Sedimentology (2019) 66, 2605–2626 doi: 10.1111/sed.12615 Diagenetic controls on the isotopic composition of carbonate- associated sulphate in the Permian Capitan Reef Complex, West Texas THEODORE M. PRESENT* , MELISSA GUTIERREZ* , GUILLAUME PARIS*1 , CHARLES KERANS† , JOHN P. GROTZINGER* and JESS F. ADKINS* *California Institute of Technology, 1200 E. California Blvd., m/c 100-23, Pasadena, CA 91125, USA (E-mail: [email protected]) †Jackson School of Geosciences, The University of Texas at Austin, 2305 Speedway Stop C9000, Austin, TX 78712, USA Associate Editor – Nicholas Tosca ABSTRACT Late Palaeozoic-age strata from the Capitan Reef in west Texas show facies- dependent heterogeneity in the sulphur isotopic composition of carbonate- associated sulphate, which is trace sulphate incorporated into carbonate min- erals that is often used to reconstruct the sulphur isotopic composition of ancient seawater. However, diagenetic pore fluid processes may influence the sulphur isotopic composition of carbonate-associated sulphate. These pro- cesses variously modify the sulphur isotopic composition of incorporated sul- phate from syndepositional seawater in shelf crest, outer shelf, shelf margin and slope depositional settings. This study used a new multicollector induc- tively-coupled plasma mass spectrometry technique to determine the sulphur isotopic composition of samples of individual depositional and diagenetic tex- tures. Carbonate rocks representing peritidal facies in the Yates and Tansill formations preserve the sulphur isotopic composition of Guadalupian seawa- ter sulphate despite alteration of the carbon and oxygen isotopic compositions by meteoric and dolomitizing diagenetic processes. However, sulphur isotopic data indicate that limestones deposited in reef and slope facies in the Capitan and Bell Canyon formations largely incorporate sulphate from anoxic marine- phreatic pore fluids isotopically modified from seawater by microbial sulphate reduction, despite generally preserving the carbon and oxygen isotopic com- positions of Permian seawater. Some early and all late meteoric calcite cements have carbonate-associated sulphate with a sulphur isotopic composi- tion distinct from that of Permian seawater. Detailed petrographic and sedi- mentary context for carbonate-associated sulphate analyses will allow for improved reconstructions of ancient seawater composition and diagenetic conditions in ancient carbonate platforms. The results of this study indicate that carbonate rocks that diagenetically stabilize in high-energy environments without pore fluid sulphate gradients can provide a robust archive of ancient seawater’s sulphur isotopic composition. Keywords Carbonate-associated sulphate, diagenesis, Guadalupe Moun- tains, Palaeozoic seawater, Permian Reef, stable isotopes. 1Present address: Centre de Recherches Petrographiques et Geochimiques, Le Centre national de la Recherche Sci- entifique, Universite de Lorraine, 15 rue Notre Dame des Pauvres BP 20, 54500 Vandœuvre-les-Nancy, France © 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists 2605 2606 T. M. Present et al. INTRODUCTION material is rare in the geological record and Sulphate – a metabolically-available, major ion absent in Precambrian strata. Therefore, CAS in in seawater – links the cycles of carbon, oxygen bulk carbonate rock is an attractive proxy for and iron through Earth’s oceans (Garrels & Ler- reconstructing the isotopic composition of man, 1984; Bottrell & Newton, 2006). Small ancient seawater because carbonates are widely- amounts are incorporated into the lattice of car- deposited in space and time in the rock record. bonate minerals [called carbonate-associated Studies of CAS in bulk carbonate material have sulphate (CAS); Burdett et al., 1989; Kamp- concluded that early diagenetic processes such as schulte & Strauss, 2004] and its sulphur isotopic neomorphism, dolomitization and authigenic composition [reported in d34S notation as part- cementation may incorporate sulphate from dia- per-thousand changes in 34S/32S from the genetically-modified or non-seawater fluids Vienna-Canyon Diablo Troilite (V-CDT) refer- (Goldberg et al., 2005; Riccardi et al., 2006; Gill ence standard] may track that of the ancient sea- et al., 2008; Loyd et al., 2012a; Rennie & water from which the minerals precipitated. A Turchyn, 2014; Baldermann et al., 2015; Present history of the d34S of seawater sulphate con- et al., 2015; Feng et al., 2016; Fichtner et al., strains ancient biogeochemical budgets because 2017). Screening for the effect of such processes secular enrichment in the d34S of seawater sul- typically entails textural, trace metal, and carbon phate represents increased burial of sulphide and oxygen isotope