Late Barremian to early Aptian calcareous nannofossil paleoceanography and paleoecology from the Ocean Drilling Program Hole 641C (Galicia Margin) Fabrizio Tremolada a, * , Elisabetta Erba b , Timothy J. Bralower c a Geological Sciences Department, Rutgers University, Piscataway, NJ 08854, USA b Dipartimento di Scienze della Terra ‘‘Ardito Desio’’, Universita ` degli Studi di Milano, 20133 Milano, Italy c Department of Geosciences, Pennsylvania State University, University Park, PA 16802, USA Received 2 March 2005; accepted in revised form 25 April 2006 Available online 15 September 2006 Abstract Calcareous nannofossil assemblages at Site 641C (Galicia Margin, North Atlantic) were investigated in order to determine changes in fertility and temperature of surface waters. Taxa such as Zeughrabdotus spp. <3.5 mm, Biscutum constans, Discorhabdus rotatorius and Diazomatolithus lehmanii, which thrived in higher fertility conditions, are particularly abundant across the CM0 interval as opposed to those with oligotrophic affinities such as Watznaueria spp. and Nannoconus spp., which are generally reduced in abundance. The abundances of nannoconids are much lower than those observed in Tethyan sections, indicating higher fertility conditions. Slumpings and low recovery prevent the identification of the onset of the ‘‘nannoconid crisis’’, but a sharp drop in nannoconid abundances, observed prior to the CM0 interval, correlates with the ‘‘nanno- conid decline’’ observed in several Tethys sections.The normalized ratio between low and high fertility taxa (Fertility Index) was used to char- acterize the nannofossil assemblages in terms of productivity changes. The highest values of the Fertility Index were observed across magnetic chron CM0. The paucity of cold water taxa such as Seribiscutum spp. and Repagulum parvidentatum suggests warm water conditions throughout the deposition of upper Barremianelower Aptian sediments on the Galicia Margin. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Calcareous nannofossils; BarremianeAptian interval; Primary productivity; Paleoecology 1. Introduction Calcareous nannoplankton are one of the most abundant marine planktonic groups. Living calcareous nannoplankton are sensitive to environmental changes, and changes in species abundance can be interpreted as a response to short-term oceanographic perturbations (e.g., Brand, 1994; Winter et al., 1994). Similarly, major changes in nannofossil assem- blages can indicate variations in the thermal and trophic re- gime of ancient oceans. Studies of modern nannoplankton reveal that most species thrive in open ocean environments where oligotrophic conditions are predominant (e.g., Bukry, 1974; Honjo and Okada, 1974). Distribution of extant nannoplankton taxa is strongly controlled by temperature (e.g., McIntyre and Be ´, 1967; Okada and Honjo, 1973; Winter et al., 1994) and fertil- ity of surface waters (e.g., Brand, 1994; Roth, 1994). Several investigations have interpreted major changes in nannofossil and nannoplankton assemblages with respect to paleoecologi- cal conditions. Knowledge of the paleoecological affinities of a number of nannofossil species was obtained from Cretaceous nannofloral assemblages (e.g., Roth, 1981, 1983, 1986; Roth and Bowdler, 1981; Roth and Krumbach, 1986; Mutterlose, 1987, 1991; Premoli Silva et al., 1989; Thierstein and Roth, 1991; Erba et al., 1992; Erba, 1994; Herrle, 2003). The aim of this study is to document the fluctuations in composition and abundance of nannofossil assemblages dur- ing the Barremian/Aptian boundary interval and to interpret * Corresponding author. E-mail address: ftremola@rci.rutgers.edu (F. Tremolada). 0195-6671/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2006.04.007 Cretaceous Research 27 (2006) 887e897 www.elsevier.com/locate/CretRes