Tracking down sea surfaces in the wind tunnel

An international team of researchers has had 20,000 litres of seawater from the North Atlantic transported by tanker to Heidelberg University for analysis - including marine researchers from the University of Oldenburg.

The scientists are analysing the seawater in the Aeolotron, a special ring-shaped wind-wave channel. The large-scale experiment marks the start of a large-scale interdisciplinary study on natural, biologically produced surface films of seas and their influence on gas exchange between the atmosphere and the ocean. Scientists from Oldenburg, Kiel, Leipzig, Lyon, Mainz, Manchester and Warnemünde are involved in the project, which is headed by Heidelberg environmental physicist Prof Dr Bernd Jähne. Marine researcher Dr Oliver Wurl, head of the "Sea Surfaces" working group at the Institute of Chemistry and Biology of the Marine Environment (ICBM), is involved in the research project on behalf of the University of Oldenburg.

More than two thirds of our planet's surface is covered by oceans. The area where seawater and atmosphere meet is therefore enormous. At the interface between these two important parts of the Earth system, exchange processes take place that determine how much heat, gases and volatile substances are transported through the ocean surface and how strongly the wind pushes the ocean currents. However, despite their importance for the climate or ocean currents, these exchange processes have so far only been understood in fragments. The research project at the Aeolotron will now help to analyse these complex processes. The main focus is on the one-millimetre-thick viscous boundary layers on both sides of the water surface.

Wurl, who set up one of the first research groups worldwide for biochemical processes on the ocean surface at the ICBM, hopes to learn more about the exchange of carbon dioxide between the atmosphere and the ocean as part of the project. "The world's oceans absorb almost a third of the carbon dioxide produced by humans. They are therefore at the centre of the global CO2 cycle," explains the marine researcher. Carbon dioxide forms weak acids when combined with water. The first signs of ocean acidification are known. The UN's Intergovernmental Panel on Climate Change (IPCC) recently announced in its latest global climate report that the acidity of the ocean has increased by 26 per cent since the beginning of the industrial age. Against this background, it is essential to better understand the process of CO2 uptake by the ocean, says Wurl. In particular, the natural organic films that form on the ocean surface and slow down gas transport play a key role in this.

However, the function of the surface films on the high seas is difficult to determine as they are exposed to constant wind and current changes. The Aeolotron wind-wave channel makes it possible to simulate uniform wind speeds and wave properties. These are ideal conditions for investigating the wafer-thin microfilms and their influence on gas exchange. In co-operation with Dr Bernd Schneider from the Leibniz Institute for Baltic Sea Research Warnemünde, Wurl and his team use the Aeolotron to test the CO2 content in the air and seawater under different wind and wave conditions and calculate the gas absorption by the seawater. The scientists skim off organic films and analyse them both chemically and microbiologically. "With increasing concentrations of organic molecules, mainly natural carbohydrates, proteins and fatty acids, the gas exchange rate in seawater decreases," reports Wurl. Initial results from Heidelberg show that the thin surface films persist at typical oceanic wind speeds and thus influence gas exchange. This is an aspect that has so far been neglected in conventional climate predictions. "We can already say that from a global perspective, the exchange of climate-relevant gases is more clearly influenced by the surface films than previously assumed," says the marine scientist.