The cosmogenic radionuclide ¹⁰Be and its stable isotope ⁹Be can be used as a tracer for water mass mixing, but such applications are rare in both open oceans and marginal seas. Here we report distributions of dissolved ⁹Be and ¹⁰Be concentrations of the seawater in the surface and water columns along a section from shelf to deep basin in the northern South China Sea (SCS). The concentrations of ⁹Be and ¹⁰Be in surface waters range between 8.8–43.6 pmol/kg and 118–576 atoms/g, respectively. The ⁹Be in the northern SCS is dominantly sourced from river waters, while ¹⁰Be has a prominent input from the western Pacific, resulting in the highest ¹⁰Be/⁹Be ratio of 10.9 × 10⁻⁸ closest to the Luzon Strait. Along the depth section, the ⁹Be concentrations decrease southward while ¹⁰Be concentrations increase with the maximum value of 1,237 atoms/g occurring at 2,843 m depth of Station A10. The ¹⁰Be/⁹Be ratios are interpreted with consideration of water mixing and boundary scavenging, which reveal evidence of a branch of the intruding Kuroshio Current (>7.7 × 10⁻⁸) in subsurface water and a deep cyclonic current (>8.9 × 10⁻⁸). A box model yields a sedimentation flux of ¹⁰Be around 1,167 atoms/m²/s, that is, 4.6 times of the atmospheric deposition flux, pointing to active boundary scavenging of ¹⁰Be. The residence time of ¹⁰Be in the deep water is estimated to be longer than 79 years in the northern SCS. The beryllium isotopes as a water mass tracer will find wide applications in basins where the endmembers are sufficiently differentiated.

中文翻译：

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用铍同位素示踪南海北部水团和评估边界清除强度

宇宙放射性核素¹⁰ Be 及其稳定同位素⁹ Be 可用作水团混合的示踪剂，但这种应用在公海和边缘海中都很少见。在这里，我们报告了南海北部 (SCS) 从陆架到深海盆地的剖面和水柱中海水中溶解的⁹ Be 和¹⁰ Be 浓度的分布。地表水中⁹ Be 和¹⁰ Be的浓度范围分别为 8.8-43.6 pmol/kg 和 118-576 atom/g。南海北部的⁹ Be 主要来自河水，而¹⁰Be 有来自西太平洋的突出输入，导致最接近吕宋海峡的¹⁰ Be/ ⁹ Be 比率最高，为 10.9 × 10 ^-8。沿深度剖面，⁹ Be 浓度向南降低，而¹⁰ Be 浓度增加，最大值为 1,237 atom/g，出现在 A10 站深度 2,843 m。在¹⁰ BE / ⁹是比被解释考虑的水混合和边界扫气，这揭示了侵入黑潮（> 7.7×10的一个分支的证据^-8）在地下水和深气旋电流（> 8.9×10 ^-8 )。箱式模型产生的沉降通量为¹⁰ Be 大约为 1,167 原子/m ² /s，即大气沉积通量的 4.6 倍，表明¹⁰ Be 的边界清除活跃。¹⁰ Be在南海北部的深水中停留时间估计超过79年。铍同位素作为水质量示踪剂将在端元充分分化的盆地中得到广泛应用。