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

The measurement of short-lived Ra-223 often involves a second measurement for supported activities, which represents Ac-227 in the sample. Here we exploit this fact, presenting a set of 284 values on the oceanic distribution of Ac-227, which was collected when analyzing water samples for short-lived radium isotopes by the radium delayed coincidence counting system. The present work compiles Ac-227 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-227 near its potential sources. The average Ac-227 activities of nearshore marine end-members range from 0.4 dpm m(-3) at the Gulf of Mexico to 3.0 dpm m(-3) in the coastal waters of the Korean Strait. In analogy to Ra-228, we find the extension of adjacent shelf regions to play a substantial role for Ac-227 activities, although less pronounced than for radium, due to its weaker shelf source. Based on previously published values, we calculate an open ocean Ac-227 inventory of 1.35 * 10(18) dpm Ac-227(ex) in the ocean, which corresponds to 37 moles, or 8.4 kg. This implies a flux of 127 dpm m(-2) y(-1) from the deep-sea floor. For the shelf regions, we obtain a global inventory of Ac-227 of 4.5 * 10(15) dpm, which cannot be converted directly into a flux value, as the regional loss term of Ac-227 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 Pa-231(ex). 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 the Hawaiian margin: Evidence for large fluid fluxes through the Puna Ridge. Marine Chemistry]. Another potential mechanism of producing deviations in Ac-227/Ra-228 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-227 in circulation models, and the overview provides values for some end-members that contribute to the global Ac distribution. (c) 2007 Elsevier B.V. All rights reserved.