Solid-solution distribution of the cosmogenic beryllium-7 in the water column

12 May 2023

This work questions the validity and limits of the hypothesis that particulate beryllium-7 (⁷Be_p) can be neglected in the different oceanographic applications of this tracer. To tackle this issue, Grenier and co-authors (2023, see reference below) measured the ⁷Be activity in suspended particles collected in and below the mixed layer in oceanic regions as different as the Mediterranean Sea, the Southern Ocean and the subpolar Atlantic (section GEOVIDE/GEOTRACES-GA01). While the ⁷Be_p activity generally monotonically decreases with depth below the mixed layer, they reveal that, at least in some oceanic regions, the removal of ⁷Be by marine particles may be significant. The ⁷Be_p fraction ranges from 2% to 32% of the total ⁷Be activity along the GEOVIDE section in the North Atlantic. In the Labrador Sea, the comparison of the ⁷Be_p inventories with the dry ⁷Be deposition fluxes estimated from aerosol samples collected during GEOVIDE suggest that a significant portion of ⁷Be_p may be removed by sinking particles. The authors conclude that future research should focus on quantifying the downward export of ⁷Be_p to deep waters, and on assessing its temporal and spatial variability. They strongly recommend that future sampling programmes should aim to collect seawater and particulate samples at the same locations, at the same depths, and at the same time.

**Figure: a)** Stations where samples for ⁷Be measurements were collected and analysed in Grenier et al. (2023). These samples were collected in the framework of different programs: OFP (Sargasso Sea), BARMED (Mediterranean Sea), KEOPS2 (Southern Ocean), and GEOVIDE/GA01 (North Atlantic). b) Compilation of estimates of the ⁷Be_p/⁷Be_tot activity ratio in the open ocean (in %, linear scale) as a function of depth (in m, log scale). The particulate ⁷Be fraction of the GEOVIDE samples accounts for 2-9% of the total ⁷Be activity at a water depth of 5 m (n = 6) and 13-32% at 20 m (n = 2). These ranges are consistent with the existing published data.

Reference:

Grenier, M., van Beek, P., Lerner, P., Sanial, V., Souhaut, M., Lagarde, M., Marchal, O., & Reyss, J. L. (2023). New insights on the 7Be cycle in the ocean. Deep Sea Research Part I: Oceanographic Research Papers, 194, 103967. doi:10.1016/j.dsr.2023.103967

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