Carbonate-associated sulfate: Experimental comparisons of common extraction methods and recommendations toward a standard analytical protocol

Chemical Geology 326–327 (2012) 132–144

Contents lists available at SciVerse ScienceDirect

Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo

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 b c

Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Strasse 49a, D-50674 Köln, Germany Department of Earth Sciences, University College London, London WC1E 6BT, UK Institut für Geologie und Paläontologie, Westfälische Wilhelms-Universität Münster, Corrensstrasse 24, D-48149 Münster, Germany

a r t i c l e

i n f o

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

a b s t r a c t 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% H2O2 (aq); and (5) pure water only. All leaching steps were performed until no dissolved sulfate was seen to precipitate on addition of BaCl2 (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 δ34SCAS values may still be significantly influenced by the non-CAS sulfate fraction if δ34SNaCl and δ34SCAS values are sufficiently different. Pre-leaching once or twice with NaOCl and/or H2O2 is shown to be insufficient to ensure elimination of reduced sulfur, e.g. in the form of pyrite, while partial oxidation 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 δ34SCRS 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 δ34SNaCl, and CRS concentrations and δ34SCRS as routine checks on possible contamination as well as tools for interpretation. Analyzing δ13Ccarb, δ18Ocarb, and elemental concentrations (Ca, Fe, Mg, Mn, Sr) of the carbonate host rock may help to constrain diagenetic alteration of the measured δ34SCAS. Published interpretations of rapidly changing seawater δ34S 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 sulfate incorporation into carbonates as well as diagenetic effects are incompletely understood, CAS is regarded as a powerful proxy material for ⁎ Corresponding author. Tel.: +49 221 4703532; fax: +49 221 4705080. E-mail address: [email protected] (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

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 exchange between pore water and seawater sulfate, causes 34S-enrichment in the residual dissolved sulfate (Canfield, 2001), while sulfide oxidation 34 results in S-depleted sulfate. If incorporated into the carbonate lattice, sulfate affected by either of these processes would alter δ34SCAS 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),

T. Wotte et al. / Chemical Geology 326–327 (2012) 132–144

pyrite, barite, gypsum and other potential sources of non-CAS sulfate (cf., Kampschulte et al., 2001; Goldberg et al., 2011). Otherwise, a mixed δ34S signal would be generated which integrates the isotopic composition of CAS and non-CAS compounds. Different methods for cleaning samples of non-CAS sulfur have been developed, primarily using solutions of sodium chloride (NaCl), sodium hypochlorite (NaOCl) or simply rinsing with deionized water. A pioneering and influential study on CAS was performed by Burdett et al. (1989). In that study, samples of foraminiferal ooze were soaked in a 5.25% NaOCl solution for 24 h prior to the CAS extraction in order to remove organic sulfur and metastable sulfides. Subsequently, the carbonate residue was rinsed with deionized water, filtered, and finally dissolved in 3.6 N HCl to liberate the CAS from the calcite lattice. After filtration, 10–20 mL of a 0.5 M BaCl2 solution was added to the filtrate in order to trigger BaSO4 precipitation (Westgate and Anderson, 1982). The final solution was heated to near boiling and kept at this temperature for 4 h before being left to cool. The precipitated BaSO4 was finally filtered through a sulfur-free ashless filter. This now quite standard NaOCl–HCl extraction technique has been adopted with slight modification by the majority of CAS workers over more than a decade (e.g., Ohkouchi et al., 1999; Loyd et al., 2012a). Hurtgen et al. (2002) were among the first to use this technique to extract CAS from bulk carbonate rock. After rinsing bleached 30–150 g carbonate samples and dissolving them with HCl, they added 6 N NaOH solution to increase the pH to between 3 and 5. Saturated bromine water (about 15 mL) was then added to trigger the precipitation of iron oxyhydroxides, which would otherwise precipitate along with BaSO4. Following filtration of the iron oxyhydroxides, 25–30 mL of an 8.5% BaCl2 solution was added and CAS precipitated as described above. A similar approach was used in subsequent studies (e.g., Hurtgen et al., 2006; Halverson and Hurtgen, 2007) but with 3 N HCl. Slight modifications were described by Gellatly and Lyons (2005) who rinsed their 100–300 g samples with deionized water before addition of 5.25% NaOCl (see also Chu et al., 2007). Samples were then dissolved slowly in 4 N HCl to completion and filtered. Precipitation of CAS as BaSO4 was achieved through addition of 125 mL of a BaCl2 solution (~250 g/L). To ensure complete precipitation, the samples were kept at room temperature for approximately 14 days. This extraction method is similar to that used by Gill et al. (2007, 2008) who soaked 60 g carbonate samples in deionized water for 24 h as a first step, followed by a single treatment with 4% NaOCl for 48 h, rinsing twice again with deionized water. After each rinse, the supernate was carefully decanted. After dissolution, filtration and BaCl2 addition, the samples were kept for at least 3 days to encourage complete precipitation of BaSO4. Loyd et al. (2012a) applied a similar leaching procedure (3 times in H2O, then 2 times in NaOCl, then HCl dissolution) in their study of CAS in carbonate concretions. Washed samples (about 150 g) were dried and weighed before acidification. A slight modification was done by Loyd et al. (2012b) who washed their powdered rock samples four times in deionized water followed by a single leaching in NaOCl. Slurries were stirred for ~2 min and kept for 8 to 12 h to allow soaking and settling. Washed samples were dried and dissolved in 3 M HCl overnight at room temperature. Loyd et al. (2012a) consider their data to have been significantly affected by pyrite oxidation, due to the predictably low initial CAS contents of diagenetic carbonates. A similar procedure was applied by Thompson and Kah (2012) who soaked their carbonate samples (~100 g) in 5.65–6% NaOCl overnight, after which they were rinsed four times with deionized water, filtered and then dried. Approximately 50–100 g of dried sample was then slowly dissolved in 3 N HCl, keeping the pH above 3 to limit pyrite oxidation and oxygen isotope exchange between sulfate and water (Chiba and Sakai, 1985; Thompson and Kah, 2012). Following filtration, the solution was brought to a pH of 9 using NaOH pellets to precipitate dissolved iron oxides. After filtration, 140 mL of a saturated BaCl2 solution (250 g/L) was added and kept at room temperature overnight.

