ANT-XXIV/3, Weekly Report No. 7

Sunday, 6 April 2008

GEOTRACES and moorings in Drake Passage

We have reached Drake Passage where the third and final part of our scientific programme takes place. Last Sunday we left King George Island with a new French/Korean group on board. In addition it was possible to load material from Jubany and Frei stations. However the bad weather conditions, which showed no hope of improvement, forced us to give up our final task of loading material from Artigas. Dense fog prevented any further flights for an unforeseeable future. During the night, we steamed to Drake Passage to continue our measurements of water mass properties and concentration of trace substances.

The aim of the GEOTRACES group is to measure the concentration and distribution of a variety of trace substances. Dissolved trace metals in seawater are the focus of their research. Iron is a very important trace metal for biological processes in the Antarctic Ocean. It is essential for all living organisms, and thus for the algae also. These algae are the basis of the food-chain of the Antarctic region and are in turn dependent on the availability of iron. However, iron is only found in extremely low concentrations of circa one hundredth of one millionth of a gram per litre seawater (10 nanogram per litre = 10 ng/L = 10-8 gram per litre). In contrast POLARSTERN is a strong ship of steel, iron is everywhere on the ship, iron is the ship. Therefore the sampling of seawater is done with the special ultraclean frame holding 24 samplers. Once this frame is back on the deck, it is immediately placed in its own laboratory container, so as to rule out direct contact with the iron of the ship. This allows us to collect the first-ever complete vertical sections, from surface to circa 5 kilometres deep bottom, in the Southern Ocean. Along the Zero meridian section we found dissolved iron is very low from 5 ng/L in surface waters increasing to 30 ng/L at great depth. In the southern part of the Weddell Gyre, between Maud Rise and the ice shelf of Antarctica, the values are even lower, from 3 ng/L in surface waters to 20 ng/L in deep waters.

It is one thing to know how much, or how little, iron there is in the seawater, but in addition we wonder where this iron has come from. Has it been blown into the ocean in dust storms carrying soil dust from land to sea? After all, soil on land contains much natural iron, about 4 percent of soils is iron. Soil also contains much aluminium (Al). Therefore we also measure dissolved Al as a source tracer for dust. Along the Greenwich meridian the dissolved aluminium in seawater is extremely low, the lowest found thus far in the world oceans. Very low levels of 6 ng/L in surface waters tell us that dust input from land is very small, if any.

Therefore the dissolved iron must come from somewhere else. In the sediments the conditions are better for iron to dissolve from the sediment particles and then enter into the bottom waters. So, perhaps that is the source of iron to the sea. We know that another element, manganese (Mn), can also be dissolved in the sediments. Consequently we use manganese as a source tracer for iron coming from below, from the bottom sediments. However the concentrations of dissolved manganese also are extremely low, from 3 ng/L in surface waters to sometimes about 10 ng/L in deeper layers. Only over the ocean ridge, formed by deep-sea volcanism, we find more manganese, and also more iron, in the deep waters. This was found before, in our summer 2007 Arctic POLARSTERN cruise. Hydrothermal circulation associated with deep-sea volcanism, is perhaps the most important source of iron in the ocean waters.

Searching for iron in the Southern Ocean is like the search for a needle (iron, of course) in a haystack. Others in the team search for their favourite metal. Zinc and copper are also necessary for all organisms and occur in very low concentrations as well. Overall the dark secrets of the deep unknown waters of the Antarctic Ocean are now being discovered for the first time.

Not only can the search for iron in the ocean be compared with the search for a needle in a haystack. Moorings can show similar properties. While the first two moorings of the Korean group in the southern Drake Passage could be recovered in spite of the unfavourable weather conditions with no problems, significant difficulties occurred with the moorings of the French group despite much better weather. We had to give up on the recovery of the first mooring after a long, unsuccessful, searching operation with a dredging wire. The second mooring was recovered after painful waiting for hours before it was finally discovered at the surface. With the help of our Posidonia system, both moorings could be monitored during all operations. It appeared that the flotation system had broken when the mooring was plunged deeper by strong currents. There remained just enough buoyancy that part of the remaining floats barely reached the surface carrying behind them the remnants of the mooring. On Sunday afternoon, luck returned to the mooring work and we could successfully recover the third French mooring without any difficulties after a quite foggy morning. Now we are in a state of high apprehension in our expectations as to what might be the fate of the other moorings in front of us.

With our best regards from all on board

Eberhard Fahrbach