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

Abstract

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.