Radiolarian stratigraphy of ODP Leg 138 sites

A group of 46 radiolarian species was used in this study of Leg 138 sites. The recovery of the sections was complete in the intervals that were cored using the APC system and nearly complete in the deeper sections. The northeastern sites (844 and 845) were sampled down through the middle Miocene into the uppermost part of the lower Miocene (middle part of the Calocycletta costata Zone). In the southeastern sites and those of the eastern transect (846 through 854) sediments were of late Miocene age (Diartus petterssoni Zone) and younger. Preservation of the radiolarian fauna was good to moderately good in most of the sites. Only in Sites 853 and 854 was the section older than late Pliocene barren of radiolarians. Reworked older radiolarians were found in the upper Miocene and Pliocene parts of the sections in most sites. Reworked upper Miocene radiolarians were even found in the upper Pliocene of Sites 853 and 854 where the upper Miocene part of the sections were barren of radiolarians. The development of an orbitally tuned time scale for the last 10 m.y. allowed the differentiation between radiolarian datums that appear to be synchronous (within 150,000 yr) in the eastern equatorial Pacific Ocean and those which appear to be diachronous. Of the 39 datums examined in this time interval, only 10 met this working definition of synchrony within the study area.