The release of dissolved actinium to the ocean: A global comparison of different end-members

The measurement of short-lived Ra often involves a second measurement for supported activities, which represents Ac in the sample. Here we exploit this fact, presenting a set of 284 values on the oceanic distribution of Ac, which was collected when analyzing water samples for short-lived radium isotopes by the radium delayed coincidence counting system. The present work compiles Ac data from coastal regions all over the northern hemisphere, including values from ground water, from estuaries and lagoons, and from marine end-members. Deep-sea samples from a continental slope off Puerto Rico and from an active vent site near Hawaii complete the overview of Ac near its potential sources. The average Ac activities of nearshore marine end-members range from 0.4 dpm m at the Gulf of Mexico to 3.0 dpm m in the coastal waters of the Korean Strait. In analogy to Ra, we find the extension of adjacent shelf regions to play a substantial role for Ac activities, although less pronounced than for radium, due to its weaker shelf source. Based on previously published values, we calculate an open ocean Ac inventory of 1.35*10 dpm Acex in the ocean, which corresponds to 37 moles, or 8.4 kg. This implies a flux of 127 dpm m y from the deep-sea floor. For the shelf regions, we obtain a global inventory of Ac of 4.5*10 dpm, which cannot be converted directly into a flux value, as the regional loss term of Ac to the open ocean would have to be included. Ac has so far been considered to behave similarly to Ra in the marine environment, with the exception of a strong Ac source in the deep-sea due to Paex. Here, we present evidence of geochemical differences between Ac, which is retained in a warm vent system, and Ra, which is readily released [Moore, W.S., Ussler, W. and Paull, C.K., 2008-this issue. Short-lived radium isotopes in ⁎ Corresponding author. School of GeoSciences, The University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK. Tel.: +49 471 4831 1287; fax: +49 471 4831 1425. E-mail addresses: [email protected] (W. Geibert), [email protected] (M. Charette), [email protected] (G. Kim), [email protected] (W.S. Moore), [email protected] (J. Street), [email protected] (M. Young), [email protected] (A. Paytan). 1 Tel.: +1 803 777 2262 (office); fax: +1 803 777 6610. 2 Tel.: +1 650 736 0655. 3 Tel.: +1 650 329 4544 (office), +1 650 387 3694 (cell). 4 Tel.: +1 650 724 4073; fax: +1 650 725 0979. 0304-4203/$ see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2007.07.005 410 W. Geibert et al. / Marine Chemistry 109 (2008) 409–420 the Hawaiian margin: Evidence for large fluid fluxes through the Puna Ridge. Marine Chemistry]. Another potential mechanism of producing deviations in Ac/Ra and daughter isotope ratios from the expected production value of lithogenic material is observed at reducing environments, where enrichment in uranium may occur. The presented data here may serve as a reference for including Ac in circulation models, and the overview provides values for some end-members that contribute to the global Ac distribution. © 2007 Elsevier B.V. All rights reserved.