Actinium is one of the rarest naturally occuring elements on earth. We measured its longest-lived isotope 227 Ac (half-life 21.77 yr) for the first time in the water column of the Southeast Pacific, the Central Arctic, the Antarctic Circumpolar Current (ACC) and the Weddell Gyre (WG). Besides the profile in the Southeast Pacific, which confirms earlier findings about the role of diapycnal mixing for 227 Ac distribution, we found three other different types of vertical profiles. These profiles point to a prominent role of advection for 227 Ac distribution, especially in the Southern Ocean. Depending on the type of profile found, 227 Ac is proposed as a tracer for different oceanographic questions. In the Southern Ocean, up to 4.93 ± 0.32 dpm m -3227 Ac is found close to the sea floor, which is the highest concentration ever observed in the ocean. Close to the sea surface in the WG, 0.46 ± 0.05 dpm m-3227 Acex (227 Ac in excess of its progenitor 231 Pa) is detected. We use 227 Acex there to determine the upwelling velocity in the Eastern WG to be about 55 m yr-1. In the ACC, Upper and Lower Circumpolar Deep Water (UCDW and LCDW) are found to differ clearly in their 227 Acex activity. High 227 Acex 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 227 Ac is compared with 228 Ra 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. Data from the Central Arctic and from a transect across the ACC confirm that 228 Ra and 227 Acex differ strongly in their sources. The first measurements of 227 Ac on suspended matter (less than 1.7% of total 227 Ac close to the sea floor) indicate that the particle reactivity of 227 Ac is negligible in the open ocean, in agreement with earlier findings [Y. Nozaki, Nature 310 (1984) 486-488]. Despite the extremely low concentrations of 227 Ac, new measurement techniques [W.S. Moore, R. Arnold, J. Geophys. Res. 101 (1996) 1321-1329] point to a comfortable and comparably simple determination of 227 Ac in the future. Finally, 227 Acex may become a widely used deep-sea specific tracer. © 2002 Elsevier Science B.V. All rights reserved.