Actinium-227 distribution traces at least three processes in the North Atlantic Ocean

9 February 2023

Le Roy and colleagues (2023, see reference below) report an oceanic section of Actinium-227 (²²⁷Ac) activities built from 9 full-depth vertical profiles determined in the North Atlantic (GEOTRACES cruise GA01). Combining the ²²⁷Ac activities with the Protactinium-231(²³¹Pa) activities provides an entire section of ²²⁷Ac in excess (²²⁷Ac_ex) activities. The main results are:

Actinium-227 activities were especially low along the section compared to other ocean basins.
In most cases, ²²⁷Ac activities reached secular equilibrium with ²³¹Pa (suggesting no external input over the last ∼100 years) in the water column 500 m above the seafloor.
The highest ²²⁷Ac activities were often found close to the seafloor, indicating that ²²⁷Ac diffuses out of the sediments.
In the upper 500 m, high ²²⁷Ac_ex activities could indicate lateral advection of waters that interacted with the margins.
In the Irminger Basin, the high ²²⁷Ac_ex activities at 2500 m were accompanied by high dissolved iron concentrations; these two enrichments may indicate the presence of a hydrothermal plume.
In the Iberian Abyssal Plain, where no bottom nepheloid layer was observed, we estimated a vertical eddy diffusivity coefficient (K_z) of 0.67 ± 0.14 cm ² s⁻¹ from the ²²⁷Ac vertical distribution.

**Figure:** Distribution of dissolved ²²⁷Ac_ex activities along the GEOVIDE section in the North Atlantic Ocean. Station numbers are found on top of the panel. The sampling depths for ²²⁷Ac are shown for each vertical profile (black dots).

Reference:

Le Roy, E., van Beek, P., Lacan, F., Souhaut, M., Sanial, V., Charette, M. A., Henderson, P. B., and Deng, F. (2023) The distribution of 227Ac along the GA01 section in the North Atlantic, Mar. Chem., 248, 104207. Access the paper: https://doi.org/10.1016/j.marchem.2023.104207

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