ARK-XXII/2, 2nd Weekly Report
6 - 12 August 2007
Clean iron measurements in the deep sea
One of the programs of our cruise aims at better understanding biogeochemical cycles in the Arctic. Many substances are carried into the Arctic through the huge river runoff from Siberia and North America or are advected from the Atlantic or the Pacific. Others are blown in with the wind. Their distribution in the water gives information about the pathways of water masses and so helps to identify ocean currents and sea ice motion. In addition, the mixture of components affects chemical and biological processes.
To determine the physical and chemical properties of water masses we measure a vertical profile of temperature, pressure and electrical conductivity (from which salinity is derived) with a ship-borne so-called CTD system (Conductivity, Temperature, Depth) every centimetre from the sea surface to the ocean floor. The CTD system is directly connected to a computer in the lab through a 6000 m electronics cable on a winch. In this way we can immediately see the profile down to three or four thousand meters depth and obtain a first idea about the stratification of the water column. Bottles for sampling water are attached to the system and are closed electronically at selected water depths. In front of the temperature and salinity profile chemists and biologists can choose which depths they want to sample. A series of these profiles are combined to give the distribution of the measurements across a section.
During our cruise we use two of these CTD systems. One is a standard system, and the other one is newly developed by the Netherlands Institute for Marine Research (NIOZ) for ultra-clean sampling. The standard system is fine for all kinds of substances, except those that are present in the ocean at very low concentrations, but are very abundant onboard a ship and in research equipment. This holds for metals like iron, copper, zinc and many others.
Even if concentrations of metals in ocean water are very low, they can play an important role for biological production. The most prominent case is that of iron. Iron limits phytoplankton growth in the Southern Ocean, even though there is an abundance of basic nutrients such as nitrogen and phosphorus. This is comparable with to junk food for us: piles of pork legs do not suit us unless we eat also a raw carrot from time to time. But no one should worry about our food here onboard - our menu offers a large variety and is full of vitamins and minerals. For example, tonight we had blue potatoes. Have you ever tried them? Delicious!
Back to the Ultra Clean system: this system consists of a titanium frame onto which the CTD probe is attached, and which holds 24 water samplers. Instead of a conventional steel-coated cable that would contaminate trace metal samples, the data cable on this system is coated with Kevlar. And to avoid contaminating the samples with the metallic surrounding of the ship as they come out of the water, the entire system is brought to an ultra clean container as soon as the system gets on deck. At the beginning of the cruise, despite great curiosity, hardly anybody dared to go on deck during the stations, but meanwhile we got more courageous and we are allowed even to enter the Ultra clean container that has an entry room that can be accessed even when dirty.
This cruise is the premiere for the Ultra Clean system. As such a system did not exist so far, little is known about trace substances in the world ocean. The survey in the Arctic is the start of the international Program GEOTRACES, which will run for the next 10 years to sample the global ocean.
We had planned to continue the section along 34E up to 87N, i.e. from the 200 m deep Barents Sea shelf across the 4000 m deep Nansen Basin until the Nansen Gakkel Ridge. But in the middle of the week we just had reached 84 30N and tried to make our way through the densely packed sea ice. The ice was not particularly thick, and flights with the helicopter carrying the EM bird, an electromagnetic probe, showed that flat ice floes had a thickness of one meter only. But light southerly winds from a high-pressure system centred at Frans Josef Land had pushed the ice towards north so that almost no leads were left between the flows. Even an ice-breaker like the Polarstern is needs some space to push the broken ice into in order to move. Furthermore, the wind pressing the ice together has piled up thick ridges that are even harder to break through than dense ice. Since we plan to cross the Nansen Gakkel Ridge further to the east later during the cruise, we decided to skip the rest of the first section and move eastward.
On our way we passed through a most beautiful "lake district". We were surrounded by thousands of large and small ponds, the shallows ones shining turquoise, the deep ones dark blue. Some ponds have a hole in the bottom that is black. The water of the ponds is melted sea ice. During summer the surface of the thick ice thaws due to the 24 hours-a-day insolation. Even a shallow melt pond reflects the sunlight less efficiently than the withered white ice surface. Instead, it absorbs the energy, which further enhances melting. Also, a satellite sees a melt pond differently than a completely frozen surface. Melt ponds on a flow can not be distinguished from open leads between flows and so in summer the real ice cover is underestimated with satellite remote sensing. A group of sea ice researchers from Hamburg is trying to tackle this problem. They measure the backscatter of radar signals with different frequencies from the helicopter. These researchers believe that wind waves on small ponds have a composition different from those on larger open leads which causes them to scatter differently. If this is true, the differences in scatterometry could be used at satellites to improve estimations of ice cover.
In the meantime our geologists have taken their first sediment samples from the deep sea north of Frans Josef Land where we move southwards along 62°E. They are interested in glaciation history; we will report more about it next week. It is snowing wet flakes and the wind is wet and cold from the north - like at Christmas in Bremerhaven!
Best regards to the south, Ursula Schauer