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.在深海钻探计划（Deep Sea Drilling Project, DSDP）第87航次中，科研人员于日本本州岛西南部四国沿岸的南海海槽（Nankai Trough）582、583两个钻探站位钻取11个钻孔，同时在本州岛东北部日本海沟（Japan Trench）的584站位钻取3个钻孔。在前者区域的厚层第四纪上部沉积物中，识别出一处被命名为伪真弯藻（Pseudoeunotia doliolus）带的低纬度硅藻带，该沉积物产出罕见但具特征性的混合类群，涵盖海洋浮游硅藻、海洋偶浮游-底栖硅藻与非海洋硅藻。在后者区域，所有采获沉积物均含有丰富至常见的硅藻，据此可识别出从第四纪上部至中中新统下部的12个连续硅藻带。该区域在更新统/上新统边界以及中新统上部和中部存在三处沉积间断。此外，本研究提出适用于早中新世至第四纪上部层位的19个修订硅藻带，这些中高纬度硅藻带以数字编码（NPD1-NPD12）标识，覆盖整个北太平洋海域。上述硅藻带的建立主要基于第87航次数据及其他DSDP资料，部分参考日本境内数套陆相地层序列。本研究通过评估既往使用的年代基准点的实操效用，尝试将新的硅藻带划分框架与已有划分框架进行对比关联。本研究首次针对性记录了角毛藻（Chaetoceros）及其相关类群的休眠孢子，并探讨了其丰度变化的潜在影响。 本研究共提出9个硅藻新种：Delphineis sheshukovae Akiba n. sp.、Denticulopsis praelauta Akiba & Koizumi n. sp.、Kisseleviella ezoensis Akiba n. sp.、Nitzschia umaoiensis Akiba n. sp.、Thalassiosira jouseae Akiba n. sp.、Thalassiosira praenidulus Akiba n. sp.、Thalassiosira sancettae Akiba n. sp.、Thalassiosira umaoiensis Akiba n. sp.以及Thalassiosira urahoroensis Akiba n. sp.。此外，本研究还提出4个分类群的系统学位置修订方案：Delphineis simonsenii (Mertz) Akiba n. comb.、Ikebea tenuis (Brun) Akiba n. comb.、Thalassiosira delicata (Barron) Akiba n. stat.以及Thalassiothrix robusta (Schrader) Akiba n. comb.。