Natürliche Radionuklide: Tracer für Transport- und Reaktionsraten im Ozean
Drei radioaktive Isotope der Elemente Uran (238U and 235U) and Thorium (232Th) haben den Zerfall seit dem Entstehen des Sonnensystems überlebt und sind überal im Ozean und in den Sedimenten zu finden. Diese Nuklide stehen an der Spitze von Zerfallsreihen mit Radionukliden mit einem weiten Spektrum von Halbwertzeiten. Eine vierte Zerfallsreihe (241Am) ist in der Natur durch Zerfall verschwunden aber kann künstlich gebildet werden und liefert eine Reihe sehr nütlicher Tracer Isotopen. Eine Übersicht der Energien aller Zerfälle ist zu finden in Walter Geiberts neuen Zerfallskarten von 241Am, 238U, 235U and 232Th.
We select nuclides of appropriate half-lives and chemistries as tools to determine transport and reaction rates in the ocean. Some examples:
The depletion of 234Th (24 days half-life) with respect to its parent 238U in the surface ocean is used to determine the export rate of particles from the euphotic zone to the deep ocean. This method provides an independent way to determine the flux of carbon and other elements from the surface water to the deep ocean. In an analogous way it is also used for the study of resuspension rates near the deep-sea floor.
230Th und 231Pa
We know the production rate of 230Th (Half-life 75400 y) and 231Pa (half-life 32500y) from Uranium dissolved in the ocean. Their activities in sediment traps and in the sediment cores are used to calibrate the collection efficiency of sediment traps and to determine to what extent sediments are redistributed before being buried at the ocean floor. Their distribution in the sediment helps us to determine the age of sediment layers and to find out whether the rain rate of particles to the seafloor has changed with the changes from glacial to interglacial climate.
210Po und 210Pb
We use Polonium-210 and Lead-210 as tracers for particle transport, too. They enable us to follow the transport of particles through the water column over a longer timescale (several months) due to their longer half life. The special chemistry of Po makes it a tracer for bacterial production, for the transfer of organic carbon to higher trophic levels, and perhaps for dimethyl sulfide (a product of algal decomposition).
das Radium Quartett
228Ra is produced everywhere in marine sediments and diffuses into the bottom water. Especially water masses that have flowed over continental shelves are spiked with the nuclide and can be followed over several half-lives (5.8 y). 228Ra can therefore serve as tracer for inputs of e.g. iron from continental shelves to the open ocean. Shorter-lived 223Ra (11 d) and 224Ra (3.7 d halflife) trace rapid mixing processes near the coast whereas long-lived 226Ra (1600 y halflife) is often more conservative.
227Ac is produced in marine sediments from the decay of 231Pa. In contrast to 228Ra, the strongest sources are in deep-sea sediments. We attempt to use this new tracer as a tool to quantify the upwelling rates of deep water masses (including NADW) that enter the Southern Ocean from the north.