Underestimation of biogenic silica sinking flux due to dissolution in sediment traps: A case study in the South China Sea

Abstract

The dissolved silicate concentrations in the supernatant of sediment trap sampling bottles retrieved from deep water (1000-3000 m) at four stations in the South China Sea (SCS) were measured to calculate the underestimated flux of biogenic silica (bSi) in sinking particles due to bSi dissolution. High dissolved silicate concentrations in the supernatant, ranging from 122.6 to 1365.8 μmol/L, indicated significant dissolution of particulate bSi in the sampling bottles. Underestimation of the bSi flux in the SCS by ~2% to ~34% (average: ~10%) due to bSi dissolution was revealed, and the degree of underestimation increased with decreasing total bSi flux. The amount of bSi dissolved within the sampling series at each station was generally positively correlated with dissolution time and to a certain extent influenced by the bSi amount collected by the sampling bottles under a low bSi sinking flux. Apparently stronger bSi dissolution was found at two of the four stations due to relatively high bSi dissolution rates, which were possibly related to a higher reactive surface area of the bSi or bacterial activity. Overall, our results demonstrated that the considerable bSi dissolution in sediment trap sampling bottles should not be ignored, especially in the low-productivity oligotrophic ocean, and bSi flux calibration via measurement of the dissolved silicate in sampling bottles is necessary. To reduce bSi flux underestimation due to the deployment of time-series sediment traps, larger-volume sampling bottles should be avoided in the oligotrophic open ocean, and sinking particle samples should be analyzed as soon as the sediment traps are recovered.