Biological and ecological insights into Ca isotopes in planktic foraminifers as a palaeotemperature proxy Simone A. Kasemann a, ⁎, Daniela N. Schmidt b, 1 , Paul N. Pearson c,2 , Chris J. Hawkesworth b,3 a School of Geosciences, Grant Institute of Earth Science, University of Edinburgh, King's Buildings, West Mains Road, Edinburgh EH9 3JW, UK b Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK c School of Earth Sciences, University of Cardiff, Main Building, PO Box 914, Cardiff CF10 3YE, UK ABSTRACT ARTICLE INFO Article history: Received 29 November 2007 Received in revised form 7 April 2008 Accepted 10 April 2008 Available online 22 April 2008 Editor: H. Elderfield Keywords: calcium isotopes planktic foraminifers proxy palaeotemperature ion microprobe Sea surface temperature (SST) is a critical variable in the Earth's climate system since it influences atmospheric circulation, the hydrological cycle and, via ocean surface density, drives ocean circulation. A detailed reconstruction of past SST's is therefore a central goal of palaeoceanographic studies. Although calcium isotopes in foraminiferal carbonate have been introduced as a new proxy for SST reconstruction, there is considerable debate about their robustness and general applicability. To resolve some of these questions, we have investigated the extent to which other environmental parameters e.g. the calcification temperature, depth stratification, growth rates and/or environmental adaptation may influence the δ 44 Ca values of planktic foraminifers in modern and Eocene samples. Geographically distributed data sets are affected by the exchange of cryptic species, i.e. morphologically similar but genetically distinct species, and by a mixing of optimal versus less optimal adaptation. Thus, we have compared species within individual samples to evaluate whether the well documented depth stratification of foraminifers is reflected in their calcium isotopes. The Eocene data set shows a general agreement between δ 44 Ca and δ 18 O-derived calcification temperatures which supports a temperature effect on Ca isotope incorporation. The vertical temperature gradient using the different depth habitats of several foraminiferal species indicates a δ 44 Ca temperature dependence of ~0.034‰ °C − 1 similar to inorganic calcite (0.015‰ °C − 1 ) and cultured O. universa (0.019‰ °C − 1 ). The gradient resembles the global sediment δ 44 Ca compilation, but it is significantly smaller than the temperature calibration of 0.22 ± 0.02‰ °C − 1 for cultured G. sacculifer . The modern data set shows a general correlation between δ 44 Ca and depth habitat reflecting a similar temperature gradient to the Eocene sample set. In contrast, the lower absolute δ 44 Ca values for the Eocene foraminifers suggest a lower seawater isotope composition. In situ analyses of individual calcite layers reveal large isotopic differences between the different calcite layers of the foraminifers highlighting the strong biological control on δ 44 Ca in foraminiferal calcite that may overprint a potential T-relationship. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The ocean's temperature influences atmospheric circulation through heat exchange and evaporation as well as through ocean circulation in response to density differences. Therefore, assessing past temperatures and the related climate change provides valuable information for future climate change predictions. Direct knowledge of past climates is restricted to the instrumental record which just reaches back to the last century. Longer records rely on proxy data for past temperatures. The precision of the temperature data will constrain climate models and hence determine the predictability of future climate projections. Several temperature proxies rely on the chemical analysis of marine carbonates, e.g. δ 18 O (e.g. Emiliani, 1955; Shackleton, 1967), Sr/Ca ratios in corals (Beck et al., 1992), Mg/Ca ratios in foraminifers (Elderfield and Ganssen, 2000; Nürnberg et al., 1996) and most recently δ 44 Ca (Nägler et al., 2000). However, all of these proxies are controlled by more than one environmental parameter (e.g. Spero et al., 1997; Meland et al., 2006) and can be modified by the carrier that incorporates the palaeotemperature signal (Allison et al., 2005; Schmidt et al., in press). Planktic foraminifers are arguably one of the most important carriers for palaeoclimate information and they have been extensively used to reconstruct past temperatures (see Henderson, 2002 for a review). The multitude of influences on the proxy, especially in regions such as high latitudes and upwelling areas (Kucera et al., 2005), make it necessary to combine a suite of proxies to carefully constrain past climate change. The shortfalls of traditional proxies, e.g. in high Earth and Planetary Science Letters 271 (2008) 292–302 ⁎ Corresponding author. Tel.: +44 1316508525; fax: +44 131 6683184. E-mail addresses: Simone.Kasemann@ed.ac.uk (S.A. Kasemann), d.schmidt@bristol.ac.uk (D.N. Schmidt), Paul.Pearson@earth.cf.ac.uk (P.N. Pearson), C.J.Hawkesworth@bristol.ac.uk (C.J. Hawkesworth). 1 Tel.: +44 117 9545414; fax: +44 117 9253385. 2 Tel.: +44 29 20874579. 3 Tel.: +44 117 9545425. 0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.04.007 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl