Oxygen isotope and Mg/Ca composition of Late Viséan (Mississippian) brachiopod shells from SW Iberia: Palaeoclimatic and palaeogeographic implications in northern Gondwana Maider Armendáriz ⁎, Idoia Rosales, Cecilio Quesada Instituto Geológico y Minero de España, Ríos Rosas 23, 28003 Madrid, Spain abstract article info Article history: Received 12 December 2007 Received in revised form 3 July 2008 Accepted 14 July 2008 Available online xxxx Keywords: Mississippian Palaeotemperatures Glaciation O-isotopes Mg/Ca ratio SW Iberia Latest Viséan brachiopod shells (Gigantoproductus sp.) from the Pedroches basin of SW Spain were sampled in order to obtain chemical and isotopic evidence that may help, first, to assess if primary geochemical and isotopical signals are preserved in these shells and, second, to reconstruct temporal variations in palaeoceanographic and palaeoclimatic conditions in northern Gondwana during this time span. Microtexture, cathodoluminescence (CL) and ultrastructure (under SEM) of the brachiopods were examined to avoid sampling of shell portions altered after deposition. Shell portions classified as non-luminescent (NL) and non-luminescent to slightly luminescent (NL–SL) present geochemical signatures within the ranges considered normal for Recent brachiopods that have not undergone significant diagenetic modification. These include low to undetectable Fe and Mn contents, high Sr values, and δ 18 O values similar to those of carbonates in equilibrium with seawater. On the other hand, valve areas that are luminescent (L) or moderately luminescent with luminescent portions (ML–L) present higher Fe and Mn contents, lower Sr concentrations, and lower δ 18 O values than those corresponding to NL and NL–SL shells. This suggests a greater diagenetic alteration of these portions and that they acted as microzones of reaction with the diagenetic fluids. Temporal trends through the studied interval show a spread in δ 13 C values much lower than that observed for contemporaneous samples (up to 1.6‰), thus obscuring the δ 13 C global signal at the time-scale resolution of this study. In contrast, the δ 18 O record of brachiopod calcite ( δ 18 O c ) shows an overall increase of ∼1‰ (relative to V-PDB) from the oldest to the youngest samples. This is accompanied by a ∼30% decrease in Mg/Ca ratios of the brachiopod calcites, confirming that at least part of this shift in δ 18 O c was related to cooling. Combination of Mg/Ca and δ 18 O c proxies established in this paper, although not full-proof for brachiopods yet, allowed for the calculation of a temperature drop of ∼3.3 °C and a change in seawater δ 18 O w that becomes ∼0.4‰ more positive during this interval. The data are consistent with regional seawater cooling that accompanied an increase in continental ice volume, over a relatively short time period of b 4 Myr. Comparison of the Carboniferous isotopic data from Spain with data compiled from Palaeotethyan basins suggests that this positive event in δ 18 O c during the Brigantian (latest Viséan) may be correlated worldwide. This synchronous global shift provides evidence for the onset of the Carboniferous glaciation in Gondwana during the Brigantian. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The Carboniferous glaciation was one of the longest and most severe palaeoclimatic crises in Earth history. It was accompanied by withdrawal of atmospheric CO 2 from extensive coal deposition and by continental drifting that culminated in amalgamation of the Pangea supercontinent (e.g. Hurley and van der Voo, 1987; Wincander and Monroe, 1993). These dramatic changes likely produced changes in the chemistry, circulation and temperature of the Carboniferous oceans, in conjunction with glacier fluctuations that are registered in the isotopic compositions of marine carbonates (e.g. Shackleton and Opdyke, 1973; Lohmann and Walker, 1989; Popp et al., 1986a; Veizer et al., 1986; Grossman, 1994; Bruckschen and Veizer, 1997). Since the dynamics of global climatic change is recorded in the oxygen isotopic evolution of the oceans, which in turn is recorded in the isotopic composition of the marine carbonates, the study of the thermal history of ancient oceans depends largely on records of the oxygen isotopic composition of their marine carbonate components. The brachiopod shells, in particular, are regarded as the most suitable material for palaeoceanographic studies in the Palaeozoic, because of their abundance in the rock record and because their low-Mg calcite composition is resistant to post-depositional diagenetic alteration and potentially reflects the isotopic composition of seawater at the time of deposition (e.g. Popp et al., 1986a; Railsback et al., 1989; Grossman et al., 1993, 2002; Bruckschen and Veizer, 1997; Marshall et al., 1997; Mii et al., Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2008) xxx–xxx ⁎ Corresponding author. Tel.: +34 91 7287288; fax: +34 91 7287202. E-mail addresses: m.armendariz@igme.es, maiderad@yahoo.es (M. Armendáriz). PALAEO-04786; No of Pages 15 0031-0182/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2008.07.008 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo ARTICLE IN PRESS Please cite this article as: Armendáriz, M., et al., Oxygen isotope and Mg/Ca composition of Late Viséan (Mississippian) brachiopod shells from SW Iberia: Palaeoclimatic and..., Palaeogeography, Palaeoclimatology, Palaeoecology (2008), doi:10.1016/j.palaeo.2008.07.008