Actinium-227 as a tracer for diapycnal mixing and deep upwellingWALTER GEIBERT1, REGINA USBECK1,2 AND MICHIEL M. RUTGERS VAN DER LOEFF1,31Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27576 Bremerhaven (wgeibert@awi-bremerhaven.de)2FIELAX Company for Scientific Data Management, Schifferstrasse 10-14, 27568 Bremerhaven, Germany3 present address: RIKZ, National Inst. For Coastal and Marine Management, P.O.Box 20907, 2500 EX The Hague, The Netherlands227Ac 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 recognized that 227Ac in excess of its progenitor 231Pa (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 of 227Ac 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 be detectable throughout the water column up to the sea surface as a consequence of intense and rapid vertical exchange of water masses.Here, we give an overview about the distribution of 227Ac in the ocean, including new results from inverse modelling. The obtained maps of the modelled global distribution of 227Acex confirm that this tracer closely reflects 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 in the near future.REFERENCESNozaki, Y. (1984), Nature 310, 486-488.Geibert, W. et al. (2002), Earth Planet. Sci. Lett. 198 (1-2), 147-165.