133

Another modification of the NaOCl–HCl CAS extraction technique was published by Newton et al. (2004) who treated their samples with 5% NaOCl before liberating CAS using 50% HCl under anaerobic conditions to avoid oxidation of surviving sulfide minerals and AVS during carbonate dissolution. The dissolved carbonate samples were filtered (again under O2-free conditions) and bromine added to the solution to oxidize any metals, which could precipitate out and be filtered off. Sulfate was precipitated from the final solution as BaSO4 following re-acidification of the solution to a pH between 2.5 and 3. In contrast to the NaOCl-based methods described above, Fike et al. (2006) and Fike and Grotzinger (2008) rinsed their 10–30 g samples 3 times and then soaked them for 24 h with deionized water before CAS extraction either by dissolution in 6 N HCl for 2 h at approximately 60 °C under a nitrogen atmosphere or for 12–24 h at room temperature. According to Fike et al. (2006) and Fike and Grotzinger (2008), these two extraction techniques did not lead to any differences in the final δ 34S values. The resultant solutions in each case were filtered, with BaSO4 precipitation triggered by addition of a 1 M BaCl2 solution. Ries et al. (2009) used considerably more carbonate sample (about 300 g), soaking in deionized water for 24 h, but otherwise applied the same technique with no pre-leach. Zhang et al. (2003) similarly dissolved their carbonate samples (approximately 50 g) directly in HCl (6 mol/L) for 12 h. After filtration, 5 g of (NH4)2(OH)Cl and 2 drops of a methylic orange solution were added. The pH of the solution was adjusted using HCl and NH4OH until the solution showed a red color. This solution was subsequently heated to near-boiling and 3 mL HCl (12 mol/L) was added, followed by the addition of about 20 mL of BaCl2 solution (100 g/L). The final solution was kept on the hot plate for 30 min and left to cool for 12 h to ensure complete precipitation of BaSO4. An alternative sequential extraction method for CAS was published by Kampschulte et al. (2001), Kampschulte and Strauss (1996, 1998, 2004) and Goldberg et al. (2005) who leached 200 g carbonate samples repeatedly in a 10% NaCl solution for 24 h. Wotte et al. (2011, 2012) used samples from 500 to 1800 g and made leaching with NaCl more effective by stirring the slurry constantly using a magnetic stirrer. The slurry was then filtered through folded paper filters and rinsed with deionized water to remove all soluble non-CAS sulfate. The rinse solution was then filtered again through cellulose nitrate membrane filters (pore diameter 0.45 μm). The resulting filtrate was acidified to a pH below 2, heated until boiling with dissolved sulfate being precipitated as BaSO4 after addition of an 8.5% BaCl2 solution (10% of the filtrate volume). In order to optimize conditions for BaSO4 crystal nucleation, the filtrate was kept at 80 °C for 3 h, before being allowed to cool overnight. NaCl leaching was repeated until no further soluble sulfate could be recovered, usually about 2–3 times. Dropping funnels were used for adding 25% HCl to the wet sample residue in order to minimize the exposure time of the sample to HCl and thereby reduce the potential for pyrite oxidation (Marenco et al., 2008b) during the sample-acid interaction. Carbonate samples were continuously stirred by magnetic stirrers and completely dissolved within 1 h. The solution was finally filtered through a cellulose nitrate membrane filter (0.45 μm) and the dissolved sulfate precipitated as BaSO4 following standard procedures (addition of 8.5–12% BaCl2). A modified NaCl leaching technique was applied by Shen et al. (2008, 2010) and also Li et al. (2010) who leached their 50–100 g samples with 10% NaCl for 24 h and subsequently washed them three times with deionized water. The samples were then stepwise dissolved in 10 M HCl. Initially, samples were immersed in a mixture of deionized water and HCl, the slurry being gently shaken for up to an hour to ensure complete reaction. During subsequent treatments (usually 2–3 times) HCl was added until the carbonate was completely dissolved. The final slurry was filtered and sulfate precipitated as BaSO4 by adding a saturated BaCl2 solution and allowing precipitation to take place over 48 h. A fourth type of CAS preparation, a combined NaCl–NaOCl leach, was applied by Gill et al. (2011a, 2011b) who immersed their samples in a 10% NaCl solution for 24 h, while agitating periodically. Subsequently,

134

T. Wotte et al. / Chemical Geology 326–327 (2012) 132–144

the supernate was decanted and samples immersed in deionized water again. This H2O leaching step was repeated twice in order to remove all sulfate liberated during the NaCl leaching. Following the second H2O rinse, samples were immersed in a 4% NaOCl solution and agitated periodically for 48 h in order to remove organically-bound sulfur that might otherwise have become oxidized to sulfate during the final carbonate dissolution. The NaOCl step was followed by two further rinses with H2O, decanting after each rinse. Samples were finally dissolved in 4 N HCl within 4 h, centrifuged and filtered through a cellulose nitrate membrane filter (0.45 μm). Sulfate was precipitated as BaSO4 by adding a saturated BaCl2 solution (250 g/L) at room temperature and kept for at least three days to ensure complete precipitation. Combined leaching procedures, comprising treatments with NaCl, deionized water, and bleach solutions, were also applied by Shen et al. (2011) and Xiao et al. (2012). In contrast to the NaCl–NaOCl method described above, Shen et al. (2011) treated their 30–50 g samples at least three times with 30% H2O2 for 48 h to completely remove pyrite and organic matter, before soaking in 10% NaCl for 24 h and washing with deionized water at least three times. Subsequently, samples were dissolved in 10 M HCl following the stepwise acidification procedure of Shen et al. (2008). Xiao et al. (2012) reversed this combined procedure, leaching 100 g samples with 10% NaCl, followed by washing with deionized water and drying. Subsequent treatment with 30% H2O2 for 48 h was carried out directly before the final carbonate dissolution step which used 3 M HCl. 10 ml of 0.1 M BaCl2 was added to 40 ml of supernate to precipitate BaSO4. Although many of these techniques appear superficially to be very similar, they vary in possibly significant ways. While some studies have used water and salt solutions only to clean carbonate samples of non-CAS sulfur, others have chosen to add powerful oxidants, such as NaOCl and H2O2, in order to remove, at least partially, the reduced sulfur component. In some studies, this bleach was added before rinsing with water or pre-leaching with NaCl solution, while in some studies it was added afterward. In several studies, workers have opted to dry out pre-leached residues, in order to weigh them accurately, or have chosen to dissolve the carbonate fraction slowly in acid, in contrast to others who avoided drying out the sample at any stage and carried out the dissolution as speedily as possible, sometimes under anaerobic conditions. It is highly probable that such contrasting methods of CAS extraction will result in significant differences in the measured δ34SCAS. Fike and Grotzinger (2008) compared their water rinsing/soaking method with a NaCl leaching method but failed to notice any difference in the δ34SCAS data generated from these two procedures. Goldberg et al. (2011) examined various methods for extracting trace sulfate from phosphorite samples, and provide some insight into CAS extraction techniques. However, no thorough evaluation or even cross-calibration comparison of the different CAS extraction methods and their effects on the resulting δ34SCAS data has been published. From the literature, it appears that bleaching carbonates prior to carbonate dissolution (the NaOCl method) represents the most commonly applied approach. However, in all cases only one, or rarely two (Shen et al., 2011; Loyd et al., 2012a) bleach steps were applied. In no case was there any verification of the extent to which reduced sulfur and organic material had been removed during pre-leaching or whether further pre-leaching treatment might have been necessary. Especially when large amounts of rock powder are used, it seems doubtful that a single application of dilute NaOCl would be sufficient to completely remove all non-CAS sulfur-bearing compounds as is frequently claimed. Moreover, the oxidizing character of the most commonly applied pre-leaches necessitates repeated rinsing and filtration after pre-leaching, in order to remove all traces of the used oxidant. In this regard, we note that the potency of NaOCl, unlike H2O2, does not decrease with time or on heating. Here we compare the major methods for CAS extraction — NaCl-, NaOCl-, H2O2- and H2O-leaching — in order to test their relative

efficacy in eliminating non-CAS sulfur-bearing phases and to evaluate the reliability of the resulting δ 34SCAS data. Using our data, we were able to identify the sources for some non-CAS phases which have the potential to affect the sulfur isotopic signal after CAS extraction. In particular, we determined the effect of pyrite oxidation and non-CAS sulfate on measured δ 34SCAS values by analyzing the isotopic composition of chromium-reducible sulfur (CRS) and of leachable non-CAS sulfate. The aim of our research is to help establish a robust, generalized protocol for CAS extraction and so enable the use of carbonate-associated sulfate sulfur isotope data as a robust seawater proxy.

