Quaternary Science Reviews 27 (2008) 956–961 Controls of shell calcification in planktonic foraminifers B. Gonzalez-Mora à , F.J. Sierro, J.A. Flores Departamento de Geologı´a, Universidad de Salamanca, Plaza de la Merced s/n., 37008 Salamanca, Spain Received 10 April 2007; received in revised form 8 January 2008; accepted 8 January 2008 Abstract In order to better constrain the factors controlling the calcification rate in planktonic foraminifera, and to assess shell weight reliability as an atmospheric CO 2 proxy, changes in shell weight in Globigerina bulloides, Neogloboquadrina pachyderma (right coiling) and Globigerinoides ruber (white) have been studied in core samples from the Alboran Sea (Western Mediterranean) between 250 and 160 ka (Marine Isotopic stages 6 and 7). These results suggest that G. bulloides and G. ruber (white) calcification rate is mainly controlled by atmospheric CO 2 and water temperature, whereas N. pachyderma (right coiling) shell weight is directly related to optimum growth conditions, since heavier shell weights coincide with more relative abundance of this species due to optimum temperature conditions. Therefore, foraminifer shell weight should be used carefully as an atmospheric CO 2 proxy. r 2008 Elsevier Ltd. All rights reserved. 1. Introduction Carbonate-bearing organisms play a very important role in the global carbon cycle since they transfer and deposit huge amounts of calcium carbonate (CaCO 3 ) on the sea floor (Milliman, 1993; Archer and Maier-Reimer, 1994; Broecker and Clark, 2001). Over the past decade, several studies have focused on the quantification of the relation- ship between the calcification of marine calcium carbonate producers and water carbonate saturation (e.g. Riebesell et al., 2000; Barker and Elderfield, 2002) with a view to evaluating the possible negative effect of today’s increasing atmospheric CO 2 levels on these organisms. Barker and Elderfield (2002) have recently suggested that water carbonate saturation is the main factor controlling the calcification rate in planktonic foraminifera (based on samples from a latitudinal transect in the North Atlantic), supported previously by culture studies (Spero et al., 1997). These authors proposed shell weight as a carbonate saturation and atmospheric CO 2 proxy. However, other studies have demonstrated that controls on the shell weight are not so simple, and that water carbonate saturation is not the only factor controlling the calcification rate in planktonic foraminifera (de Villiers, 2003, 2004). This latter author suggested that the environmental factors and optimum growth conditions are very important for higher calcification rates and no relationship between water [CO 3 2À ] and calcification rate was detected in that work. Since conflicting results have emerged from different studies addressing the reliability of shell weight as an atmospheric CO 2 proxy, our aim here is to gain a better understanding of the control of calcification rate in different foraminifer species. We report that the main control factor is water temperature in the case of Globigerina bulloides, Neogloboquadrina pachyderma (right coiling) and Globigerinoides ruber (white). 2. Regional setting The Mediterranean Sea is a semi-enclosed marginal basin with a negative water balance, which results in an anti-estuarine circulation and hence oligotrophic condi- tions (Be´thoux, 1979). The Alboran Sea is the westernmost basin in the Mediterranean Sea and is connected to the Atlantic through the Strait of Gibraltar (Fig. 1). The Atlantic Water flows into the Alboran Sea at the surface (down to 220 m), whereas the Mediterranean Outflow Water (MOW) enters the Atlantic at depth (Millot, 1999). In the Alboran basin, the Atlantic Water flows eastwards and is gradually modified due to vertical water mixing and heat interchange with the atmosphere resulting in lower ARTICLE IN PRESS 0277-3791/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2008.01.008 à Corresponding author. Tel.: +34 923 29 44 97; fax: +34 923 29 45 14. E-mail address: mora@usal.es (B. Gonzalez-Mora).