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

Abstract

The measurement of short-lived 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 231 Pa 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 et al., submitted). Another potential mechanism of producing deviations in 227 Ac/ 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 227 Ac in circulation models, and the overview provides values for some end-members that contribute to the global Ac distribution. IntroductionActinium-227 is a naturally occurring radioisotope with a half-life of 21.77 years. It is generated by decay of protactinium-231, which is particularly abundant in deep-sea sediments. Actinium has a prominent source at the deep ocean floor, in conjunction with a half-life in the range of ocean mixing time scales, which makes it particularly useful as a tracer for deep mixing and upwelling studies (Geibert et al., 2002;Nozaki, 1984;Nozaki, 1993 So far, actinium has been mainly considered to behave in the ocean similarly to radium (Nozaki et al., 1990). Similar distribution coefficients of Ra and Ac in open ocean environments justify this assumption in many cases. However, actinium is expected to resemble the lanthanides in some aspects, like in its affinity for iron hydroxide, while radium is an alkaline earth metal that tends to be less particle reactive, but co-precipitates with Material and Methods Sampling and MeasurementOur data collection was produced with the radium delayed coincidence counting (RaDeCC) system (Moore and Arnold, 1996), which is routinely operated for the radiometrical determination of short-lived Ac data that was produced in different locations where short-lived Ra isotopes were measured (Figure 1). Sampling locations HawaiiTwo different types of samples were collected in the vicinity of Hawaii (Figure 4). A set of samples was taken in nearshore surface waters, at locations that are influenced by freshwater inputs to various degrees, as seen in the salinities of these samples. For a more detailed description of these sampling locations and the radium data, see ( Yucatan/ MexicoBrackish groundwater, lagoon water, and water from the shallow coastal ocean were Puerto RicoOff the north eastern coast of Puerto Rico, a number of Ac data from deep waters near the sea floor along the slope of the island are presented (Figure 6). In contrast to most other sites shown here, these sample locations are not influenced by release from the continental shelf or freshwater/groundwater mixing effects, but they are representing open ocean values in the vicinity of slope sediments. Israel, Gulf of AqabaSamples from the Israeli coast of the Red Sea near Elat ( Figur...

Composition and fluxes of submarine groundwater along the Caribbean coast of the Yucatan Peninsula

“…These particles may not have been separated efficiently from the fibers with our coarse filter or washed off efficiently from the fibers with Ra free water before analyses. This fine particle-associated Th may be contributing to the relatively high ARs observed in offshore waters because 224 Ra will be generated from 228 Th on these particles faster than 223 Ra is generated from 227 Ac as suggested for other locations (Geibert et al, 2008;Moore et al, 2008 Ra to offshore waters (Moore et al, 1973).…”

228Ra, 226Ra, 224Ra and 223Ra in potential sources and sinks of land-derived material in the German Bight of the North Sea: implications for the use of radium as a tracer

“…Blank determination was performed using a manganese-oxideimpregnated fibre that had been flushed with de-ionised water. A final measurement of the samples was conducted after several weeks in order to determine the supported Ac, has been shown to be somewhat elevated in coastal areas, potentially causing a small but regionally variable amount of supported 223 Ra (Geibert et al 2008). 222 …”

Using the radium quartet (228Ra, 226Ra, 224Ra, and 223Ra) to estimate water mixing and radium inputs in Loch Etive, Scotland

“…Fibers were then stored for 2-3 weeks before a second count on the same device to determine how much of the 224 Ra was supported by thorium-228 ( 228 Th). When required, as a result of still-elevated activities in the second count, a third and final count was performed to assess how much of the 223 Ra was supported by actinium-227 ( 227 Ac) (Geibert et al 2008 (Scholten et al 2010). As a result of the relatively low activities of short-lived Ra for a coastal environment, the overall uncertainties of 224 Ra and 223 Ra are 10% to 40% (2 standard errors [SE]).…”

“…Fibers were then stored for 2-3 weeks before a second count on the same device to determine how much of the 224 Ra was supported by thorium-228 ( 228 Th). When required, as a result of still-elevated activities in the second count, a third and final count was performed to assess how much of the 223 Ra was supported by actinium-227 ( 227 Ac) (Geibert et al 2008). Supported 223 Ra and 224 Ra activities, as measured on the second and third counts, were subtracted to give the excesses of 223 Ra and 224 Ra ( 223 Ra ex and 224 Ra ex , respectively) in each sample.…”