooperation partners from INDP. The Cape Verdean colleagues, who are operating a 22-m research vessel, where deeply impressed by the size and standard of the R/V Polarstern. After a short introduction to the scientific program of our current cruise and a short interview with the national Cape Verdean TV station the time had come to say Goodbye and depart for immediate continuation of the tight schedule of cruise ANT-XXVI/4.

Besides an intensive sampling program we also had to accomplish an exciting task at the time-series station north of Cape Verde: A swarm of gliders from the Kiel Leibniz Institute for Marine Sciences had to be recovered. Like glider planes these sophisticated instruments “fly” through the ocean and measure temperature, salinity, oxygen, chlorophyll and turbidity. The instruments, which generate propulsion through their wings only by means of changing their specific weight, can operate for months on a few Watt of power only. After several hours and a couple of zodiac trips all four gliders had been found and safely recovered from the ocean. Once onboard, Mario Müller – a glider specialist from the Leibniz Institute for Marine Sciences in Kiel – thoroughly inspected and cleaned the instruments and secured the valuable data sets.

The working group that I would like to present in this week’s report is my own group at the IFM-GEOMAR and its close cooperation partners at the Institute for Coastal Research of the GKSS in Geesthacht and the Institute for Physical Chemistry at the University of Kiel (Steffen Assmann, Meike Becker, Björn Fiedler, Gernot Friedrichs, Arne Körtzinger). We concern ourselves with the global carbon cycle and its anthropogenic perturbation. Human activitites have – largely through burning of fossil fuels such as gas, oil, and coal – caused atmospheric CO₂ concentrations to rise by nearly 40 % since preindustrial times. The additional atmospheric greenhouse effect associated with this CO₂ increase is responsible for the major part of the observed climate change. The world’s ocean is a giant carbon reservoir and provides a huge uptake capacity for anthropogenic CO₂ and thereby strongly dampens climate change. Projections of future climate change therefore depend on robust understanding of the oceanic carbon sink and its future development under changing climate forcing. This requires a detailed knowledge about trends and inherent natural variability in marine carbon reservoirs and their physical and biological drivers. Marine carbon research is confronted with the challenging task of providing highly precise measurements at high spatial and temporal resolution – at task that can only be accomplished in international collaboration.

The aspect of high measurement quality is addressed by improvement of the chemical methods employed and the development and testing of new instruments and sensors. This is always done in view of their use on autonomous platforms such as deep-sea moorings, profiling floats and merchant vessels which help to strongly enhance spatiotemporal coverage. Along this line a new sensor for measurement of the CO₂ partial pressure, which was developed – in cooperation with us – by the Kiel company Contros, is tested in continuous mode throughout the cruise. Further tests in the water column help to evaluate the sensor performance under rapid temperature change and high pressure. After the successful test this sensor will soon be deployed on profiling floats near Cape Verde. Another example is an automated system for high-precision flow-through measurements of the pH of seawater that has reached a respectable precision of a few tenthousands of a pH unit during this cruise. The same system is now tested in a different setup for the feasibility of alkalinity measurements, another important quantity of the marine CO₂ system. A third example, finally, is a Cavity Ring-Down Spectrometer, which is employed to measure the ¹³C isotope signature of the marine CO₂ system in surface waters. This rather young technology provides extremely sensitive absorption measurement capabilites through an effective optical path length of several kilometers. In combination with a gas-water equilibration system precise measurements of the ¹³C/¹²C ratio of dissolved inorganic carbon, which provide different insight into major processes of the marine carbon cycle, can now be made at sea. After the very promising results of our cruise the system is planned to be deployed in fully autonomous mode on a merchant vessel in the near future.

With best regards from a very pleased chief scientist,
Arne Körtzinger