An approach to estimate Lower Jurassic seawater oxygen- isotope composition using d 18 O and Mg/Ca ratios of belemnite calcites (Early Pliensbachian, northern Spain) Maider Armend ariz, 1 Idoia Rosales, 1 Beatriz B adenas, 2 Laura Pi~ nuela, 3,4 Marc Aurell 2 and Jos e Carlos Garc  ıa-Ramos 3 1 Instituto Geol  ogico y Minero de Espa~ na, R ıos Rosas 23, Madrid 28003, Spain; 2 Departamento Ciencias de la Tierra, Universidad de Zaragoza, Zaragoza 50009, Spain; 3 Museo del Jur  asico de Asturias (MUJA), 33328 Colunga, Asturias, Spain; 4 Departamento de Geolog ıa, Universidad de Oviedo, Oviedo 33005, Spain ABSTRACT Palaeotemperature estimates from the oxygen-isotope compo- sitions of belemnites have been hampered by not knowing ancient seawater isotope compositions well enough. We have tackled this problem using Mg/Ca as a proxy for temperature and here, we present a ~2 Ma record of paired Mg/Ca and d 18 O measurements of Jurassic (Early Pliensbachian) belem- nites from the Asturian basin as a palaeo-proxy of seawater oxygen-isotope composition. From the combined use of the two approaches, we suggest a d 18 O w composition of about À0.1& for the Jamesoni–Ibex zones. This value may have been increased by about 0.6& during the Davoei Zone due to the effect of waters with a different d 18 O w composition. These findings illustrate the inaccuracy of using a globally homogeneous ice-free value of d 18 O w = À1& for d 18 O carb - based palaeotemperature reconstructions. Our data suggest that previous palaeotemperatures calculated in the region from d 18 O values of belemnites may have been underesti- mated as the seawater oxygen isotopic composition could have been higher. Terra Nova, 25, 439–445, 2013 Introduction The use of oxygen-isotope ratios of belemnites for palaeotemperature cal- culations has been widely used in the last six decades. However, these palaeotemperature calculations have been used with some uncertainties due to the poor knowledge of the ancient seawater oxygen isotopic composition (d 18 O w ), and the unknown species-specific biological fractionation effects and belemnite life habitat and habits (e.g. Sælen et al., 1996; Bailey et al., 2003; Rosales et al., 2004a; Dutton et al., 2007; McArthur et al., 2007a; Mut- terlose et al., 2010). In more recent years, the Mg/Ca palaeotemperature proxy has been used in belemnites as an alternative palaeothermometer due to the existence of reasonably good correlation between Mg/Ca and d 18 O bel (e.g. Bailey et al., 2003; Ro- sales et al., 2004b; Nunn and Price, 2010; Price et al., 2011; Armend ariz et al., 2012). Moreover, the combina- tion of these two palaeo-proxies has the potential to be used to estimate changes in sea-water oxygen isotopic composition and polar ice volume, although its efficacy has been ques- tioned by some authors due to signif- icant differences in the Mg/Ca concentration and a lack of Mg/Ca and d 18 O correlation between some belemnite species of the Toarcian (McArthur et al., 2007a; Li et al., 2012). To shed light on this issue, we examine the potential and consis- tency of d 18 O and Mg/Ca values of well-preserved Jurassic belemnites of the Early Pliensbachian Boreal Realm of the Asturian basin (Figs 1 and 2) as palaeoclimatic and palaeoenviron- mental proxies. To obtain more con- sistent data avoiding, as much as possible, the noise introduced by inter- specimen variability, we used chemo- stratigraphic records based on curves obtained from running average values, which are commonly used in time ser- ies data to filter short-term fluctua- tions and highlight the long-term trends. The use of these curves has pro- vided a long-term record (~2 Ma according to Gradstein et al., 2012) of Mg/Ca ratios of the investigated be- lemnites that parallel the d 18 O carb val- ues. Assuming a temperature dependence of Mg/Ca in belemnites within the range observed in most modern biogenic calcites, the signifi- cance of combining the two palaeo- proxies is that it provides an opportu- nity to calibrate the local oxygen isoto- pic composition of seawater for the Lower Jurassic. Geological setting, materials and methods The Pliensbachian succession of the Asturian basin (northern Spain) belongs to the Santa Mera Member of the Rodiles Formation and corre- sponds to a rhythmic alternation of limestone and marl deposited in an epeiric platform (Su arez Vega, 1974; Valenzuela et al., 1986). The samples for this study were collected from two well-exposed sections of the Asturian sea cliffs: Punta La Llastra and Playa de Vega (Fig. 2A). In these sections, the Pliensbachian suc- cession comprises ~80 m of marl/cal- careous mudstones interbedded with bioclastic tempestites and organic facies (black shales) at certain strati- graphic horizons (Fig. 2B; Valenzuela et al., 1986; Aurell et al., 2003; B adenas et al., 2009, 2012; Garc  ıa- Ramos and Pi~ nuela, 2010). The suc- cession contains relatively abundant para-autochthonous and autochtho- nous fossils (ammonites, belemnites, Correspondence: Maider Armend ariz, C/ Calera 1, Tres Cantos, Madrid 28760, Spain. Tel.: +34-91-7287273; fax: +34-91- 7287202; e-mail: m.armendariz@igme.es © 2013 John Wiley & Sons Ltd 439 doi: 10.1111/ter.12054