Calcareous nannofossil biohorizons of ODP Leg 167 sites

Selected calcareous nannofossils were investigated by means of quantitative and semiquantitative methods in middle Miocene to Pleistocene sediments from the California margin (Pacific Ocean) recovered during Ocean Drilling Program Leg 167. The goal of the work was to provide detailed dating and correlations of the successions recovered that span a wide latitudinal transect (from 29°N to 40°N) affected by strongly variable ecological conditions. The standard zonations are not easily applied in these sediments; hence, additional biohorizons have been adopted that are useful in the area. Specifically, the reliability of 50 biohorizons has been evaluated by considering their mode of occurrence, ranking, and spacing.
For the Pleistocene interval, the following six biohorizons are considered reliable: last occurrence (LO) of Pseudoemiliania lacunosa, first occurrence (FO) of Gephyrocapsa sp. 3, LO and FO of large Gephyrocapsa, FO of Gephyrocapsa oceanica s.l., and LO of Reticulofenestra asanoi. The acme end (AE) of small Gephyrocapsa spp. and the LOs of Helicosphaera sellii and Calcidiscus macintyrei do not seem to be reliable in the study area. For the Pliocene interval, the following nine biohorizons are considered reliable: LO of Discoaster pentaradiatus, LO of Discoaster surculus, LO of Discoaster tamalis, LO and first common and continuous occurrence (FCO) of Discoaster asymmetricus, paracme beginning (PB) and paracme end (PE) of Discoaster pentaradiatus, LO of Reticulofenestra pseudoumbilicus, and LO of Amaurolithus delicatus. The LOs of Discoaster brouweri and Discoaster triradiatus and the FCO of P. lacunosa seem to be moderately reliable. For the late Miocene interval, the following eight biohorizons are considered reliable: LO of Discoaster quinqueramus, PB and PE of R. pseudoumbilicus, FO of Amaurolithus primus, FO and LO of Minylitha convallis, LO of Catinaster calyculus, and FO of Catinaster spp. Sediments of middle Miocene age were recovered only at low-latitude Site 1010. Therefore, the reliability of the six biohorizons identified (Discoaster kugleri FO and LO, Calcidiscus macintyrei FO, Cyclicargolithus floridanus last common and continuous occurrence [LCO], Calcidiscus premacintyrei LO, and Sphenolithus heteromorphus LO) cannot be fully evaluated.
These events have been correlated with the global chronostratigraphic scale and calibrated to the geomagnetic polarity time scale. This integrated time frame has been used for dating the successions recovered during Leg 167. The biozones proposed for the Pleistocene seem to be valid globally, and they are proposed as an alternative to the standard zonation.

本研究针对太平洋加利福尼亚边缘海区大洋钻探计划（Ocean Drilling Program）第167航次所获取的中中新统至更新统沉积物，采用定量与半定量方法对其中精选的钙质超微化石（calcareous nannofossils）开展了分析。本次研究的核心目标是对该横跨29°N至40°N宽纬度断面、受强烈变化生态条件影响的沉积序列进行精细定年与地层对比。由于标准生物地层分带在这批沉积物中难以直接应用，因此研究采用了适配该区域的额外生物界标（biohorizons）。具体而言，研究通过考量其产出模式、分级与分布间距，对50个生物界标的可靠性进行了系统评估。
针对更新统沉积区间，本次研究认定以下6个生物界标具备可靠可靠性：穴状假埃米利藻（Pseudoemiliania lacunosa）的末次出现（last occurrence, LO）、桥石藻属未定种3（Gephyrocapsa sp. 3）的首现（first occurrence, FO）、大型桥石藻（large Gephyrocapsa）的末次出现与首现、大洋桥石藻广义群（Gephyrocapsa oceanica s.l.）的首现，以及浅网球石（Reticulofenestra asanoi）的末次出现。小型桥石藻属（small Gephyrocapsa spp.）的顶峰末期（acme end, AE）以及Sellii颗石藻（Helicosphaera sellii）与Macintyre钙盘藻（Calcidiscus macintyrei）的末次出现，在本研究区域内可靠性欠佳。
针对上新统沉积区间，本次研究认定以下9个生物界标具备可靠可靠性：五辐射盘星藻（Discoaster pentaradiatus）的末次出现、Surculus盘星藻（Discoaster surculus）的末次出现、Tamalis盘星藻（Discoaster tamalis）的末次出现、不对称盘星藻（Discoaster asymmetricus）的末次出现与首次普遍连续出现（first common and continuous occurrence, FCO）、五辐射盘星藻的副始期（paracme beginning, PB）与副末期（paracme end, PE）、假脐浅网球石（Reticulofenestra pseudoumbilicus）的末次出现，以及精致暗石藻（Amaurolithus delicatus）的末次出现。Brouwer盘星藻（Discoaster brouweri）与三辐射盘星藻（Discoaster triradiatus）的末次出现，以及穴状假埃米利藻的首次普遍连续出现，在本区域内仅具备中等可靠性。
针对晚中新统沉积区间，本次研究认定以下8个生物界标具备可靠可靠性：五肋盘星藻（Discoaster quinqueramus）的末次出现、假脐浅网球石的副始期与副末期、原始暗石藻（Amaurolithus primus）的首现、Convallis小颗石藻（Minylitha convallis）的首现与末次出现、Calyculus帽星藻（Catinaster calyculus）的末次出现，以及帽星藻属未定种（Catinaster spp.）的首现。
中中新世沉积物仅在低纬度站位1010处被获取，因此本次识别出的6个生物界标的可靠性无法得到全面评估：库格勒盘星藻（Discoaster kugleri）的首现与末次出现、Macintyre钙盘藻的首现、佛罗里达环形颗石藻（Cyclicargolithus floridanus）的末次普遍连续出现（last common and continuous occurrence, LCO）、前Macintyre钙盘藻（Calcidiscus premacintyrei）的末次出现，以及异形斯氏颗石藻（Sphenolithus heteromorphus）的末次出现。
上述生物事件已与全球年代地层标尺（chronostratigraphic scale）进行对比，并校准至地磁极性年表（geomagnetic polarity time scale）。该综合年代框架已被用于对第167航次获取的沉积序列进行定年。本次为更新统提出的生物地层分带具备全球适用性，可作为标准分带的替代方案。