Tracing Water Masses and Assessing Boundary Scavenging Intensity With Beryllium Isotopes in the Northern South China Sea

Abstract

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.