AbstractActinium is one of the rarest naturally occurring elements on earth. We measured its longest-lived isotope 227Ac (half-life 21.77 years) for the first time in the water column of the Southeast Pacific, the central Arctic, the Antarctic Circumpolar Current (ACC) and the Weddell Gyre. Besides the profile in the Southeast Pacific, which confirms earlier findings about the role of diapycnal mixing for 227Ac distribution, we found three other different types of vertical profiles. These profiles point to a prominent role of advection for 227Ac distribution, especially in the Southern Ocean. Depending on the type of profile found, 227Ac is proposed as a tracer for different oceanographic questions.In the Southern Ocean, up to 4.93 ± 0.32 dpm*m3 227Acex (227Ac in excess of its progenitor 231Pa) are found close to the sea floor, which are the highest concentrations ever observed in the ocean. Close to the sea surface in the Weddell Gyre (WG), 0.46 ± 0.05 dpm*m-3 227Acex are detected. We use 227Acex there to determine the upwelling velocity in the Eastern Weddell Gyre to be about 55 m*y-1.In the ACC, Upper and Lower Circumpolar Deep Water (UCDW and LCDW) are found to differ clearly in their 227Acex activity. High 227Acex activities are therefore a promising tracer for recent inputs of LCDW to the sea surface, which may help to understand the role of deep upwelling for iron inputs into Antarctic surface waters.The expected release of 227Ac is compared with 228Ra to make sure that the large near-surface excess in the water column of the Southern Ocean is not due to lateral inputs by isopycnal mixing. Results of a model indicate that the 227Ac/228Ra flux ratio is about 50 times lower on shelves than in the deep-sea. Data from the Central Arctic and from a transect across the ACC confirm that 228Ra and 227Acex differ strongly in their sources.The first measurements of 227Ac on suspended matter (less than 1.7% of total 227Ac close to the sea floor) indicate that the particle-reactivity of 227Ac is negligible in the open ocean, in agreement with earlier findings [Nozaki 1984].Despite the extremely low concentrations of 227Ac, new measurement techniques [Moore and Arnold 1996] point to a comfortable and comparably simple determination of 227Ac in the future. Finally, 227Acex may become a widely used deep-sea specific tracer.