Actinium-227 as a tracer for diapycnal mixing and deep upwelling

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Geibert, W. , Usbeck, R. and Rutgers v. d. Loeff, M. (2003): Actinium-227 as a tracer for diapycnal mixing and deep upwelling , Keynote Lecture, 13th Annual Goldschmidt Conference, Kurashiki, Japan, Sept. 7-12 .
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227Ac is a naturally occuring radioactive tracer (half-life 21.8 years) that is continuously released into the overlying water by deep-sea sediments. Since the pioneering work of Nozaki (1984), it has been recognizedthat 227Ac in excess of its progenitor231Pa (227Acex) has a huge potential as a tracer for diapycnal mixing in the deep sea. However, data on the distribution of 227Ac are still scarce due to the difficult sampling and measurement.Recently, some additional information on the global distribution of227Ac has become available (Geibert et al. 2002), confirming the results of Nozaki, and adding new insights to the role of deep upwelling for its distribution in the Southern Ocean. There, 227Acex has been shown to bedetectable throughout the water column up to the sea surface as a consequence of intense and rapid verticalexchange of water masses.Here, we give an overview about the distribution of 227Ac in the ocean, including new results from inversemodelling. The obtained maps of the modelled global distribution of227Acex confirm that this tracer closelyreflects the underlying patterns of circulation and mixing.Additionally, we give an introduction to the available measurement techniques (different a-spectrometric techniques, delayed coincidence counting of its daughter nuclides), and present the potential applications of 227Ac inthe near future.

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