Radiolarian biostratigraphy of ODP Site 111-677

Well-preserved late Miocene through Pleistocene age radiolarian assemblages were recovered during ODP Leg 111 at Site 677, on the southern flank of the Costa Rica Rift in the eastern equatorial Pacific. Radiolarian "event" biostratigraphy (first and last morphotypic appearances) was established for Holes 677A and 677B using 21 species yielding 24 reliable datum levels. The cold upwelling waters above this site have prevented many typical tropical Pacific stratigraphic radiolarians from being useful age indicators. Biostratigraphic datum levels were assigned absolute ages based on previous work and were used to date the cores. Sedimentation rates varied from 3.7 cm/1000 yr in the late Pleistocene to 6.0 cm/1000 yr in the late Miocene. The age of the oldest sediments at this site is estimated as 5.89-6.37 Ma, which indicates that Site 677 is between magnetic anomalies 3A and 4. A total of 67 taxa were assessed for stratigraphic relevance at this site and are listed in the Appendix. One previously unknown Pliocene radiolarian stratigraphic indicator, Botryostrobus euporus (Ehrenberg), is identified.