Nannofossil assemblages of mid-Pliocene sediments from the equatorial Atlantic

Downcore cyclic variation in high-resolution nannofossil abundance records from mid-Pliocene equatorial Atlantic ODP Sites 662 and 926 demonstrate the direct response by several Pliocene taxa (notably Discoaster, Sphenolithus and Florisphaera profunda) to orbitally forced climatic variation. In particular, these records display strong obliquity and precessional signals reflecting primarily high latitude, Southern hemisphere changes influencing upwelling intensity and local low-latitude, insolation-driven climatic changes (via the productivity and/or turbidity influence of Amazon-sourced terrigenous material) at Sites 622 and 926 respectively. In seasonal studies of coccolithophorid assemblages, only part of the variation observed can be explained by abiotic processes, so it is perhaps not surprising that in this study few Pliocene nannofossil taxa demonstrate significant correlations with each other or with physical environmental parameters. Only some variance in nannofossil abundances can be explained by the primary controls of temperature and productivity. The rest is attributed to nonlinear responses to climatic changes; biotic processes such as grazing, predation, viral infection and competition, and/or, abiotic factors for which there is no readily available proxy (e.g. salinity). The lack of strong, consistent intra- and inter-relationships of the nannoflora and the environment reflects an ecologically complex, differentiated original community producing a complex integrated signal transmitted into the fossil record.来自上新世中期赤道大西洋大洋钻探计划（Ocean Drilling Program, ODP）站位662与926的高分辨率钙质超微化石（nannofossil）丰度记录中的岩芯旋回变化，表明多个上新世化石类群（尤以盘星石藻属（Discoaster）、楔石藻属（Sphenolithus）与深水花球藻（Florisphaera profunda）为代表）对轨道驱动的气候变化存在直接响应。具体而言，这些记录呈现出显著的黄赤交角与岁差信号，分别反映了两类气候变化驱动过程：一类是影响上升流强度的南半球高纬度变化，另一类则是通过亚马逊陆源物质的生产力和/或浊度效应，驱动站位622与926处局地低纬度日照相关的气候变化。 针对颗石藻（coccolithophorid）群落开展的季节性研究显示，仅能通过非生物过程解释观测到的部分变异，因此本研究中仅有少量上新世钙质超微化石类群彼此间或与物理环境参数间存在显著相关性，便不足为奇。 钙质超微化石丰度的变异仅能部分通过温度与生产力这两大核心控制因子得到阐释，剩余变异则归因于对气候变化的非线性响应；其中涵盖牧食、捕食、病毒感染与种间竞争等生物过程，以及缺乏有效代用指标（proxy）的非生物因素（例如盐度）。 钙质超微植物群（nannoflora）与环境之间缺乏强且一致的种内、种间关联，这一现象反映出一个生态复杂且具有分化特征的原始群落，其产生的复杂整合信号被完整保存于化石记录之中。