2. Material and methods Recently, Wotte et al. (2012) analyzed carbonates from three Early– Middle Cambrian carbonate successions of western Gondwana (Crémenes and Genestosa from NW Spain; Ferrals-les-Montagnes from S France; for more information about location, litho- and biofacies, see Wotte, 2005, 2009) for their δ34S from CAS and CRS. The most pristine isotope values were specified by evaluating their trace elemental composition, comparing CAS and CRS isotopic compositions and concentrations, and considering the sulfur isotopic composition of the leached non-CAS. Leaching was performed using a 10% NaCl solution as described by Kampschulte and Strauss (2004). Rigidly applying this procedure, Wotte et al. (2012) could demonstrate that the δ 34S data from the different steps of NaCl leaching differ significantly from the final δ34SCAS values (for data of the Crémenes section see Fig. 7b and Table 5 of Wotte et al., 2012), thus indicating that non-CAS phases do not overprint the final δ 34SCAS values, but nonetheless have the potential to contaminate measured δ34SCAS. For our present study we re-analyzed three carbonate samples from the Spanish Crémenes section (CR 4, CR 16, and CR 18A; see Wotte et al., 2012; their Tables 2 and 5). These samples were selected because although their non-CAS and CAS δ 34S values were distinct, final CAS δ 34S was lower than average. Sample CR 4 represents a laminated mudstone with laminoid-fenestral fabrics (Wotte, 2009). The matrix is composed of micrite with sparry fenestrae. Samples CR 16 and CR 18A are from oolitic-bioclastic floatstones, composed of ooids (about 0.7 mm in diameter) and bioclasts (fragments of echinoderms, brachiopods, and trilobites) within a microsparitic matrix (Wotte, 2009). The content of abiogenetic minerals of all three samples is below 2%, composed of clay minerals, pyrite grains (max. diameter of 4 μm), and glauconite (max. diameter of 20 μm). Pyrite crystals are concentrated along sutures of pressure solution or are finely dispersed. Sample material devoid of weathered crusts and calcite veins was crushed and pulverized to b63 μm. We compared the following leaching methods designed to eliminate non-CAS: (1) NaCl, (2) combined NaCl–NaOCl, (3) NaOCl, (4) deionized water, and (5) combined NaCl–H2O2. To enhance the efficiency of all leaching steps, samples were constantly agitated using magnetic stirrers. Following the numerous leaching and soaking applications, all pre-leached carbonate samples were dissolved in 25‰ HCl. The insoluble residue was separated from the solution by filtration. Liberated CAS was precipitated as BaSO4 from the solution by adding a 12‰ BaCl2 solution (10% of the solution volume). The final acid-insoluble residue was dried and analyzed for its chromium-reducible sulfur (CRS) content and sulfur isotopic composition. The extraction of CRS was performed as described by Canfield et al. (1986). BaSO4 and Ag2S precipitates were analyzed for their δ 34S composition using an Elemental Analyzer connected to a ThermoFinnigan Delta Plus mass spectrometer. For calibration, international standards IAEA-S-1, IAEA-S-2, IAEA-S-3 and internal lab standards (Ag2S and CdS) were measured. Results are reported in the usual delta notation as per mil difference from the V-CDT standard. The analytical reproducibility was generally better than ±0.3‰.

T. Wotte et al. / Chemical Geology 326–327 (2012) 132–144

135

2.1. NaCl leaching

3. Results

Samples were leached repeatedly with 10% NaCl solution for 24 h under constant magnetic stirring in order to remove any soluble sulfate. Initial weights were 902 g for sample CR 4, 923 g for sample CR 16, and 935 g for sample CR 18A (Table 1). The NaCl leaching step needed to be repeated three times for sample CR 4, five times for sample CR 16, and nine times for sample CR 18A until no further soluble sulfate was recovered from the second to last pre-leach. For each step BaSO4 was precipitated following the standard procedure (Wotte et al., 2011, 2012). In order to verify the complete removal of non-CAS compounds by NaCl leaching, we repeated this approach using less sample powder (250 g for each sample) as a crosscheck.

For each leaching step, BaSO4 precipitates were weighed. Mean isotopic compositions given below refer, therefore, to the integrated mean of all leach steps (Table 1), and allow the relative importance of non-CAS relative to CAS and CRS to be estimated (Table 1). Where three isotopic compositions are given in brackets below they refer to the integrated mean values for the NaCl crosscheck (250 g samples), combined NaCl–NaOCl and NaCl–H2O2 procedures, respectively.

2.2. Combined NaCl–NaOCl leaching 250 g of sample powder were leached as above with 10% NaCl until no non-CAS could be recovered as BaSO4. Subsequently, samples were leached repeatedly with 10% NaOCl solution for 24 h. The slurry was carefully decanted and the solution vacuum filtered through a 0.45 μm cellulose membrane filter. Dissolved sulfate was precipitated as BaSO4. Caution is needed during the acidification of the solution prior to BaSO4 precipitation as chlorine gas is generated following a vigorous reaction. Consequently, this part of the procedure should only be performed under a fume hood and wearing proper safety equipment. NaOCl leaching was also repeated until no further sulfate was liberated: seven times for sample CR 4 and nine times each for samples CR 16 and CR 18A (Table 1).

2.3. NaOCl leaching In another experiment, samples were leached repeatedly with NaOCl for 24 h without prior NaCl leaching. Unequal amounts of rock powder were used due to availability: 200 g for CR 4, 250 g for CR 16, and 88 g for CR 18A (Table 1). Slurries were vacuum filtered and acidification of the solution was done under the fume hood. Nine repeats of the leaching step were necessary for samples CR 4 and CR 16, and seven repeats for sample CR 18A until no further BaSO4 was observed to precipitate from the solution (Table 1).

2.4. Combined NaCl–H2O2 leaching Following the initial NaCl leachings, carbonate samples (all 250 g) were leached repeatedly with 10% H2O2 for 24 h. Samples were heated to 50 °C during this process to accelerate the reaction (although we acknowledge that both heating and stirring will effectively reduce the oxidizing power of H2O2, and has likely extended leaching times (e.g., Dimitrijević et al., 1999). Solutions were decanted and filtered through cellulose nitrate membrane filters (0.45 μm pore size). Even for large amounts of sample, membrane filtering was required because folded paper filters are easily dissolved in H2O2. Sulfate was precipitated from the solution after acidification. The leaching step was repeated five times for CR 4, eight times for CR 16, and eleven times for CR 18A to ensure that all non-CAS was removed (Table 1).

2.5. Soaking and rinsing with deionized water In order to evaluate the efficiency of deionized water for eliminating non-CAS compounds from carbonate samples, 6.25 g of CR 4, 223.52 g of CR 16, and 3.44 g of CR 18A were soaked in 200 mL (CR 4 and CR 18A) and 250 mL (CR 16) of deionized water for 24 h. Subsequently, samples were filtered and solid samples rinsed with 500 mL (CR 4, CR 18A) and 1000 mL (CR 16) of deionized water.

3.1. NaCl leaching One consequence of the reduction in initial sample weights (from > 900 g to 250 g; Table 1) was a corresponding reduction in the number of leaching steps required. In contrast to Wotte et al. (2012) only three leaching steps were necessary for samples CR 4 and CR 16 (cf., three and five steps, respectively, in Wotte et al., 2012) and four leaching steps for sample CR 18A (previously ten leaching steps) (Fig. 1; Table 1). δ 34SNaCl data for sample CR 18A from the study by Wotte et al. (2012) showed an increase from 14.0 to 15.1‰ during the first five leaching steps followed by a decrease towards 10.9‰ during the subsequent three leaches (mean = 14.0‰). A similar trend could be observed for the δ 34SNaCl data generated here, increasing from 14.4 to 18.2‰ during the first two analyses, followed by a decrease towards 13.0‰ (mean = 15.1‰, 15.0‰, 15.0‰, respectively). An increase in δ 34SNaCl values from 2.9 to 10.5‰ for sample CR 16 was observed for our NaCl crosscheck (mean for NaCl leaches = 4.0‰, 3.5‰ and 4.3‰, respectively), whereas the corresponding δ 34SNaCl values reported in Wotte et al. (2012) show an increase from 3.9 to 14.2‰ and a slightly higher mean value of 5.0‰. δ 34SNaCl data from sample CR 4 showed a decrease during pre-leaching from 17.8 to 16.9‰ in Wotte et al. (2012), with an integrated mean isotopic composition of 17.7‰. The NaCl leaches in our experiments produced consistently lower mean δ 34SNaCl values of 17.0‰, 16.9‰ and 16.9‰, respectively. The δ 34SCAS data generated following the NaCl leaching steps are more positive (by up to 2.8‰) for the NaCl crosscheck than the values published by Wotte et al. (2012). δ 34SCRS data, however, are similar in both studies with values of − 5.1‰ for CR 4, 10.3‰ for CR 16, and 13.4‰ for CR 18A (Wotte et al., 2012), compared with − 4.7‰ for CR 4, 10.0‰ for CR 16, and 12.3‰ for CR 18A (herein). 3.2. Combined NaCl–NaOCl leaching The δ34SNaCl data of the combined NaCl–NaOCl leaching method are almost identical (in both trends and values) with the data from the NaCl crosscheck. The sulfur isotopic composition of the non-CAS compounds extracted during the subsequent NaOCl leaching differ however, from these δ34SNaCl values. For samples CR 4 and CR 18A the first δ34SNaOCl values are lower than the final and mean δ34SNaCl values (Fig. 1; Table 1), but this is not the case for sample CR 16, which exhibits a more positive initial δ34SNaOCl value (12.9‰ compared with 11.0‰). For sample CR4, δ34SNaOCl values are almost 20‰ lower than the δ34SNaCl values. The consecutive change in δ34SNaOCl values is also not the same for all samples. Whereas sample CR 16 shows a continuous decrease in δ34SNaOCl from 12.9 to 11.9‰, the data for sample CR 18A increase from 11.6 to 15.0‰ during four consecutive leaching steps followed by a decrease to 13.9‰ and a subsequent increase to 14.6‰. The δ34SNaOCl data for CR 4 exhibit an initial decrease from −1.9 to −2.9‰, then a gap, followed by a decrease from 19.5 to 8.7‰. These higher and missing values can be ignored as they were caused by inadvertent addition of acid that dissolved some of the carbonate after the third NaOCl leach. δ34SNaOCl values show little variance from their integrated mean values of −2.4‰ (CR 4), 12.5‰ (CR 16), and 12.8‰ (CR 18A), respectively, reflecting only minor isotopic inhomogeneity in the NaOCl-leachable sulfur component in these samples.

