Zheyu Tian , Graham Anthony Shields , Ying Zhou

Carbonate-associated sulfate (CAS) refers to trace amounts of sulfate incorporated into carbonate minerals during precipitation. CAS has been the most commonly used approach to recover the paleo-seawater sulfate sulfur isotope composition (δ34Ssw) as carbonate rocks are more common and occur in less restricted marine environments than alternative sulfate-bearing minerals (such as gypsum and anhydrite). However, uncertainties remain about the reliability of preparation techniques due to the inadvertent inclusion of contaminant sulfurous species. This study applied three oxidative leaching CAS extraction methods and compared their final δ34SCAS values with those following repeated single leaching in 10% NaCl (aq). The final δ34SCAS values after sequential leaching by a combined 12% NaOCl, 1% H2O2, and 10% NaCl approach were systematically higher by between 0.65‰ and 0.90‰ than rival methods. Our experiments indicate that contamination can affect measured δ34SCAS even given short reaction times (<30 min). A single leach with standard oxidizing reagents may not entirely eliminate contamination when handling organic and pyrite-rich carbonate samples.