Middle Miocene to Quaternary diatoms in the Nankai Trough and Japan Trench

During Leg 87 of the Deep Sea Drilling Project, eleven holes were drilled at Sites 582 and 583 in the Nankai Trough, off Shikoku, southwestern Honshu, and three holes at Site 584 in the Japan Trench, off northeastern Honshu, Japan. In the former area, a low-latitude diatom zone called the Pseudoeunotia doliolus Zone is recognized in thick upper Quaternary sediments, which yield rare but characteristic admixtures of marine planktonic, marine tychopelagic-tobenthic, and nonmarine diatoms. In the latter area, all the sediments recovered contain abundant to common diatoms, allowing recognition of 12 continuous diatom zones from upper Quaternary through lower middle Miocene. Three hiatuses occur in this area around the Pleistocene/Pliocene boundary and in the upper and middle Miocene. In addition, 19 modified diatom zones for a lower Miocene through upper Quaternary interval are proposed. These middle-to-highlatitude zones are numerically coded (NPD1-NPD12) and represent the entire North Pacific. The establishment of these zones is based primarily on Leg 87 data and other DSDP materials and partially on several Japanese subaerial sequences. Correlation of the new zonal framework with previously established frameworks is attempted by the evaluation of operational usefulness of previously used datums. Resting spores of Chaetoceros and its related forms are recorded with specific intent for the first time, and possible ramifications of its frequency variation are presented. Nine new species are proposed: Delphineis sheshukovae Akiba n. sp., Denticulopsis praelauta Akiba and Koizumi n. sp., Kisseleviella ezoensis Akiba n. sp., Nitzschia umaoiensis Akiba n. sp., Thalassiosira jouseae Akiba n. sp., T. praenidulus Akiba n. sp., T. sancettae Akiba n. sp., T. umaoiensis Akiba n. sp., and T. urahoroensis Akiba n. sp. Transfers of systematic positions of the following four taxa are also proposed: Delphineis simonsenii (Mertz) Akiba n. comb., Ikebea tenuis (Brun) Akiba n. comb., Thalassiosira delicata (Barron) Akiba n. stat., and Thalassiothrixrobusta (Schrader) Akiba n. comb.