Actinium-227 as a deep-sea tracer: sources, distribution and applications

Actinium is one of the rarest naturally occurring elements on earth. We measured its longest-lived isotope Ac-227 (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 diapyenal mixing for Ac-227 distribution, we found three other different types of vertical profiles, These profiles point to a prominent role of advection for Ac-227 distribution, especially in the Southern Ocean. Depending on the type of profile found, Ac-227 is proposed as a tracer for different oceanographic questions. In the Southern Ocean, up to 4.93+/-0.32 dpm m(-3) Ac-227 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(-3) Ac-227(ex) (Ac-227 in excess of its progenitor Pa-231) is detected. We use Ac-227(ex) 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 Ac-227(ex) activity. High Ac-227(ex) 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 Ac-227 is compared with Ra-228 to make sure that the large near-surface excess in the water column of the Southern Ocean is not due to lateral inputs by isopyenal mixing. Data from the Central Arctic and from a transect across the ACC confirm that Ra-228 and Ac-227(ex) differ strongly in their sources. The first measurements of Ac-227 on Suspended matter (less than 1.7% of total Ac-227 close to the sea floor) indicate that the particle reactivity of Ac-227 is negligible in the open ocean, in agreement with earlier findings [Y. Nozaki, Nature 310 (1984) 486-488]. Despite the extremely low concentrations of Ac-227, new measurement techniques [W.S. Moore, R. Arnold. J. Geophys. Res. 101 (1996) 1321-1329] point to a comfortable and comparably simple determination of Ac-227 in the future. Finally, Ac-227(ex) may become a widely used deep-sea specific tracer. (C) 2002 Elsevier Science B.V. All rights reserved.