136

T. Wotte et al. / Chemical Geology 326–327 (2012) 132–144

Table 1 Sulfur isotopic composition and sulfate/sulfur abundances of the non-CAS compounds, CAS and CRS, and initial weights for CAS and CRS extractions from the different leaching methods. Numbers correspond to the leaching steps of the NaCl leaching, letters to the NaOCl- respectively the NaOCl- and H2O2 leaching steps subsequent to the NaCl leachings. Furthermore, the integrated mean values of all leaching steps are given. CAS concentrations marked with asterisks are minimum values due to the inadvertent addition of HCl before the leaching process was completed; δ34SCAS values are unaffected.

T. Wotte et al. / Chemical Geology 326–327 (2012) 132–144

137

Table 1 (continued)

δ 34SCAS data for samples CR 16 and CR 18A are slightly higher than those resulting from the NaCl leaching procedures (Fig. 1; Table 1). However, this is not the case for sample CR 4, which has significantly lower δ 34SCAS values (by 1.5‰). In comparison to the δ 34SCRS values extracted after the NaCl leaching, the δ 34SCRS data from the combined NaCl–NaOCl leaching method (Fig. 1; Table 1) are significantly more negative for samples CR 4 (− 7.0‰) and CR 16 (8.3‰), but more positive for sample CR 18A (15.2‰). 3.3. NaOCl leaching The δ 34S data generated from the NaOCl- and the combined NaCl– NaOCl leaching procedures are similar for sample CR 18A with overall mean values for all leaching steps being 13.7‰ and 13.4‰, respectively. The δ 34SNaOCl values for sample CR 16 show an increase from 6.1 to 11.4‰ which opposes the decreasing trend generated from the NaOCl leaching steps of the combined NaCl–NaOCl method. However, overall mean values are similar, being 7.4‰ and 8.1‰, respectively. δ 34S values for the NaOCl leaching show a continuous decrease from 3.4 to −1.0‰ for sample CR 4. This trend as well as the range in δ 34S values is different from those shown in the combined NaCl–NaOCl leaching procedure (cf., Fig. 1; Table 1), but comparable mean values show that this difference can be explained by the inadvertent addition of acid in the latter (see Section 3.2). δ 34SCAS data are less positive than the δ 34SCAS data generated from the NaCl leaching for samples CR 4 (25.0‰) and CR 18A (21.8‰), but, 1.0‰ more positive for sample CR 16 (21.7‰). δ 34SCRS are more positive than the corresponding data generated during the NaCl and NaCl–NaOCl leaching (Fig. 1; Table 1), showing values of 11.5‰ for sample CR 16 and 16.0‰ for sample CR 18A. By contrast, the δ 34SCRS value of sample CR 4 represents the most negative value (− 8.5‰). 3.4. Combined NaCl–H2O2 leaching The combined NaCl–H2O2 method represented the most timeconsuming of the applied leaching procedures due to the oxidative destruction of paper filters which necessitated filtration through cellulose nitrate filters for all steps. As with the combined NaCl–NaOCl leaching, the consecutive change in δ34S of leached non-CAS phases is not the same for all samples. δ34S data generated during the H2O2 leaching display a greater variation than corresponding data from the NaCl–NaOCl leaching. For sample CR 18A, the δ34S data show an increase from 12.3

to 16.6‰ during seven consecutive H2O2 leaching steps followed by a decrease to 13.5‰ (mean=14.1‰). A general increase from 10.5 to 13.5‰ is observed for corresponding data from sample CR 16 (mean=11.3‰). A slight increase from −0.4 to −0.2‰ is observed for sample CR 4 (mean=−0.3‰). Integrated mean values for the H2O2 leaches are similar, but nonetheless significantly different from the NaOCl leaches. The δ 34SCAS values of samples CR4 and CR 18A generated from the combined NaCl–NaOCl leaching method are the same within error as the δ 34SCAS values from the combined NaCl–H2O2 leaching method (Fig. 1; Table 1). In contrast, the δ 34SCAS value of sample CR 16 is slightly more positive for the NaCl–NaOCl leaching (0.4‰). The δ 34SCRS value of sample CR 4 is more positive (− 6.5‰) than the corresponding value following NaOCl and NaCl–NaOCl leaching (Fig. 1; Table 1). By contrast, the δ 34SCRS value of sample CR 18A is clearly more negative (12.4‰), whereas the δ 34SCRS value of sample CR 16 (10.3‰) lies between the corresponding data from the NaOCl and the NaCl–NaOCl leaching (Fig. 1; Table 1).

3.5. Soaking and rinsing with deionized water Due to the small amounts of available material, only limited δ 34S data for leached non-CAS phases could be generated after soaking and rinsing with deionized water. It was not possible to generate data for sample CR 4 and only one single value (13.9‰) was obtained from sample CR 18A. Soaking sample CR 16 resulted in a δ 34S value of 0.5‰, whereas the rinsing process resulted in a more positive value of 1.3‰ (Fig. 1; Table 1). The samples soaked and rinsed with deionized H2O show by far the lowest δ 34SCAS values (Fig. 1; Table 1). δ 34SCRS values are −4.5‰ for sample CR 4, 10.9‰ for sample CR 16, and 14.8‰ for sample CR 18A. In summary, the application of the consecutive leaching steps resulted in a successive decrease in concentration of the non-CAS compounds following each leaching step (Table 1). Although this was always the case for the NaCl leaches, for the oxidant leaching procedures, overall decrease was punctuated by an increase during the first two leaching steps (samples CR 16, CR 18A during H2O2 leaching) or even irregular shifts towards higher concentrations, indicative of active leaching/oxidation. For example, sample CR 18A (steps C to D, and G to H during the H2O2 leaching: 366 ppm to 494 ppm, and 156 ppm to 257 ppm, respectively); and CR 4 (step B to C during the NaOCl leaching step: 22 ppm to 65 ppm).

138

T. Wotte et al. / Chemical Geology 326–327 (2012) 132–144

Fig. 1. Variation in δ34S values generated during the various leaches, δ34SCAS, and δ34SCRS. The δ34SCAS values are significantly more positive than the δ34S data generated during leaching, indicating different and isotopically distinct sources for CAS and non-CAS sulfate. The δ34SNaOCl data of leaches E–G from sample CR 4 are not illustrated as they are caused by an inadvertent addition of acid that dissolved some of the carbonate.

