Diatom biostratigraphy and datum levels of sediments from ODP Leg 167 sites

Ocean Drilling Program Leg 167 represents the first time since 1978 that the North American Pacific margin was drilled to study ocean history. More than 7500 m of Quaternary to middle Miocene (14 Ma) sediments were recovered from 13 sites, representing the most complete stratigraphic sequence on the California margin. Diatoms are found in most samples in variable abundance and in a moderately well-preserved state throughout the sequence, and they are often dominated by robust, dissolution-resistant species. The Neogene North Pacific diatom zonation of Yanagisawa and Akiba (1998, doi:10.5575/geosoc.104.395) best divides the Miocene to Quaternary sequences, and updated ages of diatom biohorizons estimated based on the geomagnetic polarity time scale of Cande and Kent (1995, doi:10.1029/94JB03098) are slightly revised to adjust the differences between the other zonations. Most of the early middle Miocene through Pleistocene diatom datum levels that have been proven to be of stratigraphic utility in the North Pacific appear to be nearly isochronous within the level of resolution constrained by sample spacing. The assemblages are characterized by species typical of middle-to-high latitudes and regions of high surface-water productivity, predominantly by Coscinodiscus marginatus, Stephanopyxis species, Proboscia barboi, and Thalassiothrix longissima. Latest Miocene through Pliocene assemblages in the region of the California Current, however, are intermediate between those of subarctic and subtropical areas. As a result, neither the existing tropical nor the subarctic (high latitude) zonal schemes were applicable for this region. An interval of pronounced diatom dissolution detected throughout the Pliocene sequence apparently correspond to a relatively warmer paleoceanographic condition resulting in a slackening of the southward flow of the California Current.

大洋钻探计划（Ocean Drilling Program, ODP）第167航次，是1978年以来首次针对北美太平洋边缘开展大洋历史研究的钻探作业。本次钻探从13个站位回收了逾7500米的第四纪至中中新世（距今14 Ma）沉积物，构成了加利福尼亚边缘最为完整的地层序列。绝大多数样品中均发现丰度不一、且在整个序列中保存状态中等完好的硅藻，这些硅藻常以抗溶蚀能力较强的粗壮物种为优势类群。Yanagisawa与Akiba（1998，doi:10.5575/geosoc.104.395）提出的新近纪北太平洋硅藻分带方案，最适用于划分中新世至第四纪地层序列；基于Cande和Kent（1995，doi:10.1029/94JB03098）地磁极性年表估算得到的硅藻生物地层界线年龄，已被小幅修订以适配不同分带方案间的差异。多数已被证实可在北太平洋用于地层学研究的中中新世早期至更新世硅藻基准面，在样品间距所限定的分辨率范围内，几乎为等时面。该硅藻组合以中高纬度及表层水高生产力区域的典型物种为特征，主要包括边缘圆筛藻（Coscinodiscus marginatus）、Stephanopyxis物种、巴博伊前棘藻（Proboscia barboi）及长海毛藻（Thalassiothrix longissima）。然而，加利福尼亚流区域的晚中新世至上新世硅藻组合，介于亚北极与亚热带区域组合之间，因此现有热带或亚北极（高纬度）分带方案均不适用于该区域。整个上新世序列中检测到的显著硅藻溶蚀区间，显然对应于相对温暖的古海洋环境，该环境导致加利福尼亚流的南向流动减弱。