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Journal Articles Deep Sea Research Part I: Oceanographic Research Papers Year : 2023

New insights on the $^7$Be cycle in the ocean

Abstract

The cosmogenic radionuclide $^7$Be 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 $^7$Be by settling particles can be neglected. In this work, we explore these assumptions and more generally assess the significance of the $^7$Be activity in the particulate fraction, through the generation of vertical profiles of particulate $^7$Be in the open ocean. From detailed measurements obtained from low-background gamma spectrometers placed in underground facilities, we report vertical profiles of $^7$Be activity in suspended particles ($^7$Be$_p$) 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, $^7$Be$_p$ 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 33.1 dpm/m$^3$ at GEOVIDE cross- over station 51/60, i.e., within a range consistent with previous $^7$Be$_p$ measurements for the open ocean. For the GEOVIDE cruise, the $^7$Be$_p$ activities measured on different filter types at different depths are corrected for filter offsets derived from multiple $^7$Be$_p$ measurements at a near-coastal station in the western Mediterranean Sea. We then combine measurements of total $^7$Be activity (Shelley et al., 2017) with our measurements of $^7$Bep activity to estimate the solid/solution partitioning of $^7$Be. On average, the particulate fraction would represent 6% of total $^7$Be activity at 5-m water depth (n = 6), 22% at 20 m (n = 2) and 9% at 70 m (n = 3). At GEOVIDE stations, $^7$Be$_p$ inventories range from 5% to 19% of the total $^7$Be inventories. In the Labrador Sea, the measured $^7$Be$_p$ in- ventories 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 K$_d$ for $^7$Be increases with depth, with log10K$_d$ averaging 5.2 ± 0.1 at 5 m and 6.1 ± 0.1 between 70 and 150 m, suggesting that scavenging of dissolved $^7$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 $^7$Be by marine particles may be significant and that it may need to be considered in applications of $^7$Be as a tracer of oceanic processes and atmospheric deposition.
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Dates and versions

hal-03969407 , version 1 (27-11-2023)

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M. Grenier, P. van Beek, P. Lerner, Virginie Sanial, M. Souhaut, et al.. New insights on the $^7$Be cycle in the ocean. Deep Sea Research Part I: Oceanographic Research Papers, 2023, 194, pp.103967. ⟨10.1016/j.dsr.2023.103967⟩. ⟨hal-03969407⟩
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