Olenekian sulfur isotope records: Deciphering global trends, links to marine redox changes and faunal evolution

The sulfur (S) isotope composition of carbonate associated sulfate (CAS) in carbonate rocks has been used to assess variations in paleo-oceanographic redox conditions and its relationship to biotic changes in Earth's history, including the Smithian - Spathian transition. However, previous CAS studies of the Olenekian are mostly based on nearshore continental shelf sections and report highly variable delta 34S values mostly offset from those of contemporaneous evaporites, casting doubt on the utility of the CAS proxy during this interval. The current study presents new CAS isotopic data from three well-dated carbonate successions which were deposited in continental shelf (Qiakong) and offshore marine (Wadi Musjah and Jebel Aweri) environments during the Olenekian (Smithian - Spathian). The aim of the study was to constrain the temporal and spatial variations in sulfur cycling and its relation to marine redox and faunal changes across the Smithian - Spathian transition (ca. 250.5-248.8 Ma). The CAS dataset is complemented by rare earth element (REE) concentration data and thin section petrography. Using a suite of optical and geochemical techniques, the preservation of near-primary CAS isotopic information in the studied samples is evaluated. Results indicate that of the three sections investigated, the offshore sections mostly preserve near-primary marine sulfate S-isotope compositions while the continental shelf Qiakong section suffers from post-depositional alteration of CAS. Comparisons of our new, as well as previously published CAS delta 34S data, with the evaporite delta 34S record suggests that although Olenekian CAS delta 34S values may have been modified by diagenetic processes, a global and primary seawater delta 34S trend can be delineated as follows: seawater delta 34S values increased across the middle Smithian and Smithian - Spathian boundary (SSB). Based on our new CAS data, this increase was in the order of 9 parts per thousand over ca. 1.14 million years. Other short-term variability in the CAS delta 34S record most likely reflects diagenetic processes. The middle Smithian to SSB delta 34S increase is attributed to a global increase in microbial sulfate reduction and pyrite burial associated with decreasing ocean dissolved oxygen during this time. Calculations of the rate of sulfur cycling and box modeling constraints indicate that Olenekian marine sulfur cycle perturbations occurred while the seawater sulfate reservoir only had between 10 and 25% of the modern marine sulfate inventory. Furthermore, results from the current study suggest that variations in ocean dissolved oxygen levels, inferred from the delta 34S and REE data, are not consistently correlated with nektonic faunal changes during the Olenekian in the studied sections. As such, faunal turnover during the Olenekian is unlikely to be explained exclusively by abiotic factors such as oceanatmosphere oxygenation levels.