This study investigates the validity of geochemical paleoredox proxies in mature organic-rich shales. Classically, Fe-S-Corg ternary diagrams and C-S crossplots have been used to differentiate between oxic, dysoxic, and anoxic / euxinic conditions, but should be limited to immature shales due to the loss of organic carbon during thermal maturation. Trace metal proxies are also used to elucidate paleoredox conditions, but their stability in mature shales has yet to be properly assessed. Due to the difficulty of finding a natural shale bed with an immature and post-mature end-member that was deposited under invariant depositional conditions, a modified version of open-system hydrous pyrolysis (OSHP) has been developed. The design of this artificial maturation apparatus differs from previous designs in that a small continuous flow of only liquid phase water is maintained throughout the entire pyrolysis experiment.
Using the OSHP apparatus, the Anna Shale (Pennsylvanian age) has been artificially matured to create a complete maturation gradient ranging from immature through the gas window. Geochemical analyses of splits along the gradient have allowed the use of certain paleoredox proxies to be assessed beyond the immature range. Preliminary geochemical analyses from this study suggest that proxies involving sulfur and molybdenum may be altered significantly as early as at the beginning of the oil window.
Varying models for organic matter (OM) accumulation argue either for enhanced preservation under existing anoxic conditions (Demaison and Moore, 1980), or for anoxia driven by high surficial productivity (Pederson and Calvert, 1990). Regardless, both models result in marine bottom waters that are oxygen-depleted, thus facilitating enhanced preservation of OM.
In recent decades, proxies for paleoredox conditions have been developed using modern anoxic environments in order to understand the conditions under which ancient black shales formed. The use of organic carbon (Corg) and pyritic sulfur (Spyr) crossplots (Berner and Raiswell, 1983, 1984; Leventhal, 1983, 1987; Morse and Berner, 1995), degree of pyritization (DOP; Raiswell et al., 1988; Hatch and Leventhal, 1992), and Fe-S-Corg ternary diagrams (Dean and Arthur, 1989; Lyons and Berner, 1992; Arthur and Sageman, 1994) have classically been used to differentiate between prevailing normal marine (oxic) and euxinic bottom-water conditions. However, due to the inherent losses of 40-60﹪ of the carbon from kerogen during thermal maturation (Raiswell and Berner, 1987), paleoredox proxies involving Corg should be limited to immature shales.
