Paleocene and Eocene nannofossil datums from Shatsky Rise and Walvis Ridge

The precision of late Paleocene to middle Eocene nannofossil datums is investigated by means of quantitative methods and correlated to the magnetic polarity stratigraphy, using sequences from the Northwest Pacific, Southeast Atlantic and Italy. It is the rule rather than the exception to find tails of very reduced abundances prior to, or after, a range of consistent and higher abundances. The absolutely first or final occurrence of a species, therefore, seldom provides a synchronous datum when material from different geographic areas are compared. On the other band, synchroneity is often confirmed when the initial sharp rise or the final sharp decline in abundance is used as datum level. The use of datums not employed in the two principal existing nannofossil zonal schemes can substantially improve the biostratigraphic resolution. Two established zonal markers show abundance patterns making them unsuitable as datums: the first occurrences of Ellipsolithus macellus (base NP4, diachronous) and Tribrachiatus nunnii (base NP10 and Paleocene/Eocene boundary, too rare and too short range in open ocean sections). The first occurrence of either Fasciculithus spp. or Sphenolithus spp. is a better marker near the base of NP4. The first occurrence of Discoaster diastypus at 56.6 Ma represents a suitable replacement for recognition of the Paleocene/Eocene boundary.

本研究采用定量方法，对晚古新世（Late Paleocene）至中始新世（Middle Eocene）超微化石（nannofossil）标志层的精度展开探究，并利用西北太平洋、东南大西洋及意大利的沉积序列，将其与磁极性地层学（magnetic polarity stratigraphy）进行对比关联。在一系列丰度稳定且较高的化石类群前后，常可观测到丰度显著降低的过渡段，这一现象普遍存在而非仅为例外。因此，当对比不同地理区域的沉积样品时，单一物种的绝对首现或末现事件极少能形成同步的标志层。反之，若以丰度的初始陡增或最终陡降作为标志层基准，则通常可确认其同步性。相较于当前主流的两类超微化石分带方案，采用未被其纳入的标志层，可显著提升生物地层分辨率（biostratigraphic resolution）。现有两类已确立的分带标志因其丰度特征，并不适合作为标志层：一是*Ellipsolithus macellus*的首现事件（对应NP4带底界，存在穿时性）；二是*Tribrachiatus nunnii*的首现事件（对应NP10带底界及古新世-始新世边界，在开阔洋区沉积序列中该物种极为稀少且分布时限极短）。在NP4带底界附近，以*Fasciculithus* spp.或*Sphenolithus* spp.的首现事件作为标志，效果更佳。*Discoaster diastypus*的首现事件发生于56.6 Ma，可作为识别古新世-始新世边界的合适替代标志。