Miocene to Pleistocene diatom biostratigraphy with a focus on Thalassiosira in sediments of ODP Leg 186 sites@en

Late Neogene biostratigraphy of diatoms has been investigated from two sites occupied during Ocean Drilling Program (ODP) Leg 186 off the coast of northeast Japan. A unique aspect of ODP Leg 186 was the installation of two permanent borehole geophysical observatories at the deep-sea terrace along the Japan Trench. The Neogene subsidence history of the forearc was documented from both Sites 1150 and 1151, and Quaternary to middle Miocene (16 Ma) sediments represent a nearly continuous stratigraphic sequence including numerous ash records, especially during the past 9 m.y. Diatoms are found in most samples in variable abundance and in a moderately well preserved state throughout the sequence. The assemblages are characterized consistently by age-diagnostic species of Denticulopsis and Neodenticula found in regions of high surface water productivity typical of middle to high latitudes. The Neogene North Pacific diatom zonation divides the Miocene to Quaternary sequences fundamentally well, except that the latest Miocene through early Pliocene Thalassiosira oestrupii Subzone is not applicable. Miocene and late Pliocene through Pleistocene diatom datum levels that have been proven to be of great stratigraphic utility in the North Pacific Ocean appear to be nearly isochronous within the level of resolution constrained by core catcher sample spacing. The taxonomy and stratigraphy of previously described species determined to be useful across the Miocene/Pliocene boundary have been investigated on the basis of the evolutionary changes within the Thalassiosira trifulta group. The biostratigraphically important forms belonging to the genus Thalassiosira have been illustrated with scanning electron micrographs.科研人员针对日本东北外海大洋钻探计划（Ocean Drilling Program, ODP）第186航次的两个站位样品，开展了晚新近纪硅藻生物地层学研究。大洋钻探计划第186航次的一项独特之处在于，其在日本海沟沿线的深海台地布设了两套永久钻孔地球物理观测站。研究通过1150和1151两个站位，阐明了弧前区的新近纪沉降历史；第四系至中中新世（16 Ma）沉积物构成了近乎连续的地层序列，其中包含大量火山灰记录，尤其在距今9百万年以来的沉积层中尤为显著。整套地层序列的绝大多数样品中均有硅藻产出，其丰度存在差异，但整体保存状态中等良好。该硅藻组合的典型特征始终为齿状藻属（Denticulopsis）与新齿状藻属（Neodenticula）的时代指示种，这些物种多分布于中高纬度表层水体生产力较高的海域。新近纪北太平洋硅藻分带方案可较好地划分中新世至第四纪地层序列，唯有晚中新世至早上新世的粗筛藻（Thalassiosira oestrupii）亚带并不适用。经证实对北太平洋地层学研究极具应用价值的中新世、晚上新世至更新世硅藻生物地层基准点，在岩心抓采样间距所限定的分辨率范围内，几乎均为等时面。研究以粗筛藻群（Thalassiosira trifulta group）内的演化变化为依据，对此前被证实对中新世/上新世界线划分具有应用价值的已描述物种的分类学与地层学特征展开了研究。具有重要生物地层学意义的粗筛藻属（Thalassiosira）类群，均通过扫描电子显微照片进行了图示说明。