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

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  (227Acex) 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 227Acex activities could indicate lateral advection of waters that interacted with the margins.
  • In the Irminger Basin, the high 227Acex 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 (Kz) of 0.67 ± 0.14 cm 2 s−1 from the 227Ac vertical distribution.
Figure: Distribution of dissolved 227Acex 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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