Fig. 1: Ocean 'landscape' with wind and waves - where is the dust? (photo: A. Körtzinger)
ANT-XXVI/4, Weekly Report No. 3
20 April - 26 April 2010
After 3900 nautical miles (or 7200 kilometres) about half of the cruise ANT-XXVI/4 lies behind us. We are now quickly approaching the equator and the sea surface temperature has climbed to over 29°C. Also, the trade winds constantly blow 28°C warm air at 4-6 Beaufort from a southeasterly direction. Soon, however, the trade wind clouds will give way to high-reaching cumulonimbus clouds – a first sign of the Intertropical Convergence Zone, the equatorial low pressure ridge also known as doldrums. More on this, however, in my next report. This week’s report is dedicated to the trace metal and dust investigations of this cruise. These are carried out by a group of scientists from the Leibniz Institute of Marine Sciences in Kiel/Germany (Thibaut Wagener, Maija Heller, Anna Dammshäuser) and an aerosol specialist from the University of Córdoba/Argentina (Diego Gaiero). Despite or even because of their extremely low concentrations trace metals play an important role in the ocean. On the one hand, a biological function has been proven for several trace metals with iron being perhaps the most prominent example. One the other hand, trace metals provide insight into several geochemical processes, e.g. on the origin, deposition and dissolution of atmospheric dust in the ocean. By delivering macro (phosphorus) and micro nutrients (iron) to the ocean this fine dust fraction from regions such as the Patagonian and Saharan deserts plays an important role for the biological productivity of the open ocean.
Fig. 2: Thibaut Wagener (left) and Diego Gaiero take water samples with a special GOFLO sampler. (photo: A. Körtzinger)
The chemistry of iron in seawater is particularly complicated and even today not fully understood. In the well-oxygenated surface ocean dissolved iron is rapidly oxidized to trivalent iron which is rather insoluble and hence precipitates as hydroxide. Therefore all processes that convert or stabilize iron in dissolved bio-available forms are biologically relevant. In these processes, which include redox reactions and iron complexation by organic substances of biological or photochemical origin, rather exotic chemical species are involved. One such exotic species is superoxide, which can be produced via photo-oxidation or organic matter. On this cruise, water samples are taken daily for measurement of iron(II) and hydrogen peroxide, a product of superoxide decomposition, as well as the different decay pathways of superoxide. Further measurements focus on the fluorescence and absorption characteristics of coloured dissolved organic matter, also known as yellow substance, which is photochemically active and responsible for the production of superoxide. Finally, experiments with the radio nuclide iron-55, which are carried in a special radio nuclide container, provide information about the solubility of the various chemical forms of iron in seawater.
Fig. 3: Anna Dammshaeuser in full outfit performing trace metal measurements in a clean air. (photo: T. Wagener)
A second research focus of the group is placed on the release of elements from atmospheric dust. This includes bio-active elements such a phosphorus or iron but also bio-inactive elements such as aluminum or titanium. While the former are known to have certain biological functions the latter provide insight into deposition and dissolution or airborne dust.
Our current cruise ANT-XXVI/4 is particularly well suited for this purpose as it provides the full contrast between the dust poor regions of the South Atlantic subtropical gyre and the dust plumes of the Patagonian and Saharan deserts. It is noteworthy that the Saharan dust plume which extends from West Africa all the way to the Caribbean is by far the largest dust source to the world ocean, delivering on the order of 200 million tons of dust annually. By means of aerosol collectors we can get hold of this dust and subject it to release experiments to gain a better understanding of the dissolution processes. Also eolisable dust samples that were taken in different Patagonian source regions are exposed to different seawater types during 10-day release experiments.
Fig. 4: Maija Heller determines the optical properties of yellow substance. (photo: T. Wagener)
Measurements of natural trace metal concentrations pose very stringent requirements on sampling and sample treatment. In the case of iron it is obvious that, because of the high contamination risk, a ship is a rather problematic but yet indispensable platform. Therefore special GOFLO water samplers are used which are deployed closed and open only under hydrostatic pressure at about 10 meters depth. Thus their ultra-clean interior does not get in contact with the contaminating ship environment. For the same reason these samplers cannot be deployed on a normal metal wire but have to be attached to a special Kevlar rope. However, all these precautionary measures are not restricted to the sampling alone. In fact all measurements and experiments have to be carried out in a special clean air container or clean bench which feature filter systems to reduce the number of particles in the air to a minimum.
In contrast to the dust-plagued European air traveller we are hoping to hit a lot of dust off West Africa after passing the Intertropical Convergence Zone. So we keep our fingers crossed and send our heartfelt regards,
Arne Körtzinger