Late Neogene radiolarian absence event in the southern South China Sea and its paleoceanographic implication

Down-core variations of radiolarians at ODP Site 1143 in the southern South China Sea (SCS) are presented for the last 12 Myr. The fluctuations of radiolarian abundances since the Late Miocene can be divided into three stages: a high abundance stage at 12–5.96 Myr, a radiolarian absent stage at 5.96–3.30 Myr and a gradual increasing stage after 3.30 Myr. The three stages correspond to the forming and vicissitudes of the Western Pacific Warm Pool (WPWP) and the Eastern Asian Summer Monsoon (EASM). The radiolarian absence event (RAE) was also absent of diatom and occurred nearly simultaneously with the closures of the Panama Isthmus and the Indonesian seaway, which probably caused the reorganization of oceanic circulation systems. Accompanied this circulation reorganization was the weakening of the WPWP and the EASM, which probably led to a weakened upwelling in the southern SCS. In addition dissolved silica (Dsi) content in surface seawater might be very low during 5.96–3.30 Myr due to the ‘‘biogenic bloom" event, which consumed a large amount of Dsi in the surface seawater. All these factors together might lead to a great decrease of siliceous production in the southern SCS and consequently caused the RAE. Moreover, the dissolution of siliceous skeletons might also influence the abundance of radiolarians.