Publication Date:
2022-05-26
Description:
Author Posting. © Elsevier B.V., 2007. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Marine Chemistry 109 (2008): 409-420, doi:10.1016/j.marchem.2007.07.005.
Description:
The measurement of short-lived 223Ra often involves a second measurement for supported
activities, which represents 227Ac in the sample. Here we exploit this fact, presenting a set of
284 values on the oceanic distribution of 227Ac, which was collected when analyzing water
samples for short-lived radium isotopes by the radium delayed coincidence counting system.
The present work compiles 227Ac data from coastal regions all over the northern hemisphere,
including values from ground water, from estuaries and lagoons, and from marine endmembers.
Deep-sea samples from a continental slope off Puerto Rico and from an active
vent site near Hawaii complete the overview of 227Ac near its potential sources.
The average 227Ac 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
228Ra, we find the extension of adjacent shelf regions to play a substantial role for 227Ac
activities, although less pronounced than for radium, due to its weaker shelf source. Based
on previously published values, we calculate an open ocean 227Ac inventory of 1.35 *
1018 dpm 227Acex 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 227Ac of 4.5 * 1015 dpm, which cannot be converted directly into a flux value, as
the regional loss term of 227Ac 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 231Paex. Here, we present evidence of
geochemical differences between Ac, which is retained in a warm vent system, and Ra,
which is readily released (Moore et al., submitted). Another potential mechanism of
producing deviations in 227Ac/228Ra 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 227Ac in
circulation models, and the overview provides values for some end-members that contribute
to the global Ac distribution.
Keywords:
Actinium
;
Radium
;
Submarine groundwater discharge
;
Ocean mixing
;
Continental shelf
;
Global circulation models
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
Format:
application/pdf
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