New insights on the 7Be cycle in the ocean

The cosmogenic radionuclide 7Be has been applied as a tracer of dynamical processes in the upper ocean and of atmospheric deposition of trace elements at the sea surface. These applications usually assume that 7Be is entirely in the dissolved form, and that scavenging and downward export of 7Be by settling particles can be neglected. In this work, we explore these assumptions and more generally assess the significance of the 7Be activity in the particulate fraction, through the generation of vertical profiles of particulate 7Be in the open ocean. From detailed measurements obtained from low-background gamma spectrometers placed in underground facilities, we report vertical profiles of 7Be activity in suspended particles (7Bep) collected in various oceanic regions: the Mediterranean Sea (DYFAMED station), the Indian Sector of the Southern Ocean (station A3-2 from the KEOPS2 cruise), the Sargasso Sea (OFP station), and the subpolar North Atlantic Ocean (GEOVIDE cruise).

We find that, in each oceanic region, 7Bep activities are generally higher in the mixed layer than in the thermocline. They vary in the mixed layer from 3.0 dpm/m3 at DYFAMED to 33.1dpm/m3 at GEOVIDE cross-over station 51/60, i.e., within a range consistent with previous 7Bep measurements for the open ocean. For the GEOVIDE cruise, the 7Bep activities measured on different filter types at different depths are corrected for filter offsets derived from multiple 7Bep measurements at a near-coastal station in the western Mediterranean Sea. We then combine measurements of total 7Be activity (Shelley et al., 2017) with our measurements of 7Bep activity to estimate the solid/solution partitioning of 7Be. On average, the particulate fraction would represent 6% of total 7Be activity at 5-m water depth (n = 6), 22% at 20 m (n = 2) and 9% at 70 m (n = 3). At GEOVIDE stations, 7Bep inventories range from 5% to 19% of the total 7Be inventories. In the Labrador Sea, the measured 7Bep inventories are lower than the dry 7Be deposition fluxes estimated from aerosol samples collected during GEOVIDE, suggesting that a significant portion of 7Bep may be removed by sinking particles. The distribution coefficient Kd for 7Be increases with depth, with log10Kd averaging 5.2 ± 0.1 at 5 m to 6.1 ± 0.1 between 70 and 150 m, suggesting that scavenging of dissolved 7Be by particles is more pronounced in the thermocline than in the mixed layer when differences in particle concentrations are taken into account. Overall, our study suggests that, at least in some oceanic regions, the removal of 7Be by marine particles may be significant and that it may need to be considered in applications of 7Be as a tracer of oceanic processes and atmospheric deposition.


Beryllium-7, Suspended particles, Vertical profiles, Open ocean, Solid, solution partitioning, GEOTRACES

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Grenier M., Van Beek P., Lerner P., Sanial V., Souhaut 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 (14p.).,

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