Impact of ash precipitation on marine environments and diatoms: A case of 1991 from the Pinatubo volcano in the South China Sea

The abundance and species composition of diatoms and sinking particles in the surface sediments of the South China Sea (SCS) before and after the 1991 eruption of Pinatubo volcano are analyzed in this work, in order to better reveal the impact of ash precipitation on marine diatoms. It is found that the precipitation of volcanic ash apparently increases the sedimentation rate, and dilutes the other components of the surface sediments, as the result, the relative abundance of diatoms in the surface sediments is significantly decreased. The records of sinking particles further suggest that the ash precipitation has a long-term effect to the sea environment. Both the total sinking particle flux and biogenic silica (BSi) flux in the deep water of central SCS increased substantially one year after the eruption, as the relative abundance and flux of diatoms and other siliceous organisms decreased significantly. The relative abundance decreased from 2.20×10⁸ to 5.48×10⁷ individuals g⁻¹, and the flux of diatoms decreased from 2.90×10⁷ to 8.57×10⁶ individuals m⁻² d⁻¹. The apparent decrease in siliceous organism fluxes is attributed to the depressed productivity in the central SCS, that might be related to Pinatubo ash falling, or the impact of the El Niño in 1991. On the other hand, the increase in “BSi” flux one year after the eruption is more likely resulted from the dissolution of volcanic glass during measurement of BSi in the lab, but not from the increase of biogenic siliceous organism frustules or skeletons. It is found that there is a large amount of volcanic glass in the sinking particles. Further works are necessary to clarify the change in the BSi flux before and after the Pinatubo eruption, the decrease of siliceous organisms, and the impact of volcanic eruption on biogeochemistry and sedimentation dynamics in the SCS.