4. Discussion The δ 34S data for all non-CAS components (δ 34SNaCl, δ 34SNaOCl, δ 34SH2O2, δ 34SH2O, δ 34SCRS) are significantly lower than the final measured δ 34SCAS values (Fig. 1; Table 1), indicating different, isotopically distinct sources for non-CAS and CAS sulfate. Variations and

trends in the δ 34S composition of the non-CAS phases indicate multiple, isotopically heterogeneous sources of non-CAS sulfur. The absence of any match between the δ 34S composition of the non-CAS phases, CAS and CRS, or between δ 34SCAS and δ 34SCRS (Fig. 2), illustrates that non-CAS has been successfully isolated from CAS to a large extent.

T. Wotte et al. / Chemical Geology 326–327 (2012) 132–144

However, it is obvious that different leaching methods result in different δ 34SCAS and δ 34SCRS values for the same sample. Moreover, the δ 34SCAS values from our NaCl crosscheck are significantly more positive than the identically derived data, albeit from considerably larger samples, published by Wotte et al. (2012) (Fig. 1; Table 1). In this light, it seems that there remains a risk of incorporating sulfate from non-CAS phases into the analyzed CAS fraction, even in cases where no precipitate could be detected after repeated leaching. Using large amounts of rock powder and inadequate grinding poses a particular problem in this regard. δ 34SCAS data generated during the NaOCl- and H2O2 leaching methods tended to be slightly more positive (up to 1‰) than the corresponding data resulting from NaCl leaching. This is especially true for samples CR 16 and CR 18A. The higher δ 34SCAS values could be a further indication that δ 34SCAS values can be affected by the sulfur isotopic composition of non-CAS compounds, despite exhaustive, consecutive leaching. Comparing the final δ 34SCAS data with the δ 34S values of the non-CAS compounds generated during the several leaching procedures, there is no obvious correlation (Fig. 2). However, when integrated means of δ 34SNaCl leaches are compared, there is indeed a linear correlation between mean δ 34SNaCl and δ 34SCAS (Fig. 3), but none between mean δ 34SNaCl and δ 34SCRS (Fig. 3) or between δ 34SCAS and δ 34SCRS. It is conceivable, therefore, that the amount of NaCl-leachable sulfate, and in particular the deviance in isotopic composition of δ 34SNaCl from δ 34SCAS are the most important factors controlling contamination of measured δ 34SCAS. It appears that rigorous leaching has eliminated most, but not all non-CAS sulfate in our experiments, but this is unlikely to be the case in many published studies. In this regard, we note that the commonly applied tests for contamination, i.e. covariation between δ 34SCAS and δ 34SCRS, or δ 34SCAS and [CAS], or δ 34SCAS and pyrite% (e.g., Shen et al., 2011) may fail to recognize contamination from non-CAS sulfate. Most of the δ 34S values generated during consecutive leaching show a successive convergence towards the sulfur isotopic composition of CRS (Fig. 1; Table 1). This is not only true for leaching with NaCl, but also for the NaOCl and H2O2 leaches. This trend probably indicates the partial dissolution of pyrite (Fig. 3d). NaOCl and H2O2, in particular, have been applied to eliminate pyrite prior to CAS extraction (cf., Hurtgen et al., 2002; Newton et al., 2004; Hurtgen et al., 2006; Gill et al., 2007; Halverson and Hurtgen, 2007; Gill et al., 2008; Xiao et al., 2012). Therefore, it is surprising that high concentrations of CRS could still be observed after consecutive leaching with these reagents. Consequently, it seems that neither NaOCl nor H2O2 is suitable for fully eliminating pyrite sulfur (cf., Brunner, 2004), while only some portion of the reduced sulfur in a sample seems to be readily available for oxidation. As the use of oxidants can only ensure removal of organic sulfur (Burdett et al., 1989), contamination from which seems to be of limited importance in a geological sample, it seems to us unwise to deliberately oxidize organic and some pyrite sulfur to sulfate. Although organic matter should be removed by the addition of chemical bleaches, oxidation of organic sulfur during carbonate dissolution seems to us a less likely source of contamination than, for example, pyrite oxidation or non-CAS sulfate. In order to avoid contamination from unintended sulfide oxidation, it then becomes crucially important that all traces of oxidants and dissolved sulfate are removed before CAS extraction, and that samples are not subjected to drying after pre-leaching, but instead dissolved quickly while still wet (Fig. 4). It also seems likely that the order in which combined bleach–NaCl leaching steps are carried out is highly significant as well as whether only decanting or complete filtration was used to separate supernate from residue after each step. In two modifications of the NaOCl method (Newton et al., 2004; Thompson and Kah, 2012), carbonate dissolution was carried out under elevated pH and O2-free conditions, respectively, in order to minimize sulfide oxidation and oxygen isotope exchange between dissolved sulfate and the aqueous medium. Fe (III) is more soluble at

139

low pH and so becomes the dominant oxidant for pyrite, rather than free oxygen, under acidic conditions (Moses and Herman, 1991), while isotopic exchange has not been demonstrated on the time scales we are considering here (Bottrell and Newton, 2006). Again, we would envisage, therefore, that rapid, incomplete dissolution of pre-leached carbonate slurry, with immediate filtering and without drying at any stage after the initial pre-leach (Fig. 4), may achieve the same goals, at least with regard to reducing inadvertent oxidation of pyrite. Incomplete dissolution has not been applied in any study to our knowledge. The application of NaOCl and H2O2 appears to result in significantly different δ34SCRS values in comparison to the δ 34SCRS data generated from the residual material after NaCl leaching (Figs. 1, 3d; Table 1). This potential effect on the final sulfur isotopic composition of CRS likely depends on isotopic inhomogeneity among sulfide mineral phases, and reflects the partial removal of some pyrite. This should not only be taken into account when interpreting the environmental significance of paired δ34SCAS and δ34SCRS data, but also when discussing the diagenetic alteration of CAS caused by bacterial sulfate reduction and sulfide oxidation (cf., Kampschulte and Strauss, 2004; Wotte et al., 2012). Comparing δ34SCRS data among the various leaches, it seems that the NaCl–H2O2 δ34SCRS values are more similar to those of the bulk pyrite (after NaCl leaching only), than are the NaCl–NaOCl δ34SCRS values. δ34SCAS data from the H2O soaking and rinsing are clearly less positive than the sulfur isotope data generated from any of the other experiments. After taking into consideration the relatively small amounts of sample used (0.65–23.46 g), CAS and CRS concentrations are higher for all three samples (Table 1) which implies the incomplete removal of the non-CAS sulfate components. This is not surprising because deionized H2O is expected to have the lowest potential for dissolving non-CAS phases in comparison to the other leaching methods. Keeping this in mind as well as the fact that even leaching with NaCl was repeated at least three times (even if the used sample amount was higher), soaking and rinsing with deionized water cannot be regarded as an effective procedure for removing non-CAS sulfate from bulk carbonates. In this regard, we note that non-CAS sulfate in our samples was considerably more abundant than CAS in two of our three samples. When regarding all of the samples from the Wotte et al. (2012) study, this was the case for 7% of samples. We note that other anomalously low δ34SCAS outliers from that study are generally associated with lower than average δ34SNaCl values. Our study may therefore have implications for the interpretation of similar low outliers in many published studies to date that are commonly interpreted to relate to shortterm fluctuations in seawater isotopic composition due to low ocean sulfate levels and/or high levels of H2S (Bottrell and Newton, 2006). One crucial aspect for the interpretation of δ 34SCAS data regards the evaluation of diagenetic alteration of the δ 34SCAS signal in particular and the analyzed rock material in general. Even after exhaustive pre-treatments, measured CAS isotopic composition may not be representative of seawater sulfate due to post-depositional alteration. Important proxies for diagenetic alteration include δ 13Ccarb- and δ 18Ocarb values, and elemental concentrations of the carbonate matrix (especially of Ca, Fe, Mn, Mg, and Sr; Fig. 4; cf., Brand and Veizer, 1980; Veizer, 1983; Kaufman et al., 1993; Derry et al., 1994; Kaufman and Knoll, 1995; Veizer et al., 1999; Shields and Veizer, 2002; Hurtgen et al., 2006; Marenco et al., 2008a; Gill et al., 2011b; Wotte et al., 2011, 2012). Furthermore, cross-plots of seawater proxies versus proxies for diagenetic alteration help to illustrate and evaluate a probable diagenetic resetting of primary marine isotope data (cf., Banner and Hanson, 1990; Jacobsen and Kaufman, 1999; Wotte et al., 2007; Derry, 2010; Gill et al., 2011b). Applying this rigid procedure to the samples studied here (cf., Wotte et al., 2012) reveals no obvious substantial diagenetic alteration of the carbonate host rock and, thus, by inference the primary marine nature of the δ 34SCAS values. We add here a word of caution, however, because the most commonly applied diagenetic proxies have so far not been shown to correlate with δ 34SCAS values in any study as far as we are aware.

