METEOR ScienceLog - Nr.7
Autor: Der gebürtige Neuseeländer Dr. Peter Croot ist wissenschaftlicher Mitarbeiter in der Sektion Chemische Ozeanographie am Ifm-GEOMAR in Kiel. Als Biogeochemiker interessiert sich Peter Croot für die Kreisläufe und Redox-Prozesse (Reduktion/Oxidation) von Spurenmetallen (z.B. Eisen, Mangan, Kupfer und einige mehr) im Meeresboden. Auf der Expedition M77/1 mißt der Meeresforscher sauerstoffempfindliche Verbindungen wie etwa Iodid, Peroxid und zweiwertiges Eisen und arbeitet mit Wissenschaftlern an Bord zusammen, die sich mit ähnlichen Redox-Prozessen beschäftigen.
Author: Dr Peter Croot (a native of the Pacific from the South Island of New Zealand) is a researcher in the chemical oceanography department at the IFM-GEOMAR and his studies focus on the marine biogeochemistry of trace metals (e.g. iron, manganese, copper, cadmium, cobalt, nickel, titanium, aluminium and zinc) and redox (reduction/oxidation) cycling in the ocean. During M77-1 he is conducting water column measurements of oxygen sensitive chemicals (Iodide, Peroxide and Fe(II)) in collaboration with the other groups onboard investigating similar redox processes in the sediment.
Wo ist der Sauerstoff/ Where ist the oxygen?
METEOR M77/1 LOG
11. November 2008:
The Meteor has been spending the last week working along 11?S off the coast of Peru. While the focus has been mainly on studying what is happening in the sediments there are also people investigating what is occurring in the water itself. This region is particularly interesting to oceanographers because in these waters just below the surface there is almost no oxygen present which strongly affects the chemistry and biology here. Here in the offshore waters of Peru, a cold current flows from the south bringing nutrients (phosphate, nitrate and silicate) into the warm sunlit Tropical surface waters along the Peruvian shelf helping to make this one of the most productive oceanic regions of the world. In the surface waters, the small plants that live in the ocean, Phytoplankton, have all the necessary ingredients for growth and this results in high primary productivity in the surface waters and a green surface ocean. The large amount of phytoplankton help supports the growth of other organisms such as zooplankton (small animals that eat phytoplankton) and fish. This constant cycle of production and consumption in surface waters leads to a steady rain of organic material (mostly from dead phytoplankton) from the surface to the deep. This organic material is broken down by bacteria and oxidized to CO2 by respiration processes in the water column. Respiration consumes O2 in the water column and through the combination of high respiration and slow ventilation, or mixing, of oxygen containing waters the underlying waters across the shelf are almost completely depleted of O2. This phenomenon is known as an Oxygen Minimum Zone (OMZ) and the Peruvian OMZ is part a larger system extending on both sides of the equator in the Eastern Pacific.
Figure 1: Overhead view of the CTD rosette on the deck of the Meteor prior to deployment in the water showing the 24 sampling bottles ready for use.
During M77 and in SFB754 researchers from IFM-GEOMAR are studying many different aspects of the chemical, biological, geological and physical oceanography of the Peruvian OMZ. During this last week of M77-1 we completed a survey using a CTD rosette (Figure 1: CTD - a device which measures temperature, salinity and pressure and especially for this cruise includes oxygen sensors. The system is also equipped with sample bottles which can be closed at different depths to bring samples back to the surface.). Our survey crossed the Peruvian continental shelf at 11?S and encountered this low oxygen zone (Figure 2) in the water column across the shelf and into the open ocean. We found that dissolved oxygen was rapidly depleted from the surface waters so that by 80 m depth there was less than 0.1% saturation (the percentage of the amount of oxygen that would be present if the water was at the surface and in equilibrium with the atmosphere). The OMZ continued to approximately 400 m depth (Figure 2) after which oxygen increased again in the water due to supply via mixing with oxygen containing deep waters.
Figure 2: Map of Oxygen concentrations across the Peruvian shelf at 11?S derived from CTD measurements obtained during M77-1.
The low oxygen waters below the surface have strong affects on some chemical cycles. This is most important for what chemists call redox (reduction/oxidation) reactions in which an element can be altered from one oxidation state to another. This is best exemplified in the case of iron where Fe(III) is present when oxygen concentrations are high and this is a highly insoluble form in water and is responsible for the formation of iron oxides we call ‘rust’. In the absence of oxygen iron can be transformed into Fe(II) which forms green coloured solutions and is highly soluble in water. Another example is the element iodine is normally present in seawater as iodate (IO3-) when oxygen is present. In the absence of oxygen it is chemically reduced to iodide (I-), the form we know and use in table salt to prevent diseases such as goitre. Presently however we know very little about how this change occurs and what organisms maybe responsible. Using water samples obtained from the CTD rosette we are currently measuring onboard the concentrations of iodide/iodate and Fe(II) to examine the influence of the OMZ on the cycling of iodine, iron and other redox (reduction/oxidation) sensitive elements. Our initial results indicate that in the OMZ iodine is only present as iodide and that in the low oxygen waters near the sediments there are high concentrations of iron present as Fe(II) which have been released from the sediments under the low oxygen waters on the chemistry of the seawater. For now our work on M77-1 continues and we are enjoying the fine Pacific weather.
Das ScienceLog ist eine Kooperation zwischen dem IFM-GEOMAR, dem SFB 754 der Christian-Albrechts-Universität Kiel und planeterde.de.