Siliceous phytoplankton response to a Middle Eocene warming event recorded in the tropical Atlantic (Demerara Rise, ODP Site 1260A) Johan Renaudie a , Taniel Danelian a,b, ⁎, Simona Saint Martin c , Laurence Le Callonnec d , Nicolas Tribovillard b a Université Pierre-et-Marie-Curie (Paris VI), Micropaléontologie, CNRS-UMR 5143 Paléobiodiversité et Paléoenvironnements, C. 104, 4 place Jussieu, 75005 Paris, France b Université Lille 1, UMR CNRS 8157 Géosystèmes, bâtiment SN5, 59655 Villeneuve d'Ascq cedex, France c Universitatea din Bucuresti, Facultatea de Geologie-Geofizica, Bulevard N. Balcescu No. 1, Bucuresti, Romania; Département d'Histoire de la Terre, Muséum National d'Histoire Naturelle, 8 rue Buffon, 75 005 Paris, France d Université Pierre-et-Marie-Curie (Paris VI), Laboratoire Biominéralisations et Environnements Sédimentaires, ISTeP - UMR 7193, C. 116, 4 place Jussieu, 75005 Paris, France abstract article info Article history: Received 15 May 2009 Received in revised form 12 November 2009 Accepted 6 December 2009 Available online 16 December 2009 Keywords: Middle Eocene Climate change Diatoms Silicoflagellates Biogenic silica Barium Tropical Atlantic 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. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Palaeogene siliceous phytoplankton have been fairly well studied from a taxonomic and biostratigraphic point of view (Mukhina, 1976; Fenner, 1978, 1984, 1985, 1991; Jousé, 1982; Perch-Nielsen et al., 1985; Fourtanier, 1991; Baldauf, 1992), but the floral response to environmental changes during the Palaeogene is less well reported. A noticeable exception is the Middle and Late Eocene increase in diatom provincialism (reported by Fenner 1985), which could be at the origin of some floristic renewal events (Baldauf, 1992; Barron, 1993). Previously, one of the main difficulties for the study of the siliceous phytoplankton response to Palaeogene climate change was due to the paucity of continuous diatomaceous sedimentary sequences recovered from the oceanic realm. The presence of well-preserved diatom as- semblages in an expanded Middle Eocene sequence recovered from Demerara Rise in the equatorial Atlantic (Danelian et al., 2007) offers the first opportunity to explore the response of the siliceous phytoplankton (mainly diatoms) in relation to a warming event recognised by Sexton et al. (2006) based on material from Demerara Rise (ODP Site 1260). The Eocene is a period of climatic transition, characterized by a progressive drop in global temperatures, between the Early Eocene Climatic Optimum and the Antarctic glaciations at the Eocene/ Oligocene transition. However, this progressive cooling trend was interrupted by several short-lived warming reversals, such as the Middle Eocene Climatic Optimum (MECO; Bohaty and Zachos, 2003), a ca. 500 Ka event that occurred at about 40.0 Ma (just above the base of magnetochron C18n.2n; Bohaty et al., 2009). The recent evaluation of the MECO's timing by Bohaty et al. (2009) suggests that the warming reversal, recognised by Sexton et al. (2006) between 44.0 and 42.0 Ma (based essentially on the magnetostratigraphic control provided by Suganuma and Ogg, 2006 and in combination with the biostratigraphic constraints published in Erbacher et al., 2004), represents an earlier warming event in the climate history of the Middle Eocene. Based on a quantitative micropalaeontological analysis of the siliceous phytoplankton record from ODP Site 1260, this study aims to describe the changes that can be observed in the structure of the siliceous floristic assemblages recorded across the 44.0–42.0 Ma warming event. Under- standing the environmental changes that may have been at the origin of the floral turnover is another question that is explored in this work using Palaeogeography, Palaeoclimatology, Palaeoecology 286 (2010) 121–134 ⁎ Corresponding author. Université Lille 1, UMR CNRS 8157 Géosystèmes, bâtiment SN5, 59655 Villeneuve d'Ascq cedex, France. E-mail address: taniel.danelian@univ-lille1.fr (T. Danelian). 0031-0182/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2009.12.004 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo