Siliceous phytoplankton in middle Eocene sediments of ODP Hole 207-1260A@en

The Middle Eocene diatom and silicoflagellate record of ODP Site 1260A (Demerara Rise) is studied quantitatively in order to throw light on the changes that siliceous phytoplankton communities experienced during a Middle Eocene warming event that occurred between 44.0 and 42.0 Ma. Both Pianka's overlap index, calculated per couple of successive samples, and cluster analysis, point to a number of significant turnover events highlighted by changes in the structure of floristic communities. The pre-warming flora, dominated by cosmopolitan species of the diatom genus Triceratium, is replaced during the warming interval by a new and more diverse assemblage, dominated by Paralia sulcata (an indicator of high productivity) and two endemic tropical species of the genus Hemiaulus. The critical warming interval is characterized by a steady increase in biogenic silica and a comparable increase in excess Ba, both reflecting an increase in productivity. In general, it appears that high productivity not only increased the flux of biogenic silica, but also sustained a higher diversity in the siliceous phytoplankton communities. The microflora preserved above the critical interval is once again of low diversity and dominated by various species of the diatom genus Hemiaulus. All assemblages in the studied material are characterized by the total absence of continental and benthic diatoms and the relative abundance of neritic forms, suggesting a transitional depositional environment between the neritic and the oceanic realms.本研究对德梅拉拉海岭大洋钻探计划（Ocean Drilling Program, ODP）1260A站位的中始新世硅藻及硅鞭藻记录开展定量分析，旨在阐明发生于44.0~42.0 Ma的中始新世变暖事件期间，硅质浮游植物（siliceous phytoplankton）群落所经历的演替变化。本研究以每对连续样品为单位计算皮安卡重叠指数（Pianka's overlap index），并辅以聚类分析（cluster analysis），二者均揭示了多起由植物群群落结构变化所体现的显著群落更替事件。变暖前的植物群以三角藻属（Triceratium）硅藻的广布种为优势类群，在变暖期则被以具槽直链藻（Paralia sulcata，高生产力指示种）及半管藻属（Hemiaulus）的两个特有热带种为优势的全新且多样性更高的化石组合所取代。关键变暖期的特征为生物成因二氧化硅（biogenic silica）持续增长，同时过剩钡（excess Ba）出现同步增长，二者均反映了海洋生产力的提升。总体而言，高生产力不仅提升了生物成因二氧化硅的沉积通量，同时也维持了硅质浮游植物群落的更高多样性。关键变暖期之上保存的微植物群再次呈现较低的多样性，且以半管藻属的多个物种为优势类群。本次研究的所有化石组合均完全不含陆生及底栖硅藻（benthic diatoms），且浅海型类群相对富集，表明该沉积环境为浅海与远洋域之间的过渡沉积环境。