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

• Published in: Earth and planetary science letters Vol. 198; no. 1; pp. 147 - 165
• Main Authors: Geibert, W., Rutgers van der Loeff, M.M., Hanfland, C., Dauelsberg, H.-J.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 30-04-2002
• Subjects: actinium; deep upwelling; Marine; mixing; ocean currents; PS, Antarctic Ocean, Antarctic Circumpolar Current; PSW, Weddell Sea; ISE, South Pacific; PN, Arctic Ocean; ocean currents; deep upwelling; actinium; mixing
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/S0012-821X(02)00512-5

Actinium is one of the rarest naturally occurring elements on earth. We measured its longest-lived isotope
227Ac (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
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 m
−3 227Ac 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 227Ac
ex (
227Ac in excess of its progenitor
231Pa) is detected. We use
227Ac
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
227Ac
ex activity. High
227Ac
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
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. Data from the Central Arctic and from a transect across the ACC confirm that
228Ra and
227Ac
ex 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 [Y. Nozaki, Nature 310 (1984) 486–488]. Despite the extremely low concentrations of
227Ac, new measurement techniques [W.S. Moore, R. Arnold, J. Geophys. Res. 101 (1996) 1321–1329] point to a comfortable and comparably simple determination of
227Ac in the future. Finally,
227Ac
ex may become a widely used deep-sea specific tracer.