140

T. Wotte et al. / Chemical Geology 326–327 (2012) 132–144

T. Wotte et al. / Chemical Geology 326–327 (2012) 132–144

141

Fig. 3. Cross-plots of δ34SCAS and δ34SCRS versus integrated mean values of δ34S generated during the different leaching procedures. (a) Cross-plot of δ34SCAS versus integrated means of δ34SNaCl exhibits a linear correlation. (b) There is neither a correlation between δ34SCRS and δ34SNaCl nor between (c) δ34SCAS and the δ34SNaOCl or the δ34SH2O2 values, respectively. (d) The linear correlation between δ34SCRS and the integrated mean values of δ34SNaOCl or δ34SH2O2 values reflects the partial dissolution of pyrite during the application of bleach solutions.

The experiments outlined here demonstrate that the NaCl-soluble, non-CAS sulfate fraction in carbonates can be proportionately far greater than, and isotopically distinct from the lattice-bound carbonate sulfate fraction. The most readily leached, and greater proportion of the non-CAS sulfate is also isotopically distinct from the pyrite sulfide fraction in two of three samples. That non-CAS sulfate can have a

marked effect on measured δ 34SCAS data can best be illustrated by sample CR 16. In this case, readily leachable sulfate is shown to be proportionately five times greater than extracted CAS, and isotopically depleted with respect to both CAS (by 16.7‰) and CRS (by 6‰). If any of the readily leachable sulfate still remained after pre-leaching, then it could have a major effect on final δ 34SCAS values. For example,

Fig. 2. Cross-plots of δ34SCAS and δ34SCRS versus δ34S values generated during leaching. (a) Data from the complete Crémenes section (cf., Wotte et al., 2012). Red circles represent the three re-analyzed samples CR 4, CR 16, and CR 18A. (b) Data plot for the three selected samples as figured in (a) and published in Wotte et al. (2012). (c) Cross-plot of the NaCl crosscheck. (d) Plot of the data, generated during the combined NaCl–NaOCl leaching. (e) Plotted data for the combined NaCl–H2O2 leaching method. (f) δ34SCAS and δ34SCRS cross-plot of the δ34SCAS and δ34SCRS versus δ34SNaOCl data. (g) Plotted data of soaking and rinsing with deionized water.

142

T. Wotte et al. / Chemical Geology 326–327 (2012) 132–144

Fig. 4. Recommended protocol for the extraction of carbonate-associated sulfate (CAS). For determining the diagenetic alteration of rock samples and isotopic composition this protocol needs also to include the analyses of δ13Ccarb, δ18Ocarb, and trace element concentrations. A further important aspect of the protocol is the associated analysis of the concentration and isotopic composition of CRS, relevant for the determination of the diagenetic alteration of CAS, but also for the discussion of paired δ34SCAS and δ34SCRS data in the context of their environmental significance.

T. Wotte et al. / Chemical Geology 326–327 (2012) 132–144

if the NaCl leach was only 94% effective in removing non-CAS, then the δ 34SCAS value for this sample could in reality be 28‰ (instead of 20.7‰; cf. 17.9‰ in Wotte et al., 2012), and so indistinguishable from other samples of the Wotte et al. (2012) study. Such low δ 34SNaCl values as in sample CR 16 suggest that the original source of this sulfur could have been a readily oxidized sulfide mineral such as pyrrhotite or greigite, rather than pyrite, but more work is clearly needed to substantiate this, while the source of readily leachable sulfate seems to us likely to be variable within and between samples. Identifying the source of this contamination should be a priority in future studies of carbonate-associated sulfate. 5. Conclusions Prior to the extraction of carbonate-associated sulfate it is essential to isolate it from all potentially soluble non-CAS sulfur-bearing compounds. Otherwise, a mixed δ 34S signal will be generated which is unlikely to represent the sulfur isotopic composition of seawater at the time of carbonate precipitation. Leaching with deionized water does not eliminate non-CAS sulfate, while a single leaching step with NaOCl or H2O2 is not sufficient to remove the entire non-CAS fraction in a carbonate sample. Our experiments demonstrate that consecutive leaching steps are necessary in order to fully eliminate all non-CAS sulfate. The number of required pre-leaches likely depends on the amount of rock powder, its particle size, and the abundance of non-CAS sulfate or readily oxidizable sulfide. Our experiments also show that leaching with NaOCl and H2O2 will generally not remove all pyrite, while both methods may substantially alter the final δ 34SCRS values, leading to uncertainties in interpreting δ 34SCAS data. Caution needs to be applied when using oxidants, such as NaOCl and H2O2 for CAS pre-leaching, as our experiments demonstrate considerable release of NaCl-insoluble sulfur derived from oxidation of reduced sulfur species, and this can overwhelm the relatively small amounts of CAS sulfate. Comparing the NaCl-, NaOCl-, H2O2 leaching methods in respect to the (1) efficiency in eliminating non-CAS compounds, (2) ease of handling, (3) toxicity, and (4) possible effects on δ 34SCRS we recommend consecutive NaCl leaching as standard protocol in future isotopic studies of carbonate-associated sulfate (Fig. 4). A rigorous extraction procedure using NaCl should be complemented by investigation of CRS (concentration and isotopic composition). δ 13Ccarb, δ 18Ocarb, and elemental concentrations (Ca, Fe, Mn, Mg, and Sr) of the carbonate host rock provide additional important information concerning the degree of diagenetic alteration and, hence, the quality of the resulting δ 34SCAS data (Fig. 4). The application of this recommended analytical protocol permits determination of the (most) primary δ 34SCAS signature and, thus, represents the method of choice for reconstructing the primary seawater sulfate sulfur isotopic composition in the geological past. The relative amount and relative isotopic deviance (from δ 34SCAS) of NaCl-leached non-CAS seem to be the strongest predictors of final measured δ 34SCAS in our study. Acknowledgments This work is part of the project WO 1215/4 generously funded by the German Research Foundation. We thank A. Cording, D. Diekrup, A. Fugmann, A. Lutter, and M. Reuschel (all Westfälische Wilhelms-Universität Münster) for their assistance in the laboratory and constructive discussions. These experiments were conceived and performed during a three-month period at the Westfälische Wilhelms-Universität Münster funding for which GS is grateful to the Alexander von Humboldt Foundation. We gratefully acknowledge the editorial guidance of Uwe Brand (Brock University) and the constructive comments of two anonymous reviewers.

