Development of east Asian summer monsoon environments in the late Miocene: radiolarian evidence from Site 1143 of ODP Leg 184

Abundant radiolarians are preserved in the relatively complete upper Miocene section of ODP Site 1143. Three radiolarian zones, RN6, RN7 and RN8, are recognized on the basis of Diartus petterssoni (total range in RN6, <11.9–8.77 Ma), Diartus hughesi (last appearance at RN7/RN8, 7.7 Ma) and Stichocorys delmontensis (RN6 to RN8, >6.7 Ma). Variations in the abundance of radiolarians, especially Pyloniid forms, radiolarian flux and species diversity are good proxies of upwelling which, similar to today’s, was likely driven by summer monsoons. These radiolarian paleomonsoon proxies indicate that the east Asian summer monsoon first initiated close to the middle/late Miocene boundary at ∼12–11 Ma and reached a maximum strength at ∼8.24 Ma. Therefore, the initiation of the east Asian summer monsoon was likely earlier than the first Indian monsoon, which appeared ∼8 myr ago.