Enhanced mesopelagic particle export in the northern South China Sea derived from 210Po/210Pb disequilibrium

Vertical distributions of dissolved and particulate 210Po and 210Pb in the water column were examined along three transects occupied on July 2016 in the northern South China Sea (SCS) to investigate particle dynamics and export. In the mixed layer, 210Po was deficient with respect to 210Pb with a 210Po/210Pb activity ratio of 0.08–0.57 and an average of 0.31 ± 0.12. The 210Po/210Pb ratio increased to 0.39–0.96 with an average of 0.71 ± 0.16 at the base of the euphotic layer (100 m). 210Po was largely depleted relative to 210Pb again in the mesopelagic layer (100–1000 m). The activity ratio of 210Po/210Pb ranged from 0.12 to 1.04 with an average of 0.42 ± 0.20, suggesting an intensified particle scavenging. Meanwhile, the residence time of particulate 210Po (PPo) in the mesopelagic layer varied from 0.01 to 0.47 yr with an average of 0.11 ± 0.02 yr, also indicating the fast removal of particles. Both the disequilibrium between 210Po and 210Pb and the short residence time of PPo in the mesopelagic water indicated a stronger scavenging and subsequent removal of 210Po relative to 210Pb throughout the water column. Based on the observed 210Po deficit, the export flux of 210Po in the mesopelagic layer was estimated in the range of 96–625 dpm m−2 d−1 with an average of 382 ± 16 dpm m−2 d−1, which was an order of magnitude higher than that in the euphotic layer. Furthermore, the export flux of total particulate matter (TPM) in mesopelagic layer was estimated between 2.15 and 30.18 g m−2 d−1. Comparing to the TPM export flux in the euphotic layer (0.44–2.67 g m−2 d−1), the significantly elevated TPM export flux in the mesopelagic waters may require extra supply of particles in addition to sinking particles from the upper water column, which is likely sourced from the surrounding shelves in the northern SCS.