143

References Banner, J.L., Hanson, G.N., 1990. Calculation of simultaneous isotopic and trace element variations during water–rock interaction with applications to carbonate diagenesis. Geochimica et Cosmochimica Acta 54, 3123–3137. Bottrell, S.H., Newton, R.J., 2006. Reconstruction of changes in global sulfur cycling from marine sulfate isotopes. Earth-Science Reviews 75, 59–83. Brand, U., Veizer, J., 1980. Chemical diagenesis of a multicomponent carbonate system — 1: Trace elements. Journal of Sedimentary Petrology 50, 1219–1236. Brunner, B., 2004. The Sulfur cycle: From bacterial microenvironment to global biogeochemical cycles. PhD dissertation ETH Zurich No. 15197, http://dx.doi.org/10.3929/ ethz-a-004597241. Burdett, J.W., Arthur, M.A., Richardson, M., 1989. A Neogene seawater sulfur isotope age curve from calcareous pelagic microfossils. Earth and Planetary Science Letters 94, 189–198. Canfield, D.E., 2001. Isotope fractionation by natural populations of sulfate reducing bacteria. Geochimica et Cosmochimica Acta 65, 1117–1124. Canfield, D.E., Raiswell, R., Westrich, J.T., Reaves, R.C., Berner, R.A., 1986. The use of chromium reduction in the analysis of reduced inorganic sulphur in sediments and shales. Chemical Geology 54, 149–155. Chiba, H., Sakai, H., 1985. Oxygen isotope exchange rate between dissolved sulfate and water at hydrothermal temperatures. Geochimica et Cosmochimica Acta 49, 993–1000. Chu, X., Zhang, T., Zhang, Q., Lyons, T.W., 2007. Sulfur and carbon isotope records from 1700 to 800 Ma carbonates of the Jixian section, northern China: implications for secular isotope variations in Proterozoic seawater and relationships to global supercontinental events. Geochimica et Cosmochimica Acta 71, 4668–4692. Derry, L.A., 2010. A burial diagenesis origin of Ediacaran Shuram–Wonoka carbon isotope anomaly. Earth and Planetary Science Letters 294, 152–162. Derry, L.A., Brasier, M.D., Corfield, R.M., Rozanov, A.Yu., Zhuravlev, A.Yu., 1994. Sr and C isotopes in Lower Cambrian carbonates from the Siberian Craton: a paleoenvironmental record during the “Cambrian explosion”. Earth and Planetary Science Letters 128, 671–681. Dimitrijević, M., Antonijević, M.M., Dimitrijević, V., 1999. Investigation of the kinetics of pyrite oxidation by hydrogen peroxide in hydrochloric acid solutions. Minerals Engineering 12, 165–174. Fike, D.A., Grotzinger, J.P., 2008. A paired sulphate–pyrite δ34S approach to understanding the evolution of the Ediacaran–Cambrian sulphur cycle. Geochimica et Cosmochimica Acta 72, 2636–2648. Fike, D.A., Grotzinger, J.P., Pratt, L.M., Summons, R.E., 2006. Oxidation of the Ediacaran Ocean. Nature 444, 744–747. Gellatly, A.M., Lyons, T.W., 2005. Trace sulfate in mid-Proterozoic carbonates and the sulfur isotope record of biospheric evolution. Geochimica et Cosmochimica Acta 69, 3813–3829. Gill, B.C., Lyons, T.W., Saltzman, M.R., 2007. Parallel, high-resolution carbon and sulphur isotope records of the evolving Paleozoic marine sulphur reservoir. Palaeogeography, Palaeoclimatology, Palaeoecology 256, 156–173. Gill, B.C., Lyons, T.W., Frank, T.D., 2008. Behavior of carbonate-associated sulfate during meteoric diagenesis and implications for the sulfur isotope paleoproxy. Geochimica et Cosmochimica Acta 72, 4699–4711. Gill, B.C., Lyons, T.W., Jenkyns, H.C., 2011a. A global perturbation to the sulfur cycle during the Toarcian Oceanic Anoxic Event. Earth and Planetary Science Letters 312, 484–496. Gill, B.C., Lyons, T.W., Young, S.A., Kump, L.R., Knoll, A.H., Saltzman, M.R., 2011b. Geochemical evidence for widespread euxinia in the Later Cambrian. Nature 269, 80–83. Goldberg, T., Poulton, S.W., Strauss, H., 2005. Sulphur and oxygen isotope signatures of late Neoproterozoic to early Cambrian sulphate, Yangtze Platform, China: diagenetic constraints and seawater evolution. Precambrian Research 137, 223–241. Goldberg, T., Shields, G.A., Newton, R.J., 2011. Analytical constraints on the measurement of the sulfur isotopic composition and concentration of trace sulfate in phosphorites: implications for sulfur isotope studies of carbonate and phosphate rocks. Geostandards and Geoanalytical Research 35, 161–174. Guo, Q., Strauss, H., Kaufman, A.J., Schröder, S., Gutzmer, J., Wing, B., Baker, M.A., Bekker, A., Jin, Q., Kim, S., Farquhar, J., 2009. Reconstructing Earth's surface oxidation across the Archean Paleoproterozoic transition. Geology 37, 399–402. Halverson, G.P., Hurtgen, M.T., 2007. Ediacaran growth of the marine sulfate reservoir. Earth and Planetary Science Letters 263, 32–44. Hurtgen, M.T., Arthur, M.A., Suits, N.S., Kaufman, A.J., 2002. The sulfur isotopic composition of Neoproterozoic seawater sulfate: implications for a snowball Earth? Earth and Planetary Science Letters 203, 413–429. Hurtgen, M.T., Halverson, G.P., Arthur, M.A., Hoffman, P.F., 2006. Sulfur cycling in the aftermath of a 635-Ma snowball glaciation: evidence for a syn-glacial sulfidic deep ocean. Earth and Planetary Science Letters 245, 551–570. Jacobsen, S.B., Kaufman, A.J., 1999. The Sr, C and O isotopic evolution of Neoproterozoic seawater. Chemical Geology 161, 37–57. Kah, L.C., Lyons, T.W., Frank, T.D., 2004. Low marine sulphate and protracted oxygenation of the Proterozoic biosphere. Nature 431, 834–837. Kampschulte, A., Strauss, H., 1996. The sulphur isotopic composition of Jurassic and Cretaceous seawater as deduced from trace sulfate in belemnites. Journal of Conference Abstracts 1, 303. Kampschulte, A., Strauss, H., 1998. The isotopic composition of trace sulphates in Palaeozoic biogenic carbonates: implications for coeval seawater and geochemical cycles. Mineralogical Magazine 62A, 744–745. Kampschulte, A., Strauss, H., 2004. The sulphur isotopic evolution of Phanerozoic seawater based on the analysis of structurally substituted sulphate in carbonates. Chemical Geology 204, 255–286.

144

T. Wotte et al. / Chemical Geology 326–327 (2012) 132–144

Kampschulte, A., Bruckschen, P., Strauss, H., 2001. The sulphur isotopic composition of trace sulphates in Carboniferous brachiopods: implications for coeval seawater, correlation with other geochemical cycles and isotope stratigraphy. Chemical Geology 175, 149–173. Kaufman, A.J., Knoll, A.H., 1995. Neoproterozoic variations in the C-isotopic composition of seawater: stratigraphic and biogeochemical implications. Precambrian Research 73, 27–49. Kaufman, A.J., Jacobsen, S.B., Knoll, A.H., 1993. The Vendian record of C- and Sr-isotopic variations: implications for tectonics and paleoclimate. Earth and Planetary Science Letters 120, 409–430. Li, C., Love, G.D., Lyons, T.W., Fike, D.A., Sessions, A.L., Chu, X., 2010. A stratified redox model for the Ediacaran ocean. Science 328, 80–83. Loyd, S.J., Berelson, W.M., Lyons, T.W., Hammond, D.E., Corsetti, F.A., 2012a. Constraining pathways of microbial mediation for carbonate concentrations of the Miocene Monterey Formation using carbonate-associated sulfate. Geochimica et Cosmochimica Acta 78, 77–98. Loyd, S.J., MArenco, P.J., Hagadorn, J.W., Lyons, T.W., Kaufman, A.J., Sour-Tovar, F., Corsetti, F.A., 2012b. Sustained low marine sulfate concentrations from the Neoproterozoic to the Cambrian: Insights from carbonates of northwestern Mexico and eastern California. Earth and Planetary Science Letters 339–340, 79–94. Lyons, T.W., Walter, L.M., Gellatly, A.M., Martini, A.M., Blake, R.E., 2004. Sites of anomalous organic remineralization in the carbonate sediments of South Florida, USA: the sulfur cycle and carbonate-associated sulfate. In: Amend, J.P., Edwards, K.J., Lyons, T.W. (Eds.), Sulfur biogeochemistry–past and present: Geological Society of America Special Paper, 379, pp. 161–176. Marenco, P.J., Corsetti, F.A., Kaufman, A.J., Bottjer, D.J., 2008a. Environmental and diagenetic variations in carbonate associated sulfate: an investigation of CAS in the Lower Triassic of the western USA. Geochimica et Cosmochimica Acta 72, 1570–1582. Marenco, P.J., Corsetti, F.A., Hammond, D.E., Kaufman, A.J., Bottjer, D.J., 2008b. Oxidation of pyrite during extraction of carbonate associated sulphate. Chemical Geology 247, 124–132. Moses, C.O., Herman, J.S., 1991. Pyrite oxidation at circumneutral pH. Geochimica et Cosmochimica Acta 55, 471–482. Newton, R.J., Pevitt, E.L., Wignall, P.B., Bottrell, S.H., 2004. Large shifts in the isotopic composition of seawater sulphate across the Permo–Triassic boundary in northern Italy. Earth and Planetary Science Letters 218, 331–345. Ohkouchi, N., Kawamura, K., Kajiwara, Y., Wada, E., Okada, M., Kanamatsu, T., Taira, A., 1999. Sulfur isotope records around Livello Bonarelli (northern Apennines, Italy) black shale at the Cenomanian-Turonian boundary. Geology 27, 535–538. Riccardi, A.L., Arthur, M.A., Kump, L.R., 2006. Sulfur isotopic evidence for chemocline upward excursions during the end-Permian mass extinction. Geochimica et Cosmochimica Acta 70, 5740–5752. Ries, J.B., Fike, D.A., Pratt, L.M., Lyons, T.W., Grotzinger, J.P., 2009. Superheavy pyrite (δ34Spyr >δ34SCAS) in the terminal Proterozoic Nama Group, southern Namibia: a consequence of low seawater sulfate at the dawn of animal life. Geology 37, 743–746. Shen, B., Xiao, S., Kaufman, A.J., Bao, H., Zhou, C., Wang, H., 2008. Stratification and mixing of a post-glacial Neoproterozoic ocean: evidence from carbon and sulfur isotopes in a cap dolostone from northwest China. Earth and Planetary Science Letters 265, 209–228. Shen, B., Xiao, S., Zhou, C., Kaufman, A.J., Yuan, X., 2010. Carbon and sulfur isotope chemostratigraphy of the Neoproterozoic Quanji Group of the Chaidam Basin, NW China. Basin stratification in the aftermath of an Ediacaran glaciations postdating the Shuram event? Precambrian Research 177, 241–252. Shen, B., Xiao, S., Bao, H., Kaufman, A.J., Zhou, C., Yuan, X., 2011. Carbon, sulfur, and oxygen isotope evidence for a strong depth gradient and oceanic oxidation after the Ediacaran Hankalchough glaciations. Geochimica et Cosmochimica Acta 75, 1357–1373.

Shields, G.A., Veizer, J., 2002. Precambrian marine carbonate isotope database: version 1.1. Geochemistry, Geophysics, Geosystems 3, 1031 http://dx.doi.org/10.1029/ 2001GC000266. Strauss, H., 1999. Geological evolution from isotope proxy signals –– sulfur. Chemical Geology 161, 89–101. Strauss, H., 2004. 4 Ga of seawater evolution: evidence from the sulphur isotopic composition of sulphate. In: Amend, J.P., Edwards, K.J., Lyons, T.W. (Eds.), Sulfur biogeochemistry–past and present: Geological Society of America Special Paper, 379, pp. 195–205. Thompson, C.K., Kah, L.C., 2012. Sulfur isotope evidence for widespread euxinia and a fluctuating oxycline in Early to Middle Ordovician greenhouse oceans. Palaeogeography, Palaeoclimatology, Palaeoecology 313–314, 189–214. Ueda, A., Campbell, F.A., Krouse, H.R., Spencer, R.J., 1987. 34S/32S variations in trace sulphide and sulphate in carbonate rocks of a Devonian reef, Alberta, Canada, and the Precambrian Siyeh Formation, Montana, U.S.A. Chemical Geology 65, 383–390. Veizer, J., 1983. Chemical diagenesis of carbonates: theory and application of trace element techniques. In: Arthur, M.A., Anderson, T.F., Kaplan, I.R., Veizer, J., Land, L.S. (Eds.), Stable isotopes in sedimentary geology: SEPM Short Course, 10, pp. 3.1–3.100. Veizer, J., Ala, D., Azmy, K., Bruckschen, P., Buhl, D., Bruhn, F., Carden, G.A.F., Diener, A., Ebneth, S., Godderis, Y., Jasper, T., Korte, C., Pawellek, F., Podlaha, O.G., Strauss, H., 1999. 87Sr/86Sr, δ13C and δ18O evolution of Phanerozoic seawater. Chemical Geology 161, 59–88. Westgate, L.M., Anderson, T.F., 1982. Extraction of various forms of sulfur from coal and shale for stable sulfur isotope analysis. Analytical Chemistry 54, 2136–2139. Wotte, T., 2005. Facies distribution patterns of the upper member of the Láncara Formation in the Somiedo-Correcilla unit (Lower–Middle Cambrian, Cantabrian zone, NW Spain) with special respect to biofacial investigations. Geosciences Journal 9, 389–402. Wotte, T., 2009. Re-interpretation of an Early–Middle Cambrian West Gondwanan ramp depositional system: a case study from the Cantabrian Zone (NW Spain). Facies 55, 473–487. Wotte, T., Álvaro, J.J., Shields, G.A., Brown, B., Brasier, M., Veizer, J., 2007. C-, O- and Srisotope stratigraphy across the Lower–Middle Cambrian transition of the Cantabrian zone (Spain) and the Montagne Noire (France), West Gondwana. Palaeogeography, Palaeoclimatology, Palaeoecology 256, 47–70. Wotte, T., Strauss, H., Sundberg, F.A., 2011. Carbon and sulfur isotopes from the Cambrian Series 2-Cambrian Series 3 of Laurentia and Siberia. In: Hollingsworth, J.S., Sundberg, F.A., Foster, J.R. (Eds.), Cambrian stratigraphy and paleontology of Northern Arizona and Southern Nevada, The 16th Field Conference of the Cambrian Stage Subdivision Working Group, International Subcommission On Cambrian Stratigraphy Flagstaff, Arizona, And Southern Nevada, United States: Museum of Northern Arizona Bulletin, 67, pp. 43–63. Wotte, T., Strauss, H., Fugmann, A., Garbe-Schönberg, D., 2012. Paired δ34S data from carbonate-associated sulfate and chromium-reducible sulfur across the traditional Lower–Middle Cambrian boundary of W-Gondwana. Geochimica et Cosmochimica Acta 85, 228–253. Xiao, S., McFadden, K.A., Peek, S., Kaufman, A.J., Zhou, C., Jiang, G., Hu, J., 2012. Integrated chemostratigraphy of the Doushantuo Formation at the northern Xiaofenghe section (Yangtze Gorges, South China) and its implication for Ediacaran stratigraphic correlation and ocean redox models. Precambrian Research 192–195, 125–141. Zhang, T., Chu, X., Zhang, Q., Feng, L., Huo, W., 2003. Variations of sulfur and carbon isotopes in seawater during the Doushantuo stage in late Neoproterozoic. Chinese Science Bulletin 48, 1375–1380.