analyses associated with minerals with lower d34S produced by anoxic meteoric or burial diagenesis that may not corre- microbial sulphate reduction (Garrels & Lerman, late with sulphur isotope alteration (Goldberg 1984). et al., 2005; Gill et al., 2011a, 2011b; Yan et al., In Precambrian through to early Mesozoic suc- 2013; Fichtner et al., 2017). Because the early dia- cessions, CAS data have highly variable isotopic genetic history of a carbonate rock typically varies compositions, especially during biotic crises (Kah with its depositional setting, a thorough account- et al., 2004; Newton et al., 2004; Gill et al., 2007, ing of early diagenetic effects on CAS d34S 2011b; Marenco et al., 2008; Li et al., 2009; John requires detailed sedimentological context. et al., 2010; Loyd et al., 2012b; Thompson & Kah, To discern the sulphur isotope composition of 2012; Wotte et al., 2012; Yan et al., 2013; Song ancient seawater when biogenic calcite is et al., 2014; Algeo et al., 2015; Sim et al., 2015; unavailable or poorly preserved, CAS analyses L. Zhang et al., 2015; Bernasconi et al., 2017; are here considered in the context of a regional Schobben et al., 2017; Witts et al., 2018). If repre- depositional facies model and diagenetic frame- sentative of seawater, these data imply a small work. The CAS and carbonate carbon and oxy- marine sulphate reservoir that could change gen isotope results are reported from the late rapidly or have spatial gradients. However, only a Guadalupian-age platform and slope carbonates few studies have systematically examined the of the Capitan Reef in the Permian Basin, West role of depositional setting, including overprint- Texas, USA. In outcrop, detailed biostratigraphic ing diagenetic regimes, on the isotopic composi- and sequence stratigraphic models tightly con- tion of CAS (Marenco et al., 2008; Wotte et al., strain shelf to basin correlations (Silver & Todd, 2012; Osburn et al., 2015), despite chemostrati- 1969; Tyrrell, 1969; Borer & Harris, 1991; Osle- graphic records being constructed from various ger, 1998; Kerans & Tinker et al., 1999; Sarg palaeoenvironments. Was Palaeozoic seawater et al., 1999; Wilde et al., 1999; Lambert et al., sulphate indeed heterogeneous? 2002; Rush & Kerans, 2010). This framework Carbonate-associated sulphate in recent bio- allows comparison of the d34S of CAS in carbon- genic carbonate accurately preserves the d34Sof ates deposited in the same body of water, but in modern seawater (Burdett et al., 1989; Kamp- different depositional settings. Further, well- schulte et al., 2001; Paris et al., 2014b; Rennie documented petrographic and geochemical fea- et al., 2018) and well-preserved biogenic carbon- tures in these carbonates record diverse diage- ate, especially low-magnesium calcite in bra- netic processes during deposition, burial and chiopods and belemnites, may robustly preserve uplift (Newell et al., 1953; Mazzullo & Cys the composition of ancient seawater (Kamp- et al., 1977; Schmidt, 1977; Yurewicz, 1977; schulte & Strauss, 2004; Gill et al., 2011a; Newton Given & Lohmann, 1986; Garber et al., 1989; et al., 2011; Wu et al., 2014; Present et al., 2015). Mruk, 1989; Scholle et al., 1992; Mutti & Simo, Unfortunately, such well-preserved fossiliferous 1993, 1994; Mazzullo, 1999; Melim & Scholle © 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology, 66, 2605–2626 Diagenesis and isotopic composition of carbonate-associated sulphate 2607 et al., 1999; Frost et al., 2012; Budd et al., 2013; 1969; Tyrrell, 1969; Borer & Harris, 1991; Osleger, Bishop et al., 2014). Understanding the deposi- 1998; Kerans & Tinker et al., 1999; Sarg et al., tional environments and diagenetic regimes that 1999; Wilde et al., 1999; Lambert et al., 2002; affect CAS allows further exploration of the Rush & Kerans, 2010). Samples for this study expression of sedimentary biogeochemical pro- were collected from the Yates, Tansill, Capitan cesses in the rock record. and Bell Canyon formations, whose stratigraphy and diagenesis are described below. The northern wall of North McKittrick Can- GEOLOGICAL AND GEOCHEMICAL yon, in Guadalupe Mountains National Park, SETTING hosts a well-exposed shelf to basin transect made accessible by the Permian Reef Geology The Capitan Reef rims a mixed carbonate and sili- Trail (Fig. 1B) (King, 1942; Newell et al., 1953; ciclastic shelf that surrounded the restricted, Bebout & Kerans, 1993). Well-bedded shelf dolo- epeiric seawater of the Delaware Basin during the mites and sandstones of the Yates Formation late Guadalupian Epoch (Fig. 1A) (Lang, 1937; interfinger with massive and poorly-bedded reef King, 1942; Ward et al., 1986). Following burial, and slope dolomitic limestones of the middle Cenozoic extension exhumed