Carbonate-associated sulfate: Experimental comparisons of common extraction methods and recommendations toward a standard analytical protocol Thomas Wotte a, ⁎, Graham A. Shields-Zhou b , Harald Strauss c a Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Strasse 49a, D-50674 Köln, Germany b Department of Earth Sciences, University College London, London WC1E 6BT, UK c Institut für Geologie und Paläontologie, Westfälische Wilhelms-Universität Münster, Corrensstrasse 24, D-48149 Münster, Germany abstract article info Article history: Received 2 February 2012 Received in revised form 25 July 2012 Accepted 27 July 2012 Available online 6 August 2012 Editor: U. Brand Keywords: Carbonate-associated sulfate Extraction methods Standard analytical protocol The aim of this study was to establish a protocol for the extraction of carbonate-associated sulfate (CAS) for the purpose of tracing the sulfur isotope composition of seawater. Existing CAS extraction methods were evaluated for their efficacy in eliminating non-CAS sulfur from the final CAS isotopic analysis. Five leaching methods were tested on three carbonate samples: (1) 10% NaCl (aq); (2) 10% NaCl (aq) followed by 10% NaOCl (aq); (3) 10% NaOCl (aq); (4) 10% NaCl (aq) followed by 10% H 2 O 2 (aq); and (5) pure water only. All leaching steps were performed until no dissolved sulfate was seen to precipitate on addition of BaCl 2 (aq). CAS was then liberated from the carbonate lattice by adding HCl from a dropping filter. All leachates, CAS fractions, and insoluble residues after CAS extraction (chromium-reducible sulfur or CRS) were analyzed for their isotopic composition. These experiments demonstrate that the leachable non-CAS sulfate fraction in carbonates can be proportionately far greater than, and isotopically distinct from the lattice-bound carbonate sulfate fraction. Here we show that some form of pre-leaching, other than with pure water, is necessary to isolate the CAS fraction in carbonates. However, even in cases of repeated pre-leaching and testing for non-CAS sulfate, measured δ 34 S CAS values may still be significantly influenced by the non-CAS sulfate fraction if δ 34 S NaCl and δ 34 S CAS values are sufficiently different. Pre-leaching once or twice with NaOCl and/or H 2 O 2 is shown to be insufficient to ensure elimination of reduced sulfur, e.g. in the form of pyrite, while partial oxi- dation of reduced sulfur during pre-leaching with these powerful oxidants extends pre-leaching times, and can thus contaminate the final CAS value. Both of these leaching methods are shown to alter final δ 34 S CRS values by partial oxidation of reduced sulfur, and so need to be applied with care. For a secure CAS extraction from carbonate rocks we recommend repeated leaching with NaCl solution as a standard protocol in future studies, with complementary analyses of pre-leach sulfate concentrations and δ 34 S NaCl , and CRS concentra- tions and δ 34 S CRS as routine checks on possible contamination as well as tools for interpretation. Analyzing δ 13 C carb , δ 18 O carb , and elemental concentrations (Ca, Fe, Mg, Mn, Sr) of the carbonate host rock may help to constrain diagenetic alteration of the measured δ 34 S CAS . Published interpretations of rapidly changing seawa- ter δ 34 S and sulfate concentrations need to be reconsidered in the light of these data. © 2012 Elsevier B.V. All rights reserved. 1. Introduction It is generally considered that the carbonate-associated sulfate (CAS) in carbonate mud but also in biogenic carbonates (e.g. shells of mollusks and brachiopods, belemnites, foraminifers, and coccoliths) archives the primary sulfate sulfur isotope composition of paleo-seawater at the time of precipitation (cf., Ueda et al., 1987; Burdett et al., 1989; Kampschulte and Strauss, 1996, 1998; Ohkouchi et al., 1999; Hurtgen et al., 2002; Kah et al., 2004; Kampschulte and Strauss, 2004; Lyons et al., 2004; Gellatly and Lyons, 2005; Riccardi et al., 2006). Although the mechanisms of sul- fate incorporation into carbonates as well as diagenetic effects are incom- pletely understood, CAS is regarded as a powerful proxy material for reconstructing the primary seawater sulfate sulfur isotope composition (Strauss, 1999; Kampschulte et al., 2001; Hurtgen et al., 2002; Kah et al., 2004; Newton et al., 2004; Strauss, 2004; Gellatly and Lyons, 2005; Guo et al., 2009). Diagenetic and analytical aspects can affect the measured isotopic composition of CAS and so both need to be carefully evaluated before using CAS as a proxy for primary seawater composition. Early diagenetic bacterial sulfate reduction, particularly under conditions of limited ex- change between pore water and seawater sulfate, causes 34 S-enrichment in the residual dissolved sulfate (Canfield, 2001), while sulfide oxidation results in 34 S-depleted sulfate. If incorporated into the carbonate lattice, sulfate affected by either of these processes would alter δ 34 S CAS during recrystallization (Kampschulte and Strauss, 2004). Regarding analytical aspects, it is essential to isolate CAS from non-CAS sulfur-bearing phases such as organic sulfur, metastable sulfides, acid volatile sulfur (AVS), Chemical Geology 326–327 (2012) 132–144 ⁎ Corresponding author. Tel.: +49 221 4703532; fax: +49 221 4705080. E-mail address: thomas.wotte@uni-koeln.de (T. Wotte). 0009-2541/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chemgeo.2012.07.020 Contents lists available at SciVerse ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo