A critical evaluation of calcium isotope ratios in tests of planktonic foraminifers Nikolaus Gussone a,b, * , Ba ¨rbel Ho ¨ nisch c , Alexander Heuser d,e , Anton Eisenhauer d , Michael Spindler f , Christoph Hemleben g a Marum, Leobener Str., 28359 Bremen, Germany b Institut fu ¨ r Mineralogie, Universita ¨t Mu ¨ nster, Corrensstr. 24, 48149 Mu ¨ nster, Germany c Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, USA d Leibniz-Institut fu ¨ r Meereswissenschaften, IFM-GEOMAR, Wischhofstr. 1-3, 24148 Kiel, Germany e Steinmann-Institut fu ¨ r Geologie, Mineralogie und Pala ¨ ontologie, Universita ¨ t Bonn, Poppelsdorfer Schloß, D-53115 Bonn, Germany f Institut fu ¨ r Polaro ¨ kologie, Universita ¨ t Kiel, Wischhofstr. 1-3, 24148 Kiel, Germany g Institut fu ¨ r Geowissenschaften, Eberhard-Karls Universita ¨t Tu ¨ bingen, Sigwartstrasse 10, 72076 Tu ¨ bingen, Germany Received 26 January 2009; accepted in revised form 31 August 2009; available online 8 September 2009 Abstract We critically evaluate the applicability of Ca-isotope ratios in planktonic foraminifers as proxy for past sea surface tem- peratures (SST) and isotope composition of paleo-seawater (d 44 Ca sw ) reconstructions. Previous studies have shown discrep- ancies regarding the temperature sensitivity of Ca isotope fractionation in foraminifers of more than one order of magnitude. We present new data from the planktonic foraminifer species Orbulina universa, Globigerinoides sacculifer and Neogloboquad- rina pachyderma (sinistral) from culture experiments, multinet deployments and coretop samples. Specimens of G. sacculifer cultured at low salinities (33–34.5) show predominantly no major temperature dependent Ca isotope fractionation, in contrast to previous individuals cultured at higher salinities of 34.5–36. The new data of O. universa are consistent with previously published results, revealing a small but signiﬁcant temperature sensitivity. Calcium isotope fractionation in tests of N. pach- yderma shows a signiﬁcant variation with temperature, which is not uniform over the total investigated temperature range (1.6 °C to +10 °C), possibly reﬂecting the inﬂuence of additional controlling factors besides temperature. Controlled disso- lution experiments in the laboratory indicate that the Ca-isotope composition of G. sacculifer and N. pachyderma is relatively insensitive to partial dissolution of their tests. Calcium isotope ratios in the planktonic foraminifers G. sacculifer and N. pachyderma (s) reveal a complex Ca isotope frac- tionation behaviour, which is not yet fully understood. Additional validation studies are crucial to enhance the basic under- standing of the calcium isotope systematics in planktic foraminifer shells, and the potential for applying Ca-isotope ratios as proxies for seawater temperature and the oceanic Ca budget. Ó 2009 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Calcium isotope ratios of foraminifer tests are mainly used for two paleoceanographic applications, i.e. the recon- struction of the oceanic Ca budget and as temperature- proxy. The use of calcium isotopes as a sea surface temper- ature (SST) proxy has attracted increasing interest since Zhu and Macdougall (1998) proposed a possible tempera- ture dependent Ca isotope fractionation in the planktonic 0016-7037/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2009.08.035 * Corresponding author. Address: Institut fu ¨r Mineralogie, Universita ¨t Mu ¨ nster, Corrensstr. 24, 48149 Mu ¨ nster, Germany. Tel.: +49 251 833 3401. E-mail address: Nikolaus.Gussone@uni-muenster.de (N. Gussone). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 73 (2009